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.

Dipolarization front (DF)—a sharp boundary with scale of ion inertial length (c/ω_pi) in the Earth's magnetotail—can also have fine structures at electron scale (c/ω_pe). Such electron‐scale structures, determining the local energy conversion, have not been revealed by multispacecraft observations so far, due to the large separation of spacecraft in previous studies. Here we report the first electron‐scale multispacecraft measurements of DF, using data from the recent Magnetospheric Multiscale mission. We find strong parallel currents only in the high‐density side of the DF but strong perpendicular currents across the whole DF. We find no parallel electric fields during the DF interval. Although DF is primarily an energy‐load region (E·J > 0), the electron‐scale currents could lead to a localized energy generation (E·J < 0). Such features are different from those reported in previous multispacecraft studies, where the currents, electric fields, and energy conversion are uniform across the DF; they also shed lights on the study of substorm current wedge, which is crucial in the magnetosphere‐ionosphere coupling.

It is still unknown nowadays whether magnetic reconnection—a process occurring both in the magnetotail and at the magnetopause—can intrinsically accelerate energetic electrons. Observations in the Earth's magnetotail usually indicate strong electron acceleration during magnetic reconnection, while observations at the Earth's magnetopause rarely show such features. With the recently launched Magnetospheric Multiscale (MMS) mission, here we report the first evidence of energetic‐electron acceleration at a reconnecting magnetopause. We find that the acceleration of electrons, with energy up to 70 times their thermal energy, occurs in the magnetosheath side of the ion diffusion region and is associated with strong whistler waves. Such acceleration—not contaminated by the magnetospheric population—is attributed to nonadiabatic wave‐particle interactions, as supported by analyses of the resonance condition. It manifests that energetic‐electron acceleration can happen at the reconnecting magnetopause, like that in the tail.

Suprathermal electrons with energy from tens to hundreds of keV are frequently observed in the Earth's magnetotail. The generation of such electrons is typically attributed to magnetic reconnection, dipolarization fronts (DFs), or flux transport. However, which of these contributes more to this generation remains unclear. In this study, we quantitatively compare the electron acceleration by these processes, using the Cluster data. We analyze an event detected in the midtail and find that the suprathermal electrons there are first accelerated by magnetic reconnection and transported earthward, and then further accelerated locally at the DF. The acceleration process by reconnection and transport, resulting in an isotropic pitch angle distribution, contributes ∼70% to the total flux enhancement, while the acceleration process by the DF, resulting in a pancake distribution, contributes ∼30% to the flux enhancement. The electron acceleration at the DF is primarily attributed to a local betatron process that is successfully reproduced using an analytic model. In order to better understand this phenomenon, we examine an additional 16 similar events and find that the DFs and magnetic reconnection statistically contribute 11% ∼ 38% and 62% ∼ 89% to the total flux enhancement, respectively. This study greatly improves our understanding of the electron energization process in the magnetotail.