# Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum

N. E. Raouafi, L. Matteini, J. Squire, S. T. Badman, M. Velli, K. G. Klein, C.

H. K. Chen, W. H. Matthaeus, A. Szabo, M. Linton, et al.

► To cite this version:

N. E. Raouafi, L. Matteini, J. Squire, S. T. Badman, M. Velli, et al. Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum. Space Science Reviews, 2023, 219, 10.1007/s11214-023-00952-4.
insu-04473218

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https://insu.hal.science/insu-04473218

Submitted on 22 Feb 2024

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# Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum

N. E. Raouafi, L. Matteini, J. Squire, S. T. Badman, M. Velli, K. G. Klein, C.

H. K. Chen, W. H. Matthaeus, A. Szabo, M. Linton, et al.

► To cite this version:

N. E. Raouafi, L. Matteini, J. Squire, S. T. Badman, M. Velli, et al. Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum. Space Science Reviews, 2023, 219, 10.1007/s11214-023-00952-4.
insu-04473218

HAL Id: insu-04473218

https://insu.hal.science/insu-04473218v1

Submitted on 22 Feb 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum

N.E. Raouafi¹ Ⓓ ID • L. Matteini² Ⓓ ID • J. Squire³ Ⓓ ID • S.T. Badman⁴,⁵ Ⓓ ID • M. Velli⁶ Ⓓ ID •

K.G. Klein⁷ Ⓓ ID • C.H.K. Chen⁸ Ⓓ ID • W.H. Matthaeus⁹ Ⓓ ID • A. Szabo¹⁰ Ⓓ ID • M. Linton¹¹ •

R.C. Allen¹ Ⓓ ID • J.R. Szalay¹² Ⓓ ID • R. Bruno¹³ Ⓓ ID • R.B. Decker¹ • M. Akhavan-Tafti¹⁴ Ⓓ ID •

O.V. Agapitov⁵ Ⓓ ID • S.D. Bale⁵,¹⁵ Ⓓ ID • R. Bandyopadhyay¹² Ⓓ ID • K. Battams¹¹ Ⓓ ID •

L. Berčič¹⁶ Ⓓ ID • S. Bourouaine¹ Ⓓ ID • T.A. Bowen⁵ Ⓓ ID • C. Cattell¹⁷ Ⓓ ID •

B.D.G. Chandran¹⁸,¹⁹ Ⓓ ID • R. Chhiber⁹,¹⁰ Ⓓ ID • C.M.S. Cohen²⁰ Ⓓ ID • R. D'Amicis¹³ Ⓓ ID •

J. Giacalone⁷ Ⓓ ID • P. Hess¹¹ Ⓓ ID • R.A. Howard¹ Ⓓ ID • T.S. Horbury² Ⓓ ID • V.K. Jagarlamudi¹ Ⓓ ID •

C.J. Joyce²¹ Ⓓ ID • J.C. Kasper¹⁴,²² Ⓓ ID • J. Kinnison¹ • R. Laker² Ⓓ ID • P. Liewer²³ Ⓓ ID •

D.M. Malaspina²⁴,²⁵ Ⓓ ID • I. Mann²⁶ Ⓓ ID • D.J. McComas¹² Ⓓ ID • T. Niembro-Hernandez⁴ Ⓓ ID •

T. Nieves-Chinchilla¹⁰ Ⓓ ID • O. Panasenco²⁷ Ⓓ ID • P. Pokorný²⁸,²⁹,³⁰ Ⓓ ID • A. Pusack²⁵ Ⓓ ID •

M. Pulupa⁵ Ⓓ ID • J.C. Perez³¹ Ⓓ ID • P. Riley³² Ⓓ ID • A.P. Rouillard³³ Ⓓ ID • C. Shi⁶ Ⓓ ID •

G. Stenborg¹ Ⓓ ID • A. Tenerani³⁴ Ⓓ ID • J.L. Verniero¹⁰ Ⓓ ID • N. Viall¹⁰ Ⓓ ID • A. Vourlidas¹ Ⓓ ID •

B.E. Wood¹¹ Ⓓ ID • L.D. Woodham² Ⓓ ID • T. Woolley² Ⓓ ID •

Received: 8 July 2022 / Accepted: 4 January 2023 / Published online: 1 February 2023
© The Author(s) 2023

## Abstract

Launched on 12 Aug. 2018, NASA's Parker Solar Probe had completed 13 of its scheduled 24 orbits around the Sun by Nov. 2022. The mission's primary science goal is to determine the structure and dynamics of the Sun's coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Parker Solar Probe returned a treasure trove of science data that far exceeded quality, significance, and quantity expectations, leading to a significant number of discoveries reported in nearly 700 peer-reviewed publications. The first four years of the 7-year primary mission duration have been mostly during solar minimum conditions with few major solar events. Starting with orbit 8 (i.e., 28 Apr. 2021), Parker flew through the magnetically dominated corona, i.e., sub-Alfvénic solar wind, which is one of the mission's primary objectives. In this paper, we present an overview of the scientific advances made mainly during the first four years of the Parker Solar Probe mission, which go well beyond the three science objectives that are: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles.

**Keywords** Sun • Corona • Solar wind • Plasma • Magnetic fields • Coronal mass ejections • Parker solar probe
# Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum

N.E. Raouafi¹ ID • L. Matteini² ID • J. Squire³ ID • S.T. Badman⁴,⁵ ID • M. Velli⁶ ID •

K.G. Klein⁷ ID • C.H.K. Chen⁸ ID • W.H. Matthaeus⁹ ID • A. Szabo¹⁰ ID • M. Linton¹¹ •

R.C. Allen¹ ID • J.R. Szalay¹² ID • R. Bruno¹³ ID • R.B. Decker¹ • M. Akhavan-Tafti¹⁴ ID •

O.V. Agapitov⁵ ID • S.D. Bale⁵,¹⁵ ID • R. Bandyopadhyay¹² ID • K. Battams¹¹ ID •

L. Berčič¹⁶ ID • S. Bourouaine¹ ID • T.A. Bowen⁵ ID • C. Cattell¹⁷ ID •

B.D.G. Chandran¹⁸,¹⁹ ID • R. Chhiber⁹,¹⁰ ID • C.M.S. Cohen²⁰ ID • R. D'Amicis¹³ ID •

J. Giacalone⁷ ID • P. Hess¹¹ ID • R.A. Howard¹ ID • T.S. Horbury² ID • V.K. Jagarlamudi¹ ID •

C.J. Joyce²¹ ID • J.C. Kasper¹⁴,²² ID • J. Kinnison¹ • R. Laker² ID • P. Liewer²³ ID •

D.M. Malaspina²⁴,²⁵ ID • I. Mann²⁶ ID • D.J. McComas¹² ID • T. Niembro-Hernandez⁴ ID •

T. Nieves-Chinchilla¹⁰ ID • O. Panasenco²⁷ ID • P. Pokorný²⁸,²⁹,³⁰ ID • A. Pusack²⁵ ID •

M. Pulupa⁵ ID • J.C. Perez³¹ ID • P. Riley³² ID • A.P. Rouillard³³ ID • C. Shi⁶ ID •

G. Stenborg¹ ID • A. Tenerani³⁴ ID • J.L. Verniero¹⁰ ID • N. Viall¹⁰ ID • A. Vourlidas¹ ID •

B.E. Wood¹¹ ID • L.D. Woodham² ID • T. Woolley² ID •

Received: 8 July 2022 / Accepted: 4 January 2023 / Published online: 1 February 2023
© The Author(s) 2023

## Abstract

Launched on 12 Aug. 2018, NASA's Parker Solar Probe had completed 13 of its scheduled 24 orbits around the Sun by Nov. 2022. The mission's primary science goal is to determine the structure and dynamics of the Sun's coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Parker Solar Probe returned a treasure trove of science data that far exceeded quality, significance, and quantity expectations, leading to a significant number of discoveries reported in nearly 700 peer-reviewed publications. The first four years of the 7-year primary mission duration have been mostly during solar minimum conditions with few major solar events. Starting with orbit 8 (i.e., 28 Apr. 2021), Parker flew through the magnetically dominated corona, i.e., sub-Alfvénic solar wind, which is one of the mission's primary objectives. In this paper, we present an overview of the scientific advances made mainly during the first four years of the Parker Solar Probe mission, which go well beyond the three science objectives that are: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles.

**Keywords** Sun • Corona • Solar wind • Plasma • Magnetic fields • Coronal mass ejections • Parker solar probe
# 1 Introduction

Parker Solar Probe (PSP; Fox et al. 2016; Raouafi 2022) is flying closer to the Sun than any previous spacecraft (S/C). Launched on 12 Aug. 2018, on 6 Dec. 2022 PSP had completed 14 of its 24 scheduled perihelion encounters (Encs.)<fnref n="1" /> around the Sun over the 7-year nominal mission duration. The S/C flew by Venus for the fifth time on 16 Oct. 2021, followed by the closest perihelion of 13.28 solar radii ($R_⊕$) on 21 Nov. 2021. The S/C will remain on the same orbit for a total of seven solar Encs. After Enc. 16, PSP is scheduled to fly by Venus for the sixth time to lower the perihelion to 11.44 $R_⊕$ for another five orbits. The seventh and last Venus gravity assist (VGA) of the primary mission is scheduled for 6 Nov. 2024. This gravity assist will set PSP for its last three orbits of the primary mission. The perihelia of orbits 22, 23, and 24 of 9.86 $R_⊕$ will be on 24 Dec. 2024, 22 Mar. 2025, and 19 Jun. 2025, respectively.

The mission's overarching science objective is to determine the structure and dynamics of the Sun's coronal magnetic field and to understand how the corona is heated, the solar wind accelerated, and how energetic particles are produced and their distributions evolve. The PSP mission targets processes and dynamics that characterize the Sun's expanding corona and solar wind, enabling the measurement of coronal conditions leading to the nascent solar wind and eruptive transients that create space weather. PSP is sampling the solar corona and solar wind to reveal how it is heated and the solar wind and solar energetic particles (SEPs) are accelerated. To achieve this, PSP measurements will be used to address the following three science goals: (1) Trace the flow of energy that heats the solar corona and accelerates the solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles. Understanding these phenomena has been a top science goal for over six decades. PSP is primarily an exploration mission that is flying through one of the last unvisited and most challenging regions of space within our solar system, and the potential for discovery is huge.

The returned science data is a treasure trove yielding insights into the nature of the young solar wind and its evolution as it propagates away from the Sun. Numerous discoveries have been made over the first three years of the prime mission, most notably the ubiquitous magnetic field switchbacks closer to the Sun, the dust-free zone (DFZ), novel kinetic aspects in the young solar wind, excessive tangential flows beyond the Alfvén critical point, dust β-streams resulting from collisions of the Geminids meteoroid stream with the zodiacal dust cloud (ZDC), and the shortest wavelength thermal emission from the Venusian surface. Since 28 Apr. 2021 (i.e., perihelion of Enc. 8), the S/C has been sampling the solar wind plasma within the magnetically-dominated corona, i.e., sub-Alfvénic solar wind, marking the beginning of a critical phase of the PSP mission. In this region solar wind physics changes because of the multi-directionality of wave propagation (waves moving sunward and anti-sunward can affect the local dynamics including the turbulent evolution, heating and acceleration of the plasma). This is also the region where velocity gradients between the fast and slow speed streams develop, forming the initial conditions for the formation, further out, of corotating interaction regions (CIRs).

The science data return (i.e., data volume) from PSP exceeded the pre-launch estimates by a factor of over four. Since the second orbit, orbital coverage extended from the nominal perihelion Enc. to over 70% of the following orbit duration. We expect this to continue

<footnote n="1">¹The PSP solar Enc. is defined as the orbit section where the S/C is below 0.25 AU from the Sun's center.</footnote>
# 1 Introduction

Parker Solar Probe (PSP; Fox et al. 2016; Raouafi 2022) is flying closer to the Sun than any previous spacecraft (S/C). Launched on 12 Aug. 2018, on 6 Dec. 2022 PSP had completed 14 of its 24 scheduled perihelion encounters (Encs.)<fnref n="1" /> around the Sun over the 7-year nominal mission duration. The S/C flew by Venus for the fifth time on 16 Oct. 2021, followed by the closest perihelion of 13.28 solar radii ($R_⊕$) on 21 Nov. 2021. The S/C will remain on the same orbit for a total of seven solar Encs. After Enc. 16, PSP is scheduled to fly by Venus for the sixth time to lower the perihelion to 11.44 $R_⊕$ for another five orbits. The seventh and last Venus gravity assist (VGA) of the primary mission is scheduled for 6 Nov. 2024. This gravity assist will set PSP for its last three orbits of the primary mission. The perihelia of orbits 22, 23, and 24 of 9.86 $R_⊕$ will be on 24 Dec. 2024, 22 Mar. 2025, and 19 Jun. 2025, respectively.

The mission's overarching science objective is to determine the structure and dynamics of the Sun's coronal magnetic field and to understand how the corona is heated, the solar wind accelerated, and how energetic particles are produced and their distributions evolve. The PSP mission targets processes and dynamics that characterize the Sun's expanding corona and solar wind, enabling the measurement of coronal conditions leading to the nascent solar wind and eruptive transients that create space weather. PSP is sampling the solar corona and solar wind to reveal how it is heated and the solar wind and solar energetic particles (SEPs) are accelerated. To achieve this, PSP measurements will be used to address the following three science goals: (1) Trace the flow of energy that heats the solar corona and accelerates the solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles. Understanding these phenomena has been a top science goal for over six decades. PSP is primarily an exploration mission that is flying through one of the last unvisited and most challenging regions of space within our solar system, and the potential for discovery is huge.

The returned science data is a treasure trove yielding insights into the nature of the young solar wind and its evolution as it propagates away from the Sun. Numerous discoveries have been made over the first three years of the prime mission, most notably the ubiquitous magnetic field switchbacks closer to the Sun, the dust-free zone (DFZ), novel kinetic aspects in the young solar wind, excessive tangential flows beyond the Alfvén critical point, dust β-streams resulting from collisions of the Geminids meteoroid stream with the zodiacal dust cloud (ZDC), and the shortest wavelength thermal emission from the Venusian surface. Since 28 Apr. 2021 (i.e., perihelion of Enc. 8), the S/C has been sampling the solar wind plasma within the magnetically-dominated corona, i.e., sub-Alfvénic solar wind, marking the beginning of a critical phase of the PSP mission. In this region solar wind physics changes because of the multi-directionality of wave propagation (waves moving sunward and anti-sunward can affect the local dynamics including the turbulent evolution, heating and acceleration of the plasma). This is also the region where velocity gradients between the fast and slow speed streams develop, forming the initial conditions for the formation, further out, of corotating interaction regions (CIRs).

The science data return (i.e., data volume) from PSP exceeded the pre-launch estimates by a factor of over four. Since the second orbit, orbital coverage extended from the nominal perihelion Enc. to over 70% of the following orbit duration. We expect this to continue

<footnote n="1">¹The PSP solar Enc. is defined as the orbit section where the S/C is below 0.25 AU from the Sun's center.</footnote>
throughout the mission. The PSP team is also looking into ways to extend the orbital coverage to the whole orbit duration. This will allow sampling the solar wind and SEPs over a large range of heliodistances.

The PSP science payload comprises four instrument suites:

1. FIELDS investigation makes measurements of the electric and magnetic fields and waves, S/C floating potential, density fluctuations, and radio emissions over 20 MHz of bandwidth and 140 dB of dynamic range. It comprises

* Four electric antennas (V1-V4) mounted at the base of the S/C thermal protection system (TPS). The electric preamplifiers connected to each antenna provide outputs to the Radio Frequency Spectrometer (RFS), the Time Domain Sampler (TDS), and the Antenna Electronics Board (AEB) and Digital Fields Board (DFB). The V1-V4 antennas are exposed to the full solar environment.

* A fifth antenna (V5) provides low (LF) and medium (MF) frequency outputs.

* Two fluxgate magnetometers (MAGs) provide data with bandwidth of ~140 Hz and at 292.97 Samples/sec over a dynamic range of ±65, 536 nT with a resolution of 16 bits.

* A search coil magnetometer (SCM) measures the AC magnetic signature of solar wind fluctuations, from 10 Hz up to 1 MHz.

V5, the MAGs, and the SCM are all mounted on the boom in the shade of the TPS. For further details, see Bale et al. (2016).

2. The Solar Wind Electrons Alphas and Protons (SWEAP) Investigation measures the thermal solar wind, i.e., electrons, protons and alpha particles. SWEAP measures the velocity distribution functions (VDFs) of ions and electrons with high energy and angular resolution. It consists of the Solar Probe Cup (SPC) and the Solar Probe Analyzers (SPAN-A and SPAN-B), and the SWEAP Electronics Module (SWEM):

* SPC is fully exposed to the solar environment as it looks directly at the Sun and measures ion and electron fluxes and flow angles as a function of energy.

* SPAN-A is mounted on the ram side and comprises an ion and electron electrostatic analyzer (SPAN-i and SPAN-e, respectively).

* SPAN-B is an electron electrostatic analyzer on the anti-ram side of the S/C.

* The SWEM manages the suite by distributing power, formatting onboard data products, and serving as a single electrical interface to the S/C.

The SPANs and the SWEM reside on the S/C bus behind the TPS. See Kasper et al. (2016) for more information.

3. The Integrated Science Investigation of the Sun (IS◎IS) investigation measures energetic particles over a very broad energy range (10 s of keV to 100 MeV). IS◎IS is mounted on the ram side of the S/C bus. It comprises two Energetic Particle Instruments (EPI) to measure low (EPI-Lo) and high (EPI-Hi) energy:

* EPI-Lo is time-of-flight (TOF) mass spectrometer that measures electrons from ~25–1000 keV, protons from ~0.04–7 MeV, and heavy ions from ~0.02–2 MeV/nuc. EPI-Lo has 80 apertures distributed over eight wedges. Their combined fields-of-view (FOVs) cover nearly an entire hemisphere.

* EPI-Hi measures electrons from ~0.5–6 MeV and ions from ~1–200 MeV/nuc. EPI-Hi consists of three telescopes: a high energy telescope (HET; double ended) and two low energy telescopes LET1 (double ended) and LET2 (single ended).

See (McComas et al. 2016) for a full description of the IS◎IS investigation.
throughout the mission. The PSP team is also looking into ways to extend the orbital coverage to the whole orbit duration. This will allow sampling the solar wind and SEPs over a large range of heliodistances.

The PSP science payload comprises four instrument suites:

1. FIELDS investigation makes measurements of the electric and magnetic fields and waves, S/C floating potential, density fluctuations, and radio emissions over 20 MHz of bandwidth and 140 dB of dynamic range. It comprises

* Four electric antennas (V1-V4) mounted at the base of the S/C thermal protection system (TPS). The electric preamplifiers connected to each antenna provide outputs to the Radio Frequency Spectrometer (RFS), the Time Domain Sampler (TDS), and the Antenna Electronics Board (AEB) and Digital Fields Board (DFB). The V1-V4 antennas are exposed to the full solar environment.

* A fifth antenna (V5) provides low (LF) and medium (MF) frequency outputs.

* Two fluxgate magnetometers (MAGs) provide data with bandwidth of ~140 Hz and at 292.97 Samples/sec over a dynamic range of ±65, 536 nT with a resolution of 16 bits.

* A search coil magnetometer (SCM) measures the AC magnetic signature of solar wind fluctuations, from 10 Hz up to 1 MHz.

V5, the MAGs, and the SCM are all mounted on the boom in the shade of the TPS. For further details, see Bale et al. (2016).

2. The Solar Wind Electrons Alphas and Protons (SWEAP) Investigation measures the thermal solar wind, i.e., electrons, protons and alpha particles. SWEAP measures the velocity distribution functions (VDFs) of ions and electrons with high energy and angular resolution. It consists of the Solar Probe Cup (SPC) and the Solar Probe Analyzers (SPAN-A and SPAN-B), and the SWEAP Electronics Module (SWEM):

* SPC is fully exposed to the solar environment as it looks directly at the Sun and measures ion and electron fluxes and flow angles as a function of energy.

* SPAN-A is mounted on the ram side and comprises an ion and electron electrostatic analyzer (SPAN-i and SPAN-e, respectively).

* SPAN-B is an electron electrostatic analyzer on the anti-ram side of the S/C.

* The SWEM manages the suite by distributing power, formatting onboard data products, and serving as a single electrical interface to the S/C.

The SPANs and the SWEM reside on the S/C bus behind the TPS. See Kasper et al. (2016) for more information.

3. The Integrated Science Investigation of the Sun (IS◎IS) investigation measures energetic particles over a very broad energy range (10 s of keV to 100 MeV). IS◎IS is mounted on the ram side of the S/C bus. It comprises two Energetic Particle Instruments (EPI) to measure low (EPI-Lo) and high (EPI-Hi) energy:

* EPI-Lo is time-of-flight (TOF) mass spectrometer that measures electrons from ~25–1000 keV, protons from ~0.04–7 MeV, and heavy ions from ~0.02–2 MeV/nuc. EPI-Lo has 80 apertures distributed over eight wedges. Their combined fields-of-view (FOVs) cover nearly an entire hemisphere.

