O+ observations in the cusp: Implications for dayside magnetic field topology

Abstract

Under conditions of sustained northward interplanetary magnetic field (IMF), magnetospheric magnetic field lines on the dayside may undergo multiple reconnection, producing closed flux tubes filled with solar wind plasma. Ionospheric O+ beams with energies of ∼1 keV/e that are propagating parallel to the magnetic field in the northern cusp region are potential evidence for this multiple reconnection process. Using observations from the Polar spacecraft, we present evidence of parallel propagating O+ in the northern cusp. A statistical study of these events indicates that they are very rare in the Polar data. Therefore, under the assumption that parallel propagating O+ beams in the northern cusp are evidence of multiple reconnection, the occurrence frequency of multiple reconnection on the dayside under northward IMF conditions appears to be small. Thus this process is insufficient to account for the nearly continuous population of solar wind plasma on closed field lines in the magnetosphere.