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This paper analyzes the frequency structure of the bands of electron-cyclotron maser instability. The calculations show that each term of the series describing the growth rate provides a double-peak structure, if we accept a nonthermal electron distribution with a two-side loss-cone. The ratio of central frequencies is found to be non-integer in general case. We conclude that the harmonic structure of solar radio spikes observed in a number of events can be imitated by electron-cyclotron maser emission of mildly relativistic electrons with a power-law momentum distribution. 相似文献
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G. D. Fleishman 《Astronomy Reports》2004,48(1):65-79
The properties of the cyclotron-maser radiation generated by anisotropic distributions of fast electrons with power-law momentum distributions are studied. The momentum and pitch-angle distributions of the electrons are chosen with regard to available observational constraints. The results obtained for these model distributions are compared with those in the literature for other distributions. The dependence of the properties of the cyclotron instability on the parameters of the distribution of fast electrons is analyzed. These properties are in good quantitative agreement with observational data on millisecond solar radio spikes. 相似文献
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Two solar radio bursts exhibiting narrow-band millisecond pulsations in intensity and polarization are analyzed. There were considerable time delays between the left-and right-circularly polarized components of the radio emission. The observed oscillations of the degree of polarization are due to the different group velocities of the ordinary and extraordinary modes in their propagation from the source to the observer; the frequency dependence of the delay is in excellent agreement with the theoretically calculated group delay in a magnetoactive plasma. It unambiguously follows that the pulsed radio emission is generated near the double upper hybrid frequency by the nonlinear plasma mechanism, since the source emission has a low degree of polarization. In addition to dispersion effects, a Fourier analysis also reveals effects associated with the source inhomogeneity. We detected a frequency drift of pulsations (autodelays) with different signs for different polarization components. This drift suggests that, apart from the dispersion effects, there are also the effects related to inhomogeneity of the radio source. It is shown, in particular, that the upper hybrid modes (generating the radio emission) are unstable in regions with enhanced gradients of the plasma density and/or magnetic field. 相似文献
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We have developed a general framework for modeling gyrosynchrotron and free–free emission from solar flaring loops and used it to test the premise that 2D maps of source parameters, particularly the magnetic field, can be deduced from spatially resolved microwave spectropolarimetry data. We show quantitative results for a flaring loop with a realistic magnetic geometry, derived from a magnetic-field extrapolation, and containing an electron distribution with typical thermal and nonthermal parameters, after folding through the instrumental profile of a realistic interferometric array. We compare the parameters generated from forward-fitting a homogeneous source model to each line of sight through the folded image data cube both with the original parameters used in the model and with parameters generated from forward-fitting a homogeneous source model to the original (unfolded) image data cube. We find excellent agreement in general, but with systematic effects that can be understood as due to the finite resolution in the folded images and the variation of parameters along the line of sight, which are ignored in the homogeneous source model. We discuss the use of such 2D parameter maps within a larger framework of 3D modeling, and the prospects for applying these methods to data from a new generation of multifrequency radio arrays now or soon to be available. 相似文献
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Marjorie L. Brooks Erica Fleishman Larry R. Brown Peggy W. Lehman Inge Werner Nathaniel Scholz Carys Mitchelmore James R. Lovvorn Michael L. Johnson Daniel Schlenk Suzanne van Drunick James I. Drever David M. Stoms Alex E. Parker Richard Dugdale 《Estuaries and Coasts》2012,35(2):603-621
Human effects on estuaries are often associated with major decreases in abundance of aquatic species. However, remediation priorities are difficult to identify when declines result from multiple stressors with interacting sublethal effects. The San Francisco Estuary offers a useful case study of the potential role of contaminants in declines of organisms because the waters of its delta chronically violate legal water quality standards; however, direct effects of contaminants on fish species are rarely observed. Lack of direct lethality in the field has prevented consensus that contaminants may be one of the major drivers of coincident but unexplained declines of fishes with differing life histories and habitats (anadromous, brackish, and freshwater). Our review of available evidence indicates that examining the effects of contaminants and other stressors on specific life stages in different seasons and salinity zones of the estuary is critical to identifying how several interacting stressors could contribute to a general syndrome of declines. Moreover, warming water temperatures of the magnitude projected by climate models increase metabolic rates of ectotherms, and can hasten elimination of some contaminants. However, for other pollutants, concurrent increases in respiratory rate or food intake result in higher doses per unit time without changes in the contaminant concentrations in the water. Food limitation and energetic costs of osmoregulating under altered salinities further limit the amount of energy available to fish; this energy must be redirected from growth and reproduction toward pollutant avoidance, enzymatic detoxification, or elimination. Because all of these processes require energy, bioenergetics methods are promising for evaluating effects of sublethal contaminants in the presence of other stressors, and for informing remediation. Predictive models that evaluate the direct and indirect effects of contaminants will be possible when data become available on energetic costs of exposure to contaminants given simultaneous exposure to non-contaminant stressors. 相似文献
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N. Raheem S. Colt E. Fleishman J. Talberth P. Swedeen K.J. Boyle M. Rudd R.D. Lopez D. Crocker D. Bohan T. O'Higgins C. Willer R.M. Boumans 《Marine Policy》2012
Regulatory agencies in the United States do not generally consider economic values of ecosystem services in their policy decisions. We report the results of a collaborative effort by a team of economists, conservation biologists, and staff members of the California Ocean Protection Council to provide spatially explicit and policy-relevant values for ecosystem services generated in coastal regions in California. We developed a matrix in which the rows are types of ecosystem services and the columns are types of marine ecosystems along the California coast. Where possible, we populated this matrix with ecosystem service values per unit of area drawn from the economics literature. We then evaluated whether the values for given services, in given ecosystems, could be reasonably approximated by applying the replacement cost or the avoided cost method. Reported values of coastal ecosystems varied widely, and much of the valuation research did not address specific ecosystem services. Even when ecosystem services were explicitly addressed, the services often were not described or valued in a spatially explicit manner. These results suggest that rigorous application of non-market values to policy decisions requires original valuation studies for specific services in specific ecosystems. Where original, place-based valuation studies are not possible, valuation by replacement or avoided cost methods is feasible for some ecosystem services. 相似文献
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Paola Del Negro Mauro Celussi Erica Crevatin Alessandro Paoli Fabrizio Bernardi Aubry Alessandra Pugnetti 《Marine Ecology》2008,29(3):375-386
The prokaryotic community, both in terms of abundance and activity (exoenzymatic hydrolysis of proteins, polysaccharides and phosphorylated molecules and leucine uptake), was investigated seasonally for a 3‐year period (2004–2006) in the Gulf of Venice (northern Adriatic Sea). By focusing on spatial and temporal variability, the prokaryote dynamics showed significant variations on a horizontal and seasonal scale, but no substantial differences were observed among years. The basin‐scale variability was mainly influenced by allochthonous inputs from the Po river and the Venice Lagoon, which were the main source of nutrients, acting as a ‘bottom up’ control on prokaryotes. On a seasonal scale, all the microbial parameters (except the polysaccharide degradation) showed significant variations following the temperature fluctuations. The annual rate of change was very low for all the chemical, physical and biological parameters and only the abundance and phosphatase activity of the prokaryotes differed significantly among years. 相似文献