Solar wind density influence on the efficiency of geomagnetic response to the interplanetary electric field

Solar wind density has been suggested to have a strong effect on Earth's magnetosphere. Elevated solar wind density often exists during intervals of enhanced magnetospheric activity, which complicates the analysis required to make this conclusion. In contrast, statistical studies have consistently shown that the independent correlation between solar wind density and magnetospheric activity is small. These two seemingly contradictory results are resolved by showing that the solar wind density affects the interplanetary electric field geoefficiency in a way that is not captured by the standard correlation or epoch averaging approach. The solar wind density influence is quantified using statistical approaches that differ from the standard treatment, including (a) data-derived impulse response functions and (b) the ratio of the integrated response of geomagnetic activity to the integrated value of the interplanetary electric field during geomagnetically active time intervals. We show that (1) it is the solar wind density, not pressure, that is the mediating factor in the case of geomagnetic activity quantified by the D_st index and (2) the geoefficiency depends on the latitude that geomagnetic activity is measured.