Energetic Ion Moments and Polytropic Index in Saturn’s Magnetosphere using Cassini/MIMI Measurements: A Simple Model Based on κ-Distribution Functions

摘要

Moments of the charged particle distribution function provide a compact way of studying the transport, acceleration, and interactions of plasma and energetic particles in the magnetosphere. We employ κ-distributions to describe the energy spectra of H~+ and O~+, based on >20 keV measurements by the three detectors of Cassini’s Magnetospheric Imaging Instrument, covering the time period from DOY 183/2004 to 016/2016, 5 < L < 20. From the analytical spectra we calculate the equatorial distributions of energetic ion moments inside Saturn’s magnetosphere and then focus on the distributions of the characteristic energy (E_c = I_E∕I_n), temperature, and κ-index of these ions. A semiempirical model is utilized to simulate the equatorial ion moments in both local time and L-shell, allowing the derivation of the polytropic index (Γ) for both H~+ and O~+. Primary results are as follows: (a) The ～9 < L < 20 region corresponds to a local equatorial acceleration region, where subadiabatic transport of H~+ (Γ ～1.25) and quasi-isothermal behavior of O~+ (Γ ～0.95) dominate the ion energetics; (b) energetic ions are heavily depleted in the inner magnetospheric regions, and their behavior appears to be quasi-isothermal (Γ <1); (c) the (quasi-) periodic energetic ion injections in the outer parts of Saturn’s magnetosphere (especially beyond 17–18 R_S) produce durable signatures in the energetic ion moments; (d) the plasma sheet does not seem to have a ground thermodynamic state, but the extended neutral gas distribution at Saturn provides an effective cooling mechanism that does not allow the plasma sheet to behave adiabatically.