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 tex2html_wrap_inline11 quadrupole lines and the fundamental and first overtone bands of H tex2html_wrap_inline13 were detected in Uranus' near-infrared spectrum from 2 - 4 tex2html_wrap_inline15 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 tex2html_wrap_inline11 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 ( tex2html_wrap_inline19 800 K) H corona. The rotational temperature for the fundamental H tex2html_wrap_inline11 band is well defined at 680 K (with 90% confidence interval 658 - 704 K; there is no evidence for the relative attenuation of H tex2html_wrap_inline11 emission at wavelengths where CH tex2html_wrap_inline25 absorbs strongly, e.g. at the H tex2html_wrap_inline11 Q branch. This indicates a lower effective emission altitude for H tex2html_wrap_inline11 than for the H corona but one which is higher than the homopause. By contrast, the H tex2html_wrap_inline13 emission lacks the bimodality seen for H tex2html_wrap_inline11 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 tex2html_wrap_inline13 is close to thermal equilibrium and represents the kinetic temperature of the ambient atmosphere. Its temperature is also close to the H tex2html_wrap_inline11 temperature, suggesting that the two species may emit from similar altitudes. However, H tex2html_wrap_inline11 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 tex2html_wrap_inline11 above the homopause. On Jupiter, this is thought to be due to the effect of precipitating auroral electrons on the H tex2html_wrap_inline11 molecules. If the same mechanism applies to Uranus, then magnetospheric precipitation on a global scale is indicated for this planet.