Electron Lifetimes from Narrowband Wave-Particle Interactions within the Plasmasphere

Abstract:

This work is devoted to the systematic study of electron lifetimes from narrowband wave-particle interactions in the plasmasphere. It relies on a new formulation of the bounce-averaged quasi-linear pitch angle diffusion coefficients parameterized by a single frequency, w, and wave-normal angle, theta [Albert, 2012]. We first show that the diffusion coefficients scale with w/W_ce, where W_ce is the equatorial electron gyrofrequency, and that maximal pitch angle diffusion occurs along the line alpha₀=pi/2-theta, where alpha₀ is the equatorial pitch angle. Lifetimes are computed for L-shell values in the range [1.5, 3.5] and energies, E, in the range [0.1, 6] MeV as a function of frequency and wave-normal angle. The lifetimes are relatively independent of frequency and wave-normal angle after taking into consideration the scaling law, with a weak dependence on wave-normal angle up to 60-70°, increasing to infinity as the wave-normal angle approaches the resonance cone. We identify regions in the (L, E) plane in which a single wave type (hiss, VLF transmitters, or lightning-generated waves) is dominant relative to the others. We find that VLF waves dominate the lifetime for 0.2-0.4 MeV at L~2 and for 0.5-0.8 MeV at L~1.5, while hiss dominates the lifetime for 2-3 MeV at L=3-3.5. The influence of lightning-generated waves is always mixed with the other two and cannot be easily differentiated. Limitations of the method for addressing effects due to restricted latitude or pitch angle domains are also discussed. Finally, for each (L, E) we search for the minimum lifetime and find that the “optimal” frequency that produces this lifetime increases as L diminishes. Restricting the search to very oblique waves, which could be emitted during the DSX satellite mission, we find that the optimal frequency is always close to 0.16W_ce.