Morphology and seasonal variations of global auroral proton precipitation observed by IMAGE-FUV

Observations with the FUV imagers on board the IMAGE satellite have been used to map the auroral electron and proton energy fluxes during the summer and winter solstices of 2000, in order to construct a statistical view of the global auroral proton precipitation. The distribution for electrons compare well both in morphology and in magnitude with those obtained previously with the Polar-UVI instruments and with an empirical auroral precipitation model based on DMSP data. The proton morphology also closely resembles the statistical ion oval derived from DMSP data, showing a "C-shaped'' morphology with a minimum located in the morning sector. The precipitation proton auroral power is on the order of 2.2 GW for an average Kp value of 2, also in close agreement with the values of the DMSP empirical model. The FUV data also reveal the presence of seasonal effects in the proton precipitation. Specifically, the latitudinal width of the proton oval is larger in summer than in winter so that the globally precipitated proton power is 1.5 times higher in summer than in winter. The occurrence probability of intense proton auroras (with energy flux >0.5 mW m(-2)) is also shown to be nearly three times higher in summer than in winter. This seasonal effect in the proton precipitation contrasts with those observed for electrons, where intense electron events occur more often in winter than in summer. We discuss a mechanism that may account for these results based on the presence of field-aligned potential drops which accelerate auroral electrons downward in regions of upward directed field-aligned current, while suppressing the precipitating magnetospheric proton flux. The presence of such field-aligned potentials is dependent on the differing solar illumination in winter and summer.