MMS Observations of Reconnection Separatrix Region in the Magnetotail at Different Distances From the Active Neutral X‐Line

The region surrounding the reconnection separatrix consists of many particle and wave transient features (electron, cold and hot ion beams, Hall E&B fields, kinetic Alfvén, LH, etc. waves) whose pattern and parameters may vary depending on the distance from active neutral line. We study nine quick MMS entries into the plasma sheet boundary layer (PSBL) from the tail lobe to address the meso‐scale pattern and other characteristics of phenomena for active separatrix crossings as deduced from particle observations. The outermost thin layer (a fraction of ion inertial scale, di) of low‐density plasma consists of accelerated electron beams and lobe cold ions and displays density depletions (EBL region). It is followed by hot proton beam (PBL region) in which the plasma density grows from lobe‐like towards plasma sheet‐like values; the beam energy‐dispersion is used to estimate the distance from the active neutral line. Thin (usually ≤ di) region containing intense Hall‐like Ez perturbations (HR) usually overlaps with EBL and PBL regions. It often includes correlated B perturbations suggesting the Alfvén wave‐related transport from the reconnection source; the estimated Alfvénic ratio δE/(VA δB) varied between 0.3 and 1.3 in studied examples. The HR is associated with profound plasma property changes, including the heating of cold ion beams in its innermost part, it hosts intense structured field‐aligned currents and intense E‐field fluctuations. Surprisingly, most of abovementioned findings are valid for crossings observed at large distances from the reconnection region (exceeding a few tens Re or >100 di) except for longer time‐scales and larger spatial scales of the pattern.