Acceleration of cometary H2O group pickup ions by obliquely propagating nonlinear magnetosonic waves

Abstract

The observations made during the encounter with comet Giacobini Zinner show that the character of MHD turbulence is governed by the magnetosonic (MS) waves generated by the pickup ions via a resonant cyclotron instability. Close to the bow shock these waves are highly nonlinear, |ΔB/B0| ∼ 1, and are propagating obliquely to the magnetic field. The interaction of cometary ions in the mass loaded solar wind with MS waves propagating away from the comet and oblique to the interplanetary magnetic field (IMF) is investigated using the test particle approach. Ion trajectories, distribution functions, widths of pitch angle scattering and energy diffusion are obtained. Because of the MS “turbulence”, the particle velocity and acceleration are found to increase with increasing wave amplitude, inclination of the wave vector to the background magnetic field, and the range of resonant mode numbers. It is found that the interaction of water group pickup ions with MS waves propagating obliquely to the IMF gives larger pitch angle scattering and acceleration than that in the case of parallel and antiparallel propagating waves. In particular, obliquely propagating MS waves at angles greater than 40° to the ambient magnetic field are very effective at accelerating particles because of a high phase velocity along the magnetic field. In the case of monochromatic MS waves, Landau damping is found to play an important role; the particles get electromagnetically trapped in a potential well due to Landau damping. In the case of MS turbulence, the particles are stochastically heated and the temperature continues to grow linearly with time. We have also investigated the relationship between pitch angle scattering and the three parameters, namely, α, the angle between the solar wind flow direction and the ambient magnetic field, θBk, the angle between the ambient magnetic field and the wave propagation vector, and the ion injection velocity. The pitch angle scattering rates are obtained using a monochromatic nonlinear MS wave as well as MS turbulence in both the quasi‐parallel (0 < α < 60°) and quasi‐perpendicular (60° < α < 90°) regimes for various values of θBk. In the case of a monochromatic MS wave, pitch angle scattering rates are found to change very slightly with respect to θBk, α and the mode number of the MS wave due to their Landau damping. In the case of MHD turbulence the pitch angle scattering rates decrease with increasing α for θBk < 40° and increase with increasing α for θBk > 40°. They are found to be independent of the injection velocity. The results are in agreement with the observations.