Coronal Mass Ejections, Corotating Interaction Regions, and Geomagnetic Storms

Abstract

The geomagnetic activity index, Dst, solar wind data, and solar coronal images have been analyzed and compared for 10 solar rotations from August 1993 to May 1994. A description of the background solar wind patterns is given for the period, and the modification of the geoeffectiveness of this pattern by seasonal effects is reviewed. Most of the geomagnetic storms are found to be associated with passage of the sector boundaries in the interplanetary magnetic field. The geoeffectiveness of these storms is determined by three components. One is the corotating interaction region (CIR) formed at the leading edge of high-speed streams from major coronal holes, a traditional source for recurrent geomagnetic storms. All of the storms near sector boundaries were followed by high-speed streams, and by assumption, were associated with CIRs. In the study period, the modification of the geoeffectiveness of these CIRs by the second component, seasonal effects, was quite strong. The final component is due to coronal mass ejection (CME) driven solar wind transients. Association of solar signatures with each storm suggests that all major storms, and probably most moderate storms, have a transient component. We conclude that major geomagnetic storms, both recurrent and nonrecurrent, are the result of the combined effects of CMEs and CIRs.