Abstract
When radiative transfer is the dominant mechanism cooling the lower thermosphere of Jupiter, CH4, (7.7μ) is probably the dominant cooling agent; however, its low turbopause mixing ratio (10−4, as compared to 10−3 in the lower atmosphere) contributes to a cooling rate small (≲10−4) compared to CO2 on Mars. This results in a Javian mesopause density ∼10 times the Martian density or ∼1014 cm−3, if radiative cooling is the primary heat transfer mechanism in the lower thermosphere. An alternate method for transporting heat is convection (forced or free), which apparently emerges as the dominant transport mechanism as the effective eddy diffusion coefficient (Kv) approaches values similar to those anticipated in the earth's lower thermosphere (106 cm see−1). Over the solar cycle, with a high heating efficiency (0.86), the temperature rise above the turbopause ranges between 19 and 53K for weak convective activity (Kv=105 cm see−1) and 7–19K for strong activity (107 cm see−1), suggesting that satellite measurements of the exospheric temperature could be used to estimate the degree of convective activity present in the upper atmosphere. Reasonable variations in the H2-He ratio and the mesopause height (∼300 km), temperature (140K) and cooling rate are of minor importance compared to the heating efficiency and the incident flux in establishing the thermospheric temperature profile via the heat conduction equation. The diurnal temperature variation in the Jovian exosphere over the solar cycle is small, probably less than 5–10K.
