9 Jupiter ’ s Thermosphere and Ionosphere

Jupiter’s upper atmosphere forms the boundary between the lower atmosphere and interplanetary space. As the topmost layer, the thermosphere absorbs solar extreme ultraviolet (EUV) radiation and charged particles from the magnetosphere that cause dissociation and ionization of thermospheric molecules. Subsequent chemistry establishes the ionosphere and the densities of minor, photoand electrochemically produced constituents. Densities are low in the upper atmosphere and the mean free path between collisions is large, varying from 10 cm near 1μbar at the base of the thermosphere to 150 km near 2.5 pbar at the exobase, the top of the thermosphere. Because of this large mean free path, molecular diffusion becomes dominant over convective mixing and thermal conduction becomes an important term in the energy balance equation. The low collision rates also imply that the population of molecular energy levels may depart from their local thermodynamic equilibrium (LTE) values.