Saturn Magnetosphere Plasma Model J. Yoshii, D. Shemansky, X. Liu SET-PSSD 06/26/11

Energy equilibrated plasma model Calculations assume the mass source is composed of H 2 O from Enceladus and HI from Saturn. Forcing is solar photon flux and energetic magnetosphere electrons passing through the plasma sheet. The magnetosphere HI population is forced as an infinite source. Current calculations are volumetric for the plasma sheet at 4 R S

Energy balance methodology Calculations establish statistical and energy balanced steady state based on selected forcing electron parameters. HI forcing is based on observed density at 4 R S. Method is currently in two stages: 1) Collisional statistical steady state using selected electron temperatures. Heavy particle mass is determined by forced electron environment and physical chemistry rates. 2) Energy balance in the plasma sheet electrons (ec) is determined through detailed quantitative energy transport rate processes using the results of (1).

Chemistry Model Chemistry model code iteratively solves volumetric rate equations for gas species populations. Current source species is H 2 O from Enceladus and HI from Saturn. Calculation initiates with a seed volume of H 2 O and an invariant volume of HI. All other species with exception of electrons have zero populations at initiation. Heavy particle species products: H 2 O, H 2, OH, O 2, OI, HI, H 2 O +, H 3 O +, OH +, O 2 +, H 2 +, OII, HII. Charge neutrality is intrinsic to the calculation.

Photoprocesses are derived from Huebner et al. (1992) Rate coefficients k(T) for chemistry are derived from experimental cross section data where available, otherwise from theoretical cross sections. Some coefficients are unpublished. Rate coefficients: Chemistry Model, cont.

Rate equation example: O 2 Reactions involving O 2 are: e + O 2 ==> O + O + e(1) ==>O e(2) hv + O 2 ==>O + O(3) ==>O 2 +(4) ==>O + O+ + e(5) H+ + O 2 ==>O H(6) O+ + O 2 ==>O O(7) O + OH==> O 2 + H(8) O 2 + H ==> O + OH(9)

Neutral species are thermal Ion rates are based on bulk flow Two populations of electrons with temperatures T eh and T ec H 2 O is diffused in to replace particle loss from the volume. Heavy species total mass is determined by the forced electron and solar environment Chemistry Model, cont.

System Energy Budget Plasma sheet electrons are created by the heterogeneous magnetosphere electron forcing and solar input In steady state, the net flow of energy in the plasma sheet electrons must be zero and self contained. This criterion defines energy balance in the plasma. Computation limits: Net electron momentum transfer losses due to bulk motion are assumed to be zero, i.e. acceleration compensation by ion drag and losses against neutrals forced by magnetic field rotation cancel; energy transfer to the gas due to electron momentum transfer is calculated on a local volume basis Bremsstrahlung losses are negligible Electron excitation of homonuclear molecule ground states is not considered to be a loss because the low density environment leaves collisional transfer in near detailed balance

Processes calculated in detail Electron-electron coulomb energy transfer Electron-ion coulomb energy transfer Electron-particle excitation-radiative loss Electron impact ionization loss Electron impact molecular dissociation loss Secondary electron energy injection through excess product and pickup energy Plasma electron recombination loss Momentum transfer to neutrals

Energy rate equations The flow of energy through all species in the volume is calculated. Net zero volumetric energy rates in the plasma electrons determine energy equilibrium in the system.

Momentum transfer energy loss rates

Loss rates for e-H 2 O processes

Electron excitation-radiative losses

Electron ionization losses

Net energy rates for plasma sheet electrons in statistical equilibrium for fixed electron densities.

Energy equilibrium neutral populations as a function of heterogeneous magnetosphere electron density (eh)

Energy equilibrium ion populations as a function of heterogeneous magnetosphere electron density (eh)

Energy equilibrium relaxation times as a function of heterogeneous magnetosphere electron density (eh)

Comparison to observation ObservedNominal model [N]/[e]60 – 30 at 4 R S 57 Tec (K) [ec] (cm -3 )50 at 4 R S 50 [H 2 O] (cm -3 ) [OH] (cm -3 ) [OI] (cm -3 ) [HI] (cm -3 )500 [H 3 O + ] (cm -3 ) [H 2 O + ] (cm -3 ) [OH + ] (cm -3 )111.9 [OII] (cm -3 )100.7 [HII] (cm -3 ) Sec (GW) S H2O (10 27 s -1 ) –

Sittler et al 2008 CAPS results

Sittler et al 2008

Conclusions The heterogenous H sourced from the Saturn atmosphere acts as a forcing function and has a significant effect on the chemistry. The level of agreement of the present pure H 2 O plasma model with observation suggests that the effect of water clusters, grains, and other species do not have a strong impact on the plasma energy budget. Further exploration of properties is required for better understanding of the system.

Inner Magnetosphere Observations Magnetospheric gas models, e.g. Sittler et al. (2008), Schippers et al. (2008)  profiles for electrons, neutrals, ions; electron energy profiles Enceladus plume, Hansen et al. (2006), Waite et al. (2006)  dominant source of H 2 O in the inner magnetosphere in the form of vapor and possibly larger particles (water clusters, nanoparticles, grains) UVIS systems scans, Melin et al. (2009)  map of atomic hydrogen indicates source at the top of Saturn’s atmosphere; [H] ~ 500 cm -3 at 4 R s