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1.
The hydrodynamic properties of a steadily expanding corona are explored for situations in which departures from spherically symmetric outflow are large, in the sense that the geometrical cross section of a given flow tube increases outward from the Sun faster than r
2 in some regions. Assuming polytropic flow, it is shown that in certain cases the flow may contain more than one critical point. We derive the criterion for determining which of these critical points is actually crossed by the transonic solution which begins at the Sun and extends continuously outward. Next, we apply the theory to geometries which exhibit rapid spreading of the flow tubes in the inner corona, followed by more-or-less radial divergence at large distances. This is believed to be the type of geometry found in coronal hole regions. The results show that, if this initial divergence is sufficiently large, the outflow becomes supersonic at a critical point encountered low in the corona in the region of high divergence, and it remains supersonic at all greater heights in the corona. This feature strongly suggests that coronal hole regions differ from other open-field regions of the corona in that they are in a fast, low density expansion state over much of their extent. Such a dynamical configuration makes it possible to reconcile the low values of electron density observed in coronal holes with the large particle fluxes in the associated high speed streams seen in the solar wind.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
2.
Subsidence profiles across 29 oil and gas fields in the 12,200-km2 Houston, Texas, regional subsidence area, which is caused by decline of ground-water level, suggest that the contribution of petroleum withdrawal to local land subsidence is small. Despite large volumes of petroleum production, subsidence at most fields was not increased by oil and gas withdrawal. Local increases of subsidence were detected at only six fields—Alco-Mag, Chocolate Bayou, Goose Creek, Hastings, Mykawa, and South Houston. With the exception of the 1-m subsidence from 1917 to 1925 at Goose Creek, differential subsidence across oil and gas fields was smaller by a factor of two or more than subsidence caused by aquifer compaction. At four fields—Barbers Hill, Cedar Bayou, Humble, and Pierce Junction—subsidence was substantially less than in the surrounding area. Except for Cedar Bayou, these fields are associated with shallow salt domes that partly occupy the aquifer system; for the three fields, subsidence during the periods of record came to less than half the subsidence in the surrounding area. In addition to land subsidence, faults with an aggregate length of more than 240 km (150 mi) have offset the land surface in historical time. Natural geologic deformation, ground-water pumping, and petroleum withdrawal have all been considered as potential causes of the historical offset across these faults. The minor amount of localized land subsidence associated with oil and gas fields, however, suggests that petroleum withdrawal is not a major cause of the historical faulting, at least by a differential compaction mechanism. 相似文献
3.
After several decades of little work, a revised tornado climatology for Austria is presented. Tornadoes seldom form in the alpine areas, however, near the eastern flanks of the Alps, favourable conditions for tornado genesis are found. Whereas in the alpine regions less than 0.3 tornadoes per 10,000 km2 a year touch down (averaged for provinces or major parts of a province), we can count 0.9 in the greater Graz area, 1.0 in the greater Linz area and 1.2 tornadoes per 10,000 km2 a year in the greater Vienna area, suggesting the existence of so-called tornado alleys. As these regions are the most populated areas of Austria, there is a possible population bias in the dataset. The overall average for Austria is 0.3 tornadoes per 10,000 km2 a year.The database consists of 89 tornadoes, one landspout and six waterspouts, with a total of 96 events. The seasonal peak is in July with a maximum probability of tornadoes in the late afternoon and early evening hours. Every fifth tornado occurs in the hour after 5 p.m. The maximum intensity determined for a tornado in Austria was T7 on the TORRO-Scale (F3 on the Fujita-Scale), the most common intensity is T2 on the TORRO-Scale (F1 on the Fujita-Scale). 相似文献
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J. L. Bertaux E. Quémerais R. Lallement E. Kyrölä W. Schmidt T. Summanen J. P. Goutail M. Berthé J. Costa T. Holzer 《Solar physics》1997,175(2):737-770
After one year of almost flawless operation on board the SOHO spacecraft poised at L1 Lagrange point, we report the main features
of SWAN observations. SWAN is mainly dedicated to the monitoring of the latitude distribution of the solar wind by the Lα
method. Maps of sky Lα emissions were recorded througout the year. The region of maximum emission, located in the upwind hemisphere,
deviates strongly from the pattern that could be expected from a solar wind constant with latitude. It is divided into two
lobes by a depression aligned with the solar equatorial plane called the Lyα groove already noted in 1976 Prognoz data. The
north lobe is much brighter than the south lobe. These two characteristics can be explained qualitatively by an enhanced ionization
along the neutral sheet where the slow solar wind is concentrated, which results from the higher low-latitude solar wind mass
flux as measured by Ulysses. The groove is the direct imprint on the sky of the enhanced carving by the slow solar wind, at
this time of solar minimum, when the tilt angle of the neutral sheet is small. The question is still pending to predict what
will happen with the ascending phase of the solar cycle. Observations of comets are briefly mentioned, with the ability of
SWAN to monitor the H2O production of many comets. Operations of the instrument are briefly described, including some instrumental problems which
could be solved by software modifications sent to the instrument.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1004979605559 相似文献
6.
