Comparison of features of all-sky imager identified substorms associated with, and not associated with, IMF northward turnings

Knowledge of the period before the auroral onset of a substorm is crucial for understanding the sequence of events leading to a substorm. It is accepted that substorm onsets are preceded by a growth-phase period of predominantly southward IMF. Such prolonged periods of southward IMF have enhanced magnetospheric convection, leading to enhanced inward motion and energization of plasma sheet particles. In a previous analysis [Hsu and McPherron, 2003] of substorm onsets identified using sudden changes in the slope of the AL index and the start of a Pi 2 pulsation burst, 60% of events were found to be associated with a partial northward recovery of the IMF Bz, indicating that an substantial number of substorms are apparently triggered by such IMF changes. Now we have the capability to study substorm onsets with the high-resolution all-sky images of the THEMIS program, obtaining onsets unambiguously, including for very weak substorms which are difficult to identify with the AL index. We identify the percentage of cases associated, and not associated, with northward turnings. Differences in optical and convection features between substorms associated, and not associated, with northward turnings are addressed. Also differences in solar wind speed are considered, because it has been found that the large IMF fluctuating during high speed streams can lead to substorms independent of the IMF direction.