AbstractThe tailward high‐speed flows, in which various kinetic processes and magnetic structures can be embedded, are usually produced by the magnetic reconnection in the Earth's magnetotail. Here, using high‐resolution measurements from the Magnetospheric Multiscale (MMS) mission, we report an intense current in the tailward high‐speed flow. The current density can reach ∼180 nA/m2 and is primarily carried by electrons. Taking advantage of the First‐Order Taylor Expansion (FOTE) method, we reveal that such an intense current is associated with a ribbon‐like magnetic structure. A large electric field, reaching ∼90 mV/m, is also observed at the magnetic structure. The Hall term dominates the electric field, however, the contribution from the pressure gradient term and the electron inertial term is nonnegligible and can lead to strong energy conversion (E ⋅ J > 2 nW/m3) through the synergistic action with the intense current. This study improves the understanding of the current behaviors and energy conversion associated with magnetic structures in the Earth's magnetotail.

Magnetic holes (MHs) are transient magnetic structures responsible for energy conversion in space plasma. Using single-spacecraft measurements from Mars Atmosphere and Volatile EvolutioN (MAVEN), the existence of MHs on Mars has been confirmed. However, due to the limitations of single-spacecraft observations, significant uncertainty also arises on the identification of the spatial scale and 3D geometry of MHs. In this study, we report a series of MHs successively detected by Tianwen-1 near the high-latitude magnetopause and by the MAVEN spacecraft near the subsolar magnetopause. The large separation between Tianwen-1 and MAVEN (similar to 4 R M) suggests these MHs are macroscale structures extending along the axial direction. Additionally, we observe whistler waves generated by electron perpendicular anisotropy in one of the macroscale MHs. This study is the first joint observation of Martian MHs, shedding light on the research of transient magnetic structures on Mars.

A whistler wave is a fundamental electromagnetic radiation. In the universe, it transmits information through diverse mediums at audio frequencies. To know the properties of such a radio, one should determine how its frequency (energy) changes with its wavelength (momentum). The relation between them is called the dispersion relation, which encodes essential information on the kinetics. Although the dispersion relation governs the majority of the whistler properties, the experimental determination of their entire dispersion is still a challenge today. Especially in high-temperature mediums, the group velocity dispersion properties of whistlers are still unverified by spacecraft experiments despite their practical importance. Here, for the first time, we experimentally determine the dispersion relation of whistler waves in superhot (T_{e}>5keV) plasmas by the magnetospheric multiscale mission located in a magnetotail plasma sheet. Our result unmasks the characteristics of whistler waves at elevated temperatures and exhibits the evolution of their group velocity dispersion from positive to negative, which agrees well with kinetic theory, opening the door to controlled radio excitation and transport.

Anti-dipolarization fronts (ADFs), characterized by the rapid increase of the negative magnetic field Bz component, are typically formed at the leading edge of the tailward reconnection jets in the Earth's magnetotail. To date, the electron-scale current structures, which govern the energy conversion at ADFs, are still barely understood due to the lack of high-resolution measurements. Here, using Magnetospheric Multiscale mission, we for the first time report a tailward ADF associated with strong field-aligned current (FAC). The FAC appears at the leading part of the ADF and its densities can reach about 200 nA/m2, which is significantly larger than those reported before. Such current is primarily contributed by the electron flow, which also forms electron beam distribution in the anti-parallel direction. Significant energy conversion (E & sdot; J, E is electric field and J is current density) is also observed at the ADF, which is mainly contributed by the FAC and the fluctuating electric fields. This study makes essential steps toward understanding the current system and the energy conversion at the ADF in the Earth's magnetotail. Dipolarization fronts (DFs) and anti-dipolarization fronts (ADFs) are the leading edge of the reconfigured magnetic field after reconnection, expanding earthward and tailward from the reconnection site respectively. The field-aligned current (FAC) at DFs play crucial roles in global mass transport and energy conversion during substorm and they have been widely studied by in situ observation and simulation. However, the FAC at ADFs and the corresponding energy conversion have not been studied so for. Here, for the first time, we report an ADF associated with strong FAC using high-resolution data from the MMS mission. We investigate the detailed features of the currents, electron distribution functions, electric fields and energy conversion at the ADF. This study shed light on how strong FAC drive energy conversion at ADFs. We for the first time report a tailward anti-dipolarization front (ADF) associated with strong field-aligned current (FAC) The FAC is primarily carried by electrons and can reach similar to 200 nA/m2, which is even stronger than the FACs reported in earthward Dipolarization fronts The strong FAC and fluctuating electric fields can drive significant energy conversion at the ADF

