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

# Electron Energization by Ion Density Enhancement in the Martian Magnetotail—MAVEN Observations

Shaosui Xu¹, David L. Mitchell¹, Jasper S. Halekas², James P. McFadden¹, Christopher M. Fowler³, Kathleen Hanley¹, Gina A. DiBraccio⁴, Christian Mazelle⁵, Hassan Akbari⁴, and Shannon M. Curry¹

¹Space Sciences Laboratory, University of California, Berkeley, CA, USA; shaosui.xu@ssl.berkeley.edu

²Department of Physics and Astronomy, University of Iowa, Iowa City, IA, USA

³Department of Physics and Astronomy, West Virginia University, Morgantown, WV, USA

⁴NASA Goddard Space Flight Center, Greenbelt, MD, USA

⁵IRAP, CNRS—University of Toulouse—UPS—CNES, Toulouse, France

Received 2024 September 8; revised 2024 September 28; accepted 2024 October 11; published 2024 November 7

## Abstract

It has been long observed at Mars that electron fluxes are enhanced during the tail current sheet crossings, of which the cause is not well understood. We use a novel approach to reveal one of the electron energization mechanisms with observations from the Mars Atmospheric and Volatile Evolution (MAVEN) mission. We find the field-aligned potential, derived from comparing electron distribution functions, to be approximately linearly correlated with the logarithmic values of the local total ion density. This is the expected behavior of an ambipolar electrostatic potential. The large amplitude of potential (tens to hundreds of V) is a result of both a significant density gradient (1 order of magnitude) and the high electron temperature (tens of eV) in the tail. Such a mechanism is not limited to ion density enhancements at current sheet crossings but can be present anywhere that large ion density gradients and hot electrons are present.

Unified Astronomy Thesaurus concepts: Planetary science (1255); Planetary magnetospheres (997)

## 1. Introduction

It has been long observed at Mars that electron fluxes are enhanced during the tail current sheet crossings, first observed by the Phobos spacecraft (E. Dubinin et al. 1993), of which the cause is not well understood (J. S. Halekas et al. 2006; 2008). Tail current sheets are formed to separate the two oppositely directed magnetotail lobes, as part of the magnetosphere of a magnetized or unmagnetized planetary body (e.g., T. I. Gombosi 1998; A. Nagy et al. 2004; A. Fedorov et al. 2008; C. Bertucci et al. 2012; G. A. DiBraccio et al. 2018; M. W. Liemohn & S. Xu 2018). These accelerated electrons carry large energy fluxes and, if precipitating onto the atmosphere (due to a change in magnetic topology), can cause impact ionization, a major source of the Mars nightside ionosphere (e.g., R. J. Lillis et al. 2009; R. J. Lillis & D. A. Brain 2013; D. Adams et al. 2018; S. Xu et al. 2020a), and possibly auroral emission (e.g., D. A. Brain et al. 2006; F. Leblanc et al. 2008; J.-C. Gérard et al. 2015; L. Soret et al. 2016, 2021; S. Xu et al. 2020b, 2022a, 2022b). Recently, the Emirates Mars Mission observed a new type of auroral emission, an elongated serpentine structure that stretches thousands of kilometers into the nightside (sinuous aurora), which is suspected to be related to electrons in the Mars magnetotail current sheet (R. J. Lillis et al. 2022).

Although ion acceleration mechanisms have been extensively studied in the magnetotails of Mars and Venus (e.g., E. Dubinin et al. 2011), electron acceleration in the same region is much less well understood. To explain how electrons are energized in the current sheet, physical mechanisms, such as magnetic reconnection, field-aligned currents, Fermi acceleration, and wave-particle interactions, were considered as

possibilities (e.g., J. S. Halekas et al. 2008; Z. Wang et al. 2019, 2023b; Y. Harada et al. 2020). In this study, we propose an acceleration mechanism that has not garnered much attention before, the ambipolar electric field/potential. The ambipolar potential ($\phi$), resulting from the electron pressure gradient ($\nabla P_e$), can be expressed as

$$e\nabla\phi = \nabla P_e/n_e = k_b T_e \nabla n_e/n_e + k_b \nabla T_e, \quad (1)$$

where $n_e$ and $T_e$ are the electron density and temperature, $k_b$ is the Boltzmann constant, and the electron pressure $P_e = n_e k_b T_e$. We note that Equation (1) is a simplified expression for the ambipolar potential, ignoring the electron inertia term and collisional processes (e.g., M. W. Liemohn et al. 1997; G. Collinson et al. 2019), but is sufficient for our discussion.

