DSMC Simulation of the Plasma Bombardment on Io’s Sublimated and Sputtered Atmosphere
Chris Moore0 and Andrew Walker1
N. Parsons2, D. B. Goldstein1, P. L. Varghese1, L. M. Trafton1, D.A. Levin2
0Sandia National Labs1University of Texas at Austin
2Penn State University
50th AIAA Aerospace Sciences Meeting1/10/2012
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company,for the United States Department of Energy’s National Nuclear Security Administration
under contract DE-AC04-94AL85000.
Supported by the NASA Planetary Atmospheres and Outer Planets Research Programs. Computations performed at the Texas Advanced Computing Center.
Outline
• Brief motivation and background information on Io• Overview of physical models in our planetary DSMC Code
• Description of new physical models– Particle description of the plasma
– Surface sputtering due to energetic ions
– Ion reaction chemistry
– Photo-chemistry
• Atmospheric Simulations• Conclusions
2
Motivation
Jupiter Io
Plasma Torus
Io FluxTube
• Jovian plasma torus sweeps past Io’s atmosphere causing:• Heating• Chemistry• Changes to the global
winds• Enhanced gas columns
due to sputtering• Observed auroral
glows • Matching obs. can
be used to probe the torus conditions
• Io supplies the Jovian plasma torus:• Surface and atmospheric sputtering• Ionization• Charge exchange
Illustration by Dr. John Spencer
3
Background Information on IoFrost patch of condensed SO2
Volcanic plume with ring deposition
Jupiter
Io
Io FluxTube
Illustration by Dr. John Spencer
• Io is the closest satellite to Jupiter• Radius ≈ 1820 km (slightly larger than our moon)• Atmosphere sustained by volcanism and sublimation from SO2 surface frosts
• Dominant dayside atmospheric species is SO2; lesser species - S, S2, SO, O, O2
• Io is the most volcanically active body in the solar system• Volcanism is due to an orbital resonance with Europa and Ganymede which
causes strong tidal forces in Io4
Brief Overview of DSMC• DSMC simulates gas
dynamics using a “large” number of representative particles– Position, velocity, internal
state, etc. stored
• Particle collisions and movement are decoupled in a given timestep
• Particles are moved by integrating F=ma
• Binary collisions allowed to occur between particles in the same “collision cell”
5
Overview of our DSMC code• Atmospheric models
– Rotational and vibrational energy states– Sub-stepped emission– Variable gravity– Simulate plasma with particles– Chemistry: neutral, photo, ion, & electron
• Surface models– Non-uniform SO2 surface frosts– Comprehensive surface thermal model– Volcanic hot spots.– Residence time on the non-frost surface– Surface sputtering by energetic ions
• Numerical models– Spatially and temporally varying
weighting functions.– Adaptive vertical grid that resolves mfp– Sample onto to uniform output grid– Separate plasma and neutral timesteps
Time scalesChemistry 10-12 seconds
Surface sputtering 10-10 seconds
Plasma Timestep 0.005 seconds
Ion-Neutral Collisions 0.01 seconds - hours
Vibrational Half-life millisecond-second
Cyclotron Gyration 0.5 seconds
Neutral Time step 0.5 seconds
Neutral Collisions 0.1 seconds - hours
Residence Time seconds - hours
Ballistic Time 2-3 minutes
Flow Evolution Several hours
Eclipse 2 hours
SO2 Photo Half-life 36 hours
Io Day 42 hours6
Overview of our DSMC code• Atmospheric models
– Rotational and vibrational energy states– Sub-stepped emission– Variable gravity– Simulate plasma with particles– Chemistry: neutral, photo, ion, & electron
• Surface models– Non-uniform SO2 surface frosts– Comprehensive surface thermal model– Volcanic hot spots.– Residence time on the non-frost surface– Surface sputtering by energetic ions
• Numerical models– Spatially and temporally varying
weighting functions.– Adaptive vertical grid that resolves mfp– Sample onto to uniform output grid– Separate plasma and neutral timesteps
Length scalesAtomic interactions~10-9 m
Sputtering radius~10-7 m
Debye Length<1 m
Electron Larmor radius 3 m
Dayside neutral m.f.p.~10 m
Volcanic plume vents0.1–10 km
Ion-neutral m.f.p.500 m
Electron-ion m.f.p.~1 km
Ion Larmor radius 3 km
Atmospheric scale height10–100 km
Nightside neutral m.f.p.~100 km
Volcanic plumes100–500 km
Io’s radius1820 km
Jovian plasma torus~105 km
7
3D / ParallelBlown-up Viewof Low AltitudeAtmosphere withMeshFull Planet
360 processors
Processor Boundary
Single ProcessorDomain
• 3D• Spherical grid – northern
hemisphere• 3°×3° latitude/longitude cells• Non-uniform radial grid
• Parallel• MPI, 900 CPU’s
• Parameters• 360 million molecules
instantaneously• Simulated 10 hours to quasi-SS• ~25,000 computational hours
8
•Ion energies in the collision cascade regime
•Little sputtering contribution from electronic excitation
•Sputtering yield proportional to incident ion energy
Surface Sputtering
x
+
x+
Incident Ion Energy, Ei (eV)S
pu
tte
rin
gY
ield
,YA
,B(#
of
Bp
er
ion
)100 200 300 400 500 600
50
100
150
200
250
300
350
YA,B/(A,B) = 0.53Ei + 29
YO , SO , O , SO = 0.256
YS , SO , S , SO = 0.223
YS , SO
YO , SO
x
+
Black Symbols - Johnson , 1984Red Symbols - Chrisey , 1987
+
+2
2
2
2+
+
et al.et al.
