The linear and quasilinear theory of perturbations in finite-β (β is the ratio of plasma pressure to magnetic energy density), collisionless plasmas, that have sheared (velocity) flows, is developed. A simple, one-dimensional magnetic field geometry is assumed to adequately represent solar wind conditions near the sun (i.e., at R ≃ 0.3 AU). Two modes are examined in detail: an ion-acoustic mode (finite-β stabilized) and a compressional Alfven mode (finite-β threshold, high-β stabilization). The role played by equilibrium temperature anisotropies, in the linear stability of these modes, is also presented. From the quasilinear theory, two results are obtained. First, the feedback of these waves on the state of the wind is such as to heat (cool) the ions in the direction perpendicular (parallel) to the equilibrium magnetic field. The opposite effect is found for the electrons. This is in qualitative agreement with the observed anisotropies of ions and electrons, in fast solar wind streams. Second, these quasilinear temperature changes are shown to result in a quasilinear growth rate that is lower than the linear growth rate, suggesting saturation of these instabilities.

We have investigated Kelvin-Helmholtz (K-H) instabilities for a homogeneous compressible plasma containing a uniform magnetic field and a linear velocity shear. A derivation of the relevant K-H dispersion equation and details regarding method of solution are given elsewhere (submitted to Mon. Not. R. Astr. Soc.). We present here an outline of our results.

Mechanisms of Alfvén wave heating in space-astrophysical plasmas are presented with particular emphasis on the parallel electric field generated in the magnetohydrodynamic perturbations due to the finite Larmor radius effects.

In a magnetohydrodynamic description of a plasma, the shear Alfvén wave is characterized by a continuous spectrum (the MHD continuum) which results from a singularity at ω ≃ k||(r)VA(r), where ω is the frequency, k|| is the wave number along the direction of the magnetic field and VA = B0/(4ρπ)1/2 is the Alfvén speed, and r is the direction of inhomo-geniety. The associated electromagnetic fields become large at this resonance layer [in fact, in ideal MHD they become singular], and if some dissipation is allowed, the transfer of energy from the waves to the electrons can take place. These continuum modes are localized in space, and can be effective in heating plasmas in narrow regions around the resonanct surface. Recently, 1-3 a new class of nonsingular global eigenmodes of the plasma have been discovered which are wide spread in the plasma, and could effectively transfer energy to the bulk of the plasma. These modes arise because of a strong coupling between the shear and the evanascent compressional mode. The coupling is provided by gradients of density, equilibrium current and nonideal effects like finite ω/ωci, where ωci is the ion cyclotron frequency. In order to understand the effectiveness of these modes to heat plasmas, i.e., to determine the effective impedance, the energy deposition profiles, parametric dependence of the heating efficiency, etc., a kinetic theory with the electron parallel dynamics is needed. We have developed a detailed kinetic theory to study the structure of the continuum as well as global eigenmodes in an inhomogenous current carrying plasma. The global modes are comparatively much less damped than the continuum modes (which become discrete in the kinetic theory) and hence are easier to excite. The implication, of course, is that the large part of energy transfer in a Alfvén Wave experiment would be mediated through these global modes. This theoretical conjecture is already confirmed in laboratory experiments on PRETEXT Tokamak.

I will present the basic theoretical analysis as well as discuss the implications of these findings.

Morphologies, energetics and nonthermal radiation emission of extragalactic radio sources can be explained in the framework of models based on the physics of supersonic jets interacting with the surrounding intergalactic medium. In this review current physical interpretation of acceleration, collimation, modulation and nonthermal radiation of these jets are discussed. We begin with an analysis of the equilibrium configurations of collimated flows; the propagation outside the galactic nuclei is shown to be modulated by fluid, MHD and resistive instabilities, which, at the same time, support charged particle acceleration by turbulent stochastic processes, and consequently nonthermal radiation emission from the radio to the X-ray range. Large scale morphologies (knots, wiggles and bendings) are considered from the point of view of their interpretation as global perturbations of the flow produced by fluid instabilities.

