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1.
The standing quasi-modes in a cylindrical incompressible flux tube with magnetic twist that undergoes a radial density structuring is considered in ideal magnetohydrodynamics (MHD). The radial structuring is assumed to be a linearly varying density profile. Using the relevant connection formulae, the dispersion relation for the MHD waves is derived and solved numerically to obtain both the frequencies and damping rates of the fundamental and first-overtone modes of both the kink (m=1) and fluting (m=2,3) waves. It was found that a magnetic twist will increase the frequencies, damping rates and the ratio of the oscillation frequency to the damping rate of these modes. The period ratio P 1/P 2 of the fundamental and its first-overtone surface waves for kink (m=1) and fluting (m=2,3) modes is lower than two (the value for an untwisted loop) in the presence of twisted magnetic field. For the kink modes, particularly, the magnetic twists B φ /B z =0.0065 and 0.0255 can achieve deviations from two of the same order of magnitude as in the observations. Furthermore, for the fundamental kink body waves, the frequency band width increases with increasing magnetic twist. 相似文献
2.
A.L. Taktakishvili A. Greco P. Veltri G. Zimbardo L.M. Zelenyi A.V. Milovanov 《Astrophysics and Space Science》2001,277(1-2):71-79
The ion dynamics in the Earth's magnetotail is studied in the case when a cross tail electric field E
0 and reconnection-driven magnetic turbulence are present in the current sheet. The magnetic turbulence observed by the Interball
spacecraft is modeled numerically by a power law magnetic fluctuation spectrum. A test particle simulation is performed for
the ions, and the distribution function moments are obtained as a function of the magnetic fluctuation level, δB/B
0, and of the value of the normal component B
n. It appears that even in the presence of magnetic turbulence, the normal component has a marked influence on particle dynamics:
the ion bulk velocity along E
y and ion temperature are almost inversely proportional to B
n. The magnetic turbulence causes the current to split in two layers, and the level of magnetic fluctuations needed to have
splitting is roughly proportional to B
n. It appears that in the relevant range of parameters, B
n and δB/B
0 have opposite effects on the current structure and on ion heating.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
The stability of a semi-infinite quasi-neutral inhomogeneous plasma with magnetic viscosity has been discussed by using JWKB approximation in which the parameters are regarded slowing varying. Dispersion relation is obtained and discussed. It is found that the inhomogeneous system is unstable for all perturbations withk
y= 0. A dispersion relation for homogeneous plasma is also obtained and discussed. It is shown that fast and slow-MHD waves propagate in the homogeneous plasma in the limit of almost perpendicular propagation under certain conditions. 相似文献
4.
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
5.
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
6.
The solar corona, modeled by a low-, resistive plasma slab, sustains MHD wave propagations due to footpoint motions in the photosphere. Simple test cases are undertaken to verify the code. Uniform, smooth and steep density, magnetic profile and driver are considered. The numerical simulations presented here focus on the evolution and properties of the Alfvén, fast and slow waves in coronal loops. The plasma responds to the footpoint motion by kink or sausage waves depending on the amount of shear in the magnetic field. The larger twist in the magnetic field of the loop introduces more fast-wave trapping and destroys initially developed sausage-like wave modes. The transition from sausage to kink waves does not depend much on the steep or smooth profile. The slow waves develop more complex fine structures, thus accounting for several local extrema in the perturbed velocity profiles in the loop. Appearance of the remnants of the ideal singularities characteristic of ideal plasma is the prominent feature of this study. The Alfvén wave which produces remnants of the ideal x
–1 singularity, reminiscent of Alfvén resonance at the loop edges, becomes less pronounced for larger twist. Larger shear in the magnetic field makes the development of pseudo-singularity less prominent in case of a steep profile than that in case of a smooth profile. The twist also causes heating at the edges, associated with the resonance and the phase mixing of the Alfvén and slow waves, to slowly shift to layers inside the slab corresponding to peaks in the magnetic field strength. In addition, increasing the twist leads to a higher heating rate of the loop. Remnants of the ideal log ¦x¦ singularity are observed for fast waves for larger twist. For slow waves they are absent when the plasma experiences large twist in a short time. The steep profiles do not favour the creation of pseudo-singularities as easily as in the smooth case. 相似文献
7.
