Giant Planet Lightning in Nonideal Gases

Abstract

Lightning has been directly observed or inferred on all giant planets, generally accepted to be occurring in their water clouds. However, much as Earth has both cloud–cloud and cloud–ground lightning, this does not mean all flashes occur in a narrow altitude range: on Jupiter, the Galileo spacecraft detected lightning flashes apparently below the cloud base, explicable as lightning due to precipitation, and the Juno SRU detected small flashes far above it, at pressures of only 1–2 bars. We use a computationally light 1D entraining plume model, incorporating particle growth and noninductive charging, which predicts this wide range of Jovian lightning provides freezing point depressions caused by ammonia, and modify it to use a van der Waals equation of state instead of an ideal gas, as well as integrating the evaporation of rain; this allows modeling of planets colder than Jupiter, where clouds and lightning occur at greater pressures. For Saturn, the uppermost lightning is predicted at 3–4 bars; unlike on Jupiter, ammonia is not required to match the Cassini observations. For Uranus and Neptune, depending on their convective structures, very high rates of lightning are possible in the deep water clouds; while deeper than on Jupiter or Saturn, lightning is predicted likely to peak above the water cloud base, at pressures around 100 bars. Voyager 2's radio observations of Uranian and Neptunian sferics may thus be either due to attenuation of deep water lightning of this type, or due to lightning in the shallow ammonia clouds; future observations are required to resolve this dichotomy.