An abrupt change in the long-term cooling of the ionosphere of Uranus

By analysis of ground-based infrared H3+ spectra, the temperature of the ionosphere of Uranus has been monitored on a semi-regular basis since 1992. Prior to 2014, the upper atmosphere was seen to cool slowly over a period of two decades, from ~750 K to ~500 K. However, observations obtained using NASA IRTF SpeX and Keck NIRSPEC during 2014 revealed a reversal in this trend, with all observations obtained in 2014 being warmer than those obtained in 2013. We discuss the importance of this reversal in the context of longterm heating effects and the energy budget of the upper atmosphere.