Propagating Alfven Waves Research Articles

Giant Chromospheric Anemone Jet Observed with Hinode and Comparison with Magnetohydrodynamic Simulations: Evidence of Propagating Alfvén Waves and Magnetic Reconnection

Hinode discovered a beautiful giant jet with both cool and hot components at the solar limb on 2007 February 9. Simultaneous observations by the Hinode SOT, XRT, and TRACE 195 satellites revealed that hot (5x10^6 K) and cool (10^4 K) jets were located side by side and that the hot jet preceded the associated cool jet (1-2 minutes). A current-sheet-like structure was seen in optical (Ca IIH), EUV (195A), and soft X-ray emissions, suggesting that magnetic reconnection is occurring in the transition region or upper chromosphere. Alfven waves were also observed with Hinode SOT. These propagated along the jet at velocities of 200 km/s with amplitudes (transverse velocity) of 5-15 km/s and a period of 200 s. We performed two-dimensional MHD simulation of the jets on the basis of the emerging flux-reconnection model, by extending Yokoyama and Shibata's model. We extended the model with a more realistic initial condition (10^6 K corona) and compared our model with multiwavelength observations. The improvement of the coronal temperature and density in the simulation model allowed for the first time the reproduction of the structure and evolution of both the cool and hot jets quantitatively, supporting the magnetic reconnection model. The generation and the propagation of Alfven waves are also reproduced self-consistently in the simulation model.

Characteristics of middle‐ to low‐latitude Pi2 excited by bursty bulk flows

This paper, by using the data of Cluster, TC‐1, GOES, and eight ground stations on 22 October 2004, studied the characteristics of low‐latitude Pi2s generated by an earthward bursty bulk flow (BBF) in the near‐Earth tail plasma sheet. The BBF excited simultaneously two distinct classes of Pi2s: one is long‐period Pi2 (90–130 s) and the other is short‐period Pi2 (∼50 s). The long‐period Pi2 is transient response type Pi2 associated with field‐aligned current produced by the braking of BBFs. The spectrum analysis show that the amplitude spectrum peak of long‐period Pi2 increases with increasing latitude, indicating that the source is at higher latitudes. The time delay for the propagation of Alfven waves from Cluster to the Earth is very close to the time difference between the onset time of the BBFs at Cluster and the starting time of the long‐period Pi2 on the ground. The short‐period Pi2 is a global cavity mode since the Pi2s in H components at eight stations have almost the same starting time, same oscillation period, and same waveform, which are all typical characteristics of cavity mode. The amplitude spectrum peak of short‐period Pi2 at NCK (N42.7) is larger than those at higher‐station UPS (N56.5) and lower‐station CST (N40.8). The polarization analysis at three lower‐latitude stations shows that the polarization underwent two reversals. The major axis of the polarization ellipse points to approximately the north, indicating that the short‐period Pi2s are not excited by nightside current system. TC‐1 observed transverse mode Pi2s. Its period is almost identical with the periods of Pi2 on the ground, indicating they belong to the same wave.

Shear Driven Kinetic Alfven Wave with General Loss-Cone Distribution Function in the Plasma Sheet Boundary Layer

Expressions for the dispersion relation and growth rate of the KAW are derived for weak and strong shear regimes using the kinetic approach in view of the simultaneous observations of the large earthward Alfvenic Poynting flux, small-scale kinetic Alfven wave (KAW), earthward flowing electrons and upward flowing ions, at the substorm event in the plasma sheet boundary layer (PSBL). General loss-cone distribution function is adopted to describe the velocity distribution of the plasma particles. The results explain the generation of the observed KAW in the PSBL by the weak shear at the substorm onset. It is found that during the substorm expansion phase the cyclotron damping of KAW may lead to the upward flowing ion. Whereas, it’s Landau damping that may lead to the parallel energisation of the electrons that explains the observed loss of Alfvenic Poynting flux. It is also noted that the loss-cone distribution index changes the profiles of the frequency and growth rate plots of the shear-driven KAW. The loss-cone distribution function is therefore, an important factor for the excitation of KAW in the active region of the magnetosphere at the PSBL. Results are consistent with the finding of Wu and Seyler (J Geophys Res 108A6:1236, 2003) concerning kinetic Alfven wave generation and its stabilization by the sheared flow.

