Hydromagnetic wave excitation by ionized interstellar hydrogen and helium in the solar wind

Interstellar atoms penetrate the heliosphere, are ionized by solar UV radiation or charge exchange with solar wind ions, and are “picked up” by the solar wind onto a ring distribution in velocity space. The ring distribution is unstable to the generation of hydromagnetic waves with growth time scales which are small compared with those of the continual pickup process and convection with the solar wind. First, the growth rates for parallel propagation are derived, presented, and compared with previous work. Then the spatially homogeneous quasi‐linear equations describing the subsequent hydromagnetic wave excitation and pickup ion velocity diffusion in pitch angle and energy are presented under the assumption of wave propagation parallel to the ambient magnetic field. Neglecting ion energy changes, analytical expressions for the time‐asymptotic wave spectra accompanying the time‐asymptotic isotropic ion distribution are derived. The results indicate that pickup helium has a very small (unobservable) effect on the solar wind wave spectrum, but that pickup hydrogen results in substantial modifications at cyclotron resonant frequencies (∼10 −2 Hz at ∼7 AU). Finally, based on the previous expressions, the radial evolution of the pickup‐hydrogen‐modified wave spectra for both polarizations and propagation directions is computed analytically including the degradation of the wave power in the divergent solar wind. The predicted modifications beyond ∼5 AU are substantial and could be observable at spacecraft frequencies greater than ∼5 × 10 −3 Hz if not degraded by turbulent wave‐wave interactions or stochastic ion acceleration. The predicted enhancement by a factor of order 10 at ∼7 AU involves ∼10% of the power in the ambient field, is left‐polarized (Alfvénic as opposed to magnetosonic) in the solar wind frame, is unpolarized in the spacecraft frame, and includes an equal admixture of propagation directions.