About foreshock phenomena at other planets:

- How do the strength of the planet's intrinsic magnetic field and ionosphere affect its fore-shock phenomena? This question arises already when magnetized planets Earth and Mercury are compared because weak Hermean magnetic field is permanently connected to the IMF. However, this question becomes more prominent when the foreshock of non-magnetized planets Venus and Mars are investigated because these planets do not have a traditional magnetosphere but their foreshock region can be magnetically connected to the planetary upper ionosphere.

- Can the dynamics in Venus and Mars foreshock propagate into the magnetic barriers and, finally, into the ionosphere along the draped magnetic field lines and what might be their ionosphere effects?

- When the non-magnetized planets Mars and Venus are considered, how do the planetary ions from the hot neutral corona and from the upper atmosphere affect the foreshock phenomena and vice versa?

# 3 Transient Dayside Magnetopause Processes and Transport

## 3.1 Surface Waves/Instabilities

Magnetopause surface waves can be generated by the solar wind dynamic pressure variations, transient phenomena near the bow shock, or the KHI on the magnetopause. The KHI can grow along the magnetopause because there exists a velocity shear between the anti-sunward streaming plasma in the magnetosheath and nearly stagnant magnetospheric plasma. The magnetopause KHI could play a key role in the transport of mass, energy, and momentum from the solar wind into the magnetosphere: the formation of the low-latitude boundary layer (Newell and Onsager 2003) and/or the cold-dense plasma sheet (e.g., Terasawa et al. 1997) often observed under northward IMF conditions, driving turbulent convection in the magnetosphere, the excitation of ULF waves in the inner magnetosphere, etc. Since extensive reviews on the magnetopause KHI have been given by Hasegawa (2012), Johnson et al. (2014), Faganello and Califano (2017), Masson and Nykyri (2018), here we only focus on recent progress on the magnetopause KHI. See, in particular, Johnson et al. (2014) for the fundamental physics underlying why and under what conditions the KHI can grow. We also point out that while spacecraft signatures of the KHI look transient because they are observable only when the boundary conditions are suitable for the KHI and spacecraft is near the KH unstable interface, the KHI itself is not necessarily a transient process. This is because KH surface waves or vortices can be generated continuously on the dayside magnetopause and advect tailward if the solar wind and magnetospheric conditions are stable and favorable for the KHI.

### 3.1.1 Recent Results on Large-Scale Properties

Kavosi and Raeder (2015) conducted a survey of the magnetopause KHI using seven years of data from the THEMIS spacecraft. They found that KH waves are present at the magnetopause approximately 19% of the time regardless of the solar wind conditions. They also showed that the occurrence rate of KH waves increases with the solar wind speed, Alfvén Mach number, and number density, but is mostly independent of the IMF magnitude. They also found that although the occurrence rate of KH waves under southward IMF is significantly higher than previously detected, it is still approximately four times less than the
occurrence rate under northward IMF. Most of the events during southward IMF are irregu-
lar, short and polychromatic as compared to regular, long lasting and monochromatic waves
under northward IMF.

Lin et al. (2014) showed, using a smaller number of KH wave events seen by THEMIS,
that the KH wave period T_KH is shorter for higher solar wind speed VSW. This is a trend
expected when the KH wavelength λ_KH is roughly constant because the advection speed of
KH waves would be proportional to the solar wind speed, i.e., T_KH ∝ λ_KH/VSW. They also
found that the KH wave period/wavelength tends to be longer for larger IMF clock angles
(smaller IMF Bz). This is a counterintuitive result because the latitudinal range where the
KHI can be unstable would be narrower and thus the allowed maximum KH wavelength is
smaller for larger clock angles (Farrugia et al. 1998). The puzzling result may be related to
how the KHI or surface waves are excited for different IMF directions. We also note that the
KH wave period is often not in the Pc 5 (>150 s) range (Fig. 6 in Lin et al. 2014) as often
assumed in the literature on ULF waves, but in the Pc 4 (45-150 s) range.

KH waves have been observed in the mid-tail as well. Wang et al. (2014) reported bursty enhancements of hot (>0.5 keV) electrons observed by the ARTEMIS spacecraft in the tail magnetosheath at lunar distance, in association with quasi-periodic plasma and field fluctuations likely due to KH surface waves. Based on these observations, they suggested that the KHI can play a role in the loss of magnetospheric particles into interplanetary space. A similar study based on MMS data for the dayside magnetosheath has been conducted by Cohen et al. (2017) which found that quiet time events peaked strongly away from local noon on the morning and afternoon sides indicating KH waves as the probable cause. Based on global MHD and energetic test-particle computations, Sorathia et al. (2017) indeed found that energetic particles escape into the magnetosheath from both dawn and dusk sides of the magnetosphere, and the KHI enhances the losses, particularly for smaller gyroradius particles such as electrons.

Ling et al. (2018) reported simultaneous ARTEMIS and Geotail observations of KH waves in the mid-tail and the near-Earth region and suggested that the KH wavelength increases as they propagate along the boundary layer toward the tail. An increasing wavelength from the subsolar region has been predicted by Otto and Fairfield (2000) based on the increasing growth time for longer wavelength or coalescence, and by Li et al. (2012) in global simulations.

Walsh et al. (2015) showed observations of KH waves along the dayside magnetopause
measured by THEMIS and Saskatoon HF radar. These observations occurred during a time
period when a cold, dense plasmaspheric plume contacted the dayside magnetopause in the
dusk sector. Theoretical calculations and observations indicate that the dense plume plasma
lowered the KH unstable threshold and permitted the waves to form on a region of the
magnetopause that is closer to the subsolar point than typically observed.

Identifying vortices from in-situ measurements is not a trivial task, although a number
of methods to identify and/or analyze magnetopause vortices exist and were reviewed by
Hasegawa (2012). Recent advances are testing and validating techniques to detect and char-
acterize vortices, which are standard tools in the field of fluid dynamics and are not well
known in the space physics community. Collado-Vega et al. (2018) applied one of such vor-
tex identification algorithms (VIAs) to velocity data from global MHD simulations of the
magnetosphere driven by real solar wind conditions. Cai et al. (2018) reviewed three VIAs
and applied to magnetic field data taken by the four Cluster spacecraft during a KH vortex
event at the duskside magnetopause (because not all four Cluster spacecraft made suffi-
ciently accurate velocity measurements), and detected a roughly equal number of clockwise
and counterclockwise magnetic field rotations.
### 3.1.2 Magnetic Reconnection Associated with KH Waves

KH waves have been largely associated with northward IMF because most observational evidence is for northward IMF. However, there is no fundamental reason that KH modes should not occur for southward IMF as well. In fact, Hwang et al. (2011) identified intermittent KH waves in a comparison of global simulations and Cluster observations. These findings were confirmed and specified by Yan et al. (2014) in THEMIS observations on the duskside flank. For southward IMF, both, magnetic reconnection and the KH instability can occur simultaneously at the Earth's magnetopause. Ma et al. (2014a,b, 2016a) discussed the interaction of the KHI and reconnection for large magnetic shear. In particular, each strongly impacts the other, with the KHI limiting the reconnected flux and modifying the dissipation region structure. It is also demonstrated that the reconnection rate maximizes for conditions that allow a strong nonlinear evolution of KH waves, i.e., fast shear flow and limited guide magnetic field (field component along the X line). However, signatures can be reminiscent of FTEs such that an identification of KH waves for southward IMF can be more difficult.

The occurrence of KH waves is not limited to the vicinity of the equatorial plane. In comparison with global MHD simulations, Hwang et al. (2012) reported the first observations of KH waves at high latitudes during strongly dawnward IMF conditions. Similarly, based on mesoscale 3D simulations, Ma et al. (2016c) re-interpreted a high latitude event, which was originally thought to represent cusp reconnection, as a boundary modulation caused by a KH wave.

Originally, KH waves have only been associated with viscous transport (Miura 1984) and not with magnetic reconnection because they were regarded as ideal instabilities. For northward IMF, first in two dimensions it was found that for an inclined magnetic field across a boundary reconnection in the twisted magnetic field has to be expected (Otto and Fairfield 2000; Nykyri and Otto 2001; Nakamura and Fujimoto 2005). Subsequently, there have been many studies using different geometries, and plasma approximations to examine two and three-dimensional properties of this vortex reconnection. Nakamura et al. (2013) performed first three-dimensional (3-D) fully kinetic simulations of secondary reconnection induced as a consequence of the KHI. They demonstrated that in the presence of guide field a number of meso-scale magnetic flux ropes can be generated along current layers compressed and thinned around the periphery of the vortex, because secondary reconnection can occur at many flux surfaces in 3-D. These flux ropes are carried by the shear flow and are entrained into the vortex. In the final stage, merging of the vortices occurs and leads to the formation of new compressed current sheets and flux ropes.

For northward IMF and three dimensions, it was realized that twisting the magnetic field in a limited region close to the equatorial plane would generate strong current layers particularly at the southern and northern boundaries of the KH unstable region. Unless these vortices unwind, reconnection was required to separate the twisted part of the flux tube from the IMF. Consequently, pairs of magnetic reconnection sites have to form, allowing the exchange of part of magnetic flux tubes between the magnetosheath and magnetosphere and thus the capture of the magnetosheath plasma (Faganello et al. 2012; Takagi et al. 2006). More recently, Borgogno et al. (2015) examined this process in three-dimensional two fluid simulations. Ma et al. (2017) showed that this double mid-latitude reconnection for northward IMF can efficiently transport the plasma with a diffusion coefficient of $1 \times 10^{10} \text{ m}^2 \text{ s}^{-1}$.
Note that this is considerably higher than estimated for reconnection within the KH vortex (Nykyri and Otto 2001). For IMF directions tilted from due north, the KHI and mid-latitude reconnection grows asymmetrically with respect to the equatorial plane and, interestingly, vortex-driven reconnection is limited to the region from the equatorial plane to mid-latitudes in the hemisphere that turns out to be less KH unstable (Fadanelli et al. 2018).
### 3.1.3 Results from the Magnetospheric Multiscale Mission

Unambiguous evidence of magnetic reconnection driven by the KHI has been identified from the MMS mission, while earlier missions did not have plasma instruments with such high time resolutions that can dissect thin current sheets formed through the KHI. Eriksen et al. (2016) reported the first direct evidence of reconnection exhausts associated with KH waves at the postnoon magnetopause observed on 8 September 2015. The evidence consists of ion jets and Hall magnetic and electric fields in agreement with strong guide-field reconnection at the trailing surfaces of the KH waves. Using the same MMS event, Li et al. (2016a) reported kinetic evidence of vortex driven reconnection, such as low-energy ($<$100 eV) magnetosheath electrons entering the magnetosphere and magnetospheric electrons escaping into the magnetosheath through reconnected field lines. Vernisse et al. (2016) further investigated magnetic topologies of magnetosheath boundary layers observed outside the KHI-corrugated magnetopause surface (Fig. 40a), and found that particle leakage features are consistent with not only local vortex induced reconnection but also mid-latitude reconnection as simulated by Borgogno et al. (2015). These results demonstrate that the 3-D development of the KHI can induce plasma entry through reconnection at both mid-latitude and equatorial regions already sunward of the dawn-dusk terminator where the KHI is in its early nonlinear phase.

Nakamura et al. (2017b) conducted a large-scale fully kinetic 3-D simulation of the KHI specifically set for the KHI event observed by MMS on 8 September 2015, and found that ion-scale jets from vortex induced reconnection rapidly decay through self-generated 3-D turbulent reconnection. More efficient plasma transfer than previously thought can occur because the surface area of the interface between the magnetosheath and magnetosphere increases as a consequence of the formation of not only MHD-scale KH vortices but also ion-scale vortices resulting from vortex induced reconnection (Figs. 40c-d). Nakamura et al. (2017a) further compared the 3-D simulation and MMS observations and suggested that in addition to a pre-existing boundary layer produced by high-latitude reconnection, the KHI can form a new mixing layer with a thickness of ~1.5 $R_E$ near the terminator (Fig. 40b).

MMS observations have also revealed properties of turbulence and electrostatic waves presumably generated in association with the KHI. Stawarz et al. (2016) demonstrated that spectra of electromagnetic fluctuations and ion and electron velocities observed in the event on 8 September 2015 were all characterized by power law, consistent with developed turbulence despite the fact that the KHI was still in an early nonlinear phase. Wilder et al. (2016) reported large-amplitude (up to 100 mV/m), parallel, electrostatic waves in the middle of a KH vortex, whose excitation appears to require mixing of multiple particle populations including cold electrons with energies <10 eV.

## 3.2 Flux Transfer Events

Flux transfer events (FTEs) observed by a spacecraft situated near the magnetopause are characterized by bipolar variations of the magnetic field component normal to the nominal magnetopause surface (Russell and Elphic 1978) and an enhancement of the field intensity at the center or a depression in the circumferential region in the case of a “crater” FTE (e.g., Sibeck et al. 2008). FTEs are believed to form through some form of time-dependent magnetic reconnection on the magnetopause and contribute to the transport of magnetic flux from the dayside magnetosphere into the magnetotail.
<figure><img src="image_45.png" /><figcaption>**Fig. 40** (a) Schematic of magnetic reconnection observed at the trailing (sunward) edge of a Kelvin-Helmholtz surface wave observed by MMS on 8 September 2015 (from Vernisse et al. 2016). Purple arrows show the direction of the spacecraft motion in the KH-wave rest frame. (b) The MMS location for 1000-1200 UT on 8 September 2015 when the KH vortices were detected, and parameters such as the particle entry rate $R_{\text{entry}}$, diffusion coefficient D, and thickness $\delta$ of the mixing layer derived from comparison between MMS observations and three-dimensional (3-D) fully kinetic simulations of the KHI (Nakamura et al. 2017a). (c) 3-D view of ion-scale vortices formed at the KH wave trailing edge through vortex induced reconnection at the early nonlinear stage of the KHI (Nakamura et al. 2017b). 3-D surfaces show ion jets from the vortex induced reconnection with large velocity components in the *L* direction. (d) Density and out-of-plane current density maps at $z = L_z$. The black curves in the top panel show the mixing surface defined as $F_e = ((n_1 - n_2))/(n_1 + n_2)) = \pm 0.99$ where $n_1$ and $n_2$ are the initial electron densities on the magnetosheath and magnetospheric sides, respectively, with $n_2/n_1 = 0.3$. In the bottom panel, the black and gray curves show contours of the *L* component of the ion bulk velocity, and green lines show the in-plane magnetic field lines</figcaption></figure>

Various fairly idealized mechanisms have been proposed for the generation of FTEs (see Scholer (1995) and Raeder (2006) for reviews of FTE models), but they can be categorized into three types: localized and transient reconnection (Russell and Elphic 1978), extended but transient reconnection (Southwood et al. 1988; Scholer 1988), and multiple X line reconnection (Lee and Fu 1985), and multiple patches (Nishida 1989). The first two types are similar in that a symmetric pair of reconnected flux bundles are generated, one on each outflow side of the reconnection line. The difference lies in the size and/or dimensionality of the reconnected flux tubes; the first type produces a 3-D elbow-shaped flux tube with a relatively small azimuthal extent and a bend near the portion where the reconnected flux tube crosses the magnetopause from the magnetosheath to the magnetosphere (in the northern hemisphere) or vice versa (in the southern hemisphere). The second type produces an az-
imuthally extended bulge in the magnetopause and thus is essentially two-dimensional. The third type is unusual in its ability to create a single magnetic flux rope when only two X lines are present, so that there can be a hemispheric or dawn-dusk asymmetry in the occurrence frequency of FTEs if there is a preferred direction in which FTEs move. The last type is a distribution of different size reconnection patches and, in principle includes the other FTE models. A concise but helpful review of FTEs has been given by Paschmann et al. (2013), thus here we focus only on recent advances in FTE studies.

The different FTE reconnection models address mostly the question of in-situ FTE sig-
natures but not issues such as the length of X lines or the total magnetic flux transport where
ground observations provide further information (see Sect. 4.2.4).