Nonrelativistic and Relativistic Treatments for Propagation of Torsional Resonant Alfven Waves in Strongly Magnetized Neutron Stars

We study the propagation and transmission of torsional Alfven waves in magnetars. First, we consider a simple two-layer model composed of an outer crust and vaccum magnetosphere. In this model, we derive the classical formulation for the transmission coefficient of Alfven waves using an MHD approximation. We then include a displacement current in Maxwell’s equations, which can be shown to become important in the magnetically relativistic region of the crust, B 4πρc. The effects of strong magnetic fields of magnetars are specifically quantified and discussed. Second, we consider a three-layer model in order to investigate the propagation properties of Alfven waves excited in the deep crust. This model is adopted for the inner crust, outer crust, and vacuum magnetosphere. It is shown that the outer crust plays a significant role as a resonant cavity if a certain condition holds. Some nonrelativistic resonance of Alfven modes with low frequencies obtained in this work is probably associated with the observed QPOs from soft gamma-ray repeaters. The estimated energy flux carried by relativistic Alfven waves is in good agreement with the observed values, E = 10–10 erg s−1, of giant flares.

Inertial effects in the Earth’s crust and magnetosphere

A review of studies devoted to the problem of exciting magnetic signals in the crust associated with the formation of the major rupture in an earthquake source and with the propagation of seismic waves was given in [Sgrigna et al., 2004]. However, this review contains incorrect citations from original papers and several erroneous statements concerning inertial and inductive mechanisms of conversion of the energy of rock motion into magnetic field energy. These mistakes are analyzed in the present paper. The formal and physical similarity between seismomagnetic waves in the crust and Alfven waves in the magnetosphere is used in the analysis. A comparative analysis of the inertial and inductive mechanisms of seismomagnetic field generation is performed. The Cherenkov criterion of Alfven wave generation due to the ionospheric effect of acoustic waves from earthquakes and explosions is derived. Attention is also given to nonlinear phenomena (nonlinearity of a mechanomagnetic conversion in the crust and anharmonicity and self-focusing of Alfven waves in the magnetosphere).

Fluid versus simulation model of solitons in solar wind: Application to Ulysses observations

This paper continues recent efforts based on Hall‐MHD theory (Sauer et al., 2007) to explain a new class of magnetically compressive solitary structures in the interplanetary space, observed by the Ulysses magnetometer (Rees et al., 2006). The theoretical basis is extended by a kinetic approach via one‐dimensional hybrid code simulations which reveal deficiencies of fluid theory in describing slow mode‐type solitons in a collisionless finite β plasma. Kinetic solitary structures, resembling obliquely propagating Alfven wave pulses with quasi‐circular or banana‐type polarization are presented which may reproduce most of the observational features. This suggests that the soliton picture provides an adequate theoretical concept for the observed events.

Parametric decays, stabilization effects, and electrostatic instabilities due to finite-amplitude Alfvén waves in a fast solar wind-like plasma

[1] The effects of a finite-amplitude Alfven wave in a plasma system with a proton and an alpha-particle beam, are studied. The system is described by a fourteenth-order polynomial providing, thereby, many decay channels for the finite-amplitude Alfven wave. Apart from the parametric decays, the finite-amplitude wave affects very drastically the stability properties of the linear system. It is shown that the large-amplitude waves can either stabilize or destabilize further linear magnetosonic and ion-cyclotron instabilities like in the case of one ion beam. However, as opposed to the case of one ion beam, for large enough amplitude of the finite-amplitude wave, the effect on the linear system is always stabilizing. The finite-amplitude waves can also trigger nonlinear purely electrostatic ion-acoustic-like instabilities. However, in the present case, even forward propagating Alfven waves can destabilize electrostatic waves supported by the beams, something that does not happen when there is only one beam. Thus the saturation mechanism and the existence of nonlinear electrostatic waves in the presence of a finite-amplitude wave is preserved in a two-ion-beam system. We believe that the present study reinforces previous results, involving only one ion-beam, relative to unresolved issues in the fast solar wind.

Correlation of longitudinal currents with Alfvén wave generation in the solar atmosphere

Current instability of Alfven waves in coronal loops is considered. Such a mechanism of Alfven wave generation is shown to be very efficient, and it can give rise to low-frequency perturbations even in the case of very weak currents, thus fostering the reconnection of magnetic fields and development of a flare. The low-frequency turbulence which arises in the process can be favorable to the heating and acceleration of particles.

