Magnetosonic waves inside and outside the plasmasphere differ statistically in occurrence rate, frequency, and intensity. How the density interface separates magnetosonic waves inside and outside the plasmasphere remains not fully understood. Here we report an experimental test made with the Van Allen Probes mission from the plasmaspheric plume through the low‐density channel to the plasmaspheric core. Our linear instability analysis and two‐dimensional full‐wave modeling support that the magnetosonic waves propagate from elsewhere to the channel, undergo reflection and transmission at the flanking plasmaspheric density interfaces and eventually exhibit drastic differences in intensity and frequency coverage between neighboring regions. Such a mesoscale (tens of wavelength wide) interface with a strong refractive index gradient allows the transformation of incident waves to surface waves and consequently filters waves in both frequency and orientation. This unexpected filtering pattern could commonly occur at the plasmaspheric boundary and eventually affect the global distribution of magnetosonic waves. Plain Language Summary Magnetosonic waves play an important role in the coupling between the ionosphere and magnetosphere and the evolution of the magnetospheric radiation environment. Previous global surveys have shown that magnetosonic waves inside and outside the plasmasphere differ significantly in occurrence rate, frequency, and intensity. In fact, magnetosonic waves are not confined near the source region but are able to propagate over a broad range of radial distances and magnetic local times. An obvious question arises as to how the density interface separates magnetosonic waves inside and outside the plasmasphere. Previous modeling and analysis suggested that the mesoscale (tens of wavelengths) density interface allowed the free penetration of magnetosonic waves from outside to inside the plasmasphere. In contrast, our data and modeling here show that such a mesoscale interface with a strong refractive index gradient allows the transformation of incident waves to surface waves and consequently filters waves in both frequency and incident angle. This unexpected filtering pattern could commonly occur at the plasmaspheric boundary and eventually affect the global distribution of magnetosonic waves. Key Points A tens of wavelength wide plasmaspheric density interface separated the magnetosonic waves with differing intensity and frequency coverage The local proton Bernstein instabilities are unable to explain the drastic differences between magnetosonic waves in neighboring regions The mesoscale plasmaspheric density interfaces can filter the inward penetrating magnetosonic waves in both frequency and incident angle