Ion Distributions in Space Plasmas

In this chapter, we present the types and properties of kappa distributions of ion velocities that describe these populations in space plasmas. We also present and interpret the measurements of kappa indices of various ion populations in space plasmas throughout the heliosphere and provide a detailed discussion of the observations in the inner heliosheath. The kappa index is the primary parameter of kappa distributions, significant for identifying the nonequilibrium stationary states and characterizing their "thermodynamic distance" from thermal equilibrium and the furthest state from thermal equilibrium, or "antiequilibrium." We examine the detailed transitions of stationary states toward the extreme state of antiequilibrium. Observations show an empirical separation of the spectrum of kappa values in ion space plasmas. The kappa spectrum is divided in two regions: the near-equilibrium region with values of kappa, 2.5 < κ < ∞ (e.g., Saturnian magnetosphere), and the far-equilibrium region with indices 1.5 < κ < 2.5 (e.g., inner heliosheath). We discuss the role of PickUp Ions (PUIs) in the transitions of kappa distributions from near-equilibrium to far-equilibrium states. With the presented statistical formulations, properties, and observations of kappa distribution of ion populations in space plasmas, the chapter provides valuable and necessary tools for space physics education and research.