Propagating Alfven Waves Research Articles

THREE-DIMENSIONAL NUMERICAL SIMULATIONS OF FAST-TO-ALFVÉN CONVERSION IN SUNSPOTS

The conversion of fast waves to the Alfven mode in a realistic sunspot atmosphere is studied through three-dimensional numerical simulations. An upward propagating fast acoustic wave is excited in the high-beta region of the model. The new wave modes generated at the conversion layer are analyzed from the projections of the velocity and magnetic field in their characteristic directions, and the computation of their wave energy and fluxes. The analysis reveals that the maximum efficiency of the conversion to the slow mode is obtained for inclinations of 25 degrees and low azimuths, while the Alfven wave conversions peaks at high inclinations and azimuths between 50 and 120 degrees. Downward propagating Alfven waves appear at the regions of the sunspot where the orientation of the magnetic field is in the direction opposite to the wave propagation, since at these locations the Alfven wave couples better with the downgoing fast magnetic wave which are reflected due to the gradients of the Alfven speed. The simulations shows that the Alfven energy at the chromosphere is comparable to the acoustic energy of the slow mode, being even higher at high inclined magnetic fields.

Phase mixing of standing Alfvén waves with shear flows in solar spicules

Alfvenic waves are thought to play an important role in coronal heating and solar wind acceleration. Here we investigate the dissipation of such waves due to phase mixing at the presence of shear flow and field in the stratified atmosphere of solar spicules. The initial flow is assumed to be directed along spicule axis and to vary linearly in the x direction and the equilibrium magnetic field is taken 2-dimensional and divergence-free. It is determined that the shear flow and field can fasten the damping of standing Alfven waves. In spite of propagating Alfven waves, standing Alfven waves in Solar spicules dissipate in a few periods. As height increases, the perturbed velocity amplitude does increase in contrast to the behavior of perturbed magnetic field. Moreover, it should be emphasized that the stratification due to gravity, shear flow and field are the facts that should be considered in MHD models in spicules.

Nonlinear evolution of torsional Alfvén waves

Aims. We study the efficiency of the energy transfer to shorter scales in the field-aligned direction – the parallel nonlinear cascade – that accompanies the propagation of torsional Alfven waves along open magnetic fields in the solar and stellar coronae, and compare it with the same effects for the shear Alfven wave. The evolution of the torsional Alfven wave is caused by the back reaction of nonlinearly induced compressive perturbations on the Alfven wave.Methods. The evolution of upwardly propagating torsional Alfven waves is considered in terms of the second-order thin flux-tube approximation in a straight untwisted and non-rotating magnetic flux-tube. The Cohen-Kulsrud equation for weakly nonlinear torsional waves is derived. In the model, the effect of the cubic nonlinearity on the propagation of long-wavelength axisymmetric torsional waves is compared with the similar effect that accompanies the propagation of plane linearly-polarised (shear) Alfven waves of small amplitude.Results. The solution to the Cohen-Kulsrud type equation for torsional waves shows that their evolution is independent of the plasma-β , which is in contrast to the shear Alfven wave. In a finite-β plasma, the nonlinear evolution of torsional Alfven waves is slower and the parallel nonlinear cascade is less efficient than those of shear Alfven waves. These results have important implications for the analysis of possible heating of the plasma and its acceleration in the upper layers of solar and stellar coronae. In particular, one-dimensional models of coronal heating and wave acceleration, which use shear Alfven waves instead of torsional Alfven waves, over-estimate the efficiency of these processes.

Ion stochastic heating by obliquely propagating magnetosonic waves

The ion motions in obliquely propagating Alfven waves with sufficiently large amplitudes have already been studied by Chen et al. [Phys. Plasmas 8, 4713 (2001)], and it was found that the ion motions are stochastic when the wave frequency is at a fraction of the ion gyro-frequency. In this paper, with test particle simulations, we investigate the ion motions in obliquely propagating magnetosonic waves and find that the ion motions also become stochastic when the amplitude of the magnetosonic waves is sufficiently large due to the resonance at sub-cyclotron frequencies. Similar to the Alfven wave, the increase of the propagating angle, wave frequency, and the number of the wave modes can lower the stochastic threshold of the ion motions. However, because the magnetosonic waves become more and more compressive with the increase of the propagating angle, the decrease of the stochastic threshold with the increase of the propagating angle is more obvious in the magnetosonic waves than that in the Alfven waves.

