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 spring-time, 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 Surveys in Geophysics (2005) 26: 135-175 Ó Springer 2005observations we illustrate the current topics under debate pertaining to the solar wind-magnetosphere interaction, for which this region is known to be of major importance.

CLUSTER encounters with the high altitude cusp: boundary structure and magnetic field depletions

“…Due to the nature of the orbit, the inbound (southern) and outbound (northern) crossings sample different regions of the high altitude cusp (e.g. Lavraud et al, 2004;Dunlop et al, 2004). We also note that, due to limited facilities for returning data from the spacecraft, there are fewer inbound than outbound crossings in 2001 and 2002, a situation remedied from early 2003.…”

Connection between bow shock and cusp energetic ions

“…It was argued that a local acceleration process involving ULF waves in the cusp [Chen et al, 1998] and ring current/ plasma sheet injection [Delcourt and Sauvaud, 1999;Blake, 1999] could be a source for the cusp energetic ions (CEPs). The presence of singly-ionized energetic oxygen may indicate a local acceleration source or leakage of magnetospheric ions [Lavraud et al, 2005]. On the other hand, observations also show that cusp energetic ion spectra agree well with those of typical bow shock diffuse ions [Chang et al, 1998], and that density and temperature distributions of CEPs are similar to those upstream values [Trattner et al, 2001].…”

Space Weather: Physics, Effects and Predictability

“…Usually, the plasma flow during northward IMF is such that the plasma mantle may not form (Sckopke et al 1976). Observations (Lavraud et al 2005) support to this hypothesis. The presence or absence of the plasma mantle region strongly depends on the nature of the IMF and the subsequent large scale plasma convection in the cusp regions.…”

“…High latitude reconnection can also open up a path for charged particles to enter or escape from the magnetosphere (Zong et al 2002). Lavraud et al (2005) have presented an overview of Cluster observations in the high latitude cusp region, showing that it is filled with magnetosheath plasma of solar wind origin. Both precipitating and mirroring components were observed.…”