NIGHTTIME IONISATION ENHANCEMENTS OBSERVED WITH THE

Six geomagnetic storms were analyzed, in a two-fold investigation of nighttime ionisation enhancement events. First, we present observational evidence that their occurrence is strongly dependent on the solar wind conditions leading to magnetic storms. Then we investigate the ionospheric structure over Athens during such events to argue on their nature, since they were always accompanied by an upwelling of the F2 layer. According to our findings, nighttime positive effects and height enhancements are attributed to two distinct mechanisms. The increased nighttime density can only be speculated to be due to increased downward plasmasheric fluxes. OCCURRENCE OF NIGHTTIME IONISATION ENHANCEMENTS Six storm periods occurred in September and October 2000 were analysed. The solar wind- magnetosphere system were studied by inspecting observations from the ACE spacecraft (solar wind magnetic field and plasma), the auroral oval and the Disturbed storm time (Dst) magnetic field, to investigate the conditions under which nighttime ionisation enhancements occur. To observe the ionospheric response at middle latitudes, data from six middle latitude stations were analysed [1]. As an indicative case, we present the main solar wind and Dst observational remarks for two successive magnetic storms with totally different characteristics. (a) 12- 13 October 2000: This storm has an initial phase associated with an SSC. The main phase lasts only 5 hours during which the IMF-Bz exhibited very sudden changes, with variations of very large magnitud