Effect of a northward turning of the interplanetary magnetic field on cusp precipitation as observed by Cluster
Publication date: 02 May 2008
Authors: Escoubet, C.P. et al.
Journal: J. Geophys. Res.
Copyright: American Geophysical UnionThe immediate effect of the rotation of the interplanetary magnetic field (IMF) from southward to northward on cusp precipitation has been rarely observed by a polar orbiting satellite in the past. The four Cluster spacecraft observed such an event on 23 September 2004 as they were crossing the polar cusp within 2-16 min from each other. Between the first three and the last spacecraft crossing the cusp, the IMF rotated from southward to northward with a dominant By (GSM) component. For the first time we can examine the changes in the particle precipitation immediately after such IMF change. The first two spacecraft observed typical IMF-southward ion dispersion, while the last one observed both an IMF-southward-like dispersion in the boundary layer and an IMF-northward dispersion in the cusp. After the IMF turning, the cusp is shown to have grown in size in both the poleward and equatorward directions. A three-dimensional magnetohydrodynamic simulation is used to determine the locations of the sources of the ions and the topology of the magnetic field during the event.