Excitation and Propagation of Magnetosonic Waves in the Earth's Dipole Magnetic Field: 3D PIC Simulation

Magnetosonic (MS) waves are common plasma waves in the Earth's magnetosphere. The self-consistent excitation of MS waves has been studied by 2D particle-in-cell simulations in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Therefore, the 3D simulation of MS waves needs to be investigated. In this paper, we investigate the excitation and evolution of MS waves in the Earth's dipole magnetic field based on a 3D general curvilinear particle-in-cell simulation. We find that the MS waves are excited primarily within 3 degrees of the equator when the thermal velocity of the ring distribution is much less than the ring velocity of the ring distribution. These waves propagate along both the radial and azimuthal directions nearly perpendicular to the background magnetic field. In the linear stage, the growth rates of MS waves are almost equal in the radial and azimuthal directions. Compared with the waves propagating along the radial direction, the waves propagating along the azimuthal direction can grow for a longer time, resulting in a larger wave amplification in this direction after saturation. The simulation results provide a valuable insight to understand the self-consistent evolution of MS waves in the Earth's dipole magnetic field, and the findings are useful for understanding the plasma wave-particle interaction in the Earth's radiation belts. Plain Language Summary The Earth's magnetosphere is a natural plasma laboratory. Since the plasma in the magnetosphere is collisionless, electromagnetic waves are significant agents for the transport of energy and momentum among different particles. The MS waves are important electromagnetic waves in the magnetosphere, which have a frequency range from several Hertz to several hundred Hertz. Recently, these waves have been receiving an increasing interest due to their potential importance in accelerating radiation belt electrons and heating plasmaspheric ions. The 2D particle-in-cell simulations have been used to investigate MS waves in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Thus, the 3D simulation of MS waves still needs to be studied. In this paper, we perform a 3D particle-in-cell simulation to investigate the excitation of the MS waves in a dipole magnetic field. A larger wave amplification is found in the azimuthal direction because they can grow for a longer time in this direction. Our results can contribute to the understanding of the spatial distribution of MS waves in the Earth's magnetosphere.