On Uranus' Near-Infrared Auroral and Ionospheric Emissions

Details On Uranus' Near-Infrared Auroral and Ionospheric Emissions On Uranus' Near-Infrared Auroral and Ionospheric Emissions

Jul 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997dps....29.1508t&link_type=abstract

American Astronomical Society, DPS meeting #29, #15.08; Bulletin of the American Astronomical Society, Vol. 29, p.993

Astronomy and Astrophysics

Astronomy

Scientific paper

We report results of spectral observations of Uranus taken at the UKIRT with CGS4 and at the IRTF with CSHELL. Emission from the fundamental band H_2 quadrupole lines and the fundamental and first overtone bands of H_3(+) were detected in Uranus' near-infrared spectrum from 2 - 4 mu m. Unlike for Jupiter, these emissions are not concentrated near the magnetic poles but are prominent over the globe. However, the morphology of the emission distribution appears to be different for these two species. The H_2 emission appears to be bimodal along the planet's central meridian indicating that it is limb brightened, suggesting that it originates from an emitting shell around the planet. Uranus is known to have a hot ( ~ 800 K) H corona. The rotational temperature for the fundamental H_2 band is well defined at 680 K (with 90% confidence interval 658 - 704 K; there is no evidence for the relative attenuation of H_2 emission at wavelengths where CH_4 absorbs strongly, e.g. at the H_2 Q branch. This indicates a lower effective emission altitude for H_2 than for the H corona but one which is higher than the homopause. By contrast, the H_3(+) emission lacks the bimodality seen for H_2 in our spectra and is more peaked towards lower latitudes. Its rotational temperature and the vibrational temperature, determined by comparing the strength of emissions in the fundamental and first overtone bands, are comparable indicating that H_3(+) is close to thermal equilibrium and represents the kinetic temperature of the ambient atmosphere. Its temperature is also close to the H_2 temperature, suggesting that the two species may emit from similar altitudes. However, H_2 is not in thermal equilibrium. As for Jupiter, its v=1 vibration level is overpopulated relative to the population expected for thermal equilibrium at this temperature and the amount of H_2 above the homopause. On Jupiter, this is thought to be due to the effect of precipitating auroral electrons on the H_2 molecules. If the same mechanism applies to Uranus, then magnetospheric precipitation on a global scale is indicated for this planet.

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