Ring Current observations by Cluster and Double Star

C. Vallat, I. Dandouras, N. Ganushkina, P. Escoubet, M. Dunlop,
P.C. Brandt, H. Reme, E. Lucek, S. McKenna-Lawlor


The highly eccentric polar orbit of the four Cluster spacecraft (perigee at 4 RE)
permits them to sample the inner magnetosphere region, allowing the study of the ring
current, the radiation belts and the outer plasmasphere.

The CIS experiment onboard Cluster samples ions with energy from ÷0 eV up to ÷40 keV
with a 4s temporal resolution and mass per charge discrimination. This allows the
detection of ions constituting the plasmasphere, the plasmasheet and the ring
current. A detailed analysis of the proton structures formed on the CIS spectrograms
at perigee, in association with particle trajectory simulations reveal the
contribution of each ion source to the ring current and the implication of large
scale electric fields in the formation of such structures.

Moreover, understanding the current circulation and closure inside the magnetosphere
as a function of geomagnetic activity is still an open question. The four
simultaneous points of measurements of the magnetic field (Cluster/FGM experiment)
allow for the first time to estimate the instantaneous ring current density. Analysis
of these data reveals the complex morphology and the large latitudinal extend of the
ring current, as well as the presence of field aligned currents at higher latitudes.
The IMAGE spacecraft can also help to place the Cluster in-situ data into a global
context: by applying an inversion technique to IMAGE/HENA neutral atom images, the
equatorial ion distribution function can be retrieved and Cluster/CIS measurements
can thus be positioned with respect to the ring current bulk. As a consequence, a 3D
mapping of the ion distribution in the inner magnetosphere is obtained. More
recently, the Chinese spacecraft TC 1 (part of the Double Star mission) has been
placed on an equatorial elliptical orbit (13.4 RE apogee). Cluster and Double Star
spacecraft orbits have been designed such that the spacecraft are almost in the same
meridian, allowing conjugate studies of the EarthÕs Magnetosphere dynamics.
Coordinated ion measurements are thus possible, and the latitudinal (Cluster) and
radial (TC1) profiles of the plasma can be studied.

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European Geosciences Union General Assembly, Vienna, Austria, 2-7 April 2006