Velocity space perturbations associated with low-frequency waves launched in a weakly collisional plasma are shown to consist of a discrete spectrum of modes. Collisions are modeled using an energy and momentum conserving one-dimensional Fokker–Planck operator. The linearized Vlasov–Poisson–Fokker–Planck system of equations is solved by expanding the perturbed ion-distribution function in terms of Hermite polynomials, from which an eigenvalue problem is set up. The eigenvalues and eigenvectors yield the ion acoustic mode that is weakly damped [J. Dougherty, Physics of Fluids 7, 1788 (1964)], as well as a discrete spectrum of kinetic modes.

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