Various instability modes can exist in the hypersonic boundary layer depending on the flow condition. Mack (Mack L. M., Special course on stability and transition of laminar flow, AGARD report, No. 709, 1984) first categorized the flow disturbances into the first mode and the second mode. In hypersonic boundary layers, the second mode plays an important role, and it behaves like a trapped acoustic wave propagating in a waveguide between the wall and the sonic line, but has a phase velocity larger than the speed of sound.