High-latitude ionosphere convection and Birkeland current response for the 15 May 2005 magnetic storm recovery phase

Abstract

The high‐latitude response of sunward E × B flow and Birkeland field‐aligned currents (FAC) is analyzed for the 15 May 2005 magnetic cloud that generated a great magnetic storm (SYM‐H = −305 nT at 0820 UT). The interplanetary magnetic field (IMF) clock angle, θ = arctan(By/Bz), gradually rotated from 65° to −80° during the 10‐h long northward IMF period and the recovery of this storm. DMSP observations confirm a dawnward migration of a Northern Hemisphere sunward E × B flow channel (FC) between a downward and upward FAC pair. This FAC system developed during southward IMF (θ = 109°) at the poleward edge of the duskside auroral oval as part of a four‐sheet FAC system 23 min before the IMF became northward. TIMED/GUVI observations show that the dawnward migration of the upward FAC coincides with a drifting transpolar auroral arc (TPA). IMAGE/WIC did not observe a TPA in the southern (winter) hemisphere. DMSP and Iridium observations are in good agreement with MHD simulation predictions of a northward IMF reorientation of high‐latitude FACs. The northern FC migration was likely due to summer hemisphere conductances, a strong average IMF Bx = −35 nT and the sunward dipole tilt angle that favor a northern high‐latitude reconnection mechanism for a well‐organized sunward FC and FAC system migration. The storm recovery rate appeared to be related with the region 2 FAC. A fast 11.4 nT/h rate was observed for a weak or nonexistent region 2 system during the high‐latitude FAC redistribution. The SYM‐H recovery slowed significantly to 0.9 nT/h following the 1800 UT region 2 system recovery.