As a variant of the multiple X line reconnection model for the FTE formation, sequential
multiple X line reconnection (SMXR) was proposed by Raeder (2006) based on global MHD
simulations of the solar wind-magnetosphere interaction under dominantly southward IMF.
In their simulations, an initial X line forms in the magnetopause current layer on the winter
hemisphere side of the point where the magnetosheath flows stagnate (see Hasegawa (2012)
for a discussion about why there may be a tendency that the initial reconnection line forms on
the winter hemisphere side). The X line then moves toward the winter hemisphere because
there is a flow directed away from the stagnation point. Subsequently, a second X line forms
in the region near the point of the initial X line formation, the result being the generation
of a flux rope between the first and second X lines. The resultant flux rope further moves
toward the winter hemisphere, and thus Raeder (2006) suggested that more FTEs would
be encountered in the winter hemisphere. The above series of processes is repeated, thus
called the SMXR model. Fear et al. (2012b) studied the seasonal control of the FTE location
using hundreds of FTEs encountered by Cluster. They found that more FTEs tend to be
observed in the winter hemisphere, although FTEs can occur even when the geomagnetic
dipole tilt is negligibly small. Their results are consistent with results based on Interball-
1 spacecraft observations of FTEs (Korotova et al. 2008), and partially support Raeder's
SMXR generation model.

Trattner et al. (2012) analyzed several cusp crossings to search for the signature of FTEs in the energy distribution of downward precipitating ions. They found several cusp events which show an energy overlap for parallel-streaming precipitating ions, that is consistent with field lines experiencing reconnection more than once. They suggested that this condition might be caused by reopening an already reconnected field line, forming a flux rope at the magnetopause.

Using a 2-D global hybrid-Vlasov simulation of the solar wind-magnetosphere inter-
action, Jarvinen et al. (2018) showed that fast-mode bow waves form ahead of poleward
propagating FTEs in the magnetosheath. Their analysis demonstrates that protons can be
energized up to 30 keV by a shock drift-like acceleration process and suggests that the re-
sulting velocity distributions may account for energy-time dispersed ion injections observed
by Cluster in the magnetosheath (Louarn et al. 2003).

Small spacecraft separations and unprecedentedly fast plasma measurements of the MMS
mission have allowed in-depth investigations of the structure and evolution of not only typi-
cal FTEs (durations >1 min or diameters >1 R<sub>E</sub>) but also smaller ones. Hwang et al. (2016)
analyzed one of the first FTEs encountered by MMS and revealed multi-layered structures
and multiple field line topologies/connectivities of the FTE, consistent with but far more
clearly than earlier observations (Pu et al. 2013; Varsani et al. 2014; Zhong et al. 2013).
Zhao et al. (2016a) used magnetometer and fast plasma instrument measurements from the
four MMS spacecraft to calculate the magnetic and plasma forces in FTEs. Their analysis
shows that some but not all FTEs are force-free structures in which the magnetic pressure
force balances the magnetic curvature force. Eastwood et al. (2016) reported MMS observations of two small-scale flux ropes with diameters of ~10 ion inertial lengths, which are seen to be force-free and not growing. They suggested that the observed flux ropes are consistent with generation on the magnetopause by secondary magnetic reconnection in the vicinity of a preexisting, primary X line (e.g., Daughton et al. 2011); secondary reconnection can be another mechanism for the FTE generation. On the other hand, Dong et al. (2017) reported a series of ion-scale FTEs that appeared to be evolving to those of a typical size following the creation by secondary reconnection. Hasegawa et al. (2016) also showed, based on simultaneous Geotail and MMS observations at different latitudes, that meso-scale FTEs with durations less than 1 min (tens of ion inertial lengths) can decay in the course of poleward propagation during quasi-continuous, spatially extended magnetopause reconnection. Hwang et al. (2018) further suggested that ion-scale FTEs could be formed by the tearing instability in the reconnection exhaust region as a consequence of the elongation of the current sheet when two X lines are being separated in the jet direction.

MMS observations also detected magnetic reconnection in a current sheet formed through the interaction of different flux tubes constituting an FTE or FTE-like structure. Such an interaction can occur when oppositely directed reconnection jets from multiple X lines collide at the center of a flux rope (Nishida 1989; Øieroset et al. 2014; Otto 1995) or when reconnected flux tubes collide with those preexisting in the current sheet with a guide field. Øieroset et al. (2016) reported the first direct evidence of guide field reconnection in a thin (2.5 ion inertial lengths) current sheet at the center of a magnetopause flux rope identified by MMS. Kacem et al. (2018) analyzed another MMS event observed in the vicinity of the magnetopause that, at first glance, appears to be a typical FTE, and found that it consisted of two distinct flux tubes with different magnetic topologies or connectivities that were separated by an ion-scale reconnecting current sheet at its center (Fig. 41). Such complex magnetic structures indicate that the three standard FTE generation mechanisms cannot simply account for the event, and suggest a 3-D interaction of reconnected field lines in the magnetopause boundary layer, as simulated by Cardoso et al. (2013) and Farinas Perez et al. (2018). Note that magnetic structures suggestive of such a 3-D interaction in the magnetopause have been reported earlier by Louarn et al. (2004) and Hasegawa et al. (2015) based on Cluster and THEMIS observations, respectively, and have been predicted by theory and simulation (Nishida and Maezawa 1971; Nishida 1989; Otto 1995). Øieroset et al. (2019) further analyzed a total of four such reconnection events in the current sheet of 3-D interlaced flux tubes, and suggested that magnetic pile up upstream of the reconnecting current sheet leads to enhanced magnetic shear and decrease in $\beta$ jump across the current sheet, both being the conditions favorable for reconnection.

The MMS mission would also be able to identify coalescence of magnetic flux ropes, but unambiguous evidence of such a process at the magnetopause remains to be reported, while Wang et al. (2016) showed Cluster observations consistent with magnetic islands coalescing in the magnetotail. In fact, Zhou et al. (2017a) showed MMS observations near the subsolar magnetopause of an FTE that they claimed are consistent with the coalescence of macroscopic flux ropes. However, the reported reconnecting current sheet was observed in a region where the GSM $B_z$ component was significantly positive, suggesting that the spacecraft were not at the center of the magnetopause current sheet, that is the expected location of the coalescence, but rather on the magnetospheric side. It is possible that what they reported are signatures of secondary reconnection, as simulated by Daughton et al. (2011), that can occur in regions somewhat away from the center of the current sheet where there exists weak but finite magnetic shear in the presence of significant guide field.
<figure><img src="image_46.png" /><figcaption>**Fig. 41** Schematic of a thin current sheet observed by MMS between two interlaced magnetic flux tubes (FTA and FTB) constituting an event that looks like a flux transfer event. An ion jet detected inside the current sheet is consistent with magnetic reconnection likely driven by flows toward the current sheet (red arrows in FTA and FTB). The P, J, and V axes are along the propagation direction of the structure, anti-parallel to the electric current in the current sheet, and along the reconnection jet, respectively. (From Kacem et al. 2018)</figcaption></figure>

## 3.3 Effects of Impulsive Events at the Magnetopause

### 3.3.1 Foreshock/Bow Shock/Magnetosheath Transients and Magnetopause Surface Waves

Foreshock transients, including HFAs, SHFAs, FBs, foreshock cavitons, and foreshock cavities, have a core with very low dynamic pressure bounded by one or two boundaries with enhanced dynamic pressure. Therefore, as they contact the bow shock, the bow shock together with the magnetopause can locally deform. For example, using multi-spacecraft observations, Sibeck et al. (1999) found that the magnetopause and magnetospheric plasma moved outward into an HFA across the original magnetosheath. Using data from multiple THEMIS spacecraft, Jacobsen et al. (2009) showed that due to an HFA the magnetopause deformed with an inward bulge and an outward bulge, which were propagating tailward with the HFA. Later, Archer et al. (2015) reported magnetopause outward motion driven by an FB. Based on observations, Archer et al. (2014) found that the pressure gradient in the magnetosheath due to a foreshock transient can accelerate magnetosheath plasma causing fast magnetosheath flow, which can approximately keep the deformed magnetopause bulge in pressure balance.

As illustrated in Fig. 42, magnetosheath jets with high dynamic pressure can penetrate deep into the magnetosheath and impinge on the magnetopause. Since magnetosheath jets are local, transient structures with typical scale sizes of 1.34 $R_E$ (azimuthal) and 0.71 $R_E$ (radial direction) (Plaschke et al. 2016, 2018a), localized boundary indentations with large amplitudes can be produced on the magnetopause (Shue et al. 2009).

The schematic diagrams in Fig. 43 illustrate the magnetopause distortion due to a magnetosheath jet. A magnetosheath jet generated an initial disturbance on the magnetopause (Fig. 43a). The distortion expanded in both azimuthal and radial directions with a maximum
<figure><img src="image_47.png" /><figcaption>**Fig. 42** Sketch illustrating magnetosheath high-speed jets which originate from the bow shock and their possible effects (Plaschke et al. 2018a)</figcaption></figure>

scale size of 2 $R_E$ wide and 1 $R_E$ deep (Fig. 43b). The flow direction of the magnetosheath jet can be reversed to sunward by the magnetopause rebound. After the flow direction change, the magnetopause gradually returned to the normal shape (Figs. 43c, d).

The dayside magnetopause can excite and trap surface waves with its own eigenmode impulsively. The magnetopause surface waves can be a standing wave pattern that is formed by the interference of surface waves propagating along the magnetospheric magnetic field lines and reflecting in the northern and southern ionospheres (Plaschke et al. 2009b,a; Plaschke and Glassmeier 2011; Archer and Plaschke 2015). The radial oscillation of the Earth's magnetopause field lines can further excite ULF waves predominantly with distinct, often called "magic", frequencies in the magnetosphere (Plaschke and Glassmeier 2011; Archer et al. 2013).

Hartinger et al. (2015) investigated the magnetospheric response to a solar wind dynamic pressure pulse using BATS-R-US global MHD simulations. The simulation results show persistent monochromatic magnetopause surface eigenmodes at a frequency of 1.8 mHz, implying that magnetopause surface eigenmodes can be a potential source of monochromatic compressional ULF waves below 2 mHz in the dayside magnetosphere and provide a seed perturbation for the tailward propagating surface waves via the KHI.

ULF waves in the magnetosphere are often observed on the ground with persistent discrete frequencies (around 0.7, 1.3, 1.9, 2.6, 3.3, and 4.8 mHz) known as "magic" frequencies (Menk 2011). Different from other standing ULF waves in the magnetosphere, there is no eigen-frequency gradient for the magnetopause standing waves (Plaschke and Glassmeier 2011).

There are many possible drivers for magnetopause surface eigenmodes including interplanetary shocks, both positive or negative solar wind dynamic pressure pulses, and magnetosheath jets. Using a simplified box model shown in Fig. 44, Archer et al. (2019) suggested that the magnetopause surface eigen-frequency should be around 1.7, 1.8 and 3.3 mHz for $n = 1$ (fundamental mode) and $n = 2$ (second harmonic mode) respectively. In addition, they provided direct observations of magnetopause eigenmodes during a magnetosheath jet event. These observations confirm that the dayside magnetopause can trap surface waves with its own eigen-frequency.
<figure><img src="image_48.png" /><figcaption>**Fig. 43** Schematic diagrams for a magnetosheath jet and distorted magnetopause observed by different THEMIS probes (colored diamonds). The black arrows indicate the directions of the magnetopause motion. (a) The small-scale, magnetosheath jets (fast anti-sunward magnetosheath flow). (b-c) The magnetopause is distorted locally. (d) The distortion is relaxing gradually. (Shue et al. 2009)</figcaption></figure>

### 3.3.2 Mode Conversion at the Magnetoopause

#### Introduction

Wave perturbations generated from the bow shock, e.g., the foreshock of the quasi-parallel shock, can propagate to the magnetopause, which in turn excite shear Alfvén waves in the magnetosphere through a wave mode conversion process in the magnetopause boundary layer (Lee et al. 1994; Johnson and Cheng 1997), in which the incoming compressional mode waves are converted to shear Alfvén mode waves through excitation of kinetic Alfvén waves (KAWs) (Hasegawa and Mima 1978; Lee et al. 1994; Johnson and Cheng 1997; Chen 1999; Johnson et al. 2001; Chaston et al. 2008; Lin et al. 2010). The KAWs excited at the magnetopause may cause a cross-field diffusive transport across the magnetopause, since the waves carry kinetic-scale electric and magnetic field variations that can break the frozen-in condition (Lee et al. 1994). Such mode conversion may, therefore, facilitate effective processes for plasma heating and transport at the magnetopause (Hasegawa and Mima 1978; Johnson and Cheng 1997; Chen 1999; Johnson and Cheng 2001; Chaston et al. 2008).

For a closed dayside magnetopause (under a northward IMF), wave-particle diffusive processes in large-amplitude ULF waves, as frequently observed at the magnetopause boundary, provide a source for plasma entry from the solar wind (Anderson et al. 1982; Tsurutani et al. 1982; Rezeau et al. 1986; LaBelle and Treumann 1988; Rezeau et al. 1989;
<figure><img src="image_49.png" /><figcaption>**Fig. 44** Diagram of the magnetopause surface eigenmode in a box model. (a) Box model equilibrium showing the magnetopause (black) which separates the magnetosheath (red) and the magnetosphere. Dark blue arrows denote the magnetic field lines between the northern and southern ionospheres (colored light blue). (b) $n = 1$ (fundamental mode) (c) $n = 2$ (second harmonic mode). The midpoint of the phase is denoted by the black dot, corresponding to the location of the magnetopause surface eigenfrequency $n = 1$ antinode and $n = 2$ node. (Archer et al. 2019)</figcaption></figure>

Engebretson et al. 1991b,a; Lin et al. 1991; Takahashi et al. 1991; Rezeau et al. 1993; Song et al. 1993a,b,c; Anderson et al. 1994; Song 1994; Song et al. 1994), in addition to the mechanisms of transport through high-latitude reconnection (Song and Russell 1992; Raeder et al. 1997; Le et al. 1996; Lavraud et al. 2006b) and Kelvin-Helmholtz vortices. Near the magnetopause boundary, a sharp transition is frequently found in wave polarization from predominantly compressional waves in the magnetosheath (e.g., due to foreshock waves of the quasi-parallel bow shock) to transverse in the boundary layer (Song et al. 1993a; Rezeau et al. 1989; Chaston et al. 2008). It has been suggested that the mode conversion from compressional to Alfvén modes provides a source of Alfvén waves at the magnetopause (Lee et al. 1994; Johnson and Cheng 1997), which can efficiently transport plasma across the magnetopause boundary (Hasegawa and Mima 1978; Lee et al. 1994; Johnson and Cheng 1997; Chen 1999; Johnson et al. 2001; Chaston et al. 2008). Multipoint measurements have verified that the dispersion of the broadband waves is consistent with the kinetic Alfvén waves (Chaston et al. 2007, 2008). Numerical simulations have been performed to investigate such mode conversion process (Lin et al. 2010, 2012; Shi et al. 2013, 2017a), including
the subsequent generation of field line resonance (FLR) associated with toroidal Alfvén waves in the dayside magnetosphere (Shi et al. 2021).

In this section, we present a review of the theoretical studies, numerical simulations, and space observations of the mode conversion process.

