Ion Composition Variations in Intense Magnetic Storms and their Relation to Storm Evolution

Abstract

AbstractIon composition variations in the inner magnetosphere during storm times are investigated using data set obtained from CRRES/ MICS. Case study of the twin storms in July, 1991 and statistical study of 12 intense storms from February to October, 1991 both indicate that the storm time ring current ions can be divided into two groups: one consists of O+, low energy H+ and He+, which are originated from the ionosphere (IOP); the other is composed of high energy H+ and He++ , which are mainly originated from the solar wind (SOP). It is shown that in quiet times the major ions of the ring current are SOPs, whereas during the main phase of large storms the major component of ring current particles is the IOP. Ring current ions of intense storms can be injected into low L‐altitudes (2 − 4). It is confirmed that the contribution of IOPs to the ring current increases with increasing L value. IOP is the major constituent of the ring current, even up to 80% of the total number density, at the Dst minimum of large storms. There is clear evidence showing that it is the rapid enhancement and reduction of the O+ flux of the ring current that leads the Dstindex to rapidly decrease during the second stage of the main phase and to quickly recover in the early recovery phase. Further analyses indicate that in weak storms (Dst >‐50 nT) the contribution of O+ ions to the ring current is negligible.