AbstractThe electron rolling‐pin distribution, showing electron pitch angles primarily at 0°, 90°, and 180°, has been widely studied in the Earth's magnetosphere, but has never been reported in other planetary environments. Here, by utilizing the Jupiter Near‐polar Orbiter (Juno) measurements, we report for the first time the electron rolling‐pin distribution in Jupiter's magnetosphere. We reveal the energy range of such distribution and find it appears only above 19.5 keV, falling well into the suprathermal energy range. Moreover, we quantitively reproduce the formation processes of such distribution by using an analytical model. Gratifyingly, the distribution derived from the analytical model agrees well with the Juno observations, indicating such distribution is formed by the combination of global‐scale Fermi acceleration and local‐scale betatron acceleration. These results, demonstrating that the electron rolling‐pin distribution exists beyond the Earth, can improve our knowledge of electron dynamics in planetary magnetosphere.

AbstractMagnetic hole (MH), sometimes referred to as magnetic bottle, exhibits strong magnetic fields at its neck but weak magnetic fields at its belly. Such structure has been widely reported in the solar wind and the Earth's magnetosphere, but has not been reported at Jupiter. Here, for the first time, we report two MHs in the Jupiter's magnetosphere by utilizing measurements of the Juno mission, with one existing in the dawn side and the other existing in the dusk side. We find that the electron pitch‐angle distribution inside the MHs can be either cigar‐type or pancake‐type. The cigar‐type distribution probably appears at the belly of the MH, whereas the pancake‐type distribution probably appears at the neck. Our analyses of the depression of magnetic fields inside the MHs support such a conjunction. These results have advanced our understanding of the transient structure and its related electron dynamics in the giant planets' magnetosphere.

AbstractThe Martian magnetotail is largely controlled by the solar wind (SW) and is modulated by variations in the upstream drivers. However, due to the limitations of single‐spacecraft observations, the effects of SW variations on the Martian magnetotail have not been fully understood so far. Here, using Tianwen‐1 and MAVEN data, we report for the first time the rapid response of Martian magnetotail to the SW disturbance. In our study, Tianwen‐1 detected the flapping of Martian magnetotail, while MAVEN monitored disturbances in the upstream SW. The results indicate that a 20% increase (or decrease) in SW dynamic pressure and a 30° (or 50°) rotation of interplanetary magnetic field clock angle could cause the Martian magnetotail to swing rapidly. These two SW disturbances could lead to oscillations of the Martian magnetotail. This study reveals the importance of joint observations for studying the interaction between the SW and Mars.

Betatron acceleration, a plasma process obtaining particle energy in the perpendicular direction but reserving energy in the field-aligned direction, is the consequence of magnetic strength enhancement when the first adiabatic invariant is conserved. Such process has been widely reported in the terrestrial magnetosphere but is barely reported in other planetary environments. Here, based on the in situ measurements from the Mars Atmosphere and Volatile Evolution mission, we report two events of betatron acceleration upstream of the Martian bow shock. In both events, betatron accelerations increase the fluxes of suprathermal electrons. The acceleration processes in these events are quantitatively reproduced with an analytical model. Gratifyingly, we find the acceleration factors derived from the analytical model are well consistent with the observations of magnetic strength enhancement. These results for the first time show that the betatron acceleration is an active upstream of the Martian bow shock and is very useful to help us understand the generation of energetic electrons in the Martian environment.

