Abstract

Plasma pressure distribution in the inner magnetosphere is one of the key parameters for understanding main processes of the magnetospheric dynamics including geomagnetic substroms. However, the pressure profiles obtained from in situ measurements by the high-altitude satellites do not allow the tracking of the magnetospheric dynamics, because a time necessary to obtain these profiles (hours) generally exceeds the characteristic times of the main magnetospheric processes (minutes or less). On contrary, fast movement of low-altitude satellites makes it possible to obtain quasi-instantaneous profiles of plasma pressure along the satellite trajectory - radial or azimuthal, depending on the satellite orbit. Precipitating particle fluxes, measured by the low-altitude Aureol-3 satellite were used. Mapping into the equatorial plane and determination of a volume of the magnetic flux tube were made using the Tsyganenko 96 and 01 geomagnetic field models. Study of radial plasma gradients showed that the inner magnetosphere is stable for development of flute (interchange) instability. Modified interchange instability related to azimuthal plasma pressure gradients and field-aligned currents in equilibrium was proposed as a source of substorm expansion phase onset. It can develop when the density of the field-aligned current reaches a definite threshold value. The growth rate of the instability is higher in the region of upward field-aligned current, where the existing field-aligned potential drop leads to the magnetosphere-ionosphere decoupling. © 2006.