It is well known that the parallel cuts of the parallel and perpendicular electric field in electron phase-space holes (electron
holes) have bipolar and unipolar structures, respectively. Recently, electron holes in the Earth’s plasma sheet have been
observed by THEMIS satellites to have detectable fluctuating magnetic field with regular structures. Du et al. (2011) investigated the evolution of a one-dimensional (1D) electron hole with two-dimensional (2D) electromagnetic particle-in-cell
(PIC) simulations in weakly magnetized plasma (Ω
e
<ω
pe
, where Ω
e
and ω
pe
are the electron gyrofrequency and electron plasma frequency, respectively), which initially exists in the simulation domain.
The electron hole is unstable to the transverse instability and broken into several 2D electron holes. They successfully explained
the observations by THEMIS satellites based on the generated magnetic structures associated with these 2D electron holes.
In this paper, 2D electromagnetic particle-in-cell (PIC) simulations are performed in the x–y plane to investigate the nonlinear evolution of the electron two-stream instability in weakly magnetized plasma, where the
background magnetic field (B0 = B0[(e)\vec] x)(\mathbf{B}_{0} =B_{0}\vec{\mathbf{e}} _{x}) is along the x direction. Several 2D electron holes are formed during the nonlinear evolution, where the parallel cuts of E
x
and E
y
have bipolar and unipolar structures, respectively. Consistent with the results of Du et al. (2011), we found that the current along the z direction is generated by the electric field drift motion of the trapped electrons in the electron holes due to the existence
of E
y
, which produces the fluctuating magnetic field δB
x
and δB
y
in the electron holes. The parallel cuts of δB
x
and δB
y
in the electron holes have unipolar and bipolar structures, respectively. 相似文献
8.
It has been hypothesized that the sustained narrowness observed in the asymptotic cylindrical region of bipolar outflows from
Young Stellar Objects (YSO) indicates that these jets are magnetically collimated. The j
z × B
ϕ force observed in z-pinch plasmas is a possible explanation for these observations. However, z-pinch plasmas are subject
to current driven instabilities (CDI). The interest in using z-pinches for controlled nuclear fusion has lead to an extensive
theory of the stability of magnetically confined plasmas. Analytical, numerical, and experimental evidence from this field
suggest that sheared flow in magnetized plasmas can reduce the growth rates of the sausage and kink instabilities. Here we
propose the hypothesis that sheared helical flow can exert a similar stabilizing influence on CDI in YSO jets. 相似文献
9.
The propagation of spiral density waves in a differentially rotating, self-gravitating, magnetoactive and highly flattened disk is investigated by using the asymptotic theory for tightly wound spirals developed by Lin and his collaborators. We adopt the continuum fluid model as the primary basis, and our treatment will be largely analytical. The disk plasma is studied in the frozen field approximation and inhomogenceous magnetic fields in the plane of the disk are considered in detail.In a differentially rotating disk with strong magnetic fields, the field lines will be considerably distorted and the mutual influence of magnetic fields and differential rotation is by no means obvious.In this paper we present a new asymptotic dispersion relation for tightly wound spiral density waves with magnetic fields along the spiral armsB
(r). The effects of the magnetic fields lead to such terms likek
2(a
2 +V
A
2
), wherek is the wave number,a represents the speed of sound,V
A = (B
2/4)1/2 is the Alfvén speed,B denotes the field strength, and is the plasma density. These terms depict the well-known magnetoacoustic waves and could have been anticipated without a detailed computation. However the interaction of magnetic fields and differential rotation may give rise to other significant terms which are not so easy to foresee.We also present a more exact local dispersion relation by using the WKB approximation and study the effects of magnetic fields on the growth rate through the parametersQ andJ defined in the literature.Although the effects of the magnetic fields are rather insignificant for applications to Galactic dynamics, the effects of the magnetic fields are important for applications to star formation and problems related to the solar nebula. 相似文献
10.
The propagation of waves in a magnetic slab embedded in a magnetic environment is investigated. The possible modes of propagation are examined from the general dispersion relation, both analytically and numerically, for disturbances which are evanescent in the environment. Approximate dispersion relations governing propagation in a slender slab of field are derived both from the general dispersion relation and from an application of the slender flux tube approximation.Several different situations, representative of both photospheric and coronal conditions, are considered. In general, the structures are found to support both fast and slow, body and surface, waves. Under coronal conditions, for two dimensional propagation, disturbances propagate as fast and slow body waves. The fast body waves are analogous to the ducted shear waves of seismology (Love waves). 相似文献
11.
B. Buti 《Astrophysics and Space Science》1982,85(1-2):35-41
The longitudinal electric field associated with the observed electrostatic turbulence in the solar wind is shown to modify the dispersive characteristics of the hydromagnetic waves propagating along the interface between the solar wind and the cometary plasma. Extremely weak turbulence has a tendency to stabilize these surface waves, whereas turbulence of moderate level can be stabilizing or destabilizing depending on the strength of the cometary magnetic fieldB
oc
relative to the interplanetary magnetic fieldB
os
. ForB
oc
B
os
, destabilization is not possible. 相似文献
12.
