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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.

Plain Language Summary Magnetosonic waves are the near-equatorially confined electromagnetic emissions between the proton gyrofrequency and the lower hybrid frequency in the magnetosphere. Theoretical and observational studies have demonstrated the potential of the magnetosonic waves to accelerate the radiation belt electrons. It is therefore important to understand the generation process and the spatiotemporal distribution of the magnetosonic waves. The substorm activities injecting hot protons into the inner magnetosphere are conventionally considered to intensify the magnetosonic waves. By analyzing the wave and particle data of the Van Allen Probes, we find a magnetosonic wave quenching region related to the distortion of the background magnetic field configuration and the cold plasma density distribution closely following the substorm injection front. We propose two possible causes for this phenomenon: (1) local weak growth or even damping of the magnetosonic waves with the reduced phase velocities and (2) suppression of the magnetosonic wave cumulative growth in the disturbed medium. This new finding may help refine the modeling of the magnetosonic waves and then the radiation belt electrons.