* EPI-Hi measures electrons from ~0.5–6 MeV and ions from ~1–200 MeV/nuc. EPI-Hi consists of three telescopes: a high energy telescope (HET; double ended) and two low energy telescopes LET1 (double ended) and LET2 (single ended).

See (McComas et al. 2016) for a full description of the IS◎IS investigation.
4. The Wide-Field Imager for Solar PRobe (WISPR) is the only remote-sensing instrument suite on the S/C. WISPR is a white-light imager providing observations of flows and transients in the solar wind over a 95° × 58° (radial and transverse, respectively) FOV covering elongation angles from 13.5° to 108°. It comprises two telescopes:

* WISPR-i covers the inner part of the FOV (40° × 40°).

* WISPR-o covers the outer part of the FOV (58° × 58°).

See Vourlidas et al. (2016) for further details.

Before tackling the *PSP* achievements during the first four years of the prime mission, a brief historical context is given in §2. §3 provides a brief summary of the *PSP* mission status. §§4-12 describe the *PSP* discoveries during the first four years of operations: switchbacks, solar wind sources, kinetic physics, turbulence, large-scale structures, energetic particles, dust, and Venus, respectively. The conclusions and discussion are given in §13.

Although Sects. 3-12 may have some overlap and cross-referencing, each section can be read independently from the rest of the paper.

## 2 Historical Context: *Mariner 2, Helios, and Ulysses*

Before *PSP*, several space missions shaped our understanding of the solar wind for decades. Three stand out as trailblazers, i.e., *Mariner 2, Helios* (Marsch and Schwenn 1990), and *Ulysses* (Wenzel et al. 1992).

*Mariner 2*, launched on 27 Aug. 1962, was the first successful mission to a planet other than the Earth (i.e., Venus). Its measurements of the solar wind are a first and among the most significant discoveries of the mission (see Neugebauer and Snyder 1962). Although the mission returned data for only a few months, the measurements showed the highly variable nature and complexity of the plasma flow expanding anti-sunward (Snyder and Neugebauer 1965). However, before the launch of *PSP*, almost everything we knew about the inner interplanetary medium was due to the double *Helios* mission. This mission set the stage for an initial understanding of the major physical processes occurring in the inner heliosphere. It greatly helped the development and tailoring of instruments onboard subsequent missions.

The two *Helios* probes were launched on 10 Dec. 1974 and 15 Jan. 1976 and placed in orbit in the ecliptic plane. Their distance from the Sun varied between 0.29 and 1 astronomical unit (AU) with an orbital period of about six months. The payload of the two *Helios* comprised several instruments:

* Proton, electron, and alpha particle analyzers;

* Two DC magnetometers;

* A search coil magnetometer;

* A radio wave experiment;

* Two cosmic ray experiments;

* Three photometers for the zodiacal light; and

* A dust particle detector

Here we provide a very brief overview of some of the scientific goals achieved by *Helios* to make the reader aware of the importance that this mission has had in the study of the solar wind and beyond.

*Helios* established the mechanisms which generate dust particles at the origin of the zodiacal light (ZL), their relationship with micrometeorites and comets, and the radial dependence of dust density (Leinert et al. 1976). Micrometeorite impacts of the dust particle
4. The Wide-Field Imager for Solar PRobe (WISPR) is the only remote-sensing instrument suite on the S/C. WISPR is a white-light imager providing observations of flows and transients in the solar wind over a 95° × 58° (radial and transverse, respectively) FOV covering elongation angles from 13.5° to 108°. It comprises two telescopes:

* WISPR-i covers the inner part of the FOV (40° × 40°).

* WISPR-o covers the outer part of the FOV (58° × 58°).

See Vourlidas et al. (2016) for further details.

Before tackling the *PSP* achievements during the first four years of the prime mission, a brief historical context is given in §2. §3 provides a brief summary of the *PSP* mission status. §§4-12 describe the *PSP* discoveries during the first four years of operations: switchbacks, solar wind sources, kinetic physics, turbulence, large-scale structures, energetic particles, dust, and Venus, respectively. The conclusions and discussion are given in §13.

Although Sects. 3-12 may have some overlap and cross-referencing, each section can be read independently from the rest of the paper.

## 2 Historical Context: Mariner 2, Helios, and Ulysses

Before *PSP*, several space missions shaped our understanding of the solar wind for decades. Three stand out as trailblazers, i.e., *Mariner 2, Helios* (Marsch and Schwenn 1990), and *Ulysses* (Wenzel et al. 1992).

*Mariner 2*, launched on 27 Aug. 1962, was the first successful mission to a planet other than the Earth (i.e., Venus). Its measurements of the solar wind are a first and among the most significant discoveries of the mission (see Neugebauer and Snyder 1962). Although the mission returned data for only a few months, the measurements showed the highly variable nature and complexity of the plasma flow expanding anti-sunward (Snyder and Neugebauer 1965). However, before the launch of *PSP*, almost everything we knew about the inner interplanetary medium was due to the double *Helios* mission. This mission set the stage for an initial understanding of the major physical processes occurring in the inner heliosphere. It greatly helped the development and tailoring of instruments onboard subsequent missions.

The two *Helios* probes were launched on 10 Dec. 1974 and 15 Jan. 1976 and placed in orbit in the ecliptic plane. Their distance from the Sun varied between 0.29 and 1 astronomical unit (AU) with an orbital period of about six months. The payload of the two *Helios* comprised several instruments:

* Proton, electron, and alpha particle analyzers;

* Two DC magnetometers;

* A search coil magnetometer;

* A radio wave experiment;

* Two cosmic ray experiments;

* Three photometers for the zodiacal light; and

* A dust particle detector

Here we provide a very brief overview of some of the scientific goals achieved by *Helios* to make the reader aware of the importance that this mission has had in the study of the solar wind and beyond.

*Helios* established the mechanisms which generate dust particles at the origin of the zodiacal light (ZL), their relationship with micrometeorites and comets, and the radial dependence of dust density (Leinert et al. 1976). Micrometeorite impacts of the dust particle
sensors allowed to study asymmetries with respect to (hereafter w.r.t.) the ecliptic plane and
the different origins related to stone meteorites or iron meteorites and suggested that many
particles run on hyperbolic orbits aiming out of the solar system (Gruen et al. 1980).

*Helios'* plasma wave experiment firstly confirmed that the generation of type III radio bursts is a two-step process, as theoretically predicted by Ginzburg and Zhelezniakov (1958) and revealed enhanced levels of ion acoustic wave turbulence in the solar wind. In addition, the radial excursion of *Helios* allowed proving that the frequency of the radio emission increases with decreasing the distance from the Sun and the consequent increase of plasma density (Gurnett et al. 1979; Kellogg 1986). The radio astronomy experiments onboard both S/C were the first to provide "three-dimensional (3D) direction finding" in space, allowing to follow the source of type III radio bursts during its propagation along the interplanetary magnetic field lines. In practice, they provided a significant extension of our knowledge of the large-scale structure of the interplanetary medium via remote sensing (Kayser and Stone 1984).

The galactic and cosmic ray experiment studied the energy spectra, charge composition, and flow patterns of both solar and galactic cosmic rays (GCRs). *Helios* was highly relevant as part of a large network of cosmic ray experiments onboard S/C located between 0.3 and 10 AU. It contributed significantly to confirming the role of the solar wind suprathermal particles as seed particles injected into interplanetary shocks to be eventually accelerated (McDonald et al. 1976). Coupling observations by *Helios* and other S/C at 1 AU allowed studying the problem of transport performing measurements in different conditions relative to magnetic connectivity and radial distance from the source region. Moreover, joint measurements between *Helios* and *Pioneer* 10 gave important results about the modulation of cosmic rays in the heliosphere (Kunow 1978; Kunow and Wibberenz 1984).

The solar wind plasma experiment and the magnetic field experiments allowed us to in-
vestigate the interplanetary medium from the large-scale structure to spatial scales of the
order of the proton Larmor radius for more than an entire 11-year solar cycle. The vary-
ing vantage point due to a highly elliptic orbit allowed us to reach an unprecedented de-
scription of the solar wind's large-scale structure and the dynamical processes that take
place during the expansion into the interplanetary medium (Schwenn et al. 1981). *Helios'*
plasma and magnetic field continuous observations allowed new insights into the study of
magneto-hydrodynamic (MHD) turbulence opening Pandora's box in our understanding of
this phenomenon of astrophysical relevance (see reviews by Tu and Marsch 1995; Bruno and
Carbone 2013). Similarly, detailed observations of the 3D velocity distribution of protons,
alphas, and electrons not only revealed the presence of anisotropies w.r.t. the local magnetic
field but also the presence of proton and alpha beams as well as electron strahl. Moreover,
these observations allowed us to study the variability and evolution of these kinetic features
with heliocentric distance and different Alfvénic turbulence levels at fluid scales (see the
review by Tu and Marsch 1995).

Up to the launch of *Ulysses* on 6 Oct. 1990, the solar wind exploration was limited to
measurements within the ecliptic plane. Like PSP, the idea of flying a mission to explore
the solar polar region dates back to the 1959 Simpson's Committee report. Using a Jupiter
gravity assist, *Ulysses* slang shot out of the ecliptic to fly above the solar poles and provide
unique measurements. During its three solar passes in 1994-95, 2000-01, and 2005, *Ulysses*
covered two minima and one maximum of the solar sunspot cycle, revealing phenomena
unknown to us before (see McComas et al. 2008). All measurements were, however, at
heliodistances beyond 1 AU and only *in situ*, as there were no remote-sensing instruments
onboard.
sensors allowed to study asymmetries with respect to (hereafter w.r.t.) the ecliptic plane and
the different origins related to stone meteorites or iron meteorites and suggested that many
particles run on hyperbolic orbits aiming out of the solar system (Gruen et al. 1980).

*Helios'* plasma wave experiment firstly confirmed that the generation of type III radio bursts is a two-step process, as theoretically predicted by Ginzburg and Zhelezniakov (1958) and revealed enhanced levels of ion acoustic wave turbulence in the solar wind. In addition, the radial excursion of *Helios* allowed proving that the frequency of the radio emission increases with decreasing the distance from the Sun and the consequent increase of plasma density (Gurnett et al. 1979; Kellogg 1986). The radio astronomy experiments onboard both S/C were the first to provide "three-dimensional (3D) direction finding" in space, allowing to follow the source of type III radio bursts during its propagation along the interplanetary magnetic field lines. In practice, they provided a significant extension of our knowledge of the large-scale structure of the interplanetary medium via remote sensing (Kayser and Stone 1984).

The galactic and cosmic ray experiment studied the energy spectra, charge composition, and flow patterns of both solar and galactic cosmic rays (GCRs). *Helios* was highly relevant as part of a large network of cosmic ray experiments onboard S/C located between 0.3 and 10 AU. It contributed significantly to confirming the role of the solar wind suprathermal particles as seed particles injected into interplanetary shocks to be eventually accelerated (McDonald et al. 1976). Coupling observations by *Helios* and other S/C at 1 AU allowed studying the problem of transport performing measurements in different conditions relative to magnetic connectivity and radial distance from the source region. Moreover, joint measurements between *Helios* and *Pioneer* 10 gave important results about the modulation of cosmic rays in the heliosphere (Kunow 1978; Kunow and Wibberenz 1984).

The solar wind plasma experiment and the magnetic field experiments allowed us to in-
vestigate the interplanetary medium from the large-scale structure to spatial scales of the
order of the proton Larmor radius for more than an entire 11-year solar cycle. The vary-
ing vantage point due to a highly elliptic orbit allowed us to reach an unprecedented de-
scription of the solar wind's large-scale structure and the dynamical processes that take
place during the expansion into the interplanetary medium (Schwenn et al. 1981). *Helios'*
plasma and magnetic field continuous observations allowed new insights into the study of
magneto-hydrodynamic (MHD) turbulence opening Pandora's box in our understanding of
this phenomenon of astrophysical relevance (see reviews by Tu and Marsch 1995; Bruno and
Carbone 2013). Similarly, detailed observations of the 3D velocity distribution of protons,
alphas, and electrons not only revealed the presence of anisotropies w.r.t. the local magnetic
field but also the presence of proton and alpha beams as well as electron strahl. Moreover,
these observations allowed us to study the variability and evolution of these kinetic features
with heliocentric distance and different Alfvénic turbulence levels at fluid scales (see the
review by Tu and Marsch 1995).

Up to the launch of *Ulysses* on 6 Oct. 1990, the solar wind exploration was limited to
measurements within the ecliptic plane. Like PSP, the idea of flying a mission to explore
the solar polar region dates back to the 1959 Simpson's Committee report. Using a Jupiter
gravity assist, *Ulysses* slang shot out of the ecliptic to fly above the solar poles and provide
unique measurements. During its three solar passes in 1994-95, 2000-01, and 2005, *Ulysses*
covered two minima and one maximum of the solar sunspot cycle, revealing phenomena
unknown to us before (see McComas et al. 2008). All measurements were, however, at
heliodistances beyond 1 AU and only *in situ*, as there were no remote-sensing instruments
onboard.
## 3 Mission Status

After a decade in the making, PSP began its 7-year journey into the Sun’s corona on 12 Aug. 2018 (Kinnison et al. 2020). Following the launch, about six weeks later, the S/C flew by Venus for the first of seven gravity assists to target the initial perihelion of $35.6 R_\odot$. As the S/C continues to perform VGAs, the perihelion has been decreased to $13.28 R_\odot$ after the fifth VGA, with the anticipation of a final perihelion of $9.86 R_\odot$ in the last three orbits. Figure 1 shows the change in perihelia as the S/C has successfully completed the VGAs and the anticipated performance in future orbits. Following the seventh VGA, the aphelion is below Venus’ orbit. So, no more VGAs will be possible, and the orbit perihelion will remain the same for a potential extended mission.

As shown in Fig. 1, the S/C had completed 13 orbits by Oct. of 2022, with an additional 11 orbits remaining in the primary mission. As designed, these orbits are separated into a solar Enc. phase and a cruise phase. Solar Encs. are dedicated to taking the data that characterize the near-Sun environment and the corona. The cruise phase of each orbit is devoted to a mix of science data downlink, S/C operations, and maintenance, and science in regions further away from the Sun.

The major engineering challenge for the mission before launch was to design and build a TPS that would keep the bulk of the S/C at comfortable temperatures during each solar Enc. period. Figure 1 also shows the temperature of the TPS’ sunward face at each perihelion, the maximum temperature in each orbit. Given the anticipated temperature at the final perihelion of nearly $1000^\circ\text{C}$, the TPS does not include sensors for the direct measurement of the TPS temperature. However, the S/C has other sensors, such as the barrier blanket temperature sensor and monitoring of the cooling system, with which the S/C’s overall thermal model has been validated. Through orbit 13, the thermal model and measured temperatures agree very well, though actual temperatures are slightly lower as the model included conservative assumptions for inputs such as surface properties. This good agreement holds throughout the orbits, including aphelion. For the early orbits, the reduced solar illumination when the S/C is further away from the Sun raised concerns before launch that the cooling system might freeze unless extra energy was provided to the S/C by tilting the system to expose more surface area to the Sun near aphelion. This design worked as expected, and temperatures near aphelion have been comfortably above the point where freezing might occur.

The mission was designed to collect science data during solar Encs. (i.e., inside 0.25 AU) and return that data to Earth during the cruise phase, when the S/C is further away from the Sun. The system was designed to do this using a Ka-band telecommunications link, one of the first uses of this technology for APL,<fnref n="2" /> with the requirement of returning an average of 85 Gbits of science data per orbit. While the pre-launch operations plan comfortably exceeded this, the mission has returned over three times the planned data volume through the first 13 orbits, with increased data return expected through the remaining orbits. The increased data return is mainly due to better than expected performance of the Ka-band telecommunications system. It has resulted in the ability to measure and return data throughout the orbit, not just in solar Encs., to characterize the solar environment fully.

Another major engineering challenge before launch was the ability of the system to detect and recover from faults and to maintain attitude control of the S/C to prevent unintended exposure to solar illumination. The fault management is, by necessity, autonomous, since the S/C spends a significant amount of time out of communication with Mission Operations during the solar Enc. periods in each orbit. A more detailed discussion of the design and

<footnote n="2">²The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland.</footnote>
## 3 Mission Status

After a decade in the making, PSP began its 7-year journey into the Sun’s corona on 12 Aug. 2018 (Kinnison et al. 2020). Following the launch, about six weeks later, the S/C flew by Venus for the first of seven gravity assists to target the initial perihelion of $35.6 R_\odot$. As the S/C continues to perform VGAs, the perihelion has been decreased to $13.28 R_\odot$ after the fifth VGA, with the anticipation of a final perihelion of $9.86 R_\odot$ in the last three orbits. Figure 1 shows the change in perihelia as the S/C has successfully completed the VGAs and the anticipated performance in future orbits. Following the seventh VGA, the aphelion is below Venus’ orbit. So, no more VGAs will be possible, and the orbit perihelion will remain the same for a potential extended mission.

As shown in Fig. 1, the S/C had completed 13 orbits by Oct. of 2022, with an additional 11 orbits remaining in the primary mission. As designed, these orbits are separated into a solar Enc. phase and a cruise phase. Solar Encs. are dedicated to taking the data that characterize the near-Sun environment and the corona. The cruise phase of each orbit is devoted to a mix of science data downlink, S/C operations, and maintenance, and science in regions further away from the Sun.

The major engineering challenge for the mission before launch was to design and build a TPS that would keep the bulk of the S/C at comfortable temperatures during each solar Enc. period. Figure 1 also shows the temperature of the TPS’ sunward face at each perihelion, the maximum temperature in each orbit. Given the anticipated temperature at the final perihelion of nearly 1000 °C, the TPS does not include sensors for the direct measurement of the TPS temperature. However, the S/C has other sensors, such as the barrier blanket temperature sensor and monitoring of the cooling system, with which the S/C’s overall thermal model has been validated. Through orbit 13, the thermal model and measured temperatures agree very well, though actual temperatures are slightly lower as the model included conservative assumptions for inputs such as surface properties. This good agreement holds throughout the orbits, including aphelion. For the early orbits, the reduced solar illumination when the S/C is further away from the Sun raised concerns before launch that the cooling system might freeze unless extra energy was provided to the S/C by tilting the system to expose more surface area to the Sun near aphelion. This design worked as expected, and temperatures near aphelion have been comfortably above the point where freezing might occur.

The mission was designed to collect science data during solar Encs. (i.e., inside 0.25 AU) and return that data to Earth during the cruise phase, when the S/C is further away from the Sun. The system was designed to do this using a Ka-band telecommunications link, one of the first uses of this technology for APL,<fnref n="2" /> with the requirement of returning an average of 85 Gbits of science data per orbit. While the pre-launch operations plan comfortably exceeded this, the mission has returned over three times the planned data volume through the first 13 orbits, with increased data return expected through the remaining orbits. The increased data return is mainly due to better than expected performance of the Ka-band telecommunications system. It has resulted in the ability to measure and return data throughout the orbit, not just in solar Encs., to characterize the solar environment fully.

Another major engineering challenge before launch was the ability of the system to detect and recover from faults and to maintain attitude control of the S/C to prevent unintended exposure to solar illumination. The fault management is, by necessity, autonomous, since the S/C spends a significant amount of time out of communication with Mission Operations during the solar Enc. periods in each orbit. A more detailed discussion of the design and

<footnote n="2">²The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland.</footnote>
<figure><img src="image_2.png" /><figcaption>**Fig. 1** (Top-blue) PSP's perihelion distance is decreased by performing gravity assists using Venus (VGAs). After seven close flybys of Venus, the final perihelion is anticipated to be 9.86 R⊕ from the Sun's center. (Top-orange) The modeled temperature of the TPS sunward face at each perihelion. The thermal sensors on the S/C (behind the TPS) confirm the TPS thermal model. It is noteworthy that there are no thermal sensors on the TPS itself. (Bottom) The trajectory of PSP during the 7-year primary mission phase as a function of days after the launch on 12 Aug. 2018. The green (red) color indicates the completed (future) part of the PSP orbit. The green dot shows the PSP heliodistance on 21 Dec. 2022</figcaption></figure>
<figure><img src="image_2.png" /><figcaption>**Fig. 1** (Top-blue) PSP's perihelion distance is decreased by performing gravity assists using Venus (VGAs). After seven close flybys of Venus, the final perihelion is anticipated to be 9.86 R⊕ from the Sun's center. (Top-orange) The modeled temperature of the TPS sunward face at each perihelion. The thermal sensors on the S/C (behind the TPS) confirm the TPS thermal model. It is noteworthy that there are no thermal sensors on the TPS itself. (Bottom) The trajectory of PSP during the 7-year primary mission phase as a function of days after the launch on 12 Aug. 2018. The green (red) color indicates the completed (future) part of the PSP orbit. The green dot shows the PSP heliodistance on 21 Dec. 2022</figcaption></figure>
operation of the S/C autonomy system is found in Kinnison et al. (2020). Through 13 orbits, the S/C has successfully executed each orbit and operated as expected in this harsh environment. We have seen some unanticipated issues, associated mainly with the larger-than-expected dust environment, that have affected the S/C. However, the autonomy system has successfully detected and recovered from all of these events. The robust design of the autonomy system has kept the S/C safe, and we expect this to continue through the primary mission.