A.S. Rodger S.W.H. Cowley M.J. Brown M. Pinnock D.A. Simmons 《Planetary and Space Science》1984,32(8):1021-1027
We describe a simple method for determining the time at which the meridian of a sub-auroral magnetic observatory crosses that of the Harang discontinuity—the separation of the eastward and westward electrojets which flow in the evening and morning sectors of the auroral oval. We then consider how this time, determined from examination of magnetograms from sub-auroral observatories varies with the dawn-dusk (y) component of the Interplanetary Magnetic Field. We find that the time at which the Harang discontinuity is identified in the Northern Hemisphere is earlier for By > 0 than the occasions when By < 0, and that the converse is observed in the Southern Hemisphere. Also we suggest that there is no significant seasonal variation in the relationship between the time of the discontinuity and By. The sense of the azimuthal shift of the auroral electrojet currents with changes in By is consistent with the theory of Cowley (1981). However, the magnitude of the observed shifts is approximately an order of magnitude greater than the theoretical predictions. We suggest that this difference between observation and theory arises from the use of a dipole magnetic field model at auroral zone latitudes in the theoretical estimation of azimuthal displacement. 相似文献
13.
The geometry of the open flux area in the polar region is computed by superposing a uniform interplanetary magnetic field (IMF) with various orientation angles to a model of the magnetosphere. It is confirmed that the IMF By component is as important as the Bz component in “opening” the magnetosphere. It is also shown that the computed area of open field lines is remarkably similar to the observed ones which were determined by using the entry of solar electrons. In particular, when the IMF vector is confined in the X-Z-plane and the Bz component has a large positive value, the open area becomes crescent-shaped, coinciding approximately with the cusp region. 相似文献
14.
A particle-mesh computer model for the simulation of the current-sheet region of the geomagnetic tail is described. Important features are (a) the use of Fast Fourier techniques for the efficient solution of Ampere's equation, (b) the incorporation of sources and sinks of particles, (c) facilities for simulating finite width effects and (d) the option of including a normal magnetic field component linking through the sheet.Simulations carried out using this model indicate that current sheets with a non-zero normal magnetic field component and an infinite width are stable. The particles trace out Speiser-like orbits in such a case. Sheets with Bnormal = 0 and a finite width are unstable with respect to the ion tearing-mode instability. However the presence of a normal magnetic field stabilises the system provided ρ0<2Ly where ρ0 is the characteristic length associated with the normal field and where Ly is the width of the sheet.On the basis of these results it is suggested that a geomagnetic substorm occurs when the normal magnetic field drops below the critical value needed for stability. 相似文献
15.
Nonlinear resonant interactions of different kinds of fast magnetosonic (FMS) waves trapped in the inhomogeneity of a low- plasma density, stretched along a magnetic field (as, for example, in coronal loops) are investigated. A set of equations describing the amplitudes of interactive modes is derived for an arbitrary density profile. The quantitative characteristics of such interactions are found. The decay instability of the wave with highest frequency is possible in the system. If amplitudes of interactive modes have close values, the long-period temporal and spatial oscillations are in the system.For a quantitative illustration, the parabolic approximation of the transverse density profile has been chosen. Dispersion relations of FMS waves trapped in a low- plasma slab with a parabolic transverse density profile are found. The transverse structure of the waves in this case can be expressed through Hermitian polynomials. The interaction of kink and sausage waves is investigated. The sausage wave, with a sufficiently large amplitude, may be unstable with respect to the decay into two kink waves, in particular. The spatial scale of a standing wave structure and the time spectrum of radiation are formed due to the nonlinear interactions of loop modes which contain information about the parameters of the plasma slab. 相似文献
16.
The oscillations of a magnetic medium periodic in the x-direction with B parallel to z, have been studied. The case with no gravity and a stepwise profile for B(x), allowing a normal mode analysis, has been examined and dispersion relations have been derived. The dispersion curves in the diagram k
z – display two types of modes, kink and sausage, like in the isolated slab, but the profiles are different and depend on Bloch's number k
0. Moreover, modes usually absent in the isolated slab (propagating and tunelling) appear here, connecting surface- or body-wave domains. The detectability of this characteristic structure of the diagnostic diagram on the observations is discussed, and prospects for a more realistic analysis including gravity are given. 相似文献
17.
