On the physics of Landau damping

The quantitative explanation of neutral sound absorption at extremely low pressures within the framework of Landau's theory (Sukhorukov and Stubbe [Phys. Plasmas 2, 4059 (1995)]) has revived the question about the physical nature of Landau damping. In the present paper it is shown in several ways that the physical mechanism behind linear Landau damping does not depend on the presence of charges. It is shown, in particular, that in the collisionless limit the damping rate of a given spatial and temporal structure is entirely determined by its spectrum, independent of whether the particles of the system are charged or uncharged. The role of charges lies in their influence on the shape of the spectrum. The conventional picture of Landau damping, involving the resonant conversion of wave energy into particle kinetic energy by electrostatic interaction, is critically investigated, and the shortcomings of this picture are unveiled. Landau damping is then described as thermal spread, which applies to both charged and uncharged gases. A correct 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)01208-2].