Existence of bulk acoustic modes in pair plasmas

In view of applications to electron-positron and fullerene pair plasmas, a thorough discussion is given of the dispersion of linear electrostatic waves, before finding the parameter ranges in which stationary nonlinear electrostatic modes can exist, when there is a thermodynamic asymmetry between both constituents. Arguments for such an asymmetry can be found in the observed dispersion in fullerene pair plasma experiments, but point to small asymmetries. The existence of solitary modes is first discussed in general terms for various polytropic pressure-density relations, showing that the solitons are always compressive in both pair components. For large thermal asymmetries, there is at most a doubling of the densities, but small asymmetries lead to weakly nonlinear amplitudes, described by familiar Korteweg-de Vries solitons. Observations of an intermediate wave in the theoretical gap between the acoustic and Langmuir branches of the linear dispersion diagram might be accounted for, at least partially, by a train of weak solitons, as the difference between weak solitary modes and linear harmonic waves is rather difficult to ascertain experimentally.