Heating of the background plasma by obliquely propagating Alfven waves excited in the electromagnetic alpha/proton instability

Previous studies have shown that obliquely propagating Alfven waves may be excited in the electromagnetic alpha/proton instability. In this paper, two-dimensional hybrid simulations are performed to investigate the nonlinear evolution of the electromagnetic alpha/proton instability. We further find that the obliquely propagating Alfven waves excited by the alpha/proton instability have nearly linear polarization. The background proton component, as well as the alpha component, can be resonantly heated by the oblique Alfven waves. The implications of our results to the unsolved solar coronal hearing problem are also discussed in this paper.

Observation of magnetic reconnection at the turbulent leading edge of an interplanetary coronal mass ejection

Magnetic Cloud Boundary Layer (MCBL) is formed by the interaction between a magnetic cloud ejected by the Sun and the ambient solar wind, which may be linked to the outer loops of an Interplanetary Coronal Mass Ejection (ICME) and often display the properties of magnetic reconnection. We study the relation between current sheets, turbulence, and magnetic reconnection at the leading edge of an ICME intercepted by the four Cluster spacecraft in the solar wind, upstream of the Earth’s bow shock, on 2005 January 21. We obtain the observational evidence of a fully-developed magnetic turbulence in the vicinity of two current sheets at the leading edge of ICME. Each current sheet shows the signatures of magnetic reconnections with oppositely propagating Alfven waves and jets. The current density of bifurcated current sheets is computed by the curlometer technique using multi-spacecraft data.

ALFVÉN WAVES IN SHEAR FLOWS REVISITED

We revisit our earlier study of the evolution of an initial propagating Alfven wave in a magnetic-field-aligned flow with a cross-field velocity shear. Our goal is to show how the Alfven wave drives up plasma density fluctuations which might be observed and serve as a signature of the presence of Alfven waves in regions such as the solar corona which are inaccessible to direct observations. Here, we introduce a new initial condition which takes into account the initial distortion of the streamlines by the Alfven wave, and we present new analytical results for the driven waves. We find that the density fluctuations of a properly placed linearly polarized Alfven wave in a shear flow are much smaller than we originally estimated.

NON-WKB MODELS OF THE FIRST IONIZATION POTENTIAL EFFECT: THE ROLE OF SLOW MODE WAVES

A model for element abundance fractionation between the solar chromosphere and corona is further developed. The ponderomotive force due to Alfven waves propagating through, or reflecting from the chromosphere in solar conditions generally accelerates chromospheric ions, but not neutrals, into the corona. This gives rise to what has become known as the First Ionization Potential (FIP) Effect. We incorporate new physical processes into the model. The chromospheric ionization balance is improved, and the effect of different approximations is discussed. We also treat the parametric generation of slow mode waves by the parallel propagating Alfven waves. This is also an effect of the ponderomotive force, arising from the periodic variation of the magnetic pressure driving an acoustic mode, which adds to the background longitudinal pressure. This can have subtle effects on the fractionation, rendering it quasi-mass independent in the lower regions of the chromosphere. We also briefly discuss the change in the fractionation with Alfven wave frequency, relative to the frequency of the overlying coronal loop resonance.

Comment on “Heating of ions by low-frequency Alfvén waves in partially ionized plasmas” [Phys. Plasmas 18, 030702 (2011)

In their paper, Dong and Paty investigated the nonresonant ion heating by low-frequency parallel propagating Alfven waves in partially ionized plasmas. The heating efficiency was found to be lower in partially ionized plasmas than that in plasmas without ion-neutral collision. Here we point out that ion heating process is different from the descriptions by Dong and Paty. Their conclusions are based on the following basic equation, which describes the perpendicular velocity component of one single ion in a spectrum of Alfven waves, whose dispersion relation is x 1⁄4 kvA (vA: Alfven speed; x: wave angular frequency; k: wave number). The equation is

HEATING OF THE SOLAR CHROMOSPHERE AND CORONA BY ALFVÉN WAVE TURBULENCE

A three-dimensional MHD model for the propagation and dissipation of Alfven waves in a coronal loop is developed. The model includes the lower atmospheres at the two ends of the loop. The waves originate on small spatial scales (less than 100 km) inside the kilogauss flux elements in the photosphere. The model describes the nonlinear interactions between Alfven waves using the reduced MHD approximation. The increase of Alfven speed with height in the chromosphere and transition region (TR) causes strong wave reflection, which leads to counter-propagating waves and turbulence in the photospheric and chromospheric parts of the flux tube. Part of the wave energy is transmitted through the TR and produces turbulence in the corona. We find that the hot coronal loops typically found in active regions can be explained in terms of Alfven wave turbulence, provided the small-scale footpoint motions have velocities of 1-2 km/s and time scales of 60-200 s. The heating rate per unit volume in the chromosphere is 2 to 3 orders of magnitude larger than that in the corona. We construct a series of models with different values of the model parameters, and find that the coronal heating rate increases with coronal field strength and decreases with loop length. We conclude that coronal loops and the underlying chromosphere may both be heated by Alfvenic turbulence.

