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

Abstract

[1] 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.

Formation of the Harang reversal and its dependence on plasma sheet conditions: Rice convection model simulations

“…[4] The Harang reversal is located near the inner edge of plasma sheet and is closely related to the region 2 FAC [Zou et al, 2009]. Moreover, recent larger-scale analyses, also of radar data [Bristow and Jensen, 2007;Zou et al, 2009], have firmly shown a relationship between the evolution of the Harang reversal and the substorm onset.…”

“…Moreover, recent larger-scale analyses, also of radar data [Bristow and Jensen, 2007;Zou et al, 2009], have firmly shown a relationship between the evolution of the Harang reversal and the substorm onset. This last feature reveals the importance of the Harang reversal, since the substorm onset mechanism is still under debate and has been theoretically predicted to be related with the reversal [Lyons et al, 2003].…”

M–I coupling across the auroral oval at dusk and midnight: repetitive substorm activity driven by interplanetary coronal mass ejections (CMEs)

“…This is the sector of the auroral oval where most vigorous M-I coupling processes involving R1 and R2 FACs are expected to take place during substorm activity (Koskinen and Pulkkinen, 1995;Lyons et al, 2003;Weygand et al, 2008;Zou et al, 2009;Nishimura et al, 2010). Thus, the Harang region is a key locus of magnetospheric convection currents where ionospheric closures of both Bostrøm type I and II currents appear along the same magnetic local time (MLT) meridian.…”

“…The Harang region, normally located in the premidnight sector, is considered to be a most interesting region for the investigation of ionospheric signatures of substorm dynamics (Lyons et al, 2003;Zou et al, 2009). This is the sector of the auroral oval where most vigorous M-I coupling processes involving R1 and R2 FACs are expected to take place during substorm activity (Koskinen and Pulkkinen, 1995;Lyons et al, 2003;Weygand et al, 2008;Zou et al, 2009;Nishimura et al, 2010).…”

On the divergence of the auroral electrojets

“…A closer investigation of the arcs inside and nearby to the HR may help to a better understanding of both arc electrodynamics and substorm physics. The auroral activity in the HR [e.g., Nielsen and Greenwald, 1979;Zou et al, 2009] is thought to be closely related to the substorm onset, but the details of this relationship are not fully understood [Weygand et al, 2008]. While meso-scale investigations of HR electrodynamics are possible based on ground data [e.g., Amm et al, 2000], high-resolution satellite data are required for the smaller scale arc studies.…”