Temperature and abundances in the Jovian auroral stratosphere: 2. Ethylene as a probe of the microbar region

Abstract

Individual emission lines of ethylene (C2H4) near 10.5 μm were measured from the equatorial and north polar regions of Jupiter. Observations were made at a spectral resolution of 0.00083 cm−1 using infrared heterodyne spectroscopy at the NASA Infrared Telescope Facility on Mauna Kea, Hawaii. The line shape information possible with this resolving power permitted the retrieval of quiescent ethylene abundances and the investigation of abundance and thermal structure variability in the polar auroral region. A rough distribution of the north polar emission as a function of longitude was obtained with an instantaneous field of view (full width at half maximum) of ∼1 arc sec on the planet. The greatest C2H4 emission was observed near the nominal north polar methane hot spot (60°N, 180° longitude, System III, 1965). It was found to be confined to <10° longitude on the planet. Using a Voyager‐derived thermal profile, retrieved ethylene mole fractions for equatorial and north polar quiescent (non‐hot spot) regions were consistent with results from existing photochemical models. At the hot spot an 18‐fold increase in abundance was required near the 10‐μbar level to reproduce the data. Alternatively varying the stratospheric thermal profile, a 67–137 K increase in temperature was required at the ∼10‐μbar level to satisfy the observed emission, if the C2H4 mole fraction is fixed to the quiescent value. These results provide the first direct probe of the upper stratosphere of Jupiter and give upper limits to the temperature increase near the source of the north polar thermal infrared aurora. Combined with results from similar measurements of auroral ethane emission (Livengood et al., this issue) probing the 1‐mbar region, altitude information on the thermal structure can be obtained for the first time. The ethylene line emission region extends to the few microbar pressure level and may overlap the region where the H2 ultraviolet aurora is formed; thus it can be used as a probe of the coupling between the ultraviolet and thermal infrared phenomena.