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TOPIC: Neptune atmosphere

Blobrana

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Posts: 131433

Date: Apr 14 06:52 2015

Neptune's atmosphere	Permalink

Title: Accurate and Approximate Calculations of Raman Scattering in the Atmosphere of Neptune Author: Lawrence Sromovsky Raman scattering by H₂ in Neptune's atmosphere has significant effects on its reflectivity for lambda< 0.5 m, producing baseline decreases of ~ 20% in a clear atmosphere and ~ 10% in a hazy atmosphere. Here we present the first radiation transfer algorithm that includes both polarisation and Raman scattering and facilitates computation of spatially resolved spectra. New calculations show that Cochran and Trafton's (1978, Astrophys. J. 219, 756-762) suggestion that light reflected in the deep CH₄ bands is mainly Raman scattered is not valid for current estimates of the CH₄vertical distribution, which implies only a 4% Raman contribution. Comparisons with IUE, HST, and groundbased observations confirm that high altitude haze absorption is reducing Neptune's geometric albedo by ~6% in the 0.22-0.26 m range and by ~13% in the 0.35-0.45 m range. We used accurate calculations to evaluate several approximations of Raman scattering. The Karkoschka (1994, Icarus 111, 174-192) method of removing Raman effects from observed spectra is shown to have limited applicability and to undercorrect the depths of weak CH4 absorption bands. The Wallace (1972, Astrophys. J. 176, 249-257) approximation produces geometric albedo values ~5% low as originally proposed, but can be much improved by adding scattering contributions from the vibrational transition. The Pollack et al. (1986, Icarus 65, 442-466) approximation is inaccurate and unstable, but can also be improved greatly by several simple modifications. A new approximation provides low errors for zenith angles below 70\deg in a clear atmosphere, although intermediate clouds present problems at longer wavelengths. Read more (544kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Dec 2 08:46 2013

Neptune atmosphere	Permalink

Title: Neptune at Summer Solstice: Zonal Mean Temperatures from Ground-Based Observations 2003-2007 Author: Leigh N. Fletcher, Imke de Pater, Glenn S. Orton, Heidi B. Hammel, Michael L. Sitko, Patrick G.J. Irwin Imaging and spectroscopy of Neptune's thermal infrared emission is used to assess seasonal changes in Neptune's zonal mean temperatures between Voyager-2 observations (1989, heliocentric longitude Ls=236) and southern summer solstice (2005, Ls=270). Our aim was to analyse imaging and spectroscopy from multiple different sources using a single self-consistent radiative-transfer model to assess the magnitude of seasonal variability. Globally-averaged stratospheric temperatures measured from methane emission tend towards a quasi-isothermal structure (158-164 K) above the 0.1-mbar level, and are found to be consistent with spacecraft observations of AKARI. This remarkable consistency, despite very different observing conditions, suggests that stratospheric temporal variability, if present, is ±5 K at 1 mbar and ±3 K at 0.1 mbar during this solstice period. Conversely, ethane emission is highly variable, with abundance determinations varying by more than a factor of two. The retrieved C2H6 abundances are extremely sensitive to the details of the T(p) derivation. Stratospheric temperatures and ethane are found to be latitudinally uniform away from the south pole (assuming a latitudinally-uniform distribution of stratospheric methane). At low and midlatitudes, comparisons of synthetic Voyager-era images with solstice-era observations suggest that tropospheric zonal temperatures are unchanged since the Voyager 2 encounter, with cool mid-latitudes and a warm equator and pole. A re-analysis of Voyager/IRIS 25-50 {\mu}m mapping of tropospheric temperatures and para-hydrogen disequilibrium suggests a symmetric meridional circulation with cold air rising at mid-latitudes (sub-equilibrium para-H2 conditions) and warm air sinking at the equator and poles (super-equilibrium para-H2 conditions). The most significant atmospheric changes are associated with the polar vortex (absent in 1989). Read more (4783kb, PDF) __________________

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