The magnetotail plasma sheet boundary layer (PSBL) is a dynamic boundary layer between the hot-denser plasma sheet and the cold-tenuous tail lobes. It plays an important role in exchanging mass and energy in the magnetotail. In previous studies, the local current carried by the electron beams has been well understood. The strong energy conversion (E & sdot; J, E is electric field and J is current density), however, is barely reported at the PSBL. Here, using magnetospheric multiscale mission, we find a strong dawn-dusk current with a magnitude of 80 nA/m(2) at the magnetotail PSBL. The strong current appears during crossing the PSBL and is primarily contributed by the perpendicular electron currents. Cooperating with an intense fluctuating electric field (reaching similar to 40 mV/m) carried by the lower-hybrid drift waves, this dawn-dusk current leads to a strong energy conversion with a magnitude larger than 3 nW/m(3). This study enhances the understanding of local energy-conversion processes at the PSBL.

We present the first comparative observation of two dipolarization fronts (DFs) in cold‐dense plasma sheet (CDPS) and hot‐tenuous plasma sheet (HTPS) conditions. We find that: (a) the CDPS and HTPS are formed under distinct solar wind conditions. (b) The DF in HTPS has a higher B z increase than the DF in CDPS; (c) DF in CDPS can transfer energy from magnetic field to particles more efficiently than the DF in HTPS; (d) the >2 keV (<2 keV) electron phase space density of flux pileup region (FPR) in HTPS is higher (lower) than that of FPR in CDPS; (e) the electron cyclotron harmonic waves are excited at DF in HTPS instead of the DF in CDPS. (f) The responses in the ionospheric currents are weak for both DFs in CDPS and HTPS. These results reveal the differences in properties and physical processes between the DFs in CDPS and HTPS. Our findings can significantly improve the understanding of physical mechanisms underlying the formation and evolution processes of DFs.

Magnetic discontinuities are fundamental structures in space and laboratory plasmas where the changes in magnetic and velocity fields are constrained by Rankine–Hugoniot relations. Due to the absence of precise measurements for particles, some issues therein are hardly investigated. The nature of discontinuities driven by the magnetohydrodynamics (MHD) turbulence, and the intermediate shock are two puzzles to be solved. The MHD turbulence generates numerous discontinuities with both small normal magnetic fields and nearly constant magnetic field magnitudes in statistics. By utilizing the data from the Parker Solar Probe, we identify among the turbulence-driven discontinuities two components that exhibit diverse statistical characteristics of the plasma density, and reveal that these discontinuities comprise 80.2% rotational and 19.8% tangential discontinuities. Then, we note a special class of discontinuities within 0.35 au that have jump conditions similar to that of the rotational discontinuity and the shock simultaneously, including (1) positively correlated jumps in the plasma density and temperature, (2) a small change in the magnetic field magnitude, and (3) opposite tangential magnetic fields on two sides. These features conform to the theoretical intermediate shock, which previous studies have found to not practically exist due to the breakdown of the evolutionary condition. By the conservation law of the mass flux across a boundary, we calculate their propagation speeds and find three intermediate shock candidates with super-Alfvénic upstream and sub-Alfvénic downstream flows. This work can improve our understanding of plasma intermittencies and suggests reassessing conclusions based on ideal MHD Rankine–Hugoniot relations.

Abstract The Earth's magnetopause is an ion‐scale boundary that separates the magnetosphere from the shocked solar wind. At such boundary, energy‐conversion processes frequently occur. Previous studies suggested that such energy conversions are related to the magnetic reconnection process. Here, we report a new mechanism that the coaction between the twisted magnetic structure and lower‐hybrid waves can also drive energy conversion (J⋅ E, J is current density and E is electric field) at the magnetopause boundary layer, by using high‐resolution measurements of the four Magnetospheric Multiscale spacecraft. We find that such energy conversion is efficient (with magnitude up to 20 nW/m3) and is attributed to an intense current filament (j ≈ 3,800 nA/m2) and a fluctuating electric field driven by lower‐hybrid waves. With the help of the First‐Order Taylor Expansion method, we find that the intense current filament is driven by a twisted magnetic structure at electron scale. Our study improves the understanding of energy‐conversion processes at the Earth's magnetopause.

