Finite Amplitude Alfven Waves Research Articles

Heating and acceleration of ions in nonresonant Alfvénic turbulence

Nonlinear scattering of protons and alpha particles during the dissipation of the finite amplitude, low-frequency Alfvenic turbulence is studied. The process discussed here is not the coherent scattering and acceleration, as those often treated in the past studies, but is an incoherent process in which it is essential that the Alfvenic turbulence has a broadband spectrum. The presence of such an Alfvenic turbulence is widely recognized observationally both in the solar corona and in the solar wind. Numerical results suggest that, although there is no apparent sign of the occurrence of any parametric instabilities, the ions are heated efficiently by the nonlinear Landau damping, i.e., trapping and phase mixing by Alfven wave packets which are generated by beating of finite amplitude Alfven waves. The heating occurs both in the parallel and in the perpendicular directions, and the ion distribution function which is asymmetric with respect to the parallel velocity is produced. Eventual perpendicular energy of ions is much influenced by the spectrum and polarization of the given Alfvenic turbulence since the turbulence initially possess transverse energy as specified by Walen’s relation.

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.

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.

Hydromagnetic discontinuities from the evolution of nonlinear Alvén waves

To study the large‐amplitude Alfvén waves that are frequently observed in the solar wind, the nonlinear equations of electron and ion fluids coupled to Maxwell equations have been solved to obtain the relationships between the fluid density and magnetic field for finite amplitude Alfvén waves in the cold plasma limit. Two regimes of the parameter ωc/Ω (ratio of characteristic frequency to the ion Larmor frequency) are examined. When ωc/Ω = 0, we obtain an exact simple wave solution for the linearly polarized Alfvén wave, and recover the earlier results of [Montgomery, 1959a]. For finite ωc/Ω geometric optics approximation which is valid for large wave numbers yields a new relationship which involves elliptically polarized Alfvén waves. Both of these waves can be shown to evolve into shocks.

Nonlinear Alfven wave propagation in nonhomogeneous collisionless media

view Abstract Citations (4) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Nonlinear Alfven Wave Propagation in Nonhomogeneous Collisionless Media Khabibrakhmanov, Ildar K. ; Summers, Danny Abstract A collisionless model of nonlinear Alfven wave propagation in a nonhomogenous medium is formulated in terms of two amplitude equations for the slow plasma parameters, the only fast parameter being the transverse component of the unit vector along the local magnetic field. The mdoel includes plasma pressure anisotropy and takes into account the relative motion of the plasma components along the magnetic field, effects which have hitherto not been considered in a self-consistent manner. Exact one-dimensional solutions are found under the restriction that the magnetic field, bulk plasma velocity vectors, and propagation direction all remain in the same plane. It is shown that these solutions reduce exactly to the standard Wentzel-Kramer-Brillouin (WKB) approximate solution, i.e., it is shown that the WKB solutions are, in fact, exact under the above stated restriction. The restriction is not related to the limitations of the WKB approximation, but merely specifies the mode of the propagating wave. This exact solution can be used as a zero-order approximation for the investigation of the stability of finite-amplitude Alfven waves. Thermal corrections to the wave amplitude solution are obtained as well as to the bulk plasma momentum equation. The wave amplitude solution shows that direct exchange of the Alfven wave energy with the kinetic energy of the particles is possible. Publication: The Astrophysical Journal Pub Date: May 1995 DOI: 10.1086/175652 Bibcode: 1995ApJ...444..796K Keywords: Collisionless Plasmas; Inhomogeneity; Magnetohydrodynamic Waves; Mathematical Models; Nonlinearity; Plasma Turbulence; Wave Propagation; Anisotropy; Kinetic Energy; Momentum Transfer; Pressure Effects; Stellar Winds; Wentzel-Kramer-Brillouin Method; Astronomy; PLASMAS; TURBULENCE; WAVES full text sources ADS |

Nonlinear Alfv�n wave modulation in the solar atmosphere

Nonlinear interaction of finite-amplitude Alfven waves with non-resonant finite-frequency electrostatic and stationary electromagnetic perturbations is considered. This interaction is governed by a pair of coupled equations consisting of nonlinear Schrodinger equation for the Alfven wave envelope and an equation for the plasma slow response that is driven by the ponderomotive force of the Alfven wave packets. The modulational instability of a constant amplitude Alfven pump is investigated and some new results for the growth rate of the instability are presented. It is found that a possible stationary state of the modulated Alfven wave packets could lead to localized structures. The relevance of our investigation to the solar atmosphere is discussed.

Evolution of polarization in localized nonlinear Alfvén waves.

A polarization change of localized and finite-amplitude Alfven waves is found by use of both MHD theory and numerical simulation. One of the transverse components of the circularly polarized Alfven wave is stable as it propagates, but the other component is unstable under both self-focusing and diffraction effects. Consequently, the wave changes its polarization from circular to linear.

Modulational instability of finite-amplitude Alfv�n waves

It is shown that a recent conclusion of Shivamaggi that ‘the modulational instability of finite amplitude Alfven waves arises when the density cavity travels at subsonic speeds’, is incorrect.