Abstract
We have identified and selected 212 geomagnetic disturbances of varying level of geomagnetic activity, based on the changes in Dst index. The selected events are divided into six different groups according to different geomagnetic activity levels. We study the average behavior of solar plasma flow speed (V), density (N), dynamic pressure (P), magnetic field (B), its north-south component (Bz) and variance (sigma(B)) and dusk-ward electric field (Ey), and their role in solar wind-magnetosphere coupling. For this purpose we adopted the method of superposed epoch (SPE) analysis. Hourly solar wind plasma and field parameters (V, B, Bz, sigma(B), N, P. Ey) together with Dst index are analyzed using SPE analysis with respect to the start of the changes in geomagnetic activity, separately for all six groups of increasing level of activity. In particular, we study the influence of the enhancements in solar plasma density/dynamic pressure and field fluctuations on the solar wind-magnetosphere coupling, during southward field orientation within the responsible interplanetary structures. In addition to studying the average properties with the use of SPE analysis, we have also considered individual events with different level of geomagnetic activity (Dst) and studied its relationship to interplanetary field parameters. We observed that the enhancement in plasma density/pressure and fluctuations in field starting several hours before the storm/activity onset, might play a role in enhancing the solar wind-magnetosphere coupling efficiency. (C) 2011 Elsevier Ltd. All rights reserved.