Effects of ion to electron temperatures on electrostatic solitons with applications to space plasma environments

Electrostatic solitary waves (ESW) and solitons are widely observed nonlinear plasma phenomena in various space environments, which may be generated by the electron streaming instability as shown in many particle simulations. The predicted electron holes associated with the ESW, however, are not observed by the recent high resolution spacecraft. This raises a possibility for the ion acoustic solitons being the potential candidate, which are described by the Sagdeev potential theory with hot electrons and cold ions being treated by the kinetic equilibrium and fluid models, respectively. The assumption of T i / T e = 0 adopted in the theoretical models for ion acoustic solitons, however, imposes a great constraint for the space applications considering that T i / T e may vary in a wide range of 0.1-10 in the Earth's space environments. This paper examines the effect of T i / T e on ion acoustic solitons by including a finite temperature in the fluid equations for the ions, which, however, can no longer be solved based on the standard Sagdeev potential method. It is shown based on the nonlinear theory that larger T i / T e may result in larger propagation speeds and the critical flow velocity for the existence of steady solitons increases with increasing T i / T e values. The nonlinear solutions for various T i / T e values may be characterized by an effective Mach number. For T i / T e ≫ 1 the hot ions and cold electrons shall be described by the kinetic and fluid models, respectively, which may result in negative electric potentials opposite to the standard ion acoustic solitons. Comparisons between the model calculations and observations are made.