Forward modeling of the coronal response to reconnection in an X-ray bright point     

Kankelborg  Charles  Longcope  Dana 《Solar physics》1999,190(1-2):59-77

We use MDI magnetic field observations and the theory of reconnection through a separator to constrain a numerical simulation of an X-ray bright point observed in EUV by TRACE. A gasdynamic model is employed to describe the corona and transition region in the bright point loop. Nonlocal effects such as opacity and ambipolar diffusion are important to the transition region; these effects are approximated locally by modification of the radiative loss and thermal conduction. A straightforward comparison of measured light curves versus those generated by the simulation shows that the reconnection model is unable to account for the observations. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005205807984  相似文献   

Chemical forms of iron in the Connecticut River estuary     

Huasheng Hong  Dana R. Kester 《Estuarine, Coastal and Shelf Science》1985,21(4):449-459

The ‘dissolved’ iron (that passed through a 0·4-μm filter) varied nonconservatively with salinity in the Connecticut River estuary. However, the total iron appeared to be conservative. Measurements of Fe(II) and Fe(III) showed that oxidation of Fe(II) was not a factor in the decrease of ‘dissolved’ iron in the low salinity region. A solubility model and analyses based on different pore-size filters indicated that a substantial amount of the ‘dissolved’ iron in the low salinity region was colloidal iron. The coagulation of fine colloidal particles led to the non-conservative behavior of ‘dissolved’ iron during estuarine mixing, but it did not necessarily lead to removal of total iron from the waters. Particulate iron was 80–90% of the total iron and it covaried with the total suspended matter during mixing and sediment resuspension. The residence time of water in the Connecticut River estuary was too short to allow removal of iron from the water column within the estuary.  相似文献   

4.8 – 20 micron imaging of orion BN/KL: luminosity sources and the role of IRc2     

Daniel Y. Gezari  Dana E. Backman 《Astrophysics and Space Science》1995,224(1-2):463-463

Mid-infrared imaging photometry of the Orion BN/KL infrared cluster at eight wavelengths between 5 and 20µm using a 58 × 62 pixel imaging array camera has revealed new compact sources and the large-scale structure of the region in diffraction-limited (1 arcsec) detail. Several new objects have been detected within a few arcsec of IRc2, widely thought to be the principal luminosity source for the entire BN/KL complex. Detailed color temperature and emission opacity images are derived from the 7.8, 12.4 and 20.0µm observations, and the 9.8µm image is used to derive an image of “silicate” dust extinction for the region. The color temperature, opacity, and extinction images show that IRc2 may not be the single dominant luminosity source for the BN/KL region; substantial contributions to the luminosity could be made by IRc7, BN, KL, and five new compact 10µm sources detected within a few arcseconds of IRc2. We suggest that a luminous, early-type star near IRc2, which is associated with the compact radio source “I” and the Orion SiO maser, is the dominant luminosity source in the BN/KL region, hidden from view by cool dust material with at least Av ～ 60 mag of visible extinction.  相似文献   

Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites   总被引：26，自引：1，他引：25  

Alberto E. Patiño Douce  A. Dana Johnston 《Contributions to Mineralogy and Petrology》1991,107(2):202-218

