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Interferometric observations of the J(0,1) CO line on Venus: upper Mesospheric winds and CO abundance.

Citation

Shah, Kathryn Pierce (1992) Interferometric observations of the J(0,1) CO line on Venus: upper Mesospheric winds and CO abundance. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:10042011-095138905

Abstract

In 1988, we observed Venus with the millimeter interferometer at the Owens Valley Radio Astronomy Observatory at 115.2712 GHz, the first rotational transition of ^(12)CO. The 33."4 diameter disk was spatially resolved by a synthesized beam with a full-width-half-maximum of 2."8. Local time ranged from afternoon on the planetary eastern limb, 2 PM, to just past local midnight on the western limb, 12:30 AM. Venus' millimeter continuum emission was measured in a 400 MHz broadband channel. More importantly, the CO absorption line was measured and finely resolved in frequency by two 32-channel filterbanks having channel widths of 50 kHz and 1 MHz.

The 400 MHz broadband channel visibility data yielded a continuum brightness map of Venus. Brightness variations across the disk were primarily caused by sidelobe beam effects associated with the incomplete (u,v) coverage of the OVRO array. However, after radial averaging, the continuum brightness map continued to show one significant trend - a nightside limb cooler than the dayside limb. The continuum channel has a weighting function which samples a wide layer in the atmosphere between 40-60 km at normal incidence and 55-75 km along the planet limb. A radiative transfer model roughly reproduced the increased nightside limb-darkening, when given a mesospheric temperature profile 40 K cooler than the nominal Pioneer Venus nightside temperature profile between 60-85 km. The strength of this result is undercut-by strong uncertainties associated with the continuum map's absolute intensity values due to the small number of baselines in the OVRO array.

The 50 KHz filterbank resolved the inner core of the CO absorption line. This yielded the first measurement of doppler shifts across Venus due to strong winds in the upper mesosphere. It is the most important result presented in the thesis. Calculated weighting functions showed sampling of the mesosphere over a 12 km layer centered at roughly 99 km. The doppler shifts have a signature which matches westward, horizontal winds - being strongly "blue" on the east/dayside limb, zero near the center and strongly "red" on the west/nightside limb of the planet. Smoothed wind measurements were best fitted in a least squares sense for a mean zonal flow of 132±10 ms^(-1). A smaller (≤40 ms^(-1))subsolar-to-antisolar flow may have been superimposed on the dominant zonal flow in 1988. These measurements indicate either a reversal of the mesospheric cyclostrophic breakdown inferred by Pioneer Venus or the influence of uninvestigated dynamical forces.

The 1 MHz and 50 kHz filter bank spectra were merged to look at the entire 115 GHz CO line. These 1988 CO spectra show a decided local time dependency, becoming progressively deeper from the afternoon to the evening hours. A constrained least-squares inversion algorithm was used to solve for the local CO mixing ratio profile over local time and latitude. The resultant CO mixing ratio remains constant with height at a value of several 10^(-5) in the late afternoon hours but increases from 10^(-4) at 80 km to 10^(-3) at 100 km in the night hours. The highest CO abundances occurred after local 10 PM and centered about the equator between 40°N and 40°S. This distribution of CO abundance fulfills predictions from research based on disk-average CO spectra and photochemical models. Only the late afternoon profiles are surprising, showing essentially little CO rather than an expected moderate CO abundance from dayside photodissociation of CO_2.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Planetary Science and Geology
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Planetary Science
Minor Option:Geology
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Muhleman, Duane Owen
Thesis Committee:
  • Ingersoll, Andrew P.
Defense Date:16 September 1991
Record Number:CaltechTHESIS:10042011-095138905
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:10042011-095138905
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6703
Collection:CaltechTHESIS
Deposited By: Dan Anguka
Deposited On:04 Oct 2011 17:52
Last Modified:25 Jan 2013 23:02

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