共查询到20条相似文献，搜索用时 46 毫秒
1.
The finite difference approximation technique using the explicit method is used for solving the unsteady flow of an electrically conducting viscous and incompressible fluid, subjected to a normal homogenous magnetic field. The flow is confined on one side of a non-magnetic infinite limiting surface (wall) which is initially at rest and then is suddenly accelerated in its own plane with a velocity which is a general function of time. The wall is porous and we assume that the Prandtl number of the fluid corresponds to the case of water and that the magnetic Prandtl number is equal to one. Quantitative discussion of the results is presented for the case of uniformly accelerated motion of the wall. 相似文献
2.
The classical Rayleigh problem has been extended to the case of the hydromagnetic free-convective flow of an electrically-conducting and incompressible viscous fluid past an infinite vertical naturally permeable wall in a rotating system. The applied transverse magnetic field is fixed with the moving wall and the magnetic Reynolds number of the flow is taken small so that the induced magnetic field can be neglected in comparison to the applied magnetic field. The permeable wall starts moving from rest in the still fluid and thus arises an initial value problem whose solution has been obtained by the Laplace transform method for two important cases impulsive as well as accelerated start of the plate. Mathematical expression for skin friction components have been also obtained in a closed form. Asymptotic behaviour of the solution is analysed for both the cases, and some interesting particular cases have also been encountered. Influence of various physical parameters occurring into the problem has been discussed with the aid of graphs and tables. 相似文献
3.
There are two distinct regimes of the first-order Fermi acceleration of shocks. The first is a linear (test-particle) regime in which most of the shock energy goes into thermal and bulk motions of the plasma. The second is an efficient regime in which the shock energy goes into accelerated particles. Although the transition region between them is narrow, we identify the factors that drive the system toward a self-organized critical state between those two regimes. Using an analytic solution, we determine this critical state and calculate the spectra and maximum energy of accelerated particles. 相似文献
4.
《天文和天体物理学研究(英文版)》2020,(3)
We have studied the dynamic proton spectra for the two solar energetic particle(SEP) events on2000 July 14(hereafter GLE59) and 2005 January 20(hereafter GLE69). The source locations of GLE59 and GLE69 are N22 W07 and N12 W58 respectively. Proton fluxes 30 Me V have been used to compute the dynamic spectral indices of the two SEP events. Our results show that spectral indices of the two SEP events increased more swiftly at early times, suggesting that the proton fluxes 30 Me V might be accelerated particularly by the concurrent flares at early times for the two SEP events. For the GLE69 with source location at N12 W58, both flare site and shock nose are well connected with the Earth at the earliest time. However, only the particles accelerated by the shock driven by eastern flank of the CME can propagate along the interplanetary magnetic field line to the Earth after the flare. For the GLE59 with source location at N22 W07, only the particles accelerated by the shock driven by western flank of the associated CME can reach the Earth after the flare. Our results also show that there was slightly more than one hour during which the proton spectra for GLE69 are softer than that for GLE59 after the flares, suggesting that the shock driven by eastern flank of the CME associated with GLE69 is weaker than the shock driven by the western flank of the CME associated with GLE59. The results support that quasi-perpendicular shock has stronger potential in accelerating particles than the quasi-parallel shock. These results also suggest that only a small part of the shock driven by western flank of the CME associated with the GLE59 is quasi-perpendicular. 相似文献
5.
A. A. Korchak 《Solar physics》1980,66(1):149-158
The influence of Coulomb collisions in two-component plasma on the nuclear composition and the charge-state of accelerated particles is investigated. The main characteristics are the location and value of the two loss maxima. It is shown that the maximum of energy losses on the electron component of plasma for flares is a high energy threshold which prevents the penetration of a large particle flux into the range 10 MeV nucl–1. The low-energy range up to the maximum is considered in detail. At preliminary or initial stage of acceleration the nuclear composition of accelerated particles is strongly dependent on their energy losses on the proton component of plasma which prevails at low energies. The conditions under which the equilibrium charge is reached are investigated. 相似文献
6.
V. N. Tsytovich R. Bingham U. de Angelis D. A. Bryant C. M. C. Nairn 《Solar physics》1993,147(2):329-335
The possibility of accelerated protons in solar flares having a sharp change in their spectral index is discussed. The analysis is based on the Tsytovich (1982, 1984, 1987a, b, c) acceleration model by MHD turbulence, which is shown to have different resonant conditions for non-relativistic and relativistic particles. The different resonant condition is shown to result in a sharp change in the accelerated proton spectral index, even in the absence of any peculiarity in the spectra of the MHD turbulence. Time scales for accelerated protons to relativistic energies are also derived, and shown to be consistent with observations. We also show that the threshold energy for electron acceleration by low frequency MHD turbulence is much greater than for proton acceleration. The turbulence therefore preferentially accelerates protons. 相似文献
7.
