共查询到20条相似文献,搜索用时 15 毫秒
1.
Lucas F. Wanex 《Astrophysics and Space Science》2005,298(1-2):337-340
A linear analysis of axial sheared flow in magnetohydrodynamic (MHD) jets with helical magnetic fields is presented. A linearized
set of ideal MHD equations allows the investigation of plasmas with both magnetic shear and flow shear included in the equilibrium
profile. These equations are integrated numerically by following the linear development in time of an initial seed perturbation.
Global instability growth rates are obtained after the numerical solution converges to the fastest growing mode. It is shown
that axial sheared flow reduces the growth of current-driven instabilities in plasma jets with constant magnetic pitch P = rB
z
/B
θ. 相似文献
2.
We study the dispersion characteristics of fast MHD surface waves travelling on a plasma slab immersed in a complex magnetic field consisting of a large longitudinal B
0z
component and a small sheared B
0y
component. The analysis shows that for typical coronal conditions both the sausage and kink waves are generally pseudo-surface waves. The tangential magnetic field, B
0y
, modifies the dispersion curves, and for sufficiently large sheared fields there is a transition from pseudo-surface to pure-surface fast kink waves.On leave from Faculty of Physics, Sofia University, BG-1126 Sofia, Bulgaria. 相似文献
3.
A. F. Kholtygin S. N. Fabrika N. A. Drake V. D. Bychkov L. V. Bychkova G. A. Chountonov T. E. Burlakova G. G. Valyavin 《Astronomy Letters》2010,36(5):370-379
Based on an analysis of the catalog of magnetic fields, we have investigated the statistical properties of the mean magnetic
fields for OB stars. We show that the mean effective magnetic field B of a star can be used as a statistically significant characteristic of its magnetic field. No correlation has been found
between the mean magnetic field strength B and projected rotational velocity of OB stars, which is consistent with the hypothesis about a fossil origin of the magnetic
field. We have constructed the magnetic field distribution function for B stars, F(B), that has a power-law dependence on B with an exponent of ≈−1.82. We have found a sharp decrease in the function F(B) for B ⩽ 400 G that may be related to rapid dissipation of weak stellar surface magnetic fields. 相似文献
4.
The magnetorotational instability (MRI) of differential rotation under the simultaneous presence of axial and azimuthal components of the (current‐free) magnetic field is considered. For rotation with uniform specific angular momentum the MHD equations for axisymmetric perturbations are solved in a local short‐wave approximation. All the solutions are overstable for Bz · Bϕ ≠ 0 with eigenfrequencies approaching the viscous frequency. For more flat rotation laws the results of the local approximation do not comply with the results of a global calculation of the MHD instability of Taylor‐Couette flows between rotating cylinders. – With Bϕ and Bz of the same order the traveling‐mode solutions are also prefered for flat rotation laws such as the quasi‐Kepler rotation. For magnetic Prandtl number Pm → 0 they scale with the Reynolds number of rotation rather than with the magnetic Reynolds number (as for standard MRI) so that they can easily be realized in MHD laboratory experiments. – Regarding the nonaxisymmetric modes one finds a remarkable influence of the ratio Bϕ/Bz only for the extrema. For Bϕ ≫ Bz and for not too small Pm the nonaxisymmetric modes dominate the traveling axisymmetric modes. For standard MRI with Bz ≫ Bϕ, however, the critical Reynolds numbers of the nonaxisymmetric modes exceed the values for the axisymmetric modes by many orders so that they are never prefered. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
5.
It has been proposed that the million-degree temperature of the corona is due to the combined effect of barely detectable
energy releases, called nanoflares, that occur throughout the solar atmosphere. Unfortunately, the nanoflare density and brightness
implied by this hypothesis means that conclusive verification is beyond present observational abilities. Nevertheless, we
investigate the plausibility of the nanoflare hypothesis by constructing a magnetohydrodynamic (MHD) model that can derive
the energy of a nanoflare from the nature of an ideal kink instability. The set of energy-releasing instabilities is captured
by an instability threshold for linear kink modes. Each point on the threshold is associated with a unique energy release;
thus we can predict a distribution of nanoflare energies. When the linear instability threshold is crossed, the instability
enters a nonlinear phase as it is driven by current sheet reconnection. As the ensuing flare erupts and declines, the field
transitions to a lower energy state, which is modelled by relaxation theory; i.e., helicity is conserved and the ratio of current to field becomes invariant within the loop. We apply the model so that all
the loops within an ensemble achieve instability followed by energy-releasing relaxation. The result is a nanoflare energy
distribution. Furthermore, we produce different distributions by varying the loop aspect ratio, the nature of the path to
instability taken by each loop and also the level of radial expansion that may accompany loop relaxation. The heating rate
obtained is just sufficient for coronal heating. In addition, we also show that kink instability cannot be associated with
a critical magnetic twist value for every point along the instability threshold. 相似文献
6.
