Magnetospheric multiscale (MMS) observed a series of isolated dipolarization fronts embedded in fast bursty bulk flows (BBFs/DFs) near 18 Earth’s radii ( R_E ) during a weak steady convection event. A number of distinctive signatures in the MMS data manifest kinetic ballooning-interchange instability (BICI) activity in the midtail whose signatures were recently identified in near-Earth THEMIS observations. For the first time we clearly identify strong electromagnetic waves near proton-cyclotron frequency also in the high-resolution plasma data. We confirm the predicted azimuthal wave propagation from four-point observations, and compare with the plasma flows’ and DFs’ motion. We use an adapted magnetospheric model to identify the ionospheric and ground footprints of the magnetic field lines. Inspecting energetic particles from conjugate low-altitude spacecraft azimuthal passage we find for the first time the azimuthal structuring of energetic proton fluxes suggesting the multiple sheets of field-aligned current. Auroral observations indicate the presence of a structured poleward azimuthal auroral arc and injection prints. We compare MMS midtail observations with previous near-Earth observations and discuss peculiarities of possible BICI generation during steady convection events. We find that the midtail BICI activity could be associated with greater temporal and spatial scales as compared to those of the near-Earth BICI activity.

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.

MMS four-spacecraft observations with high temporal and spatial resolution made it possible to study the characteristics of intense superthin current sheets (SCSs) with a current density J > 30 nA/m2, formed in bursty bulk flows (BBFs), propagating in the Plasma Sheet (PS) of the magnetotail from the remote X line. Statistical analysis of >1000 SCSs observations showed that, in the majority of cases, the current in the SCSs is parallel to the ambient magnetic field and is carried by field-aligned accelerated electron beams. The half-thickness of the SCSs is several electron gyroradii, and in such thin layers the electric current is carried by demagnetized electrons. Bursts of strong nonideal electric fields E' > 10 mV/m are often observed at the edges and/or inside the SCSs. The generation of such fields brings about energy conversion in the SCSs of hundreds of pW/m3, and in some cases up to several nW/m3, which is comparable to the energy conversion in the electron diffusion region of magnetic reconnection. The strongest energy release is observed in the SCSs formed in the fastest BBFs and under strong variations of the magnetic field in the tail lobes.

Plasmoids and magnetic field dipolarizations are reconnection-related phenomena often resulting in reconfiguration of the magnetic field and energetic particle acceleration in planetary magnetotail. Building on the work of Bl & ouml;cker et al. (2023) (), we selected seven specific events from their magnetic field dipolarization analysis, each exhibiting distinct ion dynamics during the time interval of the magnetic field dipolarizations. To gain further insights into the understanding why certain events were associated with ion intensity variations while others were not, we analyzed plasma moments, specifically ion flow velocity and density, for these selected events. Our findings revealed that certain magnetic field dipolarizations within our database exhibit sub-Alfv & eacute;nic flows and lack the properties typically associated with reconnection-related magnetic field dipolarizations. These magnetic field dipolarizations also do not accelerate ions. Furthermore, we present a survey of Jovian plasmoids and magnetic field dipolarizations during the first 47 orbits of Juno. Applying Juno magnetic field data, we identified 119 magnetic field dipolarizations and 94 plasmoids within a local time range of 18:00-06:00. The majority of plasmoids were detected in the predawn sector, whereas magnetic field dipolarizations were observed closer to Jupiter and were not limited to a specific local time. Combining the statistics of plasmoids and dipolarizations is useful for contextualizing them within the framework of reconnection. Certain magnetic field dipolarizations exhibit sub-Alfv & eacute;nic flows and deviate from typical reconnection-related properties Nonadiabatic ion acceleration is only observed in magnetic field dipolarizations with super-Alfv & eacute;nic ion flows Plasmoids are concentrated in the predawn sector, while dipolarizations are distributed from 20:00 to 5:00 local time closer to Jupiter

