Alfven Wave Damping Research Articles

Thermal Misbalance Influence on the Nonlinear Shear Alfvén Waves Under Solar Atmosphere Conditions

The self-interaction of the Alfven wave associated with the parametric interaction between the Alfven wave and longitudinal plasma motion induced by this Alfven wave is investigated in a dissipative plasma with a thermal misbalance. The thermal misbalance caused by plasma heating and radiation cooling leads to changes in the steepening rate and the resulting amplitude of Alfven waves. The characteristic times of steepening and changes of amplitude are obtained analytically and subsequently calculated for the initial stage of self-interaction of sinusoidal Alfven waves applying the upper chromosphere and coronal hole conditions. Comparison with characteristic times of viscous and Ohmic dissipation is conducted as well. It is revealed that for the chromosphere conditions there are temperature ranges where Alfven waves can steepen faster and, therefore, the nonlinear Alfven dissipation observed by Grant et al. (Nat. Phys. 14(5), 480, 2018) could be stronger. Moreover, depending on the temperature range, the steepening of the wave can occur both in front and behind. In addition, temperature ranges were found in which the Alfven wave can be strongly absorbed due to the thermal misbalance. It should be mentioned that regions of fast steepening and fast damping due to the thermal misbalance intersect. Therefore, in these regions, the damping of Alfven waves can be most significant. We also show that for coronal hole conditions the thermal misbalance only affects the Alfven wave steepening.

Phase mixing of Alfvén waves propagating in non-reflective magnetic plasma configurations

The ability of phase mixing to provide efficient damping of Alfven waves even in weakly dissipative plasmas made it a popular mechanism for explaining the solar coronal heating. Initially it was studied in the equilibrium configurations with the straight magnetic field lines and the Alfven speed only varying in the direction perpendicular to the magnetic field. Later the analysis of the Alfven wave phase mixing was extended in various directions. In particular it was studied in two-dimensional planar magnetic plasma equilibria. Analytical investigation was carried out under the assumption that the wavelength is much smaller than the characteristic scale of the background quantity variation. This assumption enabled using the Wentzel, Kramers, and Brillouin (WKB) method. When it is not satisfied the study was only carried out numerically. In general, even the wave propagation in a one-dimensional inhomogeneous equilibrium can be only studied numerically. However there is one important exception, so-called non-reflective equilibria. In these equilibria the wave equation with the variable phase speed reduces to the Klein-Gordon equation with constant coefficients. In this paper we apply the theory of non-reflective wave propagation to studying the Alfven wave phase mixing in two-dimensional planar magnetic plasma equilibria. Using curvilinear coordinates we reduce the equation describing the Alfven wave phase mixing to the equation that becomes a one-dimensional wave equation in the absence of dissipation. This equation is further reduced to the equation which is the one-dimensional Klein-Gordon equation in the absence of dissipation. Then we show that this equation has constant coefficients when a particular relation between the plasma density and magnetic field magnitude is satisfied. Using the derived Klein-Gordon-type equation we study the phase mixing in various non-reflective equilibria. We emphasise that our analysis is valid even when the wavelength is comparable with the characteristic scale of the background quantity variation. In particular, we study the Alfven wave damping due to phase mixing in an equilibrium with constant plasma density and exponentially divergent magnetic field lines. We confirm the result previously obtained in the WKB approximation that there is enhanced Alfven wave damping in this equilibrium with the damping length proportional to ln(Re), where Re is the Reynolds number. Our theoretical results are applied to heating of coronal plumes. We show that, in spite of enhanced damping, Alfven waves with periods of the order of one minute can be efficiently damped in the lower corona, at the height about 200 Mm, only if the shear viscosity is increased by about 6 orders of magnitude in comparison with its value given by the classical plasma theory. We believe that such increase of the shear viscosity can be provided by the turbulence.

