POSITION

Professor Aerospace Engineering
University of Southern California

PROFESSIONAL AFFILIATIONS

American Geophysical Union
American Astronomical Society
American Physical Society

AWARDS

RESEARCH AFFILIATIONS

RESEARCH ACTIVITY

Planetary Atmospheres

Mercury/Moon
The results of basic research on atmospheric source processes, morphology, and evolution have been published in several papers, (14, 15, 18, 25, 33, 47, 65, 68, 72, 76). Much of the work concentrates on microscopic gas-surface interaction characteristics, a seriously misunderstood subject in solar system research. A book chapter on Mercury (65) reviews the state of research up to 1987, and includes references to work on the Moon. The most recent publication is a paper on evolution of the Lunar atmosphere, coauthored with T. Morgan (76). We argue that atmospheric species such as oxygen and sodium have very high loss rates, apparently driven by strong surface reaction branches to low energy activated chemistry. Theoretical work on gas surface reactions related to this subject has been in carried out in collaboration with J. Kunc(33, 47).

Earth
Several papers on auroral and airglow excitation processes primarily in early research work have been published (1, 2, 6, 10, 11)

Jupiter, Saturn, Uranus, Neptune
Research on the excitation of the upper atmospheres is supported primarily by NASA grants. The subjects of auroral source processes, airglow, and anomalous atmospheric heating have been pursued. Many of the conclusions in this work depend on model analysis of Pioneer 10 and Voyager EUV, IUE, and HST remote sensing data. A subject of primary interest is the anomalously high upper thermospheric temperatures on all of the outer giant planets. The work in this program has concluded that heating is driven by electron excitation in the general vicinity of the exobase (49, 54, 66, 80). A mechanism for delivery of sufficiently excited electrons, however, has not been satisfactorily developed (49, 79). Results from the Galileo orbiter, in the next two years may provide further insight. Cassini will be injected into orbit at Saturn in the year 2002. See also papers 17, 23, 24, 28, 30, 31, 32, 35, 38, 42, 43, 53, 55, 58, 59, 60, 66, 84.

Titan and Triton
Voyager 1 observations of Titan and Voyager 2 data at Triton provide the primary information on the nitrogen dominated atmospheres. The data base for Triton is severely restricted by observational conditions, and a search for further information than has already been discussed in the Science 30 day report has not been successful. The Titan data base is more extensive and further work should be carried out, beyond the most recent data analysis effort (81). Research is supported primarily by NASA grants, although some indirect support from NSF is obtained in research on the properties of N_{2} (93, 94), because of atmospheric commonality with the Earth. The understanding of processes exciting the observed atmospheric emissions is incomplete and conflicting. The mean altitude of origin of the Titan emission in limb scan observations (80) is in disagreement with inference from recent measurements of the level of predissociation in the N_{2} c^{'}_{4} - X (0,0) band transition (94). See 32, 35, 36, 37, 66, 80, 94.

Magnetospheres

Jupiter
Extensive research on the Io plasma torus, a remarkable nebular phenomenon, has been carried out with NASA Grant support for several years. This work includes studies of the energy budget, and sources, as well as detailed physical chemistry modeling. The most important contribution is considered to be the determination that the plasma state could not be sustained from sources internal to the main plasma volume (62). Energy source injection mechanisms have not been satisfactorily identified. Extensive atomic structure models have been developed for the prediction of atomic and ion spectra in fine structure, calculated in collisional radiative non- LTE equilibrium. See papers 26, 27, 29, 34, 40, 41, 44, 57, 62, 71, 86, 92. Recent observations have been obtained using HST during the Shoemaker-Levy Comet 9 impact, but no effect on the Io torus was detected in any of the observational programs. Spectra of the Io plasma torus have been obtained in the Galileo UVS experiment in late 1995.
Saturn
The Saturn magnetosphere has been found to be spectacularly different from conditions at Jupiter. In the fall of 1992 our observation of OH (using the Hubble Space Telescope (HST)) in the Saturn magnetosphere profoundly changed the understanding of the state of the system (88). Our previous work with Voyager observations (79) claimed that atomic hydrogen filled the magnetosphere, inferring that neutral gas dominated the plasma, and furthermore predicted that OH and O would contribute substantially to the total population. Prominent theoretical considerations that could not tolerate significant amounts of neutral gas raised doubt that the analysis was correct. The observation of OH in 1992 and again in 1994, however, removed doubt about the constitution of the magnetosphere. The neutral/ion ratio may be as high as 100 in the Saturn plasma-sheet, compared to 0.01 in the Io plasma torus. Previous plasma-sheet theory limited the neutral/ion ratio to ~0.03. The source for the gas is presumed to be H_{2}O from the icy satellites, but mechanisms for production are not resolved. Further observations have been obtained using the IUE facility (M. Festou, PI) and an HST Cycle 6 program has been approved for 1996.

Local Interstellar Medium

Research on the properties of the Local Interstellar medium have been carried out in scattered periods beginning in 1978. The NASA Space Physics Division has shown a persistent pernicious bias against work on the effects of the neutral gas in the LISM in the United States, from the time of the formation of the Division. The dominant role of neutral hydrogen in the formation of the termination shock in the collision of the solar wind with the LISM has only recently been recognized by the particles and fields research community, which has been supported primarily by the Space Science Division. The most important contributions to research in this program are papers (48), which presents a calibration independent method of determining absolute LISM density, and (89), which presents the first evidence for a large increase in the LISM neutral atomic hydrogen density from Voyager measurements of the 50 AU region, suggesting the approach to the termination shock (89). See 19, 20, 21, 48, 64, 82, 89.

Laboratory Astrophysics

Atomic and Molecular Properties
Significant early work has been published on laboratory measurement and analysis of properties of molecules of interest to the planetary atmospheres community. More recent work has been accomplished mainly through collaboration with researchers at the Jet Propulsion Laboratory. See 3, 4, 5, 7, 8, 9, 12, 13, 16, 50, 51, 52, 56, 61, 63, 67, 68, 70, 71, 75, 76, 78, 83, 85, 86, 87, 93, 94, 95. Extensive work has been done on reaction physical chemistry, and emission modeling of the H_{2} emission systems.

Atomic electron collisional properties
Extensive work has been ongoing for several years in establishing collision properties of atoms and ions of interest to research in astrophysics, planetary atmospheres and magnetospheres. The data is compiled in formatted files for the prediction of emission spectra (26, 27, 29, 40, 41). This work has been done with the collaboration of researchers in laboratory astrophysics, as well as atomic physics theorists; J. Ajello, A. Chutjian, G. James, I Kanik, R. J. W. Henry, H. Wu, and S. Tayal. Modeled species include H, Li, He, Ar, C, O, S, N, Mg, Na, K, and their ions to charge states as high as 5. Most of this work remains unpublished. Recent advances have been made with theory and experiment on some of the more important species, and formatting for several published papers is underway.

Gas-Surface Interactions

Early work on the atmosphere of Mercury has led to an interest in gas-surface interactions. Physical and chemical gas-surface interactions at low densities are of importance in planetary work and astrophysics in general as well as in other fields. However these collisional processes have been ill understood in application to astrophysics and a theoretical program of calculating the microscopic gas-surface interaction was developed. This program has been supported off and on with some support from NASA and DOE (15, 16, 18, 25, 33, 47, 65, 68, 73, 76). Calculations have been made to establish physical potential interaction curves and accommodation coefficients for sodium, potassium, oxygen, and hydrogen, subsequent to the discovery of sodium and potassium as atmospheric species of the Moon and Mercury (33, 47, 68).

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