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Using measurements from the Mars Atmosphere and Volatile EvolutioN mission, we investigate the densities of H+ (nH+${n}_{{\\mathrm{H}}^{+}}$ ), O+ (nO+${n}_{{\\mathrm{O}}^{+}}$ ), and O2+ (no2+${n}_{{\\mathrm{o}}_{2}^{+}}$ ), respectively, in the Martian magnetotail current sheet. We find that the current sheet when it is closer to the terminator than 0.75 Mars radii is mostly dominated by heavy ions ((nO++no2+${n}_{{\\mathrm{O}}^{+}}+{n}_{{\\mathrm{o}}_{2}^{+}}$ )>2 nH+${n}_{{\\mathrm{H}}^{+}}$ ), regardless of the variation of the upstream solar wind, but that it is sometimes dominated by H+ (nH+${n}_{{\\mathrm{H}}^{+}}$>2(nO++no2+${n}_{{\\mathrm{O}}^{+}}+{n}_{{\\mathrm{o}}_{2}^{+}}$ )) at downstream distances exceeding 0.75 Mars radii. The occurrence rate of the dominant H+ weakly increases (and that of the heavy ions decreases) with solar wind density and dynamic pressure. Our results suggest that solar wind protons could enter the Martian tail and may become the dominant ion species in the current sheet, particularly when the solar wind density or dynamic pressure is high. Plain Language Summary The current sheet of the Martian magnetotail is a major channel for the escape of planetary ions. The ion composition in the current sheet is essential to our understanding of this escape, as well as the magnetotail plasma dynamics. Our current knowledge, however, is poor. Based on the measurements of the ion density of different species in the current sheet from the Mars Atmosphere and Volatile EvolutioN spacecraft, we report that the current sheets we have surveyed are dominated by either the heavy ions from the planet or H+ (mostly) from the solar wind. We find that the downstream distance and the variation of the upstream solar wind are the two key factors that account for which ion species dominates in the tail current sheet. Key Points Current sheets are mostly dominated by heavy ions but are sometimes dominated by H+ at the downstream distance exceeding 0.75 Mars radii The occurrence rate of current sheets with dominant H+ (heavy ions) weakly increases (decreases) with solar wind density and dynamic pressure Our results suggest that the dominant H+ in the current sheet could originate from solar wind

Onset of reconnection in the tail requires the current sheet thickness to be of the order of the ion thermal gyroradius or smaller. However, existing isotropic plasma models cannot explain the formation of such thin sheets at distances where the X‐lines are typically observed. Here we reproduce such thin and long sheets in particle‐in‐cell simulations using a new model of their equilibria with weakly anisotropic ion species assuming quasi‐adiabatic ion dynamics, which substantially modifies the current density. It is found that anisotropy/agyrotropy contributions to the force balance in such equilibria are comparable to the pressure gradient in spite of weak ion anisotropy. New equilibria whose current distributions are substantially overstretched compared to the magnetic field lines are found to be stable in spite of the fact that they are substantially longer than isotropic sheets with similar thickness. Plain Language Summary Ion scale current sheets forming sufficiently far from Earth are necessary to explain its stretched magnetic field reconfiguration on the night side. However, isotropic plasmas form magnetic fields that inflate with distance from Earth and cannot reproduce the observed stretched geometry. We present kinetic simulations of current sheets that inflate more gradually with distance due to slight field‐aligned anisotropy of the ion species. Their formation is provided by a special population of suprathermal ions with figure‐of‐eight orbits. We find that the resulting current sheets are stable over a long time scale and have a thickness comparable to the size of these orbits. Key Points Two‐dimensional ion‐scale current sheets stretched way beyond the isotropic limit are reproduced in particle‐in‐cell simulations Weak ion anisotropy and agyrotropy substantially modify the current density and the isotropic force balance Ion‐scale current sheets are stable in spite of the fact that they are longer compared to isotropic sheets with similar thickness

