2. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 1 Oscillation of Venus’ Upper Atmosphere* Jeffrey M. Forbes Department of Aerospace Engineering Sciences University of Colorado, Boulder, CO, 80309 USA Alex Konopliv Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA A large (~±30-50%) density oscillation at a period near 9 days is discovered in Venus’ upper atmosphere (164-184 km; 11 o N), based on radar tracking and precise orbit determination of the Magellan spacecraft between 15 September 1992 and 24 May 1993. *Geophysical Research Letters, in press, 2007

3. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 2 Due to periodic (~7 days) adjustments in periapsis of Pioneer Venus Orbiter, analyses of these data for short-term cyclic variations is problematic Data Source: NASA NSSDC Master Catalog (http://nssdc.gsfc.nasa.gov/database/MasterCatalog?ds=PSPA-00109).http://nssdc.gsfc.nasa.gov/database/MasterCatalog?ds=PSPA-00109

4. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 3 Height and local time variations for Magellan occur over much greater time scales, and thus can be separated from much shorter-period variations height local time density densities every orbit, or  3.2 h

5. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 4 After removal of the height-local time trend, analysis of the residual densities reveals ~9-10 day-oscillation day night

6. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 5 Relative density oscillation during daytime is small Band-pass filtering around 9-10 day period Wave cut-off ~ 0200 LT “Confinement”

7. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 6 Why a 9-day period? Why confinement to 1700-0200 LT? Local time effect? (source, dissipation, mean winds) Temporal effect-- an “event”? It is not possible to answer these questions with the data at hand. Some plausible options are considered.

8. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 7 days longitude zonal wavenumber Within a 9-day period,  constant (slow rotation) Doppler-shifting negligible, i.e., 9 days is “true” wave period (at least for s = 0,1,2). s is indeterminable s = 1  Equatorial zonal phase speed  40 ms -1 (westward) If background U  60-100 ms -1 (westward),  Doppler-shifted phase speeds 20-60 ms -1 (eastward),  vertically-propagating Rossby wave with  30-60 km (Del Genio & Rossow, 1990) s = 0 also possible  zonally-symmetric oscillation Other zonal wavenumbers also mathematically possible

9. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 8 Periodicities in Venus’ atmosphere so far identified between the cloud tops (ca 65 km) and 80 km:  4-day Kelvin wave*  5-day Rossby wave*  2.8-day wave** *(e.g., Del Genio and Rossow, 1982, 1990; Rossow et al., 1990) **(e.g., Del Genio and Rossow, 1982; Apt and Leung, 1982; Covey and Schubert, 1982) Why a 9-day periodicity?

10. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 9 One Possibility Non-linear wave-wave interaction, yielding “sum and difference” secondary waves, similar to established examples in Earth’s atmosphere (e.g., Teitelbaum and Vial, 1991; Pancheva et al., 2000; Pogoreltsev et al., 2002) Possible source for 9-day wave (2.8d, s = 1) X (4.0d, s = 1) --> (1.6d, s = 2) + (9.3d, s = 0) Venus atmosphere GCM (Yamamoto and Tanaka, 1997) (4.0d, s = 1) X (5.7d, s = 1) --> (2.4d, s = 2) + (13.4d, s = 0) Other Possibilities Baroclinic instability or resonance Possible Sources of Excitation

11. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 10 Day-night differences Source of Excitation Differing mean thermal and wind structures between day and night could affect the generation of waves arising from resonance or instability: This remains a possibility. Radiative Damping May be greater during the day when O/CO 2 densities are higher. Molecular Dissipation: Density Scale Height (Day > Night) “Exponential growth” vs. “altitude at which growth ceases due to molecular dssipation”: tend to cancel. Rate of decrease of wave amplitude above the peak: Does favor higher amplitudes during night, but extent unknown due to dependence on vertical wavelength. height Wave amplitude However, none of these can address the “cutoff” near 0200 LT

12. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 11 Schematic of wind profiles in Venus’ Equatorial atmosphere and effects on vertical propagation of several waves Based on model simulations (Bougher et al., 1988) Based on “available knowledge” “smooth transition” between lower and upper atmosphere regimes Only the 9-day wave finds a “window” to Magellan altitudes (~1200 - 2400 LT), and with a cutoff near midnight Critical level wave phase speed = background wind speed

13. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 12 Conclusions A large 9-day density oscillation existed in Venus’ thermosphere during “cycle 4” of the Magellan mission Its origin is unknown, but is plausibly excited by wave-wave interactions Its zonal wavenumber and propagation characteristics are indeterminable from available data Amplitude of the wave is much smaller during daytime Cutoff of the wave after 0200 LT at night is plausibly explained by mean wind filtering Dissipation of the wave could significantly affect the zonal mean circulation. Venus Express may be able to further elucidate this wave, its origins, propagation characteristics, and its role in the dynamics of Venus’ atmosphere.

14. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 13 Additional slides

15. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 14 Night-time Data Densities after normalization to 170 km using Hedin Venus model 11-day running mean (red) 1-day averages of residuals from running mean, every 6 hours, in preparation for band-pass filtering

16. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 15 Daytime Data

17. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 16 Spectrum of Daytime Density Residuals

18. EGU General Assembly 2007 Vienna, Austria, 15 – 20 April 2007 17 Demonstration that periapsis motion of PVO contaminates densities and exosphere temperatures. Spectral analysis reveals 9-day periodicity here, but below the 95% confidence level.