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 betw een the ionosphere and the magnetosphere, but when the transpolar pote ntial drop increases, region 2 currents transfer energy away from the ionosphere into the magnetosphere. Both properties are counterintuitiv e and differ from the ''normal'' properties exhibited by region 1 curr ents, which in steady state or otherwise consistently deliver energy t o the ionosphere to feed Joule dissipation. We extend the theory to in clude the effects of charge exchange so that it can be applied to magn etic storms. We conclude that charge exchange also extracts energy fro m the ionosphere via region 2 currents. To test these predictions of u nexpected properties, we use the technique of assimilative mapping of ionospheric electrodynaics (AMIE). From AMIE-derived values of ionosph eric electrical potentials and field-aligned currents for a well docum ented magnetic storm, we compute the direction and flux of energy flow ing between the ionosphere and the magnetosphere. Region 2 currents in deed 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 recove ry phase when conditions approximate steady state. Also the data are c onsistent with charge exchange acting to extract energy from the ionos phere via region 2 currents. Thus the test confirms two of the predict ions and is consistent with the third. For this storm the region 2 upw ard energy flux is at most about 30% of the energy that goes into the ring current to;account for the growth of Dst.