Post-equinoctial observations of the ionosphere of Uranus

We analyse ground-based infrared observations of H3+ emission from the upper atmosphere of Uranus using Gemini North/GNIRS, NASA IRTF/SpeX and VLT/CRIRES. These observations were obtained on 15 different nights in late 2011, between day-of-year 251 (5th of September) and 340 (6th of December). The equinox of Uranus occurred in late 2007 and these recent observations quantify the behaviour of the planet’s upper atmosphere 4 years after equinox, equivalent to 15° of circumsolar rotation.The mean temperature of the ionosphere from these measurements is 520 ± 32 K, which is cooler than any of the temperatures determined by the precursor to this study (Melin, H., Stallard, T., Miller, S., Trafton, L.M., Encrenaz, T., Geballe, T.R. [2011b]. Astrophys. J. 729, 134). Thus, the cooling trend that has been observed since the first H3+ observation in 1992 has continued, even as the planet traversed equinox. This suggests that the driver of the elevated thermospheric temperatures cannot be linked to purely seasonal mechanisms, and we consider other sources of variability, such as the changing geometry between the planet, magnetosphere and solar wind.

► Ground-based infrared observations of H3+ emission from Uranus obtained in 2011. ► The long-term cooling observed between 1992 and 2008 continued to 2011. ► The measured ionospheric temperatures in 2011 are the lowest ever recorded at Uranus.