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 H2O 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 RS
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 RS.
• 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 H2O from Enceladus and HI from Saturn.
• Calculation initiates with a seed volume of H2O and an
invariant volume of HI. All other species with exception of
electrons have zero populations at initiation.
• Heavy particle species products: H2O, H2, OH, O2, OI,
HI, H2O+, H3O+, OH+, O2+, H2+, OII, HII. Charge
neutrality is intrinsic to the calculation.
Chemistry Model, cont.
•Rate coefficients:
• 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 Some coefficients are unpublished.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 equation example: O2
• Reactions involving O2 are:
e + O2
hv + O2
H+ + O2 ==>
O+ + O2 ==>
O + OH
O2 + H
==>
==>
==>
==>
==>
O2+ + H
O2+ + O
==>
==>
O+O+e
O2+ + 2e (2)
O+O
O2+
O + O+ + e
(6)
(7)
O2 + H
O + OH
(1)
(3)
(4)
(5)
(8)
(9)
Chemistry Model, cont.
•Neutral species are thermal
•Ion rates are based on bulk flow
•Two populations of electrons with temperatures
Teh and Tec
•H2O is diffused in to replace particle loss from
the volume.
•Heavy species total mass is determined by the
forced electron and solar environment
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-H2O 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
Observed
Nominal model
[N]/[e]
60 – 30 at 4 RS
57
Tec (K)
---------------
16343
[ec] (cm-3)
50 at 4 RS
50
[H2O] (cm-3)
------------
1128
[OH] (cm-3)
670
594
[OI] (cm-3)
750
602
[HI] (cm-3)
500
500
[H3O+] (cm-3)
23
19.2
[H2O+] (cm-3)
7.5
27.5
[OH+] (cm-3)
11
1.9
[OII] (cm-3)
10
0.7
[HII] (cm-3)
-------------
0.5
Sec (GW)
SH2O (10 27 s-1)
------------6. – 7.
4.1
4.
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 H2O 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 H2O 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 Rs