Unusual characteristics of electromagnetic waves excited by cometary newborn ions with large perpendicular energies

Summary

Solution of the linear kinetic dispersion equation shows that cometary new-born ions with large perpendicular energies (large initial angles between the solar wind velocity and the IMF) can excite a wave mode with rest frame frequencies of the order of the heavy ion cyclotron frequency which exhibits maximum growth slightly away (6°–15°) from parallel propagation, together with high mass density compression ratios and almost linear polarization. The association of these properties is relevant to space observations of MHD-like waves at comets and in planetary foreshocks and has not been reported in the vast literature on possible instability mechanisms. Dilute drifting oxygen ion and photoelectron rings with finite thermal spreads model the free energy source in the solar wind frame. For parallel propagation, the mode of interest is left-hand circularly polarized, moves in the direction of the newborn particles, and is predominantly fed by the perpendicular energy of the heavy ions; as obliquity increases, its polarization and compression change rapidly. Modification of the free energy source parameters (ring density, drift velocity, perpendicular speed and thermal spread) determines their relative influence, defines the domain of existence of the mode, and shows that it might evolve into a nonoscillatory, purely growing structure distinct from the mirror wave. The results are also applicable to non-cometary environments.