Dependence of the Solar Wind Plasma Density on Moderate Geomagnetic Activity Elucidated by Potential Learning

The relationship between a moderate level of geomagnetic activity, represented by the Kp index (2- - 5+), and solar wind conditions were revealed based on Potential Learning (PL), and dependence of particular solar wind plasma density (NP) on moderate geomagnetic conditions was discussed. It has poorly been understood from what stage of geomagnetic activity NP begins to control the Kp level. Previously, PL succeeded to extract the most influential solar wind parameters at an extremely low (0 - 1+) and high (6- - 9) Kp ranges under southward interplanetary magnetic field (IMF) conditions. The IMF three components, solar wind flow speed (VX), and NP obtained from the 21-year OMNI solar wind database were used as the PL input parameters. PL extracted VX as the most significant parameter for the moderate Kp level under southward IMF conditions and NP is the second most influential parameter. Based on the examination of the statistical relationship between VX and NP under extremely low, high, and moderate Kp levels using the PL database, geomagnetic conditions remain high while NP becomes large, even if VX decreases (or remains similar). This shows that both VX and NP govern geomagnetic activity, following the relational equation between Kp and the solar wind plasma parameter. Based on the relation between VX and NP clarified by PL and incidental statistical studies using PL input data, we also revealed that NP begins to affect the Kp level from moderate geomagnetic activity level. These results suggest that even under southward IMF conditions, the geomagnetic activity might be influenced by fast (slow) VX and low (high) NP. Our results would greatly help understand general relationship between solar wind conditions and geomagnetic activity under various IMF conditions, and forecast the geomagnetic activity under extreme solar wind conditions.