H2 fluorescence spectrum from 1200 to 1700 A by electron impact - Laboratory study and application to Jovian aurora

Abstract

A by electron impact and its application to modeling the Jovian aurora have been carried out. Our laboratory data suggest that at 100 eV the relative cross sections for direct excitation of Lya, Lyman bands (B 1~u +-x 1~/), and Werner bands (C 1 '1Tu-X 1~/) are 1, 2.3±0.6, and 2.6±0.5, respectively, in conflict with Stone and Zipfs (1972) results for the Werner bands. Cascade from E, F 1~g + states contributes an additional 31% to the B 1~u + state population. It is shown that the most likely fate for the metastable H(2 2 S) atoms produced in the Jovian aurora is collisional quenching to H(2 2 P), and this could add as much as 60% to the predicted Lya emission. On the basis of detailed atmospheric and radiative transfer modeling, we conclude that the recent JUE and Voyager observations are consistent with precipitation of electrons with energy in the range of 1-30 keV or other energetic particles that penetrate to number densities of 4 X 10 10-5 X 10 13 cm-3 or column densities of 5 X 10 17-2 X 10 20 cm-2 in the atmosphere. The globally averaged energy flux and production of hydrogen atoms are 0.5-2 ergs cm-2 s-1 and 1-4 X 10 10 atoms cm-2 s-1, respectively.

Spatially Resolved Far Ultraviolet Spectroscopy of the Jovian Aurora

“…Since the first spectrum is partly absorbed by methane while the second one is not, it is possible to derive a color ratio R = I (1550−1620Å)/I (1230−1300Å) from these spectra. It should be noted that the intensities are expressed in rayleighs, so that R is 1.25 times the original color ratio defined by Yung et al (1982), expressed in energy units. Since the bandwidths of the first pair of observed spectra are 1194-1250Å and 1540-1595Å, respectively, we used an unabsorbed synthetic spectrum to extrapolate their total intensities to the bandpasses relevant for R. The numerator of R has been directly estimated from the second (unabsorbed) observed spectrum, using a synthetic unattenuated spectrum.…”

“…The H 2 FUV color ratio C = I (1550−1620Å)/I (1230− 1300Å) (Yung et al 1982) is used as a measure of the attenuation of the H 2 emission by methane and other hydrocarbons overlying the emission layer. It is directly related to the drop in the CH 4 absorption cross section at wavelengths below 1350Å, which leaves the bulk of the long-wavelength H 2 Lyman bands unabsorbed, while the short-wavelength Lyman and Werner bands are absorbed by methane.…”

Characteristics of north jovian aurora from STIS FUV spectral images

“…Methane (CH 4 ) attenuates the emission at wavelengths below 1400 Å, ethane (C 2 H 6 ), which has a continuous absorption cross-section shortward of 1550 Å, has a typical signature between 1400 and 1480 Å in the case of strongly attenuated spectra, and acetylene (C 2 H 2 ) which has a more peaked cross-section and has a significant effect at 1480 Å and 1520 Å. This very wavelength-65 dependent absorption is measured by the color ratio CR = I(1550-1620Å) / I (1230-1300Å) with I the brightness in photons unit, originally introduced in a slightly different form by Yung et al (1982). The CR is directly correlated with the amount of hydrocarbon at or above the H 2 auroral emission and is related to the altitude of the aurora relative to the hydrocarbons.…”

The color ratio-intensity relation in the Jovian aurora: Hubble observations of auroral components

“…The second approach is based on the characteristics of the emitted far ultraviolet auroral spectrum (Yung et al, 1982). Comparison between the observed UV spectrum and a reference H 2 laboratory spectrum is made to locate the bulk of the auroral emission relative to the hydrocarbon homopause.…”