Space Physics Research Group at UCB/SSL

For the modelling of the dynamics and evolution of the radiation belts, it is necessary to possess precise estimates of the lifetime of energetic particles and of the characteristic time of particle acceleration for different energies and pitch-angles in a wide range of L-shells. The lifetimes of electrons trapped in Earth's radiation belts are supposed to be determined by a process of quasi-linear pitch-angle diffusion by whistler-mode waves, assuming that their frequency spectrum is broad enough and/or their average amplitudes are not too large. We calculated the bounce-averaged electron pitch-angle diffusion coefficients using the statistical characteristics of lower band chorus activity collected by the Cluster mission from 2001-2010. These ten years of Cluster observations provide the statistical distribution of the angle between wave vectors and the background magnetic field, as well as the distributions of the wave total intensity for relatively wide ranges of magnetic latitude, magnetic local time, and Kp index. We show that the presence of oblique waves is responsible for a very important effect on particle diffusion. We found that inclusion of oblique whistler wave propagation leads to a significant increase in pitch-angle diffusion rates over those calculated under the assumption of parallel whistler wave propagation. The effect was pronounced for electrons with small equatorial pitch-angles close to the loss cone and could result in as much as an order of magnitude decrease of the electron lifetimes. We show that the intensification of pitch-angle diffusion can be explained by the contribution of higher-order cyclotron resonances.
We demonstrate that the most important effect on particle diffusion is associated with the part of wave angular distribution located between the Gendrin angle and the resonance cone angle. A separation of the wave distribution into two distinct parts: "quasi-parallel" and "strongly oblique" allows one to obtain semi-analytical lifetime estimates. We present extensive comparisons between improved analytical lifetime estimates and full numerical calculations in a broad parameter range representative of a large part of the magnetosphere from L = 2 to 7. The effects of observed very oblique whistler waves are taken into account in both numerical and analytical calculations. Analytical lifetimes (and pitch-angle diffusion coefficients) are found to be in good agreement with full numerical calculations based on CRRES and Cluster hiss and lightning-generated wave measurements inside the plasmasphere and Cluster lower-band chorus wave measurements in the outer belt for electron energies ranging from 100 keV to 5 MeV. Comparisons with lifetimes recently obtained from electron flux measurements on SAMPEX, SCATHA, SAC-C and DEMETER also show reasonable agreement.
We have also evaluated the effect of oblique wave propagation on energy diffusion of particles. A comparative analysis of the effects of angular diffusion and energy diffusion provides some indications on the possible mechanisms of particle acceleration and the growth of energetic particle fluxes during magnetic storms.