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

HAL
open science

MAVEN Observations of the Interloop Magnetic
Reconnections at Mars

Guo Chen, Can Huang, Ying Zhang, Yasong Ge, Aimin Du, Rongsheng Wang, Lei Wang, Lican Shan, Christian Mazelle, Hao Luo

► To cite this version:

Guo Chen, Can Huang, Ying Zhang, Yasong Ge, Aimin Du, et al. MAVEN Observations of the Interloop Magnetic Reconnections at Mars. The Astrophysical Journal, 2023, 952, 10.3847/1538-4357/acda90. insu-04473176

HAL Id: insu-04473176

https://insu.hal.science/insu-04473176

Submitted on 23 Feb 2024

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

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

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_1.png" /></figure>

# MAVEN Observations of the Interloop Magnetic Reconnections at Mars

Guo Chen<sup>1,2,3,4</sup>①, Can Huang<sup>1,3,4</sup>①, Ying Zhang<sup>1,3,4</sup>①, Yasong Ge<sup>1,3,4</sup>①, Aimin Du<sup>1,3,4</sup>①, Rongsheng Wang<sup>5,6</sup>①, Lei Wang<sup>1,3,4</sup>①, Lican Shan<sup>1,3,4</sup>①, Christian Mazelle<sup>7</sup>①, and Hao Luo<sup>1,3,4</sup>①

## Abstract

Magnetic reconnection between neighboring magnetic field loops, the so-called interloop reconnection, is a common process to drive flares in the solar atmosphere. However, there is no direct evidence that a similar but less explosive process can take place on planets. The strong crustal fields on Mars generate plenty of magnetic loops in the near-Mars regions, providing a unique environment to research the interloop reconnection on a planet. Here, we report magnetic reconnection events between crustal field loops in the Martian ionosphere observed by Mars Atmosphere and Volatile Evolution (MAVEN) for the first time. During the current layer crossing, MAVEN recorded the characteristic signals of collisionless magnetic reconnection, including the Hall magnetic field, Alfvénic outflow, and electron energization. This finding implies that the interloop reconnection in the Martian ionosphere could contribute to the localized energy deposition and particle energization, which provides the seed source for aurora in the Martian atmosphere.

*Unified Astronomy Thesaurus concepts: Mars (1007); Magnetic fields (994); Plasma physics (2089)*

## 1. Introduction

As a fundamental process occurs in the universe, magnetic reconnection converts electromagnetic energy into plasma kinetic energy rapidly (Dungey 1961; Tsurutani & Meng 1972; Vasyliunas 1975; Biskamp 1996; Oieroset et al. 2001; Fu et al. 2013; Wang et al. 2016). In black hole accretion disks, the relativistic temperature plasmons and the jet's limb brightening are considered to be associated with magnetic reconnection (de Gouveia dal Pino & Lazarian 2005; Ripperda et al. 2020). It also causes violent eruptions on the star's surface, e.g., solar flares and mass ejections (Tanaka 1991; Masuda et al. 1994; Somov & Kosugi 1997; Schmieder et al. 2009). Magnetic reconnection is also believed to play an essential role in the solar wind and planet interactions (Deng & Matsumoto 2001; Angelopoulos et al. 2008, 2020; Zhang et al. 2012; Phan et al. 2018; Liu et al. 2019; Lu et al. 2019; Wang et al. 2023).

Mars, an unmagnetized planet, with not only an induced magnetosphere, but also strong crustal magnetic fields up to several thousand nanoteslas (nT) in some limited regions (Acuna et al. 1999), provides a unique and dynamic environment to study mass loss and energy exchange in the solar system (Eastwood et al. 2008; Jakosky et al. 2018; Ramstad et al. 2020). In the Martian magnetosphere, magnetic reconnection plays an important role in producing flux ropes (Brain et al. 2010; Beharrell & Wild 2012; Hara et al. 2017; Bowers et al. 2021; Wang et al. 2022), generating ion jets (Harada et al. 2020), and accelerating auroral electrons (Brain

et al. 2006; Xu et al. 2020). In the near-Mars regions, where the crustal magnetic field dominates, strong crustal fields are capable of generating magnetic loops (Connerney et al. 2001) and “mini-cusp” regions (Lundin et al. 2006). The strong crustal magnetic fields produce abundant magnetic loops on Mars, resembling solar magnetic loops. However, the footpoints of the Martian crustal magnetic fields are rooted to the surface, and are unable to move like that in the solar atmosphere, and the magnetic field strengths of the loops on Mars are orders of magnitude weaker than on the Sun (Shibata et al. 1995). Recent studies have shown the crustal magnetic field can directly reconnect with the draped interplanetary magnetic field (Harada et al. 2018). Moreover, the upper part of the loop can be stretched by the shocked solar wind (de Oliveira et al. 2021), and reconnection can happen in the stretched loop (Brain et al. 2010). A question of whether magnetic reconnection can take place between neighboring loops on Mars naturally emerges. As illustrated in Figure 1, if the upper part of the original crustal field loop (Figure 1(a)) is sheared by the shocked solar wind, the free energy within the crustal field loops could be accumulated and a current layer could form between two loops (Figure 1(b)). The subsequent interloop reconnection (Figure 1(c)), like the reconnection scenario on the Sun, could take place on Mars. In this study, we present evidence of interloop reconnection in the Martian ionosphere for the first time.

## 2. Observations

### 2.1. Crustal Magnetic Loops Observed by MAVEN

The Mars Atmosphere and Volatile Evolution (MAVEN) mission, which was designed to study the variability of the

<figure><img src="image_2.png" /></figure>

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ CAS Engineering Laboratory for Deep Resources Equipment and Technology, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, People's Republic of China; huangcan@mail.iggcas.ac.cn</footnote>

<footnote n="2">² State Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing 100190, People's Republic of China</footnote>

<footnote n="3">³ Innovation Academy for Earth Science, CAS, Beijing 100029, People's Republic of China</footnote>

<footnote n="4">⁴ College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China</footnote>

<footnote n="5">⁵ CAS Key Laboratory of Geospace Environment, Department of Geophysics and Planetary Science, University of Science and Technology of China, Hefei 230026, People's Republic of China</footnote>

<footnote n="6">⁶ CAS Center for Excellence in Comparative Planetology, Hefei 230026, People's Republic of China</footnote>

<footnote n="7">⁷ Institut de Recherche en Astrophysique et Planétologie, University Paul Sabatier, CNRS, CNES, Toulouse F-31400, France
*Received 2023 May 14; revised 2023 May 23; accepted 2023 May 30; published 2023 July 17*</footnote>
# MAVEN Observations of the Interloop Magnetic Reconnections at Mars

Guo Chen<sup>1,2,3,4</sup>①, Can Huang<sup>1,3,4</sup>①, Ying Zhang<sup>1,3,4</sup>①, Yasong Ge<sup>1,3,4</sup>①, Aimin Du<sup>1,3,4</sup>①, Rongsheng Wang<sup>5,6</sup>①, Lei Wang<sup>1,3,4</sup>①, Lican Shan<sup>1,3,4</sup>①, Christian Mazelle<sup>7</sup>①, and Hao Luo<sup>1,3,4</sup>①

## Abstract

Magnetic reconnection between neighboring magnetic field loops, the so-called interloop reconnection, is a common process to drive flares in the solar atmosphere. However, there is no direct evidence that a similar but less explosive process can take place on planets. The strong crustal fields on Mars generate plenty of magnetic loops in the near-Mars regions, providing a unique environment to research the interloop reconnection on a planet. Here, we report magnetic reconnection events between crustal field loops in the Martian ionosphere observed by Mars Atmosphere and Volatile Evolution (MAVEN) for the first time. During the current layer crossing, MAVEN recorded the characteristic signals of collisionless magnetic reconnection, including the Hall magnetic field, Alfvénic outflow, and electron energization. This finding implies that the interloop reconnection in the Martian ionosphere could contribute to the localized energy deposition and particle energization, which provides the seed source for aurora in the Martian atmosphere.

