Pickup ion acceleration by turbulent electric fields in the slow solar wind

Recent spacecraft observations reveal that energetic tails in ion spectra are present continually in the slow low-latitude solar wind, even in the absence of nearby shocks. To explain this phenomenon we take into account growing evidence that MHD turbulence in the solar wind has a strong two-dimensional (2D) component. 2D MHD turbulence simulations exhibit large-scale field-aligned electric field fluctuations which are also observable in the solar wind. We explore pickup ion propagation and acceleration between the Sun and Earth with a newly developed kinetic numerical model which includes a random distribution of large-scale field-aligned electric field fluctuations consistent with observations in the low-latitude solar wind. The model is found to qualitatively reproduce the accelerated interstellar pickup He+ spectra observed at Earth. Like the observations, the simulated accelerated "tail" is approximately a power law f (v) proportional to v(-4.5) until it reaches particle speeds about ten times the solar wind speed where a rollover occurs.