Abstract
The dominant loss processes for terrestrial ring current ions are collisional, including charge exchange with the neutral geocorona and Coulomb collisions within the plasmasphere. In addition, 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. Resonant interaction between energetic ring current ions and electromagnetic ion cyclotron (EMIC) waves results in pitch angle scattering and subsequent precipitation of the energetic ions into the upper atmosphere. 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 EMIC waves. 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 energetic 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. Conversely, arcs which occur during quiet times tend to be located at higher latitudes, and their relationship with regions of enhanced cold plasma density remains unclear. Wave data available for two of the detached arc events indicate the presence of strong ion cyclotron waves near the equator in the vicinity of the proton precipitation region. The afternoon subauroral proton arcs provide an excellent basis for comparison with predicted proton precipitation patterns from wave scattering included in the increasingly sophisticated global ring current models.algorithms, and describe the results of the associated experiments.