*Unified Astronomy Thesaurus concepts: Mars (1007); Magnetic fields (994); Plasma physics (2089)*

## 1. Introduction

As a fundamental process occurs in the universe, magnetic reconnection converts electromagnetic energy into plasma kinetic energy rapidly (Dungey 1961; Tsurutani & Meng 1972; Vasyliunas 1975; Biskamp 1996; Oieroset et al. 2001; Fu et al. 2013; Wang et al. 2016). In black hole accretion disks, the relativistic temperature plasmons and the jet's limb brightening are considered to be associated with magnetic reconnection (de Gouveia dal Pino & Lazarian 2005; Ripperda et al. 2020). It also causes violent eruptions on the star's surface, e.g., solar flares and mass ejections (Tanaka 1991; Masuda et al. 1994; Somov & Kosugi 1997; Schmieder et al. 2009). Magnetic reconnection is also believed to play an essential role in the solar wind and planet interactions (Deng & Matsumoto 2001; Angelopoulos et al. 2008, 2020; Zhang et al. 2012; Phan et al. 2018; Liu et al. 2019; Lu et al. 2019; Wang et al. 2023).

Mars, an unmagnetized planet, with not only an induced magnetosphere, but also strong crustal magnetic fields up to several thousand nanoteslas (nT) in some limited regions (Acuna et al. 1999), provides a unique and dynamic environment to study mass loss and energy exchange in the solar system (Eastwood et al. 2008; Jakosky et al. 2018; Ramstad et al. 2020). In the Martian magnetosphere, magnetic reconnection plays an important role in producing flux ropes (Brain et al. 2010; Beharrell & Wild 2012; Hara et al. 2017; Bowers et al. 2021; Wang et al. 2022), generating ion jets (Harada et al. 2020), and accelerating auroral electrons (Brain

et al. 2006; Xu et al. 2020). In the near-Mars regions, where the crustal magnetic field dominates, strong crustal fields are capable of generating magnetic loops (Connerney et al. 2001) and “mini-cusp” regions (Lundin et al. 2006). The strong crustal magnetic fields produce abundant magnetic loops on Mars, resembling solar magnetic loops. However, the footpoints of the Martian crustal magnetic fields are rooted to the surface, and are unable to move like that in the solar atmosphere, and the magnetic field strengths of the loops on Mars are orders of magnitude weaker than on the Sun (Shibata et al. 1995). Recent studies have shown the crustal magnetic field can directly reconnect with the draped interplanetary magnetic field (Harada et al. 2018). Moreover, the upper part of the loop can be stretched by the shocked solar wind (de Oliveira et al. 2021), and reconnection can happen in the stretched loop (Brain et al. 2010). A question of whether magnetic reconnection can take place between neighboring loops on Mars naturally emerges. As illustrated in Figure 1, if the upper part of the original crustal field loop (Figure 1(a)) is sheared by the shocked solar wind, the free energy within the crustal field loops could be accumulated and a current layer could form between two loops (Figure 1(b)). The subsequent interloop reconnection (Figure 1(c)), like the reconnection scenario on the Sun, could take place on Mars. In this study, we present evidence of interloop reconnection in the Martian ionosphere for the first time.

## 2. Observations

### 2.1. Crustal Magnetic Loops Observed by MAVEN

The Mars Atmosphere and Volatile Evolution (MAVEN) mission, which was designed to study the variability of the

<figure><img src="image_3.png" /></figure>

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ CAS Engineering Laboratory for Deep Resources Equipment and Technology, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, People's Republic of China; huangcan@mail.iggcas.ac.cn</footnote>

<footnote n="2">² State Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing 100190, People's Republic of China</footnote>

<footnote n="3">³ Innovation Academy for Earth Science, CAS, Beijing 100029, People's Republic of China</footnote>

<footnote n="4">⁴ College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China</footnote>

<footnote n="5">⁵ CAS Key Laboratory of Geospace Environment, Department of Geophysics and Planetary Science, University of Science and Technology of China, Hefei 230026, People's Republic of China</footnote>

<footnote n="6">⁶ CAS Center for Excellence in Comparative Planetology, Hefei 230026, People's Republic of China</footnote>

<footnote n="7">⁷ Institut de Recherche en Astrophysique et Planétologie, University Paul Sabatier, CNRS, CNES, Toulouse F-31400, France
*Received 2023 May 14; revised 2023 May 23; accepted 2023 May 30; published 2023 July 17*</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Schematic illustration of the magnetic reconnection between the crustal field loops on Mars. The crustal field $B_r$ component, computed from the Langlais et al. (2019) model, is shown with color-filled contours on the spherical surface of Mars at 200 km in MSO coordinates. The underpainting of the dayside and the nightside is white and gray, respectively. (a) Configuration of the crustal field loops not influenced by the shocked solar wind. The field lines (the black lines with arrows) are reconstructed by the integral of the magnetic fields, based on the Langlais et al. (2019) model, starting from the points on the MAVEN trajectory during 09:53:00–09:55:30 UT. (b) A current layer (the red bar) forms between the neighboring crustal field loops by shocked solar wind shearing (the gray arrows). (c) The subsequent magnetic reconnection occurs in the current layer between two crustal field loops. The blue lines with arrows denote the representative field lines near the reconnection site (the red dashed box). (d) MAVEN trajectory when crossing the crustal field loops from 09:44:30 to 10:04:30 UT on 2019 May 29 (the green line). The yellow line denotes the satellite trajectory between 09:53:00 and 09:55:30 UT when it observed the interloop reconnection on Mars. The colors at the bottom of the loops mean that the magnetic field points in (blue) or out (red) of the sphere. The red dashed box in panel (d) presents the spatial range of panels (a)–(c).</figcaption></figure>

upper atmosphere, ionosphere, and magnetosphere of Mars and their interactions with the solar wind, has been providing nearly continuous observations since 2014 November (Jakosky et al. 2015). In this study, we use the data from the Magnetic Field (MAG) instrument (Connerney et al. 2015), the SupraThermal and Thermal Ion Composition (STATIC) instrument (McFaden et al. 2015), and the Solar Wind Electron Analyzer (SWEA) instrument (Mitchell et al. 2016) on board MAVEN. On 2019 May 29, MAVEN crossed the crustal field loops (see the Langlais et al. 2019 crustal field model value in Figure 1(d) and the satellite observations in Figure 2(a)) while traveling from the nightside (09:44:30 UT) to the dayside (10:04:30 UT) around the strong crustal anomalies in the southern hemisphere. MAVEN was in the ionosphere, which was identified by much higher densities of $O_2^+$ and $O^+$, and less $H^+$ (Figures 2(b)–(c)).

In recent studies, a pitch-angle-resolved shape parameter has been used to identify the Martian low-altitude magnetic topology, based on several distinct features of Martian ionospheric photoelectrons (Weber et al. 2017; Harada et al. 2018; Xu et al. 2019). In the study of Xu et al. (2017), based on 60 manually selected photoelectron energy spectra, 60 calculated derivatives of the electron fluxes with respect to energy in log space ($d(\log F)/d(\log E)$) are obtained and the average of the 60 derivatives produces a template. The shape parameter is defined as the sum of the absolute differences between the measured electron flux derivative and the template

from 20 eV to 80 eV. Small values (<1) of the shape parameter mean that the ionospheric photoelectrons are the dominant population. In Loop 1 and Loop 2, shape parameters in both the “Away” and the “Toward” directions (relative to Mars) were mostly less than 1, which supported the magnetic field lines being closed (Figure 2(f)). It is worth noting that the electrons in two directions were greater than 1 in the regions between Loop 1 and Loop 2, indicating that the state of electrons was significantly changed.

