Radially Full Reflection of Fast Magnetosonic Waves Near the Cut‐Off Frequency

Fast magnetosonic (MS) waves have been found to propagate mainly in the radial direction inside the plasmasphere. One main topic about the radial propagation of MS waves is whether they would propagate down to the ground or be reflected at some regions. Since WKB assumption might become invalid near the reflection point (if the reflection happens), an analytic approach has been utilized to investigate the propagation behavior of MS waves in this study. It is found that as MS waves propagate inward into the Earth, they might reach the cut‐off regions when their frequencies become close to the cut‐off frequency. Moreover, as demonstrated by our theoretical model, the behavior of MS waves around these regions can be described by the Airy's equation. Our results find that MS waves would be fully reflected near the cut‐off point, since only evanescent waves are allowed in the cut‐off regions. Plain Language Summary After generated, plasma waves would propagate away from the source region. During their propagation paths, these waves can interact with magnetospheric particles, which is thought as an important way to affect the dynamic evolution of the terrestrial magnetosphere. Fast magnetosonic (MS) waves are found to be available of propagating in both the radial and azimuthal directions. Particularly, they are propagating dominantly along the radial direction inside the plasmasphere. Thus, whether they could propagate down to the ground becomes an interesting topic. To investigate this topic, ray tracing, based on the WKB assumption, might cease to work when certain wave reflections occurs. Instead, an analytical approach is adopted in this study, which can give the detailed MS wave behaviors near the reflection region. Key Points Dispersive cut‐off profile of magnetosonic (MS) waves has been reported with observations of the Van Allen Probe Analytical approach is utilized to obtain wave behaviors of MS waves around the cut‐off frequency Wave fields of perpendicular MS waves near the cut‐off point can be described via an Airy's equation