Other
Scientific paper
Sep 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt.........7w&link_type=abstract
Thesis (PhD). THE JOHNS HOPKINS UNIVERSITY, Source DAI-B 60/03, p. 1127, Sep 1999, 187 pages.
Other
Hydrogen Fluorescence
Scientific paper
H2 fluorescence is the primary source of far ultraviolet (FUV) emissions from Jupiter and the other giant planets, arising as a result of charged particle impact (aurorae) and solar excitation via photons and photoelectrons (dayglow). The advent of space based ultraviolet spectrometers, both in earth orbit (IUE, HST/GHRS, HUT) and aboard the Voyager and Galileo spacecraft, has provided an opportunity for astronomers to observe these emissions over extended periods of time, under a variety of conditions. Hydrocarbon absorption, while limiting the depth to which we can see in this region of the spectrum, provides an additional diagnostic tool for studies of the upper atmosphere. Since hydrocarbon cross sections typically show strong variations with wavelength in the FUV, the emissions from different wavelength regions can be used to sample different regions of the atmosphere. This absorption also eliminates, for all practical purposes, the contribution from Rayleigh-scattered sunlight at wavelengths less than 1520 Å. The remaining FUV emission is almost exclusively from atomic and molecular hydrogen. The energy source responsible for the observed ``dayglow'' emissions on Jupiter and the other giant planets has been a source of controversy for many years, as the observed emission seemed to be substantially brighter than could be accounted for based on the solar EUV and FUV energy input. Using revised models for the solar spectrum, updated H2 transition data and excitation cross sections, and newly available data on the thermal structure and composition of Jupiter's upper atmosphere, models have been created to calculate the expected dayglow emission near the maximum and minimum points in the solar activity cycle. It now appears that both the dayglow intensity and its long term temporal variations can be accounted for based on changes in solar input. Changes in the vertical structure of the atmosphere may also be indicated, but large uncertainties in the solar spectrum model make such a conclusion tenuous, at best. Other aspects of H2 fluorescence of particular note are the strong influence of self-absorption by H2(ν <= 2) on the emission spectrum at wavelengths less than 1150 Å and the role of vibrationally excited H2 in the process of fluorescent excitation, due to near coincidences of H2 transitions with strong solar emission lines. Detailed modeling of H2 emission spectra and absorption cross sections has demonstrated the value of FUV spectroscopy as a probe of the upper atmospheres of our solar system's giant planets. Study of these emissions may one day provide additional information on conditions in the atmospheres of H 2-rich extrasolar giant planets.
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