Propagation and evolution of a rising-tone chorus wave investigated by particle-in-cell simulations

Abstract:

Chorus waves are ubiquitous and significant in the Earth’s magnetosphere which play a crucial role in controlling electron dynamics of the Van Allen radiation belt through wave-particle interactions. They are typically characterized by rising-tone or falling-tone elements in the frequency-time spectrogram, and usually occur in two frequency bands: lower band (0.1-0.5fce) or upper band (0.5-0.8fce), where fce is electron gyrofrequency. Chorus statistics shows that lower-band chorus waves are much more than upper-band chorus waves, and there are a few percent of chorus waves over a wide frequency range without a gap at 0.5fce. To study propagation and evolution of a rising-tone chorus wave in the Earth’s magnetosphere, we have performed a series of two-dimensional (2-D) particle-in-cell (PIC) simulations in a dipole magnetic field by launching a field-aligned rising-tone chorus wave over a wide frequency range crossing 0.5fce in the equatorial plane. Simulation results show that the rising-tone chorus wave leaves away the equator and then propagates toward high-latitude regions with increasing wave normal angle as a function of the magnetic latitude. When propagating to middle and high latitudes, the upper band of the rising-tone chorus wave experiences more severely attenuation or less growth than the lower band does. Consequently, a plateau-like electron population at parallel velocities close to 2V_Ae (V_Ae is electron Alfven velocity) has been formed by Landau resonance with the chorus wave.