High spectral resolution observations of Uranus' near-IR thermospheric $H_2$ emission spectrum using the IGRINS spectrograph during the 2018 and 2023 apparition

TL;DR

This study uses high spectral resolution near-IR observations to measure Uranus's thermospheric temperature and detect auroras, revealing ongoing cooling of its upper atmosphere over recent years.

Contribution

First high-resolution spectroscopic observations of Uranus's H2 emission spectrum, providing detailed thermospheric temperature measurements and insights into atmospheric cooling trends.

Findings

Uranus' thermospheric temperature was 542 K in 2018 and 397 K in 2023.

Detection of near-IR H2 auroras at magnetic poles.

Evidence of continued cooling of Uranus's thermosphere through 2023.

Abstract

Ground-based near-IR observations have revealed that Uranus anomalously hot upper atmosphere, detected by Voyager 2, has been steadily cooling. The observed $H_3^+$ and $H_2$ 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 $H_2$, 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 $H_2$ 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 $H_2$ 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.