Diffusive Equilibrium: Modeling Anisotropic Maxwellian and Kappa Field Line Distributions in Io's Plasma Torus Using Multi-Fluid and Kinetic Approaches

Abstract

Modeling density distributions along Jupiter's magnetic field lines is essential for understanding the Io plasma torus, moon plasma interactions, and plasma throughout the magnetosphere. This study compares multi-fluid and kinetic approaches to diffusive equilibrium and the effects of different plasma distribution functions and anisotropy. We establish a nominal equatorial centrifugal radial model of plasma densities and temperatures in the Io plasma torus (5–10 $R_J$) and define six cases representing combinations of distribution functions (Maxwellian, standard Kappa, product Kappa, and “Fried-Egg”) and anisotropy parameters. We are the first to apply this to the product bi-kappa distribution to determine the steady-state variation of densities and temperatures along field lines. Our results show that the choice of plasma distribution function significantly affects predicted densities and temperatures along field lines. Anisotropy in temperatures influences plasma density distributions, impacting scale heights and peak densities. Different assumptions lead to different predictions for plasma conditions at various latitudes, especially along field lines intersecting Io's and Europa's orbits. These findings highlight the importance of selecting appropriate plasma distribution models tailored to Jupiter's magnetospheric conditions. Accurate plasma modeling is crucial for interpreting observations and understanding wave propagation, energy transport, and magnetospheric processes. We demonstrate practical applications by calculating Alfvén speeds and travel times, simulating spectral emissions using the CHIANTI atomic database, and calculating $N{L}^2$ profiles to investigate diffusion processes. Our analysis aids in selecting suitable plasma distribution models for different regions within Jupiter's magnetosphere, supporting future studies of the Io plasma torus and its interactions with Jupiter's moons.