Limitations of the utility of CMEs for forecasting timings and magnitudes of geomagnetic Dst storms

Abstract

Results of the examination of data for about 100 events in solar cycle 23 (1996 onwards), when CMEs (Coronal Mass Ejections) and IP (Interplanetary) shocks could be matched, are presented. The CMEs had a large range of speed (200-4000 km/s), but the slow CMEs seemed to be accelerated and the fast ones decelerated during the transit from Sun to Earth. Hence, IP shock speed near the Earth was in a narrower range (350-2000 km/s). A regression equation can be established between the CME lateral expansion speed and the corresponding IP shock speed. But observed values have a considerable scatter and can have extreme deviations of ~ 35% from the predicted values. Similarly, the transit times from Sun to Earth can have extreme deviations of ~ 35% from the predicted values. The transit times can be as low as 25 h (extreme uncertainty ~ 5 h) to as high as 100 h (extreme uncertainty ~35 h) and have no one-to-one relationship with the magnitudes of the Dst storms that follow, or with the time intervals between the IP shock and the following maxima of negative Bz or negative Dst. The time intervals between maximum negative Bz and maximum negative Dst are mostly in the range of 0-5 h. The magnitudes of the maxima of negative Bz and the following negative Dst are highly correlated, indicating this as a major, overpowering relationship for determining the severeness of geomagnetic storms.