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Ann. Geophys., 23, 1849-1865, 2005
www.ann-geophys.net/23/1849/2005/
© European Geosciences Union 2005
First current density measurements in the ring current region using simultaneous multi-spacecraft CLUSTER-FGM data
C. Vallat1, I. Dandouras1, M. Dunlop2, A. Balogh3, E. Lucek3, G. K. Parks4, M. Wilber4, E. C. Roelof5, G. Chanteur6, and H. Rème1
1Centre d’Etude Spatiale des Rayonnements, Toulouse, France
2Rutherford Appleton Laboratory, Didcot, UK
3Blackett Laboratory, Imperial College, London, UK
4Space Sciences Laboratory, University of California, Berkeley, CA, USA
5Applied Physics Laboratory, Johns Hopkins University, Baltimore, Maryland, USA
6Centre d’Etude des Environnements Terrestre et Planétaires, Vélizy, France
Abstract. The inner magnetosphere's current mapping is one of the key elements for
current loop closure inside the entire magnetosphere. A method for directly
computing the current is the multi-spacecraft curlometer technique, which is
based on the application of Maxwell-Ampère's law. This requires the use of
four-point magnetic field high resolution measurements. The FGM experiment on
board the four Cluster spacecraft allows, for the first time, an instantaneous
calculation of the magnetic field gradients and thus a measurement of the
local current density. This technique requires, however, a careful study
concerning all the factors that can affect the accuracy of the J estimate,
such as the tetrahedral geometry of the four spacecraft, or the size and
orientation of the current structure sampled. The first part of this paper is
thus providing a detailed analysis of the method accuracy, and points out the
limitations of this technique in the region of interest. The second part is
an analysis of the ring current region, which reveals, for the first time, the
large latitudinal extent of the ring current, for all magnetic activity
levels, as well as the latitudinal evolution of the perpendicular (and
parallel) components of the current along the diffuse auroral zone. Our
analysis also points out the sharp transition between two distinct plasma
regions, with the existence of high diamagnetic currents at the interface, as
well as the filamentation of the current inside the inner plasma sheet. A
statistical study over multiple perigee passes of Cluster (at about 4 RE
from the Earth) reveals the azimuthal extent of the partial ring
current. It also reveals that, at these distances and all along the evening
sector, there isn't necessarily a strong dependence of the local current
density value on the magnetic activity level. This is a direct consequence of
the ring current morphology evolution, as well as the relative positioning of
the spacecraft with respect to the bulk of the ring current. It also proves
the existence of a substantial ring current at these distances, all over the
evening and the post-midnight sector.
Keywords. Magnetospheric physics (Current systems; Energetic
particles, trapped; Magnetospheric configuration and
dynamics)
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