Retrieval of CH4 effective temperature in Jupiter’s auroral regions using Juno/JIRAM data

Despite the multiple evidence of the diffuse presence of methane in Jupiter’s auroral regions, the mechanisms leading to the CH₄ brightening observed both from ground- and space-based platforms are not yet fully understood. During the first NASA/Juno’s orbit (JM0003), the on-board imager/spectrometer JIRAM (Jovian Infrared Auroral Mapper) allowed the detection of methane near both Jupiter’s poles. A very first analysis of the data acquired by the spectrometer, extending from 2.0 mm to 5.0 mm, showed that the peak of 3mm-CH₄ emission was mainly confined within the main auroral oval at the north pole. The same was supposed to occur in the southern aurora, although it could not be verified due to the lack of spectra above 80°S (Moriconi et al., 2017). According to previous studies (Kim et al., 2015; Altieri et al., 2016), 3μm-methane emissions are likely originated by auroral particle precipitation, albeit an auroral forcing of the atmosphere might be considered as an alternative explanation of the large abundances of methane measured over the Jovian poles. In order to address this controversy, a more detailed analysis of JIRAM data was performed. Taking advantage of the revised retrieval code developed to analyse the JIRAM spectra (Adriani et al., 2017; Dinelli et al. 2017; Moriconi et al. 2017), in this work we derived the effective temperature of methane in Jupiter’s auroral regions, being a key information to comprehend the origin of its fluorescence. A larger number of JIRAM spectra were explored, including data acquired during orbits JM0071 (11 July 2017) and JM0081 (2 September 2017) for investigating the spatial distribution of methane at Jupiter’s south poles, finally making it possible to observe the peak of methane emission well inside the southern main oval (Figure 1c,d). The new result allowed to identify the spectra showing larger CH₄ concentrations both in the southern and northern auroral region: these constituted the best candidates for the retrieval of the retrieval of methane temperature, being the measured signal mostly attributable to methane only. An additional analysis was hence performed on the selected CH₄ spectra properly adjusting the retrieval code. As a result of this study, we were able to estimate the effective temperature of methane at Jupiter’s poles, which represents a crucial parameter when constraining on the mechanisms acting in Jupiter’s polar regions.

Figure 1 Maps of the column density (mol/cm²) of CH₄ at both Jupiter’s poles. (a) orbit JM0003 North Pole; (b) orbit JM0003 South Pole; (c) orbit JM0071 South Pole; (d) orbit JM0081 South Pole.

 

References

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