### Theoretical studies

Based on the MHD linear dispersion relation in a uniform plasma, there are three fundamental modes, including the fast/slow magnetosonic mode and the shear Alfvén mode (intermediate mode). In a nonuniform plasma, however, the MHD modes are coupled. In a plasma with a one-dimensional (1-D) inhomogeneity along the direction perpendicular to the background magnetic field, the linearized MHD equations can be expressed in terms of $\delta p_1 = B_0 \delta B_\parallel + \delta p$ and $\delta B_\perp$ by a coupled system of equations (Lin et al. 2010),

$$-ik_\parallel \left(1 - \frac{k_\parallel^2 C_s^2}{\omega^2}\right) \delta p_1 = \left(1 + \frac{C_s^2}{V_A^2} - \frac{k_\parallel^2 C_s^2}{\omega^2}\right) B_0^2 \nabla_\perp \cdot \left(\frac{\delta \mathbf{B}_\perp}{B_0}\right), \quad (3)$$

and

$$(\omega^2 - k_\parallel^2 V_A^2) B_0 \delta \mathbf{B}_\perp = ik_\parallel V_A^2 \nabla_\perp \delta p_1, \quad (4)$$

with $\omega$ being the wave frequency, $V_A$ the Alfvén speed, and $k_\parallel$ the parallel wave number. Singular behavior occurs at either the Alfvén resonance (Tamao 1965; Uberoi 1972; Chen and Hasegawa 1974; Southwood 1974; Hasegawa et al. 1983), $\omega^2 = k_\parallel^2 V_A^2$, or at the sound resonance, where the compressional wave couples with the Alfvén or sound wave. In higher frequency cases, the Alfvén resonance condition is modified as $\omega^2 = k_\parallel^2 V_A^2 (1 - \omega^2/\Omega_i^2)^2$ due to the finite ion Larmor radius effects (Stix 1992), where $\Omega_i$ is the ion gyrofrequency. The Alfvén resonance singularity can be removed by including non-MHD effects such as electron inertia or ion Larmor radius corrections in Eq. (4). Near the magnetopause, electron inertial effects are typically not important.

This Alfvén resonance singularity can be removed by including kinetic effects (Chen and Hasegawa 1974; Southwood 1974; Hasegawa and Chen 1976a,b; Johnson and Cheng 1997; Johnson et al. 2001; Chen 2008; Lin et al. 2010), and the wave coupling is through the mode conversion to kinetic Alfvén waves (Hasegawa and Mima 1978; Hasegawa and Chen 1976a,b; Lee et al. 1994; Johnson and Cheng 1997) for cases with an electron $\beta_e > m_e/m_i$. As a result, mode conversion occurs where the frequency matches the continuous spectrum. For magnetospheric plasma, the KAWs are low frequency ($\omega < \omega_i$, where $\omega_i$ is the ion cyclotron frequency) Alfvén mode with $k_\perp \rho_i \sim k_\perp \rho_s \sim 1$, where $\rho_i$ is the ion Larmor radius, $\rho_s = \sqrt{T_e/T_i} \rho_i$, $T_e$ and $T_i$ are the electron and ion temperatures, respectively, and $k_\perp$ is the perpendicular wave number. On the short wavelength scale, ion motion decouples from the electron motion. KAWs can develop parallel electric fields $E_\parallel$ through charge separation, which facilitates particle heating and transport on scales where the ion motion decouples from the field lines due to polarization drift (Johnson and Cheng 1997).

Typically, the Alfvén velocity across the magnetopause increases by a factor of 10 such that an entire decade (in frequency) of wave power can be captured by the mode conversion process and localized in the boundary layer leading to massive particle transport. At the Alfvén resonance point, transversely polarized waves are expected to be generated and radiate away from the mode conversion location.

Analytical linear theories have been derived for mode conversion at the magnetopause (Johnson and Cheng 1997; Johnson et al. 2001) based on a kinetic-MHD model, which uses Ohm's law and the momentum equations obtained from the gyrokinetic equations. Based on
the quasilinear theory, the diffusion coefficient for the KAWs under typical magnetopause conditions is on the order of $D_⊥ \sim 10^{9–10}m^2/s$ (Johnson and Cheng 1997). Theoretically, ions may be nonlinearly heated by resonance between the polarization drift and gyro-motion above a threshold wave amplitude (Johnson et al. 2001; Chen et al. 2001). The effects of IMF orientation have been calculated by Johnson and Cheng (1997). While the KAWs radiate back into the magnetosheath from the Alfvén resonance location under a northward IMF, they can propagate through the $k_‖ = 0$ location due to the existence of magnetic shear, which may lead to magnetic islands (Johnson and Cheng 1997).

Johnson et al. (2001) carried out a theoretical investigation of ion motion in the presence of large-amplitude KAWs with wavelength on the order of $\rho_i$ and demonstrated that for sufficiently large wave amplitude the ion orbits become stochastic. As a result, low energy particles in the core of the ion distribution can migrate to higher energy through the stochastic sea leading to an increase in the perpendicular ion temperature and a broadening of the distribution.

### *Observations of mode conversion and associated KAWs*

Large-amplitude Alfvén waves have been observed around the magnetopause (Anderson et al. 1982; Song 1994). Cluster multipoint measurements have found the dispersion of broadband waves consistent with KAWs (Chaston et al. 2007). The deposition of energy into the auroral ionosphere from KAWs in the cusp and low-latitude boundary layer has been reported (Chaston et al. 2005), which results in precipitating electron fluxes sufficient to drive bright aurora and cause outflows of energized electrons and $O^+$ ions from the ionosphere.

Kinetic Alfvén waves also provide a natural explanation for the observed dawn-dusk asymmetry in plasma entry during northward IMF (Hasegawa et al. 2003; Wing et al. 2005) because they result from mode conversion of compressional foreshock waves, which typically bathe the dawn flank for the typical Parker spiral configuration. The nonlinear development of the magnetopause spectrum observed by Cluster (Chaston et al. 2007) and THEMIS (Chaston et al. 2008; Yao et al. 2011) has shown intriguing features. The global distribution indeed indicates that the spectral energy densities across the magnetopause are larger on the dawn than dusk side (Yao et al. 2011). The THEMIS observations (Chaston et al. 2008) show direct evidence of a turbulent spectrum of KAWs at the magnetopause with sufficient power to provide massive particle transport. The diffusion coefficients based on the observed spectra are about $D_⊥ \sim 10^{9–10}m^2/s$, consistent with the estimates of Johnson and Cheng (1997) based on the quasilinear theory. By examining the global distribution and properties of kinetic-scale magnetopause waves observed by THEMIS, Yao et al. (2011) have shown that the KAW wave activity increases with the magnetic shear across the magnetopause, under both southward and northward IMF. Such observation is consistent with the quasi-linear theory by Johnson and Cheng (1997), although it is still unclear whether the observed electromagnetic fluctuations are resulting from KAWs due to mode conversion or KAWs that are generated by magnetic reconnection (Chaston et al. 2009; Gershman et al. 2017).

Heated ions are commonly observed at the magnetopause in conjunction with waves that satisfy the threshold condition for chaotic ion heating (Chaston et al. 2008). THEMIS events show strong evidence of perpendicular heating of the magnetosheath ions through interaction with the KAWs near the dayside and flank magnetopause (Chaston et al. 2014). The wave spectra ($B^2$) associated with KAWs at the magnetopause typically exhibits a breakpoint around the ion gyroradius (Chaston et al. 2008), where the spectrum becomes more electrostatic.
Local and global simulations of mode conversion and associated KAWs

The mode conversion process and associated KAWs have been tackled with kinetic sim-
ulations. Lin et al. (2010) performed a 2-D hybrid simulation for a current sheet in slab
geometry to investigate how monotonic fast-mode compressional waves incident on a mag-
netopause current layer mode convert to short wavelength ($k_{\perp} \rho_i \sim 1$) kinetic Alfvén waves
near the Alfvén resonance surface, under a northward IMF. Structure of the mode conver-
sion was examined for various driver wave frequency $\omega_0$, wave amplitude, wave vector,
magnetopause half-width $D_0$, ion $\beta$ in the magnetosheath, and electron-to-ion temperature
ratio. Figure 45 depicts contours of various quantities around the magnetopause, obtained
by Lin et al. (2010) in a case with a northward IMF. The simulation is performed in the xz
plane, where x is normal to the magnetopause and the magnetosheath field $\mathbf{B}_0 = B_0\mathbf{z}$. At an
early time, $t = 80$ (normalized to the inverse of the magnetosheath ion gyrofrequency),
the incident compressional wave has just reached the magnetopause boundary from the
magnetosheath (left side). The incident wave has wave numbers $k_{x0} = k_{\perp 0} = 0.262$ and
$k_{z0} = k_{\parallel 0} = 0.196$ (normalized to the inverse of the ion inertial length). At later time $t = 240$,
larger-amplitude, short perpendicular wavelength KAWs are generated in the transition layer
by mode conversion, where the background density and magnetic field have a large gradi-
ent. In these waves, the shear Alfvénic components $B_y$, $E_x$, ion flow $V_{iy}$, and the parallel
current density $J_\parallel$ are well correlated. Strongly enhanced parallel electric field $E_\parallel$ is present
in the resulting waves. Meanwhile, wavy perturbations in local ion temperature $T_i$ are also
observed, correlated with the variations in $B_y$. On the right edge of the KAWs, the enhanced
perpendicular wave vector has reversed direction to $k_x < 0$, as seen from the tilt angle of
the wave fronts, where the KAWs radiate back to the magnetosheath. The dependence of
the compressional wave absorption rate on $\omega_0$, $\theta$, and $D_0$ was obtained and compared with
the linear cold-fluid theory. The absorption rate $A$ was found to have a sensitive dependence
on $\omega_0$ and the orientation of the magnetosheath $\mathbf{B}_0$. Because the group velocity of these
waves is much smaller than the compressional wave, the amplitude of the transverse fluc-
tuations is typically much larger than the amplitude of the compressional driver, consistent
with magnetopause observations (Johnson et al. 2001). Although the 2D mode conversion is
predominantly of linear physics, nonlinear harmonic generation was also found. As the inci-
dent wave amplitude is increased, harmonics of $\omega_0$ are generated, leading to the broadening
of the mode conversion region and its spectral width. The presence of the higher frequency
harmonic modes may significantly lower the threshold of the stochastic ion heating due to
KAWs (Johnson and Cheng 2001).

When extending the hybrid simulation of Lin et al. (2010) to 3-D, Lin et al. (2012) found
that the 3-D, nonlinear physics is fundamentally important to the mode conversion process.
The simulation showed that following a stage of mode conversion to KAWs with $k_x \rho_i \sim 1$
(dominated by the linear physics), the growth of KAW modes with $k_y \rho_i \sim 1$ (with $k_y$ being
perpendicular to both the background $\mathbf{B}_0$ and the wave vector $\mathbf{k}_0$ of the incident compres-
sional wave) was observed in the nonlinear stage. Figure 46 shows the time evolution of the
modes dominated by $k_x$ (left column) and modes dominated by $k_y$ (right column), for the
same case shown in Fig. 45 but run in 3-D. The first-stage growth of KAWs from $t = 40-80$,
shown in the $k_x$ dominant mode, is due to the linear physics of mode conversion from the
fast wave to KAWs, which results in the growth of $E_x$ and $E_\parallel$ when the incident fast wave
reaches the magnetopause boundary. The later stage of evolution in $t > 80$ is due to the non-
linear physics in which the mode coupling to $k_y$ plays a significant role. In $t = 80-145$, the
strong growth in $B_x$, $E_y$, and $E_\parallel$ is consistent with KAWs that are dominated by $k_y$, which
nonlinearly co-exist with the KAWs dominated by $k_x$. Finally for $t > 145$, the wave pertur-
bations of $k_x$ and $k_y$ modes become isotropic with both $E_x$ and $E_y$ growing at the same rate.
<figure><img src="image_50.png" /><figcaption>**Fig. 45** Contours of various quantities around the magnetopause boundary layer obtained from hybrid simulation in slab geometry at (a) an early time $t = 80$ and (b) $t = 240$. Short wavelength KAWs are generated by interaction between the incident wave and the magnetopause. (Lin et al. 2010)</figcaption></figure>
<figure><img src="image_51.png" /><figcaption>**Fig. 46** 3-D physics of mode conversion: Time evolution of $B_x$ (black), $E_x$ (green), $E_y$ (blue), and $E_\|$(red) for the KAWs modes dominated by $k_x$ (left column) and for those dominated by $k_y$ (right column) during the mode conversion in the inhomogeneous magnetopause (Lin et al. 2012)</figcaption></figure>

The generation of the perpendicular and azimuthal $k_y$ spectrum results when the amplitude of KAWs generated by linear mode conversion becomes large enough to drive a nonlinear parametric decay process, accompanied by a simultaneous excitation of zonal flow modes with similar large $k_y$. Since the diffusion coefficient across the gradient is proportional to the azimuthal $k_y^2$ (Johnson and Cheng 1997; Chen 1999), the KAWs generated in the nonlinear stage may lead to massive particle cross-field line transport.

The self-consistent interaction of foreshock waves with the dayside magnetosphere and the associated mode conversion process have been investigated by Lin and Wang (2005) and Shi et al. (2013, 2017a) using 3-D global-scale hybrid simulations. In the case with a radial IMF (Lin and Wang 2005; Shi et al. 2013), the quasi-parallel shock dominates the dayside. Large-amplitude compressional waves of the foreshock lead to the magnetopause surface perturbations. The results show that as the compressional pulses/packets, with a broadband frequency, propagate from the magnetosheath to the magnetopause, short wavelength structures of $k\rho_i \sim 1$ with enhanced parallel electric field $E_\|$ are excited in the magnetopause boundary layer (MPBL). The wave phase relationship between the magnetic field and the density changes from in-phase in the magnetosheath to anti-phase in the short-wavelength MPBL perturbations. The wave polarization is predominantly compressive in the magnetosheath, whereas strong transverse wave powers appear abruptly around the MPBL. The mode conversion from the compressional pulses to KAWs is identified around the predicted Alfvén resonance points in the MPBL (Shi et al. 2013). The short-wavelength KAWs are identified by the sharp increases in $E_\|$ and the transverse electromagnetic field polarization relation of Alfvén mode. These KAW perturbations propagate poleward into the cusps along the MPBL, carrying Poynting fluxes and field-aligned currents. Excitation of the azimuthal wave numbers following the growth of KAWs was also shown. The global hybrid simulation was further performed for cases with various IMF and solar wind conditions (Shi et al. 2017a). Figure 47 shows the spatial profiles of various quantities along the Sun-Earth line in a time sequence of $t = 40 – 150$ (normalized to inverse of the solar wind ion gyrofre-
<figure><img src="image_52.png" /><figcaption>**Fig. 47** Time evolution of spatial profiles of various quantities along the Sun-Earth line in a case of 3-D global hybrid simulation with an oblique IMF shows the interaction between compressional waves from the foreshock and the magnetopause, with the circles highlighting the areas of the KAW structures with locally excited $E_\|$ around the MPBL. (Shi et al. 2017a)</figcaption></figure>

quency) obtained from a case with an oblique IMF in the $B_x-B_y$ plane (cone angle of 30°) and Mach number $M_A = 5$ obtained from the 3-D global hybrid simulation of Shi et al. (2017a). Incoming foreshock compressional wave pulses characterized by the perturbations in magnetic field $B$ and ion density $N$ repeatedly propagate onto the magnetopause from the bow shock. The stronger wave pulses also lead to the generation of strong perturbations of $E_\|$ and $J_\|$. Correspondingly, the wave polarization relation is shown in Fig. 48 by plotting the relation between the transverse electric field and magnetic field for locations in the MPBL where $E_\|$ is enhanced. The results are nearly consistent with the theoretical prediction for KAWs. Around the subsolar region at the magnetopause, it can be distinguished that two wave branches propagate northward and southward, separated around the equatorial plane. For cases with an oblique IMF, the amplitude of magnetosheath compressional waves is larger at the quasi-parallel shock than at the quasi-perpendicular shock. The KAWs generated from the magnetopause mode conversion can be carried to the regions downstream of the quasi-perpendicular shock owing to the flow convection at the magnetopause. The KAWs are more intense in the downstream of quasi-parallel shocks than downstream of quasi-perpendicular shocks. It was also found that under a reduced, near-critical Mach number, KAW structures due to mode conversion exhibit a feature of broader excitation regions in the MPBL.
<figure><img src="image_53.png" /><figcaption>**Fig. 48** Global hybrid simulation results for the case shown in Fig. 47. (Top left) Spatial contours of $E_\|$ around the MPBL in the noon-meridian plane. (Top right) Corresponding polarization relation in the circled regions in the top left plot showing Alfvén modes propagating in different directions along the field lines. (Bottom row) Similar to the top row, except for the equatorial plane. The violet dashed lines in the right column represent the theoretically predicted polarization relation of KAWs. (Shi et al. 2017a)</figcaption></figure>
The mode conversion process has been suggested as a directly-driven mechanism for the generation of the frequently observed discrete harmonic frequencies of shear Alfvénic field line resonances in the dipolar field region (Engebreftson et al. 1991b; Chi et al. 1994; Clausen et al. 2009; Wang et al. 2018c), including kinetic FLRs (Chaston et al. 2014). As shown in the global hybrid simulation of Lin and Wang (2005), in addition to the resonance absorption at the magnetopause, part of the energy will be transmitted into the magnetosphere. As predicted by theories discussed above, a standing wave pattern of shear Alfvén waves is developed, leading to the resonance of closed field lines. A fundamental odd resonance mode is obtained.

