Modeling of Titan atmosphere/ionospheric chemistry : Modeling of Titan atmosphere/ionospheric chemistry Marek Banaszkiewicz
Slide2 : Introduction: Saturn magnetosphere and Titan´s magnetospheric interaction.
Theoretical approach: ionization by EUV photons and by Saturnian magnetospheric electrons.
Chemistry.
Model results: neutral atmosphere,
EUV fluxes,
Sub-ram and Solar zenith angle.
Summary and Conclusions.
Slide3 : Saturn magnetosphere and Titan magnetospheric interaction
Titan has no intrinsic magnetic field, but Saturn has an important magnetic field, with its magnetic axis aligned (within 1o ) with the rotation axis .
Titan “lives” in/out the Saturnian magnetosphere, very close to the magnetopause (at 21 RS). If the solar wind pressure is high, the magnetospheric boundary moves toward Saturn and Titan appears in the magnetosheath region of compressed solar wind plasma the interaction of the charged particles in Titan’s atmosphere with this variable environment can be extremely complex and accounting for all of the effects is rather difficult. Additionally, there is (on/off, i.e. day/night) ionization due to the solar EUV radiation.
Slide4 : Pattern of magnetic lines piled up near the subram point on the leading side of Titan and then turn gently toward the flanks of the planet.
Magnetospheric plasma, which moves along magnetic field lines, interact with the neutral atmosphere and contributes to the production of ionospheric species. Comparison of the (a) Venus-type solar wind interaction and (b) the magnetospheric interaction of Titan.
Slide5 : Ambipolar diffusion determines the transport processes of ions. In the case of Titan’s upper ionosphere, and for the subram configuration of B we apply, the ambipolar diffusion would be perpendicular to the B and not very effective vi can be neglected. Study of steady-state solutions
ni = Pi/li
Chemical equilibrium for ions, that is, the number density of ions at each atmospheric level is the quotient between the production (cm-3 s-1) and the specific loss (s-1).
Slide6 : Geometry assumed in the computations:
(i) The subram angle ,
(ii) the solar zenith angle
(iii) the observer-subram distance
1.- relation: + =
(previous models assumed = )
2.- photoionisation is calculated
for a given as a function of
height
3.- photoelectron production is
calculated along
parabolae (shadow included)
Slide7 : Ionization by EUV and UV solar photons:
spectral bins and lines (*) employed
three different models of solar flux tested: (a) Richards (1994), (b) Galand et al. (1999), (c) Keller, Gan & Cravens (1992) and
various solar zenith angles. Ionization by secondary electrons: photoelectrons generated along a magnetic field line ionise neutral atmospheric components.
Slide8 : Ionospheric chemistry: How to understand it?? 1) N2 reacts only with H2+, H+, NH+. 2) CH4 reacts with Hy+, CHx+ (x<5), and C2Hx+ (x<4) but not with heavier hydrocarbon ions. 3) CH4 reacts with N+, N2+, NxHy+, HCN+, HC3N+,
but not with HCNH+. 4) C2H2 is less reactive than C2H4 in reactions with nitrilic
ions, but more reactive in reactions with hydrocarbon ions. 5) C2H6 is slightly less reactive than C2H2 and C2H4. 6) HCN reacts very fast with H+ and H3+, fast with HCN+
and CH4+, and slow with N+, N2+ and CHx+. 7) Heavy ions rather associate than dissociate in reactions
with neutral constituents.
Slide9 : Chain of reactions CH4 N2 CH2+ CH4+ N+ CH5+ N2+ C2H5+ CxHy+ HCNH+ CxHyNz+ C3H2+ CH3+ HC3N+ HCN+ C4H2+ C4H3+ C4H4+ C2H6+ C3H3+ C3H5+ C2H4+ C2H3+ C2H2+ N2H+ CH3+ C2H4 Ionisation C2H4 C2H4 Seed ions Main neutrals Less abundant ions Most abundant ions
Slide10 : Atmosphere-Ionosphere Coupling Estimates: neutral-neutral reaction rate k 10-11 cm3 s-1,
ion-neutral reaction rate k 10-9 cm3 s-1 P = k n1 n2 Methane case But, even major neutrals can be influenced by reactions with ions,
provided that the competing neutral reactions are not very efficient:
Slide11 : Results 1) Flux of magnetospheric electrons and photoelectron flux.
calculated in a two stream approximation along each magnetic field line (parabola) from z = 725 km to 2425 km (step 100 km),
(ii) processes: ionisation of neutral components, photoelectron production, elastic and inelastic collisions (energy degradation),
(iii) boundary conditions: a Maxwellian distribution of in-going electron fluxes at the end of parabolas.
Slide12 : 2) Production rate of ions as a function of altitude:
(i) calculated explicitly for N2 and CH4 (photoionisation and electron impact)
(ii) calculated from electron-impact cross sections for other neutral species
(iii) calculated from photoionisation rates and N2 + CH4 production for other neutral components
Slide13 : 3) N2+ and CH4+ production rate by photoionisation and by magnetospheric electrons and photoelectrons.
