The standard text-book picture of the Earth’s magnetosphere is well known, and its dynamical response to periods of southward IMF (interplanetary magnetic field) has been well studied. However, half of the time the IMF is northward, and under these conditions uncharacteristic plasma has been observed in the Earth’s magnetotail lobes which is contrary to the standard text-book structure. Such signatures have occasionally been reported, and have been variously attributed to ‘filaments’ somehow protruding from the plasma sheet (though quite how was unclear) or direct solar wind entry. These signatures have also been referred to as the ‘ghost plasma sheet’ in reference to their similar characteristics to plasma at much lower latitudes. We present observations of such plasma made by Cluster on 15th September 2005. The presence of a double loss cone indicates that the plasma signatures are on closed field lines and hence incompatible with direct solar wind entry. Simultaneous observations reveal the presence of a transpolar arc (another northward IMF phenomenon – an auroral feature which extends into and often across the polar cap). We show that the plasma observations and the conjunction with the transpolar arc are predicted and explained by a mechanism based on magnetotail reconnection during periods of northward IMF.
Reference: Fear, R. C., S. E. Milan, R. Maggiolo, A. N. Fazakerley, I. Dandouras and S. B. Mende, Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere, Science, 346, 1506-1510, doi:10.1126/science.1257377 (2014)