Energy transfer via region 2 currents: A test of the standard magnetosphere-ionosphere coupling theory

The magnetosphere-ionosphere coupling theory associated with the names of Fejer, Vasyliunas, Swift, Wolf, and others can be used to predict that in steady state the region 2 currents transfer no net energy between the ionosphere and the magnetosphere, but when the transpolar potential drop increases, region 2 currents transfer energy away from the ionosphere into the magnetosphere. Both properties are counterintuitive and differ from the "normal" properties exhibited by region 1 currents, which in steady state or otherwise consistently deliver energy to the ionosphere to feed Joule dissipation. We extend the theory to include the effects of charge exchange so that it can be applied to magnetic storms. We conclude that charge exchange also extracts energy from the ionosphere via region 2 currents. To test these predictions of unexpected properties, we use the technique of assimilative mapping of ionospheric electrodynaics (AMIE). From AMTE-derived values of ionospheric electrical potentials and field-aligned currents for a well documented magnetic storm, we compute the direction and flux of energy flowing between the ionosphere and the magnetosphere. Region 2 currents indeed transfer energy out of the ionosphere during the storm main phase, when the transpolar potential drop increases to its storm time value. The rate of energy transfer is greatly suppressed late in the recovery phase when conditions approximate steady state. Also the data are consistent with charge exchange acting to extract energy from the ionosphere via region 2 currents. Thus the test confirms two of the predictions and is consistent with the third. For this storm the region 2 upward energy flux is at most about 30% of the energy that goes into the ring current to account for the growth of Dst.