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Energetic Ion Moments and Polytropic Index in Saturn's Magnetosphere using Cassini/MIMI Measurements: A Simple Model Based on κ ‐Distribution Functions
Author(s) -
Dialynas Konstantinos,
Roussos Elias,
Regoli Leonardo,
Paranicas Christopher P.,
Krimigis Stamatios M.,
Kane Mark,
Mitchell Donald G.,
Hamilton Douglas C.,
Krupp Norbert,
Carbary James F.
Publication year - 2018
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1029/2018ja025820
Subject(s) - magnetosphere , physics , polytropic process , saturn , ion , plasma sheet , distribution function , atomic physics , spectral line , plasma , computational physics , astrophysics , nuclear physics , astronomy , quantum mechanics , planet
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.