CORONAL MASS EJECTION AND STREAM INTERACTION REGION CHARACTERISTICS AND THEIR POTENTIAL GEOMAGNETIC EFFECTIVENESS

Previous studies have indicated that the largest geomagnetic storms are caused by extraordinary increases in the solar wind velocity and/or southward interplanetary magnetic field (IMF) produced by coronal mass ejections (CMEs) and their associated interplanetary shocks. However, much more frequent small to moderate increases in solar wind velocity and compressions in the IMF can be caused by either coronal mass ejections or fast/slow stream interactions. This study examines the relative statistics of the magnitudes of disturbances associated with the passage of both inter-planetary coronal mass ejections and stream interaction regions, using an exceptionally continuous interplanetary database from the Pioneer Venus Orbiter at 0.7 AU throughout most of solar cycle 21 (1979-1988). It is found that both stream interactions and CMEs produce magnetic fields significantly larger than the nominal IMF. Increases in field magnitude that are up to 2 and 3 times higher than the ambient field are observed for stream interaction regions and CMEs, respectively. Both stream interactions and CMEs produce large positive and negative B-Z components at 0.7 AU (similar to 14 nT on average), but only CMEs produce B-Z magnitudes greater than 35 nT, CMEs are often associated with sustained periods of positive or negative B-Z whereas stream interaction regions are more often associated with fluctuating B-Z. CMEs tend to produce larger solar wind electric fields than stream interactions. Yet stream interactions tend to produce larger dynamic pressures than CMEs. Dst predictions based on the solar wind duskward electric field and dynamic pressure indicate that CMEs produce the largest geomagnetic disturbances while the low-speed portion of stream interaction regions are least geomagnetically effective. Both stream interaction regions and CMEs contribute to low and moderate levels of activity with relative importance determined by their solar-cycle-dependent occurrence rates.