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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS2001

Resonance Scattering of Fe XVII X-Ray and EUV Lines

Over the years a number of calculations have been carried out to derive intensities of various X-ray and EUV lines in Fe XVII to compare with observed spectra. The predicted intensities have not agreed with solar observations, particularly for the line at 15.02 Angstroms; resonance scattering has been suggested as the source for much of the disagreement. The atomic data calculated earlier used seven configurations, 2s(sup 2) 2p(sup 6),2s(sup 2) 2p(sup 5)3s, 2s(sup 2) 2p(sup 5)3p, 2s(sup 2)2p(sup 5)3d, 2s2 p(sup 6)3s, 2s2p(sup 6)3p, and 2s2p(sup 6)3d, having n=3 orbitals and the scattering calculations in the distorted wave approximation were carried out only for incident energies above the threshold of the highest fine-structure level. These calculations have now been extended to thirteen configurations by adding six more configurations having n=4 orbitals, namely 2s(sup 2) 2p(sup 5)4s, 2s(sup 2) 2p(sup 5)4p, 2s(sup 2) 2p(sup 5)4d, 2s2p(sup 6)4s, 2s2p(sup 6)4p, and 2s2p(sup 6)4d, giving rise to 73 fine structure levels. The scattering calculations are carried out below as well as above the threshold of the highest fine-structure level. The incident energies below the threshold are 55.8, 65, 70, and 76 Ry and those above are 85, 127.5, 170, 212.5, 255, 340, and 425 Ry. The collision strengths are calculated up to total angular momentum L(sup T) = 33. Level populations and intensity ratios are calculated at various electron temperatures and densities. It is not possible to predict how the ratios would change when configurations with n=5 and 6 orbitals are added but some estimates have been made for a few transitions by Liedhal, who indicates an asymptotic convergence when n=5 and 6 orbitals are added. These improved calculations of Fe XVII change the intensity ratios compared to those obtained earlier, bringing the optically thin F(15.02)/F(16.78) ratio and several other ratios closer to the observed values: However, some disagreement with the solar observations still persists, even though the agreement of the presently calculated optically thin F(15.02)/F(15.26) ratio with the experimental results of Brown et al. and Laming et al. has improved. Some of the remaining discrepancy is still thought to be the effect of opacity, which is consistent with expected physical conditions for solar sources. EUV intensity ratios are also calculated and compared with observations. Level populations and intensity ratios are calculated, as a function of column density of Fe XVII, in the slab and cylindrical geometries. As found previously, the predicted intensities for the resonance lines at 15.02 and 15.26 Angstroms exhibit initial increases in flux relative to the forbidden line at 17.10 Angstroms and the resonance line at 16.78 Angstroms as optical thickness increases. The same behavior is predicted for the lines at 12.262 and 12.122 Angstroms. Predicted intensities for some of the allowed EUV lines are also affected by opacity.

Bhatia, A. K.↗
NASA NTRS2005

Near Infrared Spectra of Mixtures Relevant to Icy Satellites

We will present near IR spectra of ice mixtures and review the differences between spectra of mixtures and those of pure solids: the creation of new features, weakening and shifting of bands, dependence on concentration, and changes with temperature. The forbidden CO2 (2nu3) overtone at 2.134 microns (4685/cm) is absent from the spectrum of pure CO2, but present in all of the following mixtures: H2O/CO2 = 5 and = 25, H2O:CH3OH:CO2 = 100:2.5:1, and CH3OH:CO2 = 5. Also, in mixtures of H2O and any other material, we see a feature at 1.89 microns (5290/cm) that is possibly related to the "dangling OH" feature at 2.73 microns (3360/cm). The features of a material in H2O are generally weaker and shifted to longer wavelength in comparison to the pure substance. For example, the largest near-IR absorption of pure solid CH4 is located at 2.324 microns (4303/cm) but is broader and at slightly longer wavelength in samples mixed with H2O. The degree of shifting and weakening depends on the ratio of the mixture. The mixture mentioned above was at a ratio of H2O/ CH4 = 3. When the ratio rises to H2O/ CH4 = 87, the CH4 feature at 2.324 microns is shifted to shorter wavelength and is much broader and weaker. In CH4/ H2O mixtures the peaks shift to higher frequency and become increasingly broad, but this trend is reversible on re-cooling, even though the phase transitions of H2O are irreversible. In short, mixtures created in the lab produce spectra that are very different from modeled combinations of end member species. Recent Cassini VIMS observations show the CO2 fundamental at 4.255 microns (2350/cm) on Iapetus [l] and at 4.26 microns (2347/cm) on Phoebe [2], while Galileo NIMS observed it at 4.25 microns (2353/cm) on Ganymede [3]. Since pure CO2 is located at 4.266 (2344/cm), the CO2 must be mixed with something else to produce the shift. A mixture of CH3OH:CO2 = 5 at 90 K shifts the fundamental to 4.262 microns (2346/cm). The shifts in the feature between satellites could indicate variations in concentration and temperature, or the introduction of a new compound, encouraging further lab investigation.

Mastrapa, Rachel M. E.↗