The atmosphere and ionosphere of Jupiter

Abstract

The properties of the atmosphere of Jupiter are reviewed in the light of observations carried out by the Voyager mission. Solar occultation measurements in the ultraviolet show that the temperature of the upper atmosphere is 1100±200K, an apparent increase of about 30% from the value obtained by the Pioneer mission in 1973. Stellar occultation in the ultraviolet indicate that the temperature gradient in the thermosphere is about 1K km −1. These results pose problems for candidate heating mechanisms because the heat input required is large (0.5 ergs cm −2 s −1) and must be deposited at high altitudes. Solar EUV, inertia gravity waves and particle and ion precipitation appear to be unsatisfactory mechanisms for the equatorial thermosphere. Joule heating remains a possibility. For Joule heating to be a viable mechanism, differential wind of several hundred meters per second between the ions and neutrals is required throughout the entire ionosphere. The stellar occultation experiment also provides a determination of the altitude of the homopause and the eddy diffusion coefficient there of about (1–2)×10 6 cm 2 s −1. This result is consistent with the value deduced from the helium 584Å airglow emission rate based on the evidence from infrared data that the He/H 2 ratio is 0.10± 0.03. A very large solar cycle variation in the atomic hydrogen Lyman alpha airglow emission rate is discussed. The stellar occultation data also provide information concerning hydrocarbons in the atmosphere. The volume mixing ratios of CH 4 and C 2H 6 are found to be 2.5×10 −5 and 2.5×10 −6 at 5μb level, while the upper limit for C 2H 2 at 10μb level is 5×10 −6. Voyager IR data yield the volume mixing ratios deeper in the stratosphere to be (1.4±0.45)×10 −3, 5×10 −6, and 3×10 −8 to 10 −7 for CH 4, C 2H 6 and C 2H 2 respectively. The CH 4 mixing ratio is thus 1.5±0.5 times the value one would expect for a solar composition ratio of the elements. The equatorial electron density profile determined by Voyager radio occultation can be explained if ion molecule reactions between H + and vibrationally excited H 2 at high temperatures are fast enough. Temperature dependence of these reactions also accounts for an observed ionospheric diurnal variation. The high latitude ionosphere indicates possible precipitation of high energy particles in the region mapped by the Io plasma torus.