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Ann. Geophys., 24, 3027-3040, 2006
www.ann-geophys.net/24/3027/2006/
© European Geosciences Union 2006
A Statistical investigation of the invariant latitude dependence of unstable magnetospheric ion populations in relation to high m ULF wave generation
M. E. Wilson1, T. K. Yeoman1, L. J. Baddeley1, and B. J. Kellet2
1Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
2Rutherford Appleton Laboratory, Didcot, UK
Abstract. A statistical study is presented of the unstable proton populations, which
contain the free energy required to drive small-scale poloidal mode ULF
waves in the magnetosphere, observed at invariant latitudes of 60° to
80°. The data are all in the form of Ion Distribution Functions (IDFs)
amassed over ~6 years using the CAMMICE (MICS) instrument on the
Polar spacecraft, and cover proton energies of 1 keV to 328 keV. The free energy
contained in the unstable, positive gradient regions of the IDFs is
available to drive resonant wave growth. The results show that positive
gradient regions in IDFs on magnetic field lines corresponding to the lower
invariant latitudes in the range under study occur predominantly in the
afternoon sector at proton energies of 5 keV to 20 keV. In the morning and
dawn sectors positive gradient regions are seen with a typical proton energy
range of 5 keV to 45 keV. While the proton energy peaks in the afternoon
sector at around ~7 keV the morning sector has two peaks occurring at
~10 keV and ~20–30 keV. The technique of Baddeley et al. (2004), employed
to quantify the free energy in each IDF, found that as invariant latitude
increased the free energy contained in the positive gradient regions fell.
Positive gradient regions in the afternoon sector decrease in number with
invariant latitude at a faster rate than those in the morning sector. The
majority of positive gradient regions had free energy values of >1010 J with many at the lowest invariant latitudes having free energies
of in excess of 1011 J. Positive gradient regions at proton energies of
>100 keV are rarely observed, and have free energies of typically <10 J, which is too small to produce high m ULF waves of significant
amplitude.
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