1,683 research outputs found

    Electron Collisional Broadening of Isolated Lines from Multiply-Ionized Atoms

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    Recent experimental and theoretical (both improved semiclassical and fully quantum-mechanical) results for B III and Ne VII delta n = 0 transitions with n = 2 or 3, respectively, are discussed. The probable reasons for disagreements by about a factor of 2 between the fully quantum-mechanically calculated and both measured and semiclassically calculated widths can be explained in terms of violation of validity criteria and non-thermal Doppler effects. More independent determinations of line widths for multiply-ionized non-hydrogenic ions are called for, but meanwhile caution should be exercised in the use of corresponding semiclassically calculated widths, e.g., in opacity calculations or for plasma density diagnostics. Only the quantum calculations allow a clear separation of elastic and inelastic scattering contributions to the width. For B III, elastic scattering contributes about 30% whereas for Ne VII inelastic scattering dominates. This allows rather direct comparisons with benchmark electron-ion scattering experiments.Comment: 13 pages, PostScript only. To appear in JQSR

    Corrections to the Asymptotic Holtsmark Formula for Hydrogen Lines Broadened by Electrons and Ions in a Plasma

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    Corrections to asymptotic Holtsmark formula for hydrogen lines broadened by electrons and ions in plasm

    Electron Impact Broadening of Isolated Ion Lines

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    Electron impact broadening of isolated ion spectral line

    Electron-impact broadening of the 3s-3p lines in low-Z Li-like ions

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    The collisional electron-impact line widths of the 3s-3p transitions in Li-like ions from B III to Ne VIII are calculated with the convergent close-coupling (CCC) method from the atomic collision theory. The elastic and inelastic contributions to the line broadening and their Z-scaling are discussed in detail, and comparisons with recent experimental and theoretical results are also presented. It is found that similar to our previous study of line broadening in Be-like ions, the difference between experimental and CCC results monotonically increases with the spectroscopic charge of an ion.Comment: 23 pages, 8 figures, 1 table. To be published in JQSR

    Study of a colliding laser-produced plasma by analysis of time and space-resolved image spectra

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    The interaction of two counter-propagating laser-produced plasmas was studied using simultaneous imaging and spectroscopic techniques. Spectrally-filtered time-gated ICCD imaging was used to obtain information about the spatial dynamics and temporal evolution of the collision process. While, time-resolved imaging spectroscopy was used to determine the spatial and temporal distributions of electron temperature and density within the interaction region. We examine specifically the interaction of plasmas whose parameters match those typically used in pulsed laser deposition of thin films. These low temperature plasmas are highly collisional leading to the creation of a pronounced stagnation layer in the interaction region

    Ocean circulation changes off southern Greenland during the abrupt climate events of mid-to-late MIS3

