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 R_E 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 P_sw 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 P_sw 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 B_y ≫ 0. These data are used to infer a source region located at ∼7-13 R_E 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 P_sw or changes in the IMF polarity.