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1.
A scenario for the production of a current sheet above an active region during the emergence of new magnetic flux is considered. The formation of a current sheet is demonstrated via a numerical solution of a system of MHD equations with dissipative terms. The flare energy is stored in the magnetic field of the current sheet. The decay of the current sheet can account for a number of solar-flare phenomena, including the observed divergence of H α ribbons. 相似文献
2.
We present the results of numerical simulations of the development of a current sheet in the solar corona over a bipolar region during the emergence of two new sunspots arranged collinearly with older spots. Two fronts of increased plasma density form at the boundary of the rising new magnetic flux. One of these is due to the generation of a current sheet, whose magnetic field accumulates energy for a flare. The other front is a branch of the density perturbation, and separates the old and new magnetic fluxes in a region where the magnetic field lines have the same direction on both sides of the boundary. The development of this perturbation is not associated with the energy accumulation in the corona, and hinders observation of the preflare state and complicates analysis of the results. This second front can be interpreted as the eruption of a filament before the onset of the flare. A scheme conservative with respect to magnetic flux was introduced in the Peresvet code that solves the MHD equations, in order to suppress numerical instabilities in regions of large magnetic-field gradients. 相似文献
3.
New data for the energy and location of the hard-emission centers of a solar flare agree with an electrodynamic model of a solar flare based on the idea of the accumulation of free magnetic energy in the field of a current sheet. Three-dimensional MHD simulations are used to show that the energy stored in the preflare magnetic field of the current sheet is sufficient for the development of a flare and a coronal mass ejection. The flare and coronal mass ejection result from the explosive decay of the current sheet. The position of the brightness-temperature maximum of the radio emission during the flare coincides with the maximum of the current in the current sheet. The exponential spectrum of relativistic protons generated during the flare is consistent with acceleration by the electric field during the current-sheet decay. 相似文献
4.
We have developed a three-dimensional numerical model and applied it to simulate plasma flows in semi-detached binary systems
whose accretor possesses a strong intrinsic magnetic field. The model is based on the assumption that the plasma dynamics
are determined by the slow mean flow, which forms a backdrop for the rapid propagation of MHD waves. The equations describing
the slow motion of matter were obtained by averaging over rapidly propagating pulsations. The numerical model includes the
diffusion of magnetic field by current dissipation in turbulent vortices, magnetic buoyancy, and wave MHD turbulence. A modified
three-dimensional, parallel, numerical code was used to simulate the flow structure in close binary systems with various accretor
magnetic fields, from 105 to 108 G. The conditions for the formation of the accretion disk and the criteria distinguishing the two types of flow corresponding
to intermediate polars and polars are discussed. 相似文献
5.
The process of post-flare loop formation, including the heating of flux tubes by hot chromospheric sources and their filling with plasma, is demonstrated by simulations in an MHD approximation. The loop is additionally heated at its apex by the interaction of oppositely directed plasma streams. Local coronal heating over the loop is also possible due to magnetic-field-line reconnection. A new version of the PERESVET code that can take into account anisotropy of the thermal conductivity of a plasma in a magnetic field was used for the computations. 相似文献
6.
Restrictions on the Parameters of Black Hole and Plasma in the Vicinity of the Source Sagittarius A*
Astronomy Reports - Estimates for the parameters of accretion plasma flows and the central black hole by comparing the MHD model numerical calculations with observations for the center of our... 相似文献
7.
A model for magnetic reconnection in high-conductivity plasma in the solar corona is analyzed in a strong-magnetic-field approximation. The model includes a Syrovatskii current layer and magnetohydrodynamic (MHD) discontinuities attached to the ends of the layer. A two-dimensional analytical solution for the magnetic field is used to compute the distributions of the plasma flow velocity and plasma density in the vicinity of the corresponding current configuration. The properties of jumps in the density and velocity along the attached discontinuities are studied. Based on the character of the variations of the magnetic field and plasma flows at the MHD discontinuities, it is shown that, with the parameter values considered, an MHDdiscontinuity can include regions of trans-Alfvénic, fast, and slowshocks. The results obtained could be useful to explain the presence of “super-hot” (with effective electron temperatures exceeding 10 keV) plasma in solar flares. Other possible applications of the theory of discontinuous flows near regions of magnetic reconnection to analogous non-stationary phenomena in astrophysical plasmas are noted. 相似文献
8.
The results of three-dimensional MHD numerical simulations are used to investigate the characteristic properties of the magnetic-field
structures in the accretion disks of semi-detached binary systems. It is assumed that the intrinsic magnetic field of the
accretor star is dipolar. Turbulent diffusion of the magnetic field in the disk is taken into account. The SS Cyg system is
considered as an example. The results of the numerical simulations show the intense generation of a predominantly toroidal
magnetic field in the accretion disk. Magnetic zones with well defined structures for the toroidal magnetic field form in
the disk, which are separated by current sheets in which there ismagnetic reconnection and current dissipation. Possible observational
manifestations of such structures are discussed. It is shown that the interaction of a spiral precessional wave with the accretor’s
magnetosphere could lead to quasi-periodic oscillations of the accretion rate. 相似文献
9.
