Fourier–Hermite spectrum of the (reduced) non-thermal distribution function, ﹩\mathrm{log}| \delta {f}_{{\boldsymbol{k}},{\boldsymbol{m}}}{| }^{2}﹩. Top to bottom: (﹩{k}_{\parallel }﹩, k⊥), (m⊥, k⊥), and (﹩{m}_{\parallel }﹩, k⊥) planes. Labeled dashed lines provide reference slopes. White solid lines in panel (a) trace ωKAW/Ωi = 0.25, 0.5, and 1 isocontours.