Seasonal dependence of localized, High Latitude Dayside Aurora (HiLDA)

Frey, H.U., N. Ostgaard, T.J. Immel, H. Korth, and S.B. Mende


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 been accelerated in a quasi-static 
electric potential.  Here we extend the previous case study to an 
investigation of the HiLDA occurrence 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 towards low solar wind 
density and IMF with positive Bz, strong positive By (clock angles around 
70 degrees) and negative Bx components.  Such IMF conditions are favorable 
for lobe reconnection in the northern summer hemisphere, and the creation of 
a strong dusk convection cell.  Additionally, we investigate the seasonal 
occurrence of this phenomenon which shows a maximum in the northern 
hemisphere during summer months, and an almost complete absence in the 
winter.  In contrast to the daylight-suppressed aurora in the auroral oval, 
the HiLDA can not 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.

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Submitted to J. Geophys. Res., 2003