Structure of disturbances in the dayside and nightside ionosphere during periods of negative interplanetary magnetic field Bz

Abstract

By comparing data from a globally distributed set of high-latitude magnetic observatories with optical data in the nightside auroral ionosphere, we studied the features of short-term activations (≤10-15 min) during periods of steady magnetospheric convection (SMC) observed between two substorms. We found that the occurrence of local auroral and current structures stretched along the meridian to several hundreds of kilometers is the characteristic feature of activations in the nightside ionosphere. At premidnight and night hours (2100-0100 LT) we observed auroral structures that expand rapidly equatorward (V≥1 km/s) from the discrete auroral oval and become north-south aligned (N-S aurora or auroral streamers). In the night-morning hours (0100-0500 LT) torch-like structures form at the poleward boundary of the diffuse auroral oval. Ionospheric currents of local auroral structures responsible for Pi3 magnetic pulsations (T≃5-15 min) are predominantly directed to the north (to the south) for auroral structures drifting azimuthally to the west (to the east). Similar auroral and current structures in the nightside ionosphere are also observed during the expansion and recovery phase of a substorm. At the dayside during SMC events at latitudes of 75°-80°, magnetic pulsations of f ∼0.5-1.0 mHz are excited and are accompanied by enhancements of the DPY equivalent current system. We also present observations which show that the N-S aurora formation is accompanied by fluctuations of the electric field (Ey) and particle fluxes directed earthward (Vx) in the plasma sheet with a quasiperiodicity of ∼5-15 min. On the basis of these observational facts we confirm that (1) auroral streamers are the optical manifestation of high-speed flow bursts propagating earthward; (2) torch-like structures are caused by generation of magnetospheric plasma tubes of increased pressure (in comparison with the background level) drifting around the Earth. Copyright 1999 by the American Geophysical Union.