Higher-order gyroresonant acceleration of electrons by superluminous (AKR) wave-modes

We consider gyroresonant acceleration of electrons by the superluminous (R-X, L-O, L-X) wave modes that are generated as AKR in the Earth's magnetosphere. This work is an extension of our previous study in which we considered the case alpha > 1, where alpha = vertical bar Omega(c)vertical bar(2)/omega(2)(pe), with the restriction that the cyclotron harmonic N = 1. Here, we consider both regimes 0 > I and alpha <= 1, and allow higher-order harmonics N > 1. For the case alpha > 1, we find that (a) the R-X mode can accelerate electrons more effectively at higher harmonics in that acceleration to higher energies (similar to MeV) is not limited to small wave angles, as is the case for N = 1, (b) the L-O mode can produce significant acceleration of electrons from similar to 10 keV to similar to MeV energies at higher harmonics over a broad range of magnetosphere and wave normal angles, as is the case for N = 1, (c) the L-X mode is less effective for electron acceleration at higher harmonics, with typical minimum resonant energies (similar to MeV) being higher than those for N = 1. For the case alpha <= 1, we find that electron gyroresonance with the superluminous wave modes can only occur at the higher harmonics, e.g., typically N >= 4 for the R-X mode, and N >= 3 for L-O and L-X modes. Significant acceleration of electrons from similar to 10 keV to similar to MeV by each of the superluminous wave modes can occur for higher harmonic resonances over a wide range of wave normal angles. (c) 2007 Elsevier Ltd. All rights reserved.