plasma-induced sputtering & heating of titan’s atmosphere r. e. johnson & o.j. tucker
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Plasma-induced Sputtering & Heating of Titan’s Atmosphere R. E. Johnson & O.J. Tucker . Goal Understand role of the plasma in the evolution of Titan’s atmosphere Pre-Cassini Understanding: Hydrogen Escape Thermal Carbon & Nitrogen Loss Non-thermal. - PowerPoint PPT PresentationTRANSCRIPT
Plasma-induced Sputtering & Heating of Titan’s Atmosphere
R. E. Johnson & O.J. Tucker
GoalUnderstand role of the plasma in the evolution of Titan’s atmosphere
Pre-Cassini Understanding: Hydrogen Escape Thermal
Carbon & Nitrogen Loss Non-thermal
Thermal &
pick-up plasma
>10keV H+
exobaseUV EUV
>10keV O+
Smith et al. 2009; Shah et al. 2009; Sillanpaa et al 2007; Ledvina 2007; Luna et al. 2005; Michael et al 2005
Hot recoil production
Thermalconduction
Average Energy DepositionHighly Variable
Titan in Plasma Sheet
Modeling of the interaction: Sillanpaa, Snowdon, Ledvina, etc.
Thermal Plasma& Pick-up Ions
Exobase
Non-thermalEscape
Plasma-Induced Escape
CoronaCollisions UnlikelyThermosphereCollisions Likely
Energetic Ions
ThermalConduction
ThermalEscape
Michael et al. 2005DeLaHaye et al. 2007
Westlake et al. 2011Bell et al. 2011
Use Direct Simulation Monte Carlo Method (DSMC)To Describe Response of Atmosphere
Non-thermal EscapeINMS data for N2 and CH4 Density
(DeLaHaye et al. 2007)
Parameter of the fit:
Texo
Hot component
Thermal component
CH4 & N2 escape significant but highly variable
DSMC
Thermal EscapeCharacterized by the Jeans Parameter,
= Gravitational Energy/Thermal Energy
Enhanced thermal escape at Titan?
Slow Hydrodynamic Escape ModelLoss of CH4 & N2 Dominated by Thermal Conduction
(Strobel 2008;2009; Cui et al 2008; Yelle et al 2009)
Hydro-like
Jeans-like
= escape rate from top of domaino,o = evaporative flux from surface
Thermal escape at Titan: N2 & CH4 ~ Jeans Rate (Tucker &Johnson 2009)
Thermal Escape Rate vs. λ for principal species
(Volkov et al. 2011: ApJ & Phys Fluids)
~exobase
Plasma Heating of the Thermosphere?
N2 Density in Thermosphere(Westlake et al. 2011)
in plasma sheetin lobe
DSMC Model of INMS DataCross sections with internal energy exchange
exobase
N2CH4H2
escape rate (s-1) N2 CH4 H2
Lobe (DSMC)[Jeans rate]
< 1023
[2.2 x103] < 1023
[3.7 x1014]1.0 x1028
[8.5 x1027]
Plasma sheet (DSMC)[Jeans rate]
< 1023
[4.1 x1010] < 1023
[5.4 x1018]1.4 x1028
[1.1 x1028]
in lobe in plasmasheet
exobase
INMS Data from J. Bell
H2 CH4 N2
DSMC
exobase
in plasma sheet
Temperatures Separate Well Below ExobaseDSMC is useful
SummaryThermal Escape: including plasma heating
No large enhancements over the Jeans rateCH4 & N2 density profiles consistent with DSMC for lobe & plasma dataH2 : agreement only for plasma sheet data?