N.A. Tsyganenko 《Planetary and Space Science》1982,30(10):1007-1012
Calculation of stationary distributions of the most important plasma parameters (particle energy, density, field-aligned and transversal pressure) is performed for a model magnetotail plasma sheet which is formed by convecting plasma mantle particles injected into the closed geomagnetic field line tubes. Computations have been done for two convection models: (i) a model of completely adiabatic particle motion with conservation of the first two invariants and (ii) a model with a strong pitch-angle diffusion which maintains isotropy. It is found that in both cases the heating and compression of the plasma are somewhat more effective than is necessary to account for the observed gradients of magnetic field in the magnetospheric tail. A leakage of accelerated particles through the dawn and dusk edges of the plasma sheet is proposed as a possible mechanism for maintenance of stationary convection in the magnetotail. The question of the dependence of the stationary magnetotail parameters on the solar wind state is discussed briefly. 相似文献
8.
9.
It is commonly believed that solar type II bursts are caused by accelerated electrons at a shock front. Holman and Pesses (1983) suggested that electrons creating type II bursts are accelerated by the shock drift mechanism. Zlobec et al. (1993) dealt with a fine structure of type II bursts (herringbones) and suggested a qualitative model where electrons are accelerated by a nearly perpendicular wavy shock front. Using this idea, we developed a model of electron acceleration by such a wavy shock front. Electrons are accelerated by the drift mechanism in the shock layer. Under simplifying assumptions it is possible to obtain an analytical solution of electron motion in the wavy shock front. The calculations show that electrons are rarely reflected more than once at the wavy shock front and that their final energy is mostly 1–3 times the initial one. Their acceleration does not depend significantly on shock spatial parameters. In the present model all electrons are eventually transmitted downstream where they form two downstream beams. Resulting spectral and angular distributions of accelerated electrons are presented and the relevance of the model to the herringbone beams is discussed. 相似文献
10.
In this paper we have discussed two models of domain walls within the framework of Lyra geometry. An exact solution is obtained
for a thick non static domain wall. The space time is non singular both in its spatial and temporal behavior and the gravitational
field experienced by a test particle is attractive. It is found that these exists no particle horizon in our case. Also we
have presented a spherical domain wall with nonvanishing stress components in the direction perpendicular to the plane of
the wall. The gravitational field of the domain wall is shown to be attractive in nature.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
In this paper, we discuss energy conditions in modified Gauss-Bonnet gravity for locally rotationally symmetric Bianchi type I universe model with perfect fluid. The matter contents are constructed to discuss the energy conditions bounds. We take two specific f(G) models along with present day values of Hubble, deceleration, jerk and snap parameters. It is found that weak and null energy conditions are satisfied while strong energy conditions are violated for both models which represents the accelerated expansion of the universe. 相似文献
12.
It is known that for a thin domain wall the pressure in the perpendicular direction to the wall is negligible. In this paper,
we have evaluated solutions for thin domain walls with spherical symmetry following two different approaches. The exact solutions
are obtained using functional separability of the metric coefficients.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
The most important feature of the microwave type III bursts on Nov. 4, 1997 is the periodically reversed drift rates, which may be contributed to a group of electron beams trapped
by a huge magnetic tube (104 km). It is suggested that these electron beams are accelerated by the same mechanism, because there is a power law distribution
with index 3.2 in the energetic spectrum of the beam. On the other hand, the energy release in each pulse is quasi-quantized,
which is confirmed by the statistical correlation between the rising time and the burst flux. Both of these two results are
based on the model of plasma instability responsible for the burst.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
We solve the self-consistent problem of the generation of a static magnetic field by the electric current of accelerated particles near a strong plane MHD shock front. We take into account the back reaction of the field on the particle diffusion tensors and the background plasma parameters near the front. Various states that differ significantly in static magnetic-field strength are shown to be possible near a strong front. If the initial field has a component normal to the front, then its components parallel to the front are suppressed by accelerated particles by several orders of magnitude. Only the component perpendicular to the front remains. This field configuration for uniform particle injection at the front does not lead to the generation of an additional field, and, in this sense, it is stable. If the initial field is parallel to the front, then either its significant enhancement by two or three orders of magnitude or its suppression by several orders of magnitude is possible. The phenomenon under consideration is an example of the self-organization of plasma with a magnetic field in a strongly nonequilibrium system. It can significantly affect the efficiency of particle acceleration by the shock front and the magnetobremsstrahlung of the accelerated particles. 相似文献
15.