As discussed in Section 4 in S. Xu et al. (2021a), the leading terms in the generalized Ohm's law give three electric fields in collisionless regions: ambipolar electric field $E_A = -\frac{\nabla\phi}{en_e}$, the Hall electric field $E_H = -\frac{J \times B}{en_e}$, and the motional electric field $E_M = -V \times B$, where B is the magnetic vector, J is the total current density, and V is the bulk flow velocity. Among these three electric fields, only $E_A$ could have a component parallel to B ($E_{A//}$) to change electrons' total energy while the other two electric fields are always perpendicular to B, causing electrons to drift across B without gaining energy. The expected directions of these three electric fields in the tail current sheet are illustrated in Figure 1. The ambipolar potential in the near-Mars environment has been mapped in the ionosphere (order of 1 V; S. Xu et al. 2018, 2024; H. Akbari et al. 2019; G. Collinson et al. 2019) and also in the magnetosphere ($-70 \sim +100$ V; S. Xu et al. 2021a). Ambipolar potential has been predicted in the current sheet by theoretical work before (e.g., L. M. Zelenyi et al. 2004) and our study will provide observational evidence of it.

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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.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** A schematic summarizes electron acceleration in the tail current sheet. The top panel shows the ideal/expected directions of ambipolar electric field *E*<sub>A</sub>, Hall electric field *E*<sub>H</sub>, and motional electric field *E*<sub>M</sub> at the current sheet. It also shows how electrons are accelerated and decelerated by the parallel component of *E*<sub>A</sub> as they move along the magnetic field line. The bottom panel shows the idealized profile of electron pressure (*P*), the total plasma density (*N*), electron temperature (*T*<sub>e</sub>), and the ambipolar potential (*U*) across the current sheet.</figcaption></figure>

In this study, we use a novel approach to reveal electron energization in the tail current sheet caused by the ambipolar potential via a case study with the comprehensive plasma and magnetic field observations provided by the Mars Atmospheric and Volatile Evolution (MAVEN) mission. This study utilizes superthermal (3–4600 eV) electron measurements from the Solar Wind Electron Analyzer (SWEA) instrument (D. L. Mitchell et al. 2016), ion measurements from the SupraThermal And Thermal Ion Composition (STATIC) instrument (J. P. McFadden et al. 2015), and magnetic field measurements from the Magnetometer (MAG) instrument (J. Connerney et al. 2015), all onboard the MAVEN spacecraft. In Section 2, we present the MAVEN observations, followed by a theoretical experiment in Section 3; we then discuss the results and conclude this study in Section 4.

## 2. MAVEN Observations

The MAVEN observations on 2017 June 30 for the case study are shown in Figure 2, when MAVEN was crossing from the magnetotail to the dayside ionosphere. The top two panels are the ion energy spectra for heavy ($M > 12$ amu, (a)) and light ($M < 10$ amu, (b)) ions. Before 10:20 UT, the magnetotail lobe is mainly populated with energetic light ions (mainly protons). During ~10:09–10:14 UT, there is a beam of ~1 keV heavy ions (a), coincident with the crossing of the tail current sheet, as indicated by the $B_x$ reversal in panel (c). The derived ion densities from the STATIC measurements (K. G. Hanley et al. 2021; C. M. Fowler et al. 2022) in panel (f) show not only an increase in the total density by about 1 order of magnitude but also the dominant ion species switching from protons to heavy ions. Near the current sheet, the dominant ion population could be either cold planetary ions (e.g., E. Dubinin et al. 2017; S. Inui et al. 2018; S. M. Curry et al. 2022) or energetic planetary ion beams (E. Dubinin et al. 1993; A. Fedorov et al. 2008; H. Nilsson et al. 2012). The cause of the ion beams within the current sheet at Mars is not fully understood but is found to be preferentially in the hemisphere of the motional electric field (C. Zhang et al. 2024), suggesting that the convection electric force plays a role, and E. Dubinin et al. (2011, 2023) explain the ion acceleration as a result of the magnetic tension force.

Across the current sheet, an obvious enhancement in superthermal electron fluxes occurs, as shown in Figure 2(e). The electron density and temperature derived from the SWEA measurements are shown in panels (g) (blue line) and (f), where the electron density is also enhanced and the electron temperature is about 30–40 eV across the current sheet. Panel (g) also includes the total ion density as the red line, which agrees well with the superthermal electron density before 10:23 UT. It means that SWEA is measuring the majority of the total electron population and that the local electrons are hot and tenuous. The ion and electron densities appear to diverge after 10:23 UT because cold electrons below SWEA’s energy measurement range begin to dominate the total population. The last panel of Figure 2(i) is the field-aligned potential derived from comparing the phase space density distributions of high-energy (50–350 eV) parallel (blue) and antiparallel (red) electrons.