Peraluminous granitoid magmas are a characteristic product of ultrametamorphism leading to anatexis of aluminous metasedimentary rocks in the continental crust. The mechanisms and characteristic length-scales over which these magmas can be mobilized depend strongly on their melt fraction, because of their high viscosities. Thus, it is of fundamental importance to understand the controls exerted by pressure, temperature and bulk composition of the source material on melt productivity. We have studied experimentally the vapour-absent melting behaviour of a natural metapelitic rock and our results differ greatly from those of previous experimental and theoretical investigations of melt productivity from metamorphic rocks. Under H₂O-undersaturated conditions, bulk composition of the source material is the overriding factor controlling melt fraction at temperatures on the order of 850–900° C. Granitoid melts formed in this temperature interval by the peritectic dehydration-melting reaction: $$\begin{gathered} Biotite + plagioclase + aluminosilicate + quartz \hfill \\ = melt + garnet \hfill \\ \end{gathered} $$ have a restricted compositional range. As a consequence, melt fractions will be maximized from protoliths whose modes coincide with the stoichiometry of the melting reaction. This “optimum mode” (approximately 38% biotite, 32% quartz, 22% plagioclase and 8% aluminosilicate) reflects the fact that generation of low-temperature granitoid liquids requires both fusible quartzo-feldspathic components and H₂O (from hydrous minerals). Metapelitic rocks rich in mica and aluminosilicate and poor in plagioclase contain an excess of refractory material (Al₂O₃, FeO, MgO) with low solubility in low-temperature silicic melts, and will therefore be poor magma sources. Melt fraction varies inversely with pressure in the range 7–13 kbar, but the effect is not strong: the decrease (at constant temperature) over this pressure range is of at most 15 vol% (absolute). The liquids produced in our experiments are silicarich (68–73 wt% SiO₂), strongly peraluminous (2–5 wt% normative corundum) and very felsic (MgO+FeO^* +TiO₂ less than 3 wt%, even at temperatures above 1000° C). The last observation suggests that peraluminous granitoids with more than 10% mafic minerals (biotite, cordierite, garnet) contain some entrained restite. Furthermore, because liquids are also remarkably constant in composition, we believe that restite separation is more important than fractional crystallization in controlling the variability within and among peraluminous granitoids. We present liquidus phase diagrams that allow us to follow the phase relationships of melting of silica-and alumina-saturated rocks at pressures corresponding to the mid- to deep-continental crust. Garnet, aluminosilicate, quartz and ilmenite are the predominant restitic phases at temperatures of about 900° C, but Ti-rich biotite or calcic plagioclase can also be present, depending on the bulk composition of the protolith. At temperatures above 950–1050° C (depending on the pressure) the restitic assemblage is: hercynitic spinel+ilmenite+quartz±aluminosilicate. Our results therefore support the concept that aluminous granulites (garnet-spinel-plagioclase-aluminosilicate-quartz) can be the refractory residuum of anatectic events.  相似文献   

Anhydrous PT phase relations of an Aleutian high-MgO basalt: an investigation of the role of olivine-liquid reaction in the generation of arc high-alumina basalts   总被引：2，自引：2，他引：0  

David S. Draper  A. Dana Johnston 《Contributions to Mineralogy and Petrology》1992,112(4):501-519

We report results of anhydrous 1 atm and piston-cylinder experiments on ID16, an Aleutian high-magnesia basalt (HMB), designed to investigate potential petrogenetic links between arc high-alumina basalts (HABs) and less common HMBs. ID16 is multiply saturated with a plagioclase/spinel iherzolite mineral assemblage (olivine, plagioclase, clinopyroxene, orthopyroxene, spinel) immediately beneath the 12 kbar liquidus. Derivative liquids produced at high temperatures in the 10–20 kbar melting interval of ID16 have compositions resembling those published of many moderate-CaO HABs, although lower-temperature liquids are poorer in CaO and richer in alkalies than are typical HABs. Isomolar pseudoternary projections and numerical mass-balance modeling suggest that derivative melts of ID16 enter into a complex reaction relationship with olivine at 10 kbar and 1,200° C–1,150° C. We sought to test such a mechanism to explain the lack of liquidus olivine in anhydrous experiments on mafic high-alumina basalts such as SSS. 1.4 (Johnston 1986). These derivative liquids, however, do not resemble typical arc high-alumina basalts, suggesting that olivine-liquid reaction does not account for Johnston's (1986) observations. Instead, we suggest that olivine can be brought onto the liquidus of such compositions only through the involvement of H₂O, which will affect the influence of bulk CaO, MgO, and Al₂O₃ contents on the identity of HAB liquidus phases (olivine or plagioclase) at pressures less than 12 kbar.  相似文献   

Experiments In Teaching College Geography: A Report To The Profession     

John M. Ball  Dana G. Kurfman  Leonard M. Lansky  Salvatore J. Natoli 《The Professional geographer》1972,24(4):350-361

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Oxygen Utilization and Organic Carbon Remineralization in the Upper Water Column of the Pacific Ocean     

Richard A. Feely  Christopher L. Sabine  Reiner Schlitzer  John L. Bullister  Sabine Mecking  Dana Greeley 《Journal of Oceanography》2004,60(1):45-52

