Temporal evolution of a staircase ion signature observed by Cluster in the mid‐altitude polar cusp

Abstract

We use the Cluster string of pearls configuration to investigate temporal variations of ion precipitation in the mid‐altitude polar cusp. On 7 Aug. 2004, Cluster 4 was moving poleward through the Northern cusp, followed by Cluster 1, Cluster 2, and finally Cluster 3. The Wind spacecraft detected a Southward turning of the Interplanetary Magnetic Field (IMF) at the beginning of the cusp crossings and IMF‐Bz stayed negative throughout. Cluster 4 observed a high energy step in the ion dispersion around 1 keV on the equatorward side of the cusp. C1, entering the cusp around 1 minute later, did not observe the high energy step anymore but a partial dispersion with a low energy cut‐off reaching 100 eV. About 9 min later, C3 entered the cusp and observed a full ion dispersion from a few keV down to around 50 eV. The open‐closed boundary, identified by electron precipitation, was initially moving equatorward at a rate of −0.43° ILAT/minute at the beginning of the event and then slowed down to −0.16° ILAT/minute, suggesting the erosion of the dayside magnetosphere under IMF Southward. This event is explained by the onset of dayside reconnection when the IMF turned southward; the step being the first signature of the reconnection that would then evolve as a full dispersion as reconnection goes on. We observed 1–3 keV ions near the open‐closed boundary on the three spacecraft crossings that suggests a continuous reconnection during about 9 minutes.