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Ann. Geophys., 22, 2115-2133, 2004
www.ann-geophys.net/22/2115/2004/
© European Geosciences Union 2004
Ion shell distributions as free energy source for plasma waves on auroral field lines mapping to plasma sheet boundary layer
A. Olsson1, P. Janhunen2, and W. K. Peterson3
1Swedish Institute of Space Physics, Uppsala Division, Uppsala, Sweden
2Finnish Meteorological Institute, Geophysical Research, Helsinki, Finland
3LASP, University of Colorado, Boulder, Colorado, USA
Abstract. Ion shell distributions are hollow spherical shells in velocity space
that can be formed by many processes and occur in several regions of
geospace. They are interesting because they have free energy that can,
in principle, be transmitted to ions and electrons. Recently, a
technique has been developed to estimate the original free energy
available in shell distributions from in-situ data, where some of the
energy has already been lost (or consumed). We report a systematic
survey of three years of data from the Polar satellite. We present an
estimate of the free energy available from ion shell distributions on
auroral field lines sampled by the Polar satellite below 6 RE
geocentric radius. At these altitudes the type of ion shells that we
are especially interested in is most common on auroral field lines close
to the polar cap (i.e. field lines mapping to the plasma
sheet boundary layer, PSBL). Our
analysis shows that ion shell distributions that have lost some of
their free energy are commonly found not only in the PSBL, but also on
auroral field lines mapping to the boundary plasma sheet (BPS),
especially in the evening sector auroral
field lines. We suggest that the PSBL ion shell
distributions are formed during the so-called Velocity Dispersed Ion
Signatures (VDIS) events. Furthermore, we find that the partly consumed
shells often occur in association with enhanced wave activity and
middle-energy electron anisotropies. The maximum downward ion energy
flux associated with a shell distribution is often 10mWm-2 and
sometimes exceeds 40mWm-2 when mapped to the ionosphere and
thus may be enough to power many auroral processes. Earlier simulation
studies have shown that ion shell distributions can excite ion
Bernstein waves which, in turn, energise electrons in the parallel
direction. It is possible that ion shell distributions are the link
between the X-line and the auroral wave activity and electron
acceleration in the energy transfer chain for stable auroral arcs.
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