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Ann. Geophys., 22, 2497-2506, 2004
www.ann-geophys.net/22/2497/2004/
© European Geosciences Union 2004
The structure of high altitude O+ energization and outflow: a case study
H. Nilsson1, S. Joko1, R. Lundin1, H. Rème2, J.-A. Sauvaud2, I. Dandouras2, A. Balogh3, C. Carr3, L. M. Kistler4, B. Klecker5, C. W. Carlson6, M. B. Bavassano-Cattaneo7, and A. Korth8
1Swedish Institute of Space Physics, Kiruna, Sweden
2Centre d’Etude Spatiale des Rayonnements, Toulouse, France
3Imperial College of Science, Technology and Medicine, London, United Kingdom
4University of New Hampshire, Durham, USA
5Max-Planck-Institut für Extraterrestriche Physik, Garching, Germany
6Space Science Laboratory, University of California, Berkeley, USA
7Instituto di Fisica dello Spazio Interplanetario, Roma, Italy
8Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany
Abstract. Multi-spacecraft observations from the CIS ion spectrometers on
board the Cluster spacecraft have been used to study
the structure of
high-altitude oxygen ion energization and outflow. A
case study
taken from 12 April 2004 is discussed in more detail. In
this case the spacecraft crossed the polar cap, mantle and high-altitude cusp
region at altitudes between 4RE and 8RE
and 2 of the spacecraft provided data. The oxygen
ions were seen as a beam with narrow energy
distribution, and increasing field-aligned velocity and temperature
at higher altitude further in the upstream flow
direction.
The peak O+ energy was
typically just above the highest energy of observed protons. The observed
energies reached the upper limit of the CIS ion spectrometer, i.e. 38keV.
Moment data from the spacecraft have been
cross-correlated to determine cross-correlation coefficients, as
well as the phase delay between the spacecraft. Structures in ion density, temperature and
field-aligned flow appear to drift with the observed
field-perpendicular drift. This, together with a
velocity dispersion analysis, indicates that much
of the structure can be
explained by transverse heating well below the
spacecraft. However, temperature
isotropy and the particle flux as a function of
field-aligned velocity are inconsistent with a single altitude Maxwellian source.
Heating over extended altitude intervals, possibly all the way up to the observation point,
seem consistent with the observations.
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