The energy problem in giant planet upper atmospheres: Observations and models

The upper atmospheres of solar system giant planets are much hotter than expected from solar heating only. This problem is particularly pronounced on Uranus where it extends to the middle atmosphere. Solutions proposed in the past include energy deposition by upward-propagating gravity or sound waves, heating by wind-driven electrodynamics or plasma waves, and auroral heating. Cassini observations of Saturn provide the most extensive dataset available to characterize the atmosphere of any giant planet, providing global coverage that enables a detailed analysis of energy sources and dynamics in the upper atmosphere. I will present a snapshot of the middle and upper atmosphere around the northern summer solstice in 2016-2017 based on Cassini observations and use results from a global circulation model of the thermosphere and ionosphere to argue that auroral Joule heating followed by global redistribution of energy from the poles to the equator is a likely solution to the energy crisis on Saturn. More recent calculations show that this solution works on Jupiter as well. Finally, I will review the properties of the middle and upper atmosphere of Uranus based on existing observations, propose a resolution to a disagreement between different observations of the lower thermosphere, and assess the feasibility of auroral Joule heating as a possible solution to the energy problem in the upper atmosphere of Uranus.