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

The high-latitude response of sunward E x 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, theta = arctan(B-y/B-z), gradually rotated from 65 degrees to -80 degrees 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 x B flow channel (FC) between a downward and upward FAC pair. This FAC system developed during southward IMF (theta = 109 degrees) 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 B-x = -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.