Ion‐acoustic‐like instabilities in the solar wind

Abstract

The linear dispersion properties of k parallel to B electrostatic instabilities driven by an electron heat flux or an ion beam are studied by use of parameters appropriate to average values in the solar wind near 1 AU. These modes are also compared with field‐aligned electromagnetic instabilities driven by the same sources of free energy. The ion acoustic heat flux instability has a high threshold relative to the whistler heat flux instability, so the latter mode is the more likely cause of the heat flux limitation near 1 AU. However, at sufficiently large T_e/T_i the electrostatic ion beam instability threshold is of the same order as that of the magnetosonic ion beam instability, implying that the former is the more likely source of ion‐acoustic‐like fluctuations observed near 1 AU.