Interhemispheric comparison of average substorm onset locations: evidence for deviation from conjugacy

Abstract. Based on 2760 well-defined substorm onsets in the Northern Hemisphere and 1432 in the Southern Hemisphere observed by the FUV Imager on board the IMAGE spacecraft, a detailed statistical study is performed including both auroral regions. This study focuses on the hemispheric comparisons. Southward pointing interplanetary magnetic field (IMF) is favorable for substorm to occur, but still 30% of the events are preceded by northward IMF. The magnetic latitude (MLat) of substorm onset depends mainly on the merging electric field (Em) with a relationship of |dMLat|= −5.2 E_m^0.5, where dMLat is the deviation from onset MLat. In addition, seasonal effects on onset MLat are also detected, with about 2 degrees higher latitudes during solstices than equinoxes. Both IMF B_y and solar illumination have a significant influence on the magnetic local time (MLT) of onsets. An average relation, dMLT=0.25 B_y between IMF B_y and the deviation from onset MLT, was found. The B_y dependence varies slightly with the onset latitude. At lower latitudes (higher activity) it is reduced. After removal of the relationship with IMF B_y a linear relationships remains between the solar zenith angle and onset MLT with dMLT=1 min/deg. Therefore, both solar illumination and IMF B_y can contribute to hemispheric longitudinal displacements of substorm onset locations from conjugacy. No indications for systematic latitudinal displacements between the hemispheres have been found.