Compression of magnetic clouds in interplanetary space and increase in their geoeffectiveness

Using a set of 54 magnetic clouds observed in the period from 1965 to 1997, we found that the magnetic cloud field and velocity strength relationship proposed by Gonzalez et al. (Geophysical Research Letters 25 (7) (1998) 963-966) is generally followed. However, for a few events, the field strength is fairly higher than that predicted by the work of Gonzalez et al. We examined some of these events and found that compression of these magnetic clouds might have occurred, due to the follow-up presence of a higher speed stream, which led to a stronger magnetic field strength at the back region of the cloud. This intensification of the magnetic field strength is closely related to the occurrence of intense geomagnetic storms when the field has a north-south polarity. Such interplanetary compression of magnetic clouds should be considered in space weather forecasting models. (C) 2001 Elsevier Science Ltd. All rights reserved.