The relation between dayside local Poynting flux enhancement and cusp reconnection

The Defense Meteorological Satellite Program (DMSP) F15 satellite frequently observes an unusually high level of local Poynting flux when the interplanetary magnetic field (IMF) is northward but also has a large By component. We use the Open Geospace General Circulation Model (OpenGGCM) coupled magnetosphere-ionosphere-thermosphere code to study several events where, under such conditions, unusually large earthward Poynting flux has been observed. We find that the Joule heating rate in the model agrees well with the observed Poynting flux. Analysis of the model results shows that the strong Poynting flux hot spots are physically linked with magnetic reconnection at the high-latitude cusps. The solar wind mechanical force and the J × B force act on the newly opened field lines created by cusp reconnection to produce a Pedersen current, which consequently generates an intense Joule heating region, and a pair of adjacent and opposite field-aligned currents (FACs) connecting to the magnetopause currents forming a closed circuit. The intense Joule heating region is also the region with strong downward Poynting flux. The distribution, scale, and magnitude of this Joule heating region and corresponding FACs in the polar regions are mainly controlled by IMF clock angle, IMF magnitude, and solar wind dynamic pressure. A northward IMF condition with a large By component will result in an extended region with intense Joule heating and FAC, thus making a spacecraft transiting the dayside region more likely to observe a strong downward Poynting flux.