Substorm correlated absorption on a 3200 km trans-auroral HF propagation path     

S. E. Milan  T. B. Jones  M. Lester  E. M. Warrington  G. D. Reeves 《Annales Geophysicae》1996,14(2):182-190

A high-frequency transmitter located at Clyde River, NWT, Canada, and a receiver located near Boston, USA, provide a 3200 km trans-auroral, near-meridional propagation path over which the propagation characteristics have been measured. Out of the fourteen frequencies in the HF band sampled every hour for the duration of the experimental campaign (16 January-8 February 1989), the signal level measurements of 6.800 MHz transmissions were selected in order to determine the extent and occurrence of auroral absorption. The median level of auroral absorption along the path is found to increase with geomagnetic activity, quantified by the index K_p, with the increase being greater in the post-midnight sector than in the pre-midnight sector. This asymmetric behaviour is attributed to the precipitation of high energy electrons into the midnight and morning sector auroral D region. The measured diurnal variation in the median level of absorption is consistent with previous models describing the extent and magnitude of auroral absorption and electron precipitation. Individual substorms, identified from geosynchronous satellite data, are found to cause short-lived absorption events in the HF signal level of \sim30 dB at 6.800 MHz. The occurrence of substorm correlated auroral absorption events is confined to the midnight and morning sectors, consistent with the location of the electron precipitation. The magnitude of absorption is related to the magnetotail stress during the substorm growth phase and the magnetotail relaxation during the substorm expansion phase onset. The absorption magnitude and the occurrence of substorms during the period of the campaign increase at times of high K_p, leading to an increase in median auroral absorption during disturbed periods.  相似文献   

The EUV chromospheric network in the quiet Sun     

E. M. Reeves 《Solar physics》1976,46(1):53-72

Investigations on the structure and intensity of the chromospheric network from quiet solar regions have been carried out with EUV data obtained from the Harvard spectroheliometer on the Apollo Telescope Mount of Skylab. The distribution of intensities within supergranulation cell interiors follows a near normal function, where the standard deviation exceeds the value expected from the counting rate, which indicates fine-scale structure below the 5 arc sec resolution of the data. The intensities from the centers of supergranulation cells appear to be the same in both quiet regions and coronal holes, although the network is significantly different in the two types of regions. The average halfwidth of the network elements was measured as 10 arc sec, and was independent of the temperature of formation of the observing line for 3.8 < log T _e < 5.8. The contrast between the network and the centers of cells is greatest for lines with log T _e 5.2, where the network contributes approximately 75% of the intensity of quiet solar regions. The contrast and fractional intensity contributions decrease to higher and lower temperatures characteristic of the corona and chromosphere.  相似文献   

Chemistry of hot springs along the Eastern Lau Spreading Center     

Michael J. Mottl  Jeffrey S. Seewald  Margaret K. Tivey  Giora Proskurowski  Eoghan Reeves  C.-F. You  Thomas Pichler 《Geochimica et cosmochimica acta》2011,75(4):1013-76

The Eastern Lau Spreading Center (ELSC) is the southernmost part of the back-arc spreading axis in the Lau Basin, west of the Tonga trench and the active Tofua volcanic arc. Over its 397-km length it exhibits large and systematic changes in spreading rate, magmatic/tectonic processes, and proximity to the volcanic arc. In 2005, we collected 81 samples of vent water from six hydrothermal fields along the ELSC. The chemistry of these waters varies both within and between vent fields, in response to changes in substrate composition, temperature and pressure, pH, water/rock ratio, and input from magmatic gases and subducted sediment. Hot-spring temperatures range from 229° to 363 °C at the five northernmost fields, with a general decrease to the south that is reversed at the Mariner field. The southernmost field, Vai Lili, emitted water at up to 334 °C in 1989 but had a maximum venting temperature of only 121 °C in 2005, due to waning activity and admixture of bottom seawater into the subseafloor plumbing system. Chloride varies both within fields and from one field to another, from a low of 528 mmol/kg to a high of 656 mmol/kg, and may be enriched by phase separation and/or leaching of Cl from the rock. Concentrations of the soluble elements K, Rb, Cs, and B likewise increase southward as the volcanic substrate becomes more silica-rich, especially on the Valu Fa Ridge. Iodine and δ⁷Li increase southward, and δ¹¹B decreases as B increases, apparently in response to increased input from subducted sediment as the arc is approached. Species that decrease southward as temperature falls are Si, H₂S, Li, Na/Cl, Fe, Mn, and ⁸⁷Sr/⁸⁶Sr, whereas pH, alkalinity, Ca, and Sr increase. Oxygen isotopes indicate a higher water/rock ratio in the three systems on Valu Fa Ridge, consistent with higher porosity in more felsic volcanic rocks. Vent waters at the Mariner vent field on the Valu Fa Ridge are significantly hotter, more acid and metal-rich, less saline, and richer in dissolved gases and other volatiles, including H₂S, CO₂, and F, than the other vent fields, consistent with input of magmatic gases. The large variations in geologic and geophysical parameters produced by back-arc spreading along the ELSC, which exceed those along mid-ocean ridge spreading axes, produce similar large variations in the composition of vent waters, and thus provide new insights into the processes that control the chemistry of submarine hot springs.  相似文献   

Amino acid geochronology of raised beaches in south west Britain     

D.Q. Bowen  G.A. Sykes  Alayne Reeves  G.H. Miller  J.T. Andrews  J.S. Brew  P.E. Hare 《Quaternary Science Reviews》1985,4(4):279-318

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Rumex acetosella L. and Minuartia verna (L.) Hiern as geobotanical and biogeochemical indicators for ore deposits in northern Greece     

