High spectral resolution observations of Uranus' near-IR thermospheric H2 emission spectrum using the IGRINS spectrograph during the 2018 and 2023 apparitions

Abstract

Ground-based near-IR observations have revealed that Uranus' anomalously hot upper atmosphere, detected by Voyager II, has been steadily cooling. The observed H₃⁺ and H₂ emission-line spectra probe Uranus' ionosphere and thermosphere, respectively. Previous observations have shown that the cooling has continued well past the 2007 vernal equinox, when the seasonal solar forcing turned positive, resulting in net heating of the IAU northern hemisphere. Most of them, especially for H₂, were obtained at moderate spectral resolution, R ∼ 1000 to 3000, which admits more sky background, with its associated noise, per spectral resolution element relative to spectrographs having higher spectral resolution. We report the first instance of high spectral resolution being used to observe Uranus' fundamental-band rovibrational quadrupole H₂ emission spectrum; where the sky background is suppressed and narrow planetary emission lines stand out against the planetary continuum. The IGRINS spectrograph with spectral resolution R ∼ 45,000 was used to observe Uranus in the K-band on Oct 26 & 27, 2018 at the Lowell Discovery Telescope, and on Nov 27, 2023 at Gemini South. These observations reveal rovibrational temperatures of Uranus' thermosphere of 542 ± 25 K and 397 ± 32 K at these two epochs, respectively. The consecutive-nights at elevated temperature observed at the Discovery Telescope suggest that Uranus' near-IR H₂ aurora was detected over each of the northern and southern magnetic poles, respectively. The collective IGRINS results support the continued cooling of Uranus' thermosphere through the 2023 apparition, 73 % through the spring season.