Unifying Kappa Distribution Models for Non-Equilibrium Space Plasmas: A Superstatistical Approach Based on Moments

Abstract: From the perspective of non-equilibrium statistical mechanics, modeling the velocity distribution of particles in non-equilibrium, steady-state plasmas presents a significant challenge. Under this context, a family of kappa distributions has been widely used to capture the high-energy tails in space plasmas. These distributions deviate from the canonical Maxwell-Boltzmann statistics and vary significantly in their interpretation of the temperature of an out-of-equilibrium system. In this letter, we establish the validity of any kappa distribution from the standpoint of superstatistics. This study unifies these models by introducing a new kappa distribution based on superstatistical parameters, providing a more general and fundamental framework to connect these distributions and the superstatistical temperature of a system. We demonstrate that the general distribution depends on the thermal characteristics of the modeled temperature distribution population. Furthermore, we present a moment-based velocity distribution that bypasses the traditional temperature debate, relying on the velocity moments. Our findings enhance the understanding of kappa distributions and offer a robust model for non-equilibrium space plasmas.