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Coronal structures receive radiation not only from the solar disc, but also from the corona. This height-dependent incident radiation plays a crucial role in the excitation and the ionisation of the illuminated plasma. The aim of this article is to present a method for computing the detailed incident radiation coming from the solar corona, which is perceived at a point located at an arbitrary height. The coronal radiation is calculated by integrating the radiation received at a point in the corona over all of the corona visible from this point. The emission from the corona at all wavelengths of interest is computed using atomic data provided by CHIANTI. We obtain the spectrum illuminating points located at varying heights in the corona at wavelengths between 100 and 912 Å when photons can ionise H or He atoms and ions in their ground states. As expected, individual spectral lines will contribute most at the height within the corona where the local temperature is closest to their formation temperature. As there are many spectral lines produced by many ions, the coronal intensity cannot be assumed to vary in the same way at all wavelengths and so must be calculated for each separate height that is to be considered. This code can be used to compute the spectrum from the corona illuminating a point at any given height above the solar surface. This brings a necessary improvement to models where an accurate determination of the excitation and ionisation states of coronal plasma structures is crucial. 相似文献
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The kink instability in a coronal loop is a possible explanation of a compact loop flare as it may cause a current sheet to form allowing reconnection to take place and release the free magnetic energy stored in the loop. However, current sheets do not form in all cases. Ali and Sneyd (2001) investigated three different classes of equilibrium (determined by the form of the twist) using a magneto-frictional code. They searched for the equilibria to which the loop might evolve once it had become unstable to the kink instability. They found indications of current-sheet formation for only one class of equilibrium studied. However, as they pointed out, since their code searched for equilibria they were unable to say for certain that the loop would evolve in this way. In this paper we have considered the same three classes of equilibria but have used a code which follows the non-linear 3D MHD (magnetohydrodynamic) evolution of the loop. We have investigated whether or not there are indications of current-sheet formation. In the cases where there is evidence of this we have found that reconnection does occur and releases sufficient magnetic energy to explain a compact loop flare. 相似文献
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A. John Gerrard 《Geomorphology》1990,3(3-4)
Considerable insight into the nature and causes of soil variations on hillslopes has been provided by studies which have incorporated both pedological and geomorphological concepts into their methodology. Even so, there is still a great deal of uncertainty concerning the relationships between soils and landforms. This paper attempts to outline the conceptual and methodological issues involved in such a synthesis by examining the nature of soil variation on specific examples of landscapes in humid temperate climates. It has not been possible to examine soils across the whole of this region but soil-landform relationships on the Chalk of southeast England, the granite uplands of Dartmoor and in the Wyre Forest of England have been used to examine a number of well-established principles. One of these principles is that many landscapes possess slopes with easily identifiable toposequences. The hillslopes analysed have shown that relationships do exist between soils and landforms but the idea that many slopes are integrated along their entire length is not necessarily true. Individual components of slopes possess soil characteristics that appear to be related to the morphological nature of those components but the slopes, as a whole, do not possess integrated soil systems. Different parts of the slope appear to act independently. This questions the validity of employing traditional concepts such as that of the catena in soil-landform relationships. 相似文献
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The kink instability may be responsible for compact loop flares since the instability is triggered once the twist in a coronal loop exceeds a critical value. During the non-linear evolution of the instability a large current builds up, reconnection can occur and the magnetic energy released due to reconnection may explain the rapid heating of the flare. However, there has been some debate over the nature of the current concentration and, in particular, whether the current saturates or whether it is a current sheet, and what influences these possible states. In this paper we consider two similar equilibria having a twist function which rises to a peak and then falls off. One is steeper than the other allowing us to investigate whether the steepness of the peak has any effect on the nature of the current. For each profile, we run the code on five different grid resolutions and see how the maximum of the current scales with grid resolution. We also look for behavior in the x-component of the velocity which might be similar to the step-function behavior associated with singularities in the linear kink instability. For both profiles we find that the current scales almost linearly with resolution and that v
