Subauroral morning proton spots (SAMPS) as a result of plasmapause-ring-current interaction

[1] The proton aurora imager SI-12 on the IMAGE spacecraft occasionally observes subauroral morning proton spots (SAMPS) that rotate with 70 - 95% of the Earth's corotation speed. Coincident particle measurements by DMSP confirm the source to be pure precipitating protons with mean energies likely above the detector limit of 30 keV. The spots appear in the recovery phase after magnetic storms and last for 1 - 4 hours in the magnetic local time region of 0300 - 1200 hours. The latitude location is strongly related to the minimum Dst of the previous geomagnetic storm with the lowest latitude observations after the strongest storms. The rotation speed is related to the latitude ( L shell) of the spots with the largest corotation lags for spots that map to the largest L shells. IMAGE-EUV observations of the plasmasphere indicate a relationship with density gradients in the expanding plasmasphere after magnetic storms. We interpret these spots as the result of wave-particle interaction. As one likely process, we suggest the interaction of ring current protons with electromagnetic ion-cyclotron (EMIC) waves as a result of the expansion and subrotation of the dense, cold plasmasphere ions. The appearance of subauroral proton spots is therefore a consequence of the plasmasphere refilling after geomagnetic storms.