Using coordinated observations in the foreshock and the magnetosphere, Wang et al. (2018c) found direct evidence of Pc 5 field line resonances driven by the foreshock cavity. ULF waves excited in the magnetosphere have a frequency similar to that of the foreshock pressure pulses. The toroidal modes are consistent with the shear Alfvénic FLRs, while poloidal modes are also present. A pair of traveling convection vortices and auroral brightening were generated in the ionosphere correspondingly. Recent 3D global-scale hybrid simulations of Shi et al. (2021) assuming a radial IMF illustrated for the first time the properties of self-consistently generated field line resonances through direct mode conversion in magnetospheric response to the foreshock disturbances. It was found that the foreshock wave spectrum covers a wide frequency range and matches the band of FLR harmonics. The fundamental harmonic of field line resonances dominates and has the strongest wave power, and the wave power decreases with the harmonic order. The eigenstructures of the harmonic modes along dipole-like field lines were obtained. The power of the toroidal Alfvén waves is steady under the steady solar wind inputs, suggesting that the FLRs are part of the steady-state magnetosphere. At the equator, an alternating feature of toroidal Alfvén wave components $E_r$ (radial) and $B_\phi$ (azimuthal) is present. As the Alfvén speed increases inwards in the magnetosphere, the frequencies of toroidal Alfvén harmonic modes increase accordingly. Figure 49 depicts the power spectral density (PSD) as a function of $L$ for $E_r$ (Fig. 49a) and $B_\phi$ (Fig. 49b) at the equator in the noon-meridian plane. The time-averaged equatorial Alfvén speed is plotted in Fig. 49c. The white dashed lines mark the first 7 orders of $n$-harmonic modes of the FLRs. The enhanced odd harmonic numbers in $E_r$ and even-harmonic numbers in $B_\phi$ are features of standing toroidal Alfvén waves. In addition, field-aligned currents in the cusp region indicative of possibly observable aurora were found to be a result of magnetopause perturbations caused by the foreshock disturbances. There exists a low $m$-number ($m \sim 5$) azimuthal mode in the dayside driven by the FLRs, and the motion of these FAC structures is both poleward and azimuthal. The simulation also showed that when the solar wind Mach number decreases, the strength of both FLRs and field-aligned currents decreases accordingly.

### 3.3.3 Triggering of Magnetopause Reconnection

It has been reported that a local magnetic reconnection can be triggered (Hietala et al. 2018; Ng et al. 2021), or an impulsive penetration may occur when a magnetosheath jet impinges on the magnetopause (Dmitriev and Suvorova 2015).

As shown in Fig. 50, the magnetopause impact by a magnetosheath jet has been examined by Hietala et al. (2018). Before the magnetosheath jet impact, the magnetopause current layer was thick ($60–70 \, d_i$) with a high magnetic shear of $140^\circ–160^\circ$, without reconnection signatures. Shortly after the magnetosheath jet impact, reconnection outflows were observed within the magnetopause layer while the magnetic shear did not change significantly from pre-impact to post-impact. Thus, it is suggested that the observed reconnection at the magnetopause has been triggered by the magnetosheath jet’s compression on the magnetopause
<figure><img src="image_54.png" /><figcaption>**Fig. 49** Generation of standing toroidal Alfvén modes in the magnetosphere by foreshock perturbations obtained from 3D global hybrid simulation: Wave power spectra of (a) radial electric field $E_r$ and (b) azimuthal magnetic field $B_\phi$ as a function of $L$ in the noon-meridian plane, and (c) the corresponding equatorial Alfvén speed. The white dashed lines mark the first 7 harmonic modes of the FLRs. (Shi et al. 2021)</figcaption></figure>

<figure><img src="image_55.png" /><figcaption>**Fig. 50** Sketches illustrating the magnetosheath jet triggering the magnetopause reconnection. Black lines indicate the magnetopause current layer, with the current sheet center marked by the dashed line and their normal vectors by blue arrows. Purple arrows indicate the magnetosheath jet and the red arrows the reconnection outflow. (After Hietala et al. 2018)</figcaption></figure>

current sheet. These observations suggest that magnetosheath jet impacts may trigger magnetopause reconnection.

The interaction of magnetosheath jets with the magnetopause has been revealed from a sudden inward motion of the magnetopause and an enhancement in the geomagnetic field. The magnetosheath plasma penetration was determined as appearance of the magnetosheath plasma against the background of the hot magnetospheric particle population (Dmitriev and Suvorova 2015). A total of 646 magnetosheath jets have been identified using THEMIS data
<figure><img src="image_56.png" /><figcaption>**Fig. 51** GSM location of 646 magnetosheath jets (gray triangles) observed by THEMIS satellites from 2007 to 2009. The red crosses indicate the jets, which result in plasma penetration across the magnetopause based on multi-spacecraft observations. The blue circles indicate the jets, which are not accompanied by the plasma penetration. (Dmitriev and Suvorova 2015)</figcaption></figure>

from 2007 to 2009. The jets were identified as the ratio *R* of the total energy density of a jet to that of the upstream solar wind to be larger than 1 for $\Delta T = 30$ s and longer. As shown in Fig. 51, the spatial range of magnetosheath jets location was rather scattered. Hence, it seems that the magnetosheath plasma penetration does not depend on the magnetosheath jets location. Apparently, not every magnetosheath jet was accompanied by a plasma penetration in the magnetosphere. It is found that 44 out of 76 jets (almost 60%) were accompanied by the plasma penetration. The other 32 jets were not accompanied by the penetration.

## 3.4 Transient Dayside Magnetopause Processes at Other Planets

Observations of transient dayside magnetopause processes at other planets have been reported. KH instabilities have been observed at Mercury (Slavin et al. 2008), Venus (Wolff et al. 1980), Mars (Penz et al. 2004), Saturn (Pu and Kivelson 1984), and cometary ion tails (Ray 1982). FTEs are prevalent at Mercury's magnetopause with a typical duration of ~1 s and spacing of 4 s (Russell and Walker 1985; Sun et al. 2020). They have been observed during both northward and southward IMF conditions with a higher occurrence rate during southward IMF. FTEs in the shower events (≥10 FTEs in a magnetopause crossing) carry most of the magnetic flux that drives Mercury's Dungey cycle (Sun et al. 2020). At giant planets, in contrast, observations of FTEs are rare. Fourteen possible FTEs were found at Jupiter (Walker and Russell 1985) and only one FTE at Saturn was reported (Jasinski et al. 2016).

The giant planet magnetospheres contain significant local time asymmetry. These pronounced dawn-dusk asymmetries indicate that the solar wind must, fundamentally, play an important role (Hill et al. 1983; Khurana 2001). For instance, Jupiter's magnetodisc is highly stretched in the midnight to dawn sector, and more dipolar in the pre-Noon to dusk sector. Even deep in the inner magnetosphere, a persistent dawn-dusk (east-west) asymmetry of the UV emissions from the Io plasma torus (Sandel and Broadfoot 1982) led two teams, Barbosa and Kivelson (1983) and Ip and Goertz (1983), to propose that flow down the magnetotail could impose weak stresses on the Io plasma torus, consistent with a dawn-to-dusk electric field (few mV/m) that would displace plasma orbits toward dawn. Delamere and Bagenal (2010) further suggested that the electric field could be imposed by a strong viscous interaction starting at the dayside magnetopause boundary. Jupiter and Saturn both exhibit significant local time asymmetry, providing compelling evidence that the solar wind interaction must be at the root of this problem.
<figure><img src="image_57.png" /><figcaption>**Fig. 52** MFLFM simulation of Jupiter's magnetosphere, showing the KH-unstable magnetopause boundary as well as dawn/dusk asymmetry (Zhang et al. 2018b)</figcaption></figure>

There has been some debate regarding the nature of the solar wind interaction with the giant planet magnetospheres. Following the New Horizons excursion down Jupiter's magnetotail, McComas and Bagenal (2007) suggested that Jupiter's interaction with the solar wind was fundamentally different from Earth due to the lack of a well-defined and tailward-extending plasma sheet. Much of the subsequent debate is summarized by Delamere (2015), comparing the relative importance of large-scale reconnection (Dungey 1961), a viscous-like interaction (Axford and Hines 1961), and the internally-driven Vasyliunas cycle (Vasyliunas 1983). Delamere and Bagenal (2010), Delamere and Bagenal (2013), and Masters (2018) argued that the viscous-like interaction may play a dominant role in the solar wind interaction due, in part, to low reconnection rates and to the large sheared flows (maximum on the dawn flank, minimum on the dusk flank) that exist on the dayside magnetopause boundaries, leading to the KH instability.

There is abundant evidence that the KH instability is active at Saturn's magnetopause boundary (Masters et al. 2009a, 2010; Wilson et al. 2012; Delamere et al. 2013). An initially surprising result was evidence for more KH activity in the post-noon sector where the sheared flow is minimized (i.e., magnetodisc corotation and sheath flows are both tailward). However, Ma et al. (2015), using a 2-D MHD simulation of Saturn's dayside magnetopause, showed that KH vortices originating in the subsolar region (roughly from 11 to 14 local times) are transported to the postnoon sector and the wavelength is enlarged due to the gradient of sheared flow. Meanwhile, KH vortices formed in the prenoon sector ($<10$ LT) tend to diffuse rapidly and form a boundary layer, precluding the existence of well-defined vortex structures. As a result, vortex structures are more likely to be observed in the post-noon sector. To validate the simulation results, Ma et al. (2015) conducted a comprehensive boundary normal analysis (minimum variance analysis, MVA) and found that indeed the boundary normal directions fluctuate in a systematic manner consistent with the 2-D simulation results. Zhang et al. (2018a) also confirmed this result with global simulations (Fig. 52).

Reduced dawn-side sheath flows at Saturn also provide compelling evidence for a viscous-like interaction (Burkholder et al. 2017). Numerically integrated moments from
**Fig. 53** Dawn-dusk asymmetry in Saturn's magnetosheath flows. The histogram is the dawn vs. dusk average, excluding 11:30 to 12:30 LT. (Burkholder et al. 2017)

<figure><img src="image_58.png" /></figure>

**Fig. 54** Selected field lines from a local MHD simulation of Saturn's KH unstable magnetopause boundary. The red and black lines are closed field lines, and the green, blue, magenta, and orange lines are open field lines. (Ma et al. 2015)

<figure><img src="image_59.png" /></figure>

Cassini Plasma Spectrometer (CAPS) data (Thomsen and Delapp 2005; Thomsen et al. 2010) through 2011 were used to calculate average flow properties in the magnetosheath. Interestingly, there is a significant dawn-dusk asymmetry, with the dawn-side flows being substantially reduced from expectation. The asymptotic terminator flank value from MHD modeling by Desroche et al. (2013) for a polar flattened obstacle to flow is 200 km/s. Burkholder et al. (2017) found dusk flank values consistent with Desroche et al. (2013), but dawn flank values were ~100 km/s. Figure 53 summarizes their result. The proximity to the magnetopause boundary and bow shock are critical for understanding the momentum transfer rates. The results are filtered based on the time elapsed from a boundary crossing (magnetopause and bow shock crossing were catalogued by Delamere et al. (2013)). Figure 53 is an average from data points within 500 minutes of the magnetopause boundary crossing (or roughly 3 $R_S$ for a stationary boundary). Burkholder et al. (2017) conducted averages based on proximity to the magnetopause and the bow shock and found that asymmetry exists for roughly half of the sheath width.
<figure><img src="image_60.png" /><figcaption>**Fig. 55** 3-D Kelvin-Helmholtz hybrid simulation. The color slices show particle mixing, with grey representing the magnetopause boundary. Magnetic field lines are traced from the bottom to the top boundary. Red isosurfaces indicate parallel electric fields. (Delamere et al. 2018)</figcaption></figure>

Unlike the evidence for the KH instability at Saturn, magnetic reconnection has proved more elusive. Walker and Russell (1985) found 14 possible FTEs at Jupiter using the magnetic field observations from Pioneer 10 and 11 and Voyager 1 and 2. Indeed, the FTEs were associated with northward magnetosheath fields, but the electric fields were small in comparison to the magnetospheric corotation electric fields, leading to the conclusion that the FTEs were unlikely to influence magnetospheric flows. Lai et al. (2012) examined Saturn's dayside magnetopause boundary using Cassini data and found no evidence for FTEs, but identified cases where the magnetic field threads the boundary, forming "magnetic bridges". On the other hand, Jasinski et al. (2016) reported one FTE observed by the Cassini spacecraft at Saturn on 2 February 2007. Fuselier et al. (2014) showed that streaming electron distributions in the boundary layer are consistent with simple models predicting the location of reconnection. However, the situation could be more complicated because in three dimensions, the KH instability and reconnection interact (Ma et al. 2014a,b). Specifically, KH can thin the spine region connecting two vortices, triggering reconnection. Conversely, magnetic reconnection can thin a boundary in the presence of a sheared flow, triggering the KH instability. Recently, KH-associated reconnection was reported by Eriksson et al. (2016) and Li et al. (2016a) using MMS mission data at Earth. Figure 54 shows the complex magnetic field structure (with open and closed field lines) that can be generated by reconnection in a KH-unstable configuration (Ma et al. 2015; Walker et al. 2011).

Delamere et al. (2018) conducted 3-D hybrid simulations of the KH instability for conditions appropriate for Saturn's dawn-side magnetopause boundary (Fig. 55). In these simulations the phase of the surface waves can be different in the y direction, allowing adjacent surface waves to twist the magnetic field into "candy wrapper" forms, promoting magnetic
reconnection (Ma et al. 2016b, 2017). The Maxwell and Reynolds shear stresses generated at the boundary are a non-negligible fraction of the momentum flux density associated with the flow and Delamere et al. (2018) showed that the momentum transfer rates are consistent with estimates for the observed flow reduction in Saturn's dawn-side sheath by Burkholder et al. (2017).

Magnetic reconnection under super-Alfvénic sheared flow conditions that are perpendicular to the reconnection plane adds further complexity to understanding the solar wind interaction with fast rotating magnetodiscs (Ma et al. 2016a). The KH instability can be suppressed for perpendicular flow shears in excess of the magnetosonic speed when reconnection operates first. When reconnection operates in sub-Alfvénic conditions, the evolution in the outflow region satisfies the Walén relation (i.e., $\Delta\mathbf{v} = \pm\Delta\mathbf{v}_A$). However, when reconnection operates in super-Alfvénic conditions, the outflow region acquires a significant magnetic field component along the sheared flow direction. The added magnetic pressure causes the outflow region to expand and the Walén relation is not satisfied. In addition, the expansion of the boundary will stabilize KH.

A critical aspect of the solar wind interaction is the role of dynamic pressure and the related variations in magnetospheric scale (Joy et al. 2002; Kanani et al. 2010; Pilkington et al. 2015). Compressions in the magnetosphere produced by, e.g., forward shocks can significantly modify azimuthal flows via conservation of angular momentum (Southwood and Kivelson 2001). Transitions from compressed to expanded configurations are poorly understood, particularly in terms of angular momentum transport within the magnetosphere.

