| 11. |
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| 12. |
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| 13. |
- Amanda Collaboration, -, et al.
(författare)
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Optical Properties of Deep Glacial Ice at the South Pole
- 2006
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202 .- 2169-897X. ; 111:D13, s. D13203-
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Tidskriftsartikel (refereegranskat)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.
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| 14. |
- Anderson, Leif G, 1951, et al.
(författare)
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Source and formation of the upper halocline of the Arctic Ocean
- 2013
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Ingår i: Journal of Geophysical Research - Oceans. - 0148-0227 .- 2156-2202. ; 118:1, s. 410-421
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Tidskriftsartikel (refereegranskat)abstract
- The upper halocline of the Arctic Ocean has a distinct chemical signature with high nutrient concentrations as well as low oxygen and pH values. This signature is formed in the Chukchi and East Siberian Seas, by a combination of mineralization of organic matter and release of decay products to the sea ice brine enriched bottom water. Salinity and total alkalinity data show that the fraction of sea ice brine in the nutrient enriched upper halocline water in the central Arctic Ocean is up to 4%. In the East Siberian Sea the bottom waters with exceptional high nutrient concentration and low pH have typically between 5 and 10% of sea ice brine as computed from salinity and oxygen-18 values. On the continental slope, over bottom depths of 15-200 m, the brine contribution was 6% at the nutrient maximum depth (50-100 m). At the same location as well as over the deeper basin the silicate maximum was found over a wider salinity range than traditionally found in the Canada Basin, in agreement with earlier observations east of the Chukchi Plateau. A detailed evaluation of the chemical and the temperature-salinity properties suggests at least two different areas for the formation of the nutrient rich halocline within the East Siberian Sea. This has not been observed before 2004 and it could be a sign of a changing marine climate in the East Siberian Sea, caused by more open water in the summer season followed by more sea ice formation and brine production in the fall/winter.
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| 15. |
- Andersson, Camilla, 1979-, et al.
(författare)
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European ozone in a future climate : The importance of changes in dry deposition and isoprene emissions
- 2010
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 115, s. D02303-
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Tidskriftsartikel (refereegranskat)abstract
- [1] Using a regional chemistry transport model (CTM) driven with meteorological data from a regional climate model (A2 emission scenario) we have investigated important processes for determining surface ozone concentration across Europe in a future climate. Anthropogenic ozone precursor emissions were kept constant (year 2000) to isolate the effect of climate change from 1961-1990 to 2021-2050 and 2071-2100. Biogenic isoprene emissions were calculated on-line. The results point to substantial increase in daily mean and daily maximum surface ozone over central and southern Europe. The importance of changes in natural isoprene emissions and dry deposition were investigated especially.[2] The isoprene emissions increased by a factor of about 1.8 from the current to the second future period. However, a sensitivity study using a sophisticated rescaling of isoprene emissions shows that the large increase in isoprene emission is of moderate importance in southern Europe for the strong increase in surface ozone (it can explain up to 30% of the change in central, southern and western Europe).[3] The soil-moisture dependent ozone dry deposition formulation and changes in snow cover, affecting the dry deposition, are more important processes: soil moisture dependence explains up to 80% of the change in Spain. Therefore it is vital to include soil moisture dependence in a model study of this type. Isoprene emissions are of less importance (0-30% in central and southern Europe), but not dismissible and should definitely be emitted on-line in climate-ozone projection studies.
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| 16. |
- Andersson, L, et al.
(författare)
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Electron signatures and Alfven waves
- 2002
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:A9
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Tidskriftsartikel (refereegranskat)abstract
- [1] We identify two distinct electron populations associated with Alfven waves in the Freja data set using the high time resolution state of the art electron detector. One of the populations, detected together with an Alfven wave, is field-aligned and can be seen as trapped within the wave. The other electron population is detected before the wave and consists of electrons which have left the wave at a point with a velocity higher than the local Alfven speed. In the paper, the electrons leaving wave are modeled for different density profiles and are compared with the observed data. Depending on the density profile, the model can produce the same energy-time and pitch angle-time dispersion that is observed in the Freja data. The conclusion of the paper is that the Alfven wave can explain the observed particle signatures. It is shown that the Alfven wave acceleration can create electron signatures similar to inverted-V structures. The density distribution along a flux tube has an important role in the type of particle signatures that can be detected at low altitudes.
