The effect of wave generation on HXR bremsstrahlung spectra from flare thick-target beams

Fast electrons in the solar atmosphere are detected by their hard X-ray bremsstrahlung and by type III radio bursts caused by ‘bump-on-tail’ plasma wave generation. This paper investigates empirically the effect of wave generation on the HXR spectrum. Purely collisional propagation of an electron beam generates a bump in the distribution function, due to stopping of low-velocity electrons. The consequent positive gradient means there is a possibility of wave generation, production of type III radio bursts, and energy redistribution of the electron beam. We have represented this relaxation parametrically and calculated the global bremsstrahlung HXR emission spectrum. We show that for a range of relaxed forms, with different local electron spectral shapes, the bremsstrahlung spectrum integrated over the whole target is identical in shape to the purely collisionally evolved beam. Our results show that spatially integrated HXR spectral measurements would be unable to distinguish between the presence or absence of relaxation effects. Only spatially resolved hard X-ray spectra, such as anticipated from the HESSI mission, will be able to remove this ambiguity in HXR diagnostics of beam relaxation.     

An indicator of galactic violence

An indicator of galactic violence is proposed, nominally equal to the total galactic mass divided by the rim radius. Four models of galaxy formation are outlined; and for each, the dependence of the rim radius on the galactic mass and the source of angular momentum is examined. If the violence indicator is small, then a normal galaxy results. If the violence indicator is large, then a galaxy with a very massive central black hole is produced (possibly a quasar). A rim radius of zero would indicate the ultimate violence and would lead directly to a non-rotating black hole of galactic mass.Los Alamos National Laboratory is operated by the University of California for the U.S. Dept. of Energy under Contract W-7405-ENG-36.     

Statistical, geochemical, and morphological analyses of stone line formation in Uganda

The resolution of conflicting fluvial (allochthonic erosion and deposition) and bioturbation (autochthonic faunal turbation) theories of stone line formation has important implications for tropical geomorphology—with the former indicating dynamic, punctuated landscape development and the latter implying relative landscape stability with ongoing moderate erosion. On a gentle hill in central Uganda, we quantitatively characterized and statistically modeled gravel distributions for 93 profiles. Elemental analysis of K, selected rare-earth elements (REE) and appropriate index elements were employed to test for compositional differences between soils above and saprolite below hilltop stone lines. We also excavated and carefully described four indicative profiles. The results of this study were difficult to reconcile with the bioturbation theory of stone line formation. Important findings included (i) the lack of a consistent gravel-free biomantle with a weak connection between biomantle thickness and stone-line depth, (ii) highly variable stone line depth with a strong inverse relationship between stone line depth and maximum gravel content, (iii) unweathered and rounded cobbles in a stone line over saprolite with angular, highly weathered quartz, and (iv) a statistically significant geochemical difference between soils and saprolites, with evidence of a less weathered surface mantle over more weathered saprolite. These findings indicated a sedimentary origin for soils at this site—even for what is presently a flat, geomorphically isolated hilltop. Evidence from this study and previous work on the geochemistry of this site [Geochim. Cosmochim. Acta 67 (2003) 2711] suggests that the contemporary hilltop once occupied a lower landscape position.     

CHANNEL CHANGES IN ARID BADLANDS,BORREGO SPRINGS,CALIFORNIA

Geometric, hydraulic, and sediment characteristics in arid badlands near Borrego Springs, California, are examined in relation to precipitation events of varying magnitude and frequency. The longitudinal and cross profiles of five ephemeral channels occupying a 2.5 km² catchment were surveyed under pre-and post-storm conditions during the February 1976-December 1978 period. Such arid region channels offer the opportunity to observe and explain rates and methods of profile change under different flow types in a short period of time. Catchment responses to light winter events include substantial lags between initial precipitation and channel runoff, the limited downstream movement of small slugs of sediment, high losses of discharge into channel alluvium, and prolonged mass movement of debris from adjacent hillslopes into the channels following the storm events thus promoting aggradation along certain channel reaches. Responses to intense summer storms include explosive channel and hillslope runoff and localized scour and fill, both during and following such events, thereby promoting substantial aggradation and erosion along portions of the channels. Although ephemeral flow conditions may produce channel profiles which are distinct from those in perennial streams, the evaluation of the methods of sediment transport and the storage of debris in arid catchments offer useful explanation for other environments.     