Finally, magnetopause boundary processes can be the source of compressional modes in the magnetosphere, leading to resonant mode coupling (Glassmeier 1995). Both Jupiter and Saturn exhibit ultra-low-frequency, quasi-periodic behavior, which is likely a signature of resonant cavity eigenoscillations (Delamere 2016; Nichols et al. 2017).

The solar wind interaction with Jupiter and Saturn is fundamentally different from Earth. A complete understanding of the viscous-like interaction must combine flow shear (KH) instability with magnetic reconnection. Understanding the system-wide response to variations in solar wind conditions (e.g., dynamic pressure) is critical.

## 3.5 Outstanding Questions

- A longstanding question of KH waves has been whether it operates symmetric at the dawn and dusk side flanks. While the typical Parker spiral configuration, properties of the magnetosheath, and kinetic physics imply that there should be an asymmetry, the question and the dominant physical mechanisms are not resolved.

- During periods of strongly northward IMF, the plasma sheet assumes a state called cold dense plasma sheet indicating the transport of cold magnetosheath material into the plasma sheet. The dominant candidates for this process are high latitude cusp reconnection or intermediate latitude KH induced reconnection at the boundary of the KH unstable flanks. Both processes have been demonstrated to be capable to generate sufficient mass transport and both processes have observational evidence for this transport. However, we do not know their relative importance and how this depends on magnetospheric and solar wind/IMF conditions.

- The specific entropy of the cold dense plasma sheet is more than an order of magnitude higher than in the magnetosheath. This amount of non-adiabatic heating represents a serious problem both for KH associated and cusp reconnection. In order to really understand the mass transport, we need to identify the mechanism that can explain the observed heating.
- An important aspect of the KH interaction with the magnetosphere is turbulence in the magnetosheath with somewhat controversial results in the past. Therefore, we need to identify the spatial distribution of turbulence in the magnetospheric flank, its temporal evolution, and relation to dense plasma distribution.

- Many papers in the past have attributed KH waves as one source of ULF waves (Pc 5 and Pc 4). However, much of this work uses correlation studies. Therefore, we need to identify the specific mechanism for the generation of these waves and their relation to the KHI/surface waves.

- As an initial value problem, the KHI depends strongly on the thickness of the initial velocity shear layer. However, KH at the flanks evolves as a result of its history further upstream. Can we define a thickness at some downstream location (based on other mechanisms) and how important is this for the evolution of the KH waves along the flanks of the magnetosphere?

- Global MHD simulations for both southward (Claudepierre et al. 2008) and northward IMF conditions (e.g., Li et al. 2012; Merkin et al. 2013) show that two KH modes may be simultaneously excited, the outer one that grows along the magnetopause and the inner one at a velocity shear layer somewhat earthward of the magnetopause. However, there is no evidence for such two KH modes in actual magnetospheres. Questions thus remain about whether the two KH modes exist in reality and, if so, whether there is any significant role played by the presence or interaction of the two modes.

- There has been some work on the interaction of KH waves and reconnection and it is highly likely that this interaction occurs under certain conditions. However, it not clear whether there is a connection between FTEs and KHI, and what the expected signatures for such events might be.

- Several studies have associated ionospheric (auroral and convection) signatures with KH waves at the magnetospheric boundaries. However, it is not clear how this coupling occurs and whether such signatures can uniquely be attributed to such waves. Can ionospheric signatures be used as a diagnostic for KH waves?

- Observations indicate that reconnection can have different scale sizes and modes. However, there is no simple picture of FTE formation emerging from observations. Therefore, it is highly important to determine the distribution of sizes and locations of FTE associated reconnection patches on the dayside magnetopause.

- There have been several observations of interlinked FTE flux ropes confirming that this situation can occur at least occasionally. We do not understand the associated flux and mass transport. How are FTE flux ropes convected toward the tail when the flux tubes are interlaced and have the “wrong” hemispheric connection? What are the properties and signatures of such interlinked flux tubes? What is the topological connection of flux ropes created by two longer X lines and how does it resolve into a simple north or southward connection?

- MMS results have shown the formation of secondary flux ropes at the magnetopause. Is there any significant role played by these flux ropes at the magnetopause? If that is the case, what is the role?

- Considering the different manifestations of FTE structures and sizes indicated by observations, it appears of major importance to identify what controls the stability, growth, and decay of FTEs.

- Early studies have focused to identify classical signatures of stationary reconnection. Often such signature and FTEs are considered as different modes of reconnection but this is by no means clear. Just based on convection any X line or reconnection patch can be expected to be blown off the dayside magnetopause within 10 to 20 minutes such that
reconnection has to reform at least on this time scale. However, any newly formed reconnection site will develop at least initially a jet that should satisfy conditions of steady state models. Therefore, we need to resolve the relation between FTEs and so-called stationary reconnection.

- Early studies (e.g., Papamastorakis et al. 1989) have demonstrated that a number of FTEs satisfy the conditions of stationarity (de Hoffmann-Teller frame) and have an Alfvénic character (Walén relation) (see also Papamastorakis et al. 1989). This seems an important tool to study the character of FTEs and their evolution. Do all FTEs satisfy these conditions?

- As outlined, magnetosheath waves and other perturbations (KH, impulsive events, etc.) can couple to kinetic Alfvén waves at the magnetospheric boundary and these waves can cause mass transport and heating. However, it is not clear how important this effect is for instance compared to mass transport associated with reconnection.

- The same question applies to other kinetic or two fluid effects. For instance, the thin boundaries that develop in KH waves have been demonstrated to allow for enhanced diffusion. What are the conditions where this mass diffusion (or transport in filamentary structures) becomes the dominant aspect of the KH wave associated transport?

# 4 Geoeffects of Dayside Transients

## 4.1 Effects in the Magnetosphere

Transient solar wind structures associated with sudden changes (either positive or negative) in the dynamic pressure compress or inflate the Earth's magnetosphere. The geomagnetic fields observed on the ground consequently increase or decrease. The magnetopause current intensifies when a positive impulse impinges on the magnetosphere. ULF waves can be excited in the inner magnetosphere as a result of the interaction between the Earth's magnetic field and solar wind pressure impulses (Zong et al. 2009, 2017; Zong 2022; Zhang et al. 2010). Furthermore, these waves can accelerate energetic particles (Zong et al. 2007, 2009, 2012).

The pressure perturbations associated with foreshock transients typically have spatial scales of a few $R_E$ but can be significantly larger in magnitude than the solar wind dynamic pressure pulses. They in turn cause large localized deformation of the magnetopause. For example, the dynamic pressure inside HFAs is lower than the ambient solar wind due to the density depletion and flow deflection. The passage of HFAs will therefore result in local negative pressure impulses. The depletion of the total pressure in HFAs leads to a local sunward expansion of the magnetopause that has been observed. In their comprehensive study of a single HFA event, Sibeck et al. (1999) reported that the magnetopause moved 5 $R_E$ outward in response to the density decrease accompanying an HFA. Similarly, foreshock bubbles and foreshock cavities may also have significant impacts on the magnetosphere due to the dynamic pressure variations in these structures.

Foreshock transients such as HFAs impact the magnetosphere in several ways. They can transmit compressional fast magnetosonic waves into the magnetosphere that can excite resonant ULF waves (Eastwood et al. 2011; Zhao et al. 2017; Shen et al. 2018), lead to plasma flow vortex structures and reconfiguration of the magnetotail, cause particle acceleration or scattering into the loss cone and precipitating into the ionosphere, generate field-aligned currents in the magnetosphere that drive magnetic impulse events in the high-latitude ionosphere (Eastwood et al. 2008), and trigger transient auroral brightenings (Sibeck et al. 1999; Fillingim et al. 2011).
Magnetospheric ULF waves have received considerable attention recently because observations have shown that ULF waves are capable of controlling and accelerating radiation belt electrons (e.g., Elkington et al. 1999; Zong et al. 2007, 2008, 2009; Mann et al. 2013), ring current ions (e.g., Yang et al. 2011; Zong et al. 2012; Ren et al. 2016), and cold plasmaspheric electrons (e.g., Zong et al. 2017; Ren et al. 2017, 2018, 2019). The shear Alfvén mode ULF waves, with frequencies comparable to the eigenfrequencies of the geomagnetic field lines, develop into standing waves as a consequence of superposition of incident waves and reflected waves from the Earth’s ionosphere. A characteristic feature of standing waves is a 90° phase difference between the transverse components of the magnetic and electric fields in the local mean-field-aligned coordinate system (Singer et al. 1982; Takahashi et al. 2011; Le et al. 2017).

The magnetic gradient and curvature drift motion of charged particles is aligned with the azimuthal electric field component of poloidal mode ULF waves, allowing fast acceleration of energetic particles to occur in the magnetosphere (e.g., Zong et al. 2011, 2017). When a particle’s drift velocity equals the wave phase velocity in the azimuthal direction, it experiences a constant electric field and gains energy from or loses energy to waves (depending on the direction of the wave electric fields), which is a process known as drift resonance (Southwood and Kivelson 1981). Zong et al. (2009) found that electron fluxes in the 30-300 keV energy range enhanced several times within an hour due to continuous and dramatic acceleration through drift resonance with fundamental-mode ULF waves. Relativistic electrons can be accelerated by low-$m$ ULF waves. Hao et al. (2019) showed that ULF waves induced by an interplanetary shock impinge on the Earth’s magnetosphere can efficiently accelerate ultrarelativistic electrons up to 3.4 MeV in the outer radiation belt. Numerical simulations confirmed the adiabatic acceleration and transport processes of outer radiation belt electrons through drift resonance (Elkington et al. 1999; Wang et al. 2018d) and revealed the possibility of locally growing phase space density peaks produced by bursts of narrow-band ULF waves (Degeling et al. 2008).

For ring current ions and cold plasmaspheric electrons, their bounce motion should be taken into consideration during the wave-particle interactions because their bounce frequency is comparable to the wave frequency (Southwood and Kivelson 1981; Zong et al. 2017). A more complicated process, drift-bounce resonance, occurs under the resonant conditions associated with harmonic mode ULF waves (Ren et al. 2016, 2017; Liu et al. 2020g). Due to their mass difference, the drift-bounce resonance condition for ring current oxygen ions is much easier to satisfy than that for the protons (Zong et al. 2012; Ren et al. 2016; Zong et al. 2017), providing a potential explanation for the stronger $O^+$ acceleration and flux enhancement during storms. Recently, interactions between ULF waves and cold plasmaspheric populations have been investigated. Observations have demonstrated that ULF waves play a crucial role not only in the energization and transport of cold electrons and ions (Yue et al. 2016; Ren et al. 2017, 2018, 2019; Liu et al. 2019d), but also in the spatial and temporal dynamics of various waves (e.g. hiss, chorus, and kinetic Alfvén waves), as well as the subsequent wave-particle interactions due to the modulation of the plasma density and the stronger anisotropy in energetic particle distributions caused by ULF waves (e.g., Breneman et al. 2015; Malaspina et al. 2015; Li et al. 2011).

### 4.1.1 Generation of ULF Waves in the Magnetosphere by Dayside Transients

Foreshock transients are potential energy sources for magnetospheric ULF waves because the localized pressure perturbations introduce seed fluctuations on the magnetopause boundary, which can trigger/generate various types of waves. It has long been known that solar
<figure><img src="image_61.png" /><figcaption>**Fig. 56** Illustration of the poloidal mode ULF waves excited by a positive/negative solar wind impulse at 12:00 LT, 6:00 LT, and 00:00 LT at geosynchronous orbit. The arrows schematically show the drift motion direction of ions (solid) and electrons (dashed) from midnight. (After Zhang et al. 2010)</figcaption></figure>

wind pressure pulses excite ULF waves in the magnetosphere. For example, Fig. 56 illustrates poloidal mode waves excited by a positive (blue) and negative (red) solar wind impulse at different local times at geosynchronous orbit (Zhang et al. 2010). However, it has only been in the last decade or so the roles of foreshock transients in the generation of magnetospheric ULF waves started to gain attention. Several studies provided direct observational evidence that foreshock transients generated magnetospheric ULF waves, in either Pc 3-4 or Pc 5 bands. However, theoretical and numerical modeling work focused on foreshock transients as the wave source is still largely lacking. The wave generation mechanisms have not been fully explored.

It has long been well known that the main source of energy for magnetospheric Pc 3-4 waves are upstream waves generated by resonant interaction between backstreaming ions in the foreshock and the incoming solar wind. The causal relationship was established by the observed correlations between the wave occurrence and the IMF direction, as well as between the wave frequency and the IMF strength (e.g., Russell and Hoppe 1981; Le and Russell 1992, 1996). However, there is a subset of Pc 3-4 waves whose frequencies are not correlated with the IMF strength, implying other mechanisms are at work.

Eastwood et al. (2011) first reported observational evidence of transient Pc 3 waves generated by an HFA in the absence of significant upstream waves in the same frequency band. They suggested that the HFA created a localized perturbation of the magnetopause because of large variations in the dynamic pressure. Compressional waves could be generated as the perturbation interacted with the magnetopause and subsequently coupled into the magnetosphere.

Recent observations (Zhao et al. 2017) presented further evidence of Pc 3 waves generated by a large HFA. In their multiple spacecraft observations as shown in the schematic in Fig. 57, a large HFA was observed upstream from the bow shock on the dawn side, and long-lasting, nearly monochromatic Pc 3 waves were generated in the magnetosphere immediately after the HFA passage and propagated dawn-to-dusk. Coordinated observations from ground-based magnetometers in Canada (noon), Alaska (dawn), and Japan (nightside) confirmed the occurrence of the Pc 3 waves and their day-to-night propagation direction.
<figure><img src="image_62.png" /><figcaption>**Fig. 57** Schematic graph of an HFA generating global ULF waves in the magnetosphere (credit: Peking University)</figcaption></figure>

Several studies also show that foreshock transients can generate ULF waves in the Pc 5 band. Hartinger et al. (2013) presented the first observation of global Pc 5 wave activities in response to a high-speed solar wind interval associated with three types of foreshock transients, including HFAs, foreshock bubbles, and the motion of foreshock compressional boundaries. In their observations, global Pc 5 waves were excited, and the strongest waves occurred at the local time correlated with the location of the ion foreshock with amplitudes as large as 10 nT and comparable to storm-time wave activities. They concluded that foreshock transients could be an important energy source for global Pc 5 waves in the magnetosphere.

In a recent study by Shen et al. (2018) using both in-situ and ground-based observations, localized Pc 5 wave activities were reported in response to a foreshock transient event, in which there is a dawn-dusk asymmetry of the wave activity and Pc 5 waves were present only in the same local time sector as the foreshock transient was. Previous statistical studies have shown that the occurrence of Pc 5 waves exhibits a distinct dawn-dusk asymmetry, which was attributed to KH instabilities at the magnetopause (e.g., Chisham and Orr 1997). This new observation demonstrated that foreshock transients could also be an important energy source for localized Pc 5 wave activities with dawn-dusk asymmetry.

In addition to the aforementioned studies, Hietala et al. (2012) reported irregular ULF wave activities in response to the effects of pressure pulses associated with localized magnetosheath jets. These magnetosheath jets originated from the quasi-parallel bow shock under steady solar wind conditions. They found that the jets caused irregular pulsations at geosynchronous orbit. Additionally, Gillis et al. (1987) found that there was a correlation between FTEs and transversely polarized Pc 4 pulsations. A case study by Glasmeier et al.
(1984) suggested that an FTE triggered Pc 5 pulsations. Recent observations by Bentley et al. (2018) also showed that FTEs can be a possible source of magnetospheric ULF waves.

