Physical processes determining the chromospheric temperature distribution

Calculations performed with several models of the solar chromosphere support Ulmschneider's conclusion that relatively short period acoustic waves heat the low chromosphere in the region just above the temperature minimum. However, these same short period waves (10 period P80 s) are not able to maintain chromospheric temperatures at heights where _5000Å(normal) < 10^-6. The calculations also show that an earlier conjecture stating that the H₂ population might influence the non-LTE chromospheric H^- population is probably not correct, due to lower values of the ratio n _e/n _H inferred from more recent observations. Finally, the calculations support Athay's contention that the Cayrel mechanism alone cannot produce the observed temperature rise, because the magnitude of the radiative cooling in the lines is too great.     

Intracratonic asthenosphere upwelling and lithosphere rejuvenation beneath the Hoggar swell (Algeria): Evidence from HIMU metasomatised lherzolite mantle xenoliths

The mantle xenoliths included in Quaternary alkaline volcanics from the Manzaz-district (Central Hoggar) are proto-granular, anhydrous spinel lherzolites. Major and trace element analyses on bulk rocks and constituent mineral phases show that the primary compositions are widely overprinted by metasomatic processes. Trace element modelling of the metasomatised clinopyroxenes allows the inference that the metasomatic agents that enriched the lithospheric mantle were highly alkaline carbonate-rich melts such as nephelinites/melilitites (or as extreme silico-carbonatites). These metasomatic agents were characterized by a clear HIMU Sr–Nd–Pb isotopic signature, whereas there is no evidence of EM1 components recorded by the Hoggar Oligocene tholeiitic basalts. This can be interpreted as being due to replacement of the older cratonic lithospheric mantle, from which tholeiites generated, by asthenospheric upwelling dominated by the presence of an HIMU signature. Accordingly, this rejuvenated lithosphere (accreted asthenosphere without any EM influence), may represent an appropriate mantle section from which deep alkaline basic melts could have been generated and shallower mantle xenoliths sampled, respectively. The available data on lherzolite xenoliths and alkaline lavas (including He isotopes, Ra < 9) indicate that there is no requirement for a deep plume anchored in the lower mantle, and that sources in the upper mantle may satisfactorily account for all the geochemical/petrological/geophysical evidence that characterizes the Hoggar swell. Therefore the Hoggar volcanism, as well as other volcanic occurrences in the Saharan belt, are likely to be related to passive asthenospheric mantle uprising and decompression melting linked to tensional stresses in the lithosphere during Cenozoic reactivation and rifting of the Pan–African basement. This can be considered a far-field foreland reaction of the Africa–Europe collisional system since the Eocene.     

Numerical modelling of depositional sequences in half-graben rift basins

ABSTRACT A three‐dimensional numerical model of sediment transport and deposition in coarse‐grained deltas is used to investigate the controls on depositional sequence variability in marine half‐graben extensional basins subject to eustatic sea‐level change. Using rates of sea‐level change, sediment supply and fault slip reported from active rift basins, the evolution of deltas located in three contrasting structural settings is documented: (1) footwall‐sourced deltas in high‐subsidence locations near the centre of a fault segment; (2) deltas fed by large drainage catchments at fault tips; and (3) deltas sourced from drainage catchments on the hangingwall dip slope. Differences in the three‐dimensional form and internal stratigraphy of the deltas result from variations in tilting of the hangingwall and the impact of border fault slip rates on accommodation development. Because subsidence rates near the centre of fault segments are greater than all but the fastest eustatic falls, footwall‐sourced deltas lack sequence boundaries and are characterized by stacked highstand systems tracts. High subsidence and steep bathymetry adjacent to the fault result in limited progradation. In contrast, the lower subsidence rate settings of the fault‐tip and hangingwall dip‐slope deltas mean that they are subject to relative sea‐level fall and associated fluvial incision and forced regression. Low gradients and tectonic tilting of the hangingwall influence the geometry of these deltas, with fault‐tip deltas preferentially prograding axially along the fault, creating elongate delta lobes. In contrast, broad, sheet‐like delta lobes characterize the hangingwall dip‐slope deltas. The model results suggest that different systems tracts may be coeval over length scales of several kilometres and that key stratal surfaces defining and subdividing depositional sequences may only be of local extent. Furthermore, the results highlight pitfalls in sequence‐stratigraphic interpretation and problems in interpreting controlling processes from the preserved stratigraphic product.     

