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1.
Many authors use magnetic-field models to extrapolate the field in the solar corona from magnetic data in the photosphere. The accuracy of such extrapolations is usually judged qualitatively by eye, where a less judgemental quantitative approach would be more desirable. In this paper, a robust method for obtaining the best fit between a theoretical magnetic field and intensity observations of coronal loops on the solar disk will be presented. The method will be applied to Yohkoh data using a linear force-free field as an illustration. Any other theoretical model for the magnetic field can be used, provided there is enough freedom in the model to optimize the fit. 相似文献
2.
Analysis of microbiological and chitinase activities relative to the white shrimpPenaeus setiferus, and its chitinoclastic endosymbionts, have demonstrated the relevance of total chitinolytic processes in penaeid biology. Microorganisms may serve as a direct source of nutrients for the animal as well as in the elaboration of extracellularin situ chitinase enzymes. The enzyme produced by the predominant gut bacteria,Beneckea neptuna, is a moderately actively inducible chitinase while the shrimp has an indigenous constitutive chitinase and chitobiase system. Factors of temperature, pH, ion inhibition and reducing sugar ratios have been compared for the bacterial and animal enzymes. This dual enzyme system suggests that metabolic chitin transformation may play a vital role in crustacean metabolism. 相似文献
3.
The normal mode spectrum for the linearized MHD equations is investigated for a cylindrical equilibrium. This spectrum is examined for zero perpendicular thermal conduction, with both zero and non-zero scalar resistivity. Particular attention is paid to the continuous branches of this spectrum, or continuous spectra. For zero resistivity there are three types of continuous spectra present, namely the Alfvén, slow, and thermal continua. It is shown that when dissipation due to resistivity is included, the slow and Alfvén continua are removed and that the thermal continuum is shifted to a different position (where the shift is independent of the exact value of resistivity). The old location of the thermal continuum is covered by a dense set of nearly singular discrete modes called a quasi-continuum. The quasi-continuum is investigated numerically, and the eigenfunctions are shown to have rapid spatial oscillating behaviour. These oscillations are confined to the most unstable part of the equilibrium based on the Field criterion, and may be the cause of fine structure in prominences. 相似文献
4.
5.
A WKB approach, based on the method of Connor, Hastie, and Taylor (1979), is used to obtain simple estimates of the critical
conditions for the onset of ideal MHD instabilities in line-tied solar coronal loops. The method is illustrated for the constant
twist, Gold-Hoyle (1960) field, and the critical conditions are compared with previous and new numerical results. For the
force-free case, the WKB estimate for the critical loop length reduces to
. For the sufficiently non-force-free case the critical length can be expressed in the forml
0 +l
1/m. The results confirm the findings of De Bruyne and Hood (1992) that for force-free fields them = 1 mode is the first mode to become unstable but for the sufficiently strong non-force-free case this reverses with them → ∞ mode being excited first. 相似文献
6.
Ideal and resistive ballooning modes are investigated for different ratios of a two-layer stratified density region representing a model for the photospheric/coronal boundary. Construction of the ballooning equations using a WKB approach is justified by comparison between the values of the growth rate obtained using Hain-Lüst and ballooning equations together with a WKB integral relation. Different values of the density ratio, radius, and resistivity are considered. Sausage-type and kink-type instabilities are found. One of these, depending on the value of r remained unstable for large density ratios. The other instability tended to marginal stability as the density ratio was increased, and allowed parallel and perpendicular flows across the boundary. This is contrary to the predictions of both the rigid-wall and flow-through conditions. 相似文献
7.
8.
Abstract The magnetohydrodynamic stability of a class of magnetohydrostatic equilibria is investigated. The effect of gravity is included as well as the stabilising influence of the dense photospheric line-tying. Although the two-dimensional equilibria exhibit a catastrophe point, when the ratio of plasma pressure to magnetic pressure exceeds a critical value, arcade structures, with both footpoints connected to the photosphere, become unstable to three-dimensional disturbances before the catastrophe point is reached. Numerical results for field lines that are open into the solar corona suggest that they are completely stable. Although there is no definite proof of stability, this would allow the point of non-equilibrium to be reached. 相似文献
9.
J. Blake Clark Wen Long Maria Tzortziou Patrick J. Neale Raleigh R. Hood 《Estuaries and Coasts》2018,41(3):708-723
Controls on organic matter cycling across the tidal wetland-estuary interface have proved elusive, but high-resolution observations coupled with process-based modeling can be a powerful methodology to address shortcomings in either methodology alone. In this study, detailed observations and three-dimensional hydrodynamic modeling are used to examine biogeochemical exchanges in the marsh-estuary system of the Rhode River, MD, USA. Analysis of observations near the marsh in 2015 reveals a strong relationship between marsh creek salinity and dissolved organic matter fluorescence (fDOM), with wind velocity indirectly driving large amplitude variation of both salinity and fDOM at certain times of the year. Three-dimensional model results from the Finite Volume Community Ocean Model implemented for the wetland system with a new marsh grass drag module are consistent with observations, simulating sub-tidal variability of marsh creek salinity. The model results exhibit an interaction between wind-driven variation in surface elevation and flow velocity at the marsh creek, with northerly winds driving increased freshwater signal and discharge out of the modeled wetland during precipitation events. Wind setup of a water surface elevation gradient axially along the estuary drives the modeled local sub-tidal flow and thus salinity variability. On sub-tidal time scales (>36 h, <1 week), wind is important in mediating dissolved organic matter releases from the Kirkpatrick Marsh into the Rhode River. 相似文献
10.
Climate change is likely to adversely affect many countries throughout the world, but the responses of different countries to this threat vary widely. Attempts to explain the differences in countries’ mitigative policies have been largely deficient. This study seeks to assess the degree to which vulnerability may improve the level of explanation of adopted mitigation policies, studying over 90 countries between 1990 and 2011. Vulnerability is defined to be comprised of two basic factors: impacts (expected damages due to climate change) and adaptive capacity (the ability to adjust to these damages). As there may be a gap between declared and implemented policies, these components of mitigation policy are examined separately. In addition, other variables which mediate between these ‘extreme ends’ of mitigation policies are tested.The effect of vulnerability on climate change mitigation policies is examined by multiple regressions, incorporating a wide range of control variables. The results indicate that climate impacts do not affect mitigation policies. Adaptive capacity has a positive effect on the level of declared policy, but this effect becomes insignificant once implemented policy is examined. However, other tests suggest a possible transition from declarations to actions by high adaptive capacity countries. This finding suggests that high adaptive capacity countries do not view mitigation and adaptation as substitutes. Further analyses indicate that the insignificancy of impacts is caused by the uncertainty in their assessment. 相似文献