Water requirements for cultivatingSalicornia bigeloviiTorr. with seawater on sand in a coastal desert environment

The forage and oilseed halophyte,Salicornia bigeloviiTorr., was grown in gravity-drained lysimeters set in open plots of the same crop over two seasons in a coastal desert environment in Sonora, Mexico. The lysimeters were irrigated daily with seawater (40 g l⁻¹salts) at rates ranging from 46–225% of potential evaporation. Biomass and seed yields increased with increasing irrigation depth over the range of treatments. Biomass yields ranged from 13·6–23·1 t DM ha⁻¹, equivalent to conventional forage crops, on seasonal water application depths of 2·3–3·8 m, but were markedly lower at lower irrigation depths. Increasing the irrigation depth lowered the soil solution salinity, resulting in greater growth and water use, and hence leaching fractions that were nearly even over irrigation treatments, averaging 0·5. Evapo-transpiration rose in direct proportion to the irrigation depth. Potential evaporation was estimated by site pan evaporation and by the Blaney-Criddle and Penman models using climatological data; the methods agreed within 15%. The ratio of evapo-transpiration to potential evaporation increased over the growing season and approached 1·5 by pan on the highest irrigation treatment due to the combined effects of high transpiration and high evaporation from the permanently moist soil surface. The best field predictor of biomass yield was the salinity of the soil moisture in the top 15 cm of soil profile, which constitutes the root zone for this crop. Root zone salinity must be kept at 70–75 g l⁻¹for high yields. Although irrigation and drainage requirements were high compared to conventional crops, seawater irrigation appears to be feasible in medium sand and could augment crop production along coastal deserts. The possibility of using this crop for animal production is discussed.     

Comparison study of methane emissions from landfills with different landfill covers

Atmospheric methane, a more effective heat-trapping gas than CO₂ that may affect climate change, has its greatest man-made source in the US from municipal solid waste (MSW) landfills. Consequently, the wise management of landfills can reduce these greenhouse gas emissions to the atmosphere. Methane from modern MSW landfills built with composite covers is frequently vented directly to the atmosphere. Biofiltration of landfill gas could oxidize CH₄ to CO₂ and water. Methane oxidation in old landfills with conventional soil covers can be effective in reducing the amount of CH₄ emitted. In this study, comparison of methane emissions from three different landfill covers was conducted. Methane emissions from old landfills constructed with conventional soil covers, modern MSW landfills constructed with composite covers, and modern MSW landfills constructed with composite covers plus biofilters were calculated using the calculated CH₄ oxidation rates. The results showed that an average of only 14% of the generated CH₄ was emitted from landfills with modern composite covers plus biofilters, and an average of 85% of the generated CH₄ was emitted from landfills with conventional covers when 100% of the generated CH₄ emissions to the atmosphere from landfills with modern composite covers was assumed. By comparing the CH₄ emission rates from three different landfill types, the use of a properly sized biofilter should be an effective technique to reduce CH₄ emissions from landfills across the USA and potentially in many other areas of the world.     

Influence of topsoil of pyroclastic origin on microbial contamination of groundwater in fractured carbonate aquifers

The aim of the research was to analyse the influence of a topsoil of pyroclastic origin on microbial contamination of groundwater in a carbonate aquifer and verify the reliability of thermotolerant coliforms and fecal enterococci as bacterial indicators. The research was carried out through hydrogeological and microbiological monitoring at an experimental field site in Italy during two hydrologic years and through column tests in a laboratory. The taxonomic classification of fecal indicators detected in spring water samples was performed using API20 galleries. Fecal enterococci were also identified by means of 16S rRNA gene sequencing. The topsoil of pyroclastic origin significantly retains both thermotolerant coliforms and fecal enterococci. Results of column tests carried out in soil blocks collected randomly within the test site suggest that Escherichia coli was more retained than Enterococcus faecalis, even though this difference is statistically significant in only two out of six soil samples. Thus, a non-uniform difference in retention is expected at field scale. This suggestion is in agreement with the results of the microbiological monitoring. In fact, fecal enterococci were a more reliable indicator than thermotolerant coliforms for detecting contamination at both seasonal springs of the aquifer system, while no significant differences were observed at the perennial spring.     

喜马拉雅地区深反射地震──揭示印度大陆北缘岩石圈的复杂结构

喜马拉雅山的崛起和青藏高原的隆升被认作是印度板块和亚洲板块中、新生代以来汇聚、碰撞、挤压的结果，是典型的陆－陆碰撞地带。此文介绍了在喜马拉雅山区进行的第一次深反射地震试验的结果。试验剖面布置在北喜马拉雅地区内，从喜马拉雅山山脊南的帕里到康马南的萨马达共中１５点（ＣＭＰ）叠加剖面上表现出如下特点：①显示了在地壳中部有一强反射带，向北缓倾斜下去，延长达１００ｋｍ以上。它可能代表了一个活动的道冲断裂或是一条巨大的拆离带，印度地壳整体或下地壳沿此拆离层俯冲到藏南之下；②上部地壳的反射，显示了上地壳存在着大规模的叠瓦状结构；③下地壳的反射显示了塑性流变特征；④在测线南部莫霍反射明显，深度达７２─７５ｋｍ，发现了南部有双莫霍层的存在；⑤试验中还取得莫霍层下面３２ｓ、３８ｓ、４８ｓ等双程走时的多条反射，均向北倾斜，反射同相轴延续较长，信息丰富，反映了上地幔的成层结构。这些结果对印度大陆地壳整体或其下地壳俯冲到藏南特提斯喜马拉雅地壳之下并导致西藏南端地壳增厚的观点给予了实质性的支持。