The fibril structure of prominences     

E. R. Priest  A. W. Hood  U. Anzer 《Solar physics》1991,132(1):199-202

We suggest that the fibril structure of prominences may be caused by filamentation during its formation by radiative instability. We also discuss the effects of other types of instability and give a mechanism for the formation of vertical threads. The models indicate that highly inhomogeneous density structures can exist in the presence of smooth profiles for the plasma pressure and magnetic field. In our particular models the plasma pressure of a fibril prominence is higher and the vertical magnetical field is weaker than in a uniform prominence model, while the mass is substantially smaller.  相似文献   

The effect of shear on numerical models of quiescent normal-polarity prominences     

R. A. S. Fiedler  A. W. Hood 《Solar physics》1993,146(2):297-311

The equilibrium structure of normal-polarity, quiescent prominences is investigated and the influence of magnetic shear in response to a slow, shearing, photospheric velocity discussed. The results show that the overall field structure predicted by Fiedler and Hood (1992) is largely unaffected but that magnetic shear reduces the plasma beta and lengthens and flattens the magnetic field when viewed from the side. The flatness of the field suggests that the initial condensation can form and, when the mass is sufficient, deform the field slightly into the equilibrium structure calculated here. Thus, it is postulated that the field must be highly sheared for the radiation (or condensation) time to be less than the free-fall time along the field. A simple estimate predicts that the field must lie close to the polarity inversion line with an angle in agreement with observations. Hence, it is apparent that normal polarity prominences will always be observed with a highly sheared field.It is shown that the line-of-sight field component depends on the imposed shear profile and the viewing angle and in certain cases it is possible for this field component to appear to increase with height. Any observed increase of the line-of-sight magnetic field with height may then be due to the angle of the prominence to the line of sight.  相似文献   

External and internal solutions for the twisted,flux-tube,prominence model     

N. Cartledge  A. W. Hood 《Solar physics》1993,148(2):253-275

In this paper the twisted flux-tube model for the support of a prominence sheet with constant axial current density, given by Ridgway, Priest, and Amari (1991), is considered.The model is extended in Section 2 to incorporate a current sheet of finite height. The sheet is supported in a constant current density force-free field in the configuration of a twisted flux tube. The mass of the prominence sheet, using a typical height and field strength, is computed. Outside the flux tube the background magnetic field is assumed to be potential but the matching of the flux tube onto this background field is not considered here.Instead our attention is focussed, in Section 3, on the interior of the prominence. An expanded scale is used to stretch the prominence sheet to a finite width. We analytically select solutions for the internal magnetic field in this region which match smoothly onto the external force-free solutions at the prominence edge.The force balance equation applied inside the prominence then yields expressions for the pressure and density and a corresponding temperature may be computed.  相似文献   

Twisted magnetic flux tubes: Effect of small twist     

R. M. Lothian  A. W. Hood 《Solar physics》1989,122(2):227-244

The shape of a magnetic flux tube is investigated when photospheric motion causes small twist at the magnetic footpoints. Using a Fourier-Bessel series expansion, the previous results of Zweibel and Boozer (1985) and Steinolfson and Tajima (1987), when the twist is small, are substantiated. A twisting motion that is restricted to a finite region is investigated. Inside the twisted region, the tube contracts, but in the outer region the field remains straight, except for a slight expansion at the outside of the loop near the footpoints. The amount of twist depends on the radial position and can in fact be larger in the contracted region with the twist decreasing as the tube expands. An axial boundary-layer region is present, as noted by the above authors, through which the field adjusts to the line-tied magnetic footpoint positions. An analysis of the boundary layer shows that the thickness remains constant as the loop-length is increased with the result that the main part of the loop has constant cross-sectional area and can be described by cylindrically-symmetric fields. This new 1-D model predicts the main behaviour of the loop without the need to solve the more complicated 2-D problem directly. It is speculated that the boundary layers will remain even when the twist becomes large and a simple example is presented. A detailed parametric study of different twist profiles shows how the central part of the loop responds.Using the result that the majority of the loop can be described by a constant cross-sectional area, a model for a toroidal loop is presented that models coronal loops in a more realistic manner. The main result from this section is that the coronal loops can only remain in equilibrium if they are confined by an external magnetic field (possibly potential in nature) and not by a gas pressure unless additional physical effects are included.  相似文献   

THE POTENTIAL OF MAIL SURVEYS IN GEOGRAPHY: SOME EMPIRICAL EVIDENCE*     

Eran Feitelson 《The Professional geographer》1991,43(2):190-205

Faced with soaring survey costs, geographers may increasingly consider mail or telephone surveys as alternatives to in-person interviews. This study goes beyond comparisons of response rates to analyze possible biases of administration methods in geographic studies. An empirical comparison between an in-person interview survey and two mail surveys of recent home buyers’ housing preferences revealed that the mail surveys provided better representation of this population, as they had a higher completion rate (though lower response rate). While respondent attributes differed across administration modes, no direct effects of administration method on responses were found. Mail surveys appear to be a viable alternative to personal interviews in a wider set of cases than geographers usually assume, though more empirical work is needed to determine the extent of such cases.  相似文献   