NONLINEAR DAMPING OF ALFVÉN WAVES IN THE SOLAR CORONA BELOW 1.5 SOLAR RADII

Nonthermal velocities measured in the solar corona imply a strong damping of upward-propagating low-frequency less than or similar to 0.01 Hz Alfven waves at heliocentric distances from 1.02 to 1.4 solar radii. We propose a vector Alfven wave decay as a feasible mechanism for the observed Alfven wave damping. Contrary to the extensively studied scalar decay, the vector decay does not depend on the wave frequency and can be efficient for low-frequency coronal Alfven waves. We show that the vector decay is much stronger than the scalar decay and can provide the observed damping of 0.01 Hz coronal Alfven waves with perpendicular wavelengths of similar to 10(4) km or less. Fully three-dimensional (3D) numerical simulations are needed to capture this decay, whose growth rate is proportional to the vector product of interacting wave vectors.

The dead zone size limits in a proto-stellar accretion disc model heated by the damping of Alfvén waves

Heating of proto-stellar accretion discs has been studied by several authors. Jatenco-Pereira (Mon. Not. R. Astron. Soc. 431:3150, 2013) proposed a disc model with two heating mechanisms: the “anomalous” viscosity considered in terms of the α-prescription and the damping of Alfven waves. As the discs are composed of dust, it was considered that when charged dust particles acquire the same (cyclotron) frequency as the waves, a resonance occurs that leads to the damping of the waves. Here we show that the increase in the temperature of the disc midplane implies in the reduction of the size of the quiescent region in proto-stellar discs and compare it with the actual position of the solar system planets.

Overdamped Alfvén waves due to ion-neutral collisions in the solar chromosphere

Alfvenic waves are ubiquitous in the solar atmosphere and their dissipation may play an important role in atmospheric heating. In the partially ionized solar chromosphere, collisions between ions and neutrals are an efficient dissipative mechanism for Alfven waves with frequencies near the ion-neutral collision frequency. The collision frequency is proportional to the ion-neutral collision cross section for momentum transfer. Here, we investigate Alfven wave damping as a function of height in a simplified chromospheric model and compare the results for two sets of collision cross sections, namely those of the classic hard-sphere model and those based on recent quantum-mechanical computations. We find important differences between the results for the two sets of cross sections. There is a critical interval of wavelengths for which impulsively excited Alfven waves are overdamped as a result of the strong ion-neutral dissipation. The critical wavelengths are in the range from 1 km to 50 km for the hard-sphere cross sections, and from 1 m to 1 km for the quantum-mechanical cross sections. Equivalently, for periodically driven Alfven waves there is an optimal frequency for which the damping is most effective. The optimal frequency varies from 1 Hz to 10^2 Hz for the hard-sphere cross sections, and from 10^2 Hz to 10^4 Hz for the quantum-mechanical cross sections. Future observations at sufficiently high spatial or temporal resolution may show the importance of high-frequency Alfven waves for chromospheric heating. For instance, the Atacama Large Millimeter/submillimeter Array (ALMA) may be able to detect the critical wavelengths and optimal frequencies and so to test the effective collision cross section in the chromospheric plasma.

Three dimensional magnetohydrodynamic simulation of linearly polarised Alfven wave dynamics in Arnold-Beltrami-Childress magnetic field

Previous studies (e.g., Malara et al., Astrophys. J. 533, 523 (2000)) considered small-amplitude Alfven wave (AW) packets in Arnold-Beltrami-Childress (ABC) magnetic field using WKB approximation. They draw a distinction between 2D AW dissipation via phase mixing and 3D AW dissipation via exponentially divergent magnetic field lines. In the former case, AW dissipation time scales as S1∕3 and in the latter as log(S), where S is the Lundquist number. In this work, linearly polarised Alfven wave dynamics in ABC magnetic field via direct 3D magnetohydrodynamic (MHD) numerical simulation is studied for the first time. A Gaussian AW pulse with length-scale much shorter than ABC domain length and a harmonic AW with wavelength equal to ABC domain length are studied for four different resistivities. While it is found that AWs dissipate quickly in the ABC field, contrary to an expectation, it is found the AW perturbation energy increases in time. In the case of the harmonic AW, the perturbation energy growth is transient in time, attaining peaks in both velocity and magnetic perturbation energies within timescales much smaller than the resistive time. In the case of the Gaussian AW pulse, the velocity perturbation energy growth is still transient in time, attaining a peak within few resistive times, while magnetic perturbation energy continues to grow. It is also shown that the total magnetic energy decreases in time and this is governed by the resistive evolution of the background ABC magnetic field rather than AW damping. On contrary, when the background magnetic field is uniform, the total magnetic energy decrease is prescribed by AW damping, because there is no resistive evolution of the background. By considering runs with different amplitudes and by analysing the perturbation spectra, possible dynamo action by AW perturbation-induced peristaltic flow and inverse cascade of magnetic energy have been excluded. Therefore, the perturbation energy growth is attributed to a new instability. The growth rate appears to be dependent on the value of the resistivity and the spatial scale of the AW disturbance. Thus, when going beyond WKB approximation, AW damping, described by full MHD equations, does not guarantee decrease of perturbation energy. This has implications for the MHD wave plasma heating in exponentially divergent magnetic fields.

