Program: GN-2023B-Q-224

We propose to investigate the response of Uranus’ excessively hot upper atmosphere to seasonal solar forcing ahead of the 2031 solstice. We request Gemini/GNIRS time to observe Uranus in the K- and L-bands to obtain near-IR emission spectra of H3+ and H2 at medium spectral resolution during the 2023B apparition. From these spectra, we will extract the ro-vib temperature and column abundance for each species to characterize Uranus’ ionosphere and neutral thermosphere, respectively. The results will update the long-term secular-downtrend temperature record for both species, now lagging two thirds of a season past the 2007 spring equinox, in spite of the solar forcing minimum for the visible disk at equinox. A seasonal reversal in these temperatures has been anticipated since equinox, and these data will help to document the evolving downtrend and possibly the incipient reversal itself. Separate epochs of the reversals are expected, revealing the characteristic seasonal response times to solar forcing for the thermosphere and ionosphere, respectively. Since the temperatures are higher than can be explained by solar heating alone, the reversal epochs and corresponding dynamical response times should help to constrain the source of Uranus’ excess upper atmospheric heating, an open question since the Voyager epoch.