Proton Ring Evolution and Its Effect on Magnetosonic Wave Excitation: Particle-In-Cell Simulation and Linear Theory

In this letter, we used one-dimensional (1-D) particle-in-cell (PIC) simulations as well as linear theory to investigate the relationship between proton ring evolution and fast magnetosonic (MS) wave excitation. Simulations demonstrate that most of the ring protons can be scattered toward lower energies via MS waves excited by the initial proton ring distribution, leading to the increase of the proton ring speed corresponding to the maximum phase space density. These protons with a higher ring speed can further excite MS waves at a lower frequency. As a result, a new "double ring" distribution is found to be formed through scattering of higher energy protons by the lower frequency MS waves. The letter demonstrates the detailed evolution process of proton ring distribution and its effect on the excitation of MS waves.