Although there are many theoretical and numerical simulation studies attempting to understand the interaction of solar wind pressure pulses with the magnetosphere (e.g., Zhu and Kivelson 1988; Lee and Lysak 1989), mechanisms for wave excitation by localized pressure perturbations have not been fully explored. The pressure perturbations associated with foreshock transients are generally localized and moving along the magnetopause whereas solar wind pressure pulses interact with the entire magnetopause. Lin and Wang (2005) simulated the linkage between foreshock waves that involve self-consistently generated foreshock cavities and ULF waves in the magnetosphere using a 3D global-scale hybrid simulation for a case with the radial IMF. The simulation domain contains the dayside bow shock/foreshock, magnetosheath, and the magnetosphere, with the foreshock waves generated self-consistently in the interaction between the solar wind and the magnetosphere. Foreshock cavities are generated in the turbulent quasi-parallel shock, and they subsequently interact with the magnetopause as pressure pulses. The coupling between the pressure pulses/compressional waves and the magnetosphere leads to the mode conversion to shear Alfvén waves and kinetic Alfvén waves. As a result, field line resonances corresponding to the fundamental odd resonance wave number are found to be generated in the magnetosphere in this case in which the source of perturbations is near the nose of the magnetopause. Shi et al. (2021) illustrated the properties of self-consistently generated field line resonances through direct mode conversion in magnetospheric response to the foreshock disturbances (see Sect. 3.3.2 for details). In addition, based on the 3D global hybrid simulation, Wang et al. (2009) has shown a connection between the foreshock waves and the ULF waves in the cusp. Further simulations are required to fully understand the ULF waves in the magnetosphere associated with the foreshock structures.

### 4.1.2 Responses of Tail Plasma Sheet

FTEs can essentially transfer geomagnetic flux from dayside to nightside and contribute to accumulation of magnetic energy in the magnetotail, which forms the basis of activities in magnetospheric substorms or the Dungey cycle, and thus can be geoeffective. Their overall significance, however, depends largely on how FTEs are generated (Sect. 3.2) and how FTE flux tubes interact with each other during the course of their tailward transport. This is because what matters from the viewpoint of geoeffectiveness is the total amount per unit time of open flux transferred to the tail, which depends on the duration or continuity of magnetopause reconnection, extent and number of the relevant X-line, and magnetic topology of FTE flux tubes. All these factors are different for different models of FTE formation and interaction (e.g., Fear et al. 2017; Hasegawa et al. 2010; Øieroset et al. 2019), and it is not until recently that the interaction processes have been revealed using the state-of-the-art MMS observations (e.g., Kacem et al. 2018). Thus, the geoeffect of FTEs needs to be further explored in the future not only using high-resolution in situ measurements but also in conjunction with low-altitude, ground-based, and/or imaging observations, while FTEs at Mercury appear to be significant enough to drive the substorm cycle in the Hermean magnetosphere (Fear et al. 2019).

The magnetopause KHI could also be geoeffective through its role in the mass and momentum transport of the solar wind into the plasma sheet especially under northward IMF. In particular, Sorathia et al. (2019) demonstrated, based on a global MHD model combined with test particle simulation, that the particle transport rate ~$10^{26}$ s⁻¹ across the flank magnetopause resulting from the KHI is consistent with observations. They further showed that
lower entropy flux tubes created by the KHI can be subject to interchange instability, a possi-
ble mechanism of filling the central plasma sheet with cold magnetosheath plasma. It should
also be pointed out that weak sunward streams of cold and dense plasma in the plasma sheet,
as observed by Geotail (Fujimoto et al. 1998), cannot be explained by high-latitude mag-
netopause reconnection and resultant convection alone (Song et al. 1999; Hasegawa 2012).
They may result from KHI-induced interchange instability as reported by Sorathia et al.
(2019) or turbulent flow as simulated by El-Alaoui et al. (2012).

We note that geomagnetic storms preceded by an extended period of northward IMF tend to be more intense than those with no such preceding northward IMF (Lavraud et al. 2006a), indicating the importance in the storm development of the cold-dense plasma sheet formed under northward IMF. Lavraud and Borovsky (2008) also show that coronal mass ejections, one of the drivers of storms, can produce magnetosheath conditions favorable for the KHI growth through their interaction with the magnetosphere. Thus, it is possible the KHI indirectly contributes to an intensification of one of the most geoeffective phenomena via the formation of the dense plasma sheet.

Sibeck (1990) proposed a model for the interaction between a dynamic pressure pulse and the magnetosphere based on the force balance between the transient solar wind and the magnetopause/magnetosphere, in which a single or double flow vortex inside the magnetosphere near the magnetopause was predicted. The ionospheric counterpart of the vortices was observed using SuperDARN radar and ground magnetometers (e.g., Sibeck et al. 2003). Global MHD simulations showed this single vortex in the dawn and dusk magnetosphere (Wang et al. 2010; Samsonov et al. 2010; Samsonov and Sibeck 2013). Samsonov and Sibeck (2013) interpreted this vortex formation as the compressional wave penetration and reflection in the magnetosphere boundaries. Shi et al. (2014) reported the first in-situ observations of a vortex related to a sudden increase of solar wind dynamic pressure. During this event, THEMIS-B, C, and D, located near the dusk flank of the magnetopause, observed an anti-clockwise rotating vortex. Equivalent ionospheric currents (EICs) derived from the THEMIS ground magnetometer array show a vortex with counterclockwise rotation around the foot points of THEMIS B, C, and D. Zhao et al. (2016b) reported an event during which a negative solar wind dynamic pressure change also induced a flow vortex inside the magnetosphere. By comparing observations from Zhao et al. (2016b) and Shi et al. (2014), one can find that the rotation sense of the vortex induced by a negative pressure pulse is opposite to that by a positive pressure pulse.

A global picture is illustrated in Fig. 58. A change of the solar wind dynamic pressure generated two vortices, one on the dawn side and the other on the dusk, and these vortices propagate toward the tail in line with the pressure change front motion of the solar wind. The rotation sense is opposite for the dawn and dusk (clockwise on the dawn side and counterclockwise on the dusk for the pressure increase situation, and opposite for the pressure decrease situation). The two vortices can be connected to the ionospheric vortices by field-aligned currents (FACs). The FACs are in line with the region I FAC sense for the pressure increase situation (downward on the dawn side and upward on the dusk), and consistent with the Region II FAC sense for the pressure decrease situation (upward on the dawn side and downward on the dusk). The vortex is related to ULF waves and auroras sometimes. Considering the recent studies by Tian et al. (2016) and Ozturk et al. (2019), this global picture could also be revised and become complicated.

The magnetotail response to foreshock transients is largely uncharted territory. Below we show an example of the magnetotail response to an extreme HFA reported by Zhang and Liu (submitted to JGR). The HFA with extremely low solar wind dynamic pressure inside caused the local bow shock to move outward, leading to the magnetopause outward motion
<figure><img src="image_63.png" /><figcaption>**Fig. 58** An illustration of the magnetospheric responses (vortex, ULF wave and FAC) for a) positive and b) negative pressure pulses (from Shi et al. 2020b)</figcaption></figure>

by several $R_E$ as observed by the THEMIS spacecraft (Zhang and Liu, submitted to JGR). The Cluster spacecraft in the magnetotail monitored the corresponding magnetotail response (Fig. 59).

Characterized by sharp changes in the plasma temperature, density, and magnetic field (Fig. 59), Cluster rapidly entered and exited the center of the magnetotail plasma sheet. The possible process is that due to the low dynamic pressure associated with the extreme HFA, the magnetopause moved outward significantly, resulting in the fast expansion of the magnetotail. As the HFA passed by, the magnetopause recovered/moved backward, and the magnetotail correspondingly contracted. As a result, Cluster, steadily located in the dawnside magnetotail, temporarily observed the magnetotail plasma sheet. Using the timing method with all four Cluster spacecraft observations, the expansion speed of the magnetotail was determined as ~50 km/s.

Moreover, the Cluster spacecraft also observed a plasma flow vortex structure in the magnetotail plasma sheet as seen from the rotation of the plasma flow $V_x$ component (yellow shaded in Fig. 59). It could be generated by the inhomogeneous dynamic pressure associated with the extreme HFA impinging on the magnetosphere. Such a vortex structure as well as the expansion and contraction of the magnetotail could excite ULF waves contributing to further geoeffects. Therefore, it is important to understand the magnetotail response to foreshock transients. Additionally, foreshock transients can propagate to the midtail foreshock and disturb the local bow shock, magnetosheath, and magnetopause (Wang et al. 2018a, 2020b; Liu et al. 2020e), which can also potentially result in magnetotail disturbances.

## 4.2 Effects in the Ionosphere and Thermosphere

### 4.2.1 Ground Magnetic Response

Long prior to the beginning of the space age, researchers employed ground magnetometers to infer the occasional occurrence of abrupt increases in the solar wind dynamic pressure that caused simple step-function enhancements of the magnetic field strength on the surface of the Earth on a global basis (Chapman and Ferraro 1931). Signatures at higher latitudes near the auroral oval were found to be more complicated, often indicating bipolar north/south variations (Matsushita 1957). The complication arises because the signatures at higher latitudes result from both fast and intermediate mode waves transmitted into the magnetosphere.
<figure><img src="image_64.png" /><figcaption>**Fig. 59** An overview plot of Cluster observations showing the magnetotail response to an extreme HFA. From top to bottom are the total pressure (the sum of the thermal pressure and the magnetic pressure), the magnetic field in GSE coordinates, the magnetic field strength, the plasma velocity in GSE coordinates, the perpendicular and parallel ion temperature, and the ion density. The time interval of the HFA observed by THEMIS is marked by two vertical lines, and the yellow shaded region indicates a vortex-like plasma structure. (Zhang and Liu, submitted to JGR)</figcaption></figure>

The fast mode waves travel across magnetic field lines while the intermediate mode waves travel along magnetic field lines (Wilson and Sugiura 1961). With the help of extended arrays of ground magnetograms, it became clear that these magnetic impulse events could often be organized into azimuthally traveling convection vortices (Friis-Christensen et al. 1988).

Analytical models successfully predicted the organization of the magnetometer event signatures into single or pairs of vortices (Southwood and Kivelson 1990; Glassmeier and Heppner 1992; Sibeck et al. 2003). Zhu et al. (1997) simulated the ground magnetic field signatures of traveling convection vortices including both ionospheric conductivity enhancements and ground induction effects. They found that including the localized conductivity enhancement can cause a significant distortion of the current system and consequently the
ground magnetic field disturbance. Events with enhanced hard precipitation are likely to exhibit this kind of distortion. As predicted by the models, events are conjugate in both hemispheres when there are no asymmetries in the ionospheric conductivity or magnetic field, yet asymmetries in the ground magnetic response can occur when these conditions are not met (Kim et al. 2015). Hartinger et al. (2017) compared the inter-hemispheric responses seen during one of the Kim et al. (2015) events with the predictions of a series of simulations with different conductivity profiles and magnetic field topologies. They found that asymmetries (e.g., in amplitude) varied with the conductivity/distortions in the magnetic field.

The dynamic pressure variations associated with transient events created by kinetic effects within the Earth's foreshock are equal to or greater than those associated with intrinsic solar wind dynamic pressure variations, and far more common, albeit less spatially extended. Consequently, one might expect a large fraction of the largest amplitude perturbations seen in both high- and low-latitude ground magnetograms to be associated with foreshock events. In the absence of in situ measurements made within the foreshock itself, the occurrence patterns of the events seen in ground magnetometers should reflect those for foreshock transient events, i.e. they should occur during intervals of low solar wind density and dynamic pressure, high solar wind velocity, be associated with intervals of radial IMF orientation that place the foreshock upstream of the dayside magnetopause (Sibeck et al. 1989a,b; Fairfield et al. 1990) or in conjunction with abrupt changes in the IMF orientation that lead to the formation of hot flow anomalies. Case and statistical studies reveal the association between the ground magnetometer events with abrupt changes in the IMF orientation and the fact that they attain largest amplitudes and tend to occur prior to local noon (Sibeck and Korotova 1996). With the help of in situ measurements of the solar wind within the foreshock, it quickly became clear that perturbations associated with kinetic events in the foreshock drive the vast majority of traveling convection vortices (Murr and Hughes 2003), precisely as does the association of transient events at high latitudes on the ground with sudden impulse signatures at lower latitudes and geosynchronous orbit (Sibeck 1993).

### 4.2.2 Riometer Response

Transient compressions of the dayside magnetosphere such as those associated with fore-shock transients adiabatically energize magnetospheric particle populations, resulting in distributions that peak more strongly at pitch angles nearly perpendicular to the magnetic field. Just as in the case of the compressions caused by intrinsic solar wind pressure variations (Perona 1972), the resulting ion and electron distributions are unstable to the growth of EMIC (for ions) or ELF/VLF (for electrons) plasma waves that can scatter the particles and cause their precipitation into the ionosphere. Precipitating electrons with energies greater 30-300 keV enhance ionospheric densities, making the ionosphere opaque to the arrival of cosmic noise. Consequently, we expect the transient events seen in high latitude dayside ground magnetograms associated with magnetospheric compressions produced by foreshock kinetic events to exhibit equally transient increases in cosmic noise absorption. Korotova et al. (1999) reported the results of a statistical study of cusp latitude South Pole Station observations indicating that this is precisely the case. About 80% of 153 magnetic impulse events were associated with a small enhancement in cosmic noise absorption. Like magnetic impulse events, the occurrence rate of events with enhanced absorption exhibits a large peak in the pre-noon ionosphere and a smaller peak in the post-noon ionosphere.
### 4.2.3 Aurora Signatures

#### Auroras associated with foreshock transients

Dayside transients associated with dynamic pressure changes can compress or expand the magnetosphere, which may enhance or weaken the scattering of magnetospheric particles into the ionosphere and thus modify the aurora intensifications. Large scale interplanetary transients like shocks or positive/negative dynamic pressure changes can invoke a global response of auroras (e.g., Tsurutani et al. 2001; Zhou et al. 2003; Sato et al. 2001; Liou et al. 2006). Unlike those large-scale solar wind changes, foreshock transients are localized and short-lived. Hence the aurora response to them is not frequently detected. Using Polar Ultraviolet Imager, Sibeck et al. (1999) reported that an aurora brightening following an HFA appeared first on the afternoon side and then on the morning side. The different auroral luminosity in the prenoon, local noon and postnoon sectors suggests the deformation of the dayside magnetosphere caused by the HFA. Observations of traveling convection vortices and pre-noon auroral brightening due to an HFA were also reported by Fillingim et al. (2011).

By using multi-satellite observations, Shen et al. (2018) studied an event of foreshock transient-generated ULF waves with a clear dawn-dusk asymmetry in the wave power. Wang et al. (2018c) presented duskward propagating auroras due to this foreshock transient from high-resolution 2D imaging of all-sky imager at South Pole on the dayside. A schematic plot is shown in Fig. 60. The field-aligned mapping of the aurora imaging into the magnetosphere was used to determine the size (a few $R_E$ in Y) and propagation (duskward) of the magnetopause disturbance caused by the foreshock transient. Equivalent currents deduced from ground magnetometers show a pair of FACs and the duskward propagation is consistent with that of the auroras.

#### Auroras associated with KH waves

In general it is difficult to establish a 1 to 1 identification of an auroral signature with a corresponding event that is observed at the magnetospheric boundary because of the rather small size of the corresponding ionospheric footprint of the event, proper timing, and uncertainties in the mapping of the magnetic field from the magnetopause to the ionosphere. A well established theoretical effect of magnetopause processes is their influence on global magnetospheric convection which is identified through radar and low altitude satellite observations and measured in terms of a cross polar cap potential. This potential responds most strongly to magnetic reconnection for southward IMF $B_z$ with typical values of 50 to 100 kV while viscous interaction believed to be caused mostly by KH boundary waves is typically 10 to 20 kV for northward $B_z$. Here we focus on auroral signatures associated with KH waves. Auroras associated with FTEs are discussed in Sect. 4.2.4 since they are tied to ionospheric flow channels.

