The role of centrifugal acceleration near the magnetopause boundary

H. Nilsson (1), M. Waara (1), O. Marghitu (2,6), M. Yamauchi (1),  R. Lundin (1), H. Rème (3),   
J.-A. Sauvaud (3),  I. Dandouras (3), E. Lucek (4), L. M. Kistler (5), B. Klecker (6), C.  
W. Carlsson (7), M. B. Bavassano-Cattaneo (8) and A. Korth (9)

(1) Swedish Institute of Space Physics, Kiruna, Sweden, (2) Institute for Space Sciences, Bucharest, Romania, (3) Centre d'Etude Spatiale des Rayonnements, Toulouse, France, (4) Imperial College of Science, Technology and Medicine, London, United Kingdom, (5) University of New Hampshire, Durham, USA, (6) Max-Planck-Institute für Extraterrestriche Physik, Garching, Germany, (7) Space Science Laboratory, University of California, Berkeley, USA, (8) Istituto di Fisica dello Spazio Interplanetario, Roma, Italy, (9) Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany  

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 subtract 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. The statistical result is compared to the immediate variation of observed parallel velocities of H+ and O+ ions, and the accumulated acceleration during each orbit is compared with the observed parallel velocities. 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, but cannot explain all of the observed parallel velocities observed at high altitude above the polar cap, consistent with previous reports based on the same data set. 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. It is also suggested that the centrifugal acceleration and associated inertial drift can play a role in forming overlapping cusp injections. The strongest centrifugal acceleration is observed clsoe to the magnetopause, and we bring up to discussion the importance of this acceleration mechanism in relation to boundary layer phenomena.