Pitch-angle scattering rates in planetary magnetospheres

The mechanism of gyroresonant scattering of charged particles by small-amplitude, broadband electromagnetic waves is examined. By means of quasi-linear theory, a simple expression for the resonant pitch-angle diffusion coefficient D_{α α} is developed, which is valid for parallel-propagating, small-amplitude, (R-mode or L-mode) electromagnetic waves of general spectral density. We calculate the average diffusion coefficient with respect to the pitch-angle α, namely < D_αα >, which provides a practical estimate for particle scattering rates as a function of particle kinetic energy. The results for < D_αα >, corresponding to a Gaussian wave frequency spectrum, are used to estimate scattering rates for several types of wave-particle interactions in the terrestrial and Jovian magnetospheres.