EVOLUTION OF THE AURORAL OVAL DURING A WEAK SUBSTORM

The present paper deals with a detailed study of the dynamics of the p olar cap boundaries. The study involves the data from two satellites o f the APEX project, and the data from three DMSP satellites. The APEX (Active Plasma Experiment) satellites moving along the polar orbit pro vide charged particle measurements in a small spatial scale (due to va riable distance between both spacecrafts), up to 2 000 km. The large s cale changes are studied by the comparison of the APEX and the DMSP (D efense Meteorological Satellite Program) data. Different regions are i dentified using the characteristics of the precipitating particles in the energy range 0.1-20 keV which have been registered onboard all sat ellites. This satellite configuration allows us to determine the evolu tion of the small structures as well as the motion of the whole precip itating region. The main attention is concentrated on the rising phase of the substorms when the width of the auroral oval decreases with th e increasing geomagnetic activity and the velocity of the auroral oval motion can reach 0.2 degrees per minute. The observed phenomena are c ompared with the changes of the interplanetary magnetic field and the solar wind parameters as provided by the IMP-8 spacecraft.