A statistical study of unstable particle populations in the global ringcurrent and their relation to the generation of high m ULF waves

Abstract. The first statistical study of the unstable proton populations which contain "free energy" required to drive small-scale poloidal mode ULF waves in the magnetosphere between L-shell locations of 6 and 9 is presented. The data examined are all in the form of Ion Distribution Functions (IDFs) covering a particle energy range of 0.025keV to 328keV, amassed over 2.5 years from the TIMAS and CAMMICE (MICS) instruments on-board the Polar spacecraft. Any free energy which is available to drive a resonant wave mode manifests itself as a positive gradient region in the IDF. A new analysis technique applied to the data, allows for the first time, the amount of free energy contained in each IDF to be quantified. The results show that IDFs are a common occurrence in the magnetosphere at these L-shells, although they are most common in the dawn/pre-noon sector. Lower energy (10–45keV) protons are the most commonly observed unstable populations and also contain the largest amounts of free energy (>10¹⁰J). Positive gradient regions at higher energies (>100keV) are rarely observed and also contain greatly reduced free energies (<10⁹J).

Key words. Magnetospheric physics (Energetic particles, trapped; MHD waves and instabilities) – Space plasma physics (wave-particle interactions)