Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p32c..02h&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P32C-02
Physics
[2704] Magnetospheric Physics / Auroral Phenomena, [5704] Planetary Sciences: Fluid Planets / Atmospheres, [5706] Planetary Sciences: Fluid Planets / Aurorae, [6293] Planetary Sciences: Solar System Objects / Uranus
Scientific paper
Extensive observations (more than 121,000 spectra) of the Uranian aurora by the ultraviolet spectrometer (UVS) on Voyager 2 have been analyzed. By exploiting earlier work 1 on the geographic distribution of the aurora, this analysis has achieved a clean separation between the superposed spectra of the sun-side airglow, the aurora, and the sky background. One surprising result is that the auroral spectrum contains the He 584 Å emission (intensity about 0.5% of the short-wavelength H2 band emission, or about 1-2 Rayleighs at the brightest part of the aurora), even though this emission is indetectable in the airglow spectrum ( ≤ 0.01 Rayleighs). This emission's weakness in the airglow spectrum is explained by the fact that mixing in Uranus' upper atmosphere at the time of observation (extreme summer solstice at Uranus) was far weaker than at any other outer planet (homopausal eddy diffusion no more than 200 cm2/s at the sunlit pole2,3, compared to 106 cm2/s or more at the other outer planets). Thus its helium component was buried under a large column density of H2 and H, both of which are strong absorbers at 584 Å. This emission's presence in the auroral spectrum, by contrast, might be evidence for increased upward mixing at the location of the aurora because of the aurora's highly localized deposition of heat there. Alternatively, it could be due to the doubled absorption path for the back-scattered solar line as compared to the auroral one. The extremely low degree of upper atmospheric mixing on Uranus might plausibly be related to Uranus' extreme solstice orientation at the time of observation. With very little diurnal variation of insolation at any location, weather might thus have been subdued, and indeed, extensive imaging of the planet at the time revealed a nearly featureless troposphere, with virtually no clouds4. Since that time, as Uranus' aspect has changed to equinox, clouds have become much more visible5, and upper-atmospheric eddy diffusion might have increased also. However, at the time of Voyager encounter, auroral heating might have been the only way to excavate helium up to a visible altitude. Another important feature of the auroral spectrum is the long-wavelength (1500-1600 Å) Lyman-band emission of H2. This emission has been seen in the airglow6, but there it was obscured by a much stronger solar reflection. In the auroral spectrum it is distinct, attaining a level of about 150 Rayleighs in the brightest part of the aurora . A comparison of long- and short-wavelength H2 band intensities appears about right for excitation by typical auroral secondary-electron energies of a few tens of eV. Modeling of this auroral spectrum may potentially constrain the depth of penetration of the primary auroral precipitating electrons, and thus their average energy, providing clues to their source mechanisms. References: 1 Herbert, JGR in press, 2 Yelle et al., GRL 14:483, 3 Ben Jaffel et al., JGR 96:9781, 4 Smith et al., Science 233:43, 5 Hammel et al., Icarus 175:284, 6 Broadfoot et al., Science 233:74
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