We study a nonlinear mechanism for the excitation of kinetic Alfvén waves (KAWs) by fast magneto-acoustic waves (FWs) in the
solar atmosphere. Our focus is on the excitation of KAWs that have very small wavelengths in the direction perpendicular to
the background magnetic field. Because of their small perpendicular length scales, these waves are very efficient in the energy
exchange with plasmas and other waves. We show that the nonlinear coupling of the energy of the finite-amplitude FWs to the
small-scale KAWs can be much faster than other dissipation mechanisms for fast wave, such as electron viscous damping, Landau
damping, and modulational instability. The nonlinear damping of the FWs due to decay FW = KAW + KAW places a limit on the
amplitude of the magnetic field in the fast waves in the solar corona and solar-wind at the level B/B
0∼10−2. In turn, the nonlinearly excited small-scale KAWs undergo strong dissipation due to resistive or Landau damping and can
provide coronal and solar-wind heating. The transient coronal heating observed by Yohkoh and SOHO may be produced by the kinetic Alfvén waves that are excited by parametric decay of fast waves propagating from
the reconnection sites. 相似文献
7.
T. Takakura 《Solar physics》1987,107(2):283-297
Numerical simulation for the dynamics of a coronal filamentary magnetic loop has been made under the assumption that the field is initially force-free and an electric resistivity suddenly increases at a given moment due to an appearance of ion sound waves, which can be excited due to a high current density if a characteristic radius r
0 of the magnetic loop is about 3 km or less in a magnetic field B
0 of 1000 G. During the resistive decay of the magnetic field a strong field-aligned electric field is created and maintained for a sufficient time to acceleratie both electrons and protons to a high energy, which is proportional to B
0/r
0 and can be 100 MeV if r
0 = 10 km and B
0 = 1000 G. If the coronal magnetic tube is composed of many such filamentary loops, the total number of accelerated electrons is consistent with the observations. 相似文献
8.
In this paper, the Space–Time Conservation Element and Solution Element (CESE) method is applied to 2.5-dimensional resistive
magnetohydrodynamics (MHD) equations in Cartesian coordinates, with the purpose of modeling the magnetic reconnection study.
To show the validity and capacity of its application to MHD reconnection problem, spontaneous fast reconnection and magnetic
reconnection in multiple heliospheric current sheets are studied, which show good consistency with those obtained formerly
by other authors. In order to assess the ∇ ⋅ B = 0 constraint numerically, the contours and evolution of ∇ ⋅ B are analyzed. The numerical results tell us that the CESE numerical scheme not only has good numerical resolution but also
can keep the divergence-free condition for magnetic fields in the reconnection problems during the evolutionary process without
any special treatment. 相似文献
9.
SOHO/MDI magnetograms have been used to analyze the longitude distribution of the squared solar magnetic field 〈B
2〉 in the activity cycle no. 23. The energy of the magnetic field (〈B
2〉) is shown to change with longitude. However, these variations hardly fit the concept of active longitudes. In the epochs
of high solar activity, one can readily see a relationship between longitude variations of the medium-strong ((|B| > 50 G or |B| > 100 G) and relatively weak (|B| ≤ 50 G or |B| ≤ 100 G) fields at all latitudes. In other periods, this relationship is revealed mainly at the latitudes not higher than
30°. The background fields (|B| ≤ 25 G) also display longitude variations, which are, however, not related to those of the strong fields. This makes us
think that the fields of solar activity are rather inclusions to the general field than the source of the latter. 相似文献
10.
Photospheric motion shears or twists solar magnetic fields to increase magnetic energy in the corona, because this process may change a current-free state of a coronal field to force-free states which carry electric current. This paper analyzes both linear and nonlinear two-dimensional force-free magnetic field models and derives relations of magnetic energy buildup with photospheric velocity field. When realistic data of solar magnetic field (B
0 103 G) and photospheric velocity field (v
max 1 km s–1) are used, it is found that 3–4 hours are needed to create an amount of free magnetic energy which is of the order of the current-free field energy. Furthermore, the paper studies situations in which finite magnetic diffusivities in photospheric plasma are introduced. The shearing motion increases coronal magnetic energy, while the photospheric diffusion reduces the energy. The variation of magnetic energy in the coronal region, then, depends on which process dominates. 相似文献
11.
