INTERPRETATION AND MODELING OF THE HIGH-LATITUDE ELECTROMAGNETIC ENERGY FLUX

An interpretation of the electromagnetic energy flux at high latitudes under steady state conditions is presented and analyzed through model ing of the large-scale coupling between the high-latitude ionosphere a nd magnetosphere. In this paper we elucidate the steady state relation ship between the electromagnetic energy flux (divergence of the de Poy nting flux), the Joule heating rate, and the mechanical energy transfe r rate in the high-latitude ionosphere. We also demonstrate the import ant role of the neutral wind and its conductivity-weighted distributio n with altitude in determining the resultant exchange of electromagnet ic energy at high latitudes. Because the Poynting flux approach accoun ts for the neutral wind implicitly and describes the net electro-magne tic energy flux between the magnetosphere and ionosphere, it is a fund amental measure of energy transfer in the system. A significant portio n of this energy transfer results in Joule heating; however, the conve rsion of electromagnetic energy flux into mechanical energy of the neu trals is also considerable and can in some regions exceed the Joule he ating rate. We will show that neglect of the neutral dynamics in calcu lations of the Joule heating rate can be misleading. To evaluate and i nterpret the electromagnetic energy flux at high latitudes, we employ the vector spherical harmonic model, which is based on the National Ce nter for Atmospheric Research thermosphere-ionosphere general circulat ion model, to provide the steady state properties of the thermosphere- ionosphere system under moderate to quiet geomagnetic activity. For th e specific geophysical conditions modeled we conclude that (1) the ele ctromagnetic energy flux is predominantly directed into the high-latit ude ionosphere with greater input in the morning sector than in the ev ening sector, as supported by DE 2 observations. (2) The Joule heating rate accounts for much of the electromagnetic energy deposited in the ionosphere with the conductivity-weighted neutral wind contributing s ignificantly to the Joule healing rate and thus affecting the net elec tromagnetic energy flux in the ionosphere. (3) On average, the mechani cal energy transfer rate amounts to about 10% to 30% of the net electr omagnetic energy flux in the auroral dawn, dusk, and polar cap regions , acting as a sink of electromagnetic energy flux in the dawn and dusk sectors and a source in the polar cap. (4) Weak regions of upward ele ctromagnetic energy flux are found near the convection reversal bounda ries where the mechanical energy transfer rate exceeds the Joule heati ng rate. In general, large upward electromagnetic energy fluxes may be rare, as the always positive-Joule heating rate increases irrespectiv e of the source of electromagnetic energy flux; that is, neutral dynam ics contribute directly to the Joule heating rate.