Low‐altitude equatorial ions: A new look with SAMPEX

Abstract

We have used the Low‐Energy Ion Composition Analyzer (LICA) instrument on the low altitude, polar orbiting SAMPEX spacecraft to survey energetic ions near the magnetic equator from late 1992 through 1998; that is, through the declining phase of Solar Cycle 22, solar minimum, and into the rise of Solar Cycle 23. This survey gives us a unique opportunity to examine both the long‐term variation in the low‐altitude equatorial ion population and short‐term enhancements that occur during magnetic storms. During the survey period, 40 storms with minimum Dst ≤ 100 nT occurred: the majority were accompanied by increases in the equatorial ion flux. Although LICA detects ions with energies far above the bulk of the ring current ion population, the times of the maximum equatorial fluxes clustered around the time of minimum Dst, i.e., the time of maximum ring current energy content. The storm associated flux maxima were unevenly distributed in geographic longitude, with the maximum flux enhancements occurring at longitudes just west of the South Atlantic Anomaly. Except for an increase in 1994, the quiet time monthly average equatorial flux declined steadily from 1992 until early 1998; then it began to rise again. The monthly average equatorial ion fluxes had a very significant correlation with the Ap index during this period (R = 0.54), indicating that geomagnetic activity dominated the long‐term variation. During the survey, we also discovered enhancements in the equatorial ion flux that occurred shortly after the onsets of three recent, large solar energetic particle events. These enhancements began well before the commencements of the associated geomagnetic storms. The major ion species present were H, He, C, and O, therefore ruling out an ionospheric source. These ions could not have penetrated directly from interplanetary space to the magnetic equator, and we do not understand the mechanism that produces the SEP‐associated enhancements.