A simulation study of the propagation of whistler-mode chorus in the Earth's inner magnetosphere

We study the propagation of whistler-mode chorus in the magnetosphere by a spatially two-dimensional simulation code in the dipole coordinates. We set the simulation system so as to assume the outside of the plasmapause, corresponding to the radial distance from 3.9 to 4.1 R_E in the equatorial plane and the latitudinal range from -15° to +15°, where R_E is the Earth's radius. We assume amodel chorus element propagating northward from the magnetic equator of the field line at L = 4 with a rising tone from 0.2 to 0.7 ω_e0 in the time scale of 5,000 ω_e0 ^-1, where ω_e0 is the electron gyrofrequency at the magnetic equator. For the initial density distribution of cold electrons, we assume three types of initial conditions in the outside of the plasmapause: without a duct (run 1), a density enhancement duct (run 2), and a density decrease duct (run 3). In run 1, the simulation result reveals that whistler-mode waves of the different wave frequencies propagate in the different ray path in the region away from the magnetic equator. In runs 2 and 3, the model chorus element propagates inside the assumed duct with changing wave normal angle. The simulation results show the different propagation properties of the chorus element in runs 2 and 3 and reveal that resultant wave spectra observed along the field line are different between the density enhancement and density decrease duct cases. The spectral modification of chorus by the propagation effect should play a significant role in the interactions between chorus and energetic electrons in the magnetosphere, particularly in the region away from the equator. The present study clarifies that the variation of propagation properties of chorus should be taken into account for the thorough understanding of resonant interactions of chorus with energetic electrons in the inner magnetosphere.