Testing Ionospheric influence on Substorm Onset Location

Substorm onset location varies over a range of magnetic local times (MLTs) and magnetic latitudes (MLats). Different studies have shown that about 5${\%}$ of the variation in onset MLT can be explained by variations in interplanetary magnetic field orientation and seasonal variations. Both parameters introduce an azimuthal component to the magnetic field in the magnetosphere such that the projection of the onset MLT in the ionosphere is shifted. Recent studies have suggested that gradients in the ionospheric Hall conductance lead to a duskward shift of the magnetotail dynamics, which could also influence the location of substorm onset. In this paper, we quantify the dependence of the spatial variation of the onset location on the geomagnetic activity level prior to onset. We find that the dependence of onset location on prior conditions is as strong as the dependence on IMF By.