We examine possible interplanetary mechanisms for the creation of the large
st magnetic storms at the Earth. We consider the effects of interplanetary
shock events on magnetic cloud and sheath plasma. We also examine the effec
ts of a combination of a long-duration southward sheath magnetic field, fol
lowed by a magnetic cloud B-S event. Examination of profiles of the most in
tense storms from 1957 to the present indicate that the latter (double IMF
B-Z events) is an important cause of superintense D-ST events. (C) 1998 Pub
lished by Elsevier Science Ltd. All rights reserved.