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| 17. |
- Andreeova, K., et al.
(författare)
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Shock propagation in the magnetosphere : Observations and MHD simulations compared
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A9, s. A09224-
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Tidskriftsartikel (refereegranskat)abstract
- We examine the propagation of disturbances in the Earth's magnetosphere caused by fast forward shock interaction with the magnetopause. Our statistical study and event analyses show that the propagation speeds are larger in the magnetosphere than in the solar wind and are larger in the nightside magnetosphere than in the dayside magnetosphere. A case study of a double shock during 9 November 2002 is examined both observationally and using the GUMICS-4 global MHD simulation. Tracing the disturbance propagation allows us to confirm that the MHD simulation results are in good agreement with the in situ observations. The simulation results show that the propagation of the disturbance occurs in the antisunward direction at all clock angles simultaneously. However, changes in the magnetosheath are largest at high latitudes, while in the magnetotail the largest variations are seen in the plasma sheet.
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| 18. |
- Andrews, David J., et al.
(författare)
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Planetary period oscillations in Saturn's magnetosphere : Evolution of magnetic oscillation properties from southern summer to post-equinox
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. A04224-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the evolution of the properties of planetary period magnetic field oscillations observed by the Cassini spacecraft in Saturn's magnetosphere over the interval from late 2004 to early 2011, spanning equinox in mid-2009. Oscillations within the inner quasi-dipolar region (L <= 12) consist of two components of close but distinct periods, corresponding essentially to the periods of the northern and southern Saturn kilometric radiation (SKR) modulations. These give rise to modulations of the combined amplitude and phase at the beat period of the two oscillations, from which the individual oscillation amplitudes and phases (and hence periods) can be determined. Phases are also determined from northern and southern polar oscillation data when available. Results indicate that the southern-period amplitude declines modestly over this interval, while the northern-period amplitude approximately doubles to become comparable with the southern-period oscillations during the equinox interval, producing clear effects in pass-to-pass oscillation properties. It is also shown that the periods of the two oscillations strongly converge over the equinox interval, such that the beat period increases significantly from similar to 20 to more than 100 days, but that they do not coalesce or cross during the interval investigated, contrary to recent reports of the behavior of the SKR periods. Examination of polar oscillation data for similar beat phase effects yields a null result within a similar to 10% upper limit on the relative amplitude of northern-period oscillations in the south and vice versa. This result strongly suggests a polar origin for the two oscillation periods.
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| 19. |
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| 20. |
- Arvidson, R. E., et al.
(författare)
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Opportunity Mars Rover mission : Overview and selected results from Purgatory ripple to traverses to Endeavour crater
- 2011
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Ingår i: Journal of Geophysical Research. - Hoboken : Wiley-Blackwell. - 0148-0227 .- 2156-2202. ; 116, s. E00F15-
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Tidskriftsartikel (refereegranskat)abstract
- Opportunity has been traversing the Meridiani plains since 25 January 2004 (sol 1), acquiring numerous observations of the atmosphere, soils, and rocks. This paper provides an overview of key discoveries between sols 511 and 2300, complementing earlier papers covering results from the initial phases of the mission. Key new results include (1) atmospheric argon measurements that demonstrate the importance of atmospheric transport to and from the winter carbon dioxide polar ice caps; (2) observations showing that aeolian ripples covering the plains were generated by easterly winds during an epoch with enhanced Hadley cell circulation; (3) the discovery and characterization of cobbles and boulders that include iron and stony-iron meteorites and Martian impact ejecta; (4) measurements of wall rock strata within Erebus and Victoria craters that provide compelling evidence of formation by aeolian sand deposition, with local reworking within ephemeral lakes; (5) determination that the stratigraphy exposed in the walls of Victoria and Endurance craters show an enrichment of chlorine and depletion of magnesium and sulfur with increasing depth. This result implies that regional-scale aqueous alteration took place before formation of these craters. Most recently, Opportunity has been traversing toward the ancient Endeavour crater. Orbital data show that clay minerals are exposed on its rim. Hydrated sulfate minerals are exposed in plains rocks adjacent to the rim, unlike the surfaces of plains outcrops observed thus far by Opportunity. With continued mechanical health, Opportunity will reach terrains on and around Endeavour's rim that will be markedly different from anything examined to date. Copyright 2011 by the American Geophysical Union.
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