Figure 3(a) illustrates how to infer field-aligned potential by comparing electron distribution functions (DFs; or phase space density), a technique used in many studies (e.g., J. Halekas et al. 2011; Xu et al. 2019, 2021b; Wang et al. 2023a). When electrons cross an electrostatic potential (*U*), the phase space density ($cm^{-3}$ ($km s^{-1})^{-3}$) is conserved, so that the DF retains the same shape while being shifted in energy by *eU*. In this particular case, the two DFs (blue and red solid lines in Figure 3(a)), taken at times indicated by the blue and red dashed lines in Figure 2, have similar shapes with a near-constant energy difference (except for very low energies). Figure 3(b) shows the same distributions in units of differential energy flux. For all spectra shown in Figures 3(a)–(b), secondary electron contamination has been removed, and the spectra have been corrected for the spacecraft potential, which is estimated from the STATIC measurements (S. Xu et al. 2018) and publicly available as part of the STATIC data product. If we shift the red DFs of parallel and antiparallel electrons by –75.7 and –61.5 eV, respectively,
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Time series of MAVEN observations on 2017 June 30. From top to bottom: ion energy spectra for heavy ($M > 12$ amu) (a) and light ($M < 10$ amu) (b) measured by the STATIC instrument, (c) magnetic field strength and components in the Mars-centered Sun orbit coordinates measured by MAG, (d) normalized electron pitch angle distribution of 111–140 eV and (e) electron energy spectra measured by SWEA, (f) compositional ion densities measured by STATIC, total ion densities calculated from STATIC measurements, and superthermal electron densities calculated from the SWEA measurements, (g) superthermal electron temperatures calculated from SWEA measurements, and (h) field-aligned potentials derived from parallel and antiparallel electron measurements by SWEA (see the main text for more details). The two black dashed lines are the time of interest of this study, and the blue and red dashed lines mark the times of electron energy spectra used in Figures 3(a)–(b). The STATIC ion measurements are unavailable after 10:26:30 UT as the instrument goes into protect mode. The pitch angle distribution in panel (d) is a normalization by the pitch-angle-averaged flux of this energy channel for each measurement.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** (a) Solid lines are electron distribution functions (DF) separated for parallel (PA < 45°, positive energies) and antiparallel (PA > 135°, negative energies) directions for the two times marked in Figure 2 (shown as red and blue lines, respectively). Dashed lines are shifted red DFs by field-aligned potentials indicated in the upper left corners. (b) The same format as panel (a) but the y-axis is the differential energy flux. All spectra in (a) and (b) are corrected for spacecraft potentials. (c) Field-aligned potentials derived from parallel (blue “+”) and antiparallel (red “-”) electron measurements, and electron temperatures (black “+”) as a function of local total ion densities. The cyan line shows the relation of $U = \langle T_e \rangle \ln(n_i)$.</figcaption></figure>

shown as the dashed red lines, the shifted DFs match well with the blue lines above 50 eV. This suggests that the blue spectra represent the source populations for the red spectra before crossing a total field-aligned potential of ~60–70 V. Note that, as shown in the top panel of Figure 1, parallel and antiparallel source electrons (blue lines) would have entered the current sheet from different sides, traveled different paths, and experienced different field-aligned electric fields, but ended up with a similar total energization (magenta lines) at a given location. Acceleration of both parallel and antiparallel electrons is supported by the observed counterstreaming pitch angle (PA) distributions at 10:11–10:12 UT in Figure 2(d). It is also worth noting that because source electrons gain little energization across the entire current sheet, the parallel and antiparallel electron distributions would be the same before entering and after exiting the current sheet, which is why we could use both parallel and antiparallel electron DF of the same measurement,

i.e., the same time and location (10:09:30 UT), as the source electrons. These points will be discussed in detail later.