A model for quiescent solar prominences with normal polarity     

A. W. Hood  U. Anzer 《Solar physics》1990,126(1):117-133

A class of 2-D models of solar quiescent prominences, with normal polarity, is presented. These represent an extension to the Kippenhahn-Schlüter model for which the prominence configuration matches smoothly onto an external non-potential coronal solution of a constant field. Using typical prominence values a model is constructed which also matches the coronal conditions. It is found that the magnetic field component along the prominence influences the internal structure of the prominence. A simple extension to the basic models is indicated as a means of taking a lower boundary of the prominence and eliminating parasitic polarities in the photosphere.  相似文献   

Geochemical study of the Uinta Basin: formation of petroleum from the Green River formation     

B. Tissot  G. Deroo  A. Hood 《Geochimica et cosmochimica acta》1978,42(10):1469-1485

Kerogen from the Green River formation is rich in lipid material and contains predominantly aliphatic chains, with subordinate saturated cyclic material. Compositional differences due to the environment of deposition are observed in the extractable hydrocarbons. The influence of microbial degradation during deposition of organic matter is intense in the stratigraphically lowest zone and decreases toward younger beds, where fossil molecules derived from higher plants, algae and other planktonic organisms and bacteria are abundant.The great depth of the oil generation stage may be explained by a combination of (a) a relatively low geothermal gradient, and (b) a high oil generation threshold. The lower part of the formation has reached the principal stage of oil generation and is responsible for most of the crude oils produced in the Uinta Basin.  相似文献   

Magnetohydrostatic structures in the solar atmosphere     

J. P. Melville  A. W. Hood  E. R. Priest 《Solar physics》1984,92(1-2):15-31

Separable two-dimensional solutions to the isothermal magnetohydrostatic equations are presented which include the effect of gravity. Examples of three types of linear solution are given in which photospheric magnetic fields are prescribed and the field topologies are discussed. In addition, a new nonlinear solution is discussed. The functional form of the pressure distribution is restricted by the separable assumption. An analysis suggests that these are the only separable analytical solutions.  相似文献   

Rates of chitin degradation in an estuarine environment     

Mary A. Hood  Samuel P. Meyers 《Journal of Oceanography》1977,33(6):328-334

In vitro chitin degradation rates in pure cultures and in mixed natural cultures have been determined and compared with those of other workers.In situ studies in the salt marsh shrimp nursery grounds along the southeastern Louisiana coast showed that chitin degradation was most rapid (118 mg d^–1 g^–1 chitin) when water temperature averaged 30C. Maximum degradation rates were noted at the water-sediment interface and when substrate particles were reduced in size (0.25 cm²). Of the several types of chitin tested, including treated and untreated, native chitin was most rapidly solubilized. Microbial populations on this substrate developed more rapidly, suggesting that chitin degradationin situ is a function of initial colonization. This is further supported by the observation that degradation rates were most rapid when total bacteria, chitinoclastic bacteria, and the ratio of chitinoclasts to total bacterial biomass (15.6 %) was greatest.  相似文献   

Monsoon-driven biogeochemical processes in the Arabian Sea   总被引：3，自引：0，他引：3  

J.D. Wiggert  R.R. Hood  K. Banse  J.C. Kindle 《Progress in Oceanography》2005,65(2-4):176

Although it is nominally a tropical locale, the semiannual wind reversals associated with the Monsoon system of the Arabian Sea result annually in two distinct periods of elevated biological activity. While in both cases monsoonal forcing drives surface layer nutrient enrichment that supports increased rates of primary productivity, fundamentally different entrainment mechanisms are operating in summer (Southwest) and winter (Northeast) Monsoons. Moreover, the intervening intermonsoon periods, during which the region relaxes toward oligotrophic conditions more typical of tropical environments, provide a stark contrast to the dynamic biogeochemical activity of the monsoons. The resulting spatial and temporal variability is great and provides a significant challenge for ship-based surveys attempting to characterize the physical and biogeochemical environments of the region. This was especially true for expeditions in the pre-satellite era.Here, we present an overview of the dynamical response to seasonal monsoonal forcing and the characteristics of the physical environment that fundamentally drive regional biogeochemical variability. We then review past observations of the biological distributions that provided our initial insights into the pelagic system of the Arabian Sea. These evolved through the 1980s as additional methodologies, in particular the first synoptic ocean color distributions gathered by the Coastal Zone Color Scanner, became available. Through analyses of these observations and the first large-scale physical–biogeochemical modeling attempts, a pre-JGOFS understanding of the Arabian Sea emerged. During the 1990s, the in situ and remotely sensed observational databases were significantly extended by regional JGOFS activities and the onset of Sea-viewing Wide Field-of-View Sensor ocean color measurements. Analyses of these new data and coupled physical–biogeochemical models have already advanced our understanding and have led to either an amplification or revision of the pre-JGOFS paradigms. Our understanding of this complex and variable ocean region is still evolving. Nonetheless, we have a much better understanding of time–space variability of biogeochemical properties in the Arabian Sea and much deeper insights about the physical and biological factors that drive them, as well as a number of challenging new directions to pursue.  相似文献