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

Abstract

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(υ) ∝ υ−4.5 until it reaches particle speeds about ten times the solar wind speed where a rollover occurs.