Using this approach, we obtain the field-aligned potentials with respect to the blue spectrum over the energy range of 50–350 eV (the reference point of a zero potential) for this orbit segment, shown in Figure 2(i). We use a PA range of PA < 45° for parallel electrons and PA > 135° for antiparallel electrons, as the source/loss cones in the PA distributions are > ~45° within the time of the interest (Figure 2(d)). Note that the spacecraft crosses flux tubes along its trajectory, rather than traveling along a single field line. We assume that the electron population at the reference point is the source population before getting accelerated into the current sheet and that this source population is the same for neighboring flux tubes. We also calculate an error to determine if the two electron DFs are similar in shape, which is the averaged absolute relative difference in the phase space densities between the blue solid
and red dashed lines over an energy range of 50–350 eV. We use an empirical threshold of an error <0.5 to filter out bad results. Between 10:00 UT and 10:25 UT, the potentials outside of the current sheet crossing event are mostly within ±20 V, likely caused by the nominal tail potential arising from the electron pressure gradients within the Mars magnetosphere (S. Xu et al. 2021a). At the current sheet, the potential increases up to ~80 V. As the potentials inferred for the parallel and antiparallel directions are not significantly different, we consider them to be the same and do not discuss them separately hereafter.

Figure 3(c) is a collection of data between the two black dashed lines in Figure 2, where the inferred field-aligned potentials (U) (red and blue “+”) roughly linearly scale with the logarithmic local total ion density (log($n_i$)). During this time range, the electron temperatures (black “+”) are ~20–50 eV, a very small variation relative to 1 order of magnitude variation in $n_i$. Assuming a constant $T_e$, Equation (1) can be simplified as

$$e \nabla \phi = k_b T_e \nabla \ln(n_e). \quad (2)$$

That is, ambipolar potential $\phi$ linearly scales with $\ln(n_e)$, assuming a negligible variation in $T_e$. Our derived field-aligned potential U linearly scales with $\log(n_i)$ ($n_i = n_e$) and is thus ambipolar potential in nature. Note that this method assumes no temporal variation of the electron distributions or potential structures within the time range of interest. This simplification is not always valid; however, the observed linear scaling of the two parameters suggests that the assumption is reasonable in this case. Thus, we infer that the dominant electron acceleration mechanism is the ambipolar potential. We can also make a rough estimate of ambipolar potential using $\Delta\phi = \langle T_e \rangle \ln(n_1/n_2)$, where $\langle T_e \rangle$ is the averaged electron temperature and $n_1/n_2$ represents the density variation. Figure 3(c) gives a density ratio of ~10 ($\ln10 = 2.3$) and $\langle T_e \rangle = 26$ eV, which gives $\Delta\phi = \sim 60$ V. This linear scaling is shown as the cyan line in Figure 3(c). $\Delta\phi$ agrees well with independently derived field-aligned potentials, further confirming these potentials are the ambipolar potential.

### 3. Simple Experiment

While we can explain the electron flux enhancement above ~50 eV by an ambipolar potential, this cannot account for the observed distributions at lower energies shown in Figures 3(a)–(b). If electrons only experience an acceleration by an electric field alone, then the electron density should decrease to conserve the total flux. Observationally, we observe an electron density enhancement instead (Figure 2(g)), which means the total flux is not conserved in this case study. We use a simple experiment to reproduce and explain the observation.

An initial electron population is set to have a kappa distribution with an initial density of $n_0 = 0.5 \text{ cm}^{-3}$, $T_e = 19$ eV, and $\kappa = 4$, shown as the blue line in Figure 4(a). We then shift this distribution by a certain potential U and then fill the phase density hole between [0, eU] with a constant phase space density to make it flat top, shown as the red line in Figure 4(a) ($U = 70$ V). The choice of a flat-top distribution is an observed signature of electrons accelerated by an electrostatic potential (e.g., W. C. Feldman et al. 1983; J. Egedal et al. 2015; S. Xu et al. 2020b, 2021b; also in Figure 3(a)). We also show a modified electron distribution with the same total flux as the initial distribution and a flat-top shape at low energy and

the same shape at high energy to test whether the total electron flux could be conserved in the process, shown as the magenta dashed line in Figure 4(a). The maximum DF is reduced from ~$10^{-11}$ (blue line) to ~$10^{-13} \text{ cm}^{-3}$ ($\text{km s}^{-1})^{-3}$ (magenta line), by assuming accelerated electrons are modulated and energy-degraded by some processes to fill in the low-energy phase space density hole and no additional electrons needed. Both the red and magenta distributions have enhanced fluxes at high energies, as shown in Figure 4(b), but the electron density is increased to $8.5 \text{ cm}^{-3}$ for the red spectrum but is decreased to $0.2 \text{ cm}^{-3}$ for the magenta spectrum (as a result of flux conservation). This means this case study cannot be simply explained by electron acceleration while conserving the total electron flux. Meanwhile, the red spectra in Figures 4(a)–(b) agree decently well with the SWEA observations in Figures 3(a)–(b).