Electromagnetic instabilities in high-β plasmas, where β is the ratio of the kinetic plasma energy to the magnetic energy, have a broad range of astrophysical applications. The presence
of temperature anisotropies T
∥
/T
⊥
>1 (where ∥ and ⊥ denote directions relative to the background magnetic field) in solar flares and the solar wind is sustained by the observations
and robust acceleration mechanisms that heat plasma particles in the parallel direction. The surplus of parallel kinetic energy
can excite either the Weibel-like instability (WI) of the ordinary mode perpendicular to the magnetic field or the firehose
instability (FHI) of the circularly polarized waves at parallel propagation. The interplay of these two instabilities is examined.
The growth rates and the thresholds provided by the kinetic Vlasov – Maxwell theory are compared. The WI is the fastest growing
one with a growth rate that is several orders of magnitude larger than that of the FHI. These instabilities are however inhibited
by the ambient magnetic field by introducing a temperature anisotropy threshold. The WI admits a larger anisotropy threshold,
so that, under this threshold, the FHI remains the principal mechanism of relaxation. The criteria provided here by describing
the interplay of the WI and FHI are relevant for the existence of these two instabilities in any space plasma system characterized
by an excess of parallel kinetic energy. 相似文献
12.
Satoshi Hinata 《Solar physics》1988,116(2):239-258
We have investigated nonlinear equilibrium states of a microscopic current filamentation (electrothermal instability) in solar atmosphere. The microscopic filamentation instability will develop for transition zone ion temperature plasmas, provided T
e/Ti > 1, where T
e and T
i are the electron and ion temperatures, respectively. Since the temperature radio for a steady-state solar atmosphere is approximately unity, the electrothermal instability will develop only in a time-dependent solar atmosphere. Indeed, such a condition is provided by time-dependent currents, which seem to exist in many magnetic loops as recent analysis by Porter et al. (1987) indicates. When the onset condition for the electrothermal instability is satisfied, the instability drives a current filamentation to a nonlinear equilibrium state with a spatially periodic electron temperature variation with the wavelength comparable to several ion-Larmor radii. The amplitude of the periodic temperature variation may be so large that the transition layer temperature and coronal temperature plasmas may exist within several Larmor radii — coexistence of the transition zone and corona within the same macro-volume. 相似文献
13.
The damping of MHD waves in solar coronal magnetic field is studied taking into account thermal conduction and compressive
viscosity as dissipative mechanisms. We consider viscous homogeneous unbounded solar coronal plasma permeated by a uniform
magnetic field. A general fifth-order dispersion relation for MHD waves has been derived and solved numerically for different
solar coronal regimes. The dispersion relation results three wave modes: slow, fast, and thermal modes. Damping time and damping
per periods for slow- and fast-mode waves determined from dispersion relation show that the slow-mode waves are heavily damped
in comparison with fast-mode waves in prominences, prominence–corona transition regions (PCTR), and corona. In PCTRs and coronal
active regions, wave instabilities appear for considered heating mechanisms. For same heating mechanisms in different prominences
the behavior of damping time and damping per period changes significantly from small to large wavenumbers. In all PCTRs and
corona, damping time always decreases linearly with increase in wavenumber indicate sharp damping of slow- and fast-mode waves. 相似文献
14.
Vladimir A. Osherovich 《Astrophysics and Space Science》1982,86(2):453-469
One procedure for solving MHD equations is to search for a solution in an area that is restricted by boundary surfaces. This procedure requires the magnetic field to be truncated on the boundary. As a result, boundary current sheets appear. This approach is certainly acceptable for laboratory plasma experiments in which these surfaces are made of metal. For astrophysically relevant plasma, an alternative approach has been formulated by the author. We require the total magnetic energy,W, to be finite and, simultaneously, the magnetic fieldB to be continuous. The proposed approach leads to an eigenvalue problem that is treated analytically. The complete set of exact MHD solutions with multi-toroidal structure is obtained. These solutions are applied to coronal loops and transients, using the similarity assumption for time-dependent solutions.The derived pressure and density excess distributions are discussed. The estimation of the total mass excess, as well as the minimum value of the magnetic field intensity, is demonstrated. An indirect way of obtaining magnetic field measurements for transients, based on the developed model, is proposed. 相似文献
15.