地球等离子体片中持续时间很短的快速流动事件最近受到很多关注，这些事件被Angelopoulos等称为爆震流，简要回顾了爆震流的理论研究。当前理论认为，爆震流是磁泡（含较少等离子体的磁流通管）在交换不稳定性的作用下在等离子体片中的流动，磁泡图像很自然地解释了等离子体片中观测到的爆震流。理论预期的快速流在横穿磁尾方向的尺度，它的速度伴随着磁场松驰，压力和密度的降低以及电离层和地面特征都已被观测证实。 相似文献
10.
Nina Kirchner Ralf Greve Arjen P. Stroeven Jakob Heyman 《Quaternary Science Reviews》2011,30(1-2):248-267
The Tibetan Plateau is a topographic feature of extraordinary dimension and has an important impact on regional and global climate. However, the glacial history of the Tibetan Plateau is more poorly constrained than that of most other formerly glaciated regions such as in North America and Eurasia. On the basis of some field evidence it has been hypothesized that the Tibetan Plateau was covered by an ice sheet during the Last Glacial Maximum (LGM). Abundant field- and chronological evidence for a predominance of local valley glaciation during the past 300,000 calendar years (that is, 300 ka), coupled to an absence of glacial landforms and sediments in extensive areas of the plateau, now refute this concept. This, furthermore, calls into question previous ice sheet modeling attempts which generally arrive at ice volumes considerably larger than allowed for by field evidence. Surprisingly, the robustness of such numerical ice sheet model results has not been widely queried, despite potentially important climate ramifications. We simulated the growth and decay of ice on the Tibetan Plateau during the last 125 ka in response to a large ensemble of climate forcings (90 members) derived from Global Circulation Models (GCMs), using a similar 3D thermomechanical ice sheet model as employed in previous studies. The numerical results include as extreme end members as an ice-free Tibetan Plateau and a plateau-scale ice sheet comparable, in volume, to the contemporary Greenland ice sheet. We further demonstrate that numerical simulations that acceptably conform to published reconstructions of Quaternary ice extent on the Tibetan Plateau cannot be achieved with the employed stand-alone ice sheet model when merely forced by paleoclimates derived from currently available GCMs. Progress is, however, expected if future investigations employ ice sheet models with higher resolution, bidirectional ice sheet-atmosphere feedbacks, improved treatment of the surface mass balance, and regional climate data and climate reconstructions. 相似文献
11.
The flare source of thermal X-rays above a magnetic arch in the corona arises from the dissipation of the magnetic energy
of the current sheet formed at the reconnection of magnetic-field lines. The sources of hard X-rays emitted from the footpoints
of the magnetic arch are beams of electrons accelerated in field-aligned currents induced by the Hall electric field generated
in the current sheet. Both the hard X-rays detected above the active region and the type III radio emission are radiated by
electrons accelerated in the field-aligned currents induced by Alfven waves. The solar cosmic rays are emitted promptly at
the instant of the flare. It is important that the Lorentz electric field accelerates protons along the singular magnetic
X line. The relativistic protons propagate along the interplanetary magnetic field. These protons have exponential spectra,
typical for acceleration occurring in current sheets. A mechanism that is relevant for the generation of delayed cosmic rays,
which demonstrate significant anisotropy and a power-law spectrum with γ ∼5, is also discussed. 相似文献
12.
The Late Quaternary sediment sequence of the continental margin in the eastern Weddell Sea is well suited for palaeoenvironmental reconstructions. Two cores from the upper slope, which contain the sedimentary record of the last 300 ky, have been sedimentologically investigated. Age models are based on lithostratigraphy and are correlated with the stable isotope record. As a result of a detailed analysis of the clay mineral composition, grain size distributions and structures, this sedimentary record provides the first marine evidence that the Antarctic ice sheet extended to the shelf edge during the last glacial.The variations in volume and size of the ice sheet were also simulated in numerical models. Changes in accumulation rate and ice temperature are of some importance, but the model revealed that fluctuations are primarily driven by changes in eustatic sea-level and that the ice edge extended to the shelf edge during the last glacial maximum. This causal relationship implies that the maximum ice extension strongly depends on the magnitude and duration of the sea-level depression during a glacial period. The results of the sedimentological investigations and of the numerical models show that the Antarctic ice sheet follows glacial events in the northern hemisphere by teleconnections of sea level.
Correspondence to: H. Grobe 相似文献
13.
Soft X-ray data for prolonged flares in subgiants in RS CVn binary systems and some other active late-type stars (AB Dor, Algol) are analyzed. During these nonstationary events, a large amount of hot plasma with temperatures exceeding 108 K exists for many hours. Numerical simulations of gas-dynamical processes in the X-ray source—giant loops—can yield reliable estimates of the plasma parameters and flare-source size. This confirms that such phenomena exist while considerable energy is supplied to the top part of a giant loop or system of loops. Refined estimates of the flare energy (up to 1037 erg) and scales contradict the widely accepted idea that prolonged X-ray flares are associated with the evolution of local magnetic fields. The energy of the current component of the large-scale magnetic field arising during the ejection of magnetic field by plasma jets or stellar wind is estimated. Two cases are considered: a global stellar field and fields connecting regions with oppositely directed unipolar magnetic fields. The inferred energy of the current component of the magnetic field associated with distortion of the initial MHD configuration is close to the total flare energy, suggesting that large-scale magnetic fields play an important role in prolonged flares. The flare process encompasses some portion of a streamer belt and may propagate along the entire magnetic equator of the star during the most powerful prolonged events. 相似文献
14.
