Efficiency of Solar Wind Energy Coupling to the Ionosphere

We investigate the variations of the ionospheric energy coupling efficiencies with the solar wind energy input, the interplanetary magnetic field (IMF) clock angle and the solar wind dynamic pressure. The ionospheric energy coupling efficiencies are defined as the ratios of the ionospheric energy deposition (namely auroral precipitation, Joule heating, both auroral precipitation and Joule heating) to the solar wind energy input. We find that the ionospheric energy coupling efficiencies decrease exponentially with the solar wind energy input, and they are significantly dependent on the IMF clock angle and almost independent of the solar wind dynamic pressure. The results will help us estimate and predict energy transfer from the solar wind to the thermosphere-ionosphere system under extreme space weather conditions, particularly severe geomagnetic storms.