The Hydrogen Upper Atmosphere of Uranus Seen Through Lyman Alpha Observations

Several aspects of Uranus’s upper atmosphere are not well understood. The temperature is substantially higher than can be explained by solar heating alone. Over the last three decades, it has been observed that the ionosphere is continuously cooling, beyond seasonal effects. Voyager 2 revealed a substantial H exosphere and atomic corona of Uranus extending several Uranus radii. Inspired by the cooling of Uranus’s ionosphere, we are interested in understanding how its H upper atmosphere changes over a long period of time. From 1998 to 2017, Uranus was observed in ultraviolet wavelengths using Hubble Space Telescope’s STIS instrument in several observing campaigns (before and after equinox at Uranus). We analyze this data at Lyman-alpha wavelength (121.56 nm) and do radiative transfer modeling to study variation in the H upper atmosphere. We present the evolution of the H upper atmosphere over this period that we understand from the preliminary radiative transfer modeling.