Comparative Model Simulations of Venus and Mars Upper Atmospheres

1. Model Simulations of the Upper Atmospheres of Venus and Mars: Processes Regulating Solar Cycle Variability Stephen Bougher (Michigan) Amanda Brecht Tami McDunn Arnaud Valeille Jared Bell (SwRI)

2. Outline • Review of Venus and Mars thermosphere-ionosphere processes (similarities and differences) • Summary of datasets reflecting Texo for Venus and Mars • Survey of Venus and Mars upper atmosphere global models • Capabilities and limitations of the modern VTGCM and coupled MGCM-MTGCM framework • Validation of VTGCM and MTGCM over the solar cycle: • Solar cycle dayside Texo variability • Dayside ionospheric structure variations (near peak) • Comparison of dayside planet thermal balances • Conclusions and comparisons

3. Venus and Mars Upper Atmospheric Regions and Processes

4. Similarities in Processes • Similar neutral-ion chemistry impacting thermosphere and lower ionosphere structure (e.g. Fox and Sung, 2001). • Dayside PCE ionosphere is dominant near the ionospheric peak and upwards to ~180-200 km for each planet. • Similar EUV-UV heating efficiencies of ~21±2% (Fox, 1988; Fox et al., 1995; Huestis et al., 2008). • IR radiator at the base of each thermosphere is CO2 15µm emission (e.g. Bougher et al. 1994; 1999; 2000; 2009) • Global winds play a significant role controlling upper atmosphere temperatures on the Venus nightside and Mars dayside plus nightside (Bougher et al., 1999; 2000; 2009) • Intermittent neutral heating from particle precipitation is likely on both planets (to be confirmed)

5. Differences in Processes • Larger EUV-UV fluxes at Venus provide more efficient CO2 photolysis, and larger dayside O abundances. Venus: ~7-20%. Mars: ~1-3% (near their F1-peaks). (e.g. Bougher et al. 1999). • CO2 15µm emission is more efficient at Venus than Mars (above), thereby acting as a thermostat to changing solar energy deposition (Bougher et al. 1999; Mueller-Wodarg et al. 2008) • Vertical winds are stronger at Mars than Venus (same horizontal winds), owing to smaller Mars radius (Bougher et al., 2000). • Mars planetary rotation and seasons (like Earth) provide global thermospheric wind patterns very different from Venus (SS-AS) (Bougher et al. 1997; 2006) • Lower boundary forcing: Venus (RSZ flow); Mars (global or regional dust storms). Both have tides and planetary waves.

6. Measured/Modeled Venus Dayside-Nightside Exospheric Temperatures(Kasprzak et al., 1997) ΔT/ΔF10.7~ 0.4 Nearly Constant

7. Mars - MGS Ls variation removed Earth - msise90 40o latitude Earth-Mars Comparative Response to Long-Term Changes in Solar flux (Forbes et al., 2007) Mars is ~36% - 50% as responsive to solar flux received at the planet, compared to Earth, consistent with Forbes et al. (2006)

8. Other Mars Dayside Exospheric Temperature Measurements

9. MRO Nightside and MGS2 Dayside Temperature Profiles Derived from Accelerometers (Keating et al., 2008; 2009) Texo ~ 130-145ºK Texo ~ 190-200ºK 7

10. Venus Upper Atmosphere GCMs • VTGCM (e.g. Bougher et al., 1988-1997; Brecht et al. 2007-2009): • Modern thermosphere-ionosphere-mesosphere model spanning ~70-250 km • Used to interpret PVO (SMAX) and VEX (SMIN) measurements • Oxford/LMD: V-GCM (development)

11. Modern VTGCM Formulation and Structure • Altitude range: ~70-250 km (dayside). Pbot ~ 44.0 mbar • 5x5º latitude-longitude grid (pole-to-pole) • Pressure vertical coordinate (1/2-H intervals): 69-levels • Major Fields: T, U, V, W, O, CO, N2, CO2, Z • PCE ions Fields: CO2+, O2+, N2+, NO+, CO+, O+, Ne • Current Minor Fields: O2, Ar, N(4S), N(2D), NO, OH • Current Nightglow: NO* UV and O2* IR (1.27 µm) • Fox & Sung (2001) ion-neutral chemical reactions & rates. • O, CO, O2, NOx sources and losses explicitly calculated. • Telec (empirically based) from Theis and Brace [1993]. • NLTE CO2 15-micron reference cooling and near IR heating rates adapted from Roldan et al. (2000).

