Using mutual information to determine geoeffectiveness of solar wind phase fronts with different front orientations

Abstract

The geoeffectiveness of solar wind shocks depends on angle with respect to the Sun‐Earth line, with highly angled solar wind shocks being less geoeffective than nearly frontal solar wind shocks. However, it is unclear whether this holds for the orientation of structures in non‐shocked solar wind. In this paper, we perform a mutual information analysis of 18 years of in‐situ solar wind and ground magnetometer data in order to investigate the effects of solar wind phase front orientation on solar wind geoeffectiveness (indicated by SuperMAG SME). Since geomagnetic response is strongly influenced by Interplanetary Magnetic Field (IMF) Bz, and IMF Bz affects phase front orientation, we use conditional mutual information to account for the effect of Bz on geomagnetic activity. In contrast to what has been found for solar wind shocks, we find that during times of IMF Bz >0, phase fronts aligned with the average Parker spiral direction (45 deg azimuth, 0 deg inclination) tend to be associated with higher geomagnetic activity (SME >500 nT) than would be expected if IMF Bz and phase front orientation quantities were unrelated. During times of IMF Bz <0, there is no connection between solar wind phase front orientation and geomagnetic activity (SME). We believe that Parker spiral aligned phase fronts being associated with higher geomagnetic activity during times of IMF Bz > 0 is due to constant phase front orientation allowing for more efficient energy transfer either through viscous interaction or high latitude reconnection.