Long-term Correlations of Polytropic Indices with Kappa Distributions in Solar Wind Plasma near 1 au

Abstract

Abstract The polytropic behavior of space plasmas defines a power law between the plasma moments during the transition of the plasma from one state to another under constant specific heat. Knowledge of the polytropic index—the power-law exponent—is essential for understanding the dynamics of plasma particles, while a full kinetic description can be established by the study of the velocity distribution of plasma particles. The particle velocities of collisionless space plasmas, such as the solar wind, follow the kappa distribution function. The kappa index, the parameter that labels and governs these distributions, is an independent variable that describes the state of plasmas and is required for a complete description of the plasma properties. Previous studies showed and demonstrated how the kappa and polytropic indices are related to each other in the presence of potential energy, and their relationship also depends on the potential degrees of freedom. This paper extends these analyses and derives the kappa and polytropic indices of the solar wind proton plasmas using Wind observations during the last two solar cycles. We examine and show the systematic long-term correlation between these indices, the magnetic field strength, and the solar activity.