Critical Reactions in Modeling the Jovian Ionosphere

Jupiter's upper atmosphere is composed mostly of H ₂ and He with significant mixing ratios of hydrocarbons near and below the homopause. Photoionization by solar EUV and photoelectron impact ionization produces an ionosphere that is characterized by H ⁺ at high altitudes and hydrocarbon ions at low altitudes. A large number of chemical reactions involving hydrogen-, helium- and carbon-containing ions have been included in models of the Jovian ionosphere. Explaining the electron density profiles measured by spacecraft radio occultation experiments has proved problematic. Some of the difficulty is due to incomplete information on chemical reactions in three categories: (1) vibrational state specific reactions of H ₂ , (2) reactions of hydrocarbons and hydrocarbon ions with more than two carbon atoms, and (3) three-body reactions of meteoric metallic ions. At high altitudes, the major ion H ⁺ , which has a long life time, can be destroyed efficiently by reaction with H ₂ (v > 3), resulting in significant reduction of the electron densities. Reliable reaction coefficients for reactions that produce, destroy, and exchange vibrational levels of H ₂ are crucial to modeling the density profile of H ₂ (v > 3). At low altitudes, complex hydrocarbon ions are expected to be produced at significant rates, especially in the auroral regions where particle precipitation produces ionization near and below the homopause. Complex hydrocarbon ions may play an important role of forming upper atmospheric haze that are observed in the Jovian polar region. Current models include explicitly reactions of hydrocarbon ions up to two carbon atoms, but the link to haze formation is speculative. Rate coefficients for various carbon-adding reactions as well as carbon bond-breaking reactions involving C _n H _m ⁺ (n > 2) are needed to understand quantitatively the polar haze formation. In addition to endogenic ions, meteoric ions have recently been included in Jovian ionospheric models to explain the large electron densities measured under predawn conditions. Since some of meteoric ions, particularly Fe ⁺ , Mg ⁺ , Na ⁺ do not react with H ₂ or hydrocarbons in two-body interactions, they dominate the ionosphere in the region of the hydrocarbon ions, which may account for the predawn electron density profiles measured by radio occultation experiments. These metallic ions, however, may react with H ₂ and/or hydrocarbons in three-body interactions, so their density profiles are uncertain. These three-body reaction rate coefficients are therefore crucial for models of meteoric ions. In addition to ion reactions, some of neutral reactions of hydrocarbons and meteoric atoms are important as well to explain the measured electron densities. We identify a list of critical reactions whose laboratory measurements could result in significant progress in understanding the Jovian ionosphere.