Ultraviolet scattering properties and the size distribution of interstellar grains     

A. N. Witt 《Astrophysics and Space Science》1979,65(1):21-28

Recent work related to the determination of the size distribution of interstellar grains is reviewed briefly. It is shown that the scattering properties of interstellar grains determined from ultraviolet observations of reflection nebulae point toward the existence of a bimodal size distribution of grains, consisting of classical wavelength sized grains and a class of much more numerous smaller particles, which scatter far ultraviolet light isotropically with near unit albedo. The implications of this result are discussed briefly.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.  相似文献   

The nature of the red disc-like galaxies at high redshift: dust attenuation and intrinsically red stellar populations     

D. Pierini  C. Maraston  K. D. Gordon  A. N. Witt 《Monthly notices of the Royal Astronomical Society》2005,363(1):131-145

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Using continuous plankton recorder data     

A.J. Richardson  A.W. Walne  T.D. Jonas  D.W. Sims  M. Witt 《Progress in Oceanography》2006,68(1):27-74

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O2 density and temperature profiles retrieving from direct solar Lyman-alpha radiation measurements     

V. Guineva  G. Witt  J. Gumbel  M. Khaplanov  R. Werner  J. Hedin  S. Neichev  B. Kirov  L. Bankov  P. Gramatikov  V. Tashev  M. Popov  K. Hauglund  G. Hansen  J. Ilstad  H. Wold 《Geomagnetism and Aeronomy》2009,49(8):1292-1295

The resonance transition ²P-²S of the atomic hydrogen (Lyman-alpha emission) is the strongest and most conspicuous feature in the solar EUV spectrum. The Lyman-alpha radiation transfer depends on the resonance scattering from the hydrogen atoms in the atmosphere and on the O₂ absorption. Since the Lyman-alpha extinction in the atmosphere is a measure for the column density of the oxygen molecules, the atmospheric O₂ density and temperature profiles can be calculated thereof. A detector of solar Lyman-alpha radiation was manufactured in the Stara Zagora Department of the Solar-Terrestrial Influences Laboratory (STIL). Its basic part is an ionization camera, filled in with NO. A 60 V power supply is applied to the chamber. The produced photoelectric current from the sensor is fed to a two-channel amplifier, providing analog signal. The characteristics of the Lyman-alpha detector were studied. It passed successfully all tests and the results showed that the so-designed instrument could be used in rocket experiments to measure the Lymanalpha flux. From the measurements of the detector, the Lyman-alpha vertical profile can be obtained. Programs are created to compute the O₂ density, atmospheric power and temperature profiles based on Lymanalpha data. The detector design appertained to ASLAF project (Attenuation of the Solar Lyman-Alpha Flux), a scientific cooperation between STIL—Bul.Acad.Sci., Stara Zagora Department and the Atmospheric Physics Group at the Department of Meteorology (MISU), Stockholm University, Sweden. The joint project was part of the rocket experiment HotPay I, in the ALOMAR eARI Project, EU’s 6th Framework Programme, Andøya Rocket Range, Andenes, Norway. The project is partly financed by the Bulgarian Ministry of Science and Education.  相似文献   

Major and trace element distributions around active volcanic vents determined by analyses of grasses: implications for element cycling and bio-monitoring   总被引：1，自引：0，他引：1  

R. S. Martin  T. A. Mather  D. M. Pyle  J. A. Day  M. L. I. Witt  S. J. Collins  R. G. Hilton 《Bulletin of Volcanology》2010,72(8):1009-1020

Samples of grass were collected at Masaya Volcano (Nicaragua; Rhynchelytrum repens and Andropogon angustatus) and the Piton de La Fournaise (around the April 2007 eruptive vent, La Réunion; Vetiveria zizanioides) to investigate the controls on major and trace element concentrations in plants around active volcanic vents. Samples were analysed using inductively coupled plasma mass spectrometry for a wide range of elements, and atomic absorption spectroscopy for Hg. At Masaya, As, Cu, Mo, Tl and K concentrations in both grass species showed a simple pattern of variability consistent with exposure to the volcanic plume. Similar variability was found in A. angustatus for Al, Co, Cs, Hg and Mg. At the Piton de La Fournaise, the patterns of variability in V. zizanioides were more complex and related to variable exposures to emissions from both the active vent and lava flow. These results suggest that exposure to volcanic emissions is, for many elements, the main control on compositional variability in vegetation growing on active volcanoes. Thus, vegetation may be an important environmental reservoir for elements emitted by volcanoes and should be considered as part of the global biogeochemical cycles.  相似文献