The Interplanetary and Solar Causes of Major Geomagnetic Storms.

We have investigated the solar wind input conditions for geomagnetic storms with Kp greater or equal 8- during the years 1966-1990 in order to clarify the interplanetary and solar causes of major geomagnetic storms. Forty-one out of the forty-three analyzed storms were found to be caused by transient shock-associated solar wind ejecta (driver gases and magnetic clouds), one storm was caused by a slow moving magnetic cloud followed by a corotating interaction region (CIR) and one by a CIR. Independent of the phase of solar activity coronal mass ejections (CMEs) CMEs are found to be the sources of major geomagnetic storms. A similar result was obtained using the Dst-index as an indicator of major geomagnetic storms. The maximum Kp-values of the individual events were directly related to the peak southward components of the interplanetary magnetic field (IMF). Draping of the IMF near the front edges of driver gases (magnetic clouds) or/and the magnetic field configurations of magnetic clouds were the sources of the extreme negative Bz values. The intensity of the southward components was often substantially amplified at the leading and rear edges of magnetic clouds due to their interaction with the ambient solar wind. This was found to be of particular importance for cases where magnetic clouds were followed by CIRs or interplanetary shocks, especially during very disturbed interplanetary conditions, i.e. during sequences of CMEs.