Geoeffectiveness of different solar drivers, and long-term variations of the correlation between sunspot and geomagnetic activity

Two types of solar agents are mainly responsible for geomagnetic disturbances-long-lived coronal holes, regions of open solar magnetic field and sources of high speed solar wind (HSS) related to recurrent geomagnetic activity, and coronal mass ejections (CME's), regions of closed solar magnetic field related to sporadic geomagnetic activity. The coronal mass ejections can be additionally divided into magnetic clouds (MC's) and non-magnetic clouds, according to the presence or absence of magnetic field rotation. We compare the geoeffectiveness of HSS's, MC's and non-MC CME's. The average geoeffectiveness is highest for MC's and lowest for non-MC CME's. The average geoeffectiveness of MC's is strongly solar cycle dependent, but their number is relatively small, except around sunspot minimum when their geoeffectiveness is low. The geoeffectiveness of HSS's and non-MC CME's practically doesn't change throughout the cycle, however their relative abundance changes, and so does their relative contribution to the overall geomagnetic activity. The long-term decrease in the correlation between solar and geomagnetic activity is mainly due to the increasing number of HSS's on the declining phase of the sunspot cycle, which we suppose to be determined by the long-term changes in the tilt angle of the heliospheric current sheet. (C) 2006 Elsevier Ltd. All rights reserved.