MHD Activity in GOL-3 at the Stage of Plasma Cooling

Transient MHD activity in the multiple-mirror trap GOL-3 at the stage of plasma cooling after decay of the net current was studied. Such events are diagnosed as short bursts (generally shorter than 5 cycles) of oscillations of azimuthal magnetic field with the frequency within 0.1 ÷ 1 MHz. Earlier transient events at the plasma decay stage were also observed. At that time an existence of long-lived internal current structures in the GOL-3 plasma was supposed.The paper presents new results from the magnetic diagnostics. Azimuthal and longitudinal mode localization agrees with the assumption that the magnetic field is generated by filaments with the current in the range of 1 ÷ 50 A in nearly uniform azimuthal distribution. Statistical analysis shows that the main oscillation frequencies are grouped near the inversed time of the Alfven wave propagation along the trap. Power dependence of the pulse probability on its amplitude with the index α = -1.87 was found. Relationships of MHD activity on the regime of plasma heating and on a degree of the beam stabilization at the exit receiver are discussed.

Freak waves in laboratory and space plasmas

Generation of large-amplitude short-lived wave groups from small-amplitude initial perturbations in plasmas is discussed. Two particular wave modes existing in plasmas are considered. The first one is the ion-sound wave. In a plasmas with negative ions it is described by the Gardner equation when the negative ion concentration is close to critical. The results of numerical solution of the Gardner equation with the modulationally unstable initial condition are presented. These results clearly show the possibility of generation of freak ion-acoustic waves due to the modulational instability. The second wave mode is the Alfven wave. When this wave propagates at a small angle with respect to the equilibrium magnetic field, and its wave length is comparable with the ion inertia length, it is described by the DNLS equation. Studying the evolution of an initial perturbation using the linearized DNLS equation shows that the generation of freak Alfven waves is possible due to linear dispersive focusing. The numerical solution of the DNLS equation reveals that the nonlinear dispersive focusing can also produce freak Alfven waves.

NONLINEAR PROPAGATION OF ALFVÉN WAVES DRIVEN BY OBSERVED PHOTOSPHERIC MOTIONS: APPLICATION TO THE CORONAL HEATING AND SPICULE FORMATION

We have performed MHD simulations of Alfven wave propagation along an open flux tube in the solar atmosphere. In our numerical model, Alfven waves are generated by the photospheric granular motion. As the wave generator, we used a derived temporal spectrum of the photospheric granular motion from G-band movies of Hinode/SOT. It is shown that the total energy flux at the corona becomes larger and the transition region height becomes higher in the case when we use the observed spectrum rather than white/pink noise spectrum as the wave generator. This difference can be explained by the Alfven wave resonance between the photosphere and the transition region. After performing Fourier analysis on our numerical results, we have found that the region between the photosphere and the transition region becomes an Alfven wave resonant cavity. We have confirmed that there are at least three resonant frequencies, 1, 3 and 5 mHz, in our numerical model. Alfven wave resonance is one of the most effective mechanisms to explain the dynamics of the spicules and the sufficient energy flux to heat the corona.

Comments on “Comment on Propagation and Dissipation of Alfven Waves in Coronal Holes” by Suresh Chandra

We comment on the recently published paper by Chandra (Open Astronomy Journal, 2009, 2, 16-18), and show that his results are erroneous in the context of the propagation and dissipation of Alfven waves in polar coronal holes under individual effects of magnetic diffusivity and viscosity.

Development of a magnetohydrodynamic simulation code satisfying the solenoidal magnetic field condition

We have developed a new magnetohydrodynamic (MHD) simulation code that automatically satisfies the solenoidal magnetic field ( B) property ∇ ⋅ B = 0 . We use the vector potential ( A) instead of the magnetic field itself in the magnetohydrodynamic equation. To solve the advection term, we adopt a Rational-CIP algorithm in the simulation code. The non-advection terms are solved by the 4th order Runge–Kutta method for time and 4th order central difference for space in a regular grid system. Code assessments are carried out to evaluate the properties of the developed code. A remarkable feature of the new code is the description of Alfven wave propagation with less numerical dispersion. After the code assessments, we apply the code to a global simulation of the planet Mercury's magnetosphere. Fundamental structures of the magnetosphere are successfully reproduced.

