The nature of auroras during High-Intensity Long-Duration Continuous AE Activity (HILDCAA) events: 1998 to 2001

Abstract

The nature of auroras during High-Intensity Long-Duration Continuous AE Activity (HILDCAA) events occurred from 1998 to 2001 is studied using auroral images in Lyman-Birge-Hopfield Long (LBHL - 160 to 180 nm) wavelengths taken by the UVI instrument onboard the POLAR satellite. It is found that during HILDCAA events, auroras are occurring not only in the midnight sector, but at all local times along the auroral oval. During some events, auroras are noted even over the polar cap as well. The typical auroral intensities observed during HILDCAA events are in the range from 30 to 60 photon cm -2 s -1 . Using a method to evaluate relative photon fluxes based on POLAR images, the photon fluxes during HILDCAAs were qualitatively compared with those during magnetic storm main and recovery phases. It is found that, for the auroral zone midnight sector, the relative photon fluxes during the main phases of storms are much higher than those during HILDCAA intervals, as expected. However, photon fluxes observed during ' HILDCAA events are generally higher than that during the recovery phase of storms. Thus, if integrated over long periods of time (a year), the energy input from particle precipitation into the ionosphere during the declining phase/ minimum phase of the solar cycle, when HILDCAAs/corotating solar wind streams dominate, could be greater than during solar maximum, when interplanetary coronal mass ejections (ICMEs) predominate. This is in agreement with several parallel studies based on the AE and Dst indices that indicate similar conclusions.