Energetic electrons have been frequently observed during magnetic reconnection in the magnetotail. The acceleration process of the energetic electrons is not fully understood. In this paper, we select for a detailed study a case of energetic electron acceleration from the earlier reported interval of turbulent magnetic reconnection in Earth’s magnetotail observed by the Magnetospheric Multiscale mission. We use the first-order Taylor expansion method to reconstruct the magnetic topology of electron acceleration sites from the data. We find that the energetic electron fluxes increase inside the flux rope forming in front of the magnetic pileup region. We show that the energetic electrons are produced by a two-step process where two different acceleration mechanisms are successively operating outside and inside the flux rope. First, the thermal electrons are energized in the field-aligned direction inside the magnetic pileup region owing to the Fermi mechanism forming a cigar-like distribution. Second, those energized electrons are further accelerated predominately antiparallel to the magnetic field direction by a parallel electric field inside the flux rope. Our findings provide information for a better understanding of the generation of energetic electrons during turbulent reconnection process.

Abstract The electron butterfly distribution, characterized by pitch angles (PA) primarily at 45° and 135°, was rarely observed in Earth's magnetotail. Here using the high‐resolution measurements from Magnetospheric Multiscale mission, we present the observation of electron butterfly distribution in a contracting dipolarization front (DF), and propose a new physical mechanism to explain its formation. Specifically, we discover that the electron butterfly distribution only exhibited in the locally contracted DF and was observed above 1.7 keV. We infer that local contraction of the DF transformed its configuration from a magnetic bottle to an hourglass‐shaped magnetic structure, and the butterfly distribution was formed by the magnetic mirror effect of this magnetic hourglass. Additionally, the theoretically estimated loss cone of the magnetic hourglass fits well with the observations of electrons, validating our inference about the formation mechanism. These findings can improve our understanding of electron dynamics in Earth's magnetosphere.

We perform a statistical investigation of the occurrence rates of energetic electron (100–500 eV) pancake pitch-angle distributions (PADs) in the Martian space environment by utilizing 6 yr of MAVEN data. In the Martian ionosphere, we find the following: (1) at the same altitude in the terminator and night regions, the occurrences rates in the center of the southern magnetic anomaly regions are very low, but at the edges of strong magnetic fields, they increase significantly; (2) the occurrence rates of energetic electron perpendicular anisotropies on the Martian dayside increase with altitude; and (3) some closed magnetic lines in the 10°S–55°S, 30°W–125°W region at 400–800 km altitude gradually become open during the rotation of Mars from duskside to dawnside, while more closed magnetic lines are produced in the 40°S–65°S, 35°E–90°E region. In the Martian induced magnetosphere, we find the following: (1) the high-energy electron perpendicular anisotropy in the magnetosheath is the most significant; (2) the occurrence rates in the southern ( Z MSO ≤−1 R M ) magnetosheath are higher than those in the northern ( Z MSO ≥ 1 R M ) magnetosheath; (3) in the region of ∣ Z MSO ∣ < 0.5 R M , these high-energy electron pancake PADs are mainly concentrated in the magnetosheath region with Y MSO ∈ [−1.4R M , 2 R M ]; (4) the occurrence rates in the dawnside ( Y MSO ≤−1 R M ) magnetosheath are higher than those in the duskside ( Y MSO ≥ 1 R M ) magnetosheath; and (5) in the region of ∣ Y MSO ∣ < 0.5 R M , the occurrence rates throughout the magnetosheath are very high.

Magnetic flux ropes or magnetic islands are important structures responsible for electron acceleration and energy conversion during turbulent reconnection. However, the evolution of flux ropes and the corresponding electron acceleration process still remain open questions. In this paper, we present a comparative study of flux ropes observed by the Magnetospheric Multiscale mission in the outflow region during an example of turbulent reconnection in Earth's magnetotail. Interestingly, we find the farther the flux rope is away from the X-line, the bigger the size of the flux rope and the slower it moves. We estimate the power density converted at the observed flux ropes via the three fundamental electron acceleration mechanisms: Fermi, betatron, and parallel electric field. The dominant acceleration mechanism at all three flux ropes is the betatron mechanism. The flux rope that is closest to the X-line, having the smallest size and the fastest moving velocity, is the most efficient in accelerating electrons. Significant energy also returns from particles to fields around the flux ropes, which may facilitate the turbulence in the reconnection outflow region.

