Preshock region acceleration of implanted cometary H+ and O+

Abstract

One of the most interesting results of the cometary flyby missions was the realization that magnetohydrodynamic turbulence generated by the pickup process of freshly ionized cometary particles results in fast pitch angle scattering of the newly implanted ions. This process rapidly modifies the newly implanted ion pitch angle distribution in the solar wind frame of reference, from the original pickup ring to a pickup shell distribution. It also appears that the implanted ions have an isotropic pitch angle distribution in the decelerating solar wind frame of reference. This paper presents a self‐consistent description of the multispecies plasma flow (composed of solar wind protons and cometary H+ and O+) in the unshocked upstream region. The solar wind is treated as a fluid, while the evolution of the implanted ion distributions is described by generalized transport equations. The ambient solar wind population is depleted by charge exchange, while implanted protons and oxygen ions are produced by photoionization and charge transfer and lost by charge exchange. The transport equations take into account the adiabatic velocity change due to the deceleration of the flow, velocity diffusion, charge exchange loss, and the pickup of newly created ions. The model equations were numerically solved for the comet Halley flyby conditions, and the results are compared with spacecraft observations. Our calculations imply that second‐order Fermi acceleration can explain the implanted spectra observed in the unshocked solar wind. The comparison of the measured and calculated distribution functions also indicates that spatial diffusion (and consequently first‐order Fermi acceleration) of implanted ions is likely to play an important role in forming the energetic particle environment in the vicinity of the shock.