We accelerate the source electron DF (Figure 4(a), blue) over a range of electrostatic potentials ($U = [0, 100]$ V) and calculate how the DF evolves as a function of U, including the addition of a flat-top distribution at energies below eU. The red spectrum in Figure 4(a) is an example of $U = 70$ V. For each applied U, we obtain a new DF and then calculate the corresponding electron density and kinetic temperature. We plot the derived electron densities and temperatures as a function of U from this simple experiment as the blue and gray lines in Figure 4(c). With an increasing U from 0 to 100 V, the electron temperature (gray line) increases from 20 to 40 eV and the electron density (blue line) increases from 0.5 to $12 \text{ cm}^{-3}$. We also overlay the observations of $U-n$ (originally blue and red “+” in Figure 3(c)) as red “+” and the observations of $U-T_e$ (originally black “+” in Figure 3(c)) as gray “*” in Figure 4(c). The gray line agrees well with the observed electron temperature and captures the observed temperature increase. Note that there is a higher-temperature (40–50 eV) branch in the data (gray “*”) below ~1 $\text{cm}^{-3}$, corresponding to the lower electron density within 10:09:00–10:09:30 UT, which is irrelevant to the current sheet event. The blue line is also a better description of the observed $U-\ln(n)$ relation than the simple linear scaling of $U = \langle T \rangle \ln(n)$ (cyan line), because the blue line takes into account the effect of the electron temperature variation ignored in Equation (3).

Using this simple experiment, we can reproduce the MAVEN observations, including the electron flux enhancement, the electron density enhancement, the observed $U-\ln(n)$ relation, and the slightly enhanced temperature. It shows that additional electrons are needed to “fill the phase space density hole” at low energy to explain the observation. Likely, the initial charge imbalance caused by the beam of ions in the tail (at the current sheet in this case study) draws in more ambient electrons and/or electrons of a solar wind origin on the same field line very quickly to balance the charge, ending up with a distribution with a flat top at low energies (probably shaped by wave-particle interactions). A potential well set up by the ambipolar electric field “traps” low-energy electrons to maintain charge balance, while electrons with energies above the potential well move through the potential structure, getting accelerated and/or decelerated correspondingly. This also explains the low-energy (<100 eV) electron flux enhancement (10:14–10:20 UT) extended much longer after the current sheet crossing (10:10–10:14 UT) in Figure 2(e), a result of a still elevated total ion density relative to the lobe total density of ~0.5 $\text{cm}^{-3}$ (Figure 2(g)).
<figure><img src="image_6.png" /><figcaption>**Figure 4.** The same format as Figure 3. The three distributions in (a) and (b) are the initial electron distribution (blue), the accelerated electron population by $U = 70$ V (red) with a filled-in flat-top DF at energies <70 eV, and the accelerated electron population by $U = 70$ V (magenta) with a filled-in flat-top DF at energies <~120 eV. (c) The blue line is the analytical solution of the ambipolar potential $U_{\text{AM}}$ as a function of plasma density, the cyan line is the simplified solution of $U_{\text{AM}} = \langle T_e \rangle \ln(n)$, the gray line is the electron temperature as a function of plasma density, red "+" are data-derived field-aligned potentials (both parallel and antiparallel), and gray "*" are data-derived electron temperatures as a function of the data-derived ion density that are shown in Figure 3(c).</figcaption></figure>

## 4. Discussion and Conclusions

In this paper, we present a case study of the MAVEN observations to reveal a dominant electron acceleration mechanism across the tail current sheet to be the ambipolar electric field/potential. During this current sheet crossing, there is a beam of planetary ions, which creates a density gradient of more than 1 order of magnitude over a confined space. The large density gradient together with a high electron temperature (tens of eV) can result in an ambipolar potential of up to ~60 V. This agrees well with the inferred field-aligned potentials by comparing electron DFs. The field-aligned potential also shows an approximately linear relation with the local ion (plasma) density, an important clue of being a result of ambipolar potential. Furthermore, we use a simple

experiment to show that additional ambient electrons are drawn in forming a flat-top distribution at low energies (in phase space) to remove the initial charge imbalance caused by the planetary ion beam.

