Temperatures of Jupiter’s Upper Atmosphere: The Role of the Planetary Magnetic Field

The primary source of heating in the upper atmospheres (thermospheres) of giant planets has been the subject of long debate. The conundrum (or “energy crisis”) consists in observed thermosphere temperatures at low and midlatitudes exceeding values expected from solar heating by several hundred kelvins, suggesting the presence of another energy source. Several theories have been proposed to explain the high temperatures, from heating by upward-propagating gravity or acoustic waves to heating of auroral regions by magnetosphere-atmosphere coupling. While auroral heating provides sufficient energy, the problem becomes one of global energy redistribution. On a rapidly rotating planet, Coriolis forces act to “trap” auroral energy in the polar regions, heating up the poles but leaving the equator cold. On Saturn, the redistribution of energy from poles to equator was recently achieved by invoking Rayleigh drag, possibly related to atmospheric gravity or acoustic waves. Using a new general circulation model of Jupiter, we reproduce well the observed low and midlatitude thermosphere temperatures without the need to invoke Rayleigh drag. Instead, the planet’s magnetic field is strong enough to cause significant ion drag and slowing down zonal winds in the nonauroral thermosphere and allow global energy redistribution. This distinguishes Jupiter from Saturn and most likely the other gas giant planets in our solar system may have implications on extrasolar planets with strong magnetic fields as well.