+
+2
2
9
•Ion energies in the collision cascade regime
•Little sputtering contribution from electronic excitation
•Sputtering yield proportional to incident ion energy
•Sputtering yield exponential with surface frost temperature
•
Surface Sputtering
SO2 sputtering yield, S, versus SO2 frost temperature. Lanzerotti et al. (1982)
1.7
50
115
s
s
TYield
TYield
10
Surface Sputtering
Sputtered SO2 energy distribution. Boring et al. (1984)
•Ion energies in the collision cascade regime
•Little sputtering contribution from electronic excitation
•Sputtering yield proportional to incident ion energy
•Sputtering yield exponential with surface frost temperature
•
•Sputtered particles leave with Thompson energy distribution
1.7
50
115
s
s
TYield
TYield
11
Charged Particle Motion•Acceleration during move:
•Use predictor-corrector integrator
•Pre-computed (MHD) fields used•Electrons are assumed to move with the ions
•Debye length << m.f.p.
BvEm
eZe
r
Rga r
Io
ˆ
2
B-FieldE-Field(Out of the page)
Simulate simple ion motion and impact onto surface:
12
Heavy Interactions: MD/QCT1
• SO2 + O collisions simulated using Molecular Dynamics/Quasi-Classical Trajectories (MD/QCT)
•RK-4 integration of Hamiltonian equations
•Particles interact via their potentials
•Cases run for range of collider velocities and initial SO2 internal energies•Each case consists of 10,000 separate trajectories: Microcanonical sample unique impact parameters and initial SO2 component coordinates
• Potential Energy Surface• Total potential of SO2 + O system is the summation of the collisional interaction potential and molecular potential of the SO2 molecule
• Collisional interaction: Lennard-Jones 6-12 potential
• SO2 molecular potential: Murrell 3-body potential
• Allows for accurate dissociation of SO2 molecule to SO + O, O2 + S, or S + 2O 131Parsons, N. and Levin, D., 50th AIAA Aerospace Sciences Meeting: 2012-0227
Heavy Interactions
•MD/QCT (fast neutrals/ions) or theoretical cross section data vs. translational and internal energy•Linearly interpolate between nearest cross section data points
•If no MD/QCT data, use Arrhenius coefficients & TCE
Relative Velocity, Vrel (km/s)
Cro
ss
Section
(cm
2 )0 20 40 60 80 100
10-17
10-16
10-15
10-14
Total SO2 + O using MD/QCTVHS fit to MD/QCT data below 16 km/sMD/QCT SO2 + O non-reactiveSO2 + O SO + 2O, Eint=4.99 eVSO2 + O S + 3O, Eint=4.99eVO2 + O 3O
•Always use the total cross section to determine the reaction rate (number of selections and fraction accepted)
•VHS cross section « Total cross section above ~20 km/s
14
Photo-chemistry
• •Rate constants, kreact,s,i, assume quite sun
•Assume gas is optically thin•Optical depth over photo-dissociation wavelengths less than 0.1
•Give dissociation products an average excess kinetic energy
•Accurate below the exobase where products are collisionally equilibrated
Time (s)
No
rma
lize
dp
art
icle
nu
mb
er
0 100000 200000 300000 400000 500000 60000010-2
10-1
100
SO2
SOO2
SO
0-D box initialized with only SO2 particles. Lines are analytic, diamonds from DSMC.