As a part of the JGOFS synthesis and modeling project, researchers have been working to synthesize the WOCE/JGOFS/DOE/NOAA global CO₂ survey data to better understand carbon cycling processes in the oceans. Working with international investigators we have compiled a Pacific Ocean data set with over 35,000 unique samples analyzed for at least two carbon species, oxygen, nutrients, chlorofluorocarbon (CFC) tracers, and hydrographic parameters. We use these data here to estimate in-situ oxygen utilization rates (OUR) and organic carbon remineralization rates within the upper water column of the Pacific Ocean. OURs are derived from the observed apparent oxygen utilization (AOU) and the water age estimates based on CFCs in the upper water and natural radiocarbon in deep waters. The rates are generally highest just below the euphotic zone and decrease with depth to values that are much lower and nearly constant in water deeper than 1200 m. OURs ranged from about 0.02–10 μmol kg⁻¹yr⁻¹ in the upper water masses from about 100–1000 m, and averaged = 0.10 μmol kg⁻¹yr⁻¹ in deep waters below 1200 m. The OUR data can be used to directly estimate organic carbon remineralization rates using the C:O Redfield ratio given in Anderson and Sarmiento (1994). When these rates are integrated we obtain an estimate of 5.3 ± 1 Pg C yr⁻¹ for the remineralization of organic carbon in the upper water column of the Pacific Ocean. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Rising heat wave trends in large US cities     

Dana Habeeb  Jason Vargo  Brian Stone Jr. 《Natural Hazards》2015,76(3):1651-1665

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Reconnaissance dating: A new radiocarbon method applied to assessing the temporal distribution of Southern Ocean deep-sea corals     

Andrea Burke  Laura F. Robinson  Ann P. McNichol  William J. Jenkins  Kathryn M. Scanlon  Dana S. Gerlach 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(11):1510-1520

We have developed a rapid ‘reconnaissance’ method of preparing graphite for ¹⁴C/¹²C analysis. Carbonate (～15 mg) is combusted using an elemental analyzer and the resulting CO₂ is converted to graphite using a sealed tube zinc reduction method. Over 85% (n=45 replicates on twenty-one individual corals) of reconnaissance ages measured on corals ranging in age from 500 to 33,000 radiocarbon years (Ryr) are within two standard deviations of ages generated using standard hydrolysis methods on the same corals, and all reconnaissance ages are within 300 Ryr of the standard hydrolysis ages. Replicate measurements on three individual aragonitic corals yielded ages of 1076±35 Ryr (standard deviation; n=5), 10,739±47 Ryr (n=8), and 40,146±3500 Ryr (n=9). No systematic biases were found using different cleaning methods or variable sample sizes. Analysis of ¹³C/¹²C was made concurrently with the ¹⁴C/¹²C measurement to correct for natural fractionation and for fractionation during sample processing and analysis. This technique provides a new, rapid method for making accurate, percent-level ¹⁴C/¹²C analyses that may be used to establish the rates and chronology of earth system processes where survey-type modes of age estimation are desirable. For example, applications may include creation of sediment core-top maps, preliminary age models for sediment cores, and growth rate studies of marine organisms such as corals or mollusks. We applied the reconnaissance method to more than 100 solitary deep-sea corals collected in the Drake Passage in the Southern Ocean to investigate their temporal and spatial distribution. The corals used in this study are part of a larger sample set, and the subset that was dated was chosen based on species as opposed to preservation state, so as to exclude obvious temporal biases. Similar to studies in other regions, the distribution of deep-sea corals is not constant through time across the Drake Passage. Most of the corals from the Burdwood Bank (continental shelf of Argentina) have ages ranging between 0 and 2500 calendar years, whereas most of the corals from the Sars Seamount in the Drake Passage have ages between 10,000 and 12,500 calendar years. Such differences may be caused in part by sampling biases, but may also be caused by changes in larval transport, nutrient supply, or other environmental pressures.  相似文献   

Eolian Deposition and Soil Fertility in a Prehistoric Agricultural Complex in Central Arizona,USA     

Dana K. Nakase  Anthony S. Hartshorn  Katherine A. Spielmann  Sharon J. Hall 《Geoarchaeology》2014,29(2):79-97

Prehistoric farmers in arid and semiarid ecosystems commonly used rock alignments to concentrate water and sediments on their fields. Previous research has emphasized the importance of runoff from organic matter‐rich uplands as a mechanism for soil nutrient replenishment. However, eolian inputs to these dryland ecosystems might also contribute substantially to mineral‐derived nutrient pools. We explored the relative importance of eolian deposition, prehistoric agriculture, and the presence of rock alignments on soil properties in a semiarid grassland in Arizona. Subsurface soils behind natural rock alignments are finer in texture than soils unbound by rock alignments, while subsurface soils behind agricultural rock alignments coarsen relative to unbound soils. Neither rock alignments nor estimated crop yields had detectable effects on mineral‐derived nutrient pools. In contrast, eolian deposition is an important source of soil mass and nutrients to modern soils. While dust deposition likely reduced soil heterogeneity across this landscape, it could have also contributed to the sustainability of prehistoric agriculture.  相似文献