Using high-resolution Solar Wind Electron Analyzer data from the Mars Atmosphere and Volatile EvolutioN spacecraft, we report two new types of electron pitch angle distributions (PADs) in the Martian plasma environment. The first type of electron PADs, showing pitch angle primally around 45 degrees, is termed funnel distribution, which was observed near the terminator and at about 1,200 km altitude. The second type of electron PADs, showing pitch angle primally around 135 degrees, is termed skirt distribution, which was detected on the nightside and at about 1,250 km altitude. The electrons of funnel and skirt distributions do not exhibit any photoelectron signatures and are shown to originate from the solar wind. Through the fitting analysis, we find that the electrons showing both funnel and skirt PADs are thermal electron populations and suprathermal electron populations. In addition, the possibilities that such two types of electron pitch angle distributions corresponding to magnetic field configurations are also discussed.

Reconnection fronts, also known as dipolarization fronts (DFs), and kinetic Alfvén waves (KAWs) are two of the widely observed structures and waves in the terrestrial magnetotail, but their correlation remains elusive in previous spacecraft measurements. Using high-resolution Magnetospheric Multiscale data, here we provide the first observational identification of KAWs behind the DF. These low-frequency dispersive KAWs propagate quasi-perpendicular to the magnetic field at phase speeds slower than local Alfvén speed, and are highly correlated with high-frequency whistlers propagating obliquely to the magnetic field. Specifically, whistlers appear at local magnetic field minima (wave troughs of KAWs) and disappear at local magnetic field maxima (wave crests of KAWs), highly consistent with the source region of whistler waves previously reported near equator and dayside magnetopause. Our study also suggests that the KAWs behind DFs could originate from the diffusion region of magnetic reconnection. These findings improve our understanding of the relationship between DFs, KAWs, and whistler waves in terrestrial magnetotail.

In this study, we apply a method for wave analyses in space plasmas, to better understand the nature of plasma waves and turbulence. Such a method is based on four-spacecraft measurements of magnetic fields or electric fields; it calculates the phase velocity of each single-frequency wave by using the timing analysis, then divides the wave frequency by the phase velocity to obtain the wavevector, and finally solves the dispersion relation ( ω – k relation) by considering all frequency channels, and thus is termed Dispersion RelAtion From Timing (DRAFT). Quantitatively, we define three parameters, match of amplitude (MOA), ratio of half-wavelength to spacecraft separation ( λ/ 2 R sc ), and correlation coefficient (CC), to judge the reliability of this method. We illustrate the usage of this method and validate the efficiency of the three parameters by analyzing the Magnetospheric Multiscale data. We find that the DRAFT results are generally accurate under conditions MOA > 0.5, λ /2 R sc > 1, CC > 0.8. As a support, we compare our method with the previous methods such as Means and singular value decomposition (SVD). We find that if the three criteria (MOA, λ /2 R sc , CC) are satisfied, the DRAFT, Means, and SVD are consistent in resolving the wave normal angle. Such a comparison robustly validates the reliability and rationality of the DRAFT method. Using DRAFT, we successfully solve the dispersion relation of an Alfvén wave in Earth’s magnetosheath and obtain its three-dimensional K -spectrum. Our method should be useful to study the low-frequency waves and weak turbulence in space plasmas.

The configuration, local motions, and related physics processes of the terrestrial magnetotail have been well studied, playing a key role in magnetotail dynamics. But characteristics of electron pitch-angle distribution (PAD), and the formation mechanisms, at different regions in the flapping magnetotail were not pointed out. Here, we study a current sheet crossing event detected by the Magnetospheric Multiscale Mission inside a magnetotail, and investigate the electron PAD of the event in detail. When the spacecraft are out of the current sheet, low-energy and high-energy electrons present field-aligned PAD and cigar-type PAD, respectively. This difference shows different motions of electrons. We proposed two possible explanation mechanisms: crossing a newly dipolarized magnetotail, or crossing the exhaust region of the reconnection region, and we discussed them in the paper adequately. Based on the dipolarized mechanism, for the first time, we point out that bouncing electrons could be the indicator of a newly dipolarized magnetotail. In addition, four other current sheet crossing cases with similar signatures are observed. Our study improves the understanding of current sheet dynamics and magnetotail configuration physics.