As discussed above, only ambipolar electric field $E_A$ could have a component parallel to $B$ ($E_{A//}$) to change electrons' total energy. Hence, the field-aligned potential obtained in this study is a result of the integration of $E_{A//}$ along the field line. If we further assume $\nabla \times E_A = 0$ and $E_A$ can thus be expressed as a potential field ($E_A = -\nabla\phi_A$), then the ambipolar potential is independent of the integration path and our derived field-aligned potential ($U$) would be the full ambipolar potential $\phi_A$. The fact that the derived potentials separately from parallel and antiparallel electrons (equivalent of different integration paths) are about the same suggests that $\nabla \times E_A = 0$ is a reasonable
assumption. Investigating the possible asymmetry in the potentials derived from parallel and antiparallel electrons ($\nabla \times E_A$ not exactly zero) is beyond the scope of this study.

Here we synthesize all the processes. Within the tail current sheet, accelerated planetary ion beams generate a density enhancement, which causes the electron density to increase to maintain charge neutrality and also the low-energy flux enhancement (the flat-top distribution). As shown in the lower panel of Figure 1, an electron density ($N$) gradient is formed from the center of the current sheet to the low-density tail lobes, together with a high temperature of ambient electrons ($T_e$), generating a large electron pressure ($P$) gradient and then ambipolar potential ($U$). This potential well traps low-energy electrons in the current sheet and helps ions diffuse outward, corresponding to the bell shape in the density structure (Figures 2(f)–(g)). Electrons with energies above the potential well can move through the current sheet and can, in turn, be used as test particles to trace the potential structure. As shown in the upper panel of Figure 1, parallel or antiparallel electrons enter from one side of the tail current sheet, first get accelerated by $E_{A//}$ on the same side, resulting in the high-energy electron flux enhancement, and then get decelerated by $E_{A//}$ (now opposite sign) on the other side before exiting the current sheet. The net result is that electrons gain little energy across the entire current sheet. Because $U$ is a potential field, any local $U$ can be derived from the energy gain of local electrons with respect to the zero-potential reference point (assumed to be the lobe regions surrounding the current sheet).

As noted in the Introduction, the ambipolar potential ($\phi_A$) is on the order of 1 V in the ionosphere but of $\sim \pm 100$ V in the shock structure and the magnetosphere. From Equations (1)–(2), the value of the ambipolar potential is on the order of the electron temperature. It means the dominant electron population has to be hot (order of 10 eV) to generate an ambipolar potential of tens of volts to significantly enhance electron fluxes at tens to hundreds of eV. In contrast, a small $\phi_A$ exists in environments dominated by the cold ($<1$ eV) electrons and has negligible effects on the hot electrons. It is worth noting that this mechanism is not limited to the tail current sheet but can occur wherever a large density gradient is created with hot ambient electrons. For example, a linear scaling between the field-aligned potential and the logarithm of the density of reflected solar wind protons in the lunar wake is reported by S. Xu et al. (2020c). We also note that other electron acceleration mechanisms might exist at the current sheet.

Electron acceleration is a fundamental part of space physics, and our study provides a novel approach to identifying one acceleration mechanism at the tail current sheet. This mechanism should be applicable to current sheets in many other planetary environments, including unmagnetized bodies (e.g., Venus and comets) and magnetized bodies (e.g., Earth). In the case of Mars, understanding electron acceleration is also important for characterizing the nightside ionosphere and auroral emissions.

## Acknowledgments

This work was supported by the National Aeronautics and Space Administration (NASA) grant NNH10CC04C to the University of Colorado and by subcontract to Space Sciences Laboratory, University of California, Berkeley. The MAVEN project is supported by NASA through the Mars Exploration Program. Parts of this work for the observations obtained with

the SWEA instrument are supported by the French space agency National Centre for Space Studies (CNES). S.X., C.F., and H.A. also thank support from NASA's Mars Data Analysis Program, grant 80NSSC22K1076.

The research described in this study utilizes publicly available data from the MAVEN mission, including data from the SWEA, MAG, and STATIC instruments (J. Connerney 2017; D. Mitchell 2017; J. P. McFadden 2023). All data were processed using the publicly available SPEDAS software (V. Angelopoulos et al. 2019).

## ORCID iDs

Shaosui Xu https://orcid.org/0000-0002-5121-600X

David L. Mitchell https://orcid.org/0000-0001-9154-7236

Jasper S. Halekas https://orcid.org/0000-0001-5258-6128

Christopher M. Fowler https://orcid.org/0000-0003-3431-0739

Kathleen Hanley https://orcid.org/0000-0001-6250-7665

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

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