H2 Quadrupole and H3+ Emission from Uranus: The Uranian Thermosphere, Ionosphere, and Aurora

We present an analysis of the near-infrared emission of Uranus, obtained from 1993 to 1995 at the United Kingdom Infrared Telescope (UKIRT) and the NASA Infrared Telescope Facility (IRTF). We report that in contrast to Jupiter and Saturn, prominent emission in the H2 quadrupole and H overtone and fundamental bands occurs globally. The rotational temperature and luminosity of H2 and H were usually found to vary mildly with longitude. The H2, and possibly the H, temperature appears to vary with apparition, indicating a long-term variability of Uranus's thermospheric structure. The thermosphere cooled between 1992 and 1995, while the ionosphere either cooled or otherwise suffered a decline in the column of excited H. The luminosity in H2(ν = 1) was observed as high as 1.6 × 1010 W in 1993 May but remained within 10% of 1.0 × 1010 W for four rotational phases observed in 1995 June near solar minimum. The luminosity in H2(ν = 0) is predicted to be 10-20 times higher and less variable. Similarly, the luminosity in H was ~1.1 × 1011 W in 1995 June, half the value near solar maximum in 1992 April. The declining temperatures and luminosities may be induced by the declining phase of the solar cycle, when the far-ultraviolet/extreme-ultraviolet flux also declines. Although we have observed evidence of auroral emission by both molecular species, auroral processes apparently play only a secondary role in the observed excitation. The observed H2 emission is consistent with an H2 population in the ν = 1 vibrational state in thermal equilibrium. But H appears to deviate significantly from thermal equilibrium in that the ν2 = 2 state is underpopulated. The central meridian H2 intensity distribution is qualitatively consistent with emission from a thick thermosphere. An important result is that the distribution of the H fundamental-band emission differs from that of H2 by having a pronounced concentration toward the subsolar point. We propose that solar extreme-ultraviolet, filtered by Uranus's H corona, is responsible for this concentration and is the dominant source of excited H on the planet. The K-band spectrum of Uranus's rings indicates a nearly constant, featureless reflectance over this band.