Statistics of substorm occurence in storm and non-storm periods

We have investigated similarities and differences between substorms occurri ng in stormtime and those in less disturbed periods, using data from the MI CS instrument on CRRES to investigate composition changes in ring current i ons in the energy range 70-400 keV/e. A statistical study shows that while substorms show some size and composition dependence on Dst, the strongest e ffect is that substorm injected energetic ion composition reflects the back ground population at energisation. These results are supported by a superposed epoch analysis which shows that the composition of storm time injections is similar to the set of all (sto rm and quiet rime) oxygen rich injections. Moreover, these injections which have a high initial oxygen background, which is the condition that pertain s in storms. We conclude that, during solar maximum when these measurements were made, substorms are not directly responsible for the injection of oxy gen ions into the ring current. (C) 1998 Elsevier Science Ltd. All rights r eserved.