The current sheet is crucial in releasing magnetic free energy in cosmic plasmas via fast magnetic reconnection or wave excitation. This research investigates the characteristics of the Martian tail current sheet by analyzing data acquired from the Mars Atmosphere and Volatile EvolutioN spacecraft over 7 years. For the first time, we find that approximately 15% of the current sheet events display reconnection signatures, including Hall magnetic fields and fast proton flows. These reconnecting current sheets are detected more frequently on Mars than on Earth. The Martian tail current sheet is first demonstrated to be on the proton scale, which can explain the high reconnection occurrence rate. The research also reveals that the current sheets are thinner in regions closer to Mars and in the –E hemisphere. On average, reconnecting current sheets carry high fluxes of protons rather than oxygen ions. Plain Language Summary As a ubiquitous plasma configuration in various cosmic plasma environments, the current sheet is critical in facilitating the release of magnetic energy through explosive processes, such as fast magnetic reconnection. On Earth, magnetic reconnection in the tail current sheet can be responsible for triggering substorms and generating auroras. Recent observations have indicated that reconnection in the Martian tail enhances ion escape from the planet’s atmosphere. However, the frequency of reconnection events in the Martian tail and their involvement in ion loss is still not fully understood based on previous few research. In this study, we use data from the Mars Atmosphere and Volatile EvolutioN spacecraft to study the Martian tail current sheet. For the first time, we find that almost 15% of the current sheet events on Mars have reconnection signatures, happening more often than on Earth. The high occurrence rate of magnetic reconnection at the Martian tail current sheet can be attributed to its extremely thin structure, which is on the scale of protons. Magnetic reconnection may drive high fluxes of hydrogen ions and thus potentially impact the evolution of the Martian atmosphere. Key Points MAVEN recorded 880 current sheet events in the Martian tail in the past 7 years and about 15% of them show reconnection features The average thickness of the current sheet is on the proton scale and is thinner in the −E hemisphere The reconnecting tail current sheet carries a higher net tailward flux of hydrogen ions

Using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, we explore the plasma properties of Martian magnetotail current sheets (CS), to further understand the solar wind interaction with Mars and ion escape. There are some CS exhibit energetic oxygen ions that show narrow beam structures in the energy spectrum, which primarily occurs in the hemisphere where the solar wind electric field (Esw) is directed away from the planet. On average, these CS have a higher escaping flux than that of the CS without energetic oxygen ion beams, suggesting different roles in ion escape. The CS with energetic oxygen ion beams exhibits different proton and electron properties to the CS without energetic oxygen ion beams, indicating their different origins. Our analysis suggests that the CS with energetic oxygen ion beams may result from the interaction between the penetrated solar wind and localized oxygen ion plumes. Plain Language Summary Ion escape into space, driven by solar wind interactions with Mars, plays a pivotal role in the evolution of the Martian atmosphere. An important escape channel of planetary oxygen ions is the current sheet in the nightside magnetotail. Yet, our existing understanding of plasma characteristics within this magnetic structure remains quite limited. Based on the MAVEN observations, we find the current sheets can be categorized into two distinct types according to the energy distribution patterns of oxygen ions: one is with the appearance of energetic oxygen ions with narrow beam structure, the other one is not. On average, the current sheets with energetic oxygen ion beams have a higher escaping flux than those without, suggesting different roles in ion escape. Furthermore, the two types of current sheets exhibit markedly distinct plasma properties, indicating that they have different origins. Here we suggest that the current sheet with energetic oxygen ion beams arise from the interaction between the penetrated solar wind and localized oxygen ion plumes. Key Points Martian magnetotail current sheets occasionally exhibit energetic oxygen ions that show beam structures in the energy spectrum The current sheets with energetic oxygen ion beam usually have a higher escaping flux than those without Plasma properties in current sheets differ significantly differences between those with and without energetic oxygen ion beams

