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Session 70 - Planetary Systems Near & Far.
Display session, Friday, January 09
Exhibit Hall,

[70.04] Doppler Measurement of Gas Dynamics -- Analysis of Beam Integrated Velocity Field Spectra from Planets and Astrophysical Sources

T. Hewagama, J. Goldstein (CCSSE), D. Buhl, F. Espenak, T. Kostiuk (GSFC), K. Fast, T. Livengood (GSFC and UMD)

Infrared Heterodyne Spectroscopy observations of ro-vibrational emission line spectra of CO_2 in Mars and Venus and of C_2H_6 in Titan have been used to study molecular abundances and atmospheric dynamics (to 2 ms^-1). These investigations were limited by the lack of a beam-integrated radiative transfer algorithm. Typically, a mean viewing angle was assumed in the analysis whereas the spectrometer FOV contains a range in velocities and viewing angles weighted by the spectrometer response. A beam-integrated spectrum is therefore quantitatively different from individual spectra at arbitrary viewing angles. For example, the 1'' FOV of the NASA/IRTF is comparable to the size of Titan's disc and the observed beam includes dynamical contributions from all regions of the disc intercepted by the beam. These differences affect molecular abundance and wind field retrievals.

We have developed analysis software which models beam-integrated observations. Our model is characterized by an effective beam response, molecular abundance, and a wind field. The model is numerically implemented by binning the beam into a grid of elements, (2) for each element, calculating the mean viewing angle and the corresponding emergent spectrum using a radiative transfer algorithm, and (3) constructing a beam-integrated spectrum by convolving the beam element spectra with the beam model. The Doppler shift due to the wind field is modeled by applying an appropriate frequency shift in each element. Using experimental errors as weights, a \chi^2 statistic characterizes the differences between the modeled and observed spectra, and a non-linear search algorithm is used to investigate the parameter space. Results from the application of the software to dynamical studies of the atmospheres in Jupiter and Titan will be discussed. These analysis techniques are general and have potential application to other astrophysical sources.

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