Abstract

Various types of instabilities have been proposed as sources of inner magnetosphere substorm expansion phase onset. Distribution of plasma pressure in the plasma sheet, ? parameter, volume of magnetic flux tube are the key parameters for understanding the processes leading the instability development. Here, the radial distribution of plasma pressure was recovered using the low-altitude polar-orbiting Aureol-3 precipitating particle data. Corresponding variations in the magnetic pressure and in the volume of magnetic flux tube were estimated using Tsyganenko 96 magnetic field model. It is shown that development of an instability responsible for the substorm expansion phase onset takes place in the regions of low ? or at the boundary of the regions of high and low ?. A possible role of the flute-like (interchange-like) instabilities is investigated. It is shown that the obtained profiles of radial plasma pressure gradients apparently are not sharp enough to develop the instability related to radial plasma pressure gradients. The role of the instability related to the existence of the azimuthal plasma pressure gradients is discussed. Such an instability can develop when the density of the field-aligned current reaches a definite threshold value. The instability develops faster in the region of upward field-aligned current, where the existing field-aligned potential drop leads to the magnetosphere-ionosphere decoupling. © 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.