Observations of polar cap arcs on FAST

Abstract

FAST satellite observations of spatially localized regions of electron and ion precipitation with concomitant paired upward and downward field‐aligned currents and duskward electric field enhancements at high (> 80° invariant latitude) magnetic latitudes are presented. Each pass of several across the polar cap was made during a period of persistent northward interplanetary magnetic field, and was characterized by the presence of several of these precipitation regions. In each case, the precipitation regions were associated with jets of enhanced antisunward convection, with electron precipitation restricted to the upward current region, and no energetic electron outflow in the downward current region, and ion precipitation spread over broader regions, possibly by time‐of‐flight effects. We compare these low‐altitude features to those expected for source regions lying in a reconnection site at the lobe magnetopause, a bursty bulk flow in the plasma sheet, and a surface wave on the low‐latitude boundary layer (LLBL). We find that the lobe reconnection model successfully describes the observed electrodynamics of the arcs, as well as the electron and ion source temperatures, but can not readily explain the presence of precipitating O+ and He+ in addition to H+ and He++. The plasma sheet source model can explain the composition of the ion precipitation but predicts higher electron and ion source temperatures than are observed, may require the coincidence of bursty bulk flows with extensions of the plasma sheet into the lobe, and may not be consistant with other observations of auroral phenomenology. The LLBL surface wave model does not readily explain the observed electrodynamics of the arcs and is not consistent with the observed offsets between electron and ion precipitation.