The Application of Quasilinear Theory to Evaluating Diffusion Coefficients: A Few Comments

A long‐standing formulation of quasilinear theory (Lyons, 1974b, https://doi.org/10.1017/s002237780002537x ), widely applied to wave‐particle interactions in the radiation belts and elsewhere, has recently been reconsidered (Cunningham, 2023, https://doi.org/10.1029/2023ja031703 ), focusing on several underexamined issues. One concerns the representation of Fourier‐transformed wave magnetic intensity as a frequency‐dependent term times a factor depending on wave normal angle, and its proper normalization. Here, the traditional version and its proposed replacement are shown to be mathematically equivalent, if their construction from individual wave measurements is done appropriately. Coordinate invertibility, behavior near the whistler‐mode resonance cone, and superposition are also addressed.