Here the first term represents the change in the perturbed distribution function, h s ≡ ≡ f s 1 + Z s e ϕ ϕ T s f s 0 , with time. The second term represents particle streaming along the magnetic field line. The third term contains the effects of cross-field particle drifts, including the curvature drift, the grad-B drift, and the lowest order E-cross-B drift. The fourth term represents the nonlinear effect of the perturbed E × × B drift interacting with the distribution function perturbation. The fifth term uses a collision operator to include the effects of collisions between particles. The sixth term represents the Maxwell–Boltzmann response to the perturbed electric potential. The last term includes temperature and density gradients of the background distribution function, which drive the perturbation. These gradients are only significant in the direction across flux surfaces, parameterized by ψ ψ , the magnetic flux.