First Cluster results of the magnetic field structure of the mid- and high-altitude cusps
Abstract. Magnetic field measurements from the four Cluster spacecraft from the mid- and high-altitude cusp are presented. Cluster underwent two encounters with the mid-altitude cusp during its commissioning phase (24 August 2000). Evidence for field-aligned currents (FACs) was seen in the data from all three operating spacecraft from northern and southern cusps. The extent of the FACs was of the order of 1 RE in the X-direction, and at least 300 km in the Y-direction. However, fine-scale field structures with scales of the order of the spacecraft separation (300 km) were observed within the FACs. In the northern crossing, two of the spacecraft appeared to lie along the same magnetic field line, and observed very well matched signals. However, the third spacecraft showed evidence for structuring transverse to the field on scales of a few hundred km. A crossing of the high-altitude cusp from 13 February 2001 is presented. It is revealed to be a highly dynamic structure with the boundaries moving with velocities ranging from a few km/s to tens of km/s, and having structure on timescales ranging from less than one minute up to several minutes. The cusp proper is associated with the presence of a very disordered magnetic field, which is entirely different from the magnetosheath turbulence.
Key words. Magnetospheric physics (current systems; magnetopause, cusp, and boundary layers) – Space plasma physics (discontinuities)