12. Modern VTGCM Texo Over the Solar Cycle(adapted from Bougher et al., 2008) Texo ~ 240-300ºK ΔT/ΔF10.7 ~ 0.4

13. Modern VTGCM Texo: SMIN/VEX Conditions at 190 km Tmax = 242ºK Umax = 310 m/s

14. VTGCM Thermal Balances(SZA ~ 0; SMIN Conditions, VEX) Texo ~ 240ºK

15. Solar Cycle Variations at theIonospheric Peak (SZA ~ 0)

16. Mars Upper Atmosphere GCMs • MGCM-MTGCM (e.g. Bougher et al., 2001-2009): • Flux coupled separate models spanning 0-300 km • NCAR (TIGCM) and NASA Ames (MGCM) heritage. • Mars Whole Atmosphere Climate Model (MWACM) (e.g. Bougher et al; 2007-2009) • Ground to exosphere code (0-300 km). Earth GITM heritage. • Under development • LMD-GCM (e.g. Angelat-i-Coll et al; 2004; Gonzalez-Galindo et al., 2005-2009) • Ground to exosphere code (0-240 km) • LMD/AOPP MGCM heritage; LMD/IAA teaming. • ASPEN (e.g. Crowley et al. 2004-2009) • Troposphere to thermosphere (14-300 km) • NCAR TIME-GCM heritage • GM3 (e.g. Moudden et al., 2004-2009) • Ground to thermosphere code (0-160 km) • Canadian MET model heritage.

17. MTGCM Formulation and Structure • Altitude range: ~70-300 km (dayside). Pbot=1.32-microbar • 5x5º latitude-longitude grid (pole-to-pole) • Pressure vertical coordinate (1/2-H intervals): 33-levels • Major Fields: T, U, V, W, O, CO, N2, CO2, Z • PCE ions Fields: CO2+, O2+, N2+, NO+, CO+, O+, Ne • Current Minor Fields: O2 and Ar • Current Nightglow: O2 IR (1.27 µm) • Future NOx Fields: N(4S), N(2D), NO (NO nightglow) • Fox & Sung (2001) ion-neutral chemical reactions & rates. • O, CO and O2 sources and losses explicitly calculated. • Tion and Telec (empirically based) from Fox [1993]. • NLTE CO2 15-micron cooling scheme and near IR heating rates adapted from M. Lopez-Valverde (pc. 2000)

18. Coupled MGCM-MTGCM • Flux coupled codes: NASA Ames MGCM (0-90 km) and NCAR- Michigan MTGCM (~70-300 km), linked across an interface at 1.32-microbars on a regular 5x5º grid. • Fields passed upward at interface (T, U, V, Z) on 2-min time-step intervals. No downward coupling enabled. • Coupling captures upward propagating migrating & non-migrating tidal oscillations, as well as in-situ solar EUV-UV-IR heating (migrating tides). • Ls = 90 (Aphelion), Ls ~ 180 (Equ), Ls = 270 (Perihelion) • Empirical TES horizontal dust distributions (LAT vs LON). • Conrath parameter scheme used to specify vertical dust distributions (mixed to ~20-60 km). • Circulation sensitive to vertical dust dist. (Bell et al. 2007)

19. Measured/Modeled Mars Dayside Exospheric Temperatures(Bougher et al., 2009) Green = MGS drag (Forbes et al. 2008) -- Periapsis ~ 370 km -- LT = 14 -- 40-60ºS Red = MTGCM (19% EUV eff.) Blue = MTGCM (22% EUV eff.)

20. Atomic O Based Cold + Hot Coronal Temperatures (Equinox, SMAX, SZA ~ 60º) (Valeille et al. 2009) Thot ~ 4600ºK (above ~700 km) Tcold ~ 300ºK (below ~400 km)

21. MTGCM Solar Cycle + Seasonal Variations: Ls =180 (SMIN, SMAX) , Ls = 90 (SMIN) to Ls = 270 (SMAX) T~ 185K T~165K T ~ 295K T~315K

22. MTGCM Thermal Balances: SZA ~0; Ls = 180; SMAX Conditions) Texo ~ 295ºK

23. Solar Cycle Variations at theIonospheric Peak (Ls ~ 180; SZA ~ 0)

24. Conclusions • Larger dayside O abundances in the Venus thermosphere determine that CO2 15µm emission is more efficient than at Mars, thereby acting as a thermostat to changing solar energy deposition (e.g. Bougher et al. 1994; 1999). • This renders Venus ~20-25% as responsive to changing solar fluxes received at the planet (over the solar cycle) compared to Mars (e.g. Forbes et al., 2008; Bougher et al., 2009). • Molecular thermal conduction and variable global winds are very important for controlling the Mars dayside upper atmosphere temperatures, unlike Venus (e.g. Bougher et al., 2009). • Must consider solar cycle + seasonal variations together when addressing Mars Texo. In addition, LAT is crucial to exclude winter polar warming features (e.g. MO) (Bougher et al., 2006). • Sub-solar ionospheric peak densities roughly vary as √Δ solar flux (over the solar cycle) as expected/observed. • SMAX conditions at Mars need to be better measured to constrain available models that address solar cycle responses by the planet’s thermosphere-ionosphere (e.g. MAVEN). • Atomic oxygen abundances at Mars need to be measured in-situ.