Central plasma sheet disruption and the formation of dayside poleward moving auroral events

Abstract

Although most dayside poleward moving auroral forms (PMAFs) seem to be reasonably explained as a result of magnetopause merging, a number of these events can hardly be explained in terms of reconnection. These events seem to appear on closed field lines, and their poleward movement seems to be controlled not only by convection due to the interplanetary magnetic field (IMF) but also to the polarization electric field appearing inside them. This type of PMAF may be observed not only for a negative IMF Bz component but also for the positive Bz and rather small By components when field line merging may be inefficient or may occur on lobe magnetic field lines, and the poleward propagation of these auroral forms may differ from the background magnetospheric convection. These events often appear at the equatorward edge of the red auroral band, and the spike structures in dayside electron precipitations which may be responsible for these auroral forms are often seen simultaneously with the plasma sheet population. Another possible mechanism for dayside poleward moving auroral events in a region of closed field lines is proposed. This mechanism is based on observations indicating that the central plasma sheet can be divided from the low‐latitude boundary layer and the magnetopause by a gap. This means that the dayside plasma sheet has an outer boundary inside the magnetosphere which is unstable with respect to the interchange (ballooning) instability. As a result, for some conditions, clouds of warm plasma may break out from the plasma sheet and move poleward because of the polarization electric field. Projection of these plasma sheet clouds on the ionosphere would have many features which are inherent for the dayside auroral events.