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Y. Bétremieux, R. V. Yelle (Northern Arizona University)
In an effort to reconcile the ammonia vertical profile inferred from an ultraviolet spectrum of Jupiter's equatorial region obtained with the Faint Object Spectrograph on June~5th~1993 (Bétremieux and Yelle 2000) with that inferred from infrared data by Lara {\itshape et al.\/} (1998), the temperature-dependence of the ultraviolet cross section of ammonia from 170 to 227~nm was investigated. By fitting model-produced NH3 cross sections to those measured at temperatures of 195 and 295~K by Chen {\itshape et al.\/} (1999), the rotational constants, band position, linewidth and Franck-Condon factors have been determined for the 14 first normal bands and the first two hot bands of the Ã1A2''~arrow~X1A1 transition. These parameters were then used to compute the ultraviolet cross section of ammonia at the cold temperatures (110-160~K) prevailing in the upper troposphere of Jupiter. Using these model-produced cross sections rather than a cross section measured by Chen {\itshape et al.\/} (1999) at a temperature of 175~K produces subtantial changes in the inferred ammonia vertical distribution: the mole fraction of ammonia at pressures below 175~mbar dropped by several orders of magnitude and is more consistent with the data of Lara {\itshape et al.\/} (1998); at pressures from 250 to 500~mbar the ammonia mole fraction increased by up to a factor of two in the deeper regions but is still undersaturated by several orders of magnitude. The distribution of tropospheric acetylene is also impacted by the changes in the ammonia cross section: the tropospheric component of the C2H2 vertical profile was shifted 100~mbar deeper. These results clearly demonstrate that the effects of temperature on ultraviolet cross sections can have a significant impact on the interpretation of ultraviolet spectra.