On the coupling between the Harang reversal evolution and substorm dynamics: A synthesis of SuperDARN, DMSP, and IMAGE observations

The Harang reversal is a prominent feature frequently observed in the electric and magnetic field patterns in the high‐latitude auroral zone and plays an important role in substorm dynamics. A comprehensive set of instruments, including Super Dual Auroral Radar Network (SuperDARN), Defense Meteorological Satellite Program (DMSP), and Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE), is used to investigate the relationship between the Harang reversal and substorms. On the basis of nine events that have been analyzed, we find that the Harang reversal forms and becomes well defined during the growth phase. Azimuthal flows equatorward of the Harang reversal, a majority of which are in the subauroral region, enhance during the growth phase. The observations indicate that subauroral polarization streams (SAPS) and the Harang reversal evolve together as part of the growth phase development of the region 2 current system. Furthermore, the substorm auroral onset is seen to occur quite near the center of the Harang flow shear, which suggests that the substorm upward field‐aligned current develops there. After onset, the evolution of convection flows in the vicinity of the Harang region depends strongly on their location relative to that of the onset. SAPS flows equatorward of the Harang reversal suddenly increase at the substorm onset; flow shear east of the auroral onset relaxes after the onset; and poleward flows, part of a clockwise vortex, are observed west of the auroral onset after the onset. These observations demonstrate the strong coupling between the Harang reversal evolution and substorm dynamics and suggest that the nightside region 2 physics is closely related to substorm dynamics.