Dynamical structure of Venus' Middle
Atmosphere constrained by Direct Wind Measurements
Venus Express – Venus Climate Orbiter Joint Science Meeting
1-3 July 2009, ISAS, Japan
Context - In the lower mesosphere of Venus (65-85 km), visible observations of Doppler shifts in solar Fraunhofer lines, have provided the only direct Doppler wind measurements near the cloud tops in recent years (Widemann et al., 2007). The region is important as it constrains the global mesospheric circulation in which zonal winds generally decrease with height while thermospheric SS-AS winds increase. Renewed interest in measuring the winds at clouds top from the ground has emerged in the course of the Venus Express mission as well as recent reanalysis of Galileo NIMS near-infrared observations and SSI images (Limaye 2007, Peralta et al. 2007a). On Venus-Express, atmospheric circulation at 70 km (and as well near 50 km) is being measured from cloud tracking by both VIRTIS-M and VMC (Markiewicz et al. 2007, PEralta et al. 2007b).The main purpose of our work is therefore to provide direct wind measurements using visible Fraunhofer lines scattered by Venus’ cloud tops.
Wind determination methods
The Doppler shift measured in solar scattered light on Venus’ dayside results from two instantaneous motions : (1) a motion between the Sun and Venus upper clouds particles ; this Doppler velocity is minimal near Venus sub-solar point ; (2) a motion between the observer and Venus clouds ; this effect is minimal at sub-terrestrial point. The measured Doppler shift, sum of those two terms, varies with planetocentric longitude and latitude. Methods developed in recent planetary wind measurements in the visible range using high resolution spectroscopy e.g. Civeit et al. 2005, Luz et al. 2006, address the problem of maintaining a stable velocity reference during the course of acquisition. At high spectral resolution dispersion law and instrumental effets cannot be considered an absolute velocity frame at an accuracy better than about 100 ms-1, while in measuring the global wind circulation at cloud tops, we are dealing with wind amplitude variations or wind latitudinal gradients on Venus of the order of 5-10 ms-1 projected on the line of sight. Therefore, we need relative Doppler shifts between two sets of absorption lines. Furthermore, the spectral calibration at high resolving powers ($>$ 80000) is naturally fluctuating because of several effects, such as mechanical or thermal variations in the spectrograph and its environment. The measured Doppler shifts using CFHT/ESPaDOnS have individual formal accuracies of 5-10 m s-1, and their signs and magnitudes appear generally consistent with the zonal winds inferred from cloud tracking. We find best-fit values of the equatorial velocity of (155 ± 39) m s-1, (114 ± 20.5) m s-1, (92 ± 14) m s-1 and (104 ± 20) m s-1 for July 2, 3, 4 and Sep. 30, 2007.
Young (1975) discussed a further systematic effect affecting the solar Fraunhofer lines, caused by the finite angular size of the Sun and its rapid equatorial rotation. Points near the terminator of Venus are unequally illuminated by the approaching and receding limbs of the Sun. As a consequence, average solar illumination is apparently shifted due to missing radiation. This effect is of the order of the Sun’s equatorial velocity (2 km s-1), multiplied by the ratio of its angular radius as seen from Venus to the angular distance from the target point to the terminator.
Aspect of Venus on July 4, 3:00 UT during east elongation observing with visible spectrograph ESPaDOnS at Canada-France-Hawaii telescope (Mauna Kea, Hawaii). The complete optical spectrum, from 370 to 1050 nm, is collected over 40 spectral orders at each point with 3-5-s exposures at a resolution of about 80000. Evening terminator is west of central meridian, toward negative RA (Widemann et al., 2008)
Doppler winds on July 3 2007, 1:57-4:34 UT. Point acquisition numbers are indicated (a) top : relative velocity to first spectrum, uncorrected for Young effect. Dotted line : trend on point 1 relative velocities ; (b) bottom : pure-zonal fit to differential wind field corrected for Young effect gives veq = (113.7 ± 20.5) m s-1, an overall fit to the Jul. 3 observations (Widemann et al., 2008)
5n3 CO2 band - 5n3 band of 12C16O2 (00051 in HITRAN database) observed spectrum shows about 30 P and R-branch lines with a folding back in the R branch, a consequence of the decrease of rotational constant B' with vibrational excitation (Widemann et al., 2007)
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