共查询到20条相似文献，搜索用时 27 毫秒
1.
J. A. McLaughlin 《Journal of Astrophysics and Astronomy》2013,34(3):223-246
The propagation of linear Alfvén wave pulses in an inhomogeneous plasma near a 2D coronal null point is investigated. When a uniform plasma density is considered, it is seen that an initially planar Alfvén wavefront remains planar, despite the varying equilibrium Alfvén speed, and that all the wave collects at the separatrices. Thus, in the non-ideal case, these Alfvénic disturbances preferentially dissipate their energy at these locations. For a non-uniform equilibrium density, it is found that the Alfvén wavefront is significantly distorted away from the initially planar geometry, inviting the possibility of dissipation due to phase mixing. Despite this however, we conclude that for the Alfvén wave, current density accumulation and preferential heating still primarily occur at the separatrices, even when an extremely non-uniform density profile is considered. 相似文献
2.
It is known that stellar winds from late type stars are of mixed thermal and magnetic origin. The stellar wind model presented in this work uses the hydrodynamic equations of mass and momentum conservation and closes the system of equations with a detailed energy equation. Both momentum and energy equations have terms due to the effects of Alfvén waves. A smooth transition between the two regimes for Alfvén wave propagation, the undamped and the damped modes, is achieved by considering the geometrical mean of both wave amplitudes. It will be shown that the initial push on the plasma is provided by the mechanical heating input, and that further out the Alfvén waves take over energetically. 相似文献
3.
The theoretical work presented here was stimulated by the interpretation of auroral field-aligned currents in terms of an Alfvén wave generated in the neutral sheet. Allowing for convection such a wave can be stationary relative to the Earth, and with an Alfvén Mach number of about 10?2, hydromagnetics predict that the wave normal should be nearly perpendicular to the magnetic field. All the theory presented here is limited to the cold plasma approximation, which is the next step after hydromagnetics, but should have validity here as the wave is propagating into the cold polar wind plasma.The approach is similar to that of Kellogg (1964) except here we consider only the Alfvén mode, and only for Alfvén Mach numbers of about 10?2. Initially a linear approach was adopted but further computation showed that non-linear effects were responsible for making the current density approximately uniform.The final section presents a plasma sheet boundary crossing selected to illustrate the theory, and is taken from ISEE 1 and 2. The data is such that it permits a first-order estimation of scale sizes to be made in the tail, which in this case was found to be about 1000 km. Subsequent mapping to ionospheric altitudes produced a scale of about a few tens of kilometers. 相似文献
4.
We suggest a two-step mechanism for the generation of the parallel electric field at the Alfvén wave. At the first step, the coupling with the compressional mode due to the magnetic field non-uniformity and finite plasma pressure provides the parallel magnetic field of Alfvén wave. At the second step, the compressional mode acquires the parallel electric field due to coupling with the electrostatic mode as required by the quasi-neutrality condition in kinetics. The parallel electric field acquired by the Alfvén mode is considerably larger than that due to the single-step coupling between the Alfvén and electrostatic modes in kinetics. 相似文献
5.
A fast-wave pulse in a simple, cold, inhomogeneous MHD model plasma is constructed by Fourier superposition over frequency
of harmonic waves that are singular at their respective Alfvén resonances. The pulse partially reflects before reaching the
resonance layer, but also partially tunnels through to it to convert to an Alfvén wave. The exact absorption/conversion coefficient
for the pulse is shown to be given precisely by a function of transverse wavenumber tabulated in Paper I of this sequence,
and to be independent of frequency and pulse width. 相似文献
6.
We consider an expanding three-dimensional (3-D) piston as a driver of an MHD shock wave. It is assumed that the source-region surface accelerates over a certain time interval to achieve a particular maximum velocity. Such an expansion creates a large-amplitude wave in the ambient plasma. Owing to the nonlinear evolution of the wavefront, its profile steepens and after a certain time and distance a discontinuity forms, marking the onset of the shock formation. We investigate how the formation time and distance depend on the acceleration phase duration, the maximum expansion velocity (defining also acceleration), the Alfvén velocity (defining also Mach number), and the initial size of the piston. The model differs from the 1-D case, since in the 3-D evolution, a decrease of the wave amplitude with distance must be taken into account. We present basic results, focusing on the timing of the shock formation in the low- and high-plasma-beta environment. We find that the shock-formation time and the shock-formation distance are (1) approximately proportional to the acceleration phase duration; (2) shorter for a higher expansion velocity; (3) larger in a higher Alfvén speed environment; (4) only weakly dependent on the initial source size; (5) shorter for a stronger acceleration; and (6) shorter for a larger Alfvén Mach number of the source surface expansion. To create a shock causing a high-frequency type II burst and the Moreton wave, the source region expansion should, according to our results, achieve a velocity on the order of 1000 km?s?1 within a few minutes, in a low Alfvén velocity environment. 相似文献
7.
