Relativistic electron microbursts induced by EMIC triggered emissions in the Earth's radiation belts

We have performed test particle simulations to demonstrate that relativistic electron microbursts are induced by electromagnetic ion cyclotron (EMIC) triggered emissions. Pitch-angle scattering of relativistic electrons arising from the anomalous cyclotron resonance with left-hand polarized EMIC waves contributes to the sharp decrease of the relativistic electron flux in the outer radiation belt in the main phase of magnetic-storms. We investigate the nonlinear dynamics of relativistic electrons interacting with a coherent EMIC wave in the radiation belts[1]. We perform test particle simulations to reproduce the time histories of pitch angles, trajectories in the theta-zeta phase space, distribution of pitch angles assuming a left-hand polarized EMIC wave with a varying frequency as observed by Cluster spacecraft[2,3]. We found that the efficiency of pitch-angle scattering depends on the frequency sweep rate, the gradient of the magnetic field, and the wave amplitude. The most effective pitch-angle scattering takes place for the case of a rising-tone emission with an enhanced magnetic field gradient. EMIC triggered emissions with frequencies typically increasing in time are generated in the equatorial region and propagate along the magnetic field line both northward and southward. Due to the short bounce time of the electrons compared to EMIC propagation time, the electrons after adiabatic bounce motion at the mirror points interact with another wave packet near the equator and scattered into the loss cone. We find that EMIC triggered emissions are very effective in precipitating the relativistic electrons from the radiation belts. [1] Y. Omura and Q. Zhao, Nonlinear pitch-angle scattering of relativistic electrons by EMIC waves in the inner magnetosphere,J. Geophys. Res., in press. [2] J. S. Pickett,, B. Grison, Y. Omura, M. J. Engebretson, I. Dandouras, A. Masson, M. L. Adrian, O. Santolik, P. M. E.Decreau, N. Cornilleau-Wehrlin, and D. Constantinescu, Cluster observations of EMIC triggered emissions in association with Pc1 waves near Earth's plasmapause, Geophys. Res. Lett., 37, L09104, doi:10.1029/2010GL042648,2010 [3] Y. Omura, J. S. Pickett, B. Grison, O. Santolik, I. Dandouras, M. Engebretson, P. M. E. Decreau, A. Masson, Theory and observation of electromagnetic ion cyclotron triggered emissions in the magnetosphere, J. Geophys.Res., 115, A07234,doi:10.1029/2010JA015300, 2010.