Astronomy Reports - We performed a three-dimensional numerical MHD simulation of the flow structure in the asynchronous polar CD Ind during the switching phases between the magnetic poles of the... 相似文献
15.
Astronomy Reports - The paper presents a modification of the numerical MHD model of extended envelopes of hot Jupiters, in which the magnetic viscosity is considered. In the new version of the... 相似文献
16.
Ice-proximal sedimentological features from the northwestern Barents Sea suggest that this region was covered by a grounded ice sheet during the Late Weichselian. However, there is debate as to whether these sediments were deposited by the ice sheet at its maximum or a retreating ice sheet that had covered the whole Barents Sea. To examine the likelihood of total glaciation of the Late Weichselian Barents Sea, a numerical ice-sheet model was run using a range of environmental conditions. Total glaciation of the Barents Sea, originating solely from Svalbard and the northwestern Barents Sea, was not predicted even under extreme environmental conditions. Therefore, if the Barents Sea was completely covered by a grounded Late Weichselian ice sheet, then a mechanism (not accounted for within the glaciological model) by which grounded ice could have formed rapidly within the central Barents Sea, may have been active during the last glaciation. Such mechanisms include (i) grounded ice migration from nearby ice sheets in Scandinavia and the central Barents Sea, (ii) the processes of sea-ice-induced ice-shelf thickening and (iii) isostatic uplift of the central Barents Sea floor. 相似文献
17.
Numerical studies of anchored sheet pile wall behavior constructed in cut and fill conditions 总被引：1，自引：0，他引：1
The construction of sheet pile walls may involve either excavation of soils in front or backfilling of soils behind the wall. These construction procedures generate different loading conditions in the soil and therefore different wall behavior should also be expected. The conventional methods, which are based on limit equilibrium approach, commonly used in the design of anchored sheet pile walls do not consider the method of construction. However, continuum mechanics numerical methods, such as finite element method, make it possible to incorporate the construction method during the analyses and design of sheet pile walls. The effect of wall construction type for varying soil conditions and wall heights were investigated using finite element modeling and analysis. The influence of construction method on soil behavior, wall deformations, wall bending moments, and anchor forces were investigated. The study results indicate that walls constructed by backfill method yield significantly higher bending moments and wall deformations. This paper presents the results of the numerical parametric study performed and comparative analyses of the anchored sheet pile walls constructed by different construction methods. 相似文献
18.
We have carried out 3D MHD modeling of the solar corona above the active region AR 0365 before a series of flares observed on May 26–27, 2003. Maps of the evolving photospheric magnetic fields preceding the flares were used as boundary conditions. An emergence of new flux equal to ～1.5 × 1022 Maxwell preceded the observed series of X-ray flares. Modeling a large region 4 × 1010 cm in size demonstrates the formation of several current sheets in the vicinities of coronal Xlines, both already existing in the initial potential field and arising due to the emergence of the new magnetic flux. Each current sheet could be responsible for an elementary flare. 相似文献
19.
We analyze the suitability of digital equipment with CCD arrays for polarization measurements of solar coronal emission during eclipses. The effect of errors in the method on the derived plane of polarization is estimated. In principle, precise measurements of the plane of polarization can be used to determine the amplitude and direction of the current of a heliospheric plasma sheet. The errors in ordinary nondigital photometric measurements (5%) are too large to enable the resolution of a number of problems having to do with the three-dimensional structure of the solar corona. 相似文献
20.
Stéphanie Gallino Jean-Yves Josnin Marc Dzikowski Fabien Cornaton Dominique Gasquet 《Hydrogeology Journal》2009,17(8):1887-1900
Numerical models of the Aix-les-Bains thermal aquifer (France) were used to investigate the influence of Quaternary paleoclimatic events on the current thermal state of the groundwater. Initial numerical tests were successful in that present-day fluid flows (heads and flow rates) and the resulting velocities were compatible with residence time data. Water flowing through an aquifer cools the rock mass; therefore, the rate of water flow governs the outlets temperature. For the Aix-les-Bains aquifer, applying present-day flow rates to the entire history of the aquifer leads to much more substantial cooling of the rock mass than is indicated by the outlets temperature (i.e. present-day flow rates are 10 times too high). This suggests that the aquifer may have gone through alternating functioning phases, during which the rock mass cooled, and blocked phases, during which the aquifer reheated. Other results indicate that the main parameters affecting thermal behavior during a functioning phase are the total inflow volume, rather than individual inflow rates, and the initial heat field. As phenomena linked to glaciation can lead to the blocking of infiltration zones and aquifer outlets, the findings suggest that the hypothesis of intermittent aquifer functioning related to glaciations is compatible with the current thermal field. 相似文献