## 2.2. Encounter of a Current Layer Embedded between Neighboring Crustal Field Loops in the Martian Ionosphere

During the crossing of Loop 1, the magnetic field observations were close to the Langlais model values, and the magnetic field $B_Y$ component was dominant (Figures 2(d)–(e)). For Loop 2, the $B_Z$ component remained dominant and positive in both observations and modeling (Figures 2(d)–(e)), though the observed magnetic $|B|$ values were ~20 nT greater than the model results. This indicated that the shape of Loop 2 was changed by the shocked solar wind and the upper part of Loop 2 was moving tailward and getting closer to the upper part of Loop 1 (Figure 1(b)). In the region between Loop 1 and Loop 2, $B_Z$ reversed from negative values to positive values, resembling a “hyperbolic tangent shape” current layer. When MAVEN crossed the layer, the electron energy fluxes were
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Schematic illustration of the magnetic reconnection between the crustal field loops on Mars. The crustal field $B_r$ component, computed from the Langlais et al. (2019) model, is shown with color-filled contours on the spherical surface of Mars at 200 km in MSO coordinates. The underpainting of the dayside and the nightside is white and gray, respectively. (a) Configuration of the crustal field loops not influenced by the shocked solar wind. The field lines (the black lines with arrows) are reconstructed by the integral of the magnetic fields, based on the Langlais et al. (2019) model, starting from the points on the MAVEN trajectory during 09:53:00–09:55:30 UT. (b) A current layer (the red bar) forms between the neighboring crustal field loops by shocked solar wind shearing (the gray arrows). (c) The subsequent magnetic reconnection occurs in the current layer between two crustal field loops. The blue lines with arrows denote the representative field lines near the reconnection site (the red dashed box). (d) MAVEN trajectory when crossing the crustal field loops from 09:44:30 to 10:04:30 UT on 2019 May 29 (the green line). The yellow line denotes the satellite trajectory between 09:53:00 and 09:55:30 UT when it observed the interloop reconnection on Mars. The colors at the bottom of the loops mean that the magnetic field points in (blue) or out (red) of the sphere. The red dashed box in panel (d) presents the spatial range of panels (a)–(c).</figcaption></figure>

upper atmosphere, ionosphere, and magnetosphere of Mars and their interactions with the solar wind, has been providing nearly continuous observations since 2014 November (Jakosky et al. 2015). In this study, we use the data from the Magnetic Field (MAG) instrument (Connerney et al. 2015), the SupraThermal and Thermal Ion Composition (STATIC) instrument (McFaden et al. 2015), and the Solar Wind Electron Analyzer (SWEA) instrument (Mitchell et al. 2016) on board MAVEN. On 2019 May 29, MAVEN crossed the crustal field loops (see the Langlais et al. 2019 crustal field model value in Figure 1(d) and the satellite observations in Figure 2(a)) while traveling from the nightside (09:44:30 UT) to the dayside (10:04:30 UT) around the strong crustal anomalies in the southern hemisphere. MAVEN was in the ionosphere, which was identified by much higher densities of $O_2^+$ and $O^+$, and less $H^+$ (Figures 2(b)–(c)).

In recent studies, a pitch-angle-resolved shape parameter has been used to identify the Martian low-altitude magnetic topology, based on several distinct features of Martian ionospheric photoelectrons (Weber et al. 2017; Harada et al. 2018; Xu et al. 2019). In the study of Xu et al. (2017), based on 60 manually selected photoelectron energy spectra, 60 calculated derivatives of the electron fluxes with respect to energy in log space ($d(\log F)/d(\log E)$) are obtained and the average of the 60 derivatives produces a template. The shape parameter is defined as the sum of the absolute differences between the measured electron flux derivative and the template

from 20 eV to 80 eV. Small values (<1) of the shape parameter mean that the ionospheric photoelectrons are the dominant population. In Loop 1 and Loop 2, shape parameters in both the “Away” and the “Toward” directions (relative to Mars) were mostly less than 1, which supported the magnetic field lines being closed (Figure 2(f)). It is worth noting that the electrons in two directions were greater than 1 in the regions between Loop 1 and Loop 2, indicating that the state of electrons was significantly changed.

## 2.2. Encounter of a Current Layer Embedded between Neighboring Crustal Field Loops in the Martian Ionosphere

During the crossing of Loop 1, the magnetic field observations were close to the Langlais model values, and the magnetic field $B_Y$ component was dominant (Figures 2(d)–(e)). For Loop 2, the $B_Z$ component remained dominant and positive in both observations and modeling (Figures 2(d)–(e)), though the observed magnetic $|B|$ values were ~20 nT greater than the model results. This indicated that the shape of Loop 2 was changed by the shocked solar wind and the upper part of Loop 2 was moving tailward and getting closer to the upper part of Loop 1 (Figure 1(b)). In the region between Loop 1 and Loop 2, $B_Z$ reversed from negative values to positive values, resembling a “hyperbolic tangent shape” current layer. When MAVEN crossed the layer, the electron energy fluxes were
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Electron energization during MAVEN's crossing of the current layer in the Martian ionosphere on 2019 May 29. Measurements recorded from 09:20 UT to 10:20 UT are presented in MSO coordinates. (a) The magnetic field magnitude (black), $B_X$ (blue), $B_Y$ (green), and $B_Z$ (red) components measured by the MAG instrument (1 s resolution). (b) Number densities of $O_2^+$ (red), $O^+$ (blue), and $H^+$ (black) measured by the STATIC instrument. (c) Mass spectrogram of ions measured by the STATIC instrument. The crossing of the ionosphere (identified by much higher densities of $O_2^+$ and $O^+$, and less $H^+$) is pointed out at the top. The orange vertical dashed line denotes the reach of the periapsis. Panels (d)–(g) show zoom-ins on the interval denoted by the gray shaded or black dashed regions from 09:50–09:58 UT in panels (a)–(c). (d)–(e) The magnetic field magnitude (black) and $B_X$ (blue), $B_Y$ (green), and $B_Z$ (red) components for Loop 1 and Loop 2, measured by MAVEN (d) and computed by the Langlais model (e). (f) Shape parameter for the determination of the geometry of the magnetic fields. (g) Electron differential energy flux (in units of eV/(s cm² ster eV)) measured by the SWEA sensor in six channels. Note that two epochs, at 09:51:39 UT and 09:52:45 UT, are not shown, because their energy fluxes are more than one order less than those of the surrounding epochs in the channels of 10 and 20 eV. Between 09:53:00 and 09:55:30 UT (the yellow shaded region), electrons are energized when MAVEN crossed the current layer between Loop 1 and Loop 2 in a candidate reconnection event.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Electron energization during MAVEN's crossing of the current layer in the Martian ionosphere on 2019 May 29. Measurements recorded from 09:20 UT to 10:20 UT are presented in MSO coordinates. (a) The magnetic field magnitude (black), $B_X$ (blue), $B_Y$ (green), and $B_Z$ (red) components measured by the MAG instrument (1 s resolution). (b) Number densities of $O_2^+$ (red), $O^+$ (blue), and $H^+$ (black) measured by the STATIC instrument. (c) Mass spectrogram of ions measured by the STATIC instrument. The crossing of the ionosphere (identified by much higher densities of $O_2^+$ and $O^+$, and less $H^+$) is pointed out at the top. The orange vertical dashed line denotes the reach of the periapsis. Panels (d)–(g) show zoom-ins on the interval denoted by the gray shaded or black dashed regions from 09:50–09:58 UT in panels (a)–(c). (d)–(e) The magnetic field magnitude (black) and $B_X$ (blue), $B_Y$ (green), and $B_Z$ (red) components for Loop 1 and Loop 2, measured by MAVEN (d) and computed by the Langlais model (e). (f) Shape parameter for the determination of the geometry of the magnetic fields. (g) Electron differential energy flux (in units of eV/(s cm² ster eV)) measured by the SWEA sensor in six channels. Note that two epochs, at 09:51:39 UT and 09:52:45 UT, are not shown, because their energy fluxes are more than one order less than those of the surrounding epochs in the channels of 10 and 20 eV. Between 09:53:00 and 09:55:30 UT (the yellow shaded region), electrons are energized when MAVEN crossed the current layer between Loop 1 and Loop 2 in a candidate reconnection event.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** The interloop reconnection features during the current layer crossing. (a) A simple schematic of the reconnection region and the satellite trajectory in LMN coordinates. The black lines with arrows present field lines. (b) The magnetic field $B_L$ (blue), $B_M$ (green), and $B_N$ (red) components. The $V_L$ (black) component of $H^+$, which was computed with the spacecraft potential and spacecraft velocity corrected. (c) Differential energy flux of the pitch-angle distributions of 100–300 eV electrons.</figcaption></figure>