A possible mechanism for the formation and heating of coronal loops through the propagation and damping of fast mode waves is proposed and studied in detail. Loop-like field structures are represented by a dipole field with the point dipole at a given distance below the solar surface. The density of the medium is determined by hydrostatic equilibrium along the field lines in an isothermal atmosphere. The fast mode waves propagating outward from the coronal base are refracted into regions with a low Alfvén speed and suffer collisionless damping when the gas pressure becomes comparable to the magnetic pressure. The propagation and damping of these waves are studied for three different cases: a uniform density at the coronal base, a density depletion within a given flux tube, and a density enhancement within a given flux tube. The fast mode waves are found to be important in the formation and heating of the loops if the wave energy flux density is of the order 105 ergs cm-2 s-1 at the coronal base.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
7.
Ionization equilibrium is a useful assumption which allows temperatures and other plasma properties to be deduced from spectral observations. Inherent to this assumption is the premise that the ion stage densities are determined solely by atomic processes which are local functions of the plasma temperature and electron density. However, if the time scale of plasma flow through a temperature gradient is less than the characteristic time scale for an important atomic process, deviations from the ionization stage densities expected for equilibrium will occur which could introduce serious errors into subsequent analyses. In the past few years, significant flow velocities in the upper solar atmosphere have been inferred from observations of emission lines originaing in the transition region (about 104–106 K) and corona. In this paper, three models of the solar atmosphere (quiet Sun, coronal hole, and a network model) are examined to determine if the emission expected from these model atmospheres could be produced from equilibrium ion populations when steady flows of several kilometers per second are assumed. If the flows are quasi-periodic instead of steady, spatial and temporal averaging inherent in the observations may allow for the construction of satisfactory models based on the assumption of ionization equilibrium. Representative emission lines are analysed for the following ions: C iii, iv, O iv, v, vi, Ne vii, viii, Mg ix, x, Si xii, and Fe ix–xiv. Two principle conclusions are drawn. First, only the iron ions are generally in equilibrium for steady flows of 20 km s–1. For carbon and oxygen, ionization equilibrium is not a valid assumption for steady flows as small as 1 km s–1. Second, the three models representing different solar conditions behave in a qualitatively similar manner, implying that these results are not particularly model dependent over the range of temperature gradients and electron densities thus far inferred for the Sun. In view of the flow velocities which have been reported for the Sun, our results strongly suggest caution in using the assumption of ionization equilibrium for interpreting spectral lines produced in the transition region.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
8.
Lites B.W. Card G. Elmore D.F. Holzer T. Lecinski A. Streander K.V. Tomczyk S. Gurman J.B. 《Solar physics》1999,190(1-2):185-206
Solar Physics - This paper presents first observations of dynamics of the white-light solar corona detected during the few minutes of totality of a solar eclipse. Perturbations of a polar plume... 相似文献
9.
Geomechanics of hazardous landslides 总被引：1，自引：1，他引：0
POISEL Rainer BEDNARIK Martin HOLZER Rudolf LISCAK Pavel 《山地科学学报》2005,2(3):211-217
A catalogue of possible landslide initial failure mechanisms, taking into account the geological setting and the geometry of the slope, the joint structure, the habitus of the rock blocks, as well as the mechanical behaviour of the rocks and of the rock mass （deformation and strength parameters）, is presented. Its aim is to give geologists as well as engineers the opportunity to compare phenomena in the field and phenomena belonging to particular mechanisms and to find the mechanism occurring. The presented catalogue of initial landslide mechanisms only comprises the mechanisms having a clearly defined mechanical model that can be divided into empirical relations and into mechanical models, as well as an overview of run out models, which can be divided into empirical relations and into mechanical models. 相似文献
10.