Spatiotemporal structure of the reconnecting magnetosphere under B y-dominated interplanetary magnetic cloud conditions

Abstract

In this study we discuss the spatiotemporal structure of the reconnecting magnetosphere under a steady external field which was pointing southward (B z = -5 nT), but with a strong eastward component (By = 20 nT; clock angle = 100°). The relevant conditions refer to a three hour long interval midway through the passage at Earth of an interplanetary magnetic cloud on 17 April 1999. The quasi-steady parameters provided by the magnetic cloud is what allow us to probe the detailed magnetospheric/ionospheric structure under these conditions. We document the presence of a specific configuration of cusp and oval-aligned polar arcs with associated merging and lobe convection cells in both hemispheres. Strong signatures of pulsed ionospheric flows (PIFs) are present in high-latitude ground magnetometer records from Svalbard and Greenland. Particle data obtained from Polar/HYDRA during an overflight of the southern hemisphere cusp at 0800-0900 MLT show the presence of polar arcs and an inverse energy versus latitude dispersion of precipitating ions on the poleward side of the merging cell and the plasma regimes of mantle, cusp, and BPS. Furthermore, observations by Polar, DMSP, and SuperDARN document the precipitation, field-aligned current and ion flow pattern in the south. The presence of "double cusp" in the north and polar arcs in both hemispheres (mirror images about noon) during the actual IMF Bz negative (By-dominated) conditions and the implications on solar wind-magnetosphere coupling are discussed. The magnetic cloud conditions at ACE included a very low proton plasma β (∼0.01) and a low Alfven Mach number (∼2) near the center of the cloud. We speculate that these conditions may have given rise to a plasma depletion at the high-latitude magnetopause and favored the excitation of lobe reconnection under the prevailing Bz negative conditions. The reported observations are placed in the context of recent studies of the spatiotemporal structure of the dayside aurora. Copyright 2006 by the American Geophysical Union.