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L. M. Trafton (U. Texas), S. Miller (UCL), T. R. Geballe (JAC), G. E. Ballester (U. Mich.), J. Tennyson (UCL)
We present further analyses of Uranus' global-scale near-IR H2 and H3+ emissions observed from 1993 to 1995 at the UKIRT and IRTF. Auroral processes apparently play only a secondary role in the excitation. The temperature of these species varies mildly with longitude. It also varies between apparitions, indicating a long-term variation of Uranus' thermospheric structure. For a relatively quiescent period in June 1995, the average H2 rotational temperature was 624 K and the nightly rotational temperature varied with rotational phase over the range, 591-641 K. An auroral component to the emission may have been detected when unusually high H2 rotational temperatures were observed for some rotational phases in 1993 and 1994, and when an unusually high vibrational temperature of H3+ was observed in 1993 while the H2 rotational temperature was also anomalously high. By including the T(P) structure determined by Voyager underneath a thick isothermal layer, it is possible to explain the observed H2 emissions in terms of a population of the rotational and vibrational levels consistent with thermal equilibrium. This is in contrast to Jupiter where the v=1 vibrational level is strongly overpopulated due to the auroral precipitation. However, H3+ appears to deviate from thermal equilibrium. Unlike the case for Jupiter, Uranus' global H3+ emission exhibits a vibrational temperature noticeably cooler than the rotational temperature, indicating a significant underpopulation in the v=2 vibrational state relative to the v=1 state. This appears to result from the low ambient ionospheric densities (<1012 cm-3, and associated low collisional excitation rates. The emission from H2 increases sharply towards the polar limbs and extends ~10% beyond them. This is consistent with high-altitude emission from a thick, vertically inhomogeneous shell spanning the ionosphere and thermosphere above the homopause. The fundamental-band H3+ emission has a pronounced concentration towards the subsolar point of the planet. This suggests that H3+ is excited primarily by the Sun; e.g., through ionization of H2 by EUV penetrating the H corona to form H2+ which then reacts exothermally with H2 to form excited H3+.