significantly enhanced (the yellow shaded region in Figure 2(g)). The enhancement of the energy flux of higher-energy (≥79 eV) electrons was about one order of magnitude greater than that of lower-energy (≤39 eV) electrons, meaning that the electrons were impressively accelerated in the current layer.

## 2.3. Signatures in the Vicinity of the Interloop Reconnection

During 09:53:00–09:55:30 UT, MAVEN crossed the current layer. The altitude of MAVEN changed from 289 km to 391 km, above the electron exobase located at ~160 km (Xu et al. 2015, 2016). By utilizing minimum variance analysis (MVA; Sonnerup & Cahill 1967) on the magnetic field measurements for a time interval, the magnetic field data can be converted from Mars Solar Orbital (MSO) coordinates into local magnetic normal (LMN) coordinates. In the LMN system, the L, M, and N directions represent the maximum, intermediate, and minimum variance directions, respectively. A time interval from 09:54:00 to 09:55:00 UT was selected to perform the MVA for this event. The LMN directions here are $L = (0.38, -0.14, 0.92)$, $M = (0.09, -0.98, -0.19)$, and $N = (-0.92, -0.15, 0.35)$, relative to the original MSO coordinates. When the satellite moved from one side (09:54:00 UT) to the other side (09:55:00 UT) of the layer, the magnetic field $B_L$ component changed from ~−20 nT to ~20 nT (Figure 3(b)). If the satellite crosses the current layer in one exhaust region of the magnetic reconnection with a guide field, a bipolar $B_M$ variation is predicted to be detected (Huang et al. 2010). Figure 3(b) shows a bipolar $B_M$ variation (from −6 nT to 4 nT) superposed upon a guide field of ~6 nT (the $B_M$ average over the MVA interval) during the current sheet crossing, which is consistent with the former prediction. The MVA was also performed for a longer interval (09:53:50–09:55:10 UT). The $B_L$, $B_M$, and $B_N$ lines for two intervals are very close, which shows that the conclusions above are robust. During the current layer crossing, the $V_L$ variation of $H^+$ is ~12 km s⁻¹ (Figure 3(b)), which is comparable to the Alfvén speed based

on $B_0$ and total ion mass density. Therefore, the outflow in the L direction is Alfvénic.

In the neighboring loops, shown in Figure 3(c), the 100–300 eV electrons presented a two-sided loss cone distribution (trapped distribution; Brain et al. 2007). According to the Langlais model, the magnetic field strength along Loop 1 was greater than that along Loop 2 below the altitude in this event. And the in situ detected strengths of the two loops were very close. Thus, the loss cone of Loop 1 was predicted to be smaller than that of Loop 2, which corresponded with the pitch-angle distributions (PADs) shown in Figure 3(c). Based on the pitch-angle distributions, a PAD score is defined as $(f_{FA} - f_{perp})/\sigma$, where $f_{FA}$ and $f_{perp}$ are the electron flux in the field-aligned direction and in the perpendicular direction, respectively (Weber et al. 2017; Xu et al. 2019). And $\sigma$ is the measurement of the propagated uncertainty in $(f_{FA} - f_{perp})$. A negative score infers a loss cone, a positive score infers a beamed distribution, and a score near 0 infers an isotropic distribution. After calculation, the PAD scores in the “Toward” and “Away” (relative to Mars) directions were less than −3 in Loop 1 and Loop 2. The above all supports their magnetic field lines being closed.

When MAVEN crossed the current layer from Loop 1 to Loop 2, electrons were significantly accelerated. The energization in the parallel and antiparallel directions was much stronger (Figure 3(c)). This indicates that there were some source processes, such as reconnection (Brain et al. 2010), to fill the loss cone. During the layer crossing, the field-aligned populations of electrons indicate they might be energized by a Fermi mechanism (Drake et al. 2006; Wang et al. 2010) as well as a parallel electric field acceleration (Huang et al. 2010, 2017; Egedal et al. 2012) in the reconnection process. Besides, the dominant population of the energized electrons concentrates in the quasi-parallel direction, which corresponds with the auroral-like peaked electron distribution reported by Brain et al. (2006). Although MAVEN does not provide electric field data, it is theoretically possible to perform a simple analysis. In the scenario shown in Figure 3(a), the theoretical reconnection electric field should be along the −M direction, which can
<figure><img src="image_6.png" /><figcaption>**Figure 3.** The interloop reconnection features during the current layer crossing. (a) A simple schematic of the reconnection region and the satellite trajectory in LMN coordinates. The black lines with arrows present field lines. (b) The magnetic field $B_L$ (blue), $B_M$ (green), and $B_N$ (red) components. The $V_L$ (black) component of $H^+$, which was computed with the spacecraft potential and spacecraft velocity corrected. (c) Differential energy flux of the pitch-angle distributions of 100–300 eV electrons.</figcaption></figure>

significantly enhanced (the yellow shaded region in Figure 2(g)). The enhancement of the energy flux of higher-energy (≥79 eV) electrons was about one order of magnitude greater than that of lower-energy (≤39 eV) electrons, meaning that the electrons were impressively accelerated in the current layer.

## 2.3. Signatures in the Vicinity of the Interloop Reconnection

During 09:53:00–09:55:30 UT, MAVEN crossed the current layer. The altitude of MAVEN changed from 289 km to 391 km, above the electron exobase located at ~160 km (Xu et al. 2015, 2016). By utilizing minimum variance analysis (MVA; Sonnerup & Cahill 1967) on the magnetic field measurements for a time interval, the magnetic field data can be converted from Mars Solar Orbital (MSO) coordinates into local magnetic normal (LMN) coordinates. In the LMN system, the L, M, and N directions represent the maximum, intermediate, and minimum variance directions, respectively. A time interval from 09:54:00 to 09:55:00 UT was selected to perform the MVA for this event. The LMN directions here are $L = (0.38, -0.14, 0.92)$, $M = (0.09, -0.98, -0.19)$, and $N = (-0.92, -0.15, 0.35)$, relative to the original MSO coordinates. When the satellite moved from one side (09:54:00 UT) to the other side (09:55:00 UT) of the layer, the magnetic field $B_L$ component changed from ~−20 nT to ~20 nT (Figure 3(b)). If the satellite crosses the current layer in one exhaust region of the magnetic reconnection with a guide field, a bipolar $B_M$ variation is predicted to be detected (Huang et al. 2010). Figure 3(b) shows a bipolar $B_M$ variation (from −6 nT to 4 nT) superposed upon a guide field of ~6 nT (the $B_M$ average over the MVA interval) during the current sheet crossing, which is consistent with the former prediction. The MVA was also performed for a longer interval (09:53:50–09:55:10 UT). The $B_L$, $B_M$, and $B_N$ lines for two intervals are very close, which shows that the conclusions above are robust. During the current layer crossing, the $V_L$ variation of $H^+$ is ~12 km s⁻¹ (Figure 3(b)), which is comparable to the Alfvén speed based

on $B_0$ and total ion mass density. Therefore, the outflow in the L direction is Alfvénic.

