Using data from the MMS mission and the First‐Order Taylor Expansion (FOTE) method, here we reveal electron distribution functions around a reconnection X‐line at the Earth's magnetopause. We find cigar distribution of electrons in both the magnetosphere‐side and magnetosheath‐side inflow regions, isotropic distribution of electrons at the separatrix, and loss of high‐energy electrons in the antiparallel direction in the magnetosheath‐side inflow region. We interpret the formation of cigar distribution in the inflow regions using the Fermi mechanism—as suggested in previous simulations, the loss of high‐energy electrons in the magnetosheath side using the parallel electric fields—which evacuate electrons to escape the diffusion region along the antiparallel direction, and the isotropic distribution at the separatrix using the pitch angle scattering by whistler waves—which exist frequently at the separatrix. We also find that the electron distribution functions can change rapidly (within 60 ms) from isotropic to cigar as the spacecraft moves slightly away from the separatrix.

On 5 May 2017, MMS observed a crater-type flux rope on the dawnside tailward magnetopause with fluctuations. The boundary-normal analysis shows that the fluctuations can be attributed to nonlinear Kelvin-Helmholtz (KH) waves. Reconnection signatures such as flow reversals and Joule dissipation were identified at the leading and trailing edges of the flux rope. In particular, strong northward electron jets observed at the trailing edge indicated midlatitude reconnection associated with the 3-D structure of the KH vortex. The scale size of the flux rope, together with reconnection signatures, strongly supports the interpretation that the flux rope was generated locally by KH vortex-induced reconnection. The center of the flux rope also displayed signatures of guide-field reconnection (out-of-plane electron jets, parallel electron heating, and Joule dissipation). These signatures indicate that an interface between two interlinked flux tubes was undergoing interaction, causing a local magnetic depression, resulting in an M-shaped crater flux rope, as supported by reconstruction.Plain Language Summary Magnetic reconnection and Kelvin-Helmholtz instability (KHI), two of the most fundamental physical processes occurring within the heliosphere and throughout the Universe, often occur simultaneously on the Earth's magnetopause. Previous studies indicate the importance of nonlinearly developed KH waves, which produce multiple kinetic layers facilitating reconnection both in and out of the velocity shear plane and resulting in the magnetic flux rope. However, these studies significantly lacked detailed in situ observations in quantity as well as appropriate 3-D analyses of the structure of the KH vortex-induced flux rope. In this paper, we use detailed observations by the MMS spacecraft to investigate both 2-D and 3-D structures of the flux rope developed along the KH waves. We found that two flux tubes interact through reconnection to form a single combined structure, which can explain the occurrence of M-shaped crater flux rope.

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Energetic electrons exist widely in the turbulent magnetosheath, but how they are generated remains unclear. Here we report a new structure, at which electrons are efficiently accelerated in the direction parallel to the magnetic field. Such a structure, formed at the edge of a high-speed jet (HSJ), is a tangential discontinuity (TD) in the MHD regime, but exhibits impulsive fine structures in the kinetic-scale regime. The pulsation of the TD, caused by time-varying size of the HSJ, leads to the energization process: when the transverse section of the HSJ increases, a magnetic mirror is formed and subsequently electrons are trapped and accelerated via the Fermi mechanism; when the transverse section of the HSJ decreases, the magnetic mirror disappears and subsequently electrons escape. Such parallel electron heating can lead to three times of parallel-temperature increase; it can shed light on the study of electron heating in the solar wind, where TDs exist extensively.

Betatron cooling, a plasma process losing particle energy in the perpendicular direction but reserving particle energy in the field‐aligned direction, is a consequence of magnetic depression under the conservation of magnetic moment. Such process has been widely studied in the Earth's magnetosphere but has never been reported in other planetary environment. Here, by utilizing the Mars Atmosphere and Volatile EvolutioN (MAVEN) measurements, we report two events of betatron cooling in the Martian magnetotail. In one of the events, betatron cooling occurs in the suprathermal and energetic ranges of electrons, whereas in the other event, it occurs in the thermal‐energy range. We quantitatively reproduce these two processes by using an analytical model. Gratifyingly, the cooling factor derived from the analytical model agrees well with the observation of magnetic depression. These results, for the first time demonstrating the betatron‐cooling effect beyond the Earth, are useful to understand the electron dynamics in the planetary magnetosphere.