Generally, the S/C has performed well within the expectations of the engineering team, who used conservative design and robust, redundant systems to build the highly capable PSP. Along with this, a major factor in the mission's success so far is the tight coupling between the engineering and operations teams and the science team. Before launch, this interaction gave the engineering team insight into this unexplored near-Sun environment, resulting in designs that were conservative. After launch, the operations and science teams have worked together to exploit this conservatism to achieve results far beyond expectations.

# 4 Magnetic Field Switchbacks

Abrupt and significant changes of the interplanetary magnetic field direction were reported as early as the mid-1960's (see McCracken and Ness 1966). The cosmic ray anisotropy remained well aligned with the field. Michel (1967) also reported increases in the radial solar wind speed accompanying the magnetic field deviations from the Parker spiral. Using *Ulysses* data recorded above the solar poles at heliodistances $\ge 1$ AU, (Balogh et al. 1999) analyzed the propagation direction of waves to show that these rotations in the magnetic field of 90° w.r.t. the Parker spiral are magnetic field line folds rather than opposite polarity flux tubes originating at the Sun. Magnetic field inversions were observed at 1 AU by the International Sun-Earth Explorer-3 (ISEE-3 [Durney 1979]; Kahler et al. 1996) and the Advanced Composition (ACE [Stone et al. 1998]; Gosling et al. 2009, Li et al. 2016). Inside 1 AU, the magnetic field reversals were also observed in the *Helios* (Porsche 1981) solar wind measurements as close as 0.3 AU from the Sun's center (Borovsky 2016; Horbury et al. 2018).

The magnetic field switchbacks took center stage recently owing to their prominence and ubiquitousness in the PSP measurements inside 0.2 AU.

## 4.1 What Is a Switchback?

Switchbacks are short magnetic field rotations that are ubiquitously observed in the solar wind. They are consistent with local folds in the magnetic field rather than changes in the magnetic connectivity to solar source regions. This interpretation is supported by the observation of suprathermal electrons (Kahler et al. 1996), the differential streaming of alpha particles (Yamauchi et al. 2004) and proton beams (Neugebauer and Goldstein 2013), and the directionality of Alfvén waves (AWs) (Balogh et al. 1999). Because of the intrinsic Alfvénic nature of these structures – implying a high degree of correlation between magnetic and velocity fluctuations in all field components – the magnetic field fold has a distinct velocity counterpart. Moreover, the so called *one-sided* aspect of solar wind fluctuations during Alfvénic streams (Gosling et al. 2009), which is a consequence of the approximate constancy of the magnetic field strength $B = |B|$ during these intervals, has a direct impact on the distribution of $B_R$ and $V_R$ in switchbacks. Under such conditions (constant B and Alfvénic fluctuations), large magnetic fields rotations, and switchbacks in particular, always
operation of the S/C autonomy system is found in Kinnison et al. (2020). Through 13 orbits, the S/C has successfully executed each orbit and operated as expected in this harsh environment. We have seen some unanticipated issues, associated mainly with the larger-than-expected dust environment, that have affected the S/C. However, the autonomy system has successfully detected and recovered from all of these events. The robust design of the autonomy system has kept the S/C safe, and we expect this to continue through the primary mission.

Generally, the S/C has performed well within the expectations of the engineering team, who used conservative design and robust, redundant systems to build the highly capable PSP. Along with this, a major factor in the mission's success so far is the tight coupling between the engineering and operations teams and the science team. Before launch, this interaction gave the engineering team insight into this unexplored near-Sun environment, resulting in designs that were conservative. After launch, the operations and science teams have worked together to exploit this conservatism to achieve results far beyond expectations.

# 4 Magnetic Field Switchbacks

Abrupt and significant changes of the interplanetary magnetic field direction were reported as early as the mid-1960's (see McCracken and Ness 1966). The cosmic ray anisotropy remained well aligned with the field. Michel (1967) also reported increases in the radial solar wind speed accompanying the magnetic field deviations from the Parker spiral. Using *Ulysses'* data recorded above the solar poles at heliodistances $\ge 1$ AU, (Balogh et al. 1999) analyzed the propagation direction of waves to show that these rotations in the magnetic field of 90° w.r.t. the Parker spiral are magnetic field line folds rather than opposite polarity flux tubes originating at the Sun. Magnetic field inversions were observed at 1 AU by the International Sun-Earth Explorer-3 (ISEE-3 [Durney 1979]; Kahler et al. 1996) and the Advanced Composition (ACE [Stone et al. 1998]; Gosling et al. 2009, Li et al. 2016). Inside 1 AU, the magnetic field reversals were also observed in the *Helios* (Porsche 1981) solar wind measurements as close as 0.3 AU from the Sun's center (Borovsky 2016; Horbury et al. 2018).

The magnetic field switchbacks took center stage recently owing to their prominence and ubiquitousness in the PSP measurements inside 0.2 AU.

## 4.1 What Is a Switchback?

Switchbacks are short magnetic field rotations that are ubiquitously observed in the solar wind. They are consistent with local folds in the magnetic field rather than changes in the magnetic connectivity to solar source regions. This interpretation is supported by the observation of suprathermal electrons (Kahler et al. 1996), the differential streaming of alpha particles (Yamauchi et al. 2004) and proton beams (Neugebauer and Goldstein 2013), and the directionality of Alfvén waves (AWs) (Balogh et al. 1999). Because of the intrinsic Alfvénic nature of these structures – implying a high degree of correlation between magnetic and velocity fluctuations in all field components – the magnetic field fold has a distinct velocity counterpart. Moreover, the so called *one-sided* aspect of solar wind fluctuations during Alfvénic streams (Gosling et al. 2009), which is a consequence of the approximate constancy of the magnetic field strength $B = |B|$ during these intervals, has a direct impact on the distribution of $B_R$ and $V_R$ in switchbacks. Under such conditions (constant B and Alfvénic fluctuations), large magnetic fields rotations, and switchbacks in particular, always
lead to bulk speed enhancements (Matteini et al. 2014), resulting in a spiky solar wind ve-
locity profile during Alfvénic periods. Since the amplitude of the velocity spikes associated
to switchbacks is proportional to the local Alfvén speed $V_A$, the speed modulation is par-
ticularly intense in fast-solar-wind streams observed inside 1 AU, where $V_A$ is larger, and it
was suggested that velocity spikes could be even larger closer-in (Horbury et al. 2018).

Despite the previous knowledge of switchbacks in the solar wind community and some expectations that they could have played some role closer to the Sun, our consideration of these structures has been totally changed by PSP, since its first observations inside 0.3 AU (Kasper et al. 2019; Bale et al. 2019). The switchback occurrence rate, morphology, and amplitude as observed by PSP, as well as the fact that they are ubiquitously observed also in slow, though mostly Alfvénic, solar wind, made them one of the most interesting and intriguing aspects of the first PSP Encs.

In this section, we summarize recent findings about switchbacks from the first PSP orbits. In Sect. 4.2 we provide an overview of the main observational properties of these structures in terms of size, shape, radial evolution, and internal and boundary properties; in Sect. 4.3 we present current theories for the generation and evolution of switchbacks, presenting different types of models, based on their generation at the solar surface or *in situ* in the wind. §4.4 contains a final discussion of the state-of-art of switchbacks’ observational and theoretical studies and a list of current open questions to be answered by PSP in future Encs.

## 4.2 Observational Properties of Switchbacks

### 4.2.1 Velocity Increase Inside Switchbacks

At first order, switchbacks can be considered as strong rotations of the magnetic-field vector **B**, with no change in the magnetic field intensity $B = |**B**|$. Geometrically, this corresponds to a rotation of **B** with its tip constrained on a sphere of constant radius **B**. Such excursions are well represented by following the **B** direction in the RT plane, during the time series of a large amplitude switchback, like in the left panels of Fig. 2. The top left panel represents the typical **B** pattern observed since Enc. 1 (Woolley et al. 2020): the background magnetic field, initially almost aligned with the radial ($B_R < 0$) in the near-Sun regions observed by PSP, makes a significant rotation in the RT plane, locally inverting its polarity ($B_R > 0$). All this occurs keeping **B** ~ const. and points follow a circle of approximately constant radius during the rotation; as a consequence this increases significantly the transverse component of **B** and $B_T \gg B_R$ when approaching 90°. Due to the high Alfvénicity of the fluctuations in near-Sun streams sampled by PSP, the same pattern is observed for the velocity vector, with similar and proportional variations in $V_R$ and $V_T$ (bottom left panel). While the magnetic field is frame-invariant, the circular pattern seen for the velocity vector is not and its center identifies the so-called de Hoffman-Teller frame (dHT): the frame in which the motional electric field associated to the fluctuations is zero and where the switchbacks magnetic structure can be considered at rest. This frame is typically traveling at the local Alfvén speed ahead of the solar wind protons, along the magnetic field. This is consistent with the velocity measurements in the bottom left panel of Fig. 2, where the local $V_A$ is of the order of ~50 km s⁻¹ and agrees well with the local of the centre of the circle, which is roughly 50 km s⁻¹ ahead of the minimum $V_R$ seen at the beginning of the interval.

Because of the geometrical property above, there is a direct relation between the **B** ex-
cursion and the resulting modulation of the flow speed in switchbacks. Remarkably, switch-
backs always lead to speed increases, characterized by a spiky, one-sided profile of $V_R$,
independent of the underlying magnetic field polarity; i.e., regardless **B** rotates from 0° to-
wards 180°, or vice-versa (Matteini et al. 2014). As a consequence, it is possible to derive
lead to bulk speed enhancements (Matteini et al. 2014), resulting in a spiky solar wind ve-
locity profile during Alfvénic periods. Since the amplitude of the velocity spikes associated
to switchbacks is proportional to the local Alfvén speed $V_A$, the speed modulation is par-
ticularly intense in fast-solar-wind streams observed inside 1 AU, where $V_A$ is larger, and it
was suggested that velocity spikes could be even larger closer-in (Horbury et al. 2018).

Despite the previous knowledge of switchbacks in the solar wind community and some expectations that they could have played some role closer to the Sun, our consideration of these structures has been totally changed by PSP, since its first observations inside 0.3 AU (Kasper et al. 2019; Bale et al. 2019). The switchback occurrence rate, morphology, and amplitude as observed by PSP, as well as the fact that they are ubiquitously observed also in slow, though mostly Alfvénic, solar wind, made them one of the most interesting and intriguing aspects of the first PSP Encs.

In this section, we summarize recent findings about switchbacks from the first PSP orbits. In Sect. 4.2 we provide an overview of the main observational properties of these structures in terms of size, shape, radial evolution, and internal and boundary properties; in Sect. 4.3 we present current theories for the generation and evolution of switchbacks, presenting different types of models, based on their generation at the solar surface or *in situ* in the wind. §4.4 contains a final discussion of the state-of-art of switchbacks’ observational and theoretical studies and a list of current open questions to be answered by PSP in future Encs.

## 4.2 Observational Properties of Switchbacks

### 4.2.1 Velocity Increase Inside Switchbacks

At first order, switchbacks can be considered as strong rotations of the magnetic-field vector **B**, with no change in the magnetic field intensity $B = |**B**|$. Geometrically, this corresponds to a rotation of **B** with its tip constrained on a sphere of constant radius **B**. Such excursions are well represented by following the **B** direction in the RT plane, during the time series of a large amplitude switchback, like in the left panels of Fig. 2. The top left panel represents the typical **B** pattern observed since Enc. 1 (Woolley et al. 2020): the background magnetic field, initially almost aligned with the radial ($B_R < 0$) in the near-Sun regions observed by PSP, makes a significant rotation in the RT plane, locally inverting its polarity ($B_R > 0$). All this occurs keeping **B** ~ const. and points follow a circle of approximately constant radius during the rotation; as a consequence this increases significantly the transverse component of **B** and $B_T \gg B_R$ when approaching 90°. Due to the high Alfvénicity of the fluctuations in near-Sun streams sampled by PSP, the same pattern is observed for the velocity vector, with similar and proportional variations in $V_R$ and $V_T$ (bottom left panel). While the magnetic field is frame-invariant, the circular pattern seen for the velocity vector is not and its center identifies the so-called de Hoffman-Teller frame (dHT): the frame in which the motional electric field associated to the fluctuations is zero and where the switchbacks magnetic structure can be considered at rest. This frame is typically traveling at the local Alfvén speed ahead of the solar wind protons, along the magnetic field. This is consistent with the velocity measurements in the bottom left panel of Fig. 2, where the local $V_A$ is of the order of ~50 km s⁻¹ and agrees well with the local of the centre of the circle, which is roughly 50 km s⁻¹ ahead of the minimum $V_R$ seen at the beginning of the interval.

Because of the geometrical property above, there is a direct relation between the **B** ex-
cursion and the resulting modulation of the flow speed in switchbacks. Remarkably, switch-
backs always lead to speed increases, characterized by a spiky, one-sided profile of $V_R$,
independent of the underlying magnetic field polarity; i.e., regardless **B** rotates from 0° to-
wards 180°, or vice-versa (Matteini et al. 2014). As a consequence, it is possible to derive
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 2** Left: Magnetic field and velocity vector rotations during a large amplitude switchback during PSP Enc. 1 (Woolley et al. 2020). Right: An example of switchback observed by PSP during Enc. 6. Top panel shows the almost complete magnetic field reversal of $B_R$ (black), while the magnetic field intensity $|B|$ (red) remains almost constant through the whole structure. The bottom panel shows the associated jump in the radial velocity $V_R$. In a full switchback the bulk speed of the solar wind protons can increase by up to twice the Alfvén speed $V_A$; as a consequence we observe a jump from ~300 km s⁻¹ to ~600 km s⁻¹ in the speed during this interval ($V_A \sim 150$ km s⁻¹)</figcaption></figure>

a simple phenomenological relation that links the instantaneous proton radial velocity $V_R$ to the magnetic field angle w.r.t. the radial $\theta_{BR}$, where $\cos \theta_{BR} = B_R/B$. Moreover, since the solar wind speed is typically dominated by its radial component, this can be considered an approximate expression for the proton bulk speed within switchbacks (Matteini et al. 2015):

$$V_p = V_0 + V_A[1 \pm \cos \theta_{BR}], \quad (1)$$

where $V_0$ is the background solar wind speed and the sign in front of the cosine takes into account the underlying Parker spiral polarity ($-\cos \theta_{BR}$ if $B_R > 0$, $+\cos \theta_{BR}$ otherwise). As apparent from Eq. (1), the speed increase inside a switchback with constant $B$ has a maximum amplitude of $2 \times V_A$. This corresponds to magnetic field rotations that are full reversals; for moderate deflections, the speed increase is smaller, typically of the order of ~$V_A$ for a 90° deflection. Also, because the increase in $V_p$ is proportional to the local Alfvén speed, larger enhancements are expected closer to the Sun.

The right panels of Fig. 2 show one of the most striking examples of switchbacks observed by PSP during Enc. 6. This corresponds to an almost full reversal of $B_R$, from approximately 100 to -100 nT, maintaining the magnetic field intensity remarkably constant during the vector $B$ rotation. As a consequence, the background bulk flow proton velocity (~300 km s⁻¹) goes up by almost 2 $V_A$, leading to a speed enhancement up to 600 km s⁻¹ inside the structure ($V_A \sim 150$ km s⁻¹). This has the impressive effect of turning the ambient slow solar wind into fast for the duration of the crossing, without a change in the connection
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 2** Left: Magnetic field and velocity vector rotations during a large amplitude switchback during PSP Enc. 1 (Woolley et al. 2020). Right: An example of switchback observed by PSP during Enc. 6. Top panel shows the almost complete magnetic field reversal of $B_R$ (black), while the magnetic field intensity $|B|$ (red) remains almost constant through the whole structure. The bottom panel shows the associated jump in the radial velocity $V_R$. In a full switchback the bulk speed of the solar wind protons can increase by up to twice the Alfvén speed $V_A$; as a consequence we observe a jump from ~300 km s⁻¹ to ~600 km s⁻¹ in the speed during this interval ($V_A \sim 150$ km s⁻¹)</figcaption></figure>

a simple phenomenological relation that links the instantaneous proton radial velocity $V_R$ to the magnetic field angle w.r.t. the radial $\theta_{BR}$, where $\cos\theta_{BR} = B_R/B$. Moreover, since the solar wind speed is typically dominated by its radial component, this can be considered an approximate expression for the proton bulk speed within switchbacks (Matteini et al. 2015):

$$V_p = V_0 + V_A[1 \pm \cos\theta_{BR}], \quad (1)$$

where $V_0$ is the background solar wind speed and the sign in front of the cosine takes into account the underlying Parker spiral polarity ($-\cos\theta_{BR}$ if $B_R > 0$, $+\cos\theta_{BR}$ otherwise). As apparent from Eq. (1), the speed increase inside a switchback with constant $B$ has a maximum amplitude of $2 \times V_A$. This corresponds to magnetic field rotations that are full reversals; for moderate deflections, the speed increase is smaller, typically of the order of ~$V_A$ for a 90° deflection. Also, because the increase in $V_p$ is proportional to the local Alfvén speed, larger enhancements are expected closer to the Sun.

The right panels of Fig. 2 show one of the most striking examples of switchbacks observed by PSP during Enc. 6. This corresponds to an almost full reversal of $B_R$, from approximately 100 to −100 nT, maintaining the magnetic field intensity remarkably constant during the vector $B$ rotation. As a consequence, the background bulk flow proton velocity (~300 km s⁻¹) goes up by almost 2 $V_A$, leading to a speed enhancement up to 600 km s⁻¹ inside the structure ($V_A \sim 150$ km s⁻¹). This has the impressive effect of turning the ambient slow solar wind into fast for the duration of the crossing, without a change in the connection
to the source. It is an open question if even larger velocity jumps could be observed closer in, when $V_A$ approaches 200 – 300 km s⁻¹ and becomes comparable to the bulk flow itself, and what would be the consequences on the overall flow energy and dynamics.

Finally, it is worth emphasizing that the velocity enhancements discussed above relate only to the main proton core population in the solar wind plasma. Other species, like proton beams and alpha particles, react differently to switchbacks and may or may not partake in the Alfvénic motion associated to these structures, depending on their relative drift w.r.t. the proton core. In fact, alpha particles typically stream faster than protons along the magnetic field in Alfvénic streams, with a drift speed that in the inner heliosphere can be quite close to $V_A$. As a consequence they sit close to the zero electric field reference frame (dHT) and display much smaller oscillations and speed variations in switchbacks (in the case they stream exactly at the same speed as the phase velocity of the switchback, they are totally unaffected and do not feel any fold in the field (see e.g., Matteini et al. 2015). Similarly, proton beams have drift speeds that exceed the local Alfvén speed close to the Sun and therefore, because they stream faster than the dHT, they are observed to oscillate out of phase with the main proton core (i.e., they get slower inside switchbacks and the core-beam structure of the proton VDF is locally reversed where $B_R$ flips sign; Neugebauer and Goldstein 2013). The same happens for the electron strahl, leading to an inversion in the electron heat-flux.

### 4.2.2 Characteristic Scales, Size and Shape

Ideally, switchbacks would be imaged from a range of angles, providing a straightforward method to visualize their shape. However, as mentioned above, these structures are Alfvénic and have little change in plasma density, which is essential for line of sight (LOS) images from remote sensing instruments. We must instead rely on the *in situ* observations from a single S/C, which are fundamentally local measurements. Therefore, it is important to understand the relationship between the true physical structure of a switchback and the data measured by a S/C, as this can influence the way in which we think about and study them. For example, a small duration switchback in the PSP time series may be due to a physically smaller switchback, or because PSP clipped the edge of a larger switchback. This ambiguity also applies to a series of multiple switchbacks, which may truly be several closely spaced switchbacks or in fact one larger, more degraded switchback (Farrell et al. 2021).

Dudok de Wit et al. (2020) provided the first detailed statistics on switchbacks for PSP’s first Enc. They showed that switchback duration could vary from a few seconds to over an hour, with no characteristic timescale. Through studying the waiting time (the time between each switchback) statistics, they found that switchbacks exhibited long term memory, and tended to aggregate, which they take as evidence for similar coronal origin. Many authors define switchbacks as deflections, above some threshold, away from the Parker spiral. The direction of this deflection, i.e. towards +T, is also interesting as it could act as a testable prediction of switchback origin theories Schwadron and McComas (2021). For Enc. 1 at least, Dudok de Wit et al. (2020) showed that deflections were isotropic about the Parker spiral direction, although they did note that the longest switchbacks displayed a weak preference for deflections in +T. Horbury et al. (2020) also found that switchbacks displayed a slight preference to deflect in T rather than N, although there was no distinction between -T or +T. The authors refer to the clock angle in an attempt to quantify the direction of switchback deflection. This is defined as the “angle of the vector projected onto a plane perpendicular to the Parker spiral that also contains N”, where 0°, 90°, 180° and 270° refer to +N, +T, -N, -T directions respectively. Unlike the entire switchback population, the longest switchbacks
to the source. It is an open question if even larger velocity jumps could be observed closer in, when $V_A$ approaches 200 – 300 km s⁻¹ and becomes comparable to the bulk flow itself, and what would be the consequences on the overall flow energy and dynamics.

