The long-term cooling of the upper atmosphere of Uranus

Abstract

 This molecular ion H₃⁺ provides a really powerful tool with which we can remotely measure the ionospheric density and thermospheric temperature of the gas giants using near-infrared ground-based telescopes. These measurements show that the upper atmosphere of all four giant planets in our solar system are much hotter than solar input along can produce - this is known as the ‘energy crisis’ - and it is currently unclear from where this additional energy is sourced from. The two major candidates are the re-distribution of auroral Joule heating and the breaking of gravity waves generated in the turbulent lower atmosphere. Emission from H₃⁺ has been observed from Uranus on a semi-regular intervals since 1992. We have previously shown that the upper atmosphere is subject to a remarkable long-term cooling, and has cooled from ~750 K in 1992 to 500 K in 2018, at a rate of ~8 K/year. This has important implications for the drivers of the elevated temperatures in the upper atmosphere, and may be related to the ever changing geometry between the offset magnetic field of Uranus the incoming  solar-wind. Here, we present the latest observations of H₃⁺ emission from Uranus, obtained using the iSHELL instrument at the NASA Infrared Telescope Facility (IRTF) in October and November 2019. We explore any potential changes in the observed cooling, and discuss how these new observations fit within our current theories  of how the upper atmosphere of Uranus is heated.

 

 Figure 1: The long-term temperature (a) and density (b) evolution of the upper atmosphere of Uranus between 1992 and 2018.