Ion composition variations in the ring current during intense magnetic storms and their relationship with evolution of storms

Ion composition variations in the inner magnetosphere during storm time are investigated using data set obtained from CRRES/MICS. Case study of the tw in storms in July,1991 and statistical study of 12 intense storms in the sa me year both indicate that the storm-time ring current particles can be div ided into two groups:one consists of O+, low energy H+ and He+,which are or iginated from the ionosphere (IOP),the other is composed of high energy Hand He++ which are originated from the solar wind (SOP). It is shown that i n quiet time the major particles of the ring current are SOP,whereas during the main phase of large storms the main component of the ring current part icles is the IOP. Ring current particles of intense storms can be injected to low L-altitudes (L similar to 3 - 4). It is confirmed that the contribut ion of IOP to the ring current increases with increasing D-st value. In lar ge storms IOP are the major constituents of the ring current,even up to 80 % of the total number density at the D-st Maximum. There is clear evidence showing that it is the rapid enhancement and reduction of the O+ flux of th e ring current that leads the D-st index to rapidly decrease during the mai n phase and to quickly recover in the early recovery phase. Further analyse s indicate that during weak storms (D-st > - 50nT) the contribution of O+ i ons to the ring current is negligible.