WARM PROTONS AT GEOSYNCHRONOUS ORBIT

Ions in the outer magnetosphere typically are observed to consist of a hot (T similar to 10 keV) component and a cool constituent with energ ies of a few eV. The hot component often is observed to be anisotropic with T-perpendicular to > T-parallel to, where the subscripts denote directions relative to the background magnetic field, whereas the cool ions exhibit a wide variety of anisotropies. The Los Alamos magnetosp heric plasma analyzer (MPA) instruments measure ion and electron veloc ity distributions from about 1 eV to about 40 keV at geosynchronous or bit. In the afternoon and evening sectors MPA observations show that c ool ions, which are assumed to be protons, are sometimes ''warm'' with temperatures of order 10 eV and T-perpendicular to > T-parallel to. T heory and simulations of the electromagnetic proton cyclotron instabil ity, which is driven by the hot proton anisotropy, have shown that the resulting enhanced field fluctuations heat initially cool protons. Mo reover, this process implies a scaling relation for the apparent tempe rature of the warm protons as a function of the relative densities of the two components. This manuscript describes an examination of MPA da ta which shows that this scaling relation provides an approximate uppe r bound for a selected set of warm proton temperatures observed at geo synchronous orbit. This result is consistent with the hypothesis that the proton cyclotron instability is an important energization source f or warm protons at geosynchronous orbit.