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

Abstract

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 (7-9R_E) 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-B_z > 0 driven patterns, including one or two lobe reconnection-cells in the dayside polar cap, according to the amplitude of the B_y 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 8-12R_E 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.