Like for the other major planets, the temperature of Uranus’ upper atmosphere (the thermosphere and ionosphere) is over 3 times higher than can be explained by solar EUV heating alone. The cause of this excessively high temperature has been an open question since the Voyager epoch. All proposed processes have serious problems in supplying enough power. However, the temperature of Uranus' upper atmosphere has been steadily dropping for almost 30 years. The drop in the thermospheric H2 and ionospheric H3+ rovib emission-line temperature is in excess of 250 K, compared to an initial 700-750 K — when observations began in 1992. The magnitude of this decline indicates a weakening of the unknown dominant source of Uranus’ excess heating. Monitoring the temporal behavior of this decline in the context of seasonal forcing and the space environment may therefore provide insight into the nature of the unknown heating source. Resolving this open question would solve a 35-year-old problem in planetary astronomy, while providing understanding of the energy balance of the atmospheres of our outer planets — and potentially of extrasolar giant planets.
The continuation of the downtrend past the 2007 equinox may be viewed as the result of a phase lag in a simple oscillating dissipative system, forced by varying insolation. If this downtrend is seasonally driven, or keyed to the season, a trend reversal should be anticipated because only a third of the current 21-yr spring season remains. The maximum phase lag of a forced oscillating system is 90 deg, or one season; which for Uranus occurs in 2028. The reversal is thought to be imminent because of the limited radiative response time of the uppermost atmosphere. The phase lag would yield the dissipative time constant of the system, which characterizes the response time of the upper atmosphere to external forcing, whether the forcing is radiative or dynamic. Determining the epoch of Uranus' trend reversal would fix this constant, which should then constrain future models of the particular, primarily dynamical, process responsible for the excess heating.
Observations of the H2 emission from the neutral thermosphere, and the H3+ emission provide a complete view of both the charged and the neutral components of Uranus’ upper atmosphere. Using observations obtained at the IRTF, and Gemini as part of a Large & Long Term Program, we have been monitoring the continued response of Uranus’ upper atmosphere to the post-2007 equinox seasonal forcing in search of the anticipated trend reversal. We report results from the 2020 apparition. This research was supported by NASA grant NSSC18K0860.