The importance of solar illumination for discrete and diffuse aurora

We present a comprehensive overview of the occurrence of discrete and diffuse aurora in the nightside Northern Hemisphere at invariant latitudes 55−75. Twenty-one months of Freja observations (1 January 1993 to 30 September 1994) from the Northern Hemisphere, obtained at ∼1700 km altitude, are included in this investigation. We investigate the importance of seasonal effects, solar illumination and geomagnetic activity for the auroral precipitation. The seasonal variations in the occurrence of discrete aurora are separated from the dependence on solar illumination of the ionosphere. When the effects of sunlight are eliminated, aurora is found to be more common during the summer. The occurrence of diffuse, as well as discrete aurora, is suppressed by solar illumination of the ionosphere. This dependence of diffuse auroral precipitation on ionospheric conditions is not predicted by theories that attribute diffuse aurora to equatorial pitch-angle diffusion of hot magnetospheric electrons.