Gyroresonant acceleration of electrons in the magnetosphere by superluminous electromagnetic waves

Superluminous auroral kilometric radiation originates in the auroral cavity of the Earth's magnetosphere as right-hand extraordinary (R-X) mode emissi ons, with additional contributions from the left-hand ordinary (L-O) and le ft-hand extraordinary (L-X) modes. The three modes can propagate into the o uter radiation belt and undergo gyroresonant interaction with trapped energ etic electrons over a broad extent of the outer magnetosphere. We develop a general theory of quasi-linear diffusion and construct resonant diffusion curves in velocity space for each superluminous wave mode. The potential fo r stochastic electron acceleration is controlled by the dispersive properti es of the waves and the ratio between the electron gyrofrequency and plasma frequency. It is found that each of the R-X, L-O, and L-X modes can produc e significant acceleration of electrons over individual regions of paramete r space. The L-O mode is found to have the potential for accelerating elect rons from similar to 10 keV to similar to MeV energies, over a broad. range of wave normal angles, in spatial regions extending from the auroral cavit y to the high-latitude (> 30 degrees) outer radiation belt. The R-X mode ap pears to be less effective for accelerating magnetospheric electrons, since acceleration to significant energies (similar to MeV) requires very small wave normal angles (< 10 degrees). The potential for significant electron a cceleration in the magnetosphere by L-X mode waves is restricted not least by the requirement of high minimum energies, e.g., 400 keV in the outer rad iation belt. To assess whether the superluminous wave modes contribute sign ificantly to the stochastic acceleration of relativistic electrons during g eomagnetic storms, the present study needs to be supplemented by ray-tracin g analyses and the calculation of energy diffusion coefficients incorporati ng data on wave power.