The Juno Waves instrument can be used to accurately determine the electron density inside Io’s orbit, the inner Io torus. These observations have revealed a local peak in the electron density just inside M=5 and at centrifugal latitudes above about 10º that is likely the ’cold torus’ as identified in Earth-based observations of S+ emissions. This peak or ’finger’ is separated from the more dense Io torus by a local minimum or ’trough’ at M ≥ 5. The electron densities are inferred by identifying characteristic frequencies of the plasma such as the low-frequency cutoff of Z-mode radiation at fL=0 and the low-frequency cutoff of ordinary mode radiation at fpe that depend on the electron density. The ’finger’ density ranges from about 0.2 to 65 cm-3 and decreases with increasing centrifugal latitude. The ’trough’ densities range from 0.05 to ~10 cm-3. This pattern of a density ’trough’ followed by the ’finger’ closer to Jupiter is found on repeated passes through the inner Io torus over a range of centrifugal latitudes. Using a simple model for the electron densities measured above about 10º centrifugal latitude, we’ve estimated the scale height of the ’finger’ densities as about 1.17 RJ with respect to the centrifugal equator, which is somewhat surprising given the expected cold temperature of the cold torus. The larger scale height suggests a population of light ions, such as protons, are elevated off the centrifugal equator. This is confirmed by a multi-species diffusive equilibrium model.

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 $NL^2$ 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.

A document by Edward Gregory Nerney. Click on the document to view its contents.