In this paper an excitation of waves is considered during the time interval in which the undisturbed magnetic field changes its direction. If this interval is taken to be 2 years, which is shorter than the 11-year cycle, then the undisturbed components of the magnetic field may be linearly dependent on time and independent of the coordinates. The excitation of waves is due to the undisturbed stationaryV
0 flow with divV
0 = 0 and with (V
0 rot0) = constant.We use the local Cartesian coordinate system, which is immovable towards the solar centre, and consider the case when the toroidal component of the undisturbed magnetic field changes its sign simultaneously with one of the axial components. The third component does not change its direction.The efficiency of the enhancement of the magnetic field and velocity disturbances depends on the Alfvén wave frequency,
A. When
A = 0, the component of the disturbed velocity, which is directed along the constant component of the undisturbed magnetic field, increases. In this case the shear waves excite the carrier (high) frequency (KV
0), whereK is the wave vector. Due to the shear instability the amplitude of the velocity increases during 1 year before the moment of reversal of the global magnetic field polarity (RGMFP) for an arbitrary latitude. It reaches a maximum at RGMFP and decreases in the next year. When
A > 0, then the amplitudes of the disturbed values reach maxima before the moment of RGMFP, and when
A < 0, they reach maxima after it.We argue that the shear waves propagate from middle latitudes to the pole and equator. Using the results of the analytical solutions and leaning on the evidence of the observational data (Gigolashvili and Japaridze, 1992), we derive the result that the component of the undisturbed magnetic field, which is perpendicular to the solar surface, changes its sign simultaneously with the toroidal component. 相似文献
18.
Satellite observations of the heliospheric current sheet indicate that the plasma flow velocity is low at the center of the current sheet and high on the two sides of current sheet. In this paper, we investigate the growth rates and eigenmodes of the sausage, kind, and tearing instabilities in the heliospheric current sheet with the observed sheared flow. These instabilities may lead to the formation of the plasmoids and kink waves in the solar wind. The results show that both the sausage and kink modes can be excited in the heliospheric current sheet with a growth time 0.05–5 day. Therefore, these modes can grow during the transit of the solar wind from the Sun to the Earth. The sausage mode grows faster than the kink mode for
< 1.5, while the streaming kink instability has a higher growth rate for
> 1.5. Here
is the ratio between the plasma and magnetic pressures away from the current layer. If a finite resistivity is considered, the streaming sausage mode evolves into the streaming tearing mode with the formation of magnetic islands. We suggest that some of the magnetic clouds and plasmoids observed in the solar wind may be associated with the streaming sausage instability. Furthermore, it is found that a large-scale kink wave may develop in the region with a radial distance greater than 0.5–1.5 AU.Also at Department of Earth and Space Science, University of Science and Technology of China, Hefei Anhui 230029, China. 相似文献
19.
The linearized theory for the parallel propagation of magnetoacoustic-gravity surface waves is developed for an interface of a horizontal magnetic field above a field-free medium. The media either side of the interface are taken to be isothermal. The dispersion relation is obtained for the case of a constant Alfvén speed. In the absence of gravity the interface may support one or two surface modes, determined by the relative temperatures and magnetism of the two media. The effect of gravity on the modes is examined and dispersion diagrams and eigenfunctions are given. In the usual - k
x diagnostic diagram, the domain of evanescence is shown to be divided into two distinct regions determining whether a given mode will have a decaying or growing vertical velocity component. In the absence of a magnetic field the transcendental dispersion relation may be rewritten as a polynomial. This polynomial possesses two acceptable solutions only one of which may exist in any given circumstances (depending on the ratio of the densities). If the gas density within the field exceeds that in the field-free medium, then the f-mode may propagate. The f-mode exists in a restricted band of horizontal wavenumber and only when the field-free medium is warmer than the magnetic atmosphere. An analytical form for the wave speed of the f-mode is obtained for small values of the Alfvén speed. It is shown that the f-mode is related to the fast magnetoacoustic surface wave, merging into that mode at short wavelengths. 相似文献
20.
Kinetic Alfven waves are examined in the presence of electron and ion beam and an inhomogeneous magnetic field with bi-Maxwellian distribution function. The theory of particle aspect analysis is used to evaluate the trajectories of the charged particles. The expressions for the field-aligned currents, perpendicular currents (with respect to B 0), dispersion relation and growth/damping rate with marginal instability criteria are derived. The effect of electron and ion beam and inhomogeneity of magnetic field are discussed. The results are interpreted for the space plasma parameter appropriate to the auroral acceleration region of the earth’s magnetoplasma. 相似文献