Small-scale filamentary structures recorded in the auroral regions connected to the plasma sheet boundary layer

We use high time resolution data from the FAST spacecraft during a moderate substorm and concentrate on the auroral regions that are magnetically linked to the plasma sheet boundary layer (PSBL). The crossing of these regions is characterized by the presence of a pair of oppositely directed, field‐aligned current sheets. The more poleward of the two current sheets is directed earthward. Strong turbulent fluctuations are detected in association with the field‐aligned currents. The low‐frequency component (∼1 Hz) of the electric field fluctuations consists of localized structures whose characteristics are not entirely consistent with the presence of Alfvén waves. They may represent a possible example of the coupling of Alfvén waves with electron acoustic waves on small length scales. In the high‐frequency range (∼1–10 kHz), the turbulent fluctuations are dominated by large amplitude (∼500 mV/m peak to peak) bipolar electric field structures whose polarity depends on the direction of the field‐aligned currents. These latter structures are moving earthward in the upward current region and antiearthward in the downward current region. The generation of both low‐ and high‐frequency filamentary nonlinear structures appears as a natural consequence of the disturbances imposed during substorms on the auroral regions connected to the PSBL when hot and cold plasmas interact. Key Points Characterization of reconnection processes Hall currents closure by field‐aligned currents Nonlinear waves associated with reconnection