[PEM16-P26] Nonlinear pitch angle scattering of low pitch angle electrons caused by whistler mode chorus emissions
Keywords:chorus emissions, pitch angle scattering, wave-particle interaction
In this study, we derive the equation of the motion of particles without the assumption of small pitch angle to consider pitch angle scattering near the loss cone in the velocity phase space. We clarify that electrons near the loss cone satisfying the cyclotron resonant condition are scattered away from the loss cone due to the Lorentz force caused by the wave magnetic field and the parallel velocity component of electrons. In order to reproduce the pitch angle scattering caused by chorus emissions, we carry out a test particle simulation using the simulation system along a dipole magnetic field line and a whistler mode wave model. Results of the test particle simulation are consistently explained by the nonlinear theory we derived, and the pitch angle variation due to the nonlinear effect strongly depends on the wave amplitude. In particular, for the case of the large amplitude wave, most of resonant electrons are trapped by the coherent wave and are efficiently scattered away from the loss cone, resulting in less precipitating electrons. Furthermore, assuming 20 keV electrons uniformly distributed in the pitch angle range from 0 to 90 degrees, we estimate the modulation of pitch angle distribution while electrons encounter one wave packet of chorus emissions. Our results indicate that most of low pitch angle electrons scattered away from the loss cone and build a bump of distribution at the moderate pitch angle satisfying the cyclotron resonant condition. These results suggest that the relation between chorus wave intensity and the flux of auroral electron precipitation is not straightforward and that the nonlinear effect newly proposed by the present study should be taken into account.