The Seasonal Variation of the Direct Current Global Electric Circuit: 2. Further Analysis Based on Simulations

Both simulations and measurements indicate the existence of stable annual behavior of the global electric circuit (GEC) intensity, but the exact pattern of variation is very difficult to reliably determine. Here we present further analysis of this problem using the results of ionospheric potential (IP) simulations with two different models of atmospheric dynamics. From a theoretical perspective, the annual variation of the GEC is eventually determined by seasonal changes in the distribution of convection, associated with the annual cycle of insolation. Simulations clearly demonstrate that the contribution to the IP from the vicinity of the equator has two maxima following the equinoxes, whereas the contribution of nonequatorial latitudes in each hemisphere has one maximum during the local summer. The resulting seasonal variation of the GEC, being the sum of three clear patterns offsetting each other, is rather subtle, and its prediction in simulations may vary depending on the model and IP parameterization. It is likely that the actual seasonal variation of the diurnal mean GEC intensity has one pronounced maximum during the Northern Hemisphere summer and one pronounced minimum during the Northern Hemisphere winter, in agreement with potential gradient measurements in Antarctica and in agreement with the larger portion of the Northern Hemisphere occupied by land. Physical parameters characterizing convection do not provide a very good measure of contributions to the IP, notably overestimating the role of equatorial latitudes; by including surface air temperature in the IP parameterization, one can achieve a better agreement between simulations and observations.