Nonmodal Linear Theory for Space Plasmas

The nonmodal approach is a linear theory formalism that emphasizes the transient evolution of a perturbed equilibrium. It differs from the normal-mode analysis by not assuming an exponential behavior of physical perturbations. We discuss works that have applied the nonmodal formalism to the problem of solar wind heating and acceleration. We briefly review the methodology of the Kelvin formalism and of the Generalized Stability Theory, and discuss the cases of both sheared and non-sheared plasmas. The results and methodology reviewed in this paper could form the basis for a trend of research in solar wind dynamics that has not been yet systematically explored.