Meteoric Ions in the Ionosphere of Jupiter

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A reanalysis of the Voyager 2 radio occultation data has recently revealed a low-altitude layer in the jovian ionosphere (Hinson et al. 1998, J. Geophys. Res. 103, 9505–9520). The peak electron density of the layer measured on egress, which was at 93° solar zenith angle near the morning terminator, was inferred to be of the order of 104 cm−3. A substantial low-altitude layer of hydrocarbon ions in the jovian ionosphere was predicted by Kim and Fox (1994), but the peak total ion density at predawn was about 102 cm−3, two orders of magnitude smaller than the noon values, due to the efficient recombination of molecular ions during the night. The existence of large electron densities in the jovian ionospheric E region at predawn suggests the presence of ions with long lifetimes and/or those produced by a source that exhibits little local time dependence, such as ions originating from meteoroid ablation in Jupiter's atmosphere. We have modeled the production rates and subsequent chemistry of seven meteoric ions, including O+, C+, Si+, Fe+, Mg+, Na+, and S+, their compounds with H, H2, and hydrocarbons, and the corresponding neutral species. The models predict a layer of meteoric ions in the altitude region of 350–450 km above the 1-bar level, with peak total ion densities of several times 104 cm−3, which are comparable to the observed values. The peak of the meteoric atomic ion layer is most apparent at predawn and is located higher than that of the hydrocarbon ion layer during the daytime and higher than the altitude of peak production of ions by meteor ablation. At the altitude of peak ablation, about 350 km, meteoric ions are mainly removed by reactions with hydrocarbons in either two-body or three-body reactions, and the molecular ions produced are neutralized efficiently by dissociative recombination. Meteoric ions may also form adduct ions by termolecular reactions with hydrogen molecules, but metallic ions, such as Na+, Mg+, and Fe+, may be reformed from the adduct ions by a series of reactions with H atoms. Thus the net ion loss process at the metal ion peak may be dominated by rediative recombination, and the meteoric ion density profiles show little diurnal variation. The predicted peak electron density and altitude and the relative densities of the ions are dependent on the rate coefficients assumed for many of the reactions involved, and measurements of key rate coefficients are needed to further constrain the models.