Science Seminars

10/9/2014 Seminar – UV Spectroscopy of the Titan Airglow and Molecular Nitrogen Gas

Speaker: Joe Ajello (JPL/ Caltech)
Date: Friday, Oct 09, 2020
Time: 4:00pm
Location: LSTB 299

Seminar Abstract:

There are four nitrogen-bearing atmospheres in the solar system: Titan, Triton, Earth, and Pluto. The complex UV airglow signature of N2 and its dissociation products from the thermosphere and exobase from these planetary atmospheres are the principal means of studying the solar and magnetospheric energy inputs, chemistry and atmospheric evolution from volatile escape. The present-day response of Titan’s upper atmosphere to the solar-cycle variation of incident solar extreme ultraviolet (EUV) photons, as well as the local magnetospheric particle injection, is a prime focus of the Cassini Mission; and these energy sources are the ultimate source of the lower atmospheric haze and surface composition of Titan. The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed photon emissions of Titan’s day and night limb-airglow on multiple occasions, including during an eclipse observation. Beyond 120o solar zenith angle, when the upper atmosphere of Titan below 1200 km is in total XUV darkness, there is an indication of weak and sporadic night side UV airglow emission excited by magnetosphere plasma collisions with ambient thermosphere gas, with similar N2 excited features as above in the daylight or twilight glow over an extended altitude range near ~1000 km. Energetic protons ( p+, i.e., H+ ) and oxygen (O+ ) ions precipitate into Titan’s atmosphere and deposit energy that produces UV and visible airglow emissions and drives ionospheric chemistry for both the recently discovered sporadic lower-atmosphere’s ionosphere and standard topside ionosphere.We have analyzed the UVIS airglow spectra with new models based on high resolution laboratory electron impact induced EUV fluorescence spectra (FWHM = 0.2 Å, 800−1350 Å). A total of 491 EUV emission features were observed at 100 eV electron impact energy from N2 electronic-vibrational transitions and atomic N I and N II multiplets, and their emission cross sections were measured to be included in electron transport models such as Atmospheric Ultraviolet Radiance Integrated Code (auric). Molecular emission was observed to vibrationally-excited ground state levels as high as v”=17, from the a1Pg , b1Πu,andb1Σu+ excited valence states and the Rydberg series c′n+11Σu+, cn1Πuand on1Πu for n between 3 and 9. The frequently-blended molecular emission bands were disentangled with the aid of a quantum mechanical model which solves the coupled-Schroedinger equation. We have also identified within the same EUV band pass approximately 100 N I,II atomic and ionic multiplets, and measured their emission cross sections.