This work follows on the papers by Henriksen, Bridle and Chan (1982) (HB) and by Eilek and Henriksen (1983). These papers introduced a comprehensive model of (hydrodynamic) turbulence driven Alfven wave, resonant, particle acceleration. Very similar ideas were introduced independently by Fedorenko (1980), but the details were sketchy and the universality of the resulting spectral index was not realized. Eichler (1979) has come closest to these ideas previously (again independently) in a very original paper directed at ion acceleration in solar flares. He includes a back reaction on the turbulent eddies however which requires an adhoc treatment and is probably unnecessary (we replace this by the Lighthill noise analysis: HBC) and he omits synchrotron and adiabatic losses. Moreover, the quasi-linear theory is not used in calculating the distribution function. Nevertheless, it is reassuring that the same intuitive physics, even to the self-similar universality, should arise independently.

A striking feature of extragalactic radio jets is that they are so narrow. Theories of collimation should account for opening angles of order 1° or less (as opposed to those of laboratory jets which become turbulent and typically open up to more than 10°). This makes instabilities particularly troublesome for theories of collimation in which they are present. While instabilities might not entirely destroy the general bipolar nature of the flow, they decollimate it by definition.

The structure of the magnetic field in radio jets is a topic of recent interest, especially due to the possibility that some high pressure jets are confined by a magnetic pinch. Several such jets have been found which cannot be confined by external cluster gas pressure, on which there are observational limits; nor can they be in free expansion, since they do not show evidence of adiabatic expansion losses. Recent radio interferometer observations of surface brightness and polarization allow the possibility of determining the magnetic field structure. In this paper I present some basic considerations of the current and field structure required if the observed jets are to be magnetically confined.

Calculations of the minimum pressure in the M87 jet show that the jet pressure is an order of magnitude in excess of the thermal pressure in the interstellar medium calculated from the X-ray emission. The continuous energy input required to produce the jet emission is incompatible with a freely expanding jet. On the other hand, the uniform jet expansion, the jet emission and the jet polarization can be explained if the jet is self-confined by j x B forces.

Accretion disk electrodynamic phenomenae are separable into two classes: 1) disks and coronae with turbulent magnetic fields; 2) disks and black holes which are connected to a large-scale external magnetic field. Turbulent fields may originate in an α - ω dynamo, provide anomalous viscous transport, and sustain an active corona by magnetic buoyancy. The large-scale field can extract energy and angular momentum from the disk and black hole, and be dynamically configured into a collimated relativistic jet.

Electrodynamic models for the activity of galactic nuclei are shown to be current systems which can be examined in terms of equivalent circuits. The resulting inductive circuit which describes the coupling of the generator (black-hole and accretion disk) to the distant load (jet plasma) is prone to various instabilities. We consider the disruption of this current system and propose that ultra-energetic cosmic rays (E~1019–1021 eV) could be produced during the discharges, which occur at distances of ~1016–1018 cm from the central massive hole (M~108 M). Such discharges will also produce variable γ-ray and X-ray activity and we discuss observations of Cen A in this regard.

The “direct interaction approximation” of Kraichnan as modified by Kadomtsev is employed to develop a two-dimensional strong turbulence theory which predicts both nonlinear frequency broadening and a power law for the spectrum of a convecting plasma containing a gravitationally induced cross field current. These results are favorably compared with experimental observations, numerical simulations, and previous studies1 of turbulent cross field convection of current-carrying plasma.

Differentially rotating accretion disks threaded by a uniform magnetic field have been numerically simulated. Fast reconnection followed by coalescence allows the magnetic field to drive small amplitude radial oscillations in the disk. These oscillations may be observable as the viscous stresses cause the disk to brighten and fade as the disk expands and contracts. Episodes of reconnection may also