Solar cycle effects in planetary geomagnetic activity: Analysis of 36-yearlong OMNI dataset

NSSDC's OMNI dataset, which now spans 1963-1999, contains a collection of h ourly means of interplanetary magnetic field (IMF) and solar wind (SW) plas ma parameters measured near the Earth's orbit, as well as some auxiliary da ta. We report a study of solar cycle effects in planetary geomagnetic activ ity in which 27-day averages of several OMNI parameters are compared with e quivalent Kp and Dst averages. Some established trends in these parameters over solar cycles are confirmed; for example, it is concluded that changes in the magnitude (rather than in direction) constitute the primary solar cy cle variation in the IMF. However, this study also reveals that long-term c hanges in planetary geomagnetic activity are driven more actively by solar wind-magnetosphere coupling of an electrodynamic nature rather than by plas ma transport into the magnetosphere. This suggests that ambient (background ) interplanetary "electric" environment (in which the Earth's magnetosphere is immersed over the solar cycles) may play a more significant role in cau sing changes in the frequency of geomagnetic storms and substorms than prev iously realized.