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Ann. Geophys., 19, 1523-1532, 2001
www.ann-geophys.net/19/1523/2001/
© European Geosciences Union 2001
Intermittent thermal plasma acceleration linked to sporadic motions of the magnetopause, first Cluster results
J.-A. Sauvaud1, R. Lundin1,2, H. Rème1, J. P. McFadden3, C. Carlson3, G. K. Parks3, E. Möbius4, L. M. Kistler4, B. Klecker5, E. Amata6, A. M. DiLellis6, V. Formisano6, J. M. Bosqued1, I. Dandouras1, P. Décréau7, M. Dunlop8, L. Eliasson2, A. Korth9, B. Lavraud1, and M. McCarthy10
1CESR, Toulouse, France
2Institut for Rymdfysik, Kiruna, Sweden
3Space Sciences Laboratory, University of California, Berkeley, USA
4EOS, University of New Hampshire, Durham, NH, USA
5MPI für extraterrestrische Physik, Garching, Germany
6IFSI, Rome, Italy
7LPCE, Orléans, France
8Imperial College, London, UK
9MPI für Aeronomie, Katlenburg-Lindau, Germany
10Space Program, University of Washington, USA
Abstract. This paper presents the
first observations with Cluster of a very dense population of thermal
ionospheric ions (H+, He+, O+) locally
"accelerated" perpendicularly to the local magnetic field in a region
adjacent to the magnetopause and on its magnetospheric side. The observation
periods follow a long period of very weak magnetic activity. Recurrent motions
of the magnetopause are, in the presented cases, unexpectedly associated with
the appearance inside closed field lines of recurrent energy structures of
ionospheric ions with energies in the 5 eV to ~1000 eV range. The
heaviest ions were detected with the highest energies. Here, the ion behaviour
is interpreted as resulting from local electric field enhancements/decreases
which adiabatically enhance/lower the bulk energy of a local dense thermal ion
population. This drift effect, which is directly linked to magnetopause motions
caused by pressure changes, allows for the thermal ions to overcome the
satellite potential and be detected by the suprathermal CIS Cluster experiment.
When fast flowing, i.e. when detectable, the density (~ 1 cm-3) of
these ions from a terrestrial origin is (in the cases presented here) largely
higher than the local density of ions from magnetospheric/plasma sheet origin
which poses again the question of the relative importance of solar and
ionospheric sources for the magnetospheric plasma even during very quiet
magnetic conditions.
Key words. Ionosphere (planetary
ionosphere; plasma convection) Magnetospheric physics (magnetopause, cusp and
boundary layers)
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