Finally, it is worth emphasizing that the velocity enhancements discussed above relate only to the main proton core population in the solar wind plasma. Other species, like proton beams and alpha particles, react differently to switchbacks and may or may not partake in the Alfvénic motion associated to these structures, depending on their relative drift w.r.t. the proton core. In fact, alpha particles typically stream faster than protons along the magnetic field in Alfvénic streams, with a drift speed that in the inner heliosphere can be quite close to $V_A$. As a consequence they sit close to the zero electric field reference frame (dHT) and display much smaller oscillations and speed variations in switchbacks (in the case they stream exactly at the same speed as the phase velocity of the switchback, they are totally unaffected and do not feel any fold in the field (see e.g., Matteini et al. 2015). Similarly, proton beams have drift speeds that exceed the local Alfvén speed close to the Sun and therefore, because they stream faster than the dHT, they are observed to oscillate out of phase with the main proton core (i.e., they get slower inside switchbacks and the core-beam structure of the proton VDF is locally reversed where $B_R$ flips sign; Neugebauer and Goldstein 2013). The same happens for the electron strahl, leading to an inversion in the electron heat-flux.

### 4.2.2 Characteristic Scales, Size and Shape

Ideally, switchbacks would be imaged from a range of angles, providing a straightforward method to visualize their shape. However, as mentioned above, these structures are Alfvénic and have little change in plasma density, which is essential for line of sight (LOS) images from remote sensing instruments. We must instead rely on the *in situ* observations from a single S/C, which are fundamentally local measurements. Therefore, it is important to understand the relationship between the true physical structure of a switchback and the data measured by a S/C, as this can influence the way in which we think about and study them. For example, a small duration switchback in the PSP time series may be due to a physically smaller switchback, or because PSP clipped the edge of a larger switchback. This ambiguity also applies to a series of multiple switchbacks, which may truly be several closely spaced switchbacks or in fact one larger, more degraded switchback (Farrell et al. 2021).

Dudok de Wit et al. (2020) provided the first detailed statistics on switchbacks for PSP's first Enc. They showed that switchback duration could vary from a few seconds to over an hour, with no characteristic timescale. Through studying the waiting time (the time between each switchback) statistics, they found that switchbacks exhibited long term memory, and tended to aggregate, which they take as evidence for similar coronal origin. Many authors define switchbacks as deflections, above some threshold, away from the Parker spiral. The direction of this deflection, i.e. towards +T, is also interesting as it could act as a testable prediction of switchback origin theories Schwadron and McComas (2021). For Enc. 1 at least, Dudok de Wit et al. (2020) showed that deflections were isotropic about the Parker spiral direction, although they did note that the longest switchbacks displayed a weak preference for deflections in +T. Horbury et al. (2020) also found that switchbacks displayed a slight preference to deflect in T rather than N, although there was no distinction between -T or +T. The authors refer to the clock angle in an attempt to quantify the direction of switchback deflection. This is defined as the “angle of the vector projected onto a plane perpendicular to the Parker spiral that also contains N”, where 0°, 90°, 180° and 270° refer to +N, +T, -N, -T directions respectively. Unlike the entire switchback population, the longest switchbacks
did show a preference for deflection direction, that often displayed clustering about a certain direction that was not correlated to the solar wind flow direction. Crucially, Horbury et al. (2020) demonstrated a correlation between the duration of a switchback and the direction of deflection. They then asserted that the duration of a switchback was related to the way in which PSP cut through the true physical shape. Since switchbacks are Alfvénic, the direction of the magnetic field deflection also creates a flow deflection. This, when combined with the S/C velocity (which had a maximum tangential component of +90 km s⁻¹ during the first Enc.), sets the direction at which PSP travels through a switchback. As a first attempt, they assumed the switchbacks were aligned with the radial direction or dHT, allowing for the angle of PSP w.r.t. the flow to be calculated. The authors then demonstrated that as the angle to the flow decreased, the switchback duration increased, implying that these structures were long and thin along the flow direction, with transverse scales around 10⁴ km.

This idea was extended by Laker et al. (2021a) to more solar wind streams across the first two Encs. Instead of assuming a flow direction, they instead started with the idea that the structures were long and thin, and attempted to measure their orientation and dimensions. Allowing the average switchback width and aspect ratio to be free parameters they fit an expected model to the distribution of switchback durations, w.r.t. the S/C cutting angle. They applied this method while varying the switchback orientation, finding the orientation that was most consistent with the long, thin model. Switchbacks were found to be aligned away from the radial direction, towards to the Parker spiral. The statistical average switchback width was around 50,000 km, with an aspect ratio of the order of 10, although there was a large variation. Laker et al. (2021a) again emphasized that the duration of a switchback is a function of how the S/C cut through the structure, which is in turn related to the switchback deflection, dimensions, orientation and S/C velocity. A similar conclusion was also reached by Macneil et al. (2020) who argued that the direction of *Helios* w.r.t. switchbacks could influence the statistics seen in the data.

Unlike the previous studies that relied on large statistics, Krasnoselskikh et al. (2020) analyzed several case study switchbacks during the first Enc., finding currents at the boundaries. They argued that these currents flowed along the switchback surface, and also imagined switchbacks to be cylindrical. Analysing the flow deflections relative to the S/C for three switchbacks, they found a transverse scale of 7,000 km and 50,000 km for a compressive and Alfvénic switchback, respectively. A similar method was applied to a larger set of switchbacks by Larosa et al. (2021), who used minimum variance analysis (MVA) to find the normal directions of the leading and trailing edge. After calculating the width of the edges, an average normal velocity was multiplied by the switchback duration to give a final width. They found that the transverse switchback scale varied from several thousand km to a solar radius (695,000 km), with the mode value lying between 10⁴ km and 10⁵ km.

A novel approach to probe the internal structure of switchbacks was provided by Bandyopadhyay et al. (2021), who studied the behavior of energetic particles during switchback periods in the first five PSP Encs. Energetic particles (80-200 MeV/nucleus) continued to stream anti-sunward during a switchback in 86% of cases, implying that the radius of magnetic field curvature inside switchbacks was smaller or comparable to the ion gyroradius. Using typical solar wind parameters (B ~ 50 nT, ion energy 100 eV) this sets an upper limit of ~ 4000 km for the radius of curvature inside a switchback. Assuming a typical S-shaped curve envisaged by Kasper et al. (2019), this would constrain the switchback width to be less than ~ 16,000 km.

A summary of the results is displayed in Table 1, which exhibits a large variation but a general consensus that the switchback transverse scale ranges from 10³ km to 10⁵ km. Future areas of study should be focused on how the switchback shape and size varies with
did show a preference for deflection direction, that often displayed clustering about a certain direction that was not correlated to the solar wind flow direction. Crucially, Horbury et al. (2020) demonstrated a correlation between the duration of a switchback and the direction of deflection. They then asserted that the duration of a switchback was related to the way in which PSP cut through the true physical shape. Since switchbacks are Alfvénic, the direction of the magnetic field deflection also creates a flow deflection. This, when combined with the S/C velocity (which had a maximum tangential component of +90 km s⁻¹ during the first Enc.), sets the direction at which PSP travels through a switchback. As a first attempt, they assumed the switchbacks were aligned with the radial direction or dHT, allowing for the angle of PSP w.r.t. the flow to be calculated. The authors then demonstrated that as the angle to the flow decreased, the switchback duration increased, implying that these structures were long and thin along the flow direction, with transverse scales around 10⁴ km.

This idea was extended by Laker et al. (2021a) to more solar wind streams across the first two Encs. Instead of assuming a flow direction, they instead started with the idea that the structures were long and thin, and attempted to measure their orientation and dimensions. Allowing the average switchback width and aspect ratio to be free parameters they fit an expected model to the distribution of switchback durations, w.r.t. the S/C cutting angle. They applied this method while varying the switchback orientation, finding the orientation that was most consistent with the long, thin model. Switchbacks were found to be aligned away from the radial direction, towards to the Parker spiral. The statistical average switchback width was around 50,000 km, with an aspect ratio of the order of 10, although there was a large variation. Laker et al. (2021a) again emphasized that the duration of a switchback is a function of how the S/C cut through the structure, which is in turn related to the switchback deflection, dimensions, orientation and S/C velocity. A similar conclusion was also reached by Macneil et al. (2020) who argued that the direction of *Helios* w.r.t. switchbacks could influence the statistics seen in the data.

Unlike the previous studies that relied on large statistics, Krasnoselskikh et al. (2020) analyzed several case study switchbacks during the first Enc., finding currents at the boundaries. They argued that these currents flowed along the switchback surface, and also imagined switchbacks to be cylindrical. Analysing the flow deflections relative to the S/C for three switchbacks, they found a transverse scale of 7,000 km and 50,000 km for a compressive and Alfvénic switchback, respectively. A similar method was applied to a larger set of switchbacks by Larosa et al. (2021), who used minimum variance analysis (MVA) to find the normal directions of the leading and trailing edge. After calculating the width of the edges, an average normal velocity was multiplied by the switchback duration to give a final width. They found that the transverse switchback scale varied from several thousand km to a solar radius (695,000 km), with the mode value lying between 10⁴ km and 10⁵ km.

A novel approach to probe the internal structure of switchbacks was provided by Bandyopadhyay et al. (2021), who studied the behavior of energetic particles during switchback periods in the first five PSP Encs. Energetic particles (80-200 MeV/nucleus) continued to stream anti-sunward during a switchback in 86% of cases, implying that the radius of magnetic field curvature inside switchbacks was smaller or comparable to the ion gyroradius. Using typical solar wind parameters (B ~ 50 nT, ion energy 100 eV) this sets an upper limit of ~ 4000 km for the radius of curvature inside a switchback. Assuming a typical S-shaped curve envisaged by Kasper et al. (2019), this would constrain the switchback width to be less than ~ 16,000 km.

A summary of the results is displayed in Table 1, which exhibits a large variation but a general consensus that the switchback transverse scale ranges from 10³ km to 10⁵ km. Future areas of study should be focused on how the switchback shape and size varies with
<table><caption>**Table 1** Summary of the results regarding switchback shape and size, including which PSP Encs. were used in the analysis</caption><thead><tr><th>Study</th><th>Enc.</th><th>Transverse Scale (km)</th><th>Aspect</th></tr></thead><tbody><tr><td>Horbury et al. (2020)</td><td>1</td><td>10<sup>4</sup> km</td><td>—</td></tr><tr><td>Laker et al. (2021a)</td><td>1, 2</td><td>50,000 km</td><td>~ 10</td></tr><tr><td>Krasnoselskikh et al. (2020)</td><td>1</td><td>7000 km for compressive</td><td>—</td></tr><tr><td>Larosa et al. (2021)</td><td>1</td><td>50,000 km for Alfvénic<br>10<sup>3</sup> km – 10<sup>5</sup> km</td><td>—</td></tr><tr><td>Bandyopadhyay et al. (2021)</td><td>1-5</td><td>&lt; 16,000 km*</td><td>—</td></tr></tbody></table>

distance from the Sun. However, a robust method for determining how PSP cut through the switchback must be found for progress to be made in this area. For example, an increased current density or wave activity at the boundary may be used a signature of when PSP is clipping the edge of a switchback. Estimates of switchback transverse scale, like Larosa et al. (2021), could be constrained with the use of energetic particle data (Bandyopadhyay et al. 2021) on a case-by-case basis, improving the link between the duration measured by a S/C and the true physical size of the switchback.

### 4.2.3 Occurrence and Radial Evolution in the Solar Wind

Understanding how switchbacks evolve with radial distance is one of the key elements not only to determine their origin, but also to understand if switchbacks may contribute to the evolution of the turbulent cascade in the solar wind and to solar wind energy budget. Simulations (§4.3.5) and observations (§4.2.5) suggest that switchbacks may decay and disrupt as they propagate in the inner heliosphere. As a consequence, it is expected that the occurrence of switchbacks decreases with radial distance in the absence of an ongoing driver capable of reforming switchbacks *in situ*. On the contrary, the presence of an efficient driving mechanism is expected to lead to an increase, or to a steady state, of the occurrence of switchbacks with heliocentric distance. Based on this idea, Mozer et al. (2021a) analyzed the occurrence rate (counts per hour) of switchbacks with radial distance using data from Encs. 3 through 7 of PSP. The authors conclude that the occurrence rate depends on the wind speed, with higher count rates for higher wind speed, and that it and does not depend on the radial distance. Based on this result, Mozer et al. (2021a) exclude *in situ* generation mechanisms. However, it is interesting to note that counts of switchbacks observed by PSP are highly scattered with radial distance, likely due to the mixing of different streams (Mozer et al. 2021a). Tenerani et al. (2021) also report highly scattered counts of switchbacks with radial distance, although they argue that the presence of decaying and reforming switchbacks might also contribute to such an effect. Tenerani et al. (2021) analyzed the count rates (counts per km) of switchbacks by complementing PSP data with *Helios* and *Ulysses*. Their analysis shows that the occurrence of switchbacks is scale-dependent, a trend that is particularly clear in *Helios* and *Ulysses* data. In particular, they found that the fraction of switchbacks of duration of a few tens of seconds and longer increases with radial distance and that the fraction of those of duration below a few tens of seconds instead decreases. The overall cumulative counts per km, two examples of which are shown in Fig. 3, show such a trend. Results from this analysis led Tenerani et al. (2021) to conclude that switchbacks in the solar wind can decay and reform in the expanding solar wind, with *in situ* generation

<footnote>*Assuming an S-shape structure</footnote>
<table><caption>**Table 1** Summary of the results regarding switchback shape and size, including which PSP Encs. were used in the analysis</caption><thead><tr><th>Study</th><th>Enc.</th><th>Transverse Scale (km)</th><th>Aspect</th></tr></thead><tbody><tr><td>Horbury et al. (2020)</td><td>1</td><td>10<sup>4</sup> km</td><td>—</td></tr><tr><td>Laker et al. (2021a)</td><td>1, 2</td><td>50,000 km</td><td>~ 10</td></tr><tr><td>Krasnoselskikh et al. (2020)</td><td>1</td><td>7000 km for compressive</td><td>—</td></tr><tr><td>Larosa et al. (2021)</td><td>1</td><td>50,000 km for Alfvénic<br>10<sup>3</sup> km – 10<sup>5</sup> km</td><td>—</td></tr><tr><td>Bandyopadhyay et al. (2021)</td><td>1-5</td><td>&lt; 16,000 km*</td><td>—</td></tr></tbody></table>

distance from the Sun. However, a robust method for determining how PSP cut through the switchback must be found for progress to be made in this area. For example, an increased current density or wave activity at the boundary may be used a signature of when PSP is clipping the edge of a switchback. Estimates of switchback transverse scale, like Larosa et al. (2021), could be constrained with the use of energetic particle data (Bandyopadhyay et al. 2021) on a case-by-case basis, improving the link between the duration measured by a S/C and the true physical size of the switchback.

### 4.2.3 Occurrence and Radial Evolution in the Solar Wind

Understanding how switchbacks evolve with radial distance is one of the key elements not only to determine their origin, but also to understand if switchbacks may contribute to the evolution of the turbulent cascade in the solar wind and to solar wind energy budget. Simulations (§4.3.5) and observations (§4.2.5) suggest that switchbacks may decay and disrupt as they propagate in the inner heliosphere. As a consequence, it is expected that the occurrence of switchbacks decreases with radial distance in the absence of an ongoing driver capable of reforming switchbacks *in situ*. On the contrary, the presence of an efficient driving mechanism is expected to lead to an increase, or to a steady state, of the occurrence of switchbacks with heliocentric distance. Based on this idea, Mozer et al. (2021a) analyzed the occurrence rate (counts per hour) of switchbacks with radial distance using data from Encs. 3 through 7 of PSP. The authors conclude that the occurrence rate depends on the wind speed, with higher count rates for higher wind speed, and that it and does not depend on the radial distance. Based on this result, Mozer et al. (2021a) exclude *in situ* generation mechanisms. However, it is interesting to note that counts of switchbacks observed by PSP are highly scattered with radial distance, likely due to the mixing of different streams (Mozer et al. 2021a). Tenerani et al. (2021) also report highly scattered counts of switchbacks with radial distance, although they argue that the presence of decaying and reforming switchbacks might also contribute to such an effect. Tenerani et al. (2021) analyzed the count rates (counts per km) of switchbacks by complementing PSP data with *Helios* and *Ulysses*. Their analysis shows that the occurrence of switchbacks is scale-dependent, a trend that is particularly clear in *Helios* and *Ulysses* data. In particular, they found that the fraction of switchbacks of duration of a few tens of seconds and longer increases with radial distance and that the fraction of those of duration below a few tens of seconds instead decreases. The overall cumulative counts per km, two examples of which are shown in Fig. 3, show such a trend. Results from this analysis led Tenerani et al. (2021) to conclude that switchbacks in the solar wind can decay and reform in the expanding solar wind, with *in situ* generation

<footnote>*Assuming an S-shape structure</footnote>
<figure><img src="image_5.png" /><figcaption>Fig. 3 Cumulative counts of switchbacks as a function of radial distance from PSP, Helios and two polar passes of *Ulysses* (in 1994 and 2006). The left plot shows counts per km of switchbacks of duration up to 30 minute, while the right plot shows the same quantity but for switchbacks of duration up to 3 hours. PSP data (43 in total) were binned in intervals of width $\Delta R = 0.05$ AU. The error bars denote the range of data points in each bin (Tenerani et al. 2021)</figcaption></figure>

being more efficient at the larger scales. They also found that the mean radial amplitude of switchbacks decays faster than the overall turbulent fluctuations, in a way that is consistent with the radial decrease of the mean radial field. They argued that this could be the result of a saturation of amplitudes and may be a signature of decay processes of switchbacks as they evolve and propagate in the inner Heliosphere.

### 4.2.4 Thermodynamics and Energetics

An important question about switchbacks is whether the plasma inside these structures is different compared to the background surrounding plasma. We have seen already that switchbacks exhibit a bulk speed enhancement in the main core proton population. As this increase in speed corresponds to a net acceleration in the center of mass frame, the plasma kinetic energy is therefore larger in switchbacks than in the background solar wind. This result suggests these structures carry a significant amount of energy with them as the solar wind flows out into the inner heliosphere. A question that directly follows is whether the plasma is also hotter inside w.r.t. outside.

Attempting to answer this important question with SPC is non-trivial, since the measurements are restricted to a radial cut of the full 3D ion VDF (Kasper et al. 2016; Case et al. 2020). While the magnetic field rotation in switchbacks enables the sampling of many different angular cuts as the S/C Encs. these structures, the cuts are not directly comparable as they represent different combinations of $T_\perp$ and $T_\parallel$ (See for example, Huang et al. 2020a):

$$w_r = \sqrt{w_{\parallel}^2 (\hat{r} \cdot \hat{b})^2 + w_{\perp}^2 \left[ 1 - (\hat{r} \cdot \hat{b})^2 \right]}, \quad (2)$$

where $w_r$ is the measured thermal speed of the ions, related to temperature by $w = \sqrt{2k_B T/m}$, and $\hat{b} = B/B$. Therefore, SPC measurements of temperature outside switchbacks, where the magnetic field is typically radial, sample the proton parallel temperature,
<figure><img src="image_5.png" /><figcaption>Fig. 3 Cumulative counts of switchbacks as a function of radial distance from PSP, Helios and two polar passes of *Ulysses* (in 1994 and 2006). The left plot shows counts per km of switchbacks of duration up to 30 minute, while the right plot shows the same quantity but for switchbacks of duration up to 3 hours. PSP data (43 in total) were binned in intervals of width $\Delta R = 0.05$ AU. The error bars denote the range of data points in each bin (Tenerani et al. 2021)</figcaption></figure>

being more efficient at the larger scales. They also found that the mean radial amplitude of switchbacks decays faster than the overall turbulent fluctuations, in a way that is consistent with the radial decrease of the mean radial field. They argued that this could be the result of a saturation of amplitudes and may be a signature of decay processes of switchbacks as they evolve and propagate in the inner Heliosphere.

### 4.2.4 Thermodynamics and Energetics

An important question about switchbacks is whether the plasma inside these structures is different compared to the background surrounding plasma. We have seen already that switchbacks exhibit a bulk speed enhancement in the main core proton population. As this increase in speed corresponds to a net acceleration in the center of mass frame, the plasma kinetic energy is therefore larger in switchbacks than in the background solar wind. This result suggests these structures carry a significant amount of energy with them as the solar wind flows out into the inner heliosphere. A question that directly follows is whether the plasma is also hotter inside w.r.t. outside.

