TIMED/GUVI observation of solar illumination effect on auroral energy deposition

Abstract

It has been established that the local nighttime aurora is more intense in winter or darkness than in summer or sunlight. Here we expand previous work to confirm empirical relationships between energy flux and average energy of aurora and solar zenith angle (SZA) and ionospheric photoconductance by analyzing ∼6 years (2002-2007) of FUV auroral images acquired from the Global Ultraviolet Imager (GUVI) on board NASA's Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite. The total number of auroral images, north and south combined, is 55,675. The B3C auroral transport code is used to extract auroral parameters, assuming pure electron precipitation. It is found that the mean energy and energy flux carried by precipitating electrons (>0.25 erg/cm2 s) increase monotonically with SZA for SZA less than ∼108° (corresponding to an EUV shadow height of ∼430 km). The rate of increase is most pronounced in the premidnight sector, where the energy flux rate is ∼0.04 erg/cm2 s/deg and the mean energy rate is ∼0.02 keV/deg. The auroral power is anticorrelated with the ionospheric Pedersen photoconductance for all local times, with the steepest drop (-0.2 gw/mho) in the midnight sector. These results are stronger than those for the simple cases of "sunlight" versus "darkness" and/or "summer" versus "winter" used in previous work for the solar illumination effect. While the present result cannot distinguish known mechanisms, we suggest that more than one mechanism is responsible for the observed sunlight effect. Copyright 2011 by the American Geophysical Union.