Monitoring Uranus’ Secular Upper-atmospheric Temperature Downtrend

Since 1992, when observations began, the temperature of Uranus’ thermosphere and ionosphere has been in a secular decline. Evidence for this is based on rotational temperatures extracted from ground-based observations of Uranus’ near-IR H₃⁺ and H₂ emission line spectrum. The H₃⁺ emission arises solely from above the homopause since this ion is destroyed by reaction with hydrocarbons below. It is thus more reflective of ionospheric conditions. The H₂ emission is more representative of the thermosphere, as it is not restricted by the homopause. Solar heating alone is not enough to explain the high temperatures observed in the upper atmosphere of Uranus and the other major planets. The source of the missing heating is an open question since the Voyager epoch. The downtrend observed since for Uranus is in the context of its 2007 equinox and 21 Earth-year season. If the process causing the downtrend is seasonally driven, there should be a reversal of trend following equinox, after a phase lag of less than a season; i.e., before 2028. Considering that the peak in temperature of the current downtrend had to occur no more than 7-10 years after the previous solstice, we anticipate that the trend reversal may be imminent. In addition to the amplitude of the response, the phase lag may be diagnostic of the missing heating process through the revealed characteristic dynamical response time of the upper atmosphere, in comparison to radiative solar heating alone. A further constraint may be obtained if the phase lag for H₂ occurs after that for H₃⁺, owing to the greater mean depth probed by the H₂ emission, and the corresponding greater dynamical time constant for the overlying atmosphere.

We report our results, updated through the 2019 apparition, obtained from observations from iSHELL at IRTF and GNIRS at Gemini as part of a Large & Long Term program. We find that the downtrend has continued through this apparition for both the rotational temperature and emission intensity of each species. This research was supported by NASA grant NSSC18K0860.