Attempting to answer this important question with SPC is non-trivial, since the measurements are restricted to a radial cut of the full 3D ion VDF (Kasper et al. 2016; Case et al. 2020). While the magnetic field rotation in switchbacks enables the sampling of many different angular cuts as the S/C Encs. these structures, the cuts are not directly comparable as they represent different combinations of $T_\perp$ and $T_\parallel$ (See for example, Huang et al. 2020a):

$$w_r = \sqrt{w_\parallel^2 (\hat{r} \cdot \hat{b})^2 + w_\perp^2 \left[ 1 - (\hat{r} \cdot \hat{b})^2 \right]}, \quad (2)$$

where $w_r$ is the measured thermal speed of the ions, related to temperature by $w = \sqrt{2k_B T/m}$, and $\hat{b} = B/B$. Therefore, SPC measurements of temperature outside switchbacks, where the magnetic field is typically radial, sample the proton parallel temperature,
<figure><img src="image_6.png" /><figcaption>**Fig. 4** Left: SPC measurements of the core proton radial temperature during a large amplitude switchback shown in the bottom panel. The measured core proton temperature (upper panel) is modulated by the B angle and it's maximum when measuring $T_{p\perp}$ at roughly 90°, consistent with a dominant $T_{p\perp} > T_{p\|}$ anisotropy in the background plasma. Right: cuts of the ion VDF made by SPC at different angles: antiparallel (anti-radial), orthogonal and parallel (radial) to **B**. The fit of the proton core is shown in pink. The bottom panel compares the radial and anti-radial VDFs, where the latter has been flipped to account for the field reversal inside the switchback. Figure adapted from Woolley et al. (2020)</figcaption></figure>

$T_{p\|}$. In contrast, as **B** rotates towards 90° within a switchback, the SPC cut typically provides a better estimate of $T_{p\perp}$. To overcome this, Huang et al. (2020a) investigated the proton temperature anisotropy statistically. They assumed that the proton VDF does not vary significantly over the SPC sampling time as **B** deflects away from the radial direction, and then solved Eq. 2 for both $w_\|$ and $w_\perp$. While this method does reveal some information about the underlying temperature anisotropy, this approach is not suitable for the comparison of anisotropy within a single switchback since it assumes, *a priori*, that the anisotropy is fixed compared to the background plasma.

Another possibility is to investigate switchbacks that exhibit a reversal in the sign of $B_R$, in other words, $\theta_{BR} \simeq 180^\circ$ inside the switchback for a radial background field. This technique provides two estimates of $T_{p\|}$: outside the switchback, when the field is close to (anti-)radial, and inside, when $B_R$ is reversed. This is the only way to compare the same radial SPC cut of the VDF inside and outside switchbacks, leading then to a direct comparison between the two resulting $T_{p\|}$ values. Woolley et al. (2020) first attempted this approach, and a summary of their results are presented in Fig. 4. A switchback with an almost complete reversal in the field direction is tracked in the left panels; the bottom panel shows the angle of the magnetic field, from almost anti-radial to radial and back again. The measured core proton temperature, $T_{cp\|}$ (upper left panel), increases with angle, $\theta_{BR}$, and reaches a maximum at $\theta_{BR} \simeq 90^\circ$, consistent with a dominant $T_{p\perp} > T_{p\|}$ anisotropy in the background plasma. On the other hand, when the SPC sampling direction is (anti-)parallel to **B** (approximately 0° and 180°), Woolley et al. (2020) find the same value for $T_{cp\|}$. Therefore, they concluded that the plasma inside switchbacks is not significantly hotter than the background plasma.

The right panels show radial cuts of the ion VDF made by SPC at different angles: anti-parallel (anti-radial), orthogonal and parallel (radial) to **B**. The fit of the proton core is shown in pink. The bottom panel compares the measurement in the radial and anti-radial direction, once the latter has been flipped to account for the field reversal inside the switchback; the two distributions fall on top of each other, suggesting that core protons undergo a rigid rotation in velocity space inside the switchback, without a significant deformation of the VDF. The comparison in the panels also shows that the core temperature is larger for
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4** Left: SPC measurements of the core proton radial temperature during a large amplitude switchback shown in the bottom panel. The measured core proton temperature (upper panel) is modulated by the B angle and it's maximum when measuring $T_{p\perp}$ at roughly 90°, consistent with a dominant $T_{p\perp} > T_{p\parallel}$ anisotropy in the background plasma. Right: cuts of the ion VDF made by SPC at different angles: antiparallel (anti-radial), orthogonal and parallel (radial) to **B**. The fit of the proton core is shown in pink. The bottom panel compares the radial and anti-radial VDFs, where the latter has been flipped to account for the field reversal inside the switchback. Figure adapted from Woolley et al. (2020)</figcaption></figure>

$T_{p\parallel}$. In contrast, as **B** rotates towards 90° within a switchback, the SPC cut typically provides a better estimate of $T_{p\perp}$. To overcome this, Huang et al. (2020a) investigated the proton temperature anisotropy statistically. They assumed that the proton VDF does not vary significantly over the SPC sampling time as **B** deflects away from the radial direction, and then solved Eq. 2 for both $w_\parallel$ and $w_\perp$. While this method does reveal some information about the underlying temperature anisotropy, this approach is not suitable for the comparison of anisotropy within a single switchback since it assumes, *a priori*, that the anisotropy is fixed compared to the background plasma.

Another possibility is to investigate switchbacks that exhibit a reversal in the sign of $B_R$, in other words, $\theta_{BR} \simeq 180^\circ$ inside the switchback for a radial background field. This technique provides two estimates of $T_{p\parallel}$: outside the switchback, when the field is close to (anti-)radial, and inside, when $B_R$ is reversed. This is the only way to compare the same radial SPC cut of the VDF inside and outside switchbacks, leading then to a direct comparison between the two resulting $T_{p\parallel}$ values. Woolley et al. (2020) first attempted this approach, and a summary of their results are presented in Fig. 4. A switchback with an almost complete reversal in the field direction is tracked in the left panels; the bottom panel shows the angle of the magnetic field, from almost anti-radial to radial and back again. The measured core proton temperature, $T_{cp\parallel}$ (upper left panel), increases with angle, $\theta_{BR}$, and reaches a maximum at $\theta_{BR} \simeq 90^\circ$, consistent with a dominant $T_{p\perp} > T_{p\parallel}$ anisotropy in the background plasma. On the other hand, when the SPC sampling direction is (anti-)parallel to **B** (approximately 0° and 180°), Woolley et al. (2020) find the same value for $T_{cp\parallel}$. Therefore, they concluded that the plasma inside switchbacks is not significantly hotter than the background plasma.

The right panels show radial cuts of the ion VDF made by SPC at different angles: anti-parallel (anti-radial), orthogonal and parallel (radial) to **B**. The fit of the proton core is shown in pink. The bottom panel compares the measurement in the radial and anti-radial direction, once the latter has been flipped to account for the field reversal inside the switchback; the two distributions fall on top of each other, suggesting that core protons undergo a rigid rotation in velocity space inside the switchback, without a significant deformation of the VDF. The comparison in the panels also shows that the core temperature is larger for
<figure><img src="image_7.png" /><figcaption>**Fig. 5** Overview of plasma properties inside a group of switchback patches. The bottom panel shows the core proton parallel and perpendicular temperatures measured by SPAN. The colours in $T_\|$ encode the deflection of **B** from the radial direction. Patches (grey sectors) exhibit systematically higher $T_\|$ than in quiet periods, while $T_\perp$ is mostly uniform throughout the interval. Figure adapted from Woodham et al. (2021)</figcaption></figure>

oblique angles ° (large $T_{cp,\perp}$) and that the proton beam switches sides during the reversal, as discussed in Neugebauer and Goldstein (2013). They conclude that plasma inside switchbacks, at least those with the largest angular deflections, exhibits a negligible difference in the parallel temperature compared to the background, and therefore, the speed enhancement of the proton core inside these structures does not follow the expected T-V relation (e.g., see Perrone et al. 2019). This scenario is consistent with studies about turbulent properties and associated heating inside and outside switchbacks (Bourouaine et al. 2020; Martinović et al. 2021).

On the other hand, SPAN measurements of the core proton parallel and perpendicular temperatures show a large-scale modulation by patches of switchbacks (Woodham et al. 2021). Figure 5 shows an overview of magnetic field and plasma properties through an interval that contains a series of switchback patches and quiet radial periods during Enc. 2. The bottom panel highlights the behavior of $T_\perp$ and $T_\|$ through the structures. The former is approximately constant throughout the interval, consistent with an equally roughly constant solar wind speed explained by the well-known speed-temperature relationship in the solar wind (for example, see Matthaeus et al. 2006, and references therein). In contrast, the latter shows large variations, especially during patches when a systematic larger $T_\|$ is observed. As a consequence, increases in $T_\|$ are also correlated with deflections in the magnetic field directions (colors refer to the instantaneous angle $\theta_{BR}$). The origin of such a correlation between $\theta_{BR}$ and $T_\|$ is not fully understood yet, although the large-scale enhancement of the parallel temperature within patches could be a signature of some preferential heating of the plasma closer to the Sun (e.g., by interchange reconnection), supporting a coronal origin for these structures.

### 4.2.5 Switchback Boundaries and Small-Scale Waves

Switchback boundaries are plasma discontinuities, which separate two plasmas inside and outside the structure moving with different velocities that may have different temperatures
<figure><img src="image_8.png" /><figcaption>**Fig. 5** Overview of plasma properties inside a group of switchback patches. The bottom panel shows the core proton parallel and perpendicular temperatures measured by SPAN. The colours in $T_\|$ encode the deflection of **B** from the radial direction. Patches (grey sectors) exhibit systematically higher $T_\|$ than in quiet periods, while $T_\perp$ is mostly uniform throughout the interval. Figure adapted from Woodham et al. (2021)</figcaption></figure>

oblique angles ° (large $T_{cp,\perp}$) and that the proton beam switches sides during the reversal, as discussed in Neugebauer and Goldstein (2013). They conclude that plasma inside switchbacks, at least those with the largest angular deflections, exhibits a negligible difference in the parallel temperature compared to the background, and therefore, the speed enhancement of the proton core inside these structures does not follow the expected T-V relation (e.g., see Perrone et al. 2019). This scenario is consistent with studies about turbulent properties and associated heating inside and outside switchbacks (Bourouaine et al. 2020; Martinović et al. 2021).

On the other hand, SPAN measurements of the core proton parallel and perpendicular temperatures show a large-scale modulation by patches of switchbacks (Woodham et al. 2021). Figure 5 shows an overview of magnetic field and plasma properties through an interval that contains a series of switchback patches and quiet radial periods during Enc. 2. The bottom panel highlights the behavior of $T_\perp$ and $T_\|$ through the structures. The former is approximately constant throughout the interval, consistent with an equally roughly constant solar wind speed explained by the well-known speed-temperature relationship in the solar wind (for example, see Matthaeus et al. 2006, and references therein). In contrast, the latter shows large variations, especially during patches when a systematic larger $T_\|$ is observed. As a consequence, increases in $T_\|$ are also correlated with deflections in the magnetic field directions (colors refer to the instantaneous angle $\theta_{BR}$). The origin of such a correlation between $\theta_{BR}$ and $T_\|$ is not fully understood yet, although the large-scale enhancement of the parallel temperature within patches could be a signature of some preferential heating of the plasma closer to the Sun (e.g., by interchange reconnection), supporting a coronal origin for these structures.

### 4.2.5 Switchback Boundaries and Small-Scale Waves

Switchback boundaries are plasma discontinuities, which separate two plasmas inside and outside the structure moving with different velocities that may have different temperatures
<figure><img src="image_8.png" /><figcaption>**Fig. 6** The magnetic field dynamics for a typical deflection (switchback) of the magnetic field observed at heliocentric distance of 35.6 $R_\odot$ during PSP's first solar Enc., on 4 Nov. 2018 (left) and at heliocentric distance of ~50 $R_\odot$ on 10 Nov. 2018 (right). The radial component of the magnetic field (red curve in panel (a)) exhibits an almost complete inversion at the switchback boundary and becomes positive (anti-sunward). The transverse components are shown in blue (T, in the ecliptic plane) and in green (N-normal component, transverse to the ecliptic plane). The magnetic field magnitude is shown in black. Panel (b) represents plasma bulk velocity components (with a separate scale for the radial component $V_z$ shown in red) with the same color scheme as in panel (a). Panels (c) and (d) represent the proton density and temperature. Panel (e) presents the magnetic field waveforms from SCM (with the instrumental power cut-off below 3 Hz). The dynamic spectrum of these waveforms are shown in Panel (f), in which the red-dashed curve indicates the local lower hybrid ($f_{LH}$) frequency. Panels (g-j) represent the magnetic and eclectic field perturbations around the switchback leading boundary, the wavelet spectrum of the magnetic field perturbation, and radial component of the Poynting flux (blue color indicates propagation from the Sun and red sunward propagation). The same parameters for the trailing boundary are presented in panels (k-n).</figcaption></figure>

and densities. Figure 6 shows a “typical” switchback, highlighting: (1) the sharp rotation of magnetic field as well as the dropouts in field intensity on the boundaries (Fig. 6a), in agreement with Farrell et al. (2020); (2) the increase of radial velocity showing the Alfvénicity (Fig. 6b); (3) the plasma density enhancements at the boundaries of the switchback (Fig. 6c), from 300 $cm^{-3}$ to ~500 and 400 $cm^{-3}$ at the leading and trailing edges respectively with some decrease of plasma density inside the structure (Farrell et al. 2020) down to 250 – 280 $cm^{-3}$; and (4) enhanced wave activity inside the switchback and at the boundaries (Fig. 6d) predominantly below $f_{LH}$ with the higher amplitude wave bursts at the boundaries. The detailed superimposed epoch analysis of plasma and magnetic field parameters presented in Farrell et al. (2020) showed that magnetic field magnitude dips and plasma density enhancement are the characteristic features associated with switchbacks boundaries.

It is further shown that wave activity decays with heliocentric distances. Together with the activity inside switchbacks, the boundaries also relax during propagation (Mozer et al. 2020b; Farrell et al. 2021; Akhavan-Tafti et al. 2021) suggesting that the switchback boundary formation process is dynamic and evolving, even occurring near the PSP observation point inside of 40 $R_\odot$ (Farrell et al. 2021).

The analysis of MHD discontinuity types was performed by Larosa et al. (2021) who found that 32% of switchbacks may be attributed to rotational discontinuities (RD), 17% to
<figure><img src="image_9.png" /><figcaption>**Fig. 6** The magnetic field dynamics for a typical deflection (switchback) of the magnetic field observed at heliocentric distance of 35.6 $R_\odot$ during PSP's first solar Enc., on 4 Nov. 2018 (left) and at heliocentric distance of ~50 $R_\odot$ on 10 Nov. 2018 (right). The radial component of the magnetic field (red curve in panel (a)) exhibits an almost complete inversion at the switchback boundary and becomes positive (anti-sunward). The transverse components are shown in blue (T, in the ecliptic plane) and in green (N-normal component, transverse to the ecliptic plane). The magnetic field magnitude is shown in black. Panel (b) represents plasma bulk velocity components (with a separate scale for the radial component $V_z$ shown in red) with the same color scheme as in panel (a). Panels (c) and (d) represent the proton density and temperature. Panel (e) presents the magnetic field waveforms from SCM (with the instrumental power cut-off below 3 Hz). The dynamic spectrum of these waveforms are shown in Panel (f), in which the red-dashed curve indicates the local lower hybrid ($f_{LH}$) frequency. Panels (g-j) represent the magnetic and eclectic field perturbations around the switchback leading boundary, the wavelet spectrum of the magnetic field perturbation, and radial component of the Poynting flux (blue color indicates propagation from the Sun and red sunward propagation). The same parameters for the trailing boundary are presented in panels (k-n).</figcaption></figure>

and densities. Figure 6 shows a “typical” switchback, highlighting: (1) the sharp rotation of magnetic field as well as the dropouts in field intensity on the boundaries (Fig. 6a), in agreement with Farrell et al. (2020); (2) the increase of radial velocity showing the Alfvénicity (Fig. 6b); (3) the plasma density enhancements at the boundaries of the switchback (Fig. 6c), from 300 $cm^{-3}$ to ~500 and 400 $cm^{-3}$ at the leading and trailing edges respectively with some decrease of plasma density inside the structure (Farrell et al. 2020) down to 250 – 280 $cm^{-3}$; and (4) enhanced wave activity inside the switchback and at the boundaries (Fig. 6d) predominantly below $f_{LH}$ with the higher amplitude wave bursts at the boundaries. The detailed superimposed epoch analysis of plasma and magnetic field parameters presented in Farrell et al. (2020) showed that magnetic field magnitude dips and plasma density enhancement are the characteristic features associated with switchbacks boundaries.

It is further shown that wave activity decays with heliocentric distances. Together with the activity inside switchbacks, the boundaries also relax during propagation (Mozer et al. 2020b; Farrell et al. 2021; Akhavan-Tafti et al. 2021) suggesting that the switchback boundary formation process is dynamic and evolving, even occurring near the PSP observation point inside of 40 $R_\odot$ (Farrell et al. 2021).

The analysis of MHD discontinuity types was performed by Larosa et al. (2021) who found that 32% of switchbacks may be attributed to rotational discontinuities (RD), 17% to
<figure><img src="image_9.png" /><figcaption>**Fig. 7 (a)** Discontinuity classification of 273 magnetic switchbacks. Scatter plot of relative normal component of magnetic field of upstream, pristine solar wind and relative variation in magnetic field intensity across switchbacks' leading (QL-to-SPIKE) transition regions. The color shading indicates the switchbacks' distance from the Sun. **(b)** Scatter plot of the ratio of number of RD events to that of ED as a function of distance from the Sun. The histogram of event count per radial distance (bin width = 1 $R_⊙$) is provided on the right y-axis in blue for reference. **(c)** Stacked bar plots of the relative ratios of RD:TD:ED:ND discontinuities at 0.2 AU (PSP; Akhavan-Tafti et al. 2021), 1.0 AU (ISEE; Neugebauer et al. 1984b), and 1.63 – 3.73 AU (Ulysses; Yamauchi et al. 2002)</figcaption></figure>

tangential discontinuities (TD), about 42% to the group of discontinuities that are difficult to unambiguously define (ED), and 9% that do not belong to any of these groups (ND). Similarly, as shown in Fig. 7, a recent study by Akhavan-Tafti et al. (2021) reported that the relative occurrence rate of RD-type switchbacks goes down with heliocentric distance (Fig. 7b), suggesting that RD-type switchbacks may fully disappear past 0.3 AU. However, RD-type switchbacks have been observed at both Earth (1 AU; Neugebauer et al. 1984b) and near Jupiter (2.5 AU; Yamauchi et al. 2002), though at smaller rates of occurrence (Fig. 7c) than that measured by PSP. Future investigations are needed to examine (1) the mechanisms via which switchbacks may evolve, and (2) whether the dominant switchback evolution mechanism changes with heliocentric distance.

Various studies have also investigated wave activity on switchback boundaries (Mozer et al. 2020b; Agapitov et al. 2020; Larosa et al. 2021): the boundary surface MHD wave (observed at the leading edge of the switchback in Fig. 6 and highlighted in panels (g-h)) and the localized whistler bursts in the magnetic dip (observed at the trailing edge of the switchback in Fig. 6 and highlighted in panels (k-n)). The whistler wave burst in Fig. 6(k-n) had Poynting flux directed to the Sun that leaded to significant Doppler downshift of wave frequency in the S/C frame (Agapitov et al. 2020). Because of their sunward propagation these whistler waves can efficiently scatter strahl electron population. These waves are often observed in the magnetic field magnitude minima at the switchback boundaries, i.e., can be considered as the regular feature associated with switchbacks.

Lastly, features related to reconnection are occasionally observed at switchback boundaries, albeit only in about 1% of the observed events (Froment et al. 2021; Phan et al. 2020). If occurring, reconnection on the boundary of switchbacks with the solar wind magnetic field may lead to the disappearance of some switchbacks (Drake et al. 2020). Surprisingly, there has been no evidence of reconnection on switchback boundaries at distances greater than 50 $R_⊙$. Phan et al. (2020) explained that the absence of reconnection at these boundaries may be due to (a) large, albeit sub-Alfvénic, velocity shears at switchback boundaries which can suppress reconnection (Swisdak et al. 2003), or that (b) switchback boundaries, commonly characterized as Alfvénic current sheets, are isolated RD-type discontinuities that do not undergo local reconnection. Akhavan-Tafti et al. (2021) similarly showed that switchback boundaries theoretically favor magnetic reconnection based on their plasma beta and
<figure><img src="image_10.png" /><figcaption>**Fig. 7** (a) Discontinuity classification of 273 magnetic switchbacks. Scatter plot of relative normal component of magnetic field of upstream, pristine solar wind and relative variation in magnetic field intensity across switchbacks' leading (QL-to-SPIKE) transition regions. The color shading indicates the switchbacks' distance from the Sun. (b) Scatter plot of the ratio of number of RD events to that of ED as a function of distance from the Sun. The histogram of event count per radial distance (bin width = 1 $R_⊙$) is provided on the right y-axis in blue for reference. (c) Stacked bar plots of the relative ratios of RD:TD:ED:ND discontinuities at 0.2 AU (PSP; Akhavan-Tafti et al. 2021), 1.0 AU (ISEE; Neugebauer et al. 1984b), and 1.63 – 3.73 AU (Ulysses; Yamauchi et al. 2002)</figcaption></figure>

tangential discontinuities (TD), about 42% to the group of discontinuities that are difficult to unambiguously define (ED), and 9% that do not belong to any of these groups (ND). Similarly, as shown in Fig. 7, a recent study by Akhavan-Tafti et al. (2021) reported that the relative occurrence rate of RD-type switchbacks goes down with heliocentric distance (Fig. 7b), suggesting that RD-type switchbacks may fully disappear past 0.3 AU. However, RD-type switchbacks have been observed at both Earth (1 AU; Neugebauer et al. 1984b) and near Jupiter (2.5 AU; Yamauchi et al. 2002), though at smaller rates of occurrence (Fig. 7c) than that measured by PSP. Future investigations are needed to examine (1) the mechanisms via which switchbacks may evolve, and (2) whether the dominant switchback evolution mechanism changes with heliocentric distance.

