Ring current response to interplanetary magnetic cloud events

Interplanetary magnetic clouds appear to have a wide range of ability to af fect the magnetospheric energetic particle populations. This "effectiveness " or lack thereof, shows up in both the electron and ion components of the radiation belts. The ring current ions are intensified and energised by the geomagnetic storm response to the cloud. The ion composition of the ring c urrent may also change due to source variations in the solar wind and ionos phere. Seven of the magnetic cloud events during the first year of the GGS Polar mission are compared as to their effectiveness to change the intensit y and composition of the ring current ion populations. Data from the CAMMIC E MICS ion composition instrument on Polar are presented to quantify the ri ng current response to each event. The January 10, 1997 event was observed to have a relatively modest effect on the ring current. In contrast, the cl oud that arrived on May 15, 1997 resulted in ring current ion energy densit y enhancements approximately one order of magnitude larger than the pre-sto rm values. (C) 1998 Elsevier Science Ltd. All rights reserved.