Transverse Wave Propagation in Relativistic Two-Fluid Plasmas around Reissner–Nordström Black Hole

We investigate transverse electromagnetic waves propagating in a plasma influenced by the gravitational field of the Reissner–Nordstrom black hole. Applying 3+1 spacetime split we reformulate 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. Using a local approximation we 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.

Stability Properties of Strongly Magnetized Spine‐Sheath Relativistic Jets

We derive linearized relativistic magnetohydrodynamic (RMHD) equations describing a uniform axially magnetized cylindrical relativistic jet spine embedded in a uniform axially magnetized relativistically moving sheath. The displacement current is retained in the equations, so that effects associated with Alfven wave propagation near light speed can be studied. A dispersion relation for the normal modes is obtained. Analytical solutions for the normal modes in the low- and high-frequency limits are found, and a general stability condition is determined. A trans-Alfvenic and even a super-Alfvenic relativistic jet spine can be stable to velocity shear-driven Kelvin-Helmholtz modes. The resonance condition for maximum growth of the normal modes is obtained in the kinetically and magnetically dominated regimes. Numerical solution of the dispersion relation verifies the analytical solutions and is used to study the regime of high sound and Alfven speeds.

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.

Stability of a system with cosmic rays and waves

Aims. We investigate the the stability of a cosmic-ray plasma system using the hydrodynamic approach. Methods. The system we are considering comprises cosmic rays and two oppositely propagating Alfven waves. The thermal plasma flow is treated as a background, such that the feedback of the cosmic rays and waves is neglected. Steady-state solutions exist for the system, and when two waves are present, the steady state is non-uniform. Linear stability analysis on the steady-state solutions shows that for short wavelengths, large cosmic ray energy density, and small wave energy density the system is stable. Results. We present in detail the case of uniform background flow. The system is most unstable in the upstream region and becomes stable in the downstream region in the linear regime for short-enough wavelengths. We also speculate on the implication of the instability.

Ion Pickup by Finite Amplitude Parallel Propagating Alfvén Waves

Two scenarios of possible ion heating due to finite amplitude parallel propagating Alfven waves in the solar atmosphere are investigated using a one-dimensional test particle approach. (1) A finite amplitude Alfven wave is instantly introduced into a plasma (or equivalently, new ions are instantly created). (2) New ions are constantly created. In both scenarios, ions will be picked up by the Alfven wave. In case 1, the wave scatters ions in the transverse direction leading to a randomization (or heating) process. This process is complete when a phase shift of ±π in the ion gyrospeed is produced between particles with characteristic parallel thermal speed and particles with zero parallel speed. This corresponds to t = π/kvth (k is the wavenumber, and vth is the ion thermal speed). A ring velocity distribution can be produced for a large wave amplitude. The process yields a mass-proportional heating in the transverse direction, a temperature anisotropy, and a bulk flow along the background magnetic field. In case 2, continuous ion creation represents a continuing phase shift in the ion gyrospeed leading to heating. New particles are picked up by the Alfven wave within one ion gyroperiod. It is speculated that the mechanism may operate in the chromosphere and active regions where transient events may generate finite amplitude Alfven waves.

Cutoff‐free Propagation of Torsional Alfven Waves along Thin Magnetic Flux Tubes

Propagation of torsional Alfv\'en waves along magnetic flux tubes has been extensively studied for many years but no conclusive results regarding the existence of a cutoff frequency for these waves have been obtained. The main purpose of this paper is to derive new wave equations that describe the propagation of linear torsional Alfv\'en waves along thin and isothermal magnetic flux tubes, and use these wave equations to demonstrate that the torsional wave propagation is not affected by any cutoff frequency. It is also shown that this cutoff-free propagation is independent of different choices of the coordinate systems and wave variables adopted in the previous studies. A brief discussion of implications of this cutoff-free propagation of torsional tube waves on theories of wave heating of the solar and stellar atmospheres is also given.

Electromagnetic global gyrokinetic simulation of shear Alfven wave dynamics in tokamak plasmas

Electromagnetic gyrokinetic simulation in toroidal geometry is developed based on a fluid-kinetic hybrid electron model. The Alfven wave propagation in a fully global gyrokinetic particle simulation is investigated. In the long-wavelength magnetohydrodynamic limit, shear Alfven wave oscillations, continuum damping, and the appearance of the frequency gap in toroidal geometries are demonstrated. Wave propagation across the magnetic field (kinetic Alfven wave) is examined by comparing the simulation results with the theoretical dispersion relation. Furthermore, finite-beta stabilization of the ion temperature gradient mode and the onset of the kinetic ballooning mode are demonstrated.