In the neighboring loops, shown in Figure 3(c), the 100–300 eV electrons presented a two-sided loss cone distribution (trapped distribution; Brain et al. 2007). According to the Langlais model, the magnetic field strength along Loop 1 was greater than that along Loop 2 below the altitude in this event. And the in situ detected strengths of the two loops were very close. Thus, the loss cone of Loop 1 was predicted to be smaller than that of Loop 2, which corresponded with the pitch-angle distributions (PADs) shown in Figure 3(c). Based on the pitch-angle distributions, a PAD score is defined as $(f_{FA} - f_{perp})/\sigma$, where $f_{FA}$ and $f_{perp}$ are the electron flux in the field-aligned direction and in the perpendicular direction, respectively (Weber et al. 2017; Xu et al. 2019). And $\sigma$ is the measurement of the propagated uncertainty in $(f_{FA} - f_{perp})$. A negative score infers a loss cone, a positive score infers a beamed distribution, and a score near 0 infers an isotropic distribution. After calculation, the PAD scores in the “Toward” and “Away” (relative to Mars) directions were less than −3 in Loop 1 and Loop 2. The above all supports their magnetic field lines being closed.

When MAVEN crossed the current layer from Loop 1 to Loop 2, electrons were significantly accelerated. The energization in the parallel and antiparallel directions was much stronger (Figure 3(c)). This indicates that there were some source processes, such as reconnection (Brain et al. 2010), to fill the loss cone. During the layer crossing, the field-aligned populations of electrons indicate they might be energized by a Fermi mechanism (Drake et al. 2006; Wang et al. 2010) as well as a parallel electric field acceleration (Huang et al. 2010, 2017; Egedal et al. 2012) in the reconnection process. Besides, the dominant population of the energized electrons concentrates in the quasi-parallel direction, which corresponds with the auroral-like peaked electron distribution reported by Brain et al. (2006). Although MAVEN does not provide electric field data, it is theoretically possible to perform a simple analysis. In the scenario shown in Figure 3(a), the theoretical reconnection electric field should be along the −M direction, which can
<figure><img src="image_6.png" /><figcaption>**Figure 4.** The interloop reconnection event near the terminator on 2018 January 10 (Event 2). (a) Magnetic field measured by MAVEN. (b) Magnetic field computed by the Langlais model. (c) Shape parameters. (d) Electron differential energy flux. Panels (e)–(f) show zoom-ins on the intervals denoted by the yellow shaded regions from 10:31:00–10:32:30 UT in panels (a)–(d). (e) Magnetic field in the LMN coordinate system. (f) Differential energy flux of the pitch-angle distributions of 100–300 eV electrons.</figcaption></figure>

accelerate electrons in the parallel direction. All of the above demonstrates that this event (Event 1) is a crossing of a current layer embedded in the reconnection diffusion region.

## 2.4. Extra Interloop Reconnection Events

Additionally, two more interloop reconnection events are shown in Figures 4 and 5. These two events were detected by MAVEN in the southern hemisphere on 2018 January 10 (Event 2) and 2020 April 13 (Event 3), respectively. For Event 2 (Figure 4), MAVEN crossed two crustal field loops (the shape parameters were mostly near 0.5 in Figure 4(c)) near the moderately strong fields around the equator (~49°W, 13°S)

between 10:29:00 UT and 10:34:00 UT in the ionosphere. When $B_Y$ and $B_Z$ crossed the null point, the magnetic field magnitude reached the minimum (~10 nT in Figure 4(a)). From 10:31:00 UT to 10:32:30 UT (the yellow shaded region in Figures 4(a)–(d)), when the current layer was detected, MAVEN's altitude changed from 295 km to 360 km (above the electron exobase) and its solar zenith angle (SZA) changed from 87° to 82° (near the terminator). To derive the LMN coordinate system of the current layer, we have performed the MVA on the 32 vector samples per second magnetic field data during 10:31:34–10:32:00 UT and obtained that $L = (-0.32, 0.65, 0.69)$, $M = (0.95, 0.22, 0.23)$, and $N = (0.00, -0.73,$
<figure><img src="image_7.png" /><figcaption>**Figure 4.** The interloop reconnection event near the terminator on 2018 January 10 (Event 2). (a) Magnetic field measured by MAVEN. (b) Magnetic field computed by the Langlais model. (c) Shape parameters. (d) Electron differential energy flux. Panels (e)–(f) show zoom-ins on the intervals denoted by the yellow shaded regions from 10:31:00–10:32:30 UT in panels (a)–(d). (e) Magnetic field in the LMN coordinate system. (f) Differential energy flux of the pitch-angle distributions of 100–300 eV electrons.</figcaption></figure>

accelerate electrons in the parallel direction. All of the above demonstrates that this event (Event 1) is a crossing of a current layer embedded in the reconnection diffusion region.

## 2.4. Extra Interloop Reconnection Events

Additionally, two more interloop reconnection events are shown in Figures 4 and 5. These two events were detected by MAVEN in the southern hemisphere on 2018 January 10 (Event 2) and 2020 April 13 (Event 3), respectively. For Event 2 (Figure 4), MAVEN crossed two crustal field loops (the shape parameters were mostly near 0.5 in Figure 4(c)) near the moderately strong fields around the equator (~49°W, 13°S)

between 10:29:00 UT and 10:34:00 UT in the ionosphere. When $B_Y$ and $B_Z$ crossed the null point, the magnetic field magnitude reached the minimum (~10 nT in Figure 4(a)). From 10:31:00 UT to 10:32:30 UT (the yellow shaded region in Figures 4(a)–(d)), when the current layer was detected, MAVEN's altitude changed from 295 km to 360 km (above the electron exobase) and its solar zenith angle (SZA) changed from 87° to 82° (near the terminator). To derive the LMN coordinate system of the current layer, we have performed the MVA on the 32 vector samples per second magnetic field data during 10:31:34–10:32:00 UT and obtained that $L = (-0.32, 0.65, 0.69)$, $M = (0.95, 0.22, 0.23)$, and $N = (0.00, -0.73,$
<figure><img src="image_7.png" /><figcaption>**Figure 5.** The interloop reconnection event on 2020 April 13 (Event 3). (a) Magnetic field measured by MAVEN. (b) Magnetic field computed by the Langlais model. (c) Electron differential energy flux. Panels (d)–(e) show zoom-ins on the intervals denoted by the yellow shaded regions from 16:09:30–16:10:20 UT in panels (a)–(c). (d) Magnetic field in the LMN coordinate system. (e) Differential energy flux of the pitch-angle distributions of 100–300 eV electrons.</figcaption></figure>

0.69). As can be seen in Figure 4(e), the $B_L$ reversal from ~10 to ~−15 nT between 10:31:34 and 10:32:00 UT meant the current layer crossing between two loops. A unipolar $B_M$ variation (10:31:34–10:32:00 UT) was superposed upon a guide field of ~5 nT (Figure 4(e)). This indicates that MAVEN crossed the diffusion region of an asymmetric magnetic reconnection with a guide field (Pritchett & Mozer 2009; Huang et al. 2014). There was no clear electron energization observed during the current layer crossing (Figures 4(d) and (f)). Surrounding the current layer, the two-sided loss cone distributions of the 100–300 eV electrons (Figure 4(f)) imply that the field lines were closed.

