Substorm onset location and the Harang discontinuity

In this study we investigate the relation of auroral substorm onset to the Harang discontinuity. With a database of over 4300 onsets determined from auroral images we will statistically examine the magnetic latitude and MLT location of the onset with respect to the location of the Harang discontinuity as determined with data from the IMAGE magnetometer network. We define the location of the Harang discontinuity as the transition from eastward to westward equivalent ionospheric currents [Amm, 1997; 1998] at the latitude of the main auroral electrojet flow. This work addresses the question of how frequently substorm onset occurs within the ionospheric Harang discontinuity as proposed by Lyons et al. [2003]. Our initial results show that for more than 75 events that occurred within the IMAGE ground magnetometer array about 56% occurred within 6 degrees of latitude of the Harang discontinuity. Another 25% of the Frey et al. onsets occur within ±15 min of a Harang discontinuity. One of the 75 onsets appears to have no HD associated with the onset and the remaining (17%) are most likely misidentified substorms or the viewing angle of the substorm is problematic. We also examine an event that has simultaneous STARE radar data, equivalent ionospheric currents, and a substorm onset. This event shows that the substorm onset occurs between the Harang discontinuity identified in the radar data and the Harang discontinuity identified with the equivalent ionospheric currents.