Oblique modulation of electrostatic modes and envelope excitations in pair-ion and electron-positron plasmas

The nonlinear amplitude modulation of electrostatic waves propagating in a collisionless two-component plasma consisting of negative and positive species of equal mass and absolute charge is investigated. Pair-ion (e.g., fullerene) and electron-positron (e-p) plasmas (neglecting recombination) are covered by this description. Amplitude perturbation oblique to the direction of propagation of the wave has been considered. Two distinct linear electrostatic modes exist, namely an acoustic lower mode and Langmuir-type optic-type upper one. The behavior of each of these modes is examined from the modulational stability point of view. The stability criteria are investigated, depending on the electrostatic carrier wave number, the angle θ between the modulation and propagation directions, and the positron-to-electron temperature ratio . The analysis shows that modulated electrostatic wavepackets associated to the lower (acoustic) mode are unstable, for small values of carrier wave number k (i.e., for large wavelength λ) and for finite (small) values of the angle θ (yet stable for higher θ), while those related to the upper (optic-like) mode are stable for large values of the angle θ only, in the same limit, yet nearly for all values of . These results are of relevance in astrophysical contexts (e.g., in pulsar environments), where e-p plasmas are encountered, or in pair fullerene-ion plasmas, in laboratory.