Quenching of Equatorial Magnetosonic Waves by Substorm Proton Injections

Near equatorial (fast) magnetosonic waves, characterized by high magnetic compressibility, are whistler-mode emissions destabilized by proton shell/ring distributions. In the past, substorm proton injections are widely known to intensify magnetosonic waves in the inner magnetosphere. Here we report the unexpected observations by the Van Allen Probes of the magnetosonic wave quenching associated with the substorm proton injections under both high- and low-density conditions. The enhanced proton thermal pressure distorted the background magnetic field configuration and the cold plasma density distribution. The reduced phase velocities locally allowed the weak growth or even damping of magnetosonic waves. Meanwhile, the spatially irregularly varying refractive indices might suppress the cumulative growth of magnetosonic waves. For intense injections, this wave quenching region could extend over 2 hr in magnetic local time and 0.5 Earth radii in radial distance. These results provide a new understanding of the generation and distribution of magnetosonic waves.