Seasonal dependence of localized, high‐latitude dayside aurora (HiLDA)

The FUV instrument on the IMAGE spacecraft frequently observes intense ultraviolet (UV) emissions from a localized High Latitude Dayside Aurora (HiLDA) poleward of the general auroral oval location [ Frey et al. , 2003a]. It has been shown that this aurora is entirely created by high‐energy precipitating electrons, which have probably been accelerated in a quasi‐static field‐aligned electric potential. Here we extend the previous case study to an investigation of the HiLDA occurrence in the Northern Hemisphere over more than 2 years and compare it with the averaged solar wind plasma and interplanetary magnetic field (IMF) properties. HiLDA occurrence is strongly biased toward low solar wind density and IMF with positive B z , strong positive B y (clock angles around 70°), and negative B x components. Such IMF conditions are favorable for lobe reconnection in the Northern Hemisphere, and the creation of a dominating dusk convection cell with an upward field‐aligned current in its center. Additionally, we investigate the seasonal occurrence of this phenomenon, which shows a maximum in the Northern Hemisphere during sunlit summer months and an almost complete absence in the dark winter. In contrast to the daylight‐suppressed aurora in the auroral oval, the HiLDA cannot be the result of ionospheric feedback due to the stronger ionospheric conductance in sunlight. Instead, in agreement with ionospheric convection models, it is caused by the asymmetry of field‐aligned currents in different seasons, which result from the different dipole tilt angles in summer and winter. We further discuss two scenarios for how the low solar wind density can enhance the field‐aligned parallel potential over the dusk convection cell.