On the physics of Landau damping

The quantitative explanation of neutral sound absorption at extremely low p ressures within the framework of Landau's theory (Sukhorukov and Stubbe [Ph ys. Plasmas 2, 4059 (1995)]) has revived the question about the physical na ture of Landau damping. In the present paper it is shown in several ways th at the physical mechanism behind linear Landau damping does not depend on t he presence of charges. It is shown, in particular, that in the collisionle ss limit the damping rate of a given spatial and temporal structure is enti rely determined by its spectrum, independent of whether the particles of th e system are charged or uncharged. The role of charges lies in their influe nce on the shape of the spectrum. The conventional picture of Landau dampin g, involving the resonant conversion of wave energy into particle kinetic e nergy by electrostatic interaction, is critically investigated, and the sho rtcomings of this picture are unveiled. Landau damping is then described as thermal spread, which applies to both charged and uncharged gases. A corre ct estimate of the damping rate, based on this picture, is given in the low -damping limit. (C) 1999 American Institute of Physics. [S1070-664X(99)0120 8-2].