High-latitude Thermosphere Response to Solar Wind Dynamic Pressure Enhancements

Using thermospheric density observations from the CHAMP and GRACE satellites with the conjunction of Poynting flux measurements from the DMSP F16 satellite, we show that the thermosphere density as well as the downward Poynting flux intensified shortly after (within ~20 min) a sudden enhancement of the solar wind dynamic pressure mostly on the dayside auroral zone and polar cap regions with peaks in the vicinity of the noon and pre-noon cusp. Simulations from the OpenGGCM global MHD model show that the ionospheric Joule heating also increases abruptly along with the sudden enhancement of the dynamic pressure in the same regions, linking the incoming Poynting flux enhancement and the resulted thermosphere density enhancement. Our results indicate that the response of the thermosphere density to the solar wind dynamic pressure enhancement at high latitude regions is nearly instantaneous. Our modeling results demonstrate that a pair of high-latitude localized FACs near noon and pre-noon regions (or the cusp FACs) that are intensified and extend azimuthally as a result of the enhanced dayside high-latitude reconnection caused by the sudden increase of the solar wind dynamic pressure as well as the ensuing large value of IMF By with high dynamic pressure plays a critical role in this energy input process. Through various numerical experiments, we are able to clearly identify and quantify the contribution of different solar wind drivers and evaluate their relative contribution to the energy input, thus tracking their roles in the solar wind and magnetosphere-ionosphere coupling. Our results indicate that the solar wind dynamic pressure completely dominates the energy input process within the initial several min of the sudden dynamic pressure increase. When the dynamic pressure maintains steadily at a high level, its contribution decreases but still keeps at an elevated level. IMF By plays a major role in affecting the azimuthal extension of the cusp FACs. Neither the individual contribution of high solar wind dynamic pressure and large value of IMF By nor the summation of the two can account for the overall energy input, suggesting that the energy input is likely a non-linear process that will amplify the individual contributions.