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[43.06] On Uranus' H2 and H3+ Emissions

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⁺.