D. E. Shemansky

Electron impact excitation of H2 - Rydberg band systems and the benchmark dissociative cross section for H Lyman-alpha

The cross sections sigma R 1 (2p) for excitation of H Ly-alpha emission produced by electron impact on H2 is reexamined. A more accurate estimate for sigma R 1 (2p) is obtained based on Born approximation estimates of the H2 Rydberg system cross sections using measured relative excitation functions. The obtained value is (8.18 + or -1.2) x 10 to the -18th sq cm at 100 eV, a factor of 0.69 below the value universally applied to cross section measurements over the past decade. Cross sections for the H2 Rydberg systems fixed in magnitude by the Born approximation have also been obtained using experimentally determined excitation functions. Accurate analytic expressions for these cross sections allow the direct calculation of rate coefficients. 30 references.

The Galileo and Pioneer Venus ultraviolet spectrometer experiments - Solar Lyman-alpha latitude variation at solar maximum from interplanetary Lyman-alpha observations

Solar Ly-alpha latitude variation at solar maximum is examined on the basis of interplanetary Ly-alpha observations made during the Galileo and Pioneer Venus UV spectrometer experiments. A comparison is made of the latitude variation of the interplanetary (IP) Ly-alpha signal in 1986 at solar minimum from Pioneer Venus and in 1990 at solar maximum from Galileo. The Galileo EUV spectrometer shows that a large enhancement of the IP Ly-alpha emission occurred over the intervening four years near the solar equator. An IP Ly-alpha model is developed which considers the latitude variation of the solar Ly-alpha flux. The model fit to the data shows a 25-percent decrease of the full disk solar Ly-alpha flux from solar equator to solar pole in 1990. A detailed study of the Galileo IP Ly-alpha observations on day-of-year 190, 193, 197, and 200 in 1990 reveals that large variations occur in response to the 27-d solar variation. Analysis of these data shows that a maximum variation of 20 percent can be expected in the IP Ly-alpha upwind intensity over this 27-d period.

Electron impact dissociative excitation of O2: 2. Absolute emission cross sections of the OI(130.4 nm) and OI(135.6 nm) lines

[1]xa0In this work, we report the OI(135.6 nm) absolute emission cross section resulting from the long-lived (180 μs) OI(5S → 3P) transition from dissociative excitation of O2. From the ratio of the integrated intensities of the OI(135.6 nm) and OI(130.4 nm) features and from the absolute emission cross section for the OI(130.4 nm) emission feature from electron impact dissociative excitation of O2 at 100 eV, the absolute emission cross section for the OI(135.6 nm) feature was determined to be 6.4 × 10−18 cm2 at 100 eV. Electron impact-induced optical excitation functions for optically allowed transitions at 115.2 nm and 130.4 nm and for an optically forbidden transition at 135.6 nm were also obtained over the electron impact energy range 0–600 eV. The OI(135.6 nm) emission cross section was measured in the laboratory utilizing a large collision chamber (1.5 m in diameter). Electrons were produced with an electrostatically focusing gun with a large focal length (50 cm). The OI(130.4 nm, 135.6 nm) excitation functions were put on an absolute scale as described in the text, and the OI(135.6 nm)/OI(130.4 nm) ratio was determined for the entire energy range (0–600 eV). The atomic O UV emission cross sections from dissociative excitation of O2 can be used to model the recent Hubble Space Telescope observations of OI(130.4 nm) and OI(135.6 nm) intensities from Ganymede [Feldman et al., 2000] and Europa [Hall et al., 1995, 1998].

Vacuum ultraviolet studies of electron impact of helium Excitation of He n1P0 Rydberg series and ionization-excitation of He(+) nl Rydberg series

Laboratory measurements of the electron excitation cross sections of emission in the He Rydberg series (1sS S-1snp P) for n = 2,3,4 have been obtained. The cross sections were estimated by two methods: (1) analysis of calibrated laboratory spectra placed on an absolute scale using the H Ly dissociative excitation standard and (2) analysis of relative cross section data using a modified Born approximation. A new method has been developed for the application of the Bethe-Born approximation using experimental relative excitation functions that does not require extrapolation in a Fano plot in the determination of the absolute cross section. The two methods agree to within 3% for the 58.4 nm line when allowance is made for cascade transitions. We find the direct excitation cross sections at 200 eV for the n = 2,3,4 members of the published electron impact experimental measurements of the two electron process of ionization-excitation impact are generally in agreement, especially on the energy dependence of excitation functions, but show significant differences with theoretical calculations.

A comparison of the Voyager 1 ultraviolet spectrometer and plasma science measurements of the Io plasma torus

We have developed a sophisticated package (Colorado Io Torus Emissions Package, or CITEP) to simulate emissions from the Io plasma torus, and have used it to examine the consistency of Voyager in situ and remote observations. CITEP merges ion composition derived from the ultraviolet spectrometer line ratios with measurements of electron densities, electron temperatures, and ion temperatures obtained by the plasma science instrument. The program then predicts the brightness and morphology of torus EUV emissions. We find that the measured brightness is approximately 2 times larger than the model predicts. When scaled up by this factor, the morphology of the model radial emission profile is consistent with the data, but somewhat less sharply peaked at the ribbon location. We examine several possible causes for the brightness discrepancy such as calibration errors, measurement accuracy, inaccuracies of the model, and torus variability, but find no definitive cause.