The intense electron-scale current structures (ECSs) with the current density J >= 30 nA/m2 are often observed in the Plasma Sheet (PS) during high-speed bulk flows. Using MMS observations we have analyzed 41 earthward and 37 tailward flow intervals and found 452 and 754 ECSs distributed over the PS region, respectively. Almost all ECSs are generated by high-speed electron beams. The duration of ECSs is <= 1 s, and many of them have a half-thickness L <= a few rho e (rho e is the gyroradius of thermal electrons). In such thin ECSs electrons become demagnetized and experience the dynamics like that observed in the electron diffusion region. Strong nonideal electric fields (E') associated with violation of frozen-in condition for electrons are observed in the ECSs. This results in the intense energy conversion with JE' up to hundreds pW/m3. The major part of the dissipating energy is transferred to electron heating and acceleration. We suggest that the ECSs are manifestations of kinetic-scale turbulence driven by the high-speed ion bulk flows. The inductive electric fields generated by the growing magnetic fluctuations accelerate electron beams which, in turn, generate the ECSs. The ECSs thinning during their evolution, probably, stops for L <= a few rho e. Further thinning leads to development of kinetic instability causing the current disruption and strong electric field generation. The last accelerates new electron beams which generate new ECSs in other locations. Thus, the life cycles of the ECSs contribute to energy cascade in turbulent plasma at electron kinetic scales. Electron-scale current structures (ECSs) with a few L/rho e thickness are observed in the plasma sheet during fast ion bulk flows The ECSs are mostly generated by demagnetized electrons experiencing the dynamics similar to that observed in electron diffusion region In the ECSs JE' reaches the values typical for EDR, thus, they significantly contribute to the turbulent energy cascade at electron scales

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

AbstractRecent MMS observations have discovered electron‐scale super‐thin current sheets (STCSs) with a partial electron demagnetization, which distinguishes them from the ion‐scale TCSs traditionally observed by the Cluster mission. Our investigation focuses on the dynamics of STCSs and reveals new aspects influencing their stability. We use the earlier proposed 1D collisionless self‐consistent equilibrium STCS model and show that the free parameters of this model, such as the relative part of demagnetized electrons, their flow velocity and the pressure anisotropy of magnetized electron population, can contribute to the development of tearing instability. With the growth of these parameters, the STCS becomes thinner, which leads to the accumulation excess of a free energy. Stabilizing energy decreases due to the increase of a relative part of demagnetized electrons. Thus, demagnetized electrons in STCSs can provide the development of fast and short‐wavelength electron tearing modes.

We report Magnetospheric Multiscale Mission (MMS) observations of accelerated field-aligned electron beams in the outer ( | B_x| ∼( 10 1pt - 1pt 20) nT ) regions of the Plasma Sheet (PS). The MMS satellites were located earthward of the magnetic reconnection X-line, inside of the bursty bulk flow. The interval corresponds to the growth phase of the substorm. Field-aligned electron beams was produced by electrons suprathermal population with energy ∼ 1pt( 1 1pt - 1pt 5) keV . Newly accelerated electron beams are gyrotropic. In the process of evolution, agyrotropy of the beam develops. This leads to the generation of intense (up to ∼80 nA/m2) field-aligned currents. We demonstrate that acceleration of the electrons is pulsed for the time < 1pt 5 s . Propagation of the high-velocity electron beam results in the development of plasma instabilities and generation of a strong (∼59 mV/m) nonideal electric field at frequencies lower than electron cyclotron frequency ω_c,e . The presence of a nonideal electric field leads to the violation of the frozen-in condition of electron plasma and energy dissipation. The energy transformation power density reaches ∼–1000 pW/m3. Fluctuations in the spectrum of a nonideal electric field experience spectral flattering between ion plasma ω_p,i and electron cyclotron ω_c,e frequencies. This evidences energy transfer from particles to waves at this spectral range. The studied phenomena may provide significant contributions to the development of turbulence on electronic kinetic scales in the outer region of the PS.

The flapping motion of the magnetotail current sheet has a valuable contribution to magnetosphere dynamics. We have investigated features of flapping motions involving single- and multi-spacecraft methods using MMS and Cluster measurements. Velocities of dawn-dusk propagation of these motions and their types have been determined using Harris current sheet model and minimum variance analysis of the magnetic field. Dispersion features of such wavy motions using the phase differencing technique were explored. This work was carried out in accordance with the Target Comprehensive Program of the SRI NASU and SSAU in plasma physics with the support of grant no. 97742 of the Volkswagen Foundation (VW-Stiftung).

This paper studies the current profile in the Сurrent Sheet during the propagation of bursty bulk flows. The presence of superthin current sheets (STCSs) with current densities up to 100 nA/m2 is shown. The current in such structures is predominantly carried by the suprathermal demagnetized electron population. The typical half-thickness is of the order of a few gyroradiuses of thermal electrons. STCSs are often nested inside magnetic islands elongated along the radial magnetotail direction. It is shown that the evolution of such islands occurs due to the development of tearing instability, which also leads to the acceleration of electron beams. Accelerated electron beams generate field-aligned currents in the Plasma Sheet, in which the strong electric fields could be generated as a result of instabilities. These electric fields cause significant energy dissipation in the STCSs.