EURD: First Year Results

EURD (Espectrógrafo Ultravioleta extremo de Radiación Difusa), one of the instruments onboard the Spanish satellite MINISAT-01, is a spectrograph specially designed to detect diffuse radiation, covering the wavelength range of 350-1100 Å, with a spectral resolution of 6-8 Å. Its main scientific objectives are the detection of the emission line spectrum from the hot phase of the interstellar medium and the spectrum of the upper atmospheric airglow. In order to reduce geocoronal noise, EURD always observes in the anti-sun direction and only when the satellite is in orbital eclipse. After more than one year of observation we have obtained the best spectrum of the upper atmospheric nightglow in this wavelength range, the spectrum of 15 OB stars and the spectrum of the full Moon throughout the year. This revised version was published online in July 2006 with corrections to the Cover Date.     

Satellite-derived vegetation index and cover type maps for estimating carbon dioxide flux for arctic tundra regions

The spatial variability and co-variability of two different types of remote sensing derivatives that portray vegetation and geomorphic patterns are analyzed in the context of estimating regional-scale CO₂ flux from land surfaces in the arctic tundra. For a study area encompassing the Kuparuk River watershed of the North Slope of Alaska, we compare satellite-derived maps of the normalized difference vegetation index (NDVI) generated at two different spatial resolutions to a map of vegetation types derived by image classification of data from the Landsat multispectral scanner (MSS). Mean values of NDVI for each cover type stratum are unique (with the exception of moist acidic tundra and shrubland types). Based on analysis of semi-variograms generated for SPOT-NDVI data, most of the vegetation cover and landform features of this arctic tundra landscape have spatial dimensions of less than 1 km. Thaw lakes on the coastal plain and glacial depositional landforms, such as moraines in the foothills, seem to be the largest features, with vegetation units having dimensions no larger than 700 m. Frequency distributions of NDVI and vegetation types extracted for sampling transects flown by an aircraft sensing CO₂ flux, relative to distributions for the entire Kuparuk River watershed, suggest a slight sampling bias towards greater cover of mesic wet sedge tundra and thaw lakes and associated lower NDVI values. The regional pattern of NDVI for the North Slope of Alaska corresponds primarily to differences between the two major physiographic provinces of this region.     

Successful Evaluation of EPA's New Screening Model

"A successful test of such a conservative screening tool would predict concentrations of pesticides that are in the upper 95 to 99% confidence interval of appropriate monitoring results."     

Dispersion Moments of Fumigating Plumes – Lidar Estimates and Pdf Model Simulations

Near-instantaneous vertical scans ofpower plant plumes, sampled by a lidar over three days as part ofthe 1995 Kwinana Coastal Fumigation Study, are analysed to obtainhourly-averaged total dispersion moments up to fourth order, andalso the relative and meander spreads. Two distinct fumigationcases, termed as neutral and stable cases, are observed dependingon whether the plume transport prior to fumigation takes place ina neutral atmosphere, or in a stable region above the neutralatmosphere. Plumes in the stable case are observed to undergoslower fumigation. The mean spreads display contrasting behavioursfor the two fumigating cases, while the vertical skewness shows anegative peak value of about -1 for the neutral case and -2for the stable case in the fumigation zone. The lateral skewnessis positive with a peak value of about unity for both cases, andis attributed to wind directional shear within the thermalinternal boundary layer. The vertical kurtosis is greater than theGaussian value of 3 in the fumigation zone, while the lateralkurtosis is scattered about this value. A recently-developedskewed probability density function model with wind shear isapplied to describe the fumigation data. Overall, the modelsimulates the observed vertical and lateral statistics well up tothe fourth order, except for the lateral skewness and kurtosisvalues in the stable case, which is probably due to the lidar'srange and sensitivity constraints and an inadequate sample size.Although the lateral turbulent diffusion in the model is Gaussian(i.e., zero skewness), the inclusion of wind shear in the model ismanifested in the predicted lateral skewness being greater thanzero in the fumigation zone, and of similar magnitude to the lidardata in the neutral case.