Capture of magnetosheath plasma by the magnetosphere during northward IMF

P. E. Sandholt, C. J. Farrugia, S. W. H. Cowley, W. F. Denig, M. Lester, J. Moen, and B. Lybekk

Department of Physics, University of Oslo, Oslo, Norway


We interpret combined auroral and simultaneous particle data from an overflight of the ground station at Svalbard by the F13 DMSP spacecraft in terms of pulsed "capture" of northward-directed magnetosheath flux tubes by the magnetosphere, due to sequential lobe reconnection in both the southern and northern hemispheres. The event refers to a ~40-min long interval characterized by strongly northward interplanetary magnetic field (IMF) (Bz = 7 nT; clock angle ~10° - 15° ). The meridian scanning photometer at the ground station records a long stepwise poleward retraction of the band of auroral emission in the cusp region. Each step is marked by an initial, brief poleward leap, followed by a ~5 min period of relatively steady auroral latitude. In the one event where simultaneous particle data are available (at 1100 MLT), the spacecraft traverses a region of auroral precipitation where electrons and ions of magnetosheath origin are present, together with equatorward convection. With a large sunward IMF tilt (Bx = 6-7 nT) in the winter hemisphere, we suppose that the process starts with reconnection poleward of the southern cusp followed by overdraped lobe flux which reconnects with magnetospheric field lines in the northern hemisphere.

Geophys. Res. Lett., Vol. 26, No. 18, 2833-2836, 1999