Non-linear damping of visco-resistive Alfvén waves in solar spicules

Interaction of Alfven waves with plasma inhomogeneities generates phase mixing which can lead to dissipate Alfven waves and to heat the solar plasma. Here we study the dissipation of Alfven waves by phase mixing due to viscosity and resistivity variations with height. We also consider nonlinear magnetohydrodynamic (MHD) equations in our theoretical model. Non-linear terms of MHD equations include perturbed velocity, magnetic field, and density. To investigate the damping of Alfven waves in a stratified atmosphere of solar spicules, we solve the non-linear MHD equations in the x-z plane. Our simulations show that the damping is enhanced due to viscosity and resistivity gradients. Moreover, energy variations is influenced due to nonlinear terms in MHD equations.

Gamma-ray properties of supernova remnants transporting into molecular clouds: the cases of IC 443 and W44

Motivated by the recent report of the detailed spectra of IC 443 and W44 based on four years of observations with the Fermi Large Area Telescope, gamma-rays produced via proton-proton interaction in the two supernova remnants (SNRs) have been investigated in the diffusive shock acceleration scenario. In the model, a part of the SNR shell transporting into ambient dense molecular clouds (MCs), and a shock with a relatively low Alfven Mach number can be produced. Relativistic protons are accelerated in the shock via the diffusive acceleration process; moreover, a break in the particle distribution is induced since the damping of Alfven waves when the shock transporting into partly ionized plasma. A spectrum of the accelerated protons in the shock can be obtained in the test-particle case, taking into account the effect of the ambient partly ionized plasma on the acceleration process. Furthermore, hadronic gamma-rays with a peak around several GeVs are reproduced as the protons inelastically colliding with the ambient matter of the crushed gas. The observed gamma-ray spectral distributions of the SNR IC 443 and W44 can be successfully reproduced using the static model in the test-particle scenario. It can be concluded that the observed gamma-rays can be explained in the diffusive shock case as the shock waves transporting into the dense MCs for the two remnants.

Effects of ion-neutral collisions on Alfvén waves: The presence of forbidden zone and heavy damping zone

Alfven waves are low-frequency transverse waves propagating in a magnetized plasma. We define the Alfven frequency ω0 as ω0=kVAcosθ, where k is the wave number, VA is the Alfven speed, and θ is the angle between the wave vector and the ambient magnetic field. There are partially ionized plasmas in laboratory, space, and astrophysical plasma systems, such as in the solar chromosphere, interstellar clouds, and the earth ionosphere. The presence of neutral particles may modify the wave frequency and cause damping of Alfven waves. The effects on Alfven waves depend on two parameters: (1) α=nn/ni, the ratio of neutral density (nn), and ion density (ni); (2) β=νni/ω0, the ratio of neutral collisional frequency by ions νni to the Alfven frequency ω0. Most of the previous studies examined only the limiting case with a relatively large neutral collisional frequency or β≫1. In the present paper, the dispersion relation for Alfven waves is solved for all values of α and β. Approximate solutions in the limit β≫1 as w...

