H-2 quadrupole and H-3(+) emission from Uranus: The uranian thermosphere, ionosphere, and aurora

We present an analysis of the near-infrared emission of Uranus, obtained fr om 1993 to 1995 at the United Kingdom Infrared Telescope (UKIRT) and the NA SA Infrared Telescope Facility (IRTF). We report that in contrast to Jupite r and Saturn, prominent emission in the H-2 quadrupole and H-3(+) overtone and fundamental bands occurs globally. The rotational temperature and lumin osity of H-2 and H-3(+) were usually found to vary mildly with longitude. T he H-2, and possibly the H-3(+), temperature appears to vary with apparitio n, 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-3(+). The luminosity in H-2(nu = 1) was observed as high as 1.6 x 10(10) W in 1993 M ay but remained within 10% of 1.0 x 10(10) W for four rotational phases obs erved in 1995 June near solar minimum. The luminosity in H-2(nu = 0) is pre dicted to be 10-20 times higher and less variable. Similarly, the luminosit y in H-3(+) was similar to 1.1 x 10(11) W in 1995 June, half the value near solar maximum in 1992 April. The declining temperatures and luminosities m ay be induced by the declining phase of the solar cycle, when the far-ultra violet/extreme-ultraviolet flux also declines. Although we have observed ev idence of auroral emission by both molecular species, auroral processes app arently play only a secondary role in the observed excitation. The observed H-2 emission is consistent with an H-2 population in the nu=1 vibrational state in thermal equilibrium. But H-3(+) appears to deviate significantly f rom thermal equilibrium in that the nu(2) = 2 state is underpopulated. The central meridian H-2 intensity distribution is qualitatively consistent wit h emission from a thick thermosphere. An important result is that the distr ibution of the H-3(+) fundamental-band emission differs from that of H-2 by having a pronounced concentration toward the subsolar point. We propose th at solar extreme-ultraviolet, filtered by Uranus's H corona, is responsible for this concentration and is the dominant source of excited H-3(+) on the planet. The K-band spectrum of Uranus's rings indicates a nearly constant, featureless reflectance over this band.