Solar zenith angle 0o, subram angle 15o, EUV flux by Keller, Gan and Cravens (1992).
Slide14 : 4) N2+ and CH4+ production rates by photoionisation and by magnetospheric electrons and photoelectrons. EUV & UV flux from Richards (1994). sza=0o subram=30o sza=30o subram=30o sza=45o subram=30o sza=60o subram=30o sza=90o subram=30o sza=75o subram=30o
Slide17 : Models without and with magn. electrons Ne=5300 cm-3 Ne=5800 cm-3
Slide18 : 4) Neutral atmosphere: hydrocarbons.
EUV & UV Richards (1994) 000: subram=sza=0o
105: subram=0o, sza=15o
545: subram=60o, sza=75o
Slide19 : Methane case 000: subram=sza=0o
105: subram=0o, sza=15o
545: subram=60o, sza=75o
206: subram=0º, sza=30º
Lara et al. (1996): sza=30º and ionospheric chemistry off CH4 lifetime in the upper atmosphere is ~108 s time dependent models are required.
Slide20 : 4) Neutral atmosphere: oxygen and nitrogen compounds.
EUV & UV Richards (1994) 000: subram=sza=0o
105: subram=0º, sza=15o
545: subram=60o, sza=75o
Slide21 : 4) Neutral atmosphere: hydrocarbons
EUV & UV from Keller, Gan and Cravens et al (1992) 000: subram=sza=0o
105: subram=0, sza=15o
426: subram=30o, sza=60o
Slide22 : 4) Neutral atmosphere: oxygen and nitrogen compounds
EUV & UV Keller, Gan and Cravens (1992) 000: subram=sza=0o
105: subram=0, sza=15o
426: subram=30o, sza=60o
Comparison with observations : Comparison with observations
Sensitivity study (1) : Sensitivity study (1) Pascal Pernot - MC approach
Alternative - systematic variation of reaction rates
nik = ni(Rk+Rk) - ni(Rk) , i=1,..,60, k=1,...,600
Program is ready, the results have been obtained.
First conclusions: two groups of ions that differ in sensitivity wr.
to reaction rates. For instance H+, H2+ are generally
insensitive, but the major hydrocarbon and nitrilic ions
are - CH5+, C2H5+, C3H5+, HCNH+, HCN+
The main reactions: N+ + N2, CH3+ + CH4, CHCCNH+ + HCN
N+ + CH4, C2H5+ + C2H2, N2H+ + CH4
N2+ + CH4, CH4+ + CH4, CH2+ + CH4
Sensitivity study (2) - 700 km : Sensitivity study (2) - 700 km ION-NEUTRAL BIMOLECULAR REACTIONS IN THE TITANS ATMOSPHERE
2 103 152 CH3+ + CH4 ==> C2H5+ + H2 1.10E-09
19 140 202 CH5+ + C2H2 ==> C2H3+ + CH4 1.48E-09
22 192 272 C2H3+ + CH4 ==> C3H5+ + H2 1.90E-10
8 214 303 C2H5+ + C2H2 ==> C2H5+ + C2H2 7.11E-10
21 217 306 C2H5+ + C2H4 ==> C3H5+ + CH4 3.55E-10
4 222 311 C2H5+ + HCN ==> HCNH+ + C2H4 2.70E-09
13 286 413 C3H5+ + C2H2 ==> C5H5+ + H2 3.80E-10
9 310 471 C4H3+ + C2H2 ==> C6H5+ + 2.20E-10
18 318 501 C5H5+ + C2H2 ==> C7H7+ + 3.10E-11
25 319 502 C5H5+ + C3H4 ==> C6H7+ + C2H2 2.80E-10
10 325 521 C6H5+ + CH4 ==> C7H7+ + H2 7.50E-11
23 338 551 C6H7+ + C3H4 ==> C7H7+ + C2H4 5.00E-10
16 340 563 C7H7+ + C3H4 ==> + H2 2.10E-10
15 341 564 C7H7+ + C3H4 ==> + 2.40E-10
20 342 565 C7H7+ + C4H2 ==> + 5.00E-10
11 344 572 N+ + CH4 ==> CH3+ + X 6.58E-10
17 347 575 N+ + CH4 ==> HCNH+ + H2 4.62E-10
1 361 589 N+ + N2 ==> N+ + N2 2.55E-10
12 402 661 N2+ + H2 ==> N2H+ + H 2.00E-09
24 403 662 N2+ + CH4 ==> CH2+ + H2 N2 7.98E-11
5 404 663 N2+ + CH4 ==> CH3+ + H N2 1.03E-09
