On the behavior of the Dst geomagnetic index in the vicinity of magnetic cloud passages at Earth

Abstract

The geomagnetic response to magnetic clouds is investigated for the 34 magnetic clouds observed between 1973 and 1982, in particular, according to cloud type: southward turning clouds as compared to northward turning clouds (a southward turning cloud is one whose Bz near cloud onset at Earth is directed northward, while a northward turning cloud is one whose initial Bz is directed southward). While, on average, the most negative Dst value associated with interplanetary magnetic clouds is the one for northward turning clouds, the difference in the depths of decrease (onset Dst value minus the most negative Dst value during cloud passage) for the two cloud types does not appear to be statistically important; hence northward turning and southward turning clouds have depths of decrease (negative excursions in the Dst index) of comparable size, differing primarily in the time of most negative Dst occurrence (relative to cloud onset at Earth). For northward turning clouds, the most negative Dst value usually occurs within 12 hours of cloud onset at Earth, while for southward turning clouds it is delayed until after 12 hours from cloud onset. Such behavior is consequential in that it is directly related to the occurrence of southward Bz.