Cluster observations of the magnetospheric low-latitude boundary layer and cusp during extreme solar wind and interplanetary magnetic field conditions: II. 7 November 2004 ICME and statistical survey

We present a study of the plasma properties inside and dynamics of the low-latitude boundary layer (LLBL)/cusp during the ICME event on 7 November 2004 based on data from the four Cluster spacecraft. The interplanetary magnetic field (IMF) is predominantly strongly northward, up to 50 nT, with some short-duration rotations. The observed LLBL/cusp is very thick (6∈-∈7° invariant latitude (ILAT)) and migrates equatorward with rates of 0.55° and 0.04° ILAT per minute during quick southward IMF rotations and stable northward IMF, respectively. The LLBL/cusp observed by Cluster 1 and Cluster 4 is in a fast transition between different states and is populated by different types of plasma injection, presumably coming from multiple reconnection sites. During a period of extremely northward IMF, signatures of pulsed dual reconnection inside the LLBL/cusp are observed by Cluster 3, suggesting that at least part of the LLBL/cusp is on closed field lines. However, analysis of the ion data implies that the boundary layer is formed in the dawn sector of the magnetosphere and does not slowly convect from the dayside as has been suggested previously. A statistical study of the location of the LLBL/cusp equatorward boundary during the ICME events on 28∈-∈29 October 2003 and 7∈-∈10 November 2004 is performed. During extreme conditions the LLBL/cusp position is offset by -7° ILAT from the location under normal conditions, which might be explained by the influence of the high solar wind dynamic pressure. The LLBL/cusp moves equatorward with increasing southward and northward IMF. However, the LLBL/cusp position under strong southward IMF is more poleward than expected from previous studies, which could indicate some saturation in the dayside reconnection process or enhancement of the nightside reconnection rate. The LLBL/cusp position under strong northward IMF is extremely low and does not agree with the location predicted in previous studies. For the events with solar wind dynamic pressure >10 nPa, the LLBL/cusp position does not depend on the solar wind dynamic pressure. This might indicate some saturation in the mechanism of how the LLBL/cusp location depends on the solar wind dynamic pressure.