Nightside Region-2 electric field evolution during substorm growth and expansion phases

Observations of the strong coupling of Harang reversal region dynamics to substorm evolutions suggest that nightside Region-2 physics is closely related with substorm dynamics. We use SuperDARN HF radar and DMSP observations to evaluate the Region-2 electric field evolution for nine substorm events. Auroral images from the IMAGE spacecraft, when available, are superposed with these ionospheric convection patterns to better illustrate the relation between the Region-2 electric fields near the Harang reversal region and the substorm dynamics. Based on these nine events, we found that the Harang reversal forms and becomes well defined during the growth phase of all the nine substorms. Azimuthal flows equatorward of the Harang reversal increase and reach at least ~500 m/s. These observational evidences indicate that shielding, subauroral ion drifts (SAIDS), and the Harang reversal evolve together as part of the growth phase development of the Region-2 system and they become well developed during the substorm growth phase. In addition, we find that substorm onset seems to occur slightly poleward of the center of the Harang flow shear and in the weak equatorward flows near the western edge of the dawn convection cell.