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

Under conditions of sustained northward interplanetary magnetic field (IMF) , magnetospheric magnetic field lines on the dayside may undergo multiple r econnection, producing closed flux tubes filled with solar wind plasma. Ion ospheric Of beams with energies of similar to1 keV/e that are propagating p arallel to the magnetic field in the northern cusp region are potential evi dence for this multiple reconnection process. Using observations from the P olar spacecraft, we present evidence of parallel propagating O+ in the nort hern cusp. A statistical study of these events indicates that they are very rare in the Polar data. Therefore, under the assumption that parallel prop agating O+ beams in the northern cusp are evidence of multiple reconnection , the occurrence frequency of multiple reconnection on the dayside under no rthward IMF conditions appears to be small. Thus this process is insufficie nt to account for the nearly continuous population of solar wind plasma on closed field lines in the magnetosphere.