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Ann. Geophys., 26, 145-157, 2008
www.ann-geophys.net/26/145/2008/
© European Geosciences Union 2008
An assessment of the role of the centrifugal acceleration mechanism in high altitude polar cap oxygen ion outflow
H. Nilsson1, M. Waara1, O. Marghitu2,3, M. Yamauchi1, R. Lundin1, H. Rème4, J.-A. Sauvaud4, I. Dandouras4, E. Lucek5, L. M. Kistler6, B. Klecker2, C. W. Carlson7, M. B. Bavassano-Cattaneo8, and A. Korth9
1Swedish Institute of Space Physics, Kiruna, Sweden
2Max-Planck-Institut für Extraterrestriche Physik, Garching, Germany
3Institute for Space Sciences, Bucharest, Romania
4Centre d'Etude Spatiale des Rayonnements, Toulouse, France
5Imperial College of Science, Technology and Medicine, London, UK
6University of New Hampshire, Durham, USA
7Space Science Laboratory, University of California, Berkeley, USA
8Istituto di Fisica dello Spazio Interplanetario, Roma, Italy
9Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany
Abstract. The role of the centrifugal acceleration mechanism for
ion outflow at high altitude above the polar cap has been investigated.
Magnetometer data
from the four Cluster spacecraft has been used to obtain an estimate of magnetic field gradients.
This is combined with ion moment data of the convection drift and the field-aligned
particle velocity. Thus all spatial terms in the expression for the centrifugal acceleration
are directly obtained from observations. The temporal variation of the unit vector of the
magnetic field is estimated by predicting consecutive measurement-points through the use of
observed estimates of the magnetic field gradients, and subtracting this from the consecutively
observed value. The calculation has been performed
for observations of outflowing O+ beams in January to May for the years 2001–2003, and covers an altitude
range of about 5 to 12 RE.
The accumulated centrifugal acceleration during each orbit is compared with the
observed parallel velocities to get an estimate of the relative role of the centrifugal acceleration.
Finally the observed spatial terms (parallel and perpendicular)
of the centrifugal acceleration are compared with the results obtained when the magnetic
field data was taken from the Tsyganenko
T89 model instead. It is found that the centrifugal acceleration mechanism is significant, and may
explain a large fraction of the parallel velocities observed at high altitude above the polar cap.
The magnetic field model
results underestimate the centrifugal acceleration at the highest altitudes investigated
and show some systematic differences as compared to the observations in the lower altitude ranges investigated.
Our results indicate that for altitudes corresponding to magnetic field values of more than 50 nT a test particle model
with a steady state magnetic field model, a realistic convection
model and an initial velocity of about 20 k m s−1 at 5 RE should be able to reproduce
the main part of our observational results.
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