Heating of ions by low-frequency Alfven waves

This paper reports a new physical mechanism that enables the heating of ions by a low-frequency parallel propagating Alfven wave of finite amplitude in a low beta plasma. The heating does not rely on ion cyclotron resonance. The process has two stages: First, ions, whose initial average velocity is zero, are picked up in the transverse direction by the Alfven wave and obtain an average transverse velocity. Second, at a given location the parallel thermal motions of ions produce phase differences (randomization) between ions leading to the heating of ions. The randomization (or heating) process saturates when phase differences are sufficiently large. The time scale over which ions are significantly heated is ∼π∕(kvth) (vth is the initial ion thermal speed and k is the wave number). The heating is dominant in the perpendicular (to the background magnetic field) direction. Subsequently, a large ion temperature anisotropy is produced. During the heating process, ions are also accelerated in the parallel direction and obtain a bulk flow speed along the background magnetic field.

Oblique propagation and dissipation of Alfvén waves in coronal holes

We investigate the effect of viscosity and magnetic diffusivity on the oblique propagation and dissipation of Alfven waves with respect to the normal outward direction, making use of MHD equations, density, temperature and magnetic field structure in coronal holes and underlying magnetic funnels. We find reduction in the damping length scale, group velocity and energy flux density as the propagation angle of Alfven waves increases inside the coronal holes. For any propagation angle, the energy flux density and damping length scale also show a decrement in the source region of the solar wind ( 1.21 R⊙), for any propagation angle, the energy flux density peaks match with the peaks of MgX 609.78 A and 624.78 A linewidths observed from the Coronal Diagnostic Spectrometer (CDS) on SOHO and the non-thermal velocity derived from these observations, justify the observed spectroscopic signature of the Alfven wave dissipation.

Magnetohydrodynamic Simulations of Solar Chromospheric Dynamics Using a Complete Electrical Conductivity Tensor

A 1.5-dimensional, time-dependent magnetohydrodynamic (MHD) model that includes an energy equation and anisotropic electrical conductivity tensor for a variably ionized, multispecies plasma is presented. The model includes an algorithm that reduces the numerical dissipation rate far below the dissipation rate determined by the conductivity tensor. This is necessary for accurate calculation of resistive heating rates. The model is used to simulate the propagation of Alfven waves launched near the base of the middle chromosphere. The background state is the FAL CM equilibrium with a vertical magnetic field. The initial magnetic energy of a wave is almost completely damped out in the chromosphere by the time the disturbance propagates a distance of one wavelength. The energy is converted mainly into thermal energy. The remainder is converted into bulk flow kinetic energy and a Poynting flux with nonzero divergence. The thermal energy is generated almost entirely by Pedersen current dissipation. The corresponding heating rates are close to the FAL CM values near the base of the middle chromosphere. Dynamo action is observed. The damping of a continuously driven Alfven wave train is also simulated, yielding results similar to those of the single wave cases. It is the strong magnetization and weak ionization of the chromosphere that allows for strong heating by Pedersen current dissipation. This distinguishes the chromosphere from the weakly magnetized and weakly ionized photosphere, and the strongly magnetized and strongly ionized corona where Pedersen current dissipation is not significant on the length and timescales simulated.

A magnetic communication scenario for hot Jupiters

The observations of enhanced chromospheric activity on HD 179949 as well as on ν And with the same periods as their close-in planets seem to indicate some kind of magnetic interaction between star and planet. A constraint to any possible model are the large phase angles of 60° and 169°, respectively. We present a simple model, which is based on the propagation of Alfven waves within the stellar wind flow relative to the planet and which can meet this restriction. In the solar system such a model is successfully used to explain the current system between lo and Jupiter.

Parallel Proton Heating in Solar Wind Using Generalized (r, q) Distribution Function

Kinetic theory is used to calculate the power dissipated by obliquely propagating Alfven waves to heat the solar wind protons, using the Generalized (r, q) distribution function. The evolution of power dissipation of protons with increasing heliocentric distance is subsequently determined. Comparison between theoretical and observational results with data shows good agreement, especially for the slow solar wind streams. Previous results where a Maxwellian distribution function was used to calculate the power dissipated did not match well with observations.