Afternoon Subauroral Proton Precipitation Resulting from Ring Current -
Plasmasphere Interaction

M. Spasojevic, M.F. Thomsen, P.J. Chi, and B.R. Sandel


Although the dominant loss processes for ring current ions are collisional,
wave-particle interactions are also believed to play an important role as they
provide a mechanism for the rapid decay of the ring current during the early
recovery phase of geomagnetic storms. Considerable attention has been given to
regions of spatial overlap between energetic, anisotropic ring current ions and
cold, dense plasmaspheric material that should be particularly conducive to the
growth of electromagnetic ion cyclotron (EMIC) waves. Resonant interaction
between ring current ions and EMIC waves results in pitch angle scattering and
subsequent precipitation of the energetic ions into the upper atmosphere. Global
imaging of the proton aurora by the Far Ultraviolet (FUV) Spectrographic Imager
(SI) on-board the IMAGE satellite has led to the identification of arcs of
precipitating protons at latitudes equatorward of and separated from the main
proton auroral oval in the afternoon local time sector. We investigate the
occurrence of these arcs and their relationship with the plasmasphere and
electromagnetic ion cyclotron waves. In a four month study interval including
sixteen events, we find that the detached proton arcs are more likely to occur
during geomagnetically disturbed periods and specifically at times when enhanced
energetic ion densities and temperature anisotropies are observed in the
equatorial magnetosphere. The disturbance-time arcs tend to be located at lower
magnetic latitudes and are consistently associated with plasmaspheric plumes as
observed by the IMAGE Extreme Ultraviolet (EUV) instrument. Wave data from the
POLAR Magnetic Field Experiment (MFE) available for two of the detached arc
events indicate the presence of strong EMIC waves near the equator in the
vicinity of the proton precipitation region.

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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004