Torsional Alfvén waves in partially ionized solar plasma: effects of neutral helium and stratification

Ion-neutral collisions may lead to the damping of Alfven waves in chromospheric and prominence plasmas. Neutral helium atoms enhance the damping in certain temperature interval, where the ratio of neutral helium and neutral hydrogen atoms is increased. Therefore, the height-dependence of ionization degrees of hydrogen and helium may influence the damping rate of Alfven waves. We aim to study the effect of neutral helium in the damping of Alfven waves in stratified partially ionized plasma of the solar chromosphere. We consider a magnetic flux tube, which is expanded up to 1000 km height and then becomes vertical due to merging with neighboring tubes, and study the dynamics of linear torsional Alfven waves in the presence of neutral hydrogen and neutral helium atoms. We start with three-fluid description of plasma and consequently derive single-fluid magnetohydrodynamic (MHD) equations for torsional Alfven waves. Thin flux tube approximation allows to obtain the dispersion relation of the waves in the lower part of tubes, while the spatial dependence of steady-state Alfven waves is governed by Bessel type equation in the upper part of tubes. Consecutive derivation of single-fluid MHD equations results in a new Cowling diffusion coefficient in the presence of neutral helium which is different from previously used one. We found that shorter-period (< 5 s) torsional Alfven waves damp quickly in the chromospheric network due to ion-neutral collision. On the other hand, longer-period (> 5 s) waves do not reach the transition region as they become evanescent at lower heights in the network cores. Propagation of torsional Alfven waves through the chromosphere into the solar corona should be considered with caution: low-frequency waves are evanescent due to the stratification, while high-frequency waves are damped due to ion neutral collisions.

SELF-CONSISTENT MODEL OF THE INTERSTELLAR PICKUP PROTONS, ALFVÉNIC TURBULENCE, AND CORE SOLAR WIND IN THE OUTER HELIOSPHERE

A self-consistent model of the interstellar pickup protons, the slab component of the Alfvenic turbulence, and core solar wind (SW) protons is presented for r ≥ 1 along with the initial results of and comparison with the Voyager 2 (V2) observations. Two kinetic equations are used for the pickup proton distribution and Alfvenic power spectral density, and a third equation governs SW temperature including source due to the Alfven wave energy dissipation. A fraction of the pickup proton free energy, fD , which is actually released in the waveform during isotropization, is taken from the quasi-linear consideration without preexisting turbulence, whereas we use observations to specify the strength of the large-scale driving, C sh, for turbulence. The main conclusions of our study can be summarized as follows. (1) For C sh 1-1.5 and fD 0.7-1, the model slab component agrees well with the V2 observations of the total transverse magnetic fluctuations starting from ~8 AU. This indicates that the slab component at low-latitudes makes up a majority of the transverse magnetic fluctuations beyond 8-10 AU. (2) The model core SW temperature agrees well with the V2 observations for r 20 AU if fD 0.7-1. (3) A combined effect of the Wentzel-Kramers-Brillouin attenuation, large-scale driving, and pickup proton generated waves results in the energy sink in the region r 10 AU, while wave energy is pumped in the turbulence beyond 10 AU. Without energy pumping, the nonlinear energy cascade is suppressed for r 10 AU, supplying only a small energy fraction into the k-region of dissipation by the core SW protons. A similar situation takes place for the two-dimensional turbulence. (4) The energy source due to the resonant Alfven wave damping by the core SW protons is small at heliocentric distances r 10 AU for both the slab and the two-dimensional turbulent components. As a result, adiabatic cooling mostly controls the model SW temperature in this region, and the model temperature disagrees with the V2 observations in the region r 20 AU.

