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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

Mercury's magnetotail hosts a thin and highly dynamic current sheet (CS), where magnetic reconnection and strong fluctuations frequently occur. Here, we statistically analyze magnetic field power spectra across 370 magnetotail CSs observed by MESSENGER. About 20% of the events are quasi‐laminar, showing single power‐law spectra, whereas ∼80% are turbulent, exhibiting a spectral break separating inertial and kinetic ranges. A dawn–dusk asymmetry is identified: inertial‐range slopes are systematically shallower on the dawnside, whereas kinetic‐range slopes are steeper, indicating more developed turbulence there, consistent with the higher occurrence of reconnection‐related processes on the dawnside. Component analysis shows that the transverse components, orthogonal to the tail‐aligned principal field (BX), display shallow slopes near −1 in the inertial range, suggesting energy injection at ion scales rather than a classical inertial range. These results demonstrate that Mercury's unique plasma environment fundamentally reshapes the initiation of turbulence and the redistribution of energy in the magnetotail.

The Martian magnetotail is a dynamic region where several processes contribute to plasma acceleration. Here, we analyze ∼5 ${\\sim} 5$ years of Mars Atmosphere and Volatile EvolutioN (MAVEN) data to evaluate the role of magnetic tension forces in driving plasma acceleration within current sheets in the tail. Based on magnetic field measurements, we identify 547 current sheet crossings that follow a Harris profile and find that the median observed current sheet density is ∼ ${\\sim} $110 nA m−2 ${\\mathrm{m}}^{-2}$, with a typical sheet width of ∼ ${\\sim} $100 km. We estimate a median normalized |〈Bn〉| $\\vert \\langle {B}_{n}\\rangle \\vert $ of ∼ ${\\sim} $0.1, and J×Bn $\\mathbf{J}\\times {\\mathbf{B}}_{n}$ force of ∼10−16 ${\\sim} 1{0}^{-16}$ N m−3 ${\\mathrm{m}}^{-3}$, capable of accelerating planetary ions within the sheets to ∼ ${\\sim} $1 keV over ∼ ${\\sim} $2 RM ${\\mathrm{R}}_{M}$. We also analyze plasma energization signatures in nine high‐Bn ${\\mathbf{B}}_{n}$ case studies and find they can be explained by work done by J×Bn $\\mathbf{J}\\times {\\mathbf{B}}_{n}$, although observed ion differential streaming suggests additional forces may be present.

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

Particles are accelerated to very high, non-thermal energies in solar and space plasma environments. While energy spectra of accelerated electrons often exhibit a power law, it remains unclear how electrons are accelerated to high energies and what processes determine the power-law index δ . Here, we review previous observations of the power-law index δ in a variety of different plasma environments with a particular focus on sub-relativistic electrons. It appears that in regions more closely related to magnetic reconnection (such as the ‘above-the-looptop’ solar hard X-ray source and the plasma sheet in Earth’s magnetotail), the spectra are typically soft ( δ ≳ 4 ). This is in contrast to the typically hard spectra ( δ ≲ 4 ) that are observed in coincidence with shocks. The difference implies that shocks are more efficient in producing a larger non-thermal fraction of electron energies when compared to magnetic reconnection. A caveat is that during active times in Earth’s magnetotail, δ values seem spatially uniform in the plasma sheet, while power-law distributions still exist even in quiet times. The role of magnetotail reconnection in the electron power-law formation could therefore be confounded with these background conditions. Because different regions have been studied with different instrumentations and methodologies, we point out a need for more systematic and coordinated studies of power-law distributions for a better understanding of possible scaling laws in particle acceleration as well as their universality.

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

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.

Poleward boundary intensifications (PBIs) are one of the most common auroral disturbances and have been shown to play a key role in substorm dynamics. PBIs are commonly interpreted as the ionospheric signature of magnetotail reconnection, providing insight into X‐line evolution. While PBIs have been linked to ionospheric flow channels using line‐of‐sight observations, their two‐dimensional flow dynamics remain poorly understood. Using coordinated THEMIS All‐Sky Imager and SuperDARN observations, we present high‐resolution two‐dimension flows derived by the spherical elementary current systems (SECS) method in association with PBI morphology. AMPERE magnetic perturbation data are presented for one event to analyze the current system associated with a dawn‐cell PBI. Ionospheric flow channel observations are used to infer plasma inflow in the magnetotail, and our results show they play a determining role in PBI morphology, including spatial extent, number, and propagation. These results suggest that inflow properties strongly influence X‐line dynamics.

Magnetic reconnection is a fundamental plasma process that alters the magnetic field topology and releases magnetic energy. Most numerical simulations and spacecraft observations assume a two‐dimensional diffusion region, with the electron diffusion region (EDR) embedded in the same plane as the ion diffusion region (IDR) and a uniform guide field throughout. Using observations from Magnetospheric Multiscale mission, we report a non‐coplanar, knotted EDR in Earth's magnetotail current sheet. The reconnection plane of the knotted EDR deviates by approximately 38° from that of the IDR, with the guide field exhibiting both a 38° directional shift and a twofold increase in amplitude. Moreover, the Hall magnetic field is bipolar in the EDR but quadrupolar in the IDR, indicating different Hall current structures at electron and ion scales. These observations highlight the importance of three‐dimensional effects and illustrate the complexity of multiscale coupling between the EDR and IDR during reconnection studies.

During geomagnetic storms, cold ions from the ionosphere substantially contribute to the plasma sheet population, which may significantly impact magnetotail energy conversion and transport. However, the energization of cold ions in the magnetotail during storm periods remains poorly understood. This paper investigates cold ion energization by secondary reconnection in a magnetotail reconnection outflow during a storm period. This secondary reconnection was triggered by the interaction between a large‐scale flux pileup region and a magnetic flux rope. Cold protons originating from the ionosphere were injected into the diffusion region from the lobe region. These cold protons gained energy mainly through the normal electric field in the flux pileup region before entering the cold ion diffusion region, where their bulk kinetic energy was mainly converted into thermal kinetic energy. These results will significantly advance our understanding of the energization and transport of cold ions in the magnetotail. Plain Language Summary It is currently unclear how cold ions from the ionosphere gain energy in the magnetotail during geomagnetic storms. Using observations from the MMS satellite, we found that secondary reconnection occurs between a large‐scale magnetic flux rope and a magnetic flux pileup region within the turbulent reconnection outflow. The cold protons from the ionosphere during storms can directly enter the magnetotail reconnection outflow region and get accelerated and heated by the secondary reconnection. We conducted a detailed quantitative analysis of the energy changes and energy gains of cold ions. These results significantly enhance our understanding of how cold ions gain energy and are transported during storms. Key Points Cold ionospheric H+ ions can enter the magnetotail reconnection jet and then be energized by secondary reconnection therein Cold ions gain bulk kinetic energy predominantly through the normal electric field outside the cold ion diffusion region (IDR) In the cold IDR, the cold ion bulk kinetic energy is mainly transferred to thermal kinetic energy