For Event 3 (Figure 5), MAVEN crossed two crustal field loops around the strong fields (~151°W, 71°S) between 16:07:00 UT and 16:13:00 UT in the ionosphere. The shape parameters cannot

be obtained, because energy flux data of <30 eV electrons have been missing since 2020. When the current layer was observed between 16:09:30 UT and 16:10:20 UT (the yellow shaded region in Figures 5(a)–(c)), MAVEN was also near the terminator (SZA from 95° to 93°), with its altitude changing from 277 km to 306 km. By utilizing the MVA on the 32 vector samples s⁻¹ magnetic field data from 16:09:47 UT to 16:09:55 UT, L = (−0.05, −0.67, 0.74), M = (−0.98, 0.18, 0.10), and N = (0.20, 0.72, 0.66). $B_N$ deviated from 0 sometimes, as the current layer may not be a quasi-1D structure between curved loops. $B_L$ changed from ~−15 nT (16:09:47 UT) to ~15 nT (16:09:55 UT) while crossing the layer (Figure 5(d)). In a symmetric reconnection, the $B_M$ variation is a quadrupole structure. A bipolar signal of $B_M$ variation can be observed when the satellite crosses the current layer in one exhaust region (Pritchett & Coroniti 2004; Huang et al. 2014).
<figure><img src="image_8.png" /><figcaption>**Figure 5.** The interloop reconnection event on 2020 April 13 (Event 3). (a) Magnetic field measured by MAVEN. (b) Magnetic field computed by the Langlais model. (c) Electron differential energy flux. Panels (d)–(e) show zoom-ins on the intervals denoted by the yellow shaded regions from 16:09:30–16:10:20 UT in panels (a)–(c). (d) Magnetic field in the LMN coordinate system. (e) Differential energy flux of the pitch-angle distributions of 100–300 eV electrons.</figcaption></figure>

0.69). As can be seen in Figure 4(e), the $B_L$ reversal from ~10 to ~−15 nT between 10:31:34 and 10:32:00 UT meant the current layer crossing between two loops. A unipolar $B_M$ variation (10:31:34–10:32:00 UT) was superposed upon a guide field of ~5 nT (Figure 4(e)). This indicates that MAVEN crossed the diffusion region of an asymmetric magnetic reconnection with a guide field (Pritchett & Mozer 2009; Huang et al. 2014). There was no clear electron energization observed during the current layer crossing (Figures 4(d) and (f)). Surrounding the current layer, the two-sided loss cone distributions of the 100–300 eV electrons (Figure 4(f)) imply that the field lines were closed.

For Event 3 (Figure 5), MAVEN crossed two crustal field loops around the strong fields (~151°W, 71°S) between 16:07:00 UT and 16:13:00 UT in the ionosphere. The shape parameters cannot

be obtained, because energy flux data of <30 eV electrons have been missing since 2020. When the current layer was observed between 16:09:30 UT and 16:10:20 UT (the yellow shaded region in Figures 5(a)–(c)), MAVEN was also near the terminator (SZA from 95° to 93°), with its altitude changing from 277 km to 306 km. By utilizing the MVA on the 32 vector samples s⁻¹ magnetic field data from 16:09:47 UT to 16:09:55 UT, L = (−0.05, −0.67, 0.74), M = (−0.98, 0.18, 0.10), and N = (0.20, 0.72, 0.66). $B_N$ deviated from 0 sometimes, as the current layer may not be a quasi-1D structure between curved loops. $B_L$ changed from ~−15 nT (16:09:47 UT) to ~15 nT (16:09:55 UT) while crossing the layer (Figure 5(d)). In a symmetric reconnection, the $B_M$ variation is a quadrupole structure. A bipolar signal of $B_M$ variation can be observed when the satellite crosses the current layer in one exhaust region (Pritchett & Coroniti 2004; Huang et al. 2014).
Figure 5(d) shows that a bipolar $B_M$ variation (~±3 nT) between 16:09:47 UT and 16:09:55 UT was superposed on a background mean field increasing from ~−25 nT to ~5 nT. The energy flux of ≥39 eV electrons increased while MAVEN crossed the layer (Figure 5(c)) and the energization of 100–300 eV electrons in the parallel direction was much stronger (Figure 5(e)), which indicates that electrons might be energized by a parallel electric field acceleration in the reconnection process. Despite missing the shape parameter, the two-sided loss cone distribution of the 100–300 eV electrons (Figure 5(e)) provided essential support to identify the reconnection event between two neighboring closed loops. Though the reconnection evidence of Events 2 and 3 is weaker than that of Event 1, caused by missing data or the absence of an electron dissipation region encounter, the three events suggest that such a phenomenon is not unique on Mars.

### 3. Discussion

Mars lacks a globally intrinsic magnetic field. However, the induced magnetosphere of Mars provides an obstacle to the solar wind flow. After the solar wind is shocked and slowed, it flows around the Martian magnetosphere. At places with small SZAs, the crustal field loops are pressed down, because of the normal compression of the shocked solar wind. With the increase of the SZAs, the crustal field loops are stretched or sheared, due to the more tangential shocked solar wind. Near the terminator, where the SZAs are about 90°, the upper part of the stretched or sheared loop moves toward the neighboring loop. A current layer could form between the nearing loops, and the subsequent interloop magnetic reconnection could take place in this current layer. This suggests that the terminator is favorable for interloop reconnection. All three interloop reconnection events reported in this paper were detected near the terminator, which is consistent with the scenario of interloop reconnection presented above. Moreover, the energized electron distribution exhibits the auroral-like peaked electron distribution reported by Brain et al. (2006), which indicates that the interloop reconnection may provide the seed sources for aurora in the Martian atmosphere as well.

### Acknowledgments

We would like to acknowledge data support from the MAVEN project team. We thank J. P. McFadden for help with the usage of MAVEN STATIC's moment data. This work is supported by the National Natural Science Foundation of China, grant Nos. 42204168, 42274224, 41974173, 42274222, and 41874080, the National Key R&D Program of China (grant No.2022YFF0503200), the Pre-research Project on Civil Aerospace Technologies No. D020103, funded by CNSA, the Strategic Priority Research Program of the Chinese Academy of Sciences (grant Nos. XDB41010304, XDA14040403, and XDA14040404), the Specialized Research Fund for State Key Laboratories, Chinese Academy of Sciences, and the Youth Innovation Promotion Association, Chinese Academy of Sciences (grant No. 2019066). Parts of this work—the observations obtained with the SWEA instrument—are supported by the French space agency CNES.

### ORCID iDs

Guo Chen https://orcid.org/0000-0001-9367-0975

Can Huang https://orcid.org/0000-0003-0223-0494

Ying Zhang https://orcid.org/0000-0001-9313-3459

Yasong Ge https://orcid.org/0000-0002-4345-522X

Rongsheng Wang https://orcid.org/0000-0002-9511-7660

Lei Wang https://orcid.org/0000-0003-0723-7775

Lican Shan https://orcid.org/0000-0002-2354-9261

Christian Mazelle https://orcid.org/0000-0001-5332-9561

Hao Luo https://orcid.org/0000-0002-8260-1533

### References

Acuna, M. H., Connerney, J. E. P., Ness, N. F., et al. 1999, *Sci*, 284, 790

Angelopoulos, V., Artemyev, A., Phan, T. D., & Miyashita, Y. 2020, *NatPh*, 16, 317