Galactic winds driven by cosmic ray streaming

Galactic winds are observed in many spiral galaxies with sizes from dwarfs up to the Milky Way, and they sometimes carry a mass in excess of that of newly formed stars by up to a factor of ten. Multiple driving processes of such winds have been proposed, including thermal pressure due to supernova-heating, UV radiation pressure on dust grains, or cosmic ray (CR) pressure. We here study wind formation due to CR physics using a numerical model that accounts for CR acceleration by supernovae, CR thermalization, and advective CR transport. In addition, we introduce a novel implementation of CR streaming relative to the rest frame of the gas. We find that CR streaming drives powerful and sustained winds in galaxies with virial masses M_200 < 10^{11} Msun. In dwarf galaxies (M_200 ~ 10^9 Msun) the winds reach a mass loading factor of ~5, expel ~60 per cent of the initial baryonic mass contained inside the halo's virial radius and suppress the star formation rate by a factor of ~5. In dwarfs, the winds are spherically symmetric while in larger galaxies the outflows transition to bi-conical morphologies that are aligned with the disc's angular momentum axis. We show that damping of Alfven waves excited by streaming CRs provides a means of heating the outflows to temperatures that scale with the square of the escape speed. In larger haloes (M_200 > 10^{11} Msun), CR streaming is able to drive fountain flows that excite turbulence. For halo masses M_200 > 10^{10} Msun, we predict an observable level of H-alpha and X-ray emission from the heated halo gas. We conclude that CR-driven winds should be crucial in suppressing and regulating the first epoch of galaxy formation, expelling a large fraction of baryons, and - by extension - aid in shaping the faint end of the galaxy luminosity function. They should then also be responsible for much of the metal enrichment of the intergalactic medium.

Damping of Alfvén waves in solar partially ionized plasmas: effect of neutral helium in multi-fluid approach

Chromospheric and prominence plasmas contain neutral atoms, which may change the plasma dynamics through collision with ions. Most of the atoms are neutral hydrogen, but a significant amount of neutral helium may also be present in the plasma with a particular temperature. Damping of MHD waves due to ion collision with neutral hydrogen is well studied, but the effects of neutral helium are largely unknown. We aim to study the effect of neutral helium in the damping of Alfven waves in solar partially ionized plasmas. We consider three-fluid magnetohydrodynamic (MHD) approximation, where one component is electron-proton-singly ionized helium and other two components are the neutral hydrogen and neutral helium atoms. We derive the dispersion relation of linear Alfven waves in isothermal and homogeneous plasma. Then we solve the dispersion relation and derive the damping rates of Alfven waves for different plasma parameters. The presence of neutral helium significantly enhances the damping of Alfven waves compared to the damping due to neutral hydrogen at certain values of plasma temperature (10000-40000 K) and ionization. Damping rates have a peak near the ion-neutral collision frequency, but decrease for the higher part of wave spectrum. Collision of ions with neutral helium atoms can be of importance for the damping of Alfven waves in chromospheric spicules and in prominence-corona transition regions.

THE INTERACTION OF COSMIC RAYS WITH DIFFUSE CLOUDS

We study the change in cosmic-ray pressure, the change in cosmic-ray density, and the level of cosmic-ray induced heating via Alfven-wave damping when cosmic rays move from a hot ionized plasma to a cool cloud embedded in that plasma. The general analysis method outlined here can apply to diffuse clouds in either the ionized interstellar medium or in galactic winds. We introduce a general-purpose model of cosmic-ray diffusion building upon the hydrodynamic approximation for cosmic rays (from McKenzie & Voelk and Breitschwerdt and collaborators). Our improved method self-consistently derives the cosmic-ray flux and diffusivity under the assumption that the streaming instability is the dominant mechanism for setting the cosmic-ray flux and diffusion. We find that, as expected, cosmic rays do not couple to gas within cool clouds (cosmic rays exert no forces inside of cool clouds), that the cosmic-ray density does not increase within clouds (it may slightly decrease in general, and decrease by an order of magnitude in some cases), and that cosmic-ray heating (via Alfven-wave damping and not collisional effects as for ~10 MeV cosmic rays) is only important under the conditions of relatively strong (10 micro-Gauss) magnetic fields or high cosmic-ray pressure (~10^{-11} ergs cm^{-3}).

