Long-period Magnetic Pulsations Associated with Detached Proton Auroral Arcs

One remarkable discovery of the NASA-IMAGE mission is the occurrence of detached proton auroral arcs. These are fairly long lived (>30 minutes) relative to other subauroral phenomena, are found separated by hundreds of kilometers from the main dusk-sector auroral oval, and generally occur several hours into a period of heightened geomagnetic activity. Interaction of 10-50 keV ring-current protons with a cold plasmaspheric population drawn outward and sunward by enhanced magnetospheric convection is generally believed to be the source of the precipitation. The cold-hot plasma interaction is prone to the growth of the ion-cyclotron instability, which subsequently scatters hot ions into the loss cone, which are then seen as precipitation of energetic protons in detached arcs by the SI-12 proton auroral imaging instrument on IMAGE. It has now been noted that most, if not all, of the detached proton arcs are associated with a strong pulsation signature with a period in the range of 300-600 seconds (Pc5), as observed by ground-based magnetometers in the vicinity of the arcs. It is not yet clear whether the Pc5 pulsations are generated at the magnetopause by solar wind disturbances, or rather within the magnetosphere by the energetic protons themselves, possibly through a bounce resonance mechanism. Whether the Pc5 waves are somehow causative in producing the precipitation, simply a result of the hot-cold plasma interactions which otherwise scatter the ring current particles, or just coincidentally present during these events is the question this investigation seeks to answer.