x drops steeply at the position of the current concentration. This suggests that, for these particular profiles, there are indications of current sheet formation and that the steepness in the peak of the twist does not affect the nature of the current. 相似文献
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A risk society? Environmental hazards, risk and resilience in the later Middle Ages in Europe 总被引：1，自引：1，他引：0
Modern society is said to have restructured in reaction to contemporary hazards with the aim of improving its management of risk. This implies that pre-industrial societies were somehow fundamentally different. In this paper, we challenge that hypothesis by examining the ways in which risks associated with environmental hazards were managed and mitigated during the Middle Ages (defined here as the period from 1000 to 1550 AD). Beginning with a review of the many case studies of rapid onset disasters across Europe, we draw upon both historical and archaeological evidence and architectural assessments of structural damage for what is a pre-instrumental period. Building upon this, the second part of the paper explores individual outlooks on risk, emphasising the diversity of popular belief and the central importance of Christianity in framing attitudes. Despite their religious perspectives, we find that medieval communities were not helpless in the face of serious environmental hazards. We argue instead that the response of society to these threats was frequently complex, considered and, at times, surprisingly modern. 相似文献
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The kink instability may be responsible for compact loop flares since the instability is triggered once the twist in a coronal loop exceeds a critical value. During the non-linear evolution of the instability a large current builds up, reconnection can occur and the magnetic energy released due to reconnection may explain the rapid heating of the flare. However, there has been some debate over the nature of the current concentration and, in particular, whether the current saturates or whether it is a current sheet, and what influences these possible states. In this paper we consider two similar equilibria having a twist function which rises to a peak and then falls off. One is steeper than the other allowing us to investigate whether the steepness of the peak has any effect on the nature of the current. For each profile, we run the code on five different grid resolutions and see how the maximum of the current scales with grid resolution. We also look for behavior in the x-component of the velocity which might be similar to the step-function behavior associated with singularities in the linear kink instability. For both profiles we find that the current scales almost linearly with resolution and that v
x drops steeply at the position of the current concentration. This suggests that, for these particular profiles, there are indications of current sheet formation and that the steepness in the peak of the twist does not affect the nature of the current. 相似文献
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John Gerrard 《自然地理学》2013,34(3):280-300
The Pleistocene periglacial legacy to the geomorphology of Dartmoor has been substantial. This paper examines some of these relict features in an area of western Dartmoor. The major features are tors, altiplanation terraces, boulder accumulations in a variety of patterns, and earth mounds. The tors and altiplanation terraces indicate the degree of slope modification created by frost action. The block-fields (clitter) are arranged into stripes, runs and garlands. Narrow stripes start and finish in midslope positions, while boulder runs converge and diverge, apparently at random. The long axis orientation of boulders in stripes is roughly in accord with the direction of the steepest slope, whereas orientation of boulders in blockfields is more variable. The altiplanation terraces and earth mounds occur on Cox Tor, which is composed of diabase. This contrast in rock type seems to explain the lack of similar features on the granite areas. The diabase is very closely jointed and weathers to a silt grade. The earth mounds are thought to be the result of frost thrusting in a silt-based soil. The general conclusion is that many of the landforms of Dartmoor are relicts from periglacial activity during the last glacial period. [Key words: periglaciation, tors, solifluction, Dartmoor.] 相似文献
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C.?Mellor C.?L.?Gerrard K.?Galsgaard A.?W.?HoodEmail author E.?R.?Priest 《Solar physics》2005,227(1):39-60
In this paper we present results from 3D MHD numerical simulations based on the flux tube tectonics method of coronal heating proposed by Priest, Heyvaerts, and Title (2002). They suggested that individual coronal loops connect to the photosphere in many different magnetic flux fragments and that separatrix surfaces exist between the fingers connecting a loop to the photosphere and between individual loops. Simple lateral motions of the flux fragments could then cause currents to concentrate along the separatrices which may then drive reconnection contributing to coronal heating. Here we have taken a simple configuration with four flux patches on the top and bottom of the numerical domain and a small background axial field. Then we move two of the flux patches on the base between the other two using periodic boundary conditions such that when they leave the box they re-enter it at the other end. This simple motion soon causes current sheets to build up along the quasi-separatrix layers and subsequently magnetic diffusion/reconnection occurs. 相似文献