Various studies have also investigated wave activity on switchback boundaries (Mozer et al. 2020b; Agapitov et al. 2020; Larosa et al. 2021): the boundary surface MHD wave (observed at the leading edge of the switchback in Fig. 6 and highlighted in panels (g-h)) and the localized whistler bursts in the magnetic dip (observed at the trailing edge of the switchback in Fig. 6 and highlighted in panels (k-n)). The whistler wave burst in Fig. 6(k-n) had Poynting flux directed to the Sun that leaded to significant Doppler downshift of wave frequency in the S/C frame (Agapitov et al. 2020). Because of their sunward propagation these whistler waves can efficiently scatter strahl electron population. These waves are often observed in the magnetic field magnitude minima at the switchback boundaries, i.e., can be considered as the regular feature associated with switchbacks.

Lastly, features related to reconnection are occasionally observed at switchback boundaries, albeit only in about 1% of the observed events (Froment et al. 2021; Phan et al. 2020). If occurring, reconnection on the boundary of switchbacks with the solar wind magnetic field may lead to the disappearance of some switchbacks (Drake et al. 2020). Surprisingly, there has been no evidence of reconnection on switchback boundaries at distances greater than 50 $R_⊙$. Phan et al. (2020) explained that the absence of reconnection at these boundaries may be due to (a) large, albeit sub-Alfvénic, velocity shears at switchback boundaries which can suppress reconnection (Swisdak et al. 2003), or that (b) switchback boundaries, commonly characterized as Alfvénic current sheets, are isolated RD-type discontinuities that do not undergo local reconnection. Akhavan-Tafti et al. (2021) similarly showed that switchback boundaries theoretically favor magnetic reconnection based on their plasma beta and
magnetic shear angle characteristics (Swisdak et al. 2003). However, the authors concluded that negligible magnetic curvature, that is highly stretched magnetic field lines (Akhavan-Tafti et al. 2019a,b), at switchback boundaries may inhibit magnetic reconnection. Further investigations are needed to explore whether and how magnetic curvature evolves with heliocentric distance.

## 4.3 Theoretical Models

In this section, we outline the collection of theoretical models that have been formulated to explain observations of switchbacks. These are based on a variety of physical effects, and there is, as of yet, no consensus about the key ingredients needed to explain observations. In the following we discuss each model and related works in turn, organized by the primary physical effect that is assumed to drive switchback formation. These are (i) Interchange reconnection (§4.3.1), (ii) Other solar-surface processes (§4.3.2), (iii) Interactions between solar-wind streams (§4.3.3), and (iv) Expanding AWs and turbulence (§4.3.4). Within each of these broad categories, we discuss the various theories and models, some of which differ in important ways. In addition, some models naturally involve multiple physical effects, which we try to note as appropriate.

The primary motivation for understanding the origin of switchbacks is to understand their relevance to the heating and acceleration of the solar-wind. As discussed in, e.g., Cranmer (2009), magnetically driven wind models fall into the two broad classes of wave/turbulence-driven (WTD) and reconnection/loop-opening (RLO) models. A natural question is how switchbacks relate to the heating mechanism and what clues they provide as to the importance of different forms of heating in different types of wind. With this in mind, it is helpful to further, more broadly, categorize the mechanisms discussed above into “*ex situ*” mechanisms (covering interchange reconnection and other solar-surface processes) – in which switchbacks result from transient, impulsive events near the surface of the sun – and “*in situ*” mechanisms (covering stream interactions and AWs), in which switchbacks result from processes within the solar wind as it propagates outwards. An *ex situ* switchback formation model, with its focus on impulsive events, naturally ties into an RLO heating scenario; an *in situ* formation process, by focusing on local processes in the extended solar wind, naturally ties into a WTD scenario. This is particularly true given the significant energy content of switchbacks in some PSP observations (see §4.2.1), although there are also important caveats in some of the models. Thus, understanding the origin of switchbacks is key to understanding the origin of the solar wind itself. How predictions from different models hold up when compared to observations may provide us with important clues. This is discussed in more detail in the summary of the implications of different models and how they compare to observations in §4.4.2.

### 4.3.1 Interchange Reconnection

Interchange reconnection refers to the process whereby a region of open magnetic-field lines reconnect with a closed magnetic loop (Fisk 2005). Since this process is expected to be explosive and suddenly change the shape and topology of the field, it is a good candidate for the origin of switchbacks and has been considered by several authors. The basic scenario is shown in Fig. 8.

Fisk and Kasper (2020) first pointed out the general applicability of interchange reconnection to the PSP observations (the possible relevance to earlier *Ulysses* observations had also been discussed in Yamauchi et al. 2004). They focus on the large transverse flows measured by PSP as evidence for the global circulation of open flux enabled by the interchange
magnetic shear angle characteristics (Swisdak et al. 2003). However, the authors concluded that negligible magnetic curvature, that is highly stretched magnetic field lines (Akhavan-Tafti et al. 2019a,b), at switchback boundaries may inhibit magnetic reconnection. Further investigations are needed to explore whether and how magnetic curvature evolves with heliocentric distance.

## 4.3 Theoretical Models

In this section, we outline the collection of theoretical models that have been formulated to explain observations of switchbacks. These are based on a variety of physical effects, and there is, as of yet, no consensus about the key ingredients needed to explain observations. In the following we discuss each model and related works in turn, organized by the primary physical effect that is assumed to drive switchback formation. These are (i) Interchange reconnection (§4.3.1), (ii) Other solar-surface processes (§4.3.2), (iii) Interactions between solar-wind streams (§4.3.3), and (iv) Expanding AWs and turbulence (§4.3.4). Within each of these broad categories, we discuss the various theories and models, some of which differ in important ways. In addition, some models naturally involve multiple physical effects, which we try to note as appropriate.

The primary motivation for understanding the origin of switchbacks is to understand their relevance to the heating and acceleration of the solar-wind. As discussed in, e.g., Cranmer (2009), magnetically driven wind models fall into the two broad classes of wave/turbulence-driven (WTD) and reconnection/loop-opening (RLO) models. A natural question is how switchbacks relate to the heating mechanism and what clues they provide as to the importance of different forms of heating in different types of wind. With this in mind, it is helpful to further, more broadly, categorize the mechanisms discussed above into “*ex situ*” mechanisms (covering interchange reconnection and other solar-surface processes) – in which switchbacks result from transient, impulsive events near the surface of the sun – and “*in situ*” mechanisms (covering stream interactions and AWs), in which switchbacks result from processes within the solar wind as it propagates outwards. An *ex situ* switchback formation model, with its focus on impulsive events, naturally ties into an RLO heating scenario; an *in situ* formation process, by focusing on local processes in the extended solar wind, naturally ties into a WTD scenario. This is particularly true given the significant energy content of switchbacks in some PSP observations (see §4.2.1), although there are also important caveats in some of the models. Thus, understanding the origin of switchbacks is key to understanding the origin of the solar wind itself. How predictions from different models hold up when compared to observations may provide us with important clues. This is discussed in more detail in the summary of the implications of different models and how they compare to observations in §4.4.2.

### 4.3.1 Interchange Reconnection

Interchange reconnection refers to the process whereby a region of open magnetic-field lines reconnect with a closed magnetic loop (Fisk 2005). Since this process is expected to be explosive and suddenly change the shape and topology of the field, it is a good candidate for the origin of switchbacks and has been considered by several authors. The basic scenario is shown in Fig. 8.

Fisk and Kasper (2020) first pointed out the general applicability of interchange reconnection to the PSP observations (the possible relevance to earlier *Ulysses* observations had also been discussed in Yamauchi et al. 2004). They focus on the large transverse flows measured by PSP as evidence for the global circulation of open flux enabled by the interchange
<figure><img src="image_10.png" /><figcaption>**Fig. 8** Graphical overview covering most of the various proposed switchback-generation mechanisms, reprinted from https://www.nasa.gov/feature/goddard/2021/switchbacks-science-explaining-parker-solar-probe-s-magnetic-puzzle. The mechanisms are classified into those that form switchbacks (1) directly through interchange reconnection (e.g., Fisk and Kasper 2020, He et al. 2021b, Sterling and Moore 2020); (2) through ejection of flux ropes by interchange reconnection (Drake et al. 2021; Agapitov et al. 2022); (3) from expanding/growing AWs and/or Alfvénic turbulence (Squire et al. 2020; Mallet et al. 2021; Shoda et al. 2021); (4) due to roll up from nonlinear Kelvin-Helmholtz instabilities (Ruffolo et al. 2020); and (5) through magnetic field lines that stretch between sources of slower and faster wind (Schwadron and McComas 2021; see also Landi et al. 2006)</figcaption></figure>

reconnection process (Fisk and Schwadron 2001; Fisk 2005). Given that switchbacks tend to deflect preferentially in the transverse direction (see §4.2.2; Horbury et al. 2020), they argue that these two observations are suggestively compatible: an interchange reconnection event that enables the transverse transport of open flux would naturally create a transverse switchback.

Other authors have focused more on the plasma-physics process of switchback formation, including the reconnection itself and the type of perturbation it creates. Drake et al. (2021) used two-dimensional (2D) particle-in-cell (PIC) simulations to study the hypothesis that switchbacks are flux-rope structures that are ejected by bursty interchange reconnection. They present two 2D simulations, the first focusing on the interchange reconnection itself and the second on the structure and evolution of a flux rope in the solar wind. They find generally positive conclusions: flux ropes with radial-field reversals, nearly constant *B*, and temperature enhancements are naturally generated by interchange reconnection; and, flux-rope initial conditions relax into structures that match *PSP* observations reasonably well. Further discussion of the evolution of such structures, in particular how they evolve and merge with radius, is given in Agapitov et al. (2022) (see also §4.3.5) who also argue that the complex internal structure of observed switchbacks is consistent with the merging process. A challenge of the scenario is to reproduce the high Alfvénicity ($\delta B \propto \delta v$) of *PSP* observations, although the merging process of Agapitov et al. (2022) naturally halts
<figure><img src="image_11.png" /><figcaption>**Fig. 8** Graphical overview covering most of the various proposed switchback-generation mechanisms, reprinted from https://www.nasa.gov/feature/goddard/2021/switchbacks-science-explaining-parker-solar-probe-s-magnetic-puzzle. The mechanisms are classified into those that form switchbacks (1) directly through interchange reconnection (e.g., Fisk and Kasper 2020, He et al. 2021b, Sterling and Moore 2020); (2) through ejection of flux ropes by interchange reconnection (Drake et al. 2021; Agapitov et al. 2022); (3) from expanding/growing AWs and/or Alfvénic turbulence (Squire et al. 2020; Mallet et al. 2021; Shoda et al. 2021); (4) due to roll up from nonlinear Kelvin-Helmholtz instabilities (Ruffolo et al. 2020); and (5) through magnetic field lines that stretch between sources of slower and faster wind (Schwadron and McComas 2021; see also Landi et al. 2006)</figcaption></figure>

reconnection process (Fisk and Schwadron 2001; Fisk 2005). Given that switchbacks tend to deflect preferentially in the transverse direction (see §4.2.2; Horbury et al. 2020), they argue that these two observations are suggestively compatible: an interchange reconnection event that enables the transverse transport of open flux would naturally create a transverse switchback.

Other authors have focused more on the plasma-physics process of switchback formation, including the reconnection itself and the type of perturbation it creates. Drake et al. (2021) used two-dimensional (2D) particle-in-cell (PIC) simulations to study the hypothesis that switchbacks are flux-rope structures that are ejected by bursty interchange reconnection. They present two 2D simulations, the first focusing on the interchange reconnection itself and the second on the structure and evolution of a flux rope in the solar wind. They find generally positive conclusions: flux ropes with radial-field reversals, nearly constant *B*, and temperature enhancements are naturally generated by interchange reconnection; and, flux-rope initial conditions relax into structures that match *PSP* observations reasonably well. Further discussion of the evolution of such structures, in particular how they evolve and merge with radius, is given in Agapitov et al. (2022) (see also §4.3.5) who also argue that the complex internal structure of observed switchbacks is consistent with the merging process. A challenge of the scenario is to reproduce the high Alfvénicity ($\delta B \propto \delta v$) of *PSP* observations, although the merging process of Agapitov et al. (2022) naturally halts
once Alfvénic structures develop, suggesting we may be observing this end result at PSP altitudes.

A somewhat modified reconnection geometry has been explored with 2D MHD simulations by He et al. (2021b). They introduce an interchange reconnection process between open and closed regions with discontinuous guide fields, which is enabled by footpoint shearing motions and favors the emission of AWs from the reconnection site. They find quasi-periodic, intermittent emission of MHD waves, classifying the open-flux regions as “un-reconnected,” “newly reconnected,” and “post-reconnected.” Impulsive AWs, which can resemble switchbacks, robustly propagate outwards in both the newly and post-reconnected regions. While both regions have enhanced temperatures, the newly-reconnected regions have more slow-mode activity and the post-reconnected regions have lower densities, features of the model that may be observable at higher altitudes by PSP. They also see that flux ropes, which are ejected into the open field lines, rapidly disappear after the secondary magnetic reconnection between the impacting flux rope and the impacted open field lines; it is unclear whether this difference with Drake et al. (2021) is a consequence of the MHD model or the different geometry.

Finally, Zank et al. (2020) focus more on the evolution of magnetic-field structures generated by the reconnection process, which would often be in clustered in time as numerous open and closed loops reconnect over a short period. They argue that the strong radial-magnetic-field perturbations associated with switchbacks imply that their complex structures should propagate at the fast magnetosonic speed (but see also §4.3.4 below), deriving an equation from WKB (i.e., the Wentzel, Kramers, and Brillouin approximation) theory for how the structures evolve as they propagate outwards from a reconnection site to PSP altitudes. The model is compared to data in more detail in Liang et al. (2021), who use a Markov Chain Monte Carlo technique to fit the six free parameters of the model (e.g., wave angles and the initial perturbation) to seven observed variables taken from PSP time-series data for individual switchbacks. They find reasonable agreement, with around half of the observed switchbacks accepted as good fits to the model. Zank et al. (2020)'s WKB evolution equation implies that $|\delta\mathbf{B}|/|\mathbf{B}_0|$ grows in amplitude out to $\sim 50 R_\odot$ (whereupon it starts decaying again), and the shape of the proposed structures implies that switchbacks should often be observed as closely spaced double-humped structures. Their assumed fast-mode polarization implies that switchbacks that are more elongated in the radial direction will also exhibit larger variation in B, because radial elongation, combined with $\nabla \cdot \mathbf{B} = 0$, implies a mostly perpendicular wavenumber. This could be tested directly (see §4.2.2) and is a distinguishing feature between the fast-mode and AW based models (which generically predict $B \sim \text{const}$; §4.3.4).

Overall, we see that the various flavors of interchange-reconnection based models have a number of attractive features, in particular their natural explanation of the likely preferred tangential deflections of large switchbacks (§4.2.2; Horbury et al. 2020; Laker et al. 2022), along with the bulk tangential flow (Kasper et al. 2019), and of the possible observed temperature enhancements (§4.2.4; although to our knowledge, there are not yet clear predictions for separate $T_\perp$ and $T_\parallel$ dynamics). However, a number of features remain unclear, including (depending on the model in question) the Alfvénicity of the structures that are produced and how they survive and evolve as they propagate to PSP altitudes (see §4.3.5).

## 4.3.2 Other Solar-Surface Processes

Sterling and Moore (2020) present a phenomenological model for how switchbacks might form from the same process that creates coronal jets, which are small-scale filament eruptions observed in X-ray and extreme ultraviolet (EUV). Their jet model (proposed in Sterling
once Alfvénic structures develop, suggesting we may be observing this end result at PSP altitudes.

A somewhat modified reconnection geometry has been explored with 2D MHD simulations by He et al. (2021b). They introduce an interchange reconnection process between open and closed regions with discontinuous guide fields, which is enabled by footpoint shearing motions and favors the emission of AWs from the reconnection site. They find quasi-periodic, intermittent emission of MHD waves, classifying the open-flux regions as “un-reconnected,” “newly reconnected,” and “post-reconnected.” Impulsive AWs, which can resemble switchbacks, robustly propagate outwards in both the newly and post-reconnected regions. While both regions have enhanced temperatures, the newly-reconnected regions have more slow-mode activity and the post-reconnected regions have lower densities, features of the model that may be observable at higher altitudes by PSP. They also see that flux ropes, which are ejected into the open field lines, rapidly disappear after the secondary magnetic reconnection between the impacting flux rope and the impacted open field lines; it is unclear whether this difference with Drake et al. (2021) is a consequence of the MHD model or the different geometry.

Finally, Zank et al. (2020) focus more on the evolution of magnetic-field structures generated by the reconnection process, which would often be in clustered in time as numerous open and closed loops reconnect over a short period. They argue that the strong radial-magnetic-field perturbations associated with switchbacks imply that their complex structures should propagate at the fast magnetosonic speed (but see also §4.3.4 below), deriving an equation from WKB (i.e., the Wentzel, Kramers, and Brillouin approximation) theory for how the structures evolve as they propagate outwards from a reconnection site to PSP altitudes. The model is compared to data in more detail in Liang et al. (2021), who use a Markov Chain Monte Carlo technique to fit the six free parameters of the model (e.g., wave angles and the initial perturbation) to seven observed variables taken from PSP time-series data for individual switchbacks. They find reasonable agreement, with around half of the observed switchbacks accepted as good fits to the model. Zank et al. (2020)'s WKB evolution equation implies that $|\delta\mathbf{B}|/|\mathbf{B}_0|$ grows in amplitude out to $\sim 50 R_\odot$ (whereupon it starts decaying again), and the shape of the proposed structures implies that switchbacks should often be observed as closely spaced double-humped structures. Their assumed fast-mode polarization implies that switchbacks that are more elongated in the radial direction will also exhibit larger variation in $B$, because radial elongation, combined with $\nabla \cdot \mathbf{B} = 0$, implies a mostly perpendicular wavenumber. This could be tested directly (see §4.2.2) and is a distinguishing feature between the fast-mode and AW based models (which generically predict $B \sim \text{const}$; §4.3.4).

Overall, we see that the various flavors of interchange-reconnection based models have a number of attractive features, in particular their natural explanation of the likely preferred tangential deflections of large switchbacks (§4.2.2; Horbury et al. 2020; Laker et al. 2022), along with the bulk tangential flow (Kasper et al. 2019), and of the possible observed temperature enhancements (§4.2.4; although to our knowledge, there are not yet clear predictions for separate $T_\perp$ and $T_\parallel$ dynamics). However, a number of features remain unclear, including (depending on the model in question) the Alfvénicity of the structures that are produced and how they survive and evolve as they propagate to PSP altitudes (see §4.3.5).

### 4.3.2 Other Solar-Surface Processes

Sterling and Moore (2020) present a phenomenological model for how switchbacks might form from the same process that creates coronal jets, which are small-scale filament eruptions observed in X-ray and extreme ultraviolet (EUV). Their jet model (proposed in Sterling
et al. 2015) involves jets originating as erupting-flux-rope ejections through a combination of internal and interchange reconnection (thus this model would also naturally belong to §4.3.1 above). Observations that suggest jets originate around regions of magnetic-flux can-cellation (e.g., Panesar et al. 2016) support this concept. Sterling and Moore (2020) propose that the process can also produce a magnetic-field twist that propagates outwards as an AW packet that eventually evolves into a switchback. Although there is good evidence for equa-torial jets reaching the outer corona (thus allowing the switchback propagation into the solar wind) their relation to switchbacks is somewhat circumstantial at the present time; further studies of this mechanism could, for instance, attempt to correlate switchback and jet occur-rences by field-line mapping.

Using 3D MHD simulations, Magyar et al. (2021a) examined how photospheric motions at the base of a magnetic flux tube might excite motions that resemble switchbacks. They in-troduced perturbations at the lower boundary of a pressure-balanced magnetic-field solution, considering either a field-aligned, jet-like flow, or a transverse, vortical flows. Switchback-like fluctuations evolve in both cases: from the jet, a Rayleigh-Taylor-like instability that causes the field to from rolls; from the vortical perturbations, large-amplitude AWs that steepen nonlinearly. However, they also conclude that such perturbations are unlikely to enter the corona: the roll-ups fall back downwards due to gravity and the torsional waves unwind as the background field straightens. They conclude that while such structures are likely to be present in the chromosphere, it is unclear whether they are related to switch-backs as observed by PSP, since propagation effects will clearly play a dominant role (see §4.3.5).

### 4.3.3 Interactions Between Wind Streams

There exist several models that relate the formation of switchbacks in some way to the interaction between neighbouring solar-wind streams with different speeds. These could be either large-scale variations between separate slow- and fast-wind regions, or smaller-scale “micro-streams,” which seem to be observed ubiquitously in imaging studies of the low corona (DeForest et al. 2018) as well as in *in situ* data (Bale et al. 2021; Fargette et al. 2021b).<fnref n="3" /> Because these models require the stream shear to overwhelm the magnetic tension, they generically predict that switchbacks start forming primarily outside the Alfvén critical zone, once $V_R \gtrsim B$, and/or once $\beta \gtrsim 1$. However, the mechanism of switchback formation differs significantly between the models.

