Beam integrated high-resolution infrared spectra: Accurate modeling of thermal emission from extended clear atmospheres

We describe a technique for analysis of high resolution thermal IR spectra measured with a finite beam (field of view) on the target planetary atmosphere. The model developed allows a beam integrated radiative transfer calculation over the varying line-of-sight velocity, temperature, and species abundance distributed over the field of view, thus recovering information on atmospheric composition, dynamics, and thermal structure. The modeling procedure is comprises four primary aspects: segmentation of the projected observational beam to a grid of beam elements; determining the geometrical properties of the planetary atmospheric region associated with each beam element; calculating the spectrum emergent from the top of the atmosphere of each element; and weighting the ensemble of spectra with the characteristic observational beam response profile and accumulating the contributions. This prescription results in a beam integrated spectrum-a more accurate representation of fully resolved IR rotation-vibration molecular spectra of planetary atmospheres observed with finite aperture telescopes. The method is implemented as a numerical code appropriate for modeling the spectroscopic observations. This paper describes the basic formalism for beam integrated radiation transfer of planetary atmospheres, discusses the salient points of the modeling process, and quantifies limitations. Application of this technique will be illustrated using high resolution atmospheric molecular spectral line measurements of Titan obtained by infrared heterodyne spectroscopy to extract zonal wind field, temperature and C2H6 abundance.