Relationship of the ionospheric convection reversal to the hard auroral precipitation boundary

Abstract

Plasma drift and particle measurements from the DMSP spacecraft for the three Geospace Environmental Modeling (GEM) campaign periods (January 27–29, March 28–29, and July 20–21, 1992) have been used to study the relationship between the plasma convection reversal and the poleward boundary of the diffuse auroral zone characterized by hard electron precipitation with energies greater than 0.45 keV. This boundary, named the hard auroral precipitation boundary (or the HAP boundary) in this paper, is often regarded as the ionospheric footprint of the boundary between the central plasma sheet (CPS) and the boundary plasma sheet (BPS). By examining simultaneous ion drift and particle measurements from about 500 satellite passes we find that the large‐scale plasma flow in the morning and evening sectors changes its direction within the auroral oval at the HAP boundary. However, exceptions are found in the early morning sector between 0300 and 0600 MLT, where the convection reversal is sometimes (in 30% of the DMSP crossings) displaced poleward relative to the HAP boundary. It is shown that the shape of the region bordered by the HAP boundary can be roughly represented by a circle, whose size is influenced by the IMF Bz component. There is roughly a linear correlation between the diameter of this circle and the cross‐polar‐cap potential drop, with the best correlation coefficient of 0.65 for winter season. Our study suggests that the HAP boundary corresponds to the magnetospheric boundary between the quasi‐dipolar region and the region with more stretched field lines, and the source of the region 1 field‐aligned current is located near the HAP boundary. A By‐dependent shift of the HAP boundary with respect to the noon‐midnight meridian is also found. In the northern hemisphere, the shift is dawnward for positive By and duskward for negative By in the southern hemisphere, the shift is opposite to that in the northern hemisphere.