The Harang discontinuity in auroral substorms
We have investigated the electrodynamics of the pre-midnight convection reversal during substorm expansion phase through early recovery phase. This is done with measurements of the magnetic field, electric field, ion drift and electron precipitation made by the Dynamics Explorer 2 (DE-2). The measured electron precipitation has been used to calculate the height integrated Pedersen and Hall conductivity (conductance) which combined with the electric field measurements enables us to calculate the ionospheric current intensities. The field aligned current (FAC) density is then calculated from the divergence of the horizontal meridional current and from the measured magnetic field perturbations. The meridional electric field was used to identify the position of the Harang discontinuity or Harang region (HR) as we prefer to address it. We find that in general 3 different types of HRs exist: (1) A well defined point separating regions of poleward and equatorward meridional electric fields. This type is typically seen far west of the surge and is located near the high latitude boundary of the enhanced conductance region. Consequently, the HR is located at the poleward boundary of the eastward electrojet while the westward electrojet is very narrow or nonexistent. In all cases, we find that the zonal component is vanishing at the HR. (2) A region of depleted meridional electric fields with a typical width of -100 km, typically located just west of the surge. The electric field and the conductance exhibit a clearly inverse relationship. (3) A wide region (~500 km) located in the surge and bulge. The three dimensional current system is filamentary within the region separating the low latitude eastward electrojet and the intense high latitude westward electrojet.
