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 upper atmosphere. sphere. Knowledge of the magnitude of this source is fundamentally important to a thorough understanding of the region's physics. However, Joule heating is currently one of the largest sources of 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 scale in response to changes in solar wind conditions. The convection patterns represent average electric potential distributions, and thus the resulting amount of Joule heating is proportional to the square of the average E-field. That method ignores the important component of Joule 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 heating 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-field requires knowledge of the statistical characteristics of E-field variability; 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 given pattern.