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    Marine sediment cores from the North Atlantic and ice cores from the Greenland Ice Sheet serve as natural archives of past climate variability. Ice cores have revealed that during the last glacial period the climate comprises both relatively stable intervals such as the Last Glacial Maximum (LGM, ca. 26-19 ka) and unstable climate intervals such as Marine Isotope Stage 3 (MIS3, ca. 59-29 ka). The climate of MIS3 is characterized by abrupt changes from colder stadial to warmer interstadial conditions, well-known as Dansgaard-Oeschger events (DO). Although DO events had repercussions over the climate system outside of the high latitude Northern Hemisphere, they are especially pronounced in records from the North Atlantic region. North Atlantic marine sediments record changes in sea ice cover, surface productivity and iceberg discharge, which are all associated with DO events. Iceberg discharge from the Northern Hemisphere ice sheets, and hence release of fresh water into the North Atlantic was especially pronounced during so-called Heinrich (H) Stadials. H-Stadials occurred approximately every 7 to 10 kyrs and are particularly cold and long stadials within the DO event oscillations. The freshwater discharge further contributed to sea ice formation in the North Atlantic and Nordic Seas. This near-perennial sea ice cover during stadials prevented ocean-atmosphere interaction while during interstadials the Nordic Seas were seasonally ice-free. These interstadial periods allowed moisture to evaporate from the oceanĀ“s surface where after it was transported towards the Northern Hemisphere Ice Sheets, and thereby sustain their growth during MIS3 and the LGM. This thesis aims to give insights into the complex interactions between ice sheets, sea ice and ocean circulation during MIS3 and the LGM. Herein, the main focus lies on changes along the South-East Greenland margin, complemented by thoroughly studied sediment core sites from the Nordic Seas and Greenland ice core records. Paper I investigates environmental conditions between 65 and 25 ka recorded by a deep-sea sediment core from the northeastern Labrador Sea. Stable isotope data, ice-rafted debris (IRD) and planktic foraminifer fluxes and assemblage counts are compared to sediment cores from the Irminger Basin and the Greenland Sea. The results are used to draw conclusions about the sensitivity of the northwest North Atlantic to changes in sea ice extent, surface hydrography and iceberg discharge during the DO events of MIS3. We find, that an expanded sea ice cover seems to be characteristic for most stadials in the Labrador Sea while climate conditions during most interstadials allowed for open waters with a productive sea surface. Major iceberg calving events associated with H-Stadials 4 and 3 are documented. The influence of Atlantic Water is enhanced during Greenland Interstadials 14, 12 and 8. Paper II takes a detailed look at the source areas that may have contributed to each IRD event observed in Paper I. X-ray spectroscopy and single grain Lead-isotope measurements are performed. Additionally, several petrologic tracers and X-ray fluorescence Calcium-to-Strontium ratios are analyzed. The main results indicate that there are no specific trends of IRD sources throughout MIS3 or during interstadials, stadials and H-Stadials. Greenland is the main contributor of the IRD deposits observed at the Eirik Drift, although icebergs from the Laurentide Ice Sheet melted at the core site during the ice sheet collapse associated with H-Stadial 4. In Paper III, the knowledge gained within the scope of Papers I and II is combined with previously published and new datasets from three regions in the Nordic Seas and the subtropical North Atlantic. The datasets comprise biomarker, IRD and planktic foraminifer concentration as well as sea surface temperature records to evaluate changes in the North Atlantic sea ice extent during late MIS3 DO events and the LGM. The results from these proxy-data are then compared to stable oxygen isotope (Ī“18O) and deuterium excess (d-excess) records from the GRIP ice core. We find a strong link between the variability of d-excess data and the location of the sea ice edge in the North Atlantic. During interstadials and the LGM, sea surface conditions were similar and allowed for a northern moisture source of precipitation as sea ice retreated to the northwestern Nordic Seas. Additionally, the investigated regions were characterized by high plankton productivity during the LGM. The surface of the subtropical North Atlantic was as warm or warmer during the LGM than during MIS3 interstadials. The results of this thesis reveal that the northeastern Labrador was a highly dynamic region during the DO events of MIS3. The high-resolution MIS3 data close the gap between the well-studied LGM and MIS5e and connect this part of the North Atlantic to the well-constrained sea surface conditions of the Nordic Seas and the mid-latitude North Atlantic. Furthermore, by mapping the North Atlantic sea surface conditions during MIS3 DO events and the LGM, a link between ice core d-excess and the Nordic Seas sea ice cover is demonstrated.Doktorgradsavhandlin

    More on the Narrowing of Impact Broadened Radio Recombination Lines at High Principal Quantum Number

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    Recently Alexander and Gulyaev have suggested that the apparent decrease in impact broadening of radio recombination lines seen at high principal quantum number n may be a product of the data reduction process, possibly resulting from the presence of noise on the telescope spectra that is not present on the calculated comparison spectra. This is an interesting proposal. However, there are serious problems with their analysis that need to be pointed out. Perhaps the most important of these is the fact that for principal quantum numbers below n = 200, where the widths are not in question, their processed generated profile widths do not fit the widths of the processed lines obtained at the telescope. After processing, the halfwidths of the generated and telescope profiles must agree below n = 200 if we are to believe that the processed generated linewidths above n = 200 are meaningful. Theirs do not. Furthermore, we find that after applying the linewidth reduction factors found by Alexander and Gulyaev for their noise added profiles to our generated profiles to simulate their noise adding effect, the processed widths we obtain still do not come close to explaining the narrowing seen in the telescope lines for n values in the range 200 < n < 250. It is concluded that what is needed to solve this mystery is a completely new approach using a different observing technique instead of simply a further manipulation of the frequency-switched data.Comment: Six pages with 4 figures. Accepted for publication in Astrophysics and Space Scienc

    New parametrization for differences between plasma kinetic codes

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    Validation and verification of plasma kinetics codes requires the development of quantitative methods and techniques for code comparisons. We describe two parameters that can be used for characterization of differences between such codes. It is shown that these parameters, which are determined from the most general results of kinetic codes, can provide important information on the differences between the basic rate coefficients employed. Application of this method is illustrated by comparisons of some results from the 3rd NLTE Code Comparison Workshop for carbon, germanium, and gold plasmas.Comment: Submitted to High Energy Density Physics, 12 pages, 2 figure
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