Hiroshi Nakajima 《Solar physics》1983,86(1-2):427-432
The large microwave burst of 1981 April 1, which was accompanied by both hard X-ray and γ-ray emissions, was analyzed to study the acceleration of particles in the impulsive phase. The analysis suggests the following results. (1) Electrons were accelerated up to energies of several hundred keV in a low loop. On the other hand, electrons were accelerated to relativistic energy without injection of pre-accelerated electrons near the top of a large loop where energetic ions were also probably accelerated. (2) The mechanism for accelerating electrons to relativistic energy and also ions was different from that for accelerating electrons up to energies of several hundred KeV and was closely related with upward motion of a flare loop. 相似文献
16.
A. A. Korchak 《Solar physics》1978,56(1):223-234
The formation of power-law energy spectrum of particles accelerated in solar flares is investigated. The distinct difference between the mechanism and the model of acceleration is pointed out. It is shown that Fermi's model is described by linear differential equation of the first order and therefore a power-law spectrum is formed only for some special conditions which apparently are not fulfilled for flares. A satisfactory alternative to Fermi's model hasn't yet been found. In conclusion the connection between the mechanism of acceleration and a charge spectrum of accelerated particles is examined. 相似文献
17.
We examine a non-linear mechanism for a solar surge in which plasma regions of high electrical conductivity and macroscopic dimension can be rapidly accelerated without diffusion of magnetic field. The mechanism is suggested by Rust's observations, which show that surges occur near sunspots in regions of reversed magnetic polarity. For the purposes of numerical calculation, we replace the magnetic field near a polarity reversal in a sunspot by magnetic fields of current loops. The relaxation of the magnetic field generated by two antiparallel coaxial current loops in an incompressible plasma is traced by computer. The results suggest that plasma in the form of a vortex ring can be expelled at the Alfvén velocity from active solar regions. 相似文献
18.
Various mechanisms have been proposed to explain how seismic waves can be generated during a solar flare, several of which
include a major role for accelerated electrons. To address this question further, we have selected two samples of white-light
flares (WLFs): one that has associated sunquakes, and one that does not. We focus particularly on the spatial characteristics
of the hard X-ray (HXR) and WL emission, and the HXR spectral characteristics associated with the flares in both samples,
including spectral hardness, HXR source size, and total injected electron power. Coupling the determined rate of energy deposition
with the area over which the energy is being deposited suggests that in general the acoustically active flares are associated
with a larger and more impulsive deposition of electron energy. However, this does not always correspond to a higher WL contrast. 相似文献
19.
We consider the relationship of electromagnetic radiation in the three most intense flares of solar cycle 23, more specifically, those of October 28, 2003, January 20, 2005, and September 7, 2005, to the acceleration and release of protons into interplanetary space. The impulsive phase of these flares lasted ～ 20 min and consisted of at least three energy release episodes, which differed by their manifestation in the soft (1–8 Å, GOES) and hard (>150 keV, INTEGRAL) X-ray ranges as well as at radio frequencies of 245 MHz and 8.8 GHz. The protons and electrons were accelerated in each episode, but with a different efficiency; the relativistic protons were accelerated only after 5–6min of impulsive-phase development after the onset of a coronal mass ejection. It is at this time that maximum hard X-ray fluxes were observed in the September 7, 2005 event, which exceeded severalfold those for the other two flares considered. We associate the record fluxes of protons with energies > 200MeV observed in the heliosphere in the September 7, 2005 event with the dynamics of the impulsive phase. The extreme intensities of the microwave emission in the October 28, 2003 and January 20, 2005 events were probably attributable to the high-energy electron trapping conditions and did not reflect the acceleration process. 相似文献
20.
In this contribution we present Viking observations of electrons and positive ions which move upward along the magnetic field lines with energies of the same order of magnitude. We propose that both ions and electrons are accelerated by an electric field which has low-frequency temporal variations such that the ions experience an average electrostatic potential drop along the magnetic field lines whereas the upward streaming electrons are accelerated in periods of downward pointing electric field which is quasi-static for the electrons and forces them to beam out of the field region before the field changes direction.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988. 相似文献