14 429 691 N2H+ + CH4 ==> CH5+ + N2 8.90E-10
7 505 801 HCNH+ + C4H2 ==> C4H3+ + HCN 1.60E-09
6 508 804 HCNH+ + HC3N ==> CHCCNH+ + HCN 3.40E-09
3 553 871 CHCCNH+ + C2H4 ==> + H 1.30E-09
Sensitivity study (3) - 1050 km : Sensitivity study (3) - 1050 km ION-NEUTRAL BIMOLECULAR REACTIONS IN THE TITANS ATMOSPHERE
21 94 132 CH2+ + CH4 ==> C2H4+ + H2 9.10E-10
2 103 152 CH3+ + CH4 ==> C2H5+ + H2 1.10E-09
18 104 153 CH3+ + C2H2 ==> C3H3+ + H2 1.15E-09
17 125 182 CH4+ + CH4 ==> CH5+ + CH3 1.14E-09
13 140 202 CH5+ + C2H2 ==> C2H3+ + CH4 1.48E-09
12 141 203 CH5+ + C2H4 ==> C2H5+ + CH4 1.50E-09
19 192 272 C2H3+ + CH4 ==> C3H5+ + H2 1.90E-10
20 200 280 C2H3+ + HCN ==> HCNH+ + C2H2 2.30E-09
5 214 303 C2H5+ + C2H2 ==> C2H5+ + C2H2 7.11E-10
11 217 306 C2H5+ + C2H4 ==> C3H5+ + CH4 3.55E-10
4 222 311 C2H5+ + HCN ==> HCNH+ + C2H4 2.70E-09
15 286 413 C3H5+ + C2H2 ==> C5H5+ + H2 3.80E-10
6 344 572 N+ + CH4 ==> CH3+ + X 6.58E-10
9 347 575 N+ + CH4 ==> HCNH+ + H2 4.62E-10
1 361 589 N+ + N2 ==> N+ + N2 2.55E-10
8 402 661 N2+ + H2 ==> N2H+ + H 2.00E-09
10 403 662 N2+ + CH4 ==> CH2+ + H2 N2 7.98E-11
3 404 663 N2+ + CH4 ==> CH3+ + H N2 1.03E-09
24 405 664 N2+ + CH4 ==> N2H+ + CH3 3.42E-11
7 429 691 N2H+ + CH4 ==> CH5+ + N2 8.90E-10
25 435 697 N2H+ + HCN ==> HCNH+ + N2 3.20E-09
16 493 783 HCN+ + CH4 ==> HCNH+ + CH3 1.14E-09
14 505 801 HCNH+ + C4H2 ==> C4H3+ + HCN 1.60E-09
23 508 804 HCNH+ + HC3N ==> CHCCNH+ + HCN 3.40E-09
22 553 871 CHCCNH+ + C2H4 ==> + H 1.30E-09
Sensitivity study (4) - 1400 km : Sensitivity study (4) - 1400 km ION-NEUTRAL BIMOLECULAR REACTIONS IN THE TITANS ATMOSPHERE
12 94 132 CH2+ + CH4 ==> C2H4+ + H2 9.10E-10
20 95 133 CH2+ + CH4 ==> C2H5+ + H 3.90E-10
1 103 152 CH3+ + CH4 ==> C2H5+ + H2 1.10E-09
21 104 153 CH3+ + C2H2 ==> C3H3+ + H2 1.15E-09
3 125 182 CH4+ + CH4 ==> CH5+ + CH3 1.14E-09
24 134 191 CH4+ + HCN ==> HCNH+ + CH3 3.23E-09
11 140 202 CH5+ + C2H2 ==> C2H3+ + CH4 1.48E-09
14 141 203 CH5+ + C2H4 ==> C2H5+ + CH4 1.50E-09
22 164 233 C2H2+ + CH4 ==> C3H5+ + H 7.03E-10
13 192 272 C2H3+ + CH4 ==> C3H5+ + H2 1.90E-10
25 200 280 C2H3+ + HCN ==> HCNH+ + C2H2 2.30E-09
16 214 303 C2H5+ + C2H2 ==> C2H5+ + C2H2 7.11E-10
23 217 306 C2H5+ + C2H4 ==> C3H5+ + CH4 3.55E-10
5 222 311 C2H5+ + HCN ==> HCNH+ + C2H4 2.70E-09
6 344 572 N+ + CH4 ==> CH3+ + X 6.58E-10
17 345 573 N+ + CH4 ==> CH4+ + N 1.40E-10
18 346 574 N+ + CH4 ==> HCN+ + H2 H 1.40E-10
7 347 575 N+ + CH4 ==> HCNH+ + H2 4.62E-10
4 361 589 N+ + N2 ==> N+ + N2 2.55E-10
10 402 661 N2+ + H2 ==> N2H+ + H 2.00E-09
9 403 662 N2+ + CH4 ==> CH2+ + H2 N2 7.98E-11
2 404 663 N2+ + CH4 ==> CH3+ + H N2 1.03E-09
19 405 664 N2+ + CH4 ==> N2H+ + CH3 3.42E-11
8 429 691 N2H+ + CH4 ==> CH5+ + N2 8.90E-10
15 493 783 HCN+ + CH4 ==> HCNH+ + CH3 1.14E-09
Results; an example : Results; an example
What one can do with the model? : What one can do with the model? Add N2++ and its reactions; but only two rates known for sure and there is Lilenstein etal study (2004)
Introduce hot ion physics/chemistry
Invert the results with respect to the reaction rates considered as free parameters