RESPONSE OF THE OUTER IONOSPHERE TO A MAGNETIC STORM AT EQUINOX

The observations of the ACTIVE Project satellites in the interval of M arch 17-23, 1990 were analyzed for the purpose of studying the respons e of the outer ionosphere to the magnetic storm with SSC on March 20 a t 22.43 UT. In particular, measurements of thermal plasma parameters w ere used, but VLF broadband measurements and data on energetic ion and electron fluxes in the range of 17.7-272 keV were also available. The results of this case study show that the observations in the morning sector at altitudes around 2000 km reflect well the state of the plasm asphere during enhanced activity, namely the depth to which the plasma sphere has been affected by enhanced magnetospheric convection. They a lso provide the possibility of monitoring the initial phase of recover y. The early evening observations of the plasma parameters ht the oute r ionosphere at altitudes of 500-1000 km indicate a distinct trough in electron concentration. In the dusk sector, the equatorward edge of t his trough can be assumed to be the projection of the equatorial plasm apause. This, combined with the occurrence of electron temperature pea ks and with the morning plasmapause position, enables one to judge whe ther the plasmaspheric bulge has formed and whether an inner plasmapau se has occurred.