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 modeling of the large-scale coupling between the high-latitude ionosphere and magnetosphere. In this paper we elucidate the steady state relationship between the electromagnetic energy flux (divergence of the de Poynting flux), the Joule heating rate, and the mechanical energy transfer rate in the high-latitude ionosphere. We also demonstrate the important role of the neutral wind and its conductivity-weighted distribution with altitude in determining the resultant exchange of electromagnetic energy at high latitudes. Because the Poynting flux approach accounts for the neutral wind implicitly and describes the net electro-magnetic energy flux between the magnetosphere and ionosphere, it is a fundamental measure of energy transfer in the system. A significant portion of this energy transfer results in Joule heating; however, the conversion of electromagnetic energy flux into mechanical energy of the neutrals is also considerable and can in some regions exceed the Joule heating rate. We will show that neglect of the neutral dynamics in calculations of the Joule heating rate can be misleading. To evaluate and interpret the electromagnetic energy flux at high latitudes, we employ the vector spherical harmonic model, which is based on the National Center for Atmospheric Research thermosphere-ionosphere general circulation model, to provide the steady state properties of the thermosphere-ionosphere system under moderate to quiet geomagnetic activity. For the specific geophysical conditions modeled we conclude that (1) the electromagnetic energy flux is predominantly directed into the high-latitude ionosphere with greater input in the morning sector than in the evening 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 significantly to the Joule healing rate and thus affecting the net electromagnetic energy flux in the ionosphere. (3) On average, the mechanical energy transfer rate amounts to about 10% to 30% of the net electromagnetic 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 electromagnetic energy flux are found near the convection reversal boundaries where the mechanical energy transfer rate exceeds the Joule heating rate. In general, large upward electromagnetic energy fluxes may be rare, as the always positive-Joule heating rate increases irrespective of the source of electromagnetic energy flux; that is, neutral dynamics contribute directly to the Joule heating rate.