Heating of the solar atmosphere by strong damping of Alfvén waves

[1] The heating of the solar atmosphere by strongly damped Alfven waves that produce heating through plasma-neutral collisions is studied by solving analytically a self-consistent one-dimensional model of the plasma-neutral-electromagnetic system. We compute the vertical profile of the wave spectrum and power by a novel method, which includes the damping effect neglected in previous treatments, and find that the damping depends on the magnetic field strength. The damping is extremely strong for weaker magnetic field and less strong for strong field. Under either condition, the high-frequency portion of the source power spectrum is strongly damped at the lower altitudes, depositing heat there, whereas the lower-frequency perturbations are nearly undamped and can be observed in the corona and above when the field is strong. The chromosphere behaves like a low-pass filter. The magnetic field strength determines the upper cutoff frequency. As a result, the power and spectrum of the waves observed above the corona is weak for regions of weaker background magnetic field and only a fraction of those at the photosphere for regions of strong magnetic field. Contrary to what was supposed in some earlier Alfven wave damping models, the spectrum observed above the chromosphere in general does not represent the energy input. We show, using the parameters of a semi-empirical model for quiet-Sun conditions, that this mechanism, without invoking any anomalous processes, can generate sufficient heat to account for the radiative losses in the atmosphere, with most of the heat deposited as required at lower altitudes.

Alfvén wave phase-mixing and damping in the ion cyclotron range of frequencies

Aims. To determine the effect of the Hall term in the generalised Ohm's law on the damping and phase mixing of Alfven waves in the ion cyclotron range of frequencies in uniform and non-uniform equilibrium plasmas. Methods. Wave damping in a uniform plasma is treated analytically, whilst a Lagrangian remap code (Lare2d) is used to study Hall effects on damping and phase mixing in the presence of an equilibrium density gradient. Results. The magnetic energy associated with an initially Gaussian field perturbation in a uniform resistive plasma is shown to decay algebraically at a rate that is unaffected by the Hall term to leading order in k^2di^2 where k is wavenumber and di is ion skin depth. A similar algebraic decay law applies to whistler perturbations in the limit k^2di^2>>1. In a non-uniform plasma it is found that the spatially-integrated damping rate due to phase mixing is lower in Hall MHD than it is in MHD, but the reduction in the damping rate, which can be attributed to the effects of wave dispersion, tends to zero in both the weak and strong phase mixing limits.

Time damping of non-adiabatic magnetohydrodynamic waves in a partially ionised prominence medium: Effect of a background flow

Context. The simultaneous occurrence of flows and time damped small-amplitude oscillations in solar prominences is a common phenomenon. These oscillations are mostly interpreted in terms of magnetohydrodynamic (MHD) waves.Aims. We study the time damping of linear non-adiabatic MHD waves in a flowing partially ionised plasma with prominence-like physical conditions.Methods. Considering non-adiabatic single fluid equations for a partially ionised hydrogen plasma, we have solved our dispersion relations for the complex frequency, ω , and we have analysed the behavior of the period, damping time and the ratio of the damping time to the period, versus the real wavenumber k , for Alfven, fast, slow, and thermal waves.Results. While in the case without flow there is a critical wavenumber at which the period of Alfven and fast waves goes to infinite, when a flow is present two different critical wavenumbers appear. The smaller wavenumber depends on the flow speed and causes the period of the high-period branch to go to infinite. When the second critical wavenumber is attained the period of both branches become equal. In general, the time damping of Alfven and fast waves is dominated by resistive effects, and its damping ratio is very inefficient when compared to observations. The damping of slow and thermal waves is basically dominated by non-adiabatic effects, and for slow waves it is possible to obtain a damping ratio close to observations, although it would correspond to long period oscillations with large damping times not often observed. The consideration of a structured medium produces new features such as the apparition of four critical wavenumbers for Alfven waves, and one critical wavenumber for slow waves. For fast waves, constrained propagation substantially improves, within the range of observed wavelengths, the ratio of the damping time to period. Conclusions. The presence of a background flow in a partially ionised plasma gives place to new interesting features when the time damping of MHD waves is studied. In general, the results point out that ion-neutral collisions are an inefficient mechanism to explain the observed time damping of prominence oscillations if they are produced by Alfven and fast waves. If the oscillations are produced by slow waves, only long period oscillations with large damping times produce damping ratios in agreement with observations.

