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Abstract Turbulent energy dissipation is a fundamental process in plasma physics that has not been settled. It is generally believed that the turbulent energy is dissipated at electron scales leading to electron energization in magnetized plasmas. Here, we propose a micro accelerator which could transform electrons from isotropic distribution to trapped, and then to stream (Strahl) distribution. From the MMS observations of an electron-scale coherent structure in the dayside magnetosheath, we identify an electron flux enhancement region in this structure collocated with an increase of magnetic field strength, which is also closely associated with a non-zero parallel electric field. We propose a trapping model considering a field-aligned electric potential together with the mirror force. The results are consistent with the observed electron fluxes from ~50 eV to ~200 eV. It further demonstrates that bidirectional electron jets can be formed by the hourglass-like magnetic configuration of the structure. 

Abstract Foreshock transients such as foreshock bubbles (FBs), hot flow anomalies (HFAs), and spontaneous hot flow anomalies (SHFAs) display heated, tenuous cores and large flow deflections bounded by compressional boundaries. THEMIS and Cluster observations show that some cores contain local density enhancements which can be studied to better understand the evolution processes of foreshock transients. However, closer examinations of these substructures were not feasible until the availability of the higher resolution data from the Magnetospheric Multiscale mission (MMS). We identify 164 FB‐like, HFA‐like, and SHFA events from two MMS dayside phases for a statistical study to investigate their solar wind conditions, properties, and substructure properties. Occurrence rates of the three event types are higher for lower magnetic field strengths, higher solar wind speeds and Mach numbers, and quasi‐parallel bow shocks. Events usually span up to 3R_Ealong the bow shock surface and extend up to 6R_Eupstream from the bow shock. Though events with and without substructures exhibit similar solar wind conditions, events with substructures are more likely to have longer core durations and larger sizes. Substructure densities display a positive correlation with bulk flows and a negative correlation with temperatures. Substructure sizes vary between 4 and 24 ion inertial lengths, indicating multiple formation mechanisms. Substructures could be the boundary between two foreshock transient events that have merged into a single event, fast‐mode variations, generated by slow or mirror mode instabilities, or produced from instabilities due to parameter gradients at the compressional boundaries or shocks.