On the location of the CH4 homopause at Jupiter's mid-to-high latitudes

The neutral atmosphere, external magnetosphere and interplanetary environment of Jupiter are coupled within Jupiter's auroral regions. Energy from the magnetosphere is deposited as deep as the 1-mbar level (or ~150 km above the ~1-bar cloud-top level) and modifies the thermal structure and chemistry of the atmosphere (Sinclair et al., 2017, Icarus 292, 182-207, Sinclair et al., 2018, Icarus 300, 305-326). Clark et al., 2018 (JGR Space Physics 123, 7554-7567) recently performed a comparative analysis of Juno-JEDI (Jovian Electron Distribution Experiment, Mauk et al., 2017, SSR 213(1-4), 289-346) and Hisaki-EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics, Yoshioka et al., 2013, P&SS 85, 250-260) ultraviolet observations of Jupiter's aurora. They found that the agreement between both datasets was optimized when CH$ _{4}$ and other hydrocarbons were allowed to be transported to higher altitudes in their atmospheric model. This suggests that a phenomenon within Jupiter's auroral regions is acting to increase the eddy diffusion coefficient and the height of the hydrocarbon homopause. The vertical profile of CH$ _{4}$ and the altitude of its homopause is also an important quantity in the analysis and inversion of Juno-UVS (Ultraviolet Spectrometer, Gladstone et al., 2017, SSR 213, 447-473) measurements of Jupiter's ultraviolet auroral emissions. In this work, we test the hypothesis that the CH$ _{4}$ homopause is higher in Jupiter's auroral regions and attempt to constrain its altitude. We present a retrieval analysis of CH3 and CH$ _{4}$ emission spectra of Jupiter's mid-to-high latitudes measured by IRTF-TEXES (Texas Echelon Cross Echelle Spectrograph, Lacy et al., 2002) on April 16th and August 20th 2019 and future spectra to be measured by SOFIA-EXES (Echelon Cross Echelle Spectrograph on the Stratospheric Observatory for Infrared Astronomy, Richter et al., 2018, JAI 7, 1840013) in April - May 2020. An initial analysis of the TEXES data demonstrates that a higher-altitude CH$ _{4}$ homopause significantly improves the fit to the observations at higher latitudes that include the ultraviolet main auroral ovals (poleward of 71°S and 60°N) compared to a lower-altitude CH$ _{4}$ homopause. In contrast, the opposite is true at lower latitudes that do not include the ultraviolet main ovals. We also find that the fit to spectra covering 68-72°N, 170-190°W (the center of northern auroral region) is improved using a higher-altitude CH$ _{4}$ homopause, however a lower-altitude CH$ _{4}$ homopause improves the fit to spectra true outside the auroral region in the same latitude band. We will expand on this analysis by testing the retrieval fits for a larger number of photochemical models with different eddy diffusion coefficients assumed in the upper stratosphere and thus different CH$ _{4}$ homopause altitudes. This will allow us to derive a best-fitting CH$ _{4}$ homopause altitude as a function of latitude and longitude across Jupiter's mid-to-high latitudes. In particular, we will attempt to constrain the height of the CH$ _{4}$ homopause over the dawn, dusk and active region sectors within the northern main oval in order to support the analyses of the ultraviolet auroral emissions measured by Juno. We will also perform a similar analysis of SOFIA-EXES data, which are of a coarser spatial resolution but are more sensitive and probe higher in the atmosphere than IRTF-TEXES data.