We study a model of particle acceleration coupled with an MHD model of magnetic reconnection in unstable twisted coronal loops. The kink instability leads to the formation of helical currents with strong parallel electric fields resulting in electron acceleration. The motion of electrons in the electric and magnetic fields of the reconnecting loop is investigated using a test-particle approach taking into account collisional scattering. We discuss the effects of Coulomb collisions and magnetic convergence near loop footpoints on the spatial distribution and energy spectra of high-energy electron populations and possible implications on the hard X-ray emission in solar flares. 相似文献
16.
Knowledge regarding the coronal magnetic field is important for the understanding of many phenomena, like flares and coronal
mass ejections. Because of the low plasma beta in the solar corona, the coronal magnetic field is often assumed to be force-free
and we use photospheric vector magnetograph data to extrapolate the magnetic field into the corona with the help of a nonlinear
force-free optimization code. Unfortunately, the measurements of the photospheric magnetic field contain inconsistencies and
noise. In particular, the transversal components (say B x and B y) of current vector magnetographs have their uncertainties. Furthermore, the magnetic field in the photosphere is not necessarily
force free and often not consistent with the assumption of a force-free field above the magnetogram. We develop a preprocessing
procedure to drive the observed non–force-free data towards suitable boundary conditions for a force-free extrapolation. As
a result, we get a data set which is as close as possible to the measured data and consistent with the force-free assumption. 相似文献
17.
Z. F. Gao N. Wang J. P. Yuan L. Jiang D. L. Song E. L. Qiao 《Astrophysics and Space Science》2011,333(2):427-435
In this paper, we consider the effect of Landau levels on the decay of superhigh magnetic fields of magnetars. Applying 3
P
2 anisotropic neutron superfluid theory yield a second-order differential equation for a superhigh magnetic field B and its evolutionary timescale t. The superhigh magnetic fields may evolve on timescales ∼(106–107) yrs for common magnetars. According to our model, the activity of a magnetar may originate from instability caused by the
high electron Fermi energy. 相似文献
18.
J. Galindo Trejo 《Solar physics》1987,108(2):265-313
Using the MHD energy principle of Bernstein et al. (1958) we develop a formalism in order to analyze the stability properties of two-dimensional magnetostatic plasma equilibria. We apply this to four models of quiescent prominences, namely those of Menzel (1951), Dungey (1953), Kippenhahn and Schlüter (1957), and finally Lerche and Low (1980). For the observed parameter range, all models are stable and they explain reasonably well the reported flare-initiated oscillations in quiescent prominences. We also investigate other parameters regions, which may be relevant in some stellar atmospheres. It is found that, with the exception of the Kippenhahn and Schlüter model, all models become unstable. The instabilities that occur show simultaneously several features of well-known MHD-instabilities. However, an unequivocal assignment of the instabilities to specific instability prototypes is not possible. Our formalism allows one to investigate not only more realistic prominence equilibria, but also arbitrary one- and two-dimensional static ideal MHD-equilibria. 相似文献
19.
M. Yu. Piotrovich Yu. N. Gnedin T. M. Natsvlishvili N. A. Silant’ev 《Astronomy Letters》2010,36(6):389-395
We analyze the spectropolarimetric observations of 12 candidates for quasars from the spectroscopic database of the SDSS Catalog.
The magnetic fields of these objects are estimated in the context of a theory that includes the Faraday rotation of the polarization
plane on the mean free path of a photon in the outflow from an accretion disk. As a result, we have determined the column
density in the outflow, N
H ∼ 6 × 1023 cm−2, and the radial, B ∼ 1 G, and toroidal, B ∼ 600 G, magnetic fields. 相似文献
20.
The waves, propagating nearly transverse to the ambient magnetic field, with frequencies near the harmonics of the proton-cyclotron frequency are studied in an inhomogeneous plasma with protons having loss-cone distributions. Three types of drift cyclotron instabilities have been studied: (i) non-flute instability; (ii) B-resonant instability; and (iii) non-resonant instability. Increases of loss-cone and density gradient increase the growth rates of all three instabilities. Increases in the positive temperature gradient and t (ratio of thermal pressure of trapped protons to magnetic field pressure) have a stabilizing effect on the non-flute and non-resonant instabilities and a destabilizing effect on the B-resonant instability. The non-resonant instability has an interesting feature: a particular harmonic can be excited in two separate bands of unstable wave numbers. These instabilities can play an important role in the dynamics of the ring current and the inner edge of the plasma sheet region of the magnetosphere. The discrete turbulence generated by them would give rise to precipitation of protons on the auroral field lines, which may contribute to the excitation of diffuse aurora. These instabilities may be relevant to the observation of harmonic waves at 6R
E by Perrautet al. (1978). 相似文献