It has long been suspected that Pc 5 waves on the dayside might be related to KH waves at the magnetospheric boundary (Walker and Greenwald 1981). Lui et al. (1989) interpreted initial observations of bright auroral spots in the dayside afternoon sector as signatures of KH waves from the low latitude boundary layer, and Rostoker et al. (1992) provided further evidence of this interpretation through the associated observation of Pc pulsations and quantitative estimates. Similarly Farrugia et al. (1994) concluded that the most likely explanation of the auroral forms are KH waves. Based on numerical simulation Wei and Lee (1993) demonstrated quantitative consistency of these waves with KH waves and illustrated that the associated field-aligned current of the waves is upward in the afternoon sector consistent with downward electron precipitation. In the morning sector, spatially quasiperiodic plasma and electromagnetic signatures, as well as pulsating auroral arcs accompanied by pulsations
<figure><img src="image_65.png" /><figcaption>**Fig. 60** The impact of one foreshock transient on the Earth's magnetosphere as seen by multi-satellites. Fore-shock transients generated localized compressional waves with clear dawn-dusk asymmetry via disturbing the magnetopause within a small spatial size, generating localized field line resonances, field aligned current and moving aurora signatures. (After Shen et al. 2018; Wang et al. 2018c)</figcaption></figure>

mostly in the Pc 5 range also map to the flank boundary layer and have been associated with KH waves as the most likely source (Hirahara et al. 1998; Famiglia et al. 2000). Morning side pulsations were also found by Ohtani et al. (1999) but they interpreted the origin as KH waves at the magnetopause rather than the inner edge of the boundary layer. For northward IMF, Polar UVI observations demonstrate that auroral intensity varies linearly with solar wind ram pressure indicating that viscous coupling likely through KH waves increases with solar wind velocity (Zhou and Tsurutani 2004). Similar properties were found when inter-planetary shocks or solar wind pressure pulses cause fast moving auroral streaks or rays at velocities consistent with the propagation of KH waves at the magnetopause (Zhou et al.
2017b). In terms of ground magnetic field perturbations, Weigel et al. (2003) demonstrated that their amplitude is primarily determined by magnetopause reconnection while their time derivative is mostly caused by KH waves at the magnetospheric boundary.

### Throat aurora

During a study on dayside diffuse aurora based on optical observations in the green line (557.7 nm), Han et al. (2015) noticed that, when a convection-aligned stripy diffuse aurora was contacting with the discrete aurora oval, a north-south-aligned discrete auroral form was often observed. This particular auroral form was named “throat aurora”, because it was only observed around the ionospheric convection throat region. It has been confirmed that throat auroras are caused by precipitation of magnetosheath particles (Han et al. 2016) and are the ionospheric signature of magnetopause indentations (Han et al. 2018). Han et al. (2017a) reported that the spatial scale of the magnetopause indentation can be as large as ~2.0-3.0 $R_E$ after mapping a throat aurora to the geomagnetic equatorial plane and the daily occurrence rate of throat aurora is higher than ~50%, which implies that magnetopause indentations with considerable spatial scales should commonly exist on the subsolar magnetopause.

While considering the generation of throat auroras, Han et al. (2017a) noticed that throat auroras were closely involved with diffuse aurora and their orientations were convection-aligned. Because both diffuse aurora and convection are linked to magnetospheric internal phenomena, the occurrence of throat aurora was suggested to be affected by factors inside the magnetosphere, such as the ionospheric feedback effect. At the same time, Han et al. (2017a) noticed that the occurrence rate of throat aurora showed a clear dependence on the IMF cone angle, which suggested that some transient processes outside the magnetosphere, such as magnetosheath jets, might be a driver for throat auroras (Han et al. 2017a; Plaschke et al. 2018a). In order to explain why the generation of throat aurora depends on factors either inside or outside the magnetosphere, Han (2019) proposed a model that is schematically illustrated in Fig. 61. The model suggests that precipitation of a stripy diffuse aurora can lead to ionospheric conductivity enhancement and thus produce a polarization electric field in the ionosphere. After this electric field mapping to the magnetosphere along closed field lines, it may guide a magnetopause reconnection to develop inward the magnetosphere and result in a throat aurora. Some observational supports for the model were presented in Han (2019). This model can explain the occurrence dependence of throat aurora on either inside or outside factors, because two types of diffuse auroras existing near magnetic local noon are related with inside and outside factors, respectively (Han et al. 2017b). Furthermore, using coordinated observations from THEMIS and all-sky camera observations at South Pole station, Wang et al. (2018b) showed that magnetosheath jets were well associated with the localized diffuse auroral brightening observed at South Pole station near local noon. This provides further support to the model of Han (2019). According to this model, the outside factors (i.e., impulsive events in the magnetosheath like magnetosheath jets) may not drive throat aurora directly, but produce diffuse aurora first and then affect the magnetopause reconnection and result in throat aurora. The cause of throat aurora is likely strongly enhanced magnetopause reconnection forced by foreshock or magnetosheath transients.

### 4.2.4 Ionospheric Flow Response

#### Ionospheric flow response to foreshock transients

Dayside foreshock transients, such as HFAs, can also trigger ionospheric flow response. Sitar et al. (1998) identified traveling convection vortices (TCVs) in the ground magnetometer data that were associated with the field-aligned currents and propagating tailward. Meanwhile, incoherent scatter radar observed strong modulation in the ionospheric plasma flow
<figure><img src="image_66.png" /><figcaption>**Fig. 61** A conceptual model of throat aurora. The red circle over the north pole indicate the discrete aurora oval. A throat aurora in north-south direction bulge out from the discrete aurora oval toward low latitude near local noon. A stripy diffuse aurora is indicated by the green region, which will cause a polarization electric field *E*<sub>p</sub>. The model suggests that this electric field can map to the magnetosphere and affect the magnetopause reconnection to produce the throat aurora. (From Han 2019)</figcaption></figure>

strength and direction. POLAR UVI observed a localized intensification of aurora emissions due to an upward field-aligned current. DMSP observations indicated that the field-aligned currents that drove TCVs were mapped to the boundary plasma sheet-low latitude boundary layer interface. Based on the coordinated set of observations, Sitar et al. (1998) suggested that the driver was the magnetopause deformation caused by an HFA (Sibeck et al. 1998).

Later observations confirmed this scenario. Using multi-point THEMIS observations, Jacobsen et al. (2009) demonstrated significant magnetopause outward deformation caused by an HFA, which propagated tailward. TCVs were observed by ground magnetometers. Using THEMIS GMAG and POLAR UVI observations, Fillingim et al. (2011) showed a TCV and auroral brightening driven by an HFA that was detected by THEMIS spacecraft. Similarly, Wang et al. (2018c) also reported observations of TCVs and auroral brightening driven by a foreshock transient. Additionally, Liu et al. (2020e) showed a TCV which might be driven by an HFA in the nightside midtail foreshock.

## Ionospheric flow response to FTEs

An important mechanism for transfer of flux from the solar wind to the magnetosphere is impulsive dayside reconnection and FTEs (Cowley and Lockwood 1992; Lockwood et al. 1995). FTEs were first discovered by Haerendel et al. (1978) and Russell and Elphic (1978, 1979). FTEs have a typical scale size of one *R*<sub>E</sub> in the boundary normal direction at the magnetopause (Saunders et al. 1984). In the ionosphere that corresponds to around 100-
200 km along the meridian (Southwood 1985, 1987). A reconnection burst propagates from the magnetopause to the ionosphere as an Alfvénic disturbance with a system of associated field-aligned currents (Glassmeier and Stellmacher 1996).

In the dayside aurora poleward-moving transients have often been interpreted as FTE signatures (Sandholt et al. 1990, 1993; Denig et al. 1993; Moen et al. 1995; Milan et al. 1999, 2000; Thorolfsson et al. 2000). The poleward moving auroral forms (PMAFs) can occur for both southward and northward IMF although FTEs are strongly related to southward IMF conditions, which indicates that FTEs may not have a simple 1 to 1 correspondence to PMAFs (Fasel 1995). In the northern hemisphere the PMAFs move west (east) for positive (negative) IMF $B_y$ (Sandholt et al. 1998, 2004a). Milan et al. (1999) and Thorolfsson et al. (2000) associated PMAFs with plasma flow. Later, Oksavik et al. (2004) used the EISCAT Svalbard Radar to identify the relative location of PMAFs and their associated flow channels.

The first ionospheric radar observations of FTE signatures were made several decades earlier by Van Eyken et al. (1984) and Goertz et al. (1985). In the following decade there were frequent reports of poleward-moving transients in literature, which depending on their specific characteristics, were called flow channel events (FCEs) (Pinnock et al. 1993, 1995; Chisham et al. 2000; Neudegg et al. 1999, 2000), pulsed ionospheric flows (PIFs) (Provan et al. 1998, 2002; Provan and Yeoman 1999; McWilliams et al. 2000), or poleward-moving radar auroral forms (PMRAFs) (Milan et al. 2000; Davies et al. 2002; Rae et al. 2004). These dayside transients are often related to each other (Wild et al. 2001) and show repetition rates (e.g., Milan et al. 1999) that are comparable to FTEs at the magnetopause. Neudegg et al. (2000) pointed out a good correspondence between FTEs at the magnetopause and discrete flow channels in the ionosphere. Provan et al. (1998, 2002) investigated the pulsed plasma flows poleward of the convection reversal boundary.

In an effort to summarize the different categories of plasma flow, Sandholt et al. (2004b) listed three key flow channels in the daytime aurora: (1) enhanced sunward return flow on closed field lines (Lockwood et al. 1993; Moen et al. 1995, 2006), (2) enhanced flow on newly open flux containing FTEs (FCEs and PIFs), and (3) enhanced flow on old open field lines due to the solar wind magnetosphere dynamo in the high-latitude boundary layer (Stern 1984; Sandholt et al. 2004b; Farrugia et al. 2004; Sandholt and Farrugia 2007). All three categories give enhanced flow in the same direction as the anti-sunward large-scale plasma flow in the polar cap.

Afterwards, a fourth category of plasma flow was added (Rinne et al. 2007; Moen et al. 2008; Oksavik et al. 2011). Rinne et al. (2007) named it a reversed flow event (RFE), because the enhanced flow was observed in the opposite direction of the general background convection, i.e. it had the effect of reversing the flow. This work was based on a series of publications that used the European Incoherent Scatter (EISCAT) Svalbard Radar (ESR) in fast scan modes to investigate flow channels in the cusp region (Carlson et al. 2004; Oksavik et al. 2004, 2005; Rinne et al. 2007, 2010; Moen et al. 2008). Rinne et al. (2007) defined the flow speed inside the RFE to be > 250 m/s for the event to qualify as an RFE. The rationale was to highlight events that clearly stood out from the background convection. Moen et al. (2008) reported that RFEs can last for 10-20 min. Rinne et al. (2007) observed RFEs 40% of the time within one hour of magnetic noon, with no apparent preference for IMF $B_y$ or $B_z$ polarity, but with a clear preference for clock angles between 40° and 240° (i.e. $|B_y| > |B_z|$). Oksavik et al. (2004, 2005) pointed out that clockwise vorticity is consistent with an upward Birkeland current (i.e., intense aurora) on one side of a flow channel, while counterclockwise vorticity is consistent with a downward Birkeland current (i.e., weak aurora) on the other side of the flow channel. Moen et al. (2008) pointed out that the RFE phenomenon is not uniquely related to PMAFs but appears to be a specific feature of Birkeland Current
Arcs (BCAs). Moen et al. (2008) suggested that the RFEs may be generated in two ways: (1) the RFE channel can be a region where two magnetosphere-ionosphere (MI) current loops, forced by independent voltage generators, couple through a poorly conducting ionosphere, or (2) the RFE channel can be the ionospheric footprint of an inverted-V-type coupling region. It remains unclear if one dominates, or if both mechanisms are closely related.

Dayside transients are known to cause significant space weather effects (Moen et al. 2013). Moen et al. (2004) presented a one-to-one relationship between PMAFs and ion up-flow events in the cusp ionosphere. For southward IMF there are also frequent reports of polar cap patches in the polar ionosphere (Crowley 1996; Tsunoda 1988; Basu and Val-ladares 1999; Dandekar and Bullett 1999). Polar cap patches are 100-1000 km wide islands in the horizontal direction (Weber et al. 1984), where the local F-region electron density can be 2-20 times larger than the surrounding plasma (Buchau et al. 1983; Weber et al. 1984; Crowley et al. 2000).

Most patches originate at lower latitudes, where photoionization from solar EUV radiation increases the plasma density (Foster 1993). Three groups of mechanisms have been suggested for formation and structuring of polar cap patches (Moen et al. 2006). The first one is formation via transient reconnection or FTEs (Lockwood and Carlson 1992; Carlson et al. 2002, 2004, 2006), where the open-closed polar cap boundary leaps equatorward to a lower latitude high-density plasma reservoir, followed by a relaxation of the boundary, which transports high-density plasma into the poleward flow. The second one is enhanced plasma depletion between patches due to enhanced recombination of plasma from enhanced ion-frictional or Joule heating due to rapid plasma drift in short-lived east-west flow channels in the cusp ionosphere (Valladares et al. 1996). The third one is changes in the plasma convection pattern, especially due to IMF $B_y$ reversals, that alternate the intake of high density (postnoon) and low density (prenoon) plasma and create isolated polar cap patches that drift into the dayside polar cap (Anderson et al. 1988; Sojka et al. 1993; Rodger et al. 1994). Walker et al. (1999) suggested that the soft particle precipitation in the cusp region may also cause significant ionization that can result in F-region plasma that drift into the polar cap. Oksavik et al. (2006) added that particle precipitation inside lobe convection cells may also lead to the formation of polar cap patches in the polar cap during northward IMF.

For southward IMF the polar cap patches can travel all the way across the polar cap with the large-scale ionospheric convection (Foster 2005; Oksavik et al. 2010; Van Der Meeren et al. 2014). While in transit, the polar cap patches are known to disrupt radio communication and navigation systems (Basu et al. 1990, 1998). Intense PMAF activity can also cause significant phase scintillation and loss of signal lock for satellite navigation signals (Oksavik et al. 2015). Recombination of the high-density plasma leads to weak 630.0 nm emissions that can be observed as airglow patches (Weber et al. 1984; Hosokawa et al. 2014, 2016, 2019). On the nightside the airglow patches leave the polar cap and enter the nightside auroral oval (Lorentzen et al. 2004; Moen et al. 2007; Van Der Meeren et al. 2015). Nishimura et al. (2014) also followed a PMAF/airglow patch that moved across the polar cap from the dayside to the nightside, where it resulted in a poleward boundary intensification (PBI). It shows that dayside transients may ultimately also play a key role in triggering localized nightside reconnection and flow bursts in the nightside plasma sheet.

An open question for all FTE models is the length of the X lines and the amount of magnetic flux actually transported in FTEs. Milan et al. (2016) used optical and SuperDARN data to examine two events to show that their ionosphere extent was very different (2 vs. 7 hours local time). Oksavik et al. (2005) presented a comprehensive set of ground-based, ionospheric, and low-altitude satellite observations surrounding a PMAF, and confirmed that narrow flow channels consistent with small-scale twin convection cells and associated
field-aligned currents were observed along with the PMAF. They estimated that the open magnetic flux transferred by the poleward flow channel is of the order of 1 MWb, which is consistent with a narrow flux bundle created by localized and transient reconnection. Other ground-based observations (Fear et al. (2017); and references therein), however, show that in very extreme cases the flux content of each FTE can be as large as 50 MWb, that is about 10% of the open flux in the polar cap. These observations indicate that the reconnection line responsible for some FTEs is long, and no single FTE formation mechanism can account for a variety of FTEs as observed.

Fear et al. (2017) also argue that when the reconnection line associated with FTEs is extended, the magnetic flux transferred by each FTE should not be estimated as the product of the field component along the FTE flux tube/rope axis and the cross sectional area of the FTE, as often done in the literature (e.g., Slavin et al. 2010). An accurate estimate is in fact given by the product of the field component normal to the magnetopause ($B_n$) within the FTE, the radius of the flux rope ($r$), and the length of the flux rope ($L$): $\Phi = B_n r L$. Fear et al. (2012a) analyzed the axial direction of a series of FTEs observed by the Cluster spacecraft during a magnetopause crossing of high magnetic shear in order to constrain the possible formation mechanisms. They found that the FTE axes are all oriented in the dawn-dusk direction, consistent with the processes involving relatively long reconnection lines (extended single or multiple X line reconnection) rather than localized, transient reconnection.

