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A statistical study is presented of the unstable proton populations, whichcontain the free energy required to drive small-scale poloidal mode ULFwaves in the magnetosphere, observed at invariant latitudes of 60° to80°. The data are all in the form of Ion Distribution Functions (IDFs)amassed over ~6 years using the CAMMICE (MICS) instrument on thePolar spacecraft, and cover proton energies of 1 keV to 328 keV. The free energycontained in the unstable, positive gradient regions of the IDFs isavailable to drive resonant wave growth. The results show that positivegradient regions in IDFs on magnetic field lines corresponding to the lowerinvariant latitudes in the range under study occur predominantly in theafternoon sector at proton energies of 5 keV to 20 keV. In the morning anddawn sectors positive gradient regions are seen with a typical proton energyrange of 5 keV to 45 keV. While the proton energy peaks in the afternoonsector 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), employedto quantify the free energy in each IDF, found that as invariant latitudeincreased the free energy contained in the positive gradient regions fell.Positive gradient regions in the afternoon sector decrease in number withinvariant latitude at a faster rate than those in the morning sector. Themajority of positive gradient regions had free energy values of >1010 J with many at the lowest invariant latitudes having free energiesof in excess of 1011 J. Positive gradient regions at proton energies of>100 keV are rarely observed, and have free energies of typically <10<sup<9 J, which is too small to produce high m ULF waves of significantamplitude

A Statistical investigation of the invariant latitude dependence of unstable magnetospheric ion populations in relation to high m ULF wave generation