Angelopoulos, V., McFadden, J. P., Larson, D., et al. 2008, *Sci*, 321, 931

Beharrell, M. J., & Wild, J. A. 2012, *JGRA*, 117, A12212

Biskamp, D. 1996, *Ap&SS*, 242, 165

Bowers, C. F., Slavin, J. A., DiBraccio, G. A., et al. 2021, *GeoRL*, 48, e93296

Brain, D. A., Baker, A. H., Briggs, J., et al. 2010, *GeoRL*, 37, L14108

Brain, D. A., Halekas, J. S., Peticolas, L. M., et al. 2006, *GeoRL*, 33, L01201

Brain, D. A., Lillis, R. J., Mitchell, D. L., Halekas, J. S., & Lin, R. P. 2007, *JGRA*, 112, A09201

Connerney, J. E. P., Acuna, M. H., Wasilewski, P. J., et al. 2001, *GeoRL*, 28, 4015

Connerney, J. E. P., Espley, J., Lawton, P., et al. 2015, *SSRv*, 195, 257

de Gouveia dal Pino, E. M., & Lazarian, A. 2005, *A&A*, 441, 845

de Oliveira, I., Franz, M., Echer, E., & Franco, A. 2021, *JGRA*, 126, e29242

Deng, X. H., & Matsumoto, H. 2001, *Natur*, 410, 557

Drake, J. F., Swisdak, M., Che, H., & Shay, M. A. 2006, *Natur*, 443, 553

Dungey, J. W. 1961, *PhRvL*, 6, 47

Eastwood, J. P., Brain, D. A., Halekas, J. S., et al. 2008, *GeoRL*, 35, L02106

Egedal, J., Daughton, W., & Le, A. 2012, *NatPh*, 8, 321

Fu, H. S., Khotyaintsev, Y. V., Vaivads, A., Retino, A., & Andre, M. 2013, *NatPh*, 9, 426

Hara, T., Harada, Y., Mitchell, D. L., et al. 2017, *GeoRL*, 44, 7653

Harada, Y., Halekas, J. S., DiBraccio, G. A., et al. 2018, *GeoRL*, 45, 4550

Harada, Y., Halekas, J. S., Xu, S., et al. 2020, *JGRA*, 125, e28576

Huang, C., Lu, Q. M., Lu, S., Wang, P., & Wang, S. 2014, *JGRA*, 119, 798

Huang, C., Lu, Q. M., Wang, R. S., et al. 2017, *ApJ*, 835, 245

Huang, C., Lu, Q. M., & Wang, S. 2010, *PhPl*, 17, 072306

Jakosky, B. M., Brain, D., Chaffin, M., et al. 2018, *Icar*, 315, 146

Jakosky, B. M., Lin, R. P., Grebowsky, J. M., et al. 2015, *SSRv*, 195, 3

Langlais, B., Thebault, E., Houliez, A., Purucker, M. E., & Lillis, R. J. 2019, *JGRE*, 124, 1542

Liu, Y. Y., Fu, H. S., Liu, C. M., et al. 2019, *ApJL*, 877, L16

Lu, S., Angelopoulos, V., Artemyev, A. V., et al. 2019, *ApJ*, 878, 109

Lundin, R., Winningham, D., Barabash, S., et al. 2006, *Sci*, 311, 980

Masuda, S., Kosugi, T., Hara, H., & Ogawara, Y. 1994, *Natur*, 371, 495

McFadden, J., Kortmann, O., Curtis, D., et al. 2015, *SSRv*, 195, 199

Mitchell, D. L., Mazelle, C., Sauvaud, J. A., et al. 2016, *SSRv*, 200, 495

Oieroset, M., Phan, T. D., Fujimoto, M., Lin, R. P., & Lepping, R. P. 2001, *Natur*, 412, 414

Phan, T. D., Eastwood, J. P., Shay, M. A., et al. 2018, *Natur*, 557, 202

Pritchett, P. L., & Coroniti, F. V. 2004, *JGRA*, 109, A01220

Pritchett, P. L., & Mozer, F. S. 2009, *JGRA*, 114, A11210

Ramstad, R., Brain, D. A., Dong, Y. X., et al. 2020, *NatAs*, 4, 979

Ripperda, B., Bacchini, F., & Philippov, A. A. 2020, *ApJ*, 900, 100

Schmieder, B., Aulanier, G., Demoulin, P., & Pariat, E. 2009, *EP&S*, 61, 565

Shibata, K., Masuda, S., Shimojo, M., et al. 1995, *ApJ*, 451, L83

Somov, B. V., & Kosugi, T. 1997, *ApJ*, 485, 859

Sonnerup, B. U., & Cahill, L. J. 1967, *JGR*, 72, 171

Tanaka, K. 1991, *SoPh*, 136, 133

Tsurutani, B. T., & Meng, C. I. 1972, *JGRA*, 77, 2964

Vasyliunas, V. M. 1975, *RvGeo*, 13, 303

Wang, L., Huang, C., Ge, Y. S., et al. 2022, *JGRE*, 127, e07181

Wang, R. S., Lu, Q. M., Du, A. M., & Wang, S. 2010, *PhRvL*, 104, 4

Wang, R. S., Lu, Q. M., Nakamura, R., et al. 2016, *NatPh*, 12, 263

Wang, Z., Vaivads, A., Fu, H. S., et al. 2023, *ApJ*, 946, 67

Weber, T., Brain, D., Mitchell, D., et al. 2017, *JGRA*, 122, 9777

Xu, S. S., Liemohn, M., Bougher, S., & Mitchell, D. 2015, *GeoRL*, 42, 9702

Xu, S. S., Liemohn, M., Bougher, S., & Mitchell, D. 2016, *JGRA*, 121, 3767

Xu, S. S., Mitchell, D., Liemohn, M., et al. 2017, *JGRA*, 122, 1831

Xu, S. S., Mitchell, D. L., McFadden, J. P., et al. 2020, *GeoRL*, 47, e87414

Xu, S. S., Weber, T., Mitchell, D. L., et al. 2019, *JGRA*, 124, 1823

Zhang, T. L., Lu, Q. M., Baumjohann, W., et al. 2012, *Sci*, 336, 567
Figure 5(d) shows that a bipolar $B_M$ variation (~±3 nT) between 16:09:47 UT and 16:09:55 UT was superposed on a background mean field increasing from ~−25 nT to ~5 nT. The energy flux of ≥39 eV electrons increased while MAVEN crossed the layer (Figure 5(c)) and the energization of 100–300 eV electrons in the parallel direction was much stronger (Figure 5(e)), which indicates that electrons might be energized by a parallel electric field acceleration in the reconnection process. Despite missing the shape parameter, the two-sided loss cone distribution of the 100–300 eV electrons (Figure 5(e)) provided essential support to identify the reconnection event between two neighboring closed loops. Though the reconnection evidence of Events 2 and 3 is weaker than that of Event 1, caused by missing data or the absence of an electron dissipation region encounter, the three events suggest that such a phenomenon is not unique on Mars.

### 3. Discussion

Mars lacks a globally intrinsic magnetic field. However, the induced magnetosphere of Mars provides an obstacle to the solar wind flow. After the solar wind is shocked and slowed, it flows around the Martian magnetosphere. At places with small SZAs, the crustal field loops are pressed down, because of the normal compression of the shocked solar wind. With the increase of the SZAs, the crustal field loops are stretched or sheared, due to the more tangential shocked solar wind. Near the terminator, where the SZAs are about 90°, the upper part of the stretched or sheared loop moves toward the neighboring loop. A current layer could form between the nearing loops, and the subsequent interloop magnetic reconnection could take place in this current layer. This suggests that the terminator is favorable for interloop reconnection. All three interloop reconnection events reported in this paper were detected near the terminator, which is consistent with the scenario of interloop reconnection presented above. Moreover, the energized electron distribution exhibits the auroral-like peaked electron distribution reported by Brain et al. (2006), which indicates that the interloop reconnection may provide the seed sources for aurora in the Martian atmosphere as well.

