Bio/Research

My research group is involved with numerical modeling of the chemical and thermal structures, luminosity, and evolution of the thermospheres-ionospheres of the Earth and planets. Recently, an important focus has been the nightside ionosphere of Venus and the ultraviolet "auroral" emissions observed there by the Pioneer Venus spacecraft. We have suggested that the emissions are produced by impact of soft electrons that have been observed in the umbra of the planet, and we are modeling the electron-energy deposition to determine their spectrum. Using a combination of modeling and analysis of ion composition data from Pioneer Venus, we hope to elucidate the relative roles of electron impact and ion transport in producing the nightside ionosphere.

We are also investigating the ionospheric structure, odd nitrogen chemistry, and evolution of the Martian atmosphere. The increased knowledge of the ionosphere of Venus gained from Pioneer Venus has enabled us to better understand the Martian ionosphere, which has a similar composition, but for which there are fewer measurements. The 15N/14N ratio measured by the Viking spacecraft showed that the ratio is enhanced over that found in the terrestrial atmosphere, presumably due to selective escape of 14N. We have modeled the non-thermal escape processes over the age of the solar system to determine the initial nitrogen inventory.

The Jovian ionosphere has also been a focus of our research, especially the hydrocarbon ion chemistry, H3+ densities and vibrational distributions. Infrared emissions, which have been attributed to vibrational transitions of H3+, have been observed from the Jovian polar regions in the 2 - 4 micrometer range. We have modeled the vibrational distribution of H3+ in order to identify the production mechanisms, and predicted the emission rates. In collaboration with Dr. Roger Yelle at the University of Arizona, we are studying the ionospheric composition and structure, odd nitrogen chemistry, and heating efficiencies in the thermospheres of Titan and Triton, which are satellites of Saturn and Neptune, respectively.

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