Behavior of the Ion and Electron Fluxes Fitted by a Kappa Distribution Function for Different Plasma Beta Parameter in Earth Magnetosphere: THEMIS 0bservation

Abstract:

We have investigated the properties of the ion and electron kappa distribution parameters in the magnetosphere of the Earth for different values of the ion and electron plasma beta (β) parameters (the ratio between the plasma and magnetic pressures). For this study we used the data from five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft during the year 2008 to 2018. The magnetosphere of the Earth represents a natural plasma laboratory allowing us to study (it in situ) the evolution of particle distribution functions in the absence of Coulomb collisions. Our study covers different regions of the magnetosphere, making it possible to find the dependencies for the main κ-parameters for a wide range of beta (β) from 10^-4 to 10², approximately. It was found that for the very low and very high plasma beta (β), the values of κ-parameter have a very reduced range, meanwhile the plasmas with β ≈ 0.1-1.0 contain particle fluxes well fitted by kappa distributions with a κ-index between 1.5 and 20, approximately. This means that the relaxation of a distribution function to the Maxwell distribution is more efficient when the β ≈ 0.1-1.0. For a fixed beta parameter β, a relation between the κ-index and the core energy is well fitted by a power law function for both species. The intercept (A) and the power law index gamma (γ) show an interesting behavior with beta. For both species the A has a minimum near to β ≈ 0.1, meanwhile gamma (γ) has a maximum for the same value of beta. Starting from β>1, both A and gamma (γ) take nearly constant values. This effect is especially notable for electrons, and might be related to their demagnetization. These results offer new insight into the dynamics of plasma in Earth's magnetosphere.