It has been proposed that the cyclic rising tone in frequency observed in rising tone quasi-periodic (QP) fast magnetosonic waves is due to the cyclic diffusion of the inner energy edge of the source proton shell distribution toward lower energies. Linear theory shows that as the inner edge of the proton shell diffuses inwards the unstable frequency rises leading to a rising tone. We present the first observations of this diffusion of the proton shell distribution for this type of wave event, observed by the HOPE instrument on Van Allen Probes B, where this diffusion is clearly observed in two back-to-back QP wave cycles. Linear instability is performed on the observed proton shell distribution, and the linear growth rate is compared with the observed wave frequency, the linear theory applied to the observed proton shell distributions was qualitatively but not quantitatively consistent with observed wave frequencies. Nearby Waveform Receiver burst mode measurements showed that the spacing of the emission bands in frequency was very close the proton cyclotron frequency
f
cP , suggesting near source. However, a ±2 nT spacecraft induced oscillation at ½ the spacecraft spin period in the background magnetic field magnitude could not allow us to conclude that the emission bands where at integer multiples of
f
cP , a stronger condition for source proximity.