Overlapping cusp ion structures under Northward IMF: signature of re-closed magnetic field lines?
Résumé
On some occasions, Cluster data in the mid-altitude cusp reveal overlapping ion populations under Northward IMF. While the poleward part of the cusp exhibits the expected reverse dispersion due to lobe reconnection, its equatorward part shows a second high-energy ion population that coexists with the low energy tail of the dispersion. This second populations is either dispersionless or slightly dispersed with energies increasing with increasing latitude, indicative of sunward convection. The analysis of data from Cluster fleet and, for one event, of data from the EISCAT Svalbard Radar in conjunction with Cluster, reveals that the second population comes very likely from the opposite hemisphere and is on closed field lines. We interpret this overlap of closed-LLBL and cusp populations in terms of magnetic field lines being opened in one hemisphere by lobe reconnection and re- closed in the other.