In collisionless space, plasma waves are important channels of energy conversion, affecting the local particle velocity distribution functions through wave–particle interactions. In this paper we present a comparative statistical analysis of the characteristics of quasi-parallel narrowband whistler waves and the properties of resonant electrons interacting with these waves during the intervals of earthward and tailward high-velocity bulk flows produced by the near-Earth X-line and observed by Magnetospheric Multiscale Mission spacecraft. We found that on both sides of the X-line, the suprathermal electrons (≥1 keV) having large pitch angles make the major contribution to the maximal growth rate ( γ ) of these waves. The whistler waves were observed almost simultaneously with strong enhancements of perpendicular magnetic gradients localized at electron scales near dipolarization fronts associated with the earthward bulk flows, and near flux ropes/magnetic islands embedded into the tailward bulk flows. Betatron energization of electrons due to the appearance of such gradients increases the perpendicular anisotropy of electron distribution, which could be responsible for the whistler wave generation. We found that in the course of electron interactions with the whistler waves the lower-energy resonant electrons can transfer a part of their kinetic energy to the higher-energy electrons, especially in the Central Plasma Sheet. This results in formation/enhancement of energy-dependent perpendicular anisotropy and power-law tails in the high-energy range of electron velocity distribution. We conclude that despite the differences in the magnetic structure of the earthward and tailward bulk flows, the mechanisms of the quasi-parallel whistler wave generation and the properties of resonant electrons are quite similar.

EDITORIAL article Front. Astron. Space Sci., 11 May 2023Sec. Space Physics Volume 10 - 2023 | https://doi.org/10.3389/fspas.2023.1195579

We report the MMS observations of the intense spikes of field-aligned current (FAC) produced by magnetic reconnection at the plasma sheet (PS) field lines. The MMS was located tailward of a near-Earth X-line and the most intense spike of FAC with an electric current density of ∼70 nA/m2 was observed near the magnetic separatrix. The FAC structures located deeper in the PS were strongly filamented and consisted of several spikes with a thickness of ∼(1–2)ρe (ρe is the gyroradius of thermal electrons). We found that the FAC in these structures was carried by unmagnetized thermal and suprathermal electron populations (≥ 1 keV), which were ∼(20–80)% of the entire electron population. Strong nonideal electric fields up to ∼100 mV/m were detected in the FAC spike near the magnetic separatrix. The generation of these fields was mainly due to the anomalous resistivity, possibly caused by the electrostatic fluctuations. As a result, a significant energy dissipation of up to 1.3 nW/m3 occurred within the electron-scale FAC structure, which caused an increase in the electron temperature by a factor of two compared with that outside the FAC. Thus, MMS observations demonstrate that during the interval of the active X-line, the outer part of the PS consists of multiple electron-scale FAC layers/filaments in which a significant energy exchange between electrons and fields occurs. To investigate the stability of these filaments and estimate their lifetime, additional observations and theoretical studies are needed.

Recent MMS observations confirmed the existence of the quasi‐stationary electron‐scale super‐thin current sheets (STCSs) with a half‐thickness of few electron gyroradii. STCSs contain the electron population consisting of magnetized and demagnetized particles. To study the role of the demagnetized electrons, we developed a self‐consistent combined model of a STCS in which the dynamics of a part of the electron population is described by the quasi‐adiabatic approximation, while the motion of the other part is described by the guiding center approximation. Demagnetized ion population can be described in a frame of quasi‐adiabatic approach. In the presented model we introduced the weight coefficient of the demagnetized electron plasma density χ depending on the magnetic field normal component B n / B 0 and compared the characteristic profiles of two STCSs differing by the presence ( χ ≠ 0) and absence ( χ = 0) of demagnetized electrons. It is shown that the presence of demagnetized electrons makes the current density profile of STCS significantly more intense and narrower. The thickness and intensity of STCS depend on the anisotropy coefficient of magnetized electrons and the incoming drift velocity of demagnetized electrons. The maximum current density continuously increases with the growth of the demagnetized electron population and is controlled by the ratio of the thermal to the incoming drift electron velocities. Thus intense current sheets can be formed even in the absence of strong electron anisotropy, which is confirmed by MMS observations.