Thin current sheets (TCS) in Earth's magnetotail are fundamental to magnetospheric dynamics. A key question concerning static magnetotail TCSs is the mechanism of radial force balance. Using the unprecedented measurements from the Magnetospheric Multiscale mission, we statistically analyze TCS crossing events from 2017 to 2020 to investigate this issue. Our analysis reveals a strong magnetic tension within TCSs, with the radial thermal pressure gradient accounts for only 10%–30% of the required balance. The off‐diagonal pressure components (Pi,xz and Pe,xz) are crucial for achieving force balance, contributing ∼55% of the required force in the further‐Earth region (−30 RE < X < −20 RE, where RE is Earth's radius), and ∼30% in the near‐Earth region (−20 RE < X < −10 RE). This work provides the first direct observational evidence demonstrating that particle kinetic effects (ion nongyrotropy and electron pressure anisotropy) play a significant role in the force balance of magnetotail TCSs.

The Martian magnetotail current sheet serves as a critical pathway for ionospheric ion escape. Contrary to the conventional view that external magnetic pressure is balanced mainly by internal ion thermal pressure, we present novel observations from the Mars Atmosphere and Volatile Evolution spacecraft of an electron‐dominated pressure balance configuration. The current sheet electrons exhibit two distinct populations: a thermal core of ionospheric origin and a suprathermal shell of magnetosheath origin. Their bulk temperature reaches up to three times higher than that outside the current sheet. Based on linear instability analysis, we propose two candidate heating mechanisms: (a) Landau resonant or transit‐time heating by magnetosonic waves likely originating from the magnetosheath, and (b) Landau or cyclotron resonant heating by whistler and electron cyclotron harmonic waves generated spontaneously from the shell‐like electron velocity distribution. These results highlight the potentially significant role of plasma waves in sustaining the Martian atmospheric escape channels.

Two types of filamentary currents (FCs) were observed inside a magnetic flux rope at the magnetopause by the Magnetospheric Multiscale mission. The first FC is identified as an electron vortex, while the other is a reconnecting current sheet. Stochastic electric fields were generated within the FCs, resulting in electron acceleration up to a few keV, similar to recent simulations of electron acceleration inside vortex, which is a second‐order Fermi acceleration. Furthermore, two FCs propagated at different speeds, causing compression in the region between them. Energetic electrons up to 200 keV were detected in the compressed region and displayed a double power‐law spectrum. Observations suggest that the electrons were mainly accelerated by betatron mechanism in the compressed region. The formation, evolution, and interaction of FCs provide a novel mechanism for electron acceleration. These results clearly show the significance of electron‐scale dynamics within flux rope. Plain Language Summary Magnetic reconnection is a fundamental plasma process by which magnetic energy is converted into the kinetic energy of charged particles. Understanding the acceleration mechanisms for the energetic electrons during magnetic reconnection is a long‐standing question in the study of space and astrophysical plasmas. Using Magnetospheric Multiscale observations at Earth's magnetopause, we present in situ evidence of electron acceleration up to 200 keV between two consecutive filamentary currents (FCs) inside a magnetic flux rope. Two FCs propagate at different speeds, with the second moving faster, thus causing a compressed region between them. These results provide an important new way for electron acceleration in magnetic reconnection. Key Points Two types of filamentary currents (FCs) were observed near the center of a magnetic flux rope Stochastic electric fields were generated inside two FCs and accelerated electrons Electrons were accelerated up to 200 keV in the compressed region between two currents by the betatron mechanism