Landi et al. (2006) presented an early proposal of this form to explain *Ulysses* observations. Using 2D MHD simulations, they studied the evolution of a large-amplitude parallel (circularly polarized) AW propagating in a region that also includes strong flow shear from a central smaller-scale velocity stream. They find that large-magnitude field reversals develop across the stream due to the stretching of the field. However, the reversals are also associated with large compressive fluctuations in the thermal pressure, *B*, and plasma $\beta$. Although these match various *Ulysses* datasets quite well, they are much less Alfvénic then most switchbacks observed by PSP.

Ruffolo et al. (2020) consider the scenario where nonlinear Kelvin-Helmholtz instabilities develop across micro-stream boundaries, with the resulting strong turbulence producing switchbacks. This is motivated in part by the Solar TERRrestrial RELations Observatory (STEREO; Kaiser et al. 2008) observations of the transition between “striated” (radially

<footnote n="3">³We also caution, however, that switchbacks themselves create large radial velocity perturbations (see §4.2.1), which clearly could not be a cause of switchbacks.</footnote>
et al. 2015) involves jets originating as erupting-flux-rope ejections through a combination of internal and interchange reconnection (thus this model would also naturally belong to §4.3.1 above). Observations that suggest jets originate around regions of magnetic-flux can-cellation (e.g., Panesar et al. 2016) support this concept. Sterling and Moore (2020) propose that the process can also produce a magnetic-field twist that propagates outwards as an AW packet that eventually evolves into a switchback. Although there is good evidence for equa-torial jets reaching the outer corona (thus allowing the switchback propagation into the solar wind) their relation to switchbacks is somewhat circumstantial at the present time; further studies of this mechanism could, for instance, attempt to correlate switchback and jet occur-rences by field-line mapping.

Using 3D MHD simulations, Magyar et al. (2021a) examined how photospheric motions at the base of a magnetic flux tube might excite motions that resemble switchbacks. They in-troduced perturbations at the lower boundary of a pressure-balanced magnetic-field solution, considering either a field-aligned, jet-like flow, or a transverse, vortical flows. Switchback-like fluctuations evolve in both cases: from the jet, a Rayleigh-Taylor-like instability that causes the field to from rolls; from the vortical perturbations, large-amplitude AWs that steepen nonlinearly. However, they also conclude that such perturbations are unlikely to enter the corona: the roll-ups fall back downwards due to gravity and the torsional waves unwind as the background field straightens. They conclude that while such structures are likely to be present in the chromosphere, it is unclear whether they are related to switch-backs as observed by PSP, since propagation effects will clearly play a dominant role (see §4.3.5).

### 4.3.3 Interactions Between Wind Streams

There exist several models that relate the formation of switchbacks in some way to the interaction between neighbouring solar-wind streams with different speeds. These could be either large-scale variations between separate slow- and fast-wind regions, or smaller-scale “micro-streams,” which seem to be observed ubiquitously in imaging studies of the low corona (DeForest et al. 2018) as well as in *in situ* data (Bale et al. 2021; Fargette et al. 2021b).<fnref n="3" /> Because these models require the stream shear to overwhelm the magnetic tension, they generically predict that switchbacks start forming primarily outside the Alfvén critical zone, once $V_R \gtrsim B$, and/or once $\beta \gtrsim 1$. However, the mechanism of switchback formation differs significantly between the models.

Landi et al. (2006) presented an early proposal of this form to explain *Ulysses* observations. Using 2D MHD simulations, they studied the evolution of a large-amplitude parallel (circularly polarized) AW propagating in a region that also includes strong flow shear from a central smaller-scale velocity stream. They find that large-magnitude field reversals develop across the stream due to the stretching of the field. However, the reversals are also associated with large compressive fluctuations in the thermal pressure, *B*, and plasma $\beta$. Although these match various *Ulysses* datasets quite well, they are much less Alfvénic then most switchbacks observed by PSP.

Ruffolo et al. (2020) consider the scenario where nonlinear Kelvin-Helmholtz instabilities develop across micro-stream boundaries, with the resulting strong turbulence producing switchbacks. This is motivated in part by the Solar TERRrestrial RELations Observatory (STEREO; Kaiser et al. 2008) observations of the transition between “striated” (radially

<footnote n="3">³We also caution, however, that switchbacks themselves create large radial velocity perturbations (see §4.2.1), which clearly could not be a cause of switchbacks.</footnote>
elongated) and “floculated” (more isotropic) structures (DeForest et al. 2016) around the surface where $\beta \approx 1$, which is around the Alfvén critical zone. Since this region is where the velocity shear starts to be able to overwhelm the stabilizing effect of the magnetic field, it is natural to imagine that the instabilities that develop will contribute to the change in fluctuation structure and the generation of switchbacks. Comparing PSP and ex situ observations with theoretical arguments and numerical simulations, Ruffolo et al. (2020) argue that this scenario can account for a range of solar-wind properties, and that the conditions – e.g., the observed Alfvén speed and prevalence of small-scale velocity shears – are conducive to causing shear-driven turbulence. Their 3D MHD simulations of shear-driven turbulence generate a significant reversed-field fraction that is comparable to PSP observations, with the distributions of $B$, radial field, and tangential flows having a promising general shape. However, it remains unclear whether turbulence generated in this way is sufficiently Alfvénic to explain observations, since they see somewhat larger variation in $B$ than observed in many PSP intervals (but see Ruffolo et al. 2021). A key prediction of this model is that switchback activity should generally increase with distance from the Sun, since the turbulence that creates the switchbacks should continue to be driven so long as there remains sufficient velocity shear between streams. This feature is a marked contrast to models that invoke switchback generation through interchange reconnection or other Solar-surface processes.

Schwadron and McComas (2021) consider a simpler geometric explanation – that switchbacks result from global magnetic-field lines that stretch across streams with different speeds, rather than due to waves or turbulence generation. This situation is argued to naturally result from the global transport of magnetic flux as magnetic-field footpoints move between sources of wind with different speeds, with the footpoint motions sustained by interchange reconnection to conserve magnetic flux (Fisk and Schwadron 2001). A field line that moves from a source of slower wind into faster wind (thus traversing faster to slower wind as it moves radially outwards) will naturally reverse its radial field across the boundary due to the stretching by velocity shear. This explanation focuses on the observed asymmetry of the switchbacks – as discussed in §4.2, the larger switchback deflections seem to show a preference to be tangential and particularly in the +T (Parker-spiral) direction, which is indeed the direction expected from the global transport of flux through interchange reconnection.<fnref n="4" /> Field reversals are argued to develop their Alfvénic characteristics beyond the Alfvén point, since the field kink produced by a coherent velocity shear does not directly produce $\delta B \propto \delta v$ or $B \sim \text{const}$ (as also seen in the simulations of Landi et al. 2006).

#### 4.3.4 Expanding Alfvén Waves and Turbulence

The final class of models relate to perhaps the simplest explanation: that switchbacks are spherically polarized ($B = \text{const}$) AWs (or Alfvénic turbulence) that have reached amplitudes $|\delta B|/|B_0| \gtrsim 1$ (where $B_0$ is the background field). The idea follows from the realisation (Goldstein et al. 1974; Barnes and Hollweg 1974) that an Alfvénic perturbation – one with $\delta v = B/\sqrt{4\pi\rho}$ and $B, \rho$, and the thermal pressure all constant – is an exact nonlinear solution to the MHD equations that propagates at the Alfvén velocity. This is true no matter the amplitude of the perturbation compared to $B_0$, a property that seems unique among the zoo of hydrodynamic and hydromagnetic waves (other waves generally form into shocks at large amplitudes). Once $|\delta B| \gtrsim |B_0|$ such states will often reverse the magnetic field in order to maintain their spherical polarization (they involve a perturbation $\delta B$ parallel to $B_0$).

<footnote n="4">⁴Note however, that Horbury et al. 2020 argue that the global tangential flow asymmetry is not a consequence of the switchbacks themselves.</footnote>
elongated) and “floculated” (more isotropic) structures (DeForest et al. 2016) around the surface where $\beta \approx 1$, which is around the Alfvén critical zone. Since this region is where the velocity shear starts to be able to overwhelm the stabilizing effect of the magnetic field, it is natural to imagine that the instabilities that develop will contribute to the change in fluctuation structure and the generation of switchbacks. Comparing PSP and ex situ observations with theoretical arguments and numerical simulations, Ruffolo et al. (2020) argue that this scenario can account for a range of solar-wind properties, and that the conditions – e.g., the observed Alfvén speed and prevalence of small-scale velocity shears – are conducive to causing shear-driven turbulence. Their 3D MHD simulations of shear-driven turbulence generate a significant reversed-field fraction that is comparable to PSP observations, with the distributions of $B$, radial field, and tangential flows having a promising general shape. However, it remains unclear whether turbulence generated in this way is sufficiently Alfvénic to explain observations, since they see somewhat larger variation in $B$ than observed in many PSP intervals (but see Ruffolo et al. 2021). A key prediction of this model is that switchback activity should generally increase with distance from the Sun, since the turbulence that creates the switchbacks should continue to be driven so long as there remains sufficient velocity shear between streams. This feature is a marked contrast to models that invoke switchback generation through interchange reconnection or other Solar-surface processes.

Schwadron and McComas (2021) consider a simpler geometric explanation – that switchbacks result from global magnetic-field lines that stretch across streams with different speeds, rather than due to waves or turbulence generation. This situation is argued to naturally result from the global transport of magnetic flux as magnetic-field footpoints move between sources of wind with different speeds, with the footpoint motions sustained by interchange reconnection to conserve magnetic flux (Fisk and Schwadron 2001). A field line that moves from a source of slower wind into faster wind (thus traversing faster to slower wind as it moves radially outwards) will naturally reverse its radial field across the boundary due to the stretching by velocity shear. This explanation focuses on the observed asymmetry of the switchbacks – as discussed in §4.2, the larger switchback deflections seem to show a preference to be tangential and particularly in the +T (Parker-spiral) direction, which is indeed the direction expected from the global transport of flux through interchange reconnection.<fnref n="4" /> Field reversals are argued to develop their Alfvénic characteristics beyond the Alfvén point, since the field kink produced by a coherent velocity shear does not directly produce $\delta B \propto \delta v$ or $B \sim \text{const}$ (as also seen in the simulations of Landi et al. 2006).

#### 4.3.4 Expanding Alfvén Waves and Turbulence

The final class of models relate to perhaps the simplest explanation: that switchbacks are spherically polarized ($B = \text{const}$) AWs (or Alfvénic turbulence) that have reached amplitudes $|\delta B|/|B_0| \gtrsim 1$ (where $B_0$ is the background field). The idea follows from the realisation (Goldstein et al. 1974; Barnes and Hollweg 1974) that an Alfvénic perturbation – one with $\delta v = B/\sqrt{4\pi\rho}$ and $B, \rho$, and the thermal pressure all constant – is an exact nonlinear solution to the MHD equations that propagates at the Alfvén velocity. This is true no matter the amplitude of the perturbation compared to $B_0$, a property that seems unique among the zoo of hydrodynamic and hydromagnetic waves (other waves generally form into shocks at large amplitudes). Once $|\delta B| \gtrsim |B_0|$ such states will often reverse the magnetic field in order to maintain their spherical polarization (they involve a perturbation $\delta B$ parallel to $B_0$).

<footnote n="4">⁴Note however, that Horbury et al. 2020 argue that the global tangential flow asymmetry is not a consequence of the switchbacks themselves.</footnote>
Moreover, as they propagate in an inhomogeneous medium, nonlinear AWs behave just like small-amplitude waves (Hollweg 1974b; Barnes and Hollweg 1974); this implies that in the expanding solar wind, where the decreasing Alfvén speed causes $|\delta\mathbf{B}|/|\mathbf{B}_0|$ to increase, waves propagating outwards from the inner heliosphere can grow to $|\delta\mathbf{B}| \gtrsim |\mathbf{B}_0|$, feasibly forming switchbacks from initially small-amplitude waves. In the process, they may develop the sharp discontinuities characteristic of PSP observations if, as they grow, the constraint of constant $\mathbf{B}$ becomes incompatible with smooth $\delta\mathbf{B}$ perturbations. However, in the more realistic scenario where there exists a spectrum of waves, this wave growth competes with the dissipation of the large-scale fluctuations due to turbulence induced by wave reflection (see, e.g., Velli et al. 1989; Verdini and Velli 2007; Chandran and Hollweg 2009; Johnston et al. 2022) or other effects (e.g., Roberts et al. 1992). If dissipation is too fast, it will stop the formation of switchbacks; so, in this formation scenario turbulence and switchbacks are inextricably linked (as is also the case in the scenario of Ruffolo et al. 2020). Thus, understanding switchbacks will require understanding and accurately modelling the turbulence properties, evolution, and amplitude (Usmanov et al. 2018; Chhiber et al. 2019a; Perez and Chandran 2013).

Several recent papers have explored the general scenario from different standpoints, finding broadly consistent results. Squire et al. (2020) and Johnston et al. (2022) used expanding-box MHD simulations to understand how AWs grow in amplitude and develop turbulence. The basic setup involves starting from a purely outwards propagating (fully imbalanced) population of moderate-amplitude randomly phased waves and expanding the box to grow the waves to larger amplitudes. Switchbacks form organically as the waves grow, with their strength (e.g., the strength and proportion of field reversals) and properties (e.g., the extent to which $\mathbf{B}$ is constant across switchbacks) depending on the expansion rate and the wave spectrum. While promising, these simulations are highly idealized – e.g., the expanding box applies only outside the Alfvén point, the equation of state was taken as isothermal. While this has hindered the comparison to some observational properties, there are also some promising agreements (Johnston et al. 2022). Similar results were found by Shoda et al. (2021) using more comprehensive and realistic simulations that capture the full evolution of the solar wind from coronal base to PSP altitudes. Their simulation matches well the bulk properties of the slow-Alfvénic wind seen by PSP and develops strong switchbacks beyond $\sim 10 - 20 R_\odot$ (where the amplitude of the turbulence becomes comparable to the mean field). They find switchbacks that are radially elongated, as observed, although the proportion of field reversals is significantly lower than observed (this was also the case in Squire et al. 2020). It is unclear whether this discrepancy is simply due to insufficient numerical resolution or a more fundamental issue with the AW scenario. Shoda et al. (2021) do not see a significant correlation between switchbacks and density perturbations (see §4.2.4 for discussion), while more complex correlations with kinetic thermal properties (Woolley et al. 2020) cannot be addressed either this model or the simpler local ones (Squire et al. 2020; Johnston et al. 2022).

Mallet et al. (2021) consider a complementary, analytic approach to the problem, studying how one-dimensional, large-amplitude AWs grow and change shape in an expanding plasma. This shows that expansion necessarily generates small compressive perturbations in order to maintain the wave’s spherical polarization as it grows to large amplitudes, providing specific $\beta$-dependent predictions for magnetic and plasma compressibility. The model has been extended to include the Parker spiral by Squire et al. (2022), who find that the interaction with a rotating background field causes the development of tangential asymmetries in the switchback deflection directions. These, and the compressive predictions of Mallet et al. (2021), seem to explain various aspects of simulations (Johnston et al. 2022). Overall, these
Moreover, as they propagate in an inhomogeneous medium, nonlinear AWs behave just like small-amplitude waves (Hollweg 1974b; Barnes and Hollweg 1974); this implies that in the expanding solar wind, where the decreasing Alfvén speed causes $|\delta\mathbf{B}|/|\mathbf{B}_0|$ to increase, waves propagating outwards from the inner heliosphere can grow to $|\delta\mathbf{B}| \gtrsim |\mathbf{B}_0|$, feasibly forming switchbacks from initially small-amplitude waves. In the process, they may develop the sharp discontinuities characteristic of PSP observations if, as they grow, the constraint of constant $\mathbf{B}$ becomes incompatible with smooth $\delta\mathbf{B}$ perturbations. However, in the more realistic scenario where there exists a spectrum of waves, this wave growth competes with the dissipation of the large-scale fluctuations due to turbulence induced by wave reflection (see, e.g., Velli et al. 1989; Verdini and Velli 2007; Chandran and Hollweg 2009; Johnston et al. 2022) or other effects (e.g., Roberts et al. 1992). If dissipation is too fast, it will stop the formation of switchbacks; so, in this formation scenario turbulence and switchbacks are inextricably linked (as is also the case in the scenario of Ruffolo et al. 2020). Thus, understanding switchbacks will require understanding and accurately modelling the turbulence properties, evolution, and amplitude (Usmanov et al. 2018; Chhiber et al. 2019a; Perez and Chandran 2013).

Several recent papers have explored the general scenario from different standpoints, finding broadly consistent results. Squire et al. (2020) and Johnston et al. (2022) used expanding-box MHD simulations to understand how AWs grow in amplitude and develop turbulence. The basic setup involves starting from a purely outwards propagating (fully imbalanced) population of moderate-amplitude randomly phased waves and expanding the box to grow the waves to larger amplitudes. Switchbacks form organically as the waves grow, with their strength (e.g., the strength and proportion of field reversals) and properties (e.g., the extent to which $\mathbf{B}$ is constant across switchbacks) depending on the expansion rate and the wave spectrum. While promising, these simulations are highly idealized – e.g., the expanding box applies only outside the Alfvén point, the equation of state was taken as isothermal. While this has hindered the comparison to some observational properties, there are also some promising agreements (Johnston et al. 2022). Similar results were found by Shoda et al. (2021) using more comprehensive and realistic simulations that capture the full evolution of the solar wind from coronal base to PSP altitudes. Their simulation matches well the bulk properties of the slow-Alfvénic wind seen by PSP and develops strong switchbacks beyond $\sim 10–20 R_\odot$ (where the amplitude of the turbulence becomes comparable to the mean field). They find switchbacks that are radially elongated, as observed, although the proportion of field reversals is significantly lower than observed (this was also the case in Squire et al. 2020). It is unclear whether this discrepancy is simply due to insufficient numerical resolution or a more fundamental issue with the AW scenario. Shoda et al. (2021) do not see a significant correlation between switchbacks and density perturbations (see §4.2.4 for discussion), while more complex correlations with kinetic thermal properties (Woolley et al. 2020) cannot be addressed either this model or the simpler local ones (Squire et al. 2020; Johnston et al. 2022).

Mallet et al. (2021) consider a complementary, analytic approach to the problem, studying how one-dimensional, large-amplitude AWs grow and change shape in an expanding plasma. This shows that expansion necessarily generates small compressive perturbations in order to maintain the wave’s spherical polarization as it grows to large amplitudes, providing specific $\beta$-dependent predictions for magnetic and plasma compressibility. The model has been extended to include the Parker spiral by Squire et al. (2022), who find that the interaction with a rotating background field causes the development of tangential asymmetries in the switchback deflection directions. These, and the compressive predictions of Mallet et al. (2021), seem to explain various aspects of simulations (Johnston et al. 2022). Overall, these
analyses provide simple, geometric explanations for various switchback properties, most importantly the preference for switchbacks to be radially elongated (§4.2.2 and Table 1); however, they are clearly highly idealised, particularly concerning the neglect of turbulence. The models also struggle to reproduce the extremely sharp switchback boundaries seen in PSP data, which is likely a consequence of considering one-dimensional (1D) waves, since much sharper structures evolve in similar calculations with 2D or 3D structure (Squire and Mallet 2022).

Overall, AW models naturally recover the Alfvénicity ($\delta B \propto v$ and nearly constant $B$) and radial elongation of switchbacks seen in PSP observations, but may struggle with some other features. It remains unclear whether detailed aspects of the preferred tangential deflections of large switchbacks can be recovered (Fargette et al. 2022; Laker et al. 2022), although large-amplitude AWs do develop tangentially asymmetries as a consequence of expansion and the rotating Parker spiral (Johnston et al. 2022; Squire et al. 2022). Similarly, further work is needed to understand the compressive properties, in particular in a kinetic plasma.<fnref n="5" /> AW models naturally predict an increase in switchback occurrence with radial distance out to some maximum (whereupon it may decrease again), although the details depend on low-coronal conditions and the influence of turbulence, which remain poorly understood (Johnston et al. 2022). Computational models have also struggled to reproduce the very high switchback fractions observed by PSP; whether this is due to numerical resolution or more fundamental issues remains poorly understood.

### 4.3.5 Propagation and Evolution of Switchbacks

A final issue to consider, particularly for understanding the distinction between *ex situ* and *in situ* generation mechanisms, is how a hypothetical switchback evolves as it propagates and is advected outwards in the solar wind. In particular, if switchbacks are to be formed at the solar surface, they must be able to propagate a long way without disrupting or dissolving. Further, different formation scenarios predict different occurrence rates and size statistics as a function of heliocentric radius (§4.2.3), and it is important to understand how they change shape and amplitude in order to understand what solar-wind observations could tell us about coronal conditions.

