Cluster observations of the high-altitude cusp for northward interplanetary magnetic field: A case study

Physics – Plasma Physics

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Magnetospheric Physics: Magnetopause, Cusp, And Boundary Layers, Magnetospheric Physics: Plasma Convection, Space Plasma Physics: Magnetic Reconnection

Scientific paper

Since January 2001, the multisatellite and multiinstrument CLUSTER mission gives a unique opportunity to study the structure and dynamics of the high-altitude polar cusp. On 17 March 2001, CLUSTER sampled the northern high-altitude cusp (7-9RE) around noon for more than 1 hour during very quiet interplanetary and magnetospheric conditions (P < 1 nPa, AE ~ 0) and for an Interplanetary Magnetic Field (IMF) directed northward and duskward. In this paper we present detailed four-point plasma observations from the electron (Plasma Electron And Current Experiment) and ion (CLUSTER Ion Spectrometer) instruments. Ion observations throughout the cusp crossing reveal repetitive ``reverse'' ion energy dispersions associated with sporadic electron injections. Ion injections are observed with a variable time repetition rate, from ~5 min to as low as ~1 min. Although highly turbulent, the observed convection flows are in close agreement with IMF-Bz > 0 driven patterns, including one or two lobe reconnection-cells in the dayside polar cap, according to the amplitude of the By component. The overall convection pattern responds in ~3-5 min to abrupt changes in the IMF orientation. Successive electron and ion patches are interpreted as signatures of pulsed, enhanced reconnection in the high-latitude magnetopause, poleward of CLUSTER, at a distance estimated to be 8-12RE and, at times, less. A four-point boundary analysis demonstrates that reconnected flux tubes (Flux Transfer Events) convect with drift directions and velocities (6-15 km/s) in close agreement with the inferred convective patterns. Furthermore, CLUSTER demonstrates that boundary cusp motions, with a velocity up to ~20 km/s, are immediately and directly induced by abrupt changes in the IMF orientation.

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