Relation of substorm onset to Harang discontinuity

In this study we investigate the relation of auroral substorm onset to the Harang discontinuity (HD). Various studies have reported that the substorm onset occurs poleward, equatorward, or within the HD. The motivation for the present study is to further investigate this relation with a much larger database. Using a database of over 3700 onsets determined from auroral images taken on the IMAGE spacecraft, we statistically examine the magnetic latitude location of the onset with respect to the location of the HD. The location of the discontinuity is determined with data from the IMAGE ground magnetometer network. We define the location of the HD as the transition from relatively strong eastward to relatively strong westward equivalent ionospheric currents at the latitude of the main nightside auroral electrojet flow. Our results show that the location of 75 onsets that occurred above the IMAGE ground array about 31% occurred within 6 degrees of latitude of the HD observed during the growth phase of the substorm and 37% occur within 5 degrees of latitude of the HD observed during the expansion phase. We also find that 23% of the onsets occur in association with vortices of ionospheric currents observed during the growth phase. The remaining 31% of the onsets do not appear to be associated with a HD or a vortex during the growth phase. Ten of the initial 75 onsets were identified as likely pseudo breakups and not fully developed substorms. Seven of the initial 75 onsets were probably poleward boundary intensifications (PBI). This study demonstrates that approximately 2/3 of the auroral substorm onsets do not occur within or near the HD identified in the growth phase. These results do not support the prediction made by Lyons et al. (2003) that substorm auroral onset should occur within the upward field aligned current in the Harang discontinuity.