### 4.2.5 Thermospheric Heating

As noted in Sects. 4.2.1 and 4.2.4, transient compressions of the dayside magnetopause and magnetosphere launch field-aligned currents that carry transient electric fields along the magnetic field lines to the high-latitude dayside ionosphere. Here the currents and electric fields drive traveling convection vortices associated with currents and plasma flow in the ionosphere. The compressions can create or enhance anisotropies in magnetospheric plasma populations that lead to the growth of plasma waves which can in turn scatter some of the magnetospheric particles into the loss cone, causing them to precipitate into the high latitude ionosphere. The transient magnetopause compression can also heat and expand the high-latitude thermosphere. Enhanced particle precipitation ionizes neutral atmosphere, increasing the ionospheric plasma density. As the result, more ionospheric plasmas experience the fast $E \times B$ drift at a speed up to several km/s and collide with the slow thermospheric neutrals at a typical speed of ~100 m/s. The neutrals and plasmas exchange energy and momentum through this collisional process, resulting in the enhancements of the neutral wind and the high-altitude thermospheric density. Some, perhaps most, of the frequently observed transient magnetopause compressions and traveling compression vortices are associated with kinetic structures generated within the foreshock. Consequently, there is a need to examine the potential effect of these transients upon the ionosphere and the thermosphere.

Schunk et al. (1994) employed the Utah State University ionospheric model to study the response of the ionosphere to representative traveling convection twin vortices for a range of background conditions. They found local, transient, ion and electron temperature enhancements, $O^+$ density depletions and $NO^+$ density increases, non-Maxwellian ion distributions including enhanced temperature anisotropies, elevated plasma scale heights, and plasma upwelling events. The effects were evident at all altitudes, but largest in the E and lower F regions, and somewhat larger in summer than winter.

Recent studies (Connor et al. 2014, 2016; Shi et al. 2017b; Ozturk et al. 2018) investigated the impact of the sudden magnetopause compression on the ionosphere-thermosphere system using a global magnetosphere-ionosphere-thermosphere coupled model. They found
that the sudden magnetopause compression leads to enhancement of dayside aurora precip-
itation, modification of ionospheric convection pattern, temperature increase of the iono-
sphere and thermosphere, and neutral density increase at 400 km altitude.

**4.3 Outstanding Questions**

Dayside transients are frequently observed throughout geospace, but there are still many
open questions. Upstream transients are localized and short-lived, just like those in the iono-
sphere. But it is not always clear which event in the ionosphere is linked to which specific
transient in the foreshock, bow shock, magnetosheath, or at the magnetopause. Observations
are scarce and typically constrained by point measurements (in-situ spacecraft) or limited
field-of-view (riometers, magnetometers, radars, and ground-based imagers). Consequently,
a complete view of the whole system is missing, because simultaneous global observations
are unavailable.

The topology of Earth's magnetic field, and especially how open magnetic field lines map through a dynamic and disturbed magnetopause, is not understood beyond a very basic conceptual level. Consequently, it is not straightforward to trace events on open magnetic field lines between the magnetosheath and the polar ionosphere. For the same reason, more work is needed in order to make robust predictions that allow transients to be followed and tracked. The Earth's magnetosphere is also a barrier that may block smaller transients from giving any detectable signatures in the ionosphere/thermosphere.

In the last 10-15 years a new understanding has emerged that geospace is asymmetric (Laundal and Østgaard 2009), which plays a key role in how the northern and southern hemispheres respond differently to solar-wind driving. Differences in sunlight exposure, magnetic field strength, and pole offsets lead to observed asymmetries in ionospheric convection, thermospheric winds, currents and magnetic field perturbations, ion outflow, electron density, and auroral emissions (Laundal et al. 2017). The relative importance of each process is still debated, including the role of asymmetric feedback. However, asymmetries should play a significant role in redistribution of ionospheric plasma, formation of ionospheric irregularities, and energetic particle precipitation.

In the polar ionosphere there are many open questions related to plasma circulation, as more instrumentation leads to better data coverage in remote locations of the Earth. A topic currently under active investigation is ionospheric flow channels (Reistad et al. 2019a,b; Herlingshaw et al. 2019, 2020). What is the role of the Earth's dipole tilt in plasma circulation? What are the size and time constraints for meso-scale ionospheric flow channels to develop? Under what IMF conditions do different types of flow channels occur? Do all flow channels transit the polar cap (Nishimura et al. 2014), or does it only apply to a subset (e.g. only the largest transients)? Consequently, it is still very difficult to predict the dynamics and implications of transients in the polar ionosphere and thermosphere.

An important method to identify boundary processes and their relative location in the magnetosphere is the mapping of ionospheric signatures into the magnetosphere and its boundary layer. However, although there is much auroral activity for higher latitudes in the giant magnetospheres, there is an open debate on where the actual open-closed boundary is located in the ionosphere. Therefore it is not well established whether auroral events map to the actual boundary or much deeper into the magnetosphere.

In summary, quite little is known about the overall geo-effective impact of dayside tran-
sients, both at Earth and at other planets. Open questions remain on properties of ULF waves
generated by dayside transients; which fraction is generated by KH waves, pressure pulses,
and foreshock transients? Nevertheless, it is evident that dayside transients must play a key
role in space weather disruptions both at Earth and at other planets. A better understanding of dayside transients is therefore fundamental for society, as everyone in their daily life becomes more dependent on advanced technology, and human activity is expected to grow, both in space and in polar regions.

# 5 Conclusions and Outstanding Questions

A veritable zoo of transient phenomena has been identified upstream from the bow shock. These phenomena include HFAs, SHFAs, foreshock bubbles, density holes, foreshock cavities, foreshock cavitons, foreshock compressional boundaries, and SLAMs. They show both similar and different characteristics (Table 1). Among these phenomena, HFAs and SHFAs have been studied extensively. HFAs and SHFAs exhibit core regions with low field strength, low density, plasma heating, and plasma deflection bounded by one or two compressional boundaries. Their typical spatial scales are several $R_E$. Occurrence patterns for the two phenomena do not show clear statistical differences except that HFAs require a TD to form. HFAs are universal phenomena which have been observed at many planets. On average, their occurrence rate is about several per day. Conditions favoring HFA formation are fast solar wind, high Mach number, low IMF strength, small IMF cone angles, large magnetic shear angles across TDs, and small TD transit speed along the bow shock surface. HFAs are very dynamic. As HFAs evolve from “young” to “mature”, foreshock ions and solar wind ions gradually merge into one diffuse population. As the plasma and magnetic field perturbations associated with HFAs strengthen, wave activity within them intensifies. HFAs are also efficient particle accelerators with multiple acceleration mechanisms proposed. Magnetic reconnection and flux ropes have also been observed inside HFAs. Foreshock bubbles, density holes, foreshock cavities, and foreshock cavitons are all characterized by core regions with low field strength and low density, but they exhibit different degrees of plasma heating and deflection and have very different formation mechanisms. Foreshock compressional boundaries can be identified as regions of enhanced field strength and density that separate the solar wind from the foreshock. Inside the foreshock, there are also ULF waves, which can further steepen into shocklets and SLAMS with enhanced fluctuations. Solar wind discontinuities or local, transient bow shock deformations, enable the occurrence of localized, transient foreshock regions.

Multiple types of transient structures have been observed in the magnetosheath. Magnetosheath jets are localized structures with enhanced dynamic pressure and typical spatial scales of less than one $R_E$. They mainly occur downstream from the quasi-parallel bow shock. They form due to bow shock ripples, solar wind discontinuities, or foreshock transients. They are associated with enhanced dynamic pressures that can compress the magnetopause, which can in turn disturb the magnetosphere and ionosphere, trigger magnetic reconnection, and result in plasma penetration across the magnetopause. Due to their fast motion, they can also drive a bow wave that can accelerate particles. In the magnetosheath, there are also magnetic holes and magnetic peaks with local field strength decreases and increases, respectively, from MHD scales to kinetic scales. They form due to mirror mode or solitary waves. Particle acceleration, deceleration, and various waves have been observed inside them.

These transient phenomena have been observed and simulated at various planets (Table 4). In principle, they very likely exist at all planets because their physical background is similar to those at Earth. Observations are needed to fill the empty slots in Table 4. Hence, Table 4 gives the future direction of research on the transient events. To study these events at
various planets and compare their features with their terrestrial counterpart will dramatically improve our understanding of them.

Surface waves at the magnetopause are excited by solar wind pressure variations, transient phenomena generated near the bow shock, or the KHI on the magnetopause, and have been observed at Earth, Mercury, Venus, Mars, and Saturn. They may play a significant role in viscous or turbulent transport of solar wind plasmas into the magnetotail and in driving magnetospheric ULF waves. A recent extensive survey using THEMIS data has shown that the terrestrial KH waves are present approximately 19% of the time and the occurrence rate increases with the solar wind speed, Alfvén Mach number, and number density. The KH wavelength increases as they propagate toward the tail. KH waves have been observed not only on the flanks, but also in the high latitude cusp region during strongly dawnward IMF conditions and even close to the subsolar point when dense plume plasma is present in the magnetosphere. During southward IMF, magnetic reconnection and KHI can occur simultaneously and interact with each other. Recent MMS observations show unambiguous evidence of magnetic reconnection and properties of turbulence/waves associated with KHI.

FTEs, with a typical duration of 1 min and spacing of 10 min, are frequently observed at Earth during southward IMF. MMS observations revealed structures and evolution of both typical and ion-scale FTEs. FTEs are very common at Mercury's magnetopause with a typical duration of ~1 s and spacing of 4 s. They have been observed during both northward and southward IMF conditions. At giant planets, in contrast, observations of FTEs are rare. The solar wind interaction with giant planets is fundamentally different from Earth and Mercury, where IMF is important in coupling solar wind to the magnetosphere. A complete understanding of the viscous-like interaction must combine KH instability with magnetic reconnection.

Dynamic pressure variations associated with dayside transients deform the magnetopause and launch field-aligned currents that drive traveling convection vortices and plasma flow in the high-latitude ionosphere. Dayside transients can also transmit compressional waves into the magnetosphere that can excite resonant ULF waves locally or globally. Analytical theories, numerical simulations, and in-situ observations have found that foreshock compressional waves can directly mode convert to shear Alfvén waves when the compressional wave frequency matches the local Alfvén resonance condition due to the nonuniformity of the magnetosphere, providing an effective mechanism of ULF wave generation. These ULF waves can interact with particles in the radiation belts, ring current, and the plasmasphere. Transient compressions by dayside transients can create or enhance plasma anisotropies that lead to the growth of plasma waves which can scatter particles into the loss cone and precipitate into the ionosphere, resulting in transient auroral brightenings. Enhanced ionospheric densities due to particle precipitation can heat and expand the high-latitude thermosphere and increase cosmic noise absorption. Dayside transients can impact not only the dayside but also the nightside magnetosphere and ionosphere since dayside transients and the compressional waves excited by them can propagate to the nightside. For example, FTEs and KH waves can transport mass and magnetic flux from dayside to the magnetotail plasma sheet. The magnetotail plasma sheet expands and contracts in response to the magnetopause perturbations driven by foreshock transients.

The forthcoming joint European Space Agency and Chinese Academy of Sciences spacecraft mission Solar wind Magnetosphere Ionosphere Link Explorer (SMILE), which is presently scheduled for launch in November 2023 will offer new opportunities to observe the effects of foreshock transients upon the Earth's magnetosphere (Branduardi-Raymont and Wang 2017). Operating from a highly inclined (70–98°) and highly elliptical orbit with an apogee of 20 $R_E$, the spacecraft will carry a magnetometer and top-hap ion analyzer capable of observing the solar wind input from a near-Earth vantage point, a wide
field-of-view soft X-ray telescope capable of identifying the location of the subsolar magnetosheath, and a far ultraviolet camera capable of observing the auroral oval. SMILE's soft X-ray telescope is a novel instrument designed to measure the emissions generated when high charge state solar wind ions exchange charges with exospheric neutrals in the dayside magnetosheath and cusps. Since high charge solar wind ions do not penetrate the dayside magnetopause, there are no emissions from the outer dayside magnetosphere. Numerical simulations have shown that the telescope is capable of distinguishing the low emission dayside magnetosphere from the high emission magnetosheath and therefore identifying the magnetopause in line-of-sight integrated images. Whether or not the cadence of the images suffices to image foreshock transient events or their effects on the magnetopause remains an open question. The typical solar wind density is $5 \text{ cm}^{-3}$ and the typical solar wind velocity is $400 \text{ km s}^{-1}$. As noted earlier in this article, foreshock events tend to occur for low solar wind densities and high solar wind velocities, perhaps densities of $2.5 \text{ cm}^{-3}$ and velocities of $800 \text{ km s}^{-1}$. In either case, the solar wind ion flux is $2.0 \times 10^8 \text{ cm}^{-2} \text{ s}^{-1}$. For fluxes like these, the integration time required for the instrument to construct an image and identify the subsolar magnetopause is on the order of 25 min, longer than the 5 to 15 min required for solar wind features to sweep past the entire dayside magnetosphere. The situation with the far ultraviolet imager on SMILE is far more favorable. The imager will distinguish the dayside auroral emissions driven by transient events at the bow shock and magnetopause from Earth's albedo and nominal dayglow by imaging with a resolution of 150 km and a cadence of 60 s in the 160-180 nm Lyman-Birge-Hopfield Long band. This will be able to detect the occurrence, extent, and propagation of the auroral brightenings associated with transient compressions of the magnetosphere driven by foreshock events.

Although significant progress has been made on dayside transients and their geoeffects, there are still many open questions. We list below some outstanding questions (in addition to the specific outstanding questions at the end of each section):

- What are the physical differences and relationships among transient phenomena upstream from the bow shock (such as hot flow anomaly, foreshock bubbles, foreshock cavities, and density holes)?

- How much energy is transported to the foreshock region and how fast is the energy coupled to the solar wind?

- How much do foreshock transients contribute to particle acceleration at shocks?

- How do they evolve with time and transition through the magnetosheath?

- Do foreshock events trigger transient features (magnetic reconnection/FTEs, surface waves etc.) at the magnetopause?

- How does the ionosphere respond to transient phenomena generated at the magnetopause and bow shock?

- How can we quantify the energy and momentum transport during the dayside transient phenomena? How deterministic is the magnetosphere?

- How much do dayside transients contribute to magnetospheric dawn-dusk asymmetry?

- Are dayside transients equally important for different magnetospheres (e.g., Saturn and Jupiter's magnetospheres)?

**Acknowledgements** QGZ acknowledges Major Project of Chinese National Programs for Fundamental Research and Development 2021YFA0718600 and China Space Agency Project D020301. HZ acknowledges NSF AGS-1352669 and NASA grant 80NSSC18K1376. BL thanks the Centre National d'Etudes Spatiales (CNES; grant nos. APR-W-EEXP/10-01-01-05 and APR-Z-ETP-E-0010/01-01-05) and GENCI Agency (DARI project; grant no. A0050400295) for their support. TZL is supported by the NASA Living With a Star Jack Eddy Postdoctoral Fellowship Program, administered by the Cooperative Programs for the Advancement of Earth System Science (CPAESs) and is partially supported by NSF award AGS-1941012.
HKC acknowledges NSF grants, OIA-1920965 and AGS-1928883 and the NASA grants, 80NSSC18K1043, 80NSSC18K1052 and 80NSSC19K0844. Work at GSFC was supported by NASA's THEMIS, MMS, and SR programs. The work of DH is supported by National Natural Science Foundation of China (42030101). The research of AK and GF has been supported by the Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network. KO is supported by the Research Council of Norway grant 223252. The authors acknowledge the International Space Science Institute-Beijing for supporting the international team "Dayside Transient Phenomena and Their Impact on the Magnetosphere-Ionosphere".

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