Cluster Observes the High-Altitude Cusp Region

Abstract

This paper gives an overview of Cluster observations in the high-altitude cusp region of the magnetosphere. The low and mid-altitude cusps have been extensively studied previously with a number of low-altitude satellites, but only little is known about the distant part of the magnetospheric cusps. During the springtime , the trajectory of the Cluster fleet is well placed for dayside, high-altitude magnetosphere investigations due to its highly eccentric polar orbit. Wide coverage of the region has resulted and, depending on the magnetic dipole tilt and the solar wind conditions, the spacecraft are susceptible to encounter: the plasma mantle, the high-altitude cusp, the dayside magnetosphere (i.e. dayside plasma sheet) and the distant exterior cusp diamagnetic cavity. The spacecraft either exit into the magnetosheath through the dayside magnetopause or through the exterior cusp-magnetosheath interface. This paper is based on Cluster observations made during three high-altitude passes. These were chosen because they occurred during different solar wind conditions and different interspacecraft separations. In addition, the dynamic nature of the cusp allowed all the aforementioned regions to be sampled with different order, duration and characteristics. The analysis deals with observations of: (1) both spatial and temporal structures at high-altitudes in the cusp and plasma mantle, (2) signatures of possible steady reconnection, flux transfer events (FTE) and plasma transfer events (PTE), (3) intermittent cold (<100 eV) plasma acceleration associated with both plasma penetration and boundary motions, (4) energetic ions (5-40 keV) in the exterior cusp diamagnetic cavity and (5) the global structure of the exterior cusp and its direct interface with the magnetosheath. The analysis is primarily focused on ion and magnetic field measurements. By use of these recent multi-spacecraft Cluster

Cluster Observations of the CUSP: Magnetic Structure and Dynamics

“…These results (those for the southern cusps of 13 and 20 February, in particular) support an interpretation in which there often is an outer, indented (with respect to the nominal magnetopause), magnetosheath/cusp interface. For other events (as for 4 April 2001, and the event discussed by Lavraud et al (2003)), indentation of the outer interface does not appear to be present, or else these events correspond to central exits through the exterior cusp/magnetosheath interface. Within this boundary (Earthward side) a region exists, which may be loosely termed the exterior cusp in the sense that inner boundaries exist.…”

“…From these figures it is seen that the plasma ion distributions show a more extended structure than the magnetic field. It is also clear that the plasma moments show no evidence of the ''stagnant'' regions observed in the events discussed by Lavraud et al (2002Lavraud et al ( , 2003. In both cases, however, the southward and dawnward directed IMF (and magnetosheath field) is likely to result in significant and perhaps continual magnetic reconnection across the dayside magnetopause, and the result of such field line merging is likely to be that field lines are continually swept tailward across the cusp.…”

“…The spacecraft therefore appear to traverse the dayside region after about 06:25 UT, which is filled with trapped plasma sheet ions on closed field lines (not shown, see the companion paper by Lavraud et al, 2003). After 07 UT and until the magnetopause exit at 09:20 UT the ion density remains low with no significant bulk flow (during this period the GSM magnetic field turns from southward to northward).…”

“…In support of this interpretation (and as indicated above), the MVA results are suggestive of a RD, although the field component normal to the boundary is small (<2nT), and the Walen test across the boundary is satisfied to a high degree . Lavraud et al (2002Lavraud et al ( , 2003 show the character of the outer boundary to be suggestive of a RD, in the case of the event of 4 February 2001, and they discuss the significance of this (see also Onsager et al, 2001, for comparative events).…”

“…(This question has also been addressed, at least in part, by Vasyliunas (1995), Russell (2000) and Onsager et al (2001)). The question is, however, related to the definition of the outer region of the cusp as an ''exterior'' cusp, and the issue is discussed at more length in Lavraud et al (2003). It appears that the nature of the outer boundary is affected by the direction of the local magnetosheath field at least; Lavraud et al (2003) discuss the case of northward IMF and magnetosheath field in particular.…”