Multipoint observations of transient reconnection signatures in the cusp precipitation: A Cluster‐IMAGE detailed case study

Abstract

This paper uses 90 min of Cluster multipoint data at ∼5 RE altitude together with global dayside imaging data provided by the IMAGE‐SI‐12 instrument to analyze the northern cusp crossed on 14 July 2001, during a period of high solar wind pressure Psw and strongly duskward interplanetary magnetic field (IMF). Simultaneous observations reveal intense cusp activity in the postnoon sector, characterized by multiple, impulsive energy‐dispersed ion injections, with a recurrence time of ∼8–10 min or less. Most of these transient signatures correspond one to one with repeated Psw enhancements. A multipoint analysis reveals that field‐aligned current sheets associated with ion steps are moving predominantly westward with a velocity, up to ∼20 km/s, in agreement with a flux tube motion controlled by magnetic tension forces when IMF By ≫ 0. These data are used to infer a source region located at ∼7–13 RE from Cluster, that is, on the dusk flank of the compressed magnetosphere, around 17–18 magnetic local time. We interpret these very dynamic and transient features as probable signatures of pulsed magnetic reconnection that is operating in a localized region of the magnetopause centered in the preferential antiparallel merging site. Our results suggest that the reconnection rate is not spontaneously self‐varying but may be directly modulated by either upstream dynamic pressure Psw or changes in the IMF polarity.