Solar wind density influence on geomagnetic storm intensity

Solar wind density has been argued to have a strong effect on geomagnetic storms. Elevated solar wind density tends to occur in time intervals when the solar wind electric field is large. This complicates the analysis required to identify a solar wind density influence because the solar wind electric field is the dominant driver of geomagnetic storms. Statistical studies have consistently shown that the independent correlation between solar wind density and geomagnetic storm intensity (via a proxy, such as the D-st index) is small. Modeling considerations predict a significant geomagnetic storm dependence on the plasma sheet density, which is indirectly connected to solar wind density. In this work, the solar wind density influence is quantified using two statistical measures: (1) data-derived impulse response functions and (2) the relationship between the integrated value of D-st to the integrated value of the solar wind electric field during geomagnetic storm intervals. Results from both approaches indicate that the solar wind density modifies the geoefficiency or the ability of a given value of the solar wind electric field to create a D-st disturbance. The impulse response method also predicts that solar wind density explains the difference in geoefficiency, as opposed to the solar wind dynamic pressure. Although the geoefficiency effect is large, its influence is shown to be small when only large storms are considered because large storms typically have large density.