SPRG Seminars
February 4, 2014:
"Wave-particle interaction in the Radiation Belts: Effects of Whistler Waves Oblique Propagation"
Oleksiy Agapitov, SSL/UCB, LPC2E - University of Orléans - France, and Taras Shevchenko National University of Kyiv - Ukraine
ELF/VLF waves play a crucial role in the dynamics of radiation belts, and are responsible for the loss and the acceleration of energetic electrons. The modelling of wave-particle interactions requires the best possible knowledge of wave energy and wave-normal directions distribution in L-shells for different magnetic latitudes and magnetic activity conditions. On the basis of ten years (2001-2010) of Cluster STAFF-SA measurements the statistical study for ELF/VLF emissions in a whistler frequency range has been carried out and the distributions of the wave parameters (amplitude and wave normal direction) are obtained. We show that whistler wave normals are directed approximately along the magnetic field (with the mean value about 10-15 degrees) in a vicinity of the geomagnetic equator but the distribution changes with magnetic latitude, the angle for a given frequency tends to the resonance cone for L-shell<5.5. As a result at latitudes about 35-40 degrees, wave-normals become very oblique to the background magnetic field. The obtained results were proved by means of numerical ray tracing simulations. Wave obliqueness results in an important effect on electron scattering in the radiation belts: the scattering rates magnitudes increase up to 2 orders (in comparison with the conventional parallel waves propagation model). This growth of the pitch-angle diffusion rates is caused by the significant increase of the contribution of higher order cyclotron resonances at large latitudes, which is the most efficient for electrons with small equatorial pitch-angles. This tends to the substantial decrease of the electron life-time in the outer radiation belt. Whistler wave obliqueness also causes the existence of electric field component along the background magnetic field. Due to this, large amplitude oblique whistler waves can effectively trap and accelerate electrons. The new acceleration mechanism of radiation belts electron by oblique whistler waves based on Landau resonance has been proposed. This mechanism can explain a very rapid energy gain up to 1 MeV for electrons.