Effect of a northward turning of the interplanetary magnetic field on cusp precipitation as observed by Cluster

The 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.