Overlapping ion structures in the mid-altitude cusp under northward IMF: signature of dual lobe reconnection?
- 1Institut de Recherche en Astrophysique et Planétologie, UMR5277, CNRS/UPS, 9 avenue du Colonel Roche, BP 44346, 31028 Toulouse cedex 4, France
- 2European Space Agency, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
- 3University of California Los Angeles, Institute of Geophysics and Planetary Physics, Los Angeles, CA 90095-1567, USA
- 4Mullard Space Science Laboratory, Dept. of Space and Climate Physics, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
Abstract. On some rare occasions, data from the Cluster Ion Spectrometer (CIS) in the mid-altitude cusp reveal overlapping ion populations under northward interplanetary magnetic field (IMF). While the poleward part of the cusp exhibits the expected reverse dispersion due to lobe reconnection, its equatorward part shows a second ion population at higher-energy that coexists with the low energy tail of the dispersion. This second population is either dispersionless or slightly dispersed with energies increasing with increasing latitudes, indicative of lobe reconnection as well. Our analysis of a case that occurred 3 September 2002 when the IMF stayed northward for more than two hours suggests that the second population comes from the opposite hemisphere and is very likely on newly-closed field lines. We interpret this overlap of cusp populations as a clear mid-altitude signature of re-closed magnetic field lines by double lobe reconnection (reconnection in both hemispheres) under northward IMF. This interpretation is supported by modelling performed with the Cooling model and an MHD model.