### Acknowledgments

We would like to acknowledge data support from the MAVEN project team. We thank J. P. McFadden for help with the usage of MAVEN STATIC's moment data. This work is supported by the National Natural Science Foundation of China, grant Nos. 42204168, 42274224, 41974173, 42274222, and 41874080, the National Key R&D Program of China (grant No.2022YFF0503200), the Pre-research Project on Civil Aerospace Technologies No. D020103, funded by CNSA, the Strategic Priority Research Program of the Chinese Academy of Sciences (grant Nos. XDB41010304, XDA14040403, and XDA14040404), the Specialized Research Fund for State Key Laboratories, Chinese Academy of Sciences, and the Youth Innovation Promotion Association, Chinese Academy of Sciences (grant No. 2019066). Parts of this work—the observations obtained with the SWEA instrument—are supported by the French space agency CNES.

### ORCID iDs

Guo Chen https://orcid.org/0000-0001-9367-0975

Can Huang https://orcid.org/0000-0003-0223-0494

Ying Zhang https://orcid.org/0000-0001-9313-3459

Yasong Ge https://orcid.org/0000-0002-4345-522X

Rongsheng Wang https://orcid.org/0000-0002-9511-7660

Lei Wang https://orcid.org/0000-0003-0723-7775

Lican Shan https://orcid.org/0000-0002-2354-9261

Christian Mazelle https://orcid.org/0000-0001-5332-9561

Hao Luo https://orcid.org/0000-0002-8260-1533

### References

Acuna, M. H., Connerney, J. E. P., Ness, N. F., et al. 1999, *Sci*, 284, 790

Angelopoulos, V., Artemyev, A., Phan, T. D., & Miyashita, Y. 2020, *NatPh*, 16, 317

Angelopoulos, V., McFadden, J. P., Larson, D., et al. 2008, *Sci*, 321, 931

Beharrell, M. J., & Wild, J. A. 2012, *JGRA*, 117, A12212

Biskamp, D. 1996, *Ap&SS*, 242, 165

Bowers, C. F., Slavin, J. A., DiBraccio, G. A., et al. 2021, *GeoRL*, 48, e93296

Brain, D. A., Baker, A. H., Briggs, J., et al. 2010, *GeoRL*, 37, L14108

Brain, D. A., Halekas, J. S., Peticolas, L. M., et al. 2006, *GeoRL*, 33, L01201

Brain, D. A., Lillis, R. J., Mitchell, D. L., Halekas, J. S., & Lin, R. P. 2007, *JGRA*, 112, A09201

Connerney, J. E. P., Acuna, M. H., Wasilewski, P. J., et al. 2001, *GeoRL*, 28, 4015

Connerney, J. E. P., Espley, J., Lawton, P., et al. 2015, *SSRv*, 195, 257

de Gouveia dal Pino, E. M., & Lazarian, A. 2005, *A&A*, 441, 845

de Oliveira, I., Franz, M., Echer, E., & Franco, A. 2021, *JGRA*, 126, e29242

Deng, X. H., & Matsumoto, H. 2001, *Natur*, 410, 557

Drake, J. F., Swisdak, M., Che, H., & Shay, M. A. 2006, *Natur*, 443, 553

Dungey, J. W. 1961, *PhRvL*, 6, 47

Eastwood, J. P., Brain, D. A., Halekas, J. S., et al. 2008, *GeoRL*, 35, L02106

Egedal, J., Daughton, W., & Le, A. 2012, *NatPh*, 8, 321

Fu, H. S., Khotyaintsev, Y. V., Vaivads, A., Retino, A., & Andre, M. 2013, *NatPh*, 9, 426

Hara, T., Harada, Y., Mitchell, D. L., et al. 2017, *GeoRL*, 44, 7653

Harada, Y., Halekas, J. S., DiBraccio, G. A., et al. 2018, *GeoRL*, 45, 4550

Harada, Y., Halekas, J. S., Xu, S., et al. 2020, *JGRA*, 125, e28576

Huang, C., Lu, Q. M., Lu, S., Wang, P., & Wang, S. 2014, *JGRA*, 119, 798

Huang, C., Lu, Q. M., Wang, R. S., et al. 2017, *ApJ*, 835, 245

Huang, C., Lu, Q. M., & Wang, S. 2010, *PhPl*, 17, 072306

Jakosky, B. M., Brain, D., Chaffin, M., et al. 2018, *Icar*, 315, 146

Jakosky, B. M., Lin, R. P., Grebowsky, J. M., et al. 2015, *SSRv*, 195, 3

Langlais, B., Thebault, E., Houliez, A., Purucker, M. E., & Lillis, R. J. 2019, *JGRE*, 124, 1542

Liu, Y. Y., Fu, H. S., Liu, C. M., et al. 2019, *ApJL*, 877, L16

Lu, S., Angelopoulos, V., Artemyev, A. V., et al. 2019, *ApJ*, 878, 109

Lundin, R., Winningham, D., Barabash, S., et al. 2006, *Sci*, 311, 980

Masuda, S., Kosugi, T., Hara, H., & Ogawara, Y. 1994, *Natur*, 371, 495

McFadden, J., Kortmann, O., Curtis, D., et al. 2015, *SSRv*, 195, 199

Mitchell, D. L., Mazelle, C., Sauvaud, J. A., et al. 2016, *SSRv*, 200, 495

Oieroset, M., Phan, T. D., Fujimoto, M., Lin, R. P., & Lepping, R. P. 2001, *Natur*, 412, 414

Phan, T. D., Eastwood, J. P., Shay, M. A., et al. 2018, *Natur*, 557, 202

Pritchett, P. L., & Coroniti, F. V. 2004, *JGRA*, 109, A01220

Pritchett, P. L., & Mozer, F. S. 2009, *JGRA*, 114, A11210

Ramstad, R., Brain, D. A., Dong, Y. X., et al. 2020, *NatAs*, 4, 979

Ripperda, B., Bacchini, F., & Philippov, A. A. 2020, *ApJ*, 900, 100

Schmieder, B., Aulanier, G., Demoulin, P., & Pariat, E. 2009, *EP&S*, 61, 565

Shibata, K., Masuda, S., Shimojo, M., et al. 1995, *ApJ*, 451, L83

Somov, B. V., & Kosugi, T. 1997, *ApJ*, 485, 859

Sonnerup, B. U., & Cahill, L. J. 1967, *JGR*, 72, 171

Tanaka, K. 1991, *SoPh*, 136, 133

Tsurutani, B. T., & Meng, C. I. 1972, *JGRA*, 77, 2964

Vasyliunas, V. M. 1975, *RvGeo*, 13, 303

Wang, L., Huang, C., Ge, Y. S., et al. 2022, *JGRE*, 127, e07181

Wang, R. S., Lu, Q. M., Du, A. M., & Wang, S. 2010, *PhRvL*, 104, 4

Wang, R. S., Lu, Q. M., Nakamura, R., et al. 2016, *NatPh*, 12, 263

Wang, Z., Vaivads, A., Fu, H. S., et al. 2023, *ApJ*, 946, 67

Weber, T., Brain, D., Mitchell, D., et al. 2017, *JGRA*, 122, 9777

Xu, S. S., Liemohn, M., Bougher, S., & Mitchell, D. 2015, *GeoRL*, 42, 9702

Xu, S. S., Liemohn, M., Bougher, S., & Mitchell, D. 2016, *JGRA*, 121, 3767

Xu, S. S., Mitchell, D., Liemohn, M., et al. 2017, *JGRA*, 122, 1831

Xu, S. S., Mitchell, D. L., McFadden, J. P., et al. 2020, *GeoRL*, 47, e87414

Xu, S. S., Weber, T., Mitchell, D. L., et al. 2019, *JGRA*, 124, 1823

Zhang, T. L., Lu, Q. M., Baumjohann, W., et al. 2012, *Sci*, 336, 567