Nonlinearly coupled electromagnetic ion-cyclotron and magnetosonic waves in astrophysical plasmas

A set of three nonlinearly coupled equations governing the interaction between electromagnetic ion-cyclotron and magnetosonic waves is derived. In appropriate limiting cases, the set yields simplified equations. On the other hand, the full set of equations is used to derive a general dispersion relation for the parametric interaction of electromagnetically modulated ion-cyclotron wave packets. An analytical expression for the growth rate of the electromagnetic modulational instability is presented. The relevance of our investigation to non-thermal electromagnetic fluctuations in astrophysical and cometary plasmas is pointed out.     

Scattering physics

The theory of polarized scattering in a stellar atmosphere is formulated, first within the framework of classical physics, then in terms of quantum mechanics. The expression for the redistribution matrix that describes partial redistribution in polarization and frequency is derived for the general case when the magnetic field is of arbitrary strength. The special cases of weak fields (the Hanle limit) and zero fields (non-magnetic scattering) are discussed. Observational examples of spectral signatures in linear polarization are presented, which show effects of hyperfine structure, interference between fine structure components, and molecular scattering.     

Search for spectral line polarization in the solar vacuum ultraviolet

An instrument designed to record polarization in the region 120–150 nm of the solar spectrum was launched on the satellite Intercosmos-16, July 27, 1976. The aim was to search for resonance-line polarization that is caused by coherent scattering. Oblique reflections at gold- and aluminium-coated mirrors in the instrument were used to analyze the polarization. The average polarization of the L solar limb was found to be less than 1%. It is indicated how future improved VUV polarization measurements may be a diagnostic tool for chromospheric and coronal magnetic fields and for the three-dimensional geometry of the emitting structures.On leave from Lund Observatory, S-22224 Lund, Sweden.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Global wave patterns in the Sun's magnetic field

When the observed pattern of solar magnetic fields is decomposed in its spherical-harmonic components and a time series analysis is performed, a resonant global wave pattern is revealed. The power spectrum indicates modes with discrete frequencies, obeying a strict parity selection rule in the case of the zonal, rotationally-symmetric modes (with spherical-harmonic orderm=0). For instance, the 22 yr resonance that dominates for the anti-symmetric modes (with odd values of the spherical-harmonic degreel) is completely absent for the symmetric modes, which instead exhibit a number of resonances having frequencies increasing withl.A more traditional way of looking at the evolution of the zonal magnetic pattern is in the form of isocontours in latitude-time space (as in the butterfly diagram of sunspots). We show how this pattern can to a good approximation be represented as a superposition of 14 discrete modes, each with a purely sinusoidal time variation, one mode for each value ofl (=1,2, ..., 14). This corresponds to the assumption that the true, fully resolved and noise-free power spectrum consists of -function peaks, one for eachl value.This approach allows us to analyse the roles of the individual discrete modes in generating the well-known features in the traditional btterfly diagrams, e.g., the drift of the sunspot zones towards the equator and the prominence zones towards the poles during the course of the 11 yr cycle. It is shown that these features are accounted for entirely by the odd parity modes with the single, sinusoidal period of 22 yr. The drifts (and thus the arrow of time) are caused by the systematic phase relations between the 22 yr modes. The even modes exhibit an entirely different pattern. Since they have considerably shorter periods, they cause an undulation of the odd-mode contour lines when superposed on the anti-symmetric pattern.The dispersion, amplitude, and phase relations of the discrete modes are given. It is indicated how they can be used in combination with spectral inversion techniques to determine the depth variation of the parameters in the governing global wave equation.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Nonlinear coupling between electromagnetic fields in a strongly magnetized electron-positron plasma

The nonlinear coupling between electromagnetic fields in a strongly magnetized electron-positron plasma is considered. We point out that compressional magnetic field perturbations are excited by the rotational part of the nonlinear current, and derive a new nonlinear system of equations that is basic for studies of modulational instabilities and coherent nonlinear structures in magnetized electron-positron plasmas.     

Parametric instabilities and nucleation of coupled Langmuir and acoustic waves in a weakly-ionized plasma

A pair of coupled equations governing the nonlinear interaction between Langmuir and acoustic waves in partially-ionized plasmas are presented. Three-wave decay interactions, modulatinal instabilities, as well as the nucleation of coupled Langmuir and acoustic waves can then be studied. The results are of relevance to the lower part of the ionosphere, radio-frequency driven gas discharges, as well as the interstellar medium.     

Modifications of the magnetospheric plasma due to ponderomotive forces

By means of two simple examples it is demonstrated that the ponderomotive force induced by the geomagnetic pulsations significantly modifies the plasma distribution in the Earth's magnetosphere. The first example considers the quasi-static plasma equilibrium at closed magnetic shells. Ponderomotive forces, averaged over the geomagnetic pulsation period, increase the plasma density at the equator of the oscillating magnetic shell. If the oscillation amplitude exceeds a certain critical level, the quasi-static solution predicts a maximum of the plasma density at the equator of the magnetic shell. The second example considers the quasi-stationary plasma flow along the open field lines stretching into the geomagnetic tail (the polar wind). The inclusion of the ponderomotive force may shift the critical point for the flow transition through the supersonic barrier closer to the Earth.     

Nonlinear Interactions Between Kinetic Alfvén and Ion-Sound Waves

The resonant interaction between kinetic Alfvén and ion-acoustic waves is considered using the Hall-MHD theory. The results of previous authors are generalized to cover both finite Larmor radius as well as the ideal MHD results. It is found that the three-wave coupling is strongest when the wavelength is comparable to the ion-sound gyroradius. Applications of our work to weak turbulence theories as well as to the heating of the solar corona are pointed out.     

A new low-frequency electrostatic mode in a nonuniform electron-positron plasma

The existence of a novel low-frequency electrostatic mode, which is due to the gravitational acceleration in a nonuniform electron-positron plasma, is pointed out. At large amplitudes, the unstable modes self-organize to form dipolar vortices.     

Gnetic wave instability in a plasma ith relativistic particles in a random magnetic field

A new mechanism, which accounts for the direct excitation of electromagnetic waves by relativistic particles moving in random magnetic fields, is considered. We are then able to study maser-type wave excitations by slightly anisotropic particle distribution functions as well as electromagnetic wave emissions in beam-plasma interactions