S. Bougher and A. Brecht (U. of Michigan) S. Rafkin and A. Stern (SwRI-Boulder)

1. VTGCM and Applications to VEX and PVO Data Analysis:Upgraded Simulations (F10.7 ~70 & 200) Venus Express Science MeetingThuile, ItalyMarch 18-24, 2007 S. Bougher and A. Brecht (U. of Michigan) S. Rafkin and A. Stern (SwRI-Boulder) Contact: rafkin@boulder.swri.edu or bougher@umich.edu This work was funded in part by the NASA Venus Express Participating Scientist Program under grant NNG06GC76G.

2. Upper Atmosphere Datasets at Venus • Pioneer Venus Orbiter (1979-1992) -- F10.7 ~ 200 (in-situ at start) and 130 (entry at end) -- OUVS (airglow, neutral density and T structure) -- ONMS (neutral density and T structure) -- OETP (electron T and density) -- RPA (ion T and density) • Venus Express (June 2006 to date) -- F10.7 ~ 70-90 (primary mission) -- SPICAV (airglow, neutral density and T structure) -- VeRA (radio occultation; T-structure; electron density profiles) -- VIRTIS (O2 airglow, T structure, inferred winds) -- VMC (visible and IR camera, O2 airglow)

3. Venus Atmosphere Regions and Processes

4. Venus Thermospheric General Circulation Model (VTGCM) Formulation and Structure • Altitude range: ~94-200 km (day); 94-150 km (night) • 5x5º latitude-longitude grid (pole-to-pole) • Pressure vertical coordinate (1/2-H intervals): 34-levels • Major Fields: T, U, V, W, O, CO, N2, CO2, Z • PCE ions Fields: CO2+, O2+, N2+, NO+, O+, Ne • Minor Fields: O2 (and O2 IR nightglow at 1.27 m) • Future Minor Fields: N(4S), N(2D) (and NO nightglow) • Full 2-hemispheric capability. Timestep = 60.0 secs. • Venus obliquity ~ 177.4º (“seasonal” cases possible) • Rayleigh friction used to slow SS-AS winds. • Weak RSZ winds prescribed at model lower boundary • Upgraded airglow capability: O2-IR, NO-UV(future)

5. VTGCM Science Goals in Support of VEX • Compare the modeled structure of the atmosphere (up to 180 km) with high vertical resolution solar/stellar occultations, especially in middle to high latitudes; • Use observed spatial distributions of NO, O, and H airglows, at spatial resolutions substantially better than existing data, and interpret these global tracers of the thermospheric circulation with the VTGCM; • Determine the dynamical processes that link the Venus middle and upper atmospheres (tides, planetary, and gravity waves, etc.) through general circulation modeling of the thermosphere; • Compare and contrast the response of the Venus atmosphere with that of Mars—where the sister spacecraft (Mars Express) and SPICAM is in orbit, and where the Mars Reconnaissance Oribiter was simultaneously aerobraking in 2006—under the same solar cycle forcing.

6. Input Parameters for VTGCM Simulations: VEX & PVO • F10.7 ~ 70, 130, 200. Solomon UV routine (0.1-175.0 nm). • Factor for heliocentric distance = 1.914. Equinox (only). • Q-Efficiency (EUV, UV) = 20, 22% (after Fox, 1988) • Slant column integration for Chapman functions. • K(O-CO2) = 3.0 x 10-12 cm3/secat 300K • CO2 15- m cooling scheme from Bougher et al., (1986). • Kzz≤ 1.0-3.0 x107 cm2/sec. Prandtl # = 10.0 • Fox & Sung (2001) ion-neutral chemical reactions & rates. • O & CO sources and losses explicitly calculated. • Te from Theis and Brace (1993). Ti = Tn. • No Venus rotation. Static (GM) LBC (densities and T)

7. T+(U,V) at Exobase VEX PVO T<230K U<185 m/s T<300K U<220 m/s

8. Temperature (K) at Equator VEX PVO T(NT)>100K T(DY)<230K T(NT)>100K T(DY)<300K

9. Solar Cycle Texo & T140Variations(LAT = 2.5N; SLT = 1200) Texo ~ 230 to 325K F10.7 ~ 70 to 240 units ΔTexo ~ 95 K * PVO *MGN *VEX? T140 ~ 200 to 240K F10.7 ~ 70 to 240 units

10. U and W at Equator VEX PVO Min: -220 m/s Max: 200 m/s Min: -186 m/s Max: 173 m/s U (m/s) Min: -0.6 m/s Max: 0.45 m/s Min: -0.35 m/s Max: 0.25 m/s W (s-1) (Multiply By SHT for m/s)

11. Concentration (#/cm3) at Equator log10[O] log10[CO] log10[CO2] Nightside Peak: 6.0x1011 cm-3 Nightside Peak: 6.7x1012 cm-3 VEX Nightside Peak: 7.9x1011 cm-3 Nightside Peak: 7.7x1012 cm-3 PVO

12. Electron Density (#/cm3) at Equator(VEX): log10[Ne] Peak [Ne] Density @ ~138 km. [Ne] < 4.0x 105

13. Peak Electron Density Variations(LAT = 2.5N; SLT = 12; ALT ~ 140 km) * PVO *Venera 9-10 *VEX? SMIN = 4.1 x 105 cm3/sec SMAX = 7.0 x 105 cm3/sec

14. Molecular O2 at Equator Molecular O2 IR Nightglow at Equator (log10 (ph/cm3/sec) log10[O2] #/cm3 Peak Intensity: ~0.8-1.0 MR Peak V.E.R. of 7.7x105 ph/cm3/sec @ 108 km.

15. Summary and Conclusions • Upgraded VTGCM codeis operational for 4-major and 1-minor species plus several photochemical ion species. • Production simulations over the solar cycle (and weak Venus seasons) are completed. Comparisons to PVO, Magellan drag (MGN), and VEX datasets are starting. • O2 IR nightglow calculations are available for new comparisons to VEX datasets (when available). • Gravity wave breaking formulations will be soon incorporated into the VTGCM: (a) to provide self-consistent RSZ winds and needed momentum drag, and (b) to produce unique circulation patterns giving rise to variable O2 and NO nightglow intensity distributions.