SURFACE ALFVÉN WAVE DAMPING IN A THREE-DIMENSIONAL SIMULATION OF THE SOLAR WIND

Here we investigate the contribution of surface Alfven wave damping to the heating of the solar wind in minima conditions. These waves are present in regions of strong inhomogeneities in density or magnetic field (e. g., the border between open and closed magnetic field lines). Using a 3-dimensional Magnetohydrodynamics (MHD) model, we calculate the surface Alfven wave damping contribution between 1-4 solar radii, the region of interest for both acceleration and coronal heating. We consider waves with frequencies lower than those that are damped in the chromosphere and on the order of those dominating the heliosphere. In the region between open and closed field lines, within a few solar radii of the surface, no other major source of damping has been suggested for the low frequency waves we consider here. This work is the first to study surface Alfven waves in a 3D environment without assuming a priori a geometry of field lines or magnetic and density profiles. We determine that waves with frequencies >2.8x10^-4 Hz are damped between 1-4 solar radii. In quiet sun regions, surface Alfven waves are damped at further distances compared to active regions, thus carrying additional wave energy into the corona. We compare the surface Alfven wave contribution to the heating by a variable polytropic index and find that it an order of magnitude larger than needed for quiet sun regions. For active regions the contribution to the heating is twenty percent. As it has been argued that a variable gamma acts as turbulence, our results indicate that surface Alfven wave damping is comparable to turbulence in the lower corona. This damping mechanism should be included self consistently as an energy driver for the wind in global MHD models.

On the origin of solar wind. Alfvén waves induced jump of coronal temperature

Absorbtion of Alfven waves is considered to be the main mechanism of heating in the solar corona. It is concluded that the sharp increase of the plasma temperature by two orders of magnitude is related to a self-induced opacity with respect to Alfven waves. The maximal frequency for propagation of Alfven waves is determined by the strongly temperature dependent kinematic viscosity. In such a way the temperature jump is due to absorption of high frequency Alfven waves in a narrow layer above the solar surface. It is calculated that the power per unit area dissipated in this layer due to damping of Alfven waves blows up the plasma and gives birth to the solar wind. A model short wave-length (WKB) evaluation takes into account the 1/f^2 frequency dependance of the transversal magnetic field and velocity spectral densities. Such spectral densities agree with old magnetometric data taken by Voyager 1 and recent theoretical calculations in the framework of Langevin-Burgers MHD. The present theory predicts existence of intensive high frequency MHD Alfven waves in the cold layer beneath the corona. It is briefly discussed how this statement can be checked experimentally. It is demonstrated that the magnitude of the Alfven waves generating random noise and the solar wind velocity can be expressed only in terms of satellite experimental data. It is advocated that investigation of properties of the solar surface as a random driver by optical methods is an important task for future solar physics. Jets of accretion disks are speculated as a special case of the wind from magnetized turbulent plasma.

Radio Spectral Index Variations and Physical Conditions in Kepler’s Supernova Remnant

A new epoch of VLA measurements of Kepler's supernova remnant (SNR) was obtained to make accurate measurements of the radio spectral index variations and polarization. We have compared these new radio images with Hα, infrared (IR), and X-ray data to better understand the three-dimensional structure and dynamics of Kepler's SNR and to better understand the physical relationships between the various nonthermal and thermal plasmas. Spatial variations in the radio spectral index from -0.85 to -0.6 are observed between 6 and 20 cm. The mean spectral index is -0.71. The mean percent polarization is 3.5% at 20 cm and 6% at 6 cm. There is a strong correspondence between the radial and azimuthal profiles of the radio, X-ray, Hα, and IR emission in different locations around the remnant, although there is no single, global pattern. Spectral tomography shows that the flat- and steep-spectrum radio emissions have distinct structures. The flat-spectrum radio emission is found at either a larger radius than or coincident with the steep-spectrum emission. We interpret these spectral components as tracing forward- and reverse-shocked material, respectively. The flat-spectrum radio emission can alternatively be interpreted as the bow shocked material (reshocked by the forward shock) from the progenitor's motion through the interstellar medium. The Hα and IR images are very similar. Their leading edges are coincident and are either in front of or coincident with the leading edges of the X-ray and radio emission. The X-ray emission matches the Hα and IR emission in places and, in other places, traces the steep-spectrum radio emission. In the north, there is also an anticorrelation in the azimuthal profiles around the remnant of the flat-spectrum radio emission and the thermal X-ray, Hα, and IR emissions. We suggest that this could be due to a relative weakening of the particle acceleration at the forward shock due to Alfven wave damping in regions of high density.