Seasonal effects on auroral particle acceleration and precipitation

Abstract

Global auroral images acquired from the Polar ultraviolet imager in the Northern Hemisphere during the winter of 1996 and the summer of 1997 (4 weeks before and after solstice) are used to study seasonal effects on auroral acceleration and precipitation. The energy flux of precipitating electrons is inferred from auroral luminosity in the long‐wavelength bands (1600–1800 Å) of N2 Lyman‐Birge‐Hopfield (LBHl) auroral emissions, and the mean energy of precipitating electrons is inferred from the intensity ratio of LBHl to LBHs (1400–1600 Å, the shorter wavelength of LBH bands) auroral emissions. Results indicate that dayside and nightside regions of aurora reveal different seasonal effects: nightside (∼ 1900–0300 MLT) auroral power is suppressed in summer, while dayside auroral power is enhanced in summer and forms the so‐called postnoon auroral hot spots, all by a factor of ∼2. The average energy of precipitating electrons is higher in the dark than in the sunlit hemisphere, while the number flux is lower in the dark than in the sunlit hemisphere for all regions. These changes, up to a factor of ∼3, are local time and latitude dependent. The suppression of the nightside auroral power in summer is associated with a large decrease in the electron energy, whereas the enhancement of dayside aurora in summer is associated with a large increase in the electron number flux. The increase of dayside auroral power in summer may be associated with the large‐scale upward field‐aligned currents, which peak in summer. Results are also discussed in the context of a conductivity feedback instability and a cyclotron maser instability. The asymmetric seasonal effects on the dayside and nightside auroras suggest a voltage generator for the dayside magnetosphere and a current generator for the nightside magnetosphere.