Various studies have focused on large-amplitude Alfvénic initial conditions, thus probing the scenario where Alfvénic switchback progenitors are released in the low corona (e.g., due to reconnection), perhaps with subsequent evolution resulting from the AW/turbulence effects considered in §4.3.4. Using 2D MHD simulations, they start from an analytic initial condition with a magnetic perturbation that is large enough to reverse the mean field and an Alfvénic velocity $\delta v = \pm \delta B / \sqrt{4\pi\rho}$. While Landi et al. (2005) showed that such structures rapidly dissolve if $B$ is not constant across the wave, Tenerani et al. (2020) reached the opposite conclusion for switchbacks with constant $B$ (as relevant to observations), with their initial conditions propagating unchanged for hundreds of Alfvén times before eventually decaying due to parametric instability. They concluded that even relatively short wavelength switchbacks can in principle survive propagating out to tens of solar radii. Using the same initial conditions, Magyar et al. (2021b) extended the analysis to include switchbacks propagating through a radially stratified environment. They considered a fixed, near-exponential density profile and a background magnetic field with different degrees of expansion, which

<footnote n="5">⁵Note, however, that AW models do not predict an *absence* of compressive features in switchbacks. Indeed, compressive flows are necessary to maintain spherical polarization as they grow in amplitude due to expansion (Mallet et al. 2021).</footnote>
analyses provide simple, geometric explanations for various switchback properties, most importantly the preference for switchbacks to be radially elongated (§4.2.2 and Table 1); however, they are clearly highly idealised, particularly concerning the neglect of turbulence. The models also struggle to reproduce the extremely sharp switchback boundaries seen in PSP data, which is likely a consequence of considering one-dimensional (1D) waves, since much sharper structures evolve in similar calculations with 2D or 3D structure (Squire and Mallet 2022).

Overall, AW models naturally recover the Alfvénicity ($\delta B \propto v$ and nearly constant $B$) and radial elongation of switchbacks seen in PSP observations, but may struggle with some other features. It remains unclear whether detailed aspects of the preferred tangential deflections of large switchbacks can be recovered (Fargette et al. 2022; Laker et al. 2022), although large-amplitude AWs do develop tangentially asymmetries as a consequence of expansion and the rotating Parker spiral (Johnston et al. 2022; Squire et al. 2022). Similarly, further work is needed to understand the compressive properties, in particular in a kinetic plasma.<fnref n="5" /> AW models naturally predict an increase in switchback occurrence with radial distance out to some maximum (whereupon it may decrease again), although the details depend on low-coronal conditions and the influence of turbulence, which remain poorly understood (Johnston et al. 2022). Computational models have also struggled to reproduce the very high switchback fractions observed by PSP; whether this is due to numerical resolution or more fundamental issues remains poorly understood.

### 4.3.5 Propagation and Evolution of Switchbacks

A final issue to consider, particularly for understanding the distinction between *ex situ* and *in situ* generation mechanisms, is how a hypothetical switchback evolves as it propagates and is advected outwards in the solar wind. In particular, if switchbacks are to be formed at the solar surface, they must be able to propagate a long way without disrupting or dissolving. Further, different formation scenarios predict different occurrence rates and size statistics as a function of heliocentric radius (§4.2.3), and it is important to understand how they change shape and amplitude in order to understand what solar-wind observations could tell us about coronal conditions.

Various studies have focused on large-amplitude Alfvénic initial conditions, thus probing the scenario where Alfvénic switchback progenitors are released in the low corona (e.g., due to reconnection), perhaps with subsequent evolution resulting from the AW/turbulence effects considered in §4.3.4. Using 2D MHD simulations, they start from an analytic initial condition with a magnetic perturbation that is large enough to reverse the mean field and an Alfvénic velocity $\delta v = \pm \delta B / \sqrt{4\pi\rho}$. While Landi et al. (2005) showed that such structures rapidly dissolve if $B$ is not constant across the wave, Tenerani et al. (2020) reached the opposite conclusion for switchbacks with constant $B$ (as relevant to observations), with their initial conditions propagating unchanged for hundreds of Alfvén times before eventually decaying due to parametric instability. They concluded that even relatively short wavelength switchbacks can in principle survive propagating out to tens of solar radii. Using the same initial conditions, Magyar et al. (2021b) extended the analysis to include switchbacks propagating through a radially stratified environment. They considered a fixed, near-exponential density profile and a background magnetic field with different degrees of expansion, which

<footnote n="5">⁵Note, however, that AW models do not predict an *absence* of compressive features in switchbacks. Indeed, compressive flows are necessary to maintain spherical polarization as they grow in amplitude due to expansion (Mallet et al. 2021).</footnote>
changes the radial profile of $V_A$ in accordance with different possible conditions in the low corona. Their basic results are consistent with the expanding-AW theory discussed above (§4.3.4), with switchbacks in super-radially expanding background fields maintaining strong field deflections, while those in radially expanding or non-expanding backgrounds unfold as they propagate outwards. The study also reveals a number of non-WKB effects from stratification, such as a gravitational damping from plasma entrained in the switchback. More generally, they point out that after propagating any significant distance in a radially stratified environment, a switchback will have deformed significantly compared to the results from Tenerani et al. (2020), either changing shape or unfolding depending on the background profile. This blurs the line between *ex situ* and *in situ* formation scenarios.

The above studies, by fixing $\delta v = \pm \delta B / \sqrt{4\pi\rho}$ and $B = \text{const}$, effectively assume that switchbacks are Alfvénic. Agapitov et al. (2022) have considered the evolution and merging of flux ropes, that are ejected from interchange reconnection sites in the scenario of Drake et al. (2021). They show that while flux ropes are likely to form initially with an aspect ratio of near unity, merging of ropes through slow reconnection of the wrapping magnetic field is energetically favorable. This merging continues until the axial flows inside the flux ropes increase to near Alfvénic values, at which point the process becomes energetically unfavorable. This process also causes flux ropes to increasingly radially elongated with distance from the sun, which is observationally testable (see §4.2.2) and may be the opposite prediction to AW based models (since the wave vector rotates towards the parallel direction with expansion). Drake et al. (2021) also argue that the complex, inner structure of observed switchbacks is consistent with this merging process.

The WKB fast-mode-like calculation of Zank et al. (2020) produces somewhat modified scalings (which nonetheless predict a switchback amplitude that increases with radius), but does not address the stability or robustness of the structures. Considerations relating to the long-time stability of switchbacks are less relevant to the shear-driven models of Ruffolo et al. (2020), Schwadron and McComas (2021), in which switchbacks are generated in the Alfvén zone and beyond (where $V_R \gtrsim V_A$), so will not have propagated a significant distance before reaching PSP altitudes.

Overall, we see that Alfvénic switchbacks are expected to be relatively robust, as are flux-rope structures, although they evolve significantly through merging. This suggests that source characteristics could be retained (albeit with significant changes in shape) as they propagate through the solar wind. If indeed switchbacks are of low-coronal origin, this is encouraging for the general program of using switchbacks to learn about the important processes that heat and accelerate the solar wind.

## 4.4 Outlook and Open Questions

### 4.4.1 Connection to Solar Sources

Because of their persistence in the solar wind, switchbacks can also be considered as tracers of processes occurring in the Solar atmosphere and therefore can be used to identify wind sources at the Sun. Recent work by Woolley et al. (2021) compared the properties of two switchback patches during different Encs. and suggested that patches could be linked to coronal hole boundary regions at the solar surface. Woolley et al. (2021) also showed that these periods, which had bulk velocities of ~300 km s⁻¹, could sometimes be characterized by a particularly low alpha abundance. The cause of this low alpha abundance is not known, but it could be related to the processes and mechanisms governing the release of the solar wind at the surface. Moreover, local modulation in the alpha fraction observed *in situ* and
changes the radial profile of $V_A$ in accordance with different possible conditions in the low corona. Their basic results are consistent with the expanding-AW theory discussed above (§4.3.4), with switchbacks in super-radially expanding background fields maintaining strong field deflections, while those in radially expanding or non-expanding backgrounds unfold as they propagate outwards. The study also reveals a number of non-WKB effects from stratification, such as a gravitational damping from plasma entrained in the switchback. More generally, they point out that after propagating any significant distance in a radially stratified environment, a switchback will have deformed significantly compared to the results from Tenerani et al. (2020), either changing shape or unfolding depending on the background profile. This blurs the line between *ex situ* and *in situ* formation scenarios.

The above studies, by fixing $\delta v = \pm \delta B / \sqrt{4\pi\rho}$ and $B = \text{const}$, effectively assume that switchbacks are Alfvénic. Agapitov et al. (2022) have considered the evolution and merging of flux ropes, that are ejected from interchange reconnection sites in the scenario of Drake et al. (2021). They show that while flux ropes are likely to form initially with an aspect ratio of near unity, merging of ropes through slow reconnection of the wrapping magnetic field is energetically favorable. This merging continues until the axial flows inside the flux ropes increase to near Alfvénic values, at which point the process becomes energetically unfavorable. This process also causes flux ropes to increasingly radially elongated with distance from the sun, which is observationally testable (see §4.2.2) and may be the opposite prediction to AW based models (since the wave vector rotates towards the parallel direction with expansion). Drake et al. (2021) also argue that the complex, inner structure of observed switchbacks is consistent with this merging process.

The WKB fast-mode-like calculation of Zank et al. (2020) produces somewhat modified scalings (which nonetheless predict a switchback amplitude that increases with radius), but does not address the stability or robustness of the structures. Considerations relating to the long-time stability of switchbacks are less relevant to the shear-driven models of Ruffolo et al. (2020), Schwadron and McComas (2021), in which switchbacks are generated in the Alfvén zone and beyond (where $V_R \gtrsim V_A$), so will not have propagated a significant distance before reaching PSP altitudes.

Overall, we see that Alfvénic switchbacks are expected to be relatively robust, as are flux-rope structures, although they evolve significantly through merging. This suggests that source characteristics could be retained (albeit with significant changes in shape) as they propagate through the solar wind. If indeed switchbacks are of low-coronal origin, this is encouraging for the general program of using switchbacks to learn about the important processes that heat and accelerate the solar wind.

## 4.4 Outlook and Open Questions

### 4.4.1 Connection to Solar Sources

Because of their persistence in the solar wind, switchbacks can also be considered as tracers of processes occurring in the Solar atmosphere and therefore can be used to identify wind sources at the Sun. Recent work by Woolley et al. (2021) compared the properties of two switchback patches during different Encs. and suggested that patches could be linked to coronal hole boundary regions at the solar surface. Woolley et al. (2021) also showed that these periods, which had bulk velocities of ~300 km s⁻¹, could sometimes be characterized by a particularly low alpha abundance. The cause of this low alpha abundance is not known, but it could be related to the processes and mechanisms governing the release of the solar wind at the surface. Moreover, local modulation in the alpha fraction observed *in situ* and
crucially during switchback patches could be a direct signature of spatial modulation in solar sources; Bale et al. (2021) have identified funnels as a possible source for these structures. Such an interpretation is consistent with the finding presented by Fargette et al. (2021b) who have identified some periodicity in patches that is consistent with that expected from Solar supergranulation, and also some smaller scale signatures potentially related to granular structures inside funnels (see Fig. 9).

Another interesting interpretation has been proposed by (Neugebauer and Sterling 2021) who suggest that patches of switchbacks observed in the inner heliosphere by PSP could then evolve with radial distance into structures with an overall higher bulk velocity, like the microstreams observed by *Ulysses* in the polar wind (Neugebauer et al. 1995). According to the authors, then microstreams might be the result of accumulated and persistent velocity enhancements resulting from a series of switchbacks or patches. At the same time, Neugebauer and Sterling (2021) also propose that the individual switchbacks inside the patches could be generated by miniflament/flux rope eruptions that cause coronal jets (Sterling and Moore 2020), so that microstreams are a consequence of a series of such jet-driven switchbacks occurring in close succession.

### 4.4.2 Implications for Our Understanding of the Solar Wind

Switchback observations hold promise as a way to better constrain our understanding of the solar-wind itself. In particular, most of the theoretical models discussed in §4.3 also suggest broader implications for coronal and solar-wind heating, and thus the origin of the solar wind. Given this, although there is currently no consensus about the key ingredients that form switchbacks, if a particular model does gain further observational support, this may lead to more profound shifts in our understanding of the heliosphere. Here, we attempt to broadly characterize the implications of the different formation scenarios in order to highlight the general importance of understanding switchbacks. Further understanding will require constant collaboration between theory and observations, whereby theorists attempt to provide the most constraining, and unique, possible tests of their proposed mechanisms in order to better narrow down the possibilities. Such a program is strongly supported by the recent observations of switchback modulation on solar supergranulation scales, which suggest a direct connection to solar-surface processes (see above §4.4.1).

Above (§4.3), we broadly categorized models into “*ex situ*” and “*in situ*,” which involved switchbacks being formed on or near the solar surface, or in the bulk solar wind, respectively. These classes then naturally tied into RLO models of coronal heating for *ex situ* models, or to WTD coronal-heating theories for *in situ* switchback formation (with some modifications for specific models). But, the significant differences between different models narrow down the correspondence further than this. Let us consider some of the main proposals and what they will imply, if correct. In the discussion below, we consider some of the same models as discussed in §4.3, but grouped together by their consequence to the solar wind as opposed to the switchback formation mechanism.

Fisk and Kasper (2020) and Schwadron and McComas (2021) propose that switchbacks are intimately related to the global transport of open magnetic flux caused by interchange reconnection, either through the ejection of waves or due to the interaction between streams. This global circulation would have profound consequences more generally, e.g., for coronal and solar-wind heating (Fisk 2003) or the magnetic-field structure of the solar wind structure (the sub- and super-Parker spiral; Schwadron and McComas 2021). Some other interchange reconnection or impulsive jet mechanisms – in particular, the ejection of flux ropes Drake et al. (2021), Agapitov et al. (2022), or MHD waves Zank et al. (2020), He et al. (2021b),
crucially during switchback patches could be a direct signature of spatial modulation in solar sources; Bale et al. (2021) have identified funnels as a possible source for these structures. Such an interpretation is consistent with the finding presented by Fargette et al. (2021b) who have identified some periodicity in patches that is consistent with that expected from Solar supergranulation, and also some smaller scale signatures potentially related to granular structures inside funnels (see Fig. 9).

Another interesting interpretation has been proposed by (Neugebauer and Sterling 2021) who suggest that patches of switchbacks observed in the inner heliosphere by PSP could then evolve with radial distance into structures with an overall higher bulk velocity, like the microstreams observed by *Ulysses* in the polar wind (Neugebauer et al. 1995). According to the authors, then microstreams might be the result of accumulated and persistent velocity enhancements resulting from a series of switchbacks or patches. At the same time, Neugebauer and Sterling (2021) also propose that the individual switchbacks inside the patches could be generated by miniflament/flux rope eruptions that cause coronal jets (Sterling and Moore 2020), so that microstreams are a consequence of a series of such jet-driven switchbacks occurring in close succession.

### 4.4.2 Implications for Our Understanding of the Solar Wind

Switchback observations hold promise as a way to better constrain our understanding of the solar-wind itself. In particular, most of the theoretical models discussed in §4.3 also suggest broader implications for coronal and solar-wind heating, and thus the origin of the solar wind. Given this, although there is currently no consensus about the key ingredients that form switchbacks, if a particular model does gain further observational support, this may lead to more profound shifts in our understanding of the heliosphere. Here, we attempt to broadly characterize the implications of the different formation scenarios in order to highlight the general importance of understanding switchbacks. Further understanding will require constant collaboration between theory and observations, whereby theorists attempt to provide the most constraining, and unique, possible tests of their proposed mechanisms in order to better narrow down the possibilities. Such a program is strongly supported by the recent observations of switchback modulation on solar supergranulation scales, which suggest a direct connection to solar-surface processes (see above §4.4.1).

Above (§4.3), we broadly categorized models into “*ex situ*” and “*in situ*,” which involved switchbacks being formed on or near the solar surface, or in the bulk solar wind, respectively. These classes then naturally tied into RLO models of coronal heating for *ex situ* models, or to WTD coronal-heating theories for *in situ* switchback formation (with some modifications for specific models). But, the significant differences between different models narrow down the correspondence further than this. Let us consider some of the main proposals and what they will imply, if correct. In the discussion below, we consider some of the same models as discussed in §4.3, but grouped together by their consequence to the solar wind as opposed to the switchback formation mechanism.

Fisk and Kasper (2020) and Schwadron and McComas (2021) propose that switchbacks are intimately related to the global transport of open magnetic flux caused by interchange reconnection, either through the ejection of waves or due to the interaction between streams. This global circulation would have profound consequences more generally, e.g., for coronal and solar-wind heating (Fisk 2003) or the magnetic-field structure of the solar wind structure (the sub- and super-Parker spiral; Schwadron and McComas 2021). Some other interchange reconnection or impulsive jet mechanisms – in particular, the ejection of flux ropes Drake et al. (2021), Agapitov et al. (2022), or MHD waves Zank et al. (2020), He et al. (2021b),
<figure><img src="image_11.png" /><figcaption>**Fig. 9** Top: Variation of plasma properties observed during patches of switchbacks during Enc. 6. The second panel shows that Helium abundance is modulated with the patch profile too, with enhanced fractional density inside patches of switchbacks. The periodicity is consistent with the crossing of funnels emerging from the Solar atmosphere. Figure adapted from Bale et al. (2021). Bottom: Cartoon showing the association between switchback patches and their periodicity with supergranular and granular structure in Corona. Figure adapted from Fargette et al. (2021b).</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 9** Top: Variation of plasma properties observed during patches of switchbacks during Enc. 6. The second panel shows that Helium abundance is modulated with the patch profile too, with enhanced fractional density inside patches of switchbacks. The periodicity is consistent with the crossing of funnels emerging from the Solar atmosphere. Figure adapted from Bale et al. (2021). Bottom: Cartoon showing the association between switchback patches and their periodicity with supergranular and granular structure in Corona. Figure adapted from Fargette et al. (2021b).</figcaption></figure>
Sterling and Moore (2020) from the reconnection site – do not necessarily involve the same open-flux-transport mechanism, but clearly favor an RLO-based coronal heating scenario, and have more specific consequences for each individual models; for example, the importance flux-rope structures to the energetics of the inner heliosphere for Drake et al. (2021), magnetosonic perturbations in Zank et al. (2020), or specific photospheric/chromospheric motions for He et al. (2021b), Sterling and Moore (2020). The model of Ruffolo et al. (2020) suggests a very different scenario, whereby shear-driven instabilities play a crucial role outside of the Alfvén critical zone (where $V_R \gtrsim V_A$), where they would set the properties of the turbulent cascade and change the energy budget by boosting heating and acceleration of slower regions. Finally, in the Alfvénic turbulence/waves scenario, switchbacks result from the evolution of turbulence to very large amplitudes $|\delta\mathbf{B}| \gtrsim |\mathbf{B}_0|$ (Squire et al. 2020; Shoda et al. 2021; Mallet et al. 2021). In turn, given the low plasma $\beta$, this implies that the energy contained in Alfvénic fluctuations is significant even at PSP altitudes (at least in switchback-filled regions), by which point they should already have contributed significantly to heating. Combined with low-coronal observations (e.g., De Pontieu et al. 2007), this is an important constraint on wave-heating models (e.g., Cranmer et al. 2007).

Finally, despite the significant differences between models highlighted above, it is also worth noting some similarities. In particular, features of various models are likely to coexist and/or feed into one another. For example, some of the explosive, *ex situ* scenarios (§4.3.1–4.3.2) propose that such events release AWs, which then clearly ties into the AW/expansion scenario of 4.3.4. Indeed, as pointed out by Magyar et al. (2021b), the subsequent evolution of switchbacks as they propagate in the solar wind cannot be *avoided*, which muddies the *in situ/ex situ* distinction. In this case, the distinction between *in situ* and *ex situ* scenarios would be more related to the relevance of distinct, impulsive events to wave launching, as opposed to slower, quasi-continuous stirring of motions. Similarly, Ruffolo et al. (2020) discuss how waves propagating upwards from the low corona could intermix and contribute to the shear-driven dynamics that form the basis for their model. These interrelationships, and the coexistence of different mechanisms, should be kept in mind moving forward as we attempt to distinguish observationally between different mechanisms.

### 4.4.3 Open Questions and Future Encs

Given their predominance in the solar wind plasma close to the Sun and because each switchback is associated with a positive increase in the bulk kinetic energy of the flow – as they imply a net motion of the centre of mass of the plasma – it is legitimate to consider whether switchbacks play any dynamical role in the acceleration of the flow and its evolution in interplanetary space. Moreover, it is an open question if the kinetic energy and Poynting flux carried by these structures have an impact on the overall energy budget of the wind. To summarize, these are some of the main currently open questions about these structures and their role in the solar wind dynamics:

* Do switchbacks play any role in solar wind acceleration?

* Does energy transported by switchbacks constitute a important possible source for plasma heating during expansion?

* Do switchbacks play an active role in driving and maintaining the turbulent cascade?

* Are switchbacks distinct plasma parcels with different properties than surrounding plasma?

* Do switchbacks continuously decay and reform during expansion?

* Are switchbacks signatures of key processes in the Solar atmosphere and tracers of specific types of solar wind sources? (Open field regions vs. streamer)