Spatial Variations in the Altitude of the CH₄ Homopause at Jupiter’s Mid-to-high Latitudes, as Constrained from IRTF-TEXES Spectra

Abstract

We present an analysis of IRTF-TEXES spectra of Jupiter’s mid-to-high latitudes in order to test the hypothesis that the CH4 homopause altitude is higher in Jupiter’s auroral regions compared to elsewhere on the planet. A family of photochemical models, based on Moses & Poppe (2017), were computed with a range of CH4 homopause altitudes. Adopting each model in turn, the observed TEXES spectra of H2 S(1), CH4, and CH3 emission measured on 2019 April 16 and August 20 were inverted, the vertical temperature profile was allowed to vary, and the quality of the fit to the spectra was used to discriminate between models. At latitudes equatorward of Jupiter’s main auroral ovals (>62°S, <54°N, planetocentric), the observations were adequately fit assuming a homopause altitude lower than ∼360 km (above 1 bar). At 62°N, inside the main auroral oval, we derived a CH4 homopause altitude of km, whereas outside the main oval at the same latitude, a 1σ upper limit of 370 km was derived. Our interpretation is that a portion of energy from the magnetosphere is deposited as heat within the main oval, which drives vertical winds and/or higher rates of turbulence and transports CH4 and its photochemical by-products to higher altitudes. Inside the northern main auroral oval, a factor of ∼3 increase in CH3 abundance was also required to fit the spectra. This could be due to uncertainties in the photochemical modeling or an additional source of CH3 production in Jupiter’s auroral regions.

Variability and Hemispheric Symmetry of the Pedersen Conductance in the Jovian Aurora

“…Missing observational information on this question, we have assumed that the vertical CH 4 distribution is spatially uniform in the auroral region. Recent ground-based infrared spectral observations by Sinclair et al (2020) suggest that the altitude of the hydrocarbon homopause may increase inside the aurora relative to lower latitude regions. They claim that the homopause altitude in the north is higher by about 100 km in the aurora than outside.…”

“…Recent ground‐based infrared spectral observations by Sinclair et al. (2020) suggest that the altitude of the hydrocarbon homopause may increase inside the aurora relative to lower latitude regions. They claim that the homopause altitude in the north is higher by about 100 km in the aurora than outside.…”

Local Time Dependence of Jupiter's Polar Auroral Emissions Observed by Juno UVS

“…The energy dependence inferred from the color ratio assumes that the vertical structure of Jupiter's atmosphere is uniform across the polar region. This assumption has recently come under scrutiny (Clark et al., 2018; Gérard et al., 2014; Sinclair et al., 2020).…”

The CH$_4$ abundance in Jupiter's upper atmosphere

“…Their model 1 is very similar to the CH 4 VMR 3 profile of Moses et al (2005), and model 5 shows a higher abundance above ∼ 10 −3 mbar, approaching the VMR 1 profile at lower pressures. Our best-fit CH 4 abundances in the VMR R 1 profile are significantly higher than those of Sinclair et al (2020) near the homopause, at pressures in the range of 10 −2 -10 −4 mbar. A possible explanation for this discrepancy could be that the mid-infrared emission measurements they analysed might be probing slightly higher atmospheric heights than ISO/SWS data (see their Fig.…”

“…Very recently, Sinclair et al (2020) used ground-based IRTF-TEXES high-resolution spectra to compare the homopause location in auroral and non-auroral regions and inferred the CH 4 abundance near the homopause. They did not use the 3.3 µm methane emission in the near-infrared, but studied midinfrared CH 4 radiances arising from the v 4 level, and the CH 3 emission near 600 cm −1 .…”