Predictors of Substorm Onset Conjugate Displacement

Abstract

Using a substorm onset list from the Imager for Magnetopause to Aurora Global Exploration (IMAGE) and Polar satellite images, we have constructed a new database of the times and locations of 63 substorm onsets observed in both hemispheres. We find that incorporating the history of the interplanetary magnetic field (IMF) via integration improves the predictive power of IMF $B_y$ in explaining conjugate displacement in Magnetic Local Time (MLT). Our data set, approximately five times larger than any previously reported, enhances statistical power and focuses on the substorm onset feature. This allows for better correlation of IMF $B_y$ with MLT conjugate displacement ($\Delta$MLT), since it has been shown that IMF-induced $B_y$ within the magnetosphere tends to diminish as substorms progress. In line with previous work, we find a linear relationship between IMF $B_y$ and $\Delta$MLT, though with a shallower gradient. Additionally, we analyze predictors of conjugate displacement in Magnetic Latitude ($\Delta$MLat), showing that dipole tilt $\left(\psi \right)$ and the $y$ component of the solar wind velocity $\left(V_y\right)$ at the bowshock are the strongest predictors. Using a multilinear fit we estimate the relationship between $\Delta$MLat and these two predictors. We suggest mechanisms by which $\psi$ may influence $\Delta$MLat. By contrast, the relationship between $V_y$ and $\Delta$MLat requires further investigation to uncover the underlying mechanism.