On Uranus' Near-Infrared Auroral and Ionospheric Emissions
L. M. Trafton (Univ.Texas), S. Miller (UCL), T. R. Geballe (JACH), J. Tennyson (UCL), G. E. Ballester (U. Mich)
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 quadrupole lines and the fundamental and first overtone bands of H were detected in Uranus' near-infrared spectrum from 2 - 4 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 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 band is well defined at 680 K (with 90% confidence interval 658 - 704 K; there is no evidence for the relative attenuation of H emission at wavelengths where CH absorbs strongly, e.g. at the H Q branch. This indicates a lower effective emission altitude for H than for the H corona but one which is higher than the homopause. By contrast, the H emission lacks the bimodality seen for H 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 is close to thermal equilibrium and represents the kinetic temperature of the ambient atmosphere. Its temperature is also close to the H temperature, suggesting that the two species may emit from similar altitudes. However, H 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 above the homopause. On Jupiter, this is thought to be due to the effect of precipitating auroral electrons on the H molecules. If the same mechanism applies to Uranus, then magnetospheric precipitation on a global scale is indicated for this planet.