A thin current sheet in the Earth’s magnetotail with characteristic thickness of one to several proton gyroradii is often observed during magnetospheric disturbances named substorms, when a relatively thick current configuration in the magnetotail is narrowed to an extremely small thickness and then can spontaneously be destroyed. The current-sheet destruction is usually accompanied by such active processes as plasma acceleration and heating, as well as generation of an induced electric field and magnetohydrodynamic waves. In this paper, we developed and investigated a model of formation of a thin current sheet in which, along with protons, we have taken into account single-charged oxygen ions coming from the ionosphere into the magnetotail current sheet during magnetically active periods. The aim of this simulation is to study the peculiarities of thin current-sheet formation in two-component plasma and investigate its structure. It is shown that equilibrium configuration can have some characteristic properties. In particular, if the system consists only of protons or heavy ions, single-scale current equilibrium supported by quasi-adiabatic particles is formed. When a current sheet is formed in plasma that consists of a mixture of protons and oxygen ions in comparable concentrations, a current sheet can be formed with heavy ions as current carriers and chaotic proton trajectories that make a negative contribution to the current, due to which the current-density profile becomes bifurcated with the minimum at the center and maxima at the periphery of the sheet. The results may be useful for the interpretation of observational data in the Earth’s magnetotail.

Energetic particle acceleration and energization in planetary magnetotails are often associated with dipolarization fronts characterized by a rapid increase of the meridional component of the magnetic field. Despite many studies of dipolarization events in Earth's magnetotail, Jupiter's magnetotail provides an almost ideal environment to study high-energetic ion acceleration by dipolarization fronts because of its large spatial scales and plasma composition of heavy and light ions. In this study, we focus on the response of different high-energetic ion intensities (H, He, S, and O) to prominent magnetic dipolarization fronts inside the Jovian magnetotail. We investigate if ion energization and acceleration are present in the observations around the identified dipolarization fronts. Therefore, we present a statistical study of 87 dipolarization front signatures, which are identified in the magnetometer data of the Juno spacecraft from July 2016 to July 2021. For the ion intensity analysis, we use the energetic particle observations from the Jupiter Energetic Particle Detector Instrument. Our statistical study reveals that less than half of the identified events are accompanied by an increase of the ion intensities, while most of the other events show no significant change in the ion intensity dynamics. In about 40% of the events located in the dawn sector a significant decrease of the energy spectral index is detected indicating ion acceleration by the dipolarization fronts.

According to various orientation of the interplanetary magnetic field (IMF), the planetary shock can be either quasi‐parallel or quasi‐perpendicular. Under quasi‐parallel conditions a significant number of solar wind suprathermal particles are reflected from the shock and drift along IMF, forming an extended and highly turbulent region called the foreshock where various nonlinear plasma phenomena are observed. In this research, a case study of the structures in the foreshock region at Mars as observed by Mars Atmosphere and Volatile Evolution is performed close to Martian aphelion, when the foreshock wave activity is usually low. Data from plasma analyzer STATIC and magnetometer MAG is used to analyze ion beams angular spectrum and magnetic field dynamics. It is shown that the observed structures are consistent with Short Large‐Amplitude Magnetic Structures (SLAMS), commonly detected in foreshock regions of magnetized and unmagnetized bodies throughout the Solar system. Finally, the Alfven Mach number is calculated to analyze characteristics of the observed foreshock structures. The analysis shows that the observed SLAMS‐like structures at Mars are steepened waves formed by the ion cyclotron resonance between plasma waves propagating along the IMF and the back‐streaming scattered solar wind H + and exospheric O + and O 2 + ions, with the dominant impact of O + ions. The steepening process is accompanied with observation of gradients in the diffuse ion density near the bowshock, like it has been previously reported near the Earth (e.g., Scholer, 1993, https://doi.org/10.1029/92JA01875 ; Tsubouchi & Lembege, 2004, https://doi.org/10.1029/2003JA010014 ).

The generation process of auroral spirals is described by different theories varying for their morphology and surrounding conditions. Here, a possible mechanism is proposed for an eastward moving auroral spiral, which was observed in Tromsø, Norway, during the expansion phase of a substorm on 18 September 2013. Measurements from the THEMIS-A and Cluster spacecraft were analyzed, which were located up to ∼10 RE duskward from the spiral generator region in the magnetosphere. Precursory to the spiral observation, concurrent magnetic field dipolarizations, flow bursts and electron injections were measured by the Cluster satellites between 13.6 and 14.2 RE radial distance from Earth. A local Kelvin-Helmholtz-like vortex street in the magnetic field was detected at the same time, which was likely caused by bursty bulk flows. The vortex street was oriented approximately in the X-Y (GSE) plane and presumably propagated towards the source region of the spiral due to a high dawnward velocity component in the flow bursts. The observations suggest that the spiral can have been generated by an associated vortex in the magnetotail and then mapped along the magnetic field lines to the ionosphere. To better understand the role of the ionosphere in auroral spiral generation, in future more mesoscale observations are required.