ON THE IMPORTANCE OF E-FIELD VARIABILITY FOR JOULE HEATING IN THE HIGH-LATITUDE THERMOSPHERE

Joule heating is known to be one of the major energy sources of the up per atmosphere. sphere. Knowledge of the magnitude of this source is f undamentally important to a thorough understanding of the region's phy sics. However, Joule heating is currently one of the largest sources o f uncertainty in the thermosphere's energy budget. In numerical models the distribution Of Joule heating is generally computed using mean or average convection patterns, which evolve on a relatively long time s cale in response to changes in solar wind conditions. The convection p atterns represent average electric potential distributions, and thus t he resulting amount of Joule heating is proportional to the square of the average E-field. That method ignores the important component of Jo ule heating due to rapid or small-scale fluctuations in E-field or ion drifts. However, E-field fluctuations are known to exist on a variety of temporal and spatial scales, and the actual amount of Joule heatin g in the thermosphere is proportional. to the average of the square of the E-field. The computation of the average of the square of the E-fi eld requires knowledge of the statistical characteristics of E-field v ariability; thus knowledge not available at present. In this paper we assess, on the bases of theoretical considerations, the importance of E-field variability as an upper-atmosphere energy source. We show that the inclusion of E-field variability in the high-latitude convection model can significantly increase the amount of Joule heating for a giv en pattern.