Radial transport of storm time ring current ions

Abstract

Radial transport of energetic ions for the development of the main phase of geomagnetic storms is investigated with data from the medium energy particle analyzer (MEPA) on the Charge Composition Explorer (CCE) spacecraft, which monitored protons (Ep > 56 keV), helium ions (EHe > 72 keV), and the carbon‐nitrogen‐oxygen group, which is mostly dominated by oxygen ions (E0 > 137 keV). From a study of four geomagnetic storms, we show that the flux increase of these ions in the inner ring current region (L ≲ 5) can be accounted for by an inward displacement of the ring current population by ∼0.5 to 3.5 RE. There is a general trend that a larger inward displacement occurs at higher L shells than at lower ones. These results are in agreement with previous findings. The radially injected population consists of the prestorm population modified by substorm injections which occur on a much shorter time scale than that for a storm main phase. It is also found that the inward displacement is relatively independent of ion mass and energy, suggesting that the radial transport of these energetic ions is effected primarily by convective motion from a large electric field or by diffusion resulting from magnetic field fluctuations.