Non-LTE effect on the estimation of H3+ vibrational temperatures from 2 and 4 micron observations of the Jovian aurora

Details Non-LTE effect on the estimation of H3+ vibrational temperatures from 2 and 4 micron observations of the Jovian aurora Non-LTE effect on the estimation of H3+ vibrational temperatures from 2 and 4 micron observations of the Jovian aurora

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005dps....37.6017k&link_type=abstract

American Astronomical Society, DPS meeting #37, #60.17; Bulletin of the American Astronomical Society, Vol. 37, p.1572

Astronomy and Astrophysics

Astronomy

Scientific paper

In Jovian auroral studies, H3+ line emissions in fundamental (v2 = 1 - 0), hot (v2 = 2 - 1), and overtone (v2 = 2 - 0) bands have been observed in the 2 micron and 4 micron wavelength regions. From the ratios of line intensities in different bands, vibrational temperatures that represent the relative populations of the v2 = 2 and v2 = 1 levels were often compared with rotational temperatures to check whether H3+ vibrational levels is in local thermodynamic equilibrium (LTE) with background thermosphere since fast radiative relaxation may cause underpopulation of the v2 = 2 level. A simple three level model is developed to estimate the non-LTE effect on the intensity ratio of R(3,4) in the hot band to Q(1,0) in the fundamental band. The simple model, unlike a similar model published previously, shows marginal change in the ratio due to non-LTE effect at ambient H2 densities of 1012 cm-3 or less. In order to estimate accurately the non-LTE effect, a comprehensive model for H3+ level populations is constructed by including 1012 ro-vibrational levels up to 10000 cm-1 and 823 measured radiative transition from recent critical compilation of H3+ spectroscopy. Because about 40%\ of the levels are not radiatively connected, which reduces effective Einstein A coefficients by a factor of about 100, the radiative relaxation of the H3+ levels is not on the average so fast as previously thought. Non-LTE effect becomes apparent only at ambient H2 densities of 1010 cm-3, 100 times less than the simple three level model predicts. This suggests that non-LTE effect is not of importance in observed Jovian H3+ auroral emission since thermospheric H2 densities are expected to be much greater than 1010 cm-3 at the emission peak. Observed line intensity ratios of the hot and fundamental bands should be close to values in LTE. This may explain that vibrational temperatures derived from the ratios of observed line intensities were not far from H3+ rotational temperatures that are in LTE with thermospheric temperatures.
This work is financially supported by KOSEF (R14-2002-043-01000-0).

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