Energy Transfer in the Solar Wind-Magnetosphere-Ionosphere System Under Radial Interplanetary Magnetic Field Conditions: Implication of Transpolar Arcs

Over the past several decades, many studies have expanded our understanding of solar wind-magnetosphere-ionosphere coupling processes, particularly for southward/northward interplanetary magnetic fields (IMFs) corresponding to the antiparallel/parallel orientations of the IMF with respect to the Earth's magnetic field at the dayside magnetopause. In this study, we turn our attention to radially directed IMF conditions and examine the possible mechanism for solar wind energy input into the magnetosphere/ionosphere when the IMF lines split and drape along the magnetopause. The auroral images acquired by the DMSP/SSUSI and TIMED/GUVI instruments on 29 October 2005 reveal that a transpolar arc (TPA), the phenomenon associated with the solar wind-magnetosphere-ionosphere coupling via magnetic reconnection with the northward IMF, can simultaneously appear in the northern and southern polar caps under a prolonged interval of earthward directed IMF conditions. Our observational result implies that the formation of TPAs is not limited to northward IMF condition and is attributable to tail reconnection, rather than lobe reconnection. Therefore, this study provides evidence that magnetic reconnection in the plasma sheet and/or plasma sheet boundary layer is the possible mechanism for solar wind energy input into the magnetosphere/ionosphere under radial IMF conditions.