The Jovian magnetosphere is highly dynamic, influenced by both solar wind and internal processes associated with the rapid planetary rotation and Io's volcanic activities. Accompanying the mass and energy circulations driven by the magnetospheric dynamics, the magnetic configuration also changes dramatically. One of the crucial parameters to characterize the magnetic configuration is magnetic field line curvature (FLC), which generally describes how stretched the field line is. The curvature is pivotal to influence particle behaviors, for example, pitch angle scattering which may lead to auroral particle precipitation. In this work, a method is proposed to investigate the real‐time magnetic FLC in Jovian current sheet using the magnetic field data from the Juno spacecraft. The results indicate that the FLC scattering of ions and relativistic electrons are common in Jovian magnetosphere, providing a crucial insight to understand the particle behaviors. Plain Language Summary Both the Earth and the Jupiter have intrinsic magnetic field. When the planetary magnetic field interacts with the solar wind, a region called magnetosphere is formed. Particle behaviors in different planetary systems are different, due to the different magnetospheric dynamics. The curvature of magnetic field, describing the stretch level of a magnetic field line, is a basic parameter to describe a planetary space system, and it can significantly influence particle behaviors, for example, to scatter the magnetospheric particles to planetary atmosphere, causing auroral emissions. In this work, we proposed a method to calculate the magnetic field line curvature (FLC) near the equatorial plane inside the Jupiter's magnetosphere using Juno data set, for the first time to provide a global picture on the magnetic FLC. By comparing with the radius of particles' gyration motions, we suggest that ions and electrons can be strongly scattered by the magnetic FLC. We believe that the results in this study provide useful information on the different particle behaviors between the terrestrial system and the Jovian system. Key Points We proposed a method to investigate the magnetic field line curvature (FLC) in Jupiter's current sheet using data from Juno data set 50 events are selected by specific criteria. The magnetic FLC and different particles' Larmor radius are investigated The FLC will scatter ions and relativistic electrons as a potential cause of auroral precipitation

We have conducted a survey of 575 slow-to-fast stream interaction regions (SIRs) using Solar Terrestrial Relations Observatory (STEREO) A and B data, analyzing their properties while extending a Level-3 data product through 2016. Among 518 pristine SIRs, 54% are associated with heliospheric current sheet (HCS) crossings, and 34% are without any HCS crossing. The other 12% of the SIRs often occur in association with magnetic sectors shorter than three days. The SIRs with HCS crossings have slightly slower speeds but higher maximum number densities, magnetic-field strengths, dynamic pressures, and total pressures than the SIRs without an HCS. The iron charge state is higher throughout the SIRs with an HCS than the SIRs without an HCS, by about 1 / 3 charge unit. In contrast with the comparable phases of Solar Cycle 23, slightly more SIRs and higher recurrence rates are observed in the years 2009 – 2016 of Cycle 24, with a lower HCS association rate, possibly attributed to persistent equatorial coronal holes and more pseudo-streamers in this recent cycle. The solar-wind speed, peak magnetic field, and peak pressures of SIRs are all lower in this cycle, but the weakening is less than for the comparable background solar-wind parameters. Before STEREO-B lost contact in October 2014, 151 SIR pairs were observed by the twin spacecraft. Of the dual observations, the maximum speed is the best correlated of the plasma parameters. We have obtained a sample of plasma-parameter differences analogous to those that would be observed by a mission at Lagrange points 4 or 5. By studying several cases with large discrepancies between the dual observations, we investigate the effects of HCS relative location, tilt of stream interface, and small transients on the SIR properties. To resolve the physical reasons for the variability of SIR structures, mesoscale multi-point observations and time-dependent solar-wind modeling are ultimately required.

We present one case study of magnetic islands and energetic electrons in the reconnection diffusion region observed by the Cluster spacecraft. The cores of the islands are characterized by strong core magnetic fields and density depletion. Intense currents, with the dominant component parallel to the ambient magnetic field, are detected inside the magnetic islands. A thin current sheet is observed in the close vicinity of one magnetic island. Energetic electron fluxes increase at the location of the thin current sheet, and further increase inside the magnetic island, with the highest fluxes located at the core region of the island. We suggest that these energetic electrons are firstly accelerated in the thin current sheet, and then trapped and further accelerated in the magnetic island by betatron and Fermi acceleration. Key Points Strong core fields, density depletion, intense currents inside magnetic islands Energetic electron increase in the thin current sheet, and magnetic island Energetic electrons are first accelerated in thin current sheet, then in island