Articles | Volume 22, issue 12
22 Dec 2004
 | 22 Dec 2004

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

L. J. Baddeley, T. K. Yeoman, D. M. Wright, K. J. Trattner, and B. J. Kellet

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)