Energetic Ion Moments and Polytropic Index in Saturn's Magnetosphere using Cassini/MIMI Measurements: A Simple Model Based on <i>κ</i>‐Distribution Functions

Abstract

Abstract 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 &gt;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 &lt; L &lt; 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 &lt; L &lt; 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&lt;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.