A.E. Kelepertsis  I. Andrulakis  R.D. Reeves 《Journal of Geochemical Exploration》1985,23(2):203-212

Copper, zinc, lead, manganese, arsenic, iron, cadmium and molybdenum were measured in specimens of Rumex acetosella and Minuartia verna and in soils where these plants grow. In Macedonia and the Chalkidiki peninsula both plants occur on newly investigated sulphide-mineralized ground. There are highly significant correlations between the levels of copper, zinc and lead in the plants and those in the soils. Where high concentrations of arsenic occur in the soil, this is also reflected in the plants. Because of the common association of gold and arsenic, it is suggested that analysis of Rumex and Minuartia for arsenic may be a useful prospecting tool since most of the ore occurrences in the Serbomacedonian massif are auriferous. The high levels of manganese in both plants from a manganese mineralized area indicate a strong possibility of using biogeochemical methods to define zones of some types of mineralization of this element also.  相似文献   

The potential of mid- and near-infrared diffuse reflectance spectroscopy for determining major- and trace-element concentrations in soils from a geochemical survey of North America   总被引：2，自引：0，他引：2  

James B. Reeves III  David B. Smith 《Applied Geochemistry》2009,24(8):1472

In 2004, soils were collected at 220 sites along two transects across the USA and Canada as a pilot study for a planned soil geochemical survey of North America (North American Soil Geochemical Landscapes Project). The objective of the current study was to examine the potential of diffuse reflectance (DR) Fourier Transform (FT) mid-infrared (mid-IR) and near-infrared (NIRS) spectroscopy to reduce the need for conventional analysis for the determination of major and trace elements in such continental-scale surveys. Soil samples (n = 720) were collected from two transects (east–west across the USA, and north–south from Manitoba, Canada to El Paso, Texas (USA), n = 453 and 267, respectively). The samples came from 19 USA states and the province of Manitoba in Canada. They represented 31 types of land use (e.g., national forest, rangeland, etc.), and 123 different land covers (e.g., soybeans, oak forest, etc.). The samples represented a combination of depth-based sampling (0–5 cm) and horizon-based sampling (O, A and C horizons) with 123 different depths identified. The set was very diverse with few samples similar in land use, land cover, etc. All samples were analyzed by conventional means for the near-total concentration of 49 analytes (C_total, C_carbonate and C_organic, and 46 major and trace elements). Spectra were obtained using dried, ground samples using a Digilab FTS-7000 FT spectrometer in the mid- (4000–400 cm⁻¹) and near-infrared (10,000–4000 cm⁻¹) at 4 cm⁻¹ resolution (64 co-added scans per spectrum) using a Pike AutoDIFF DR autosampler. Partial least squares calibrations were develop using: (1) all samples as a calibration set; (2) samples evenly divided into calibration and validation sets based on spectral diversity; and (3) samples divided to have matching analyte concentrations in calibration and validation sets. In general, results supported the conclusion that neither mid-IR nor NIRS would be particularly useful in reducing the need for conventional analysis of soils from this continental-scale geochemical survey. The extreme sample diversity, likely caused by the widely varied parent material, land use at the site of collection (e.g., grazing, recreation, agriculture, etc.), and climate resulted in poor calibrations even for C_total, C_organic and C_carbonate. The results indicated potential for mid-IR and NIRS to differentiate soils containing high concentrations (>100 mg/kg) of some metals (e.g., Co, Cr, Ni) from low-level samples (<50 mg/kg). However, because of the small number of high-level samples, it is possible that differentiation was based on factors other than metal concentration. Results for Mg and Sr were good, but results for other metals examined were fair to poor, at best. In essence, it appears that the great variation in chemical and physical properties seen in soils from this continental-scale survey resulted in each sample being virtually unique. Thus, suitable spectroscopic calibrations were generally not possible.  相似文献   

A Semi‐Structured MODFLOW‐USG Model to Evaluate Local Water Sources to Wells for Decision Support          下载免费PDF全文

Daniel T. Feinstein  Michael N. Fienen  Howard W. Reeves  Christian D. Langevin 《Ground water》2016,54(4):532-544

In order to better represent the configuration of the stream network and simulate local groundwater‐surface water interactions, a version of MODFLOW with refined spacing in the topmost layer was applied to a Lake Michigan Basin (LMB) regional groundwater‐flow model developed by the U.S. Geological. Regional MODFLOW models commonly use coarse grids over large areas; this coarse spacing precludes model application to local management issues (e.g., surface‐water depletion by wells) without recourse to labor‐intensive inset models. Implementation of an unstructured formulation within the MODFLOW framework (MODFLOW‐USG) allows application of regional models to address local problems. A “semi‐structured” approach (uniform lateral spacing within layers, different lateral spacing among layers) was tested using the LMB regional model. The parent 20‐layer model with uniform 5000‐foot (1524‐m) lateral spacing was converted to 4 layers with 500‐foot (152‐m) spacing in the top glacial (Quaternary) layer, where surface water features are located, overlying coarser resolution layers representing deeper deposits. This semi‐structured version of the LMB model reproduces regional flow conditions, whereas the finer resolution in the top layer improves the accuracy of the simulated response of surface water to shallow wells. One application of the semi‐structured LMB model is to provide statistical measures of the correlation between modeled inputs and the simulated amount of water that wells derive from local surface water. The relations identified in this paper serve as the basis for metamodels to predict (with uncertainty) surface‐water depletion in response to shallow pumping within and potentially beyond the modeled area, see Fienen et al. (2015a).  相似文献