1 Io Day
reactionsN
iisreactsreact tkP ,,, exp1
Sunlighttime
15
Longitude (degrees)
La
titu
de
(de
ge
es
)
03060901201501802102402703003303600
30
60
9090 92 94 96 98 100 102 104 106 108 110 112 114
Surface (frost)Temperature (K)
Subsolar Point
Dawn TerminatorDusk Terminator
Leading HemisphereTrailingHemisphere
XY
Z
Y, Subplasma Point (270 )
Constant NightsideTemperature (90 K)
Subsolar Point(351.1 )
Upstream PlasmaCorotational Direction
Trailing Hemisphere
X (0 )
Leading Hemisphere
Simulation Conditions
•Io just before ingress → Plasma incident onto dusk terminator•Assume uniform SO2 frost → No rock surface or residence time
•Assume simple radiative equilibrium surface temperature model•Do not account for Io’s rotation, thermal inertia
Y
Jupiter
Eclipse
Sunlight
Plasma Flow
Io
8.9°Io
Sub-Jovian spot; 0° longitude
Io’s orbit
X
DuskTerminator
DawnTerminatorPlasma
Flow
Sunlight
16
3D Results: SO2
•SO2 number density peaks near the subsolar point
•Day-to-night near surface flow develops from subsolar point
•Retrograde wind forms and high density “finger” extends past the dawn terminator due to plasma pressure
•Slight increase in the polar atmosphere due to preferential polar sputtering
Direction of Io’s rotation
17
3D Results: O2
•O2 produced via photo-dissociation on dayside
•Non-condensable O2 gas dynamics very different, but day-to-night flow still present
•O2 “finger” extends much further onto the nightside, ≈ to the dusk terminator
•Retrograde flow across nightside meets day-to-night flow at dusk terminator
•O2 diffuses towards the poles where it is stripped away or destroyed by the plasma
Dawn Terminator
18
3D Results: O+
•O+ density contours 4 km above Io’s surface
•High altitude ions stream along field lines to surface
•On the nightside, ions stream to the surface
•Upstream torus O+
density 2400 cm-3
•Dense dayside atmosphere prevents plasma penetration
•Enhancement on the dayside from plasma flow
19
Surface Sputtering of SO2 Frost
•Sputtering primarily on the nightside and at high latitudes•Dense atmospheric columns (> 1015 cm-2) block energetic ions from reaching the surface
•Obs. show green auroral glow only on Io’s nightside•Sodium is believed to be sputtered off Io’s surface
•Simulated SO2 sputtering map suggests Na is the source of green aurora with sputtering blocked on dayside
Longitude (degrees)
La
titu
de
(de
ge
es
)
03060901201501802102402703003303600
30
60
901.0E+25 1.9E+25 3.7E+25 7.2E+25 1.4E+26 2.7E+26 5.2E+26 1.0E+27
SputteringRate (s-1 km-2)
Subsolar Point
Dawn Terminator
Dusk Terminator
Subplasma Point
Nightside
Dawn Terminator
Nightside Na aurora?
20
Longitude (degrees)
La
titu
de
(de
ge
es
)
03060901201501802102402703003303600
30
60
900.0010 0.0040 0.0158 0.0631 0.2512 1.0000
NormalizedSputtering Rate
Subsolar Point
Dawn Terminator
Dusk Terminator
Subplasma Point
Nightside
Discussion•Direction of plasma flow relative to subsolar point important
•Subsolar point changes during Io’s orbit → Atmospheric dynamics will change as Io orbits Jupiter
•Sputtering only occurring near night time temperatures implies preferential scouring of surface by plasma from 270°–360°
•Eclipse inhibits formation of dayside atmosphere
•Plasma directly impacts this quadrent
•Io’s surface frost poor in this region
Prior to ingress
Y
Jupiter
Eclipse
Sunlight
Plasma Flow
Io
8.9°Io
Io’s orbit
X
Sunlight
Eastern Elongation
Plasma Flow
Plasma Flow
Sunlight
Dusk Terminator
Dusk Terminator
DuskTerminator
Io
Current simulation
21
Conclusions•The interaction of the Jovian plasma torus with Io’s atmosphere was simulated using the DSMC method.
•A sub-stepping method was used to time-resolve the movement and collisions of energetic ions and electrons from the Jovian plasma torus
•MD/QCT simulations were used to compute the cross-sections for heavy reactions
•Sputtering from Io’s surface by energetic ions and fast neutrals was included
•Formation of high density “finger” onto the nightside near the dawn terminator due to plasma pressure
•Interesting O2 flow feature generated at the dusk terminator
•Non-condensable O2 pushed across the nightside to the dusk terminator where it meets the opposite day-to-night flow
•O2 stagnates and forced to diffuse slowly towards the pole until it is stripped away and/or dissociated
•Sensitivity of sputtering on surface temperature can lead to sharp gradients in sputtering column density — Sputtering blocked by large columns > 1015 cm-2
•Concentrated at high latitudes and on low density nightside
•Possible cause of observed (Voyager, Galileo) frost-poor region of Io’s surface22
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