Uppsala University

About: Uppsala University is a education organization based out in Uppsala, Sweden. It is known for research contribution in the topics: Population & Insulin. The organization has 36485 authors who have published 107509 publications receiving 4220668 citations. The organization is also known as: Uppsala universitet & uu.se.

Optical properties of deep glacial ice at the South Pole

Abstract: We have remotely mapped optical scattering and absorption in glacial ice at the South Pole for wavelengths between 313 and 560 nm and depths between 1100 and 2350 m. We used pulsed and continuous light sources embedded with the AMANDA neutrino telescope, an array of more than six hundred photomultiplier tubes buried deep in the ice. At depths greater than 1300 m, both the scattering coefficient and absorptivity follow vertical variations in concentration of dust impurities, which are seen in ice cores from other Antarctic sites and which track climatological changes. The scattering coefficient varies by a factor of seven, and absorptivity (for wavelengths less than ∼450 nm) varies by a factor of three in the depth range between 1300 and 2300 m, where four dust peaks due to stadials in the late Pleistocene have been identified. In our absorption data, we also identify a broad peak due to the Last Glacial Maximum around 1300 m. In the scattering data, this peak is partially masked by scattering on residual air bubbles, whose contribution dominates the scattering coefficient in shallower ice but vanishes at ∼1350 m where all bubbles have converted to nonscattering air hydrates. The wavelength dependence of scattering by dust is described by a power law with exponent -0.90 ± 0.03, independent of depth. The wavelength dependence of absorptivity in the studied wavelength range is described by the sum of two components: a power law due to absorption by dust, with exponent -1.08 ± 0.01 and a normalization proportional to dust concentration that varies with depth; and a rising exponential due to intrinsic ice absorption which dominates at wavelengths greater than ∼500 nm. Copyright 2006 by the American Geophysical Union.

Reverse Monte Carlo modelling

Abstract: Reverse Monte Carlo (RMC) modelling is a general method of structural modelling based on experimental data. RMC modelling can be applied to many different sorts of data, simultaneously if wished. Powder and single-crystal neutron diffraction (including isotopic substitution), x-ray diffraction (including anomalous scattering) and electron diffraction, extended x-ray absorption fine structure and nuclear magnetic resonance (magic angle spinning and second moment) have already been used to provide data. RMC modelling can also be applied to many different types of system - liquids, glasses, polymers, crystals and magnetic materials. This article outlines the RMC method and discusses some of the common misconceptions about it. It is stressed that RMC models are neither unique nor `correct'. However, they are often useful for aiding our understanding either of the structure itself, or of the relationships between local structure and other physical properties. Examples are given and the possibilities for further development of the RMC method are discussed.