Seasonal Variations in Diffuse, Monoenergetic, and Broadband Aurora

Here we investigate the separate seasonal dependence of both types of electron accelerated aurora - broadband (or wave) in addition to the previously studied monoenergetic - and both ion and electron diffuse aurora. Dayside and nightside variations are separately considered, as are conditions of low and high solar wind driving. Several clear patterns emerge. One is that the dayside tends to maximize precipitation in the summer, and much more so for low solar wind driving. Nightside precipitation is higher in the winter, and much more so for high solar wind driving. Dayside effects are strongest in number flux, and stronger in diffuse aurora than accelerated aurora. The ease of ion entry through the summer cusp, along with the constraints of charge quasi-neutrality, and the rise in dayside currents in the summer hemisphere adequately explains much (perhaps all) of the dayside behavior. Nightside effects are more apparent in energy flux, with the winter/summer ratio of monoenergetic aurora the largest, 1.70 for high solar wind driving. However both other types of electron aurora, diffuse (1.30) and broadband (1.26), also have winter/summer energy fluxes well above unity for high solar wind driving. The nightside seasonal variation of ions is much smaller, with slightly more energy flux postmidnight in the winter, but with slightly higher energy fluxes premidnight in the summer. Since the increased nightside fluxes into the winter hemisphere occurs primarily under strong solar wind driving, and is much more prominent in energy flux than number flux, it likely reflects increased energization in the winter ionosphere when stronger currents are being driven into the ionosphere from the magnetosphere. Equinoctial behavior tends to lie between the summer and winter hemisphere values, but typically closer to the latter.