NON-WKB MODELS OF THE FIRST IONIZATION POTENTIAL EFFECT: IMPLICATIONS FOR SOLAR CORONAL HEATING AND THE CORONAL HELIUM AND NEON ABUNDANCES

We revisit in more detail a model for element abundance fractionation in the solar chromosphere, that gives rise to the "FIP Effect" in the solar corona and wind. Elements with first ionization potential below about 10 eV, i.e. those that are predominantly ionized in the chromosphere, are enriched in the corona by a factor 3-4. We model the propagation of Alfven waves through the chromosphere using a non-WKB treatment, and evaluate the ponderomotive force associated with these waves. Under solar conditions, this is generally pointed upwards in the chromosphere, and enhances the abundance of chromospheric ions in the corona. Our new approach captures the essentials of the solar coronal abundance anomalies, including the depletion of He relative to H, and also the putative depletion of Ne, recently discussed in the literature. We also argue that the FIP effect provides the strongest evidence to date for energy fluxes of Alfven waves sufficient to heat the corona. However it appears that these waves must also be generated in the corona, in order to preserve the rather regular fractionation pattern without strong variations from loop to loop observed in the solar corona and slow speed solar wind.

Comment on Propagation and Dissipation of Alfven Waves in Coronal Holes

Dwivedi and Srivastava (1) (DS) investigated the propagation and dissipation of Alfven waves in coronal holes after accounting for the viscosity and magnetic diffusivity. After solving a set of equations with the help of computer results are reported by them. We find that the same set of equations can be solved even analytically. Since DS have not reported any values of physical parameters used by them except their expressions, we could not trace out the source of error. One reason for the difference in our results and those of DS can be assigned to some mistakes in their computer program or to the values of parameters used.

Upward propagating high-frequency Alfvén waves as identified from dynamic wave-like spicules observed by SOT onHinode

Aims. We identify high-frequency Alfven waves propagating upward in the solar chromosphere and transition region from observation by Solar Optical Telescope (SOT) onboard Hinode .Methods. The spicule shape is enhanced through application of a normal radial gradient filter and an un-sharp mask on the images taken by SOT. The displaced position of the spicule is at each height obtained by tracing the maximum intensity after image processing. The dominant wave period is obtained by the FFT method applied to the time variations of the displaced position at a certain height. The phase speed is estimated with the help of a cross-correlation analysis of two temporal sequences of the displaced positions at two heights along the spicule.Results. We find in four cases that the spicules are modulated by high-frequency (≥ 0.02 Hz) transverse fluctuations. Such fluctuations are suggested to be Alfven waves that propagate upward along the spicules with phase speed ranges from 50 to 150 km s-1 . Three of the modulated spicules show clear wave-like shapes with short wavelengths less than 8 Mm.Conclusions. Our work identified directly upward propagation of Alfven waves in the solar chromosphere and transition region. In addition to the recently reported Alfven waves with very long wavelength and wave period, we find here four examples of Alfven waves with shorter wavelengths and periods. These findings shed new light on the wave origin and on coronal and solar-wind heating.

Transverse Wave Propagation in Relativistic Two-Fluid Plasmas around Schwarzschild-de Sitter Black Hole

We investigate transverse electromagnetic waves propagating in a plasma influenced by the gravitational field of the Schwarzschild-de Sitter black hole. Applying 3+1 spacetime split we derive the relativistic two-fluid equations to take account of gravitational effects due to the event horizon and describe the set of simultaneous linear equations for the perturbations. We use a local approximation to investigate the one-dimensional radial propagation of Alfven and high frequency electromagnetic waves. We derive the dispersion relation for these waves and solve it for the wave number $k$ numerically.

Polar-Cap Accelerator and Radio Emission from Pulsars

Electric currents j flow along the open magnetic field lines from the polar caps of neutron stars. Activity of a polar cap depends on the ratio \alpha=j/c\rho_GJ, where \rho_GJ is the corotation charge density. The customary assumption \alpha\approx 1 is not supported by recent simulations of pulsar magnetospheres and we study polar caps with arbitrary \alpha. We argue that no significant activity is generated on field lines with 0 1 or \alpha<0, a high voltage is generated, leading to unsteady e^+- discharge on a scale-height smaller than the size of the polar cap. The discharge can power observed pulsars. Voltage fluctuations in the discharge imply unsteady twisting of the open flux tube and generation of Alfven waves. These waves are ducted along the tube and converted to electromagnetic waves, providing a new mechanism for pulsar radiation.