Plasmaspheric Density Structures and Dynamics: Properties Observed by the CLUSTER and IMAGE Missions

Abstract

Plasmaspheric density structures have been studied since the discovery of the plasmasphere in the late 1950s. But the advent of the CLUSTER and IMAGE missions in 2000 has added substantially to our knowledge of density structures, thanks to the new 56 F. Darrouzet et al.capabilities of those missions: global imaging with IMAGE and four-point in situ measurements with CLUSTER. The study of plasma sources and losses has given new results on refilling rates and erosion processes. Two-dimensional density images of the plasmasphere have been obtained. The spatial gradient of plasmaspheric density has been computed. The ratios between H + , He + and O + have been deduced from different ion measurements. Plasmaspheric plumes have been studied in detail with new tools, which provide information on their morphology, dynamics and occurrence. Density structures at smaller scales have been revealed with those missions, structures that could not be clearly distinguished before the global images from IMAGE and the four-point measurements by CLUSTER became available. New terms have been given to these structures, like "shoulders", "channels", "fingers" and "crenulations". This paper reviews the most relevant new results about the plasmaspheric plasma obtained since the start of the CLUSTER and IMAGE missions.

Detection of a plasmaspheric wind in the Earth's magnetosphere by the Cluster spacecraft

“…Volland, 1973;McIlwain, 1986;Pierrard et al, 2008). This convection electric field configuration has been often used to explain the erosion of the plasmasphere and the formation of plumes, with the most salient feature the formation of a large-scale plume in the afternoon sector Darrouzet et al, 2009). The here-observed outward plasma flow, in the outer plasmasphere, constitutes apparently another mode for plasmaspheric material release into the magnetosphere, which operates continuously and in all local-time sectors.…”

“…Considering the following, we can calculate the resulting plasma loss rate from the plasmasphere to the outer magnetosphere: (a) a total plasma density of the order of 100 cm −3 , in the outer plasmasphere and at a geocentric distance of typically 4 R E , as deduced from the measurements of the WHIS-PER sounder experiment onboard Cluster Darrouzet et al, 2009); (b) a continuous radial outflow that at these distances is essentially for the plasma between 30 • north and 30 • south geomagnetic latitude, i.e. over 2π sr (cf.…”

“…The in situ observations of the outer plasmasphere obtained by the Cluster constellation provide some novel views of this region (Dandouras et al, 2005;Darrouzet et al, 2009).…”

The relationship between plasmapause, solar wind and geomagnetic activity between 2007 and 2011

“…Each spacecraft carries 11 instruments. The WHISPER data allow determining the electron density inside and outside the plasmasphere (Décréau et al, 2001;Darrouzet et al, 2009b;Lointier et al, 2013). The frequency spectra obtained during the passive and active (sounding) operation modes of the instrument carry out a direct or indirect determination of the electron plasma frequency F P .…”

Lower hybrid resonances stimulated by the four CLUSTER relaxation sounders deep inside the plasmasphere: observations and inferred plasma characteristics

“…Perigee is placed not far from the magnetic equator, and it happens that the orbit grazes the plasmasphere boundary layer along a significant distance (∼1 R E ) below and above the equatorial plane, within a narrow local time sector . More generally, the WHISPER frequency range, although excluding density values above 80 cm −3 , describes adequately the outer plasmasphere region where plasma plumes and other irregularities are encountered (Darrouzet et al, , 2009b. A systematic study of the 2002data (El-Lemdani Mazouz et al, 2009, gives a precise view of the circumstances in which WHISPER upper density threshold is exceeded.…”