Non-thermal escape: expansion in corona implies non-thermal escape
Projection of the Electric Field on the Equatorial Plane (S. Ledvina)
Ion flow across exobase is non-uniform
Need 2&3D Simulations
Sputtering & Heating of CoronaSlow ion-neutral collision cross sections are large
+
Exiting, Pick-up Ions
N2,CH4,H2N2,CH4,H2
+
Incident Ions
Non-thermal escape: is non-uniform & variable 1. Need morphology of the local plasma flux fora number of passes 2. Need to re-analyze the INMS data in the corona
Effect of Neutral-Neutral Cross Sections on H2 profile and escape
collision model Rate x 1028 H2 s-1
hard sphere (HS) .98HS with internal energy 1.1variable (HS) with internal energy 1.04
To ~132 K
Cassini Plasma Data: Ta
M~16
M~28
M~28M~16
M=2M=1
M=1M=2
Energy flux ratio (egress/ingress) near exobase ~ 1.3
1679kmegress
Analytic Model
Struck neutrals have a spectrum of recoil energies, E ~ Edeposited / E2
Recoils ejected if direction is up and E > Eescape
Number ejected per ion incident ~ E deposited / Eescape
Tested by simulations
+
Monte Carlo Simulations (e.g. Bird; Shematovich et al. 2003)
• Track representative particles under gravity• Monte Carlo choice of collision outcome • Simulate an atmosphere
• Inject ions• Change in atmospheric structure• Count ejected molecules
• Equivalent to solving the Boltzmann equation for a gas• Limiting factors: cross sections and range of densities
+
Used sputter models
Best Fit Energy Distributions
These only give bounds for E > ~ 0.2eV
Maxwellian + Analytic
Kappa Function
Fits to Hot Corona (De La Haye et al. 2007) Tx Energy Deposition Escape Flux (K) (eV/cm3/s) (109 amu/cm2/s)
TA ingress 150 100 1.5 ( <18)
egress 157 78 1.1 (<14)
TB egress 149 290 4.0 (<48)
T5 ingress 162 ~0 ~0
egress 154 60 0 .9 (<12)
0.2 (<5) x1010 amu/cm2/s (DeLaHaye et al. 2007) 4-5 x1010 amu/cm2/s (Yelle et al. 2007)* 5 x1010 amu/cm2/s (Strobel 2007)
CH4 Escape 1/7 the photo destruction rate (Yelle et al 2007)Total atmospheric mass lose present atmosphere in ~4.5Gyr
Atmospheric Loss Rate
• 0.2 - 5 x1010 amu/cm2/s (DeLaHaye et al. 2007)• 5 x1010 amu/cm2/s (Strobel 2007)• 4-5 x1010 amu/cm2/s (Yelle et al. 2007).
INMS EXOSPHERE DATA De La Haye et al. 2007
Therefore: Invert data
Simulate the corona get best fit energy spectrum Power Law ~E-x
Kappa Distributions
Obtain heating rate
Energetic Neutrals Image Part of the CoronaH+ (10’s keV) + H2 H + H2
+
(MIMI Instrument: I. Dandouras et al,)
Plasma is variable but not unlike Voyager (Hartle et al. 2006)
Area 2 x10^18
• Sillanpaa O+ 4 x10^9eV/cm^2/s global• Teng (Pick-up) 5.6 x10^7• Ledvina 5.6 x10^8/cm^2/s
Incident Flux
~16 amu (< ~ 0.75 keV) --> O+ (CHx+,N+)
~28 amu ( < ~1.25 keV) --> N2+ (HCNH+,C2H5
+)
Energy Flux
EUV ~ 2 x1010 eV/cm2/s Plasma ~1.5-0.5 x1010 eV/cm2/sEnergetic Ions ~0.5 x1010 eV/cm2/s
Sillanpaa et al 2007; Ledvina 2007; Michael et al 2005
Incident Ions N (x1025 s-1) N2 (x1025 s-1) Net N as N and N2
(x1025 s-1)O+ , N2
+ 2.5 0.7 3.9
Model Global Average Escape Rates
Escape of N atoms as N or N2 is ~ 4x1025 N s-1
Flux = 2 x107 N/cm2/s~ 10% CH4 ~ 2 x106 /cm2/s
~ 10% H2 ~ 2 x107 /cm2/s
corresponds to < 1% of present atmosphere in 4GyrFor comparison
If Io had a Titan like atmosphereLose ~ 100% in 0.14 Gyr
(Johnson, 2004)
Incident Ions Energy Flux Net Ejecta
(14 + 28 amu)
O+, N2+ ~5x109eV/cm2/s ~6 x1026 amu/s
3 x108 amu/cm2/s
Model Global Average Escape Rates
Flux ≈
Plasma ions (14, 28)
>10keV H+
Ledvina, Tucker
exobase
Average Energy Deposition
UV+EUV
>10keV O+
Ledvina, Tucker
UV-EUV ~ 2 x1010 eV/cm2/s Plasma ions ~0.4 (1.5) x1010 eV/cm2/sEnergetic Ions (>10keV) ~0.5 x1010 eV/cm2/s Sillanpaa et al 2007; Ledvina 2007; Michael et al 2005
Effects
• Chemistry: dissociation, ionization & O+ implantation
• Heating• Atmospheric loss: thermal & nonthermal
Source for MagnetosphereEvolution of atmosphere
Goal: accurately describe escape processes
Simulations Energy spectra of N2 in the Transition Region & Corona
Thermal core + suprathermal tail
Below exobase
Above exobase
Hot N2 populates corona
Some Energy Deposition Rate Estimates
Smith et al 2009
Luna et al 2005
Shah et al 2009
Shah et al 2009
Michael et al 2005
Strobel 2009
In Plasma Sheet Thermal & Pick-up
UV/EUV Solar med.
Mimi O+ H+ max
H+ O+ Mimi Median