Ion aurora and its seasonal variations

[1] Recent studies have shown that intense discrete aurora, auroral kilometric radiation, upflowing ion beams, and downward directed electric fields are more intense in the winter hemisphere than in the summer. This is particularly true of the dusk to midnight sector where intense electron aurora are most common. Here we use one solar cycle of DMSP satellite particle data to investigate the seasonality of the ion aurora. The ion aurora proves to be approximately equal in the summer and winter hemispheres in the dusk- midnight sector ( with the summer hemisphere favored by 0 to 4%). However, in the MLT hours from midnight to dawn, the ion precipitating energy flux is 15 - 40% higher in winter than in summer. The absolute magnitude of the ion effect is smaller than was found for discrete electron aurora ( which show a threefold difference between winter and summer). The seasonal behavior of the ions may reflect the observation that diverging electric fields, which accelerate ions downward, are found mainly postmidnight, and are stronger in the winter. The relative weakness of the seasonal effects in ions may reflect their high average energy ( many tens of keV), which is substantially larger than typical of electric potentials found in the auroral circuit. Ions in the dusk to midnight sector are most intense equatorward of the region of discrete aurora, and are thus probably not much affected by the seasonality of the field- aligned electric fields which exist there ( and which are of the sense to retard ion precipitation). Interestingly, ion average energies are higher in the winter hemisphere than in the summer hemisphere at all local times, regardless of whether or not energy fluxes are enhanced. The above results apply to the actual range of ion energies measured by the DMSP electrostatic analyzers ( 32 eV to 30 keV). A sample extrapolation to 100 keV energy, assuming the spectra are Maxwellian, showed the same overall pattern. The percentage by which the winter hemisphere was favored over the summer for precipitating ion energy flux was moderately larger when using extrapolation, and a larger difference was still seen postmidnight than premidnight.