Multiple fluorescent scattering of N2 ultraviolet emissions in the atmospheres of the Earth and Titan

Abstract

Absorption and reemission by N2 of the Lyman‐Birge‐Hopfield (LBH) and Birge‐Hopfield (BH) band systems in the earth aurora and Titan dayglow has been calculated by using multiple scattering in a plane‐parallel radiative transfer model and a line‐by‐line synthesis of individual bands. The effects of rotational structure are included through the use of temperature‐dependent band escape functions and band transmission functions. Iteration through successive scatterings shows that fewer than ten scatterings are important, even for the thickest bands. The excitation rate caused by multiple scattering is sensitive to the vibrational population distribution of the ambient N2, especially for the BH system in the earth aurora. For a vibrational temperature of 2000 K, the fluorescent scattering contribution at 110 km is 15 times that of a vibrational temperature of 300 K. Comparison of theoretical intensities to rocket observations of the BH (1, 10) band indicates a vibrational temperature of 1000 K, but the corresponding (1, 3) band intensity is brighter than the observation. Scattering on Titan was modeled for both a high‐ and low‐altitude source and for various vibrational temperatures. The LBH system has an enhanced sensitivity to vibrational temperature on Titan because of the rapidly changing photoabsorption cross section of CH4 around 1400 Å. It is argued that the c4′ ¹Σu+ Rydberg bands should have an intensity profile similar to the BH bands. The c4′ (0, 0) band limb brightening inferred from the full‐disk and bright limb spectra observed by Voyager indicates a source around 4000 km, but this is in conflict with the observed intensity peak at 3660 km in the limb scan data.