Dispersion properties of kinetic Alfvén wave in quantum magnetoplasma are derived. The quantum contribution to the Landau damping of kinetic Alfvén wave is also derived by using linearized Vlasov equation which contains the Bohm quantum potential. Classical Landau damped kinetic Alfvén waves play an important role in turbulence of astrophysical plasmas. The quantum modification in Landau damping of kinetic Alfvén wave can also play a significant role in changing the scaling law of turbulent spectra as well as the formation of damped localized Alfvénic structures in dense astrophysical plasmas. 相似文献
8.
A study is made of the hydromagnetic wave activity observed on the ground during the sub-auroral red (SAR) arc event of 17–18 December 1971. The available wave energy flux in the magnetosphere, inferred from the observed wave amplitude on the ground using the present understandings of wave localization and ionosphere wave attenuation is sufficient to produce the SAR arc. This finding supports kinetic Alfvén wave heating as a production mechanism for SAR arc optical emissions. 相似文献
9.
S. Parhi B.P. Pandey M. Goossens G.S. Lakhina P. De Bruyne 《Astrophysics and Space Science》1997,250(1):147-162
The solar corona, modelled by a low β, resistive plasma slab sustains MHD wave propagations due to footpoint motions in the
photosphere. The density, magnetic profile and driver are considered to be neither very smooth nor very steep. The numerical
simulation presents the evolution of MHD waves and the formation of current sheet. Steep gradients in slow wave at the slab
edges which are signature of resonance layer where dissipation takes place are observed. Singularity is removed by the inclusion
of finite resistivity. Dissipation takes place around the resonance layer where the perturbation develops large gradients.
The width of the resonance layer is calculated. The thickness of the Alfvén resonance layer is more than that of the slow
wave resonance layer. Attempt is made to distinguish between slow and Alfvén wave resonance layers. Fast waves develop into
kink modes. As plasma evolves the current sheets which provide the heating at the edges gets distorted and fragment into two
current sheets at each edge which in turn come closer when the twist is enhanced.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
Alfvén waves play three related roles in the impulsive phase of a solar flare: they transport energy from a generator region to an acceleration region; they map the cross-field potential (associated with the driven energy release) from the generator region onto the acceleration region; and within the acceleration region they damp by setting up a parallel electric field that accelerates electrons and transfers the wave energy to them. The Alfvén waves may also be regarded as setting up new closed-current loops, with field-aligned currents that close across field lines at boundaries. A model is developed for large-amplitude Alfvén waves that shows how Alfvén waves play these roles in solar flares. A picket-fence structure for the current flow is incorporated into the model to account for the “number problem” and the energy of the accelerated electrons. 相似文献
11.
We underline the importance of Alfvén wave dissipation in the magnetic funnels through the viscous and resistive plasma. Our
results show that Alfvén waves are one of the primary energy sources in the innermost part of coronal holes where the solar
wind outflow starts. 相似文献
12.
Properties of dispersive Alfvén waves: 1. Kinetics (very low, intermediate, and low density plasmas)
P. P. Malovichko 《Kinematics and Physics of Celestial Bodies》2013,29(6):269-284
The work is devoted to the study of the behavior of dispersive Alfvén waves, including inertial and kinetic Alfvén waves, in astrophysical plasma of very low, intermediate, and low pressure. New and full solutions were obtained. The solutions for “ordinary” and inertial Alfvén waves were shown to be particular cases of the general solution. The effect of the parameters of the astrophysical medium on the behavior and properties of dispersive Alfvén waves was analyzed. All the main wave characteristics were obtained, namely, the dispersion, fading, polarization, density perturbations, and charge density perturbations, whose consideration is essential for observation and detection of these waves and for more adequate understanding of their role in different astrophysical processes that occur in the space environment. 相似文献
13.
A. N. Kryshtal’ E. K. Sirenko S. V. Gerasimenko 《Kinematics and Physics of Celestial Bodies》2007,23(3):89-96
We examine the physical conditions for the origin of the decay instability of kinetic Alfvén waves in loop plasmas at the early flare stages. The synchronism conditions are used to derive a modified expression for the nonlinear growth rate of the process of the decay of the primary kinetic Alfvén wave (KAW) into an ion-acoustic wave and a secondary KAW. The threshold amplitude of the primary KAW is calculated in units of the background magnetic field strength in the chromospheric section of loop current circuit. 相似文献
14.
In the present paper, we have investigated nonlinear interaction of three dimensional kinetic Alfvén wave with perpendicularly propagating magnetosonic wave for intermediate β-plasma (m e /m i ?β?1). We have developed the set of dimensionless equations in the presence of ponderomotive nonlinearity due to three dimensional kinetic Alfvén wave in the dynamics of perpendicularly propagating magnetosonic wave. Numerical simulation has been carried out to study the effect of nonlinear coupling of three dimensional kinetic Alfvén wave with perpendicularly propagating magnetosonic wave on power spectrum for the plasma parameters applicable to solar wind around 1 AU. Relevance of the obtained results is pointed out with observation received by Cluster spacecraft for the solar wind around 1 AU. 相似文献
15.
L.C. Lee 《Planetary and Space Science》1982,30(11):1127-1132
The reflection and refraction of MHD waves through an “open” magnetopause (rotational discontinuity) is studied. It is found that most of the incident wave energy can be transmitted through the open magnetopause. A transverse Alfvén wave (or a compressional magnetosonic wave) from the solar wind incident upon the open magnetopause would generally lead to the generation of both the transverse Alfvén and compressional magnetosonic waves in the magnetosphere. Transmission of Alfvén waves in the coplanar rotational discontinuity is studied in detail. The integral power of the Alfvén-wave transfer is found to be proportional to the open magnetic flux of the magnetosphere and is typically ～ 1% of the power of the total electromagnetic energy transfer through the open magnetopause. The transmitted wave power may contribute significantly to the geomagnetic pulsations observed on the ground, especially in the open-field-line region. 相似文献
16.
P. P. Malovichko 《Kinematics and Physics of Celestial Bodies》2014,30(1):22-31
The work is devoted to the study of the behavior of dispersive Alfvén waves in astrophysical plasma of finite and high pressure. All the main wave characteristics were obtained, namely, the dispersion, fading, polarization, density perturbations, and charge density perturbations. The effect of the parameters of the space environment on the behavior and properties of dispersive Alfvén waves was analyzed. The wave behavior in finite and high-pressure plasmas is shown to differ appreciably from the behavior in very low, intermediate, and low-pressure plasmas. 相似文献
17.
The efficacy of fast?–?slow MHD mode conversion in the surface layers of sunspots has been demonstrated over recent years using a number of modelling techniques, including ray theory, perturbation theory, differential eigensystem analysis, and direct numerical simulation. These show that significant energy may be transferred between the fast and slow modes in the neighbourhood of the equipartition layer where the Alfvén and sound speeds coincide. However, most of the models so far have been two dimensional. In three dimensions the Alfvén wave may couple to the magnetoacoustic waves with important implications for energy loss from helioseismic modes and for oscillations in the atmosphere above the spot. In this paper, we carry out a numerical “scattering experiment,” placing an acoustic driver 4 Mm below the solar surface and monitoring the acoustic and Alfvénic wave energy flux high in an isothermal atmosphere placed above it. These calculations indeed show that energy conversion to upward travelling Alfvén waves can be substantial, in many cases exceeding loss to slow (acoustic) waves. Typically, at penumbral magnetic field strengths, the strongest Alfvén fluxes are produced when the field is inclined 30°?–?40° from the vertical, with the vertical plane of wave propagation offset from the vertical plane containing field lines by some 60°?–?80°. 相似文献
18.
Inertial Alfvén waves are investigated using Maxwell-Boltzmann-Vlasov equation to evaluate the dispersion relation and growth/damping rate in inhomogeneous plasma. Expressions for the dispersion relation and growth/damping rate are evaluated in inhomogeneous plasma. The effects of density, temperature and velocity gradient are included in the analysis. The results are interpreted for the space plasma parameters appropriate to the plasma sheet boundary layer. It is found that the inhomogeneities of plasma contribute significantly to enhance the growth rate of inertial Alfvén wave. The applicability of this model is assumed for auroral acceleration region and plasma sheet boundary layer. 相似文献
19.
Magnetospheric Alfvén waves are reflected by the ionosphere. We investigate the effect of horizontally varying ionospheric conductivity on the process of Alfvén wave reflection. Four idealised ionospheric models are considered in detail. We find that the reflection process is strongly dependent on the orientation of the wave electric field vector with respect to the boundary between high and low conductivities, and under certain conditions subsidiary Alfvén waves are generated. The field-aligned currents in the subsidiary Alfvén waves serve to close divergent horizontal currents resulting from the non-uniform ionospheric conductivity. The implications for ground-based pulsation studies are discussed. 相似文献
20.
A simulation based on a pseudo-spectral method has been performed in order to study particle acceleration. A model for the acceleration of charged particles by field localization is developed for the low-\(\upbeta\) plasma. For this purpose, a fractional diffusion approach has been employed. The nonlinear interaction between a 3D inertial Alfvén wave and a slow magnetosonic wave has been examined, and the dynamical equations of these two waves in the presence of ponderomotive nonlinearity have been solved numerically. The nonlinear evolution of the inertial Alfvén wave in the presence of slow magnetosonic wave undergoes a filamentation instability and results in field intensity localization. The results obtained show the localization and power spectrum of inertial Alfvén wave due to nonlinear coupling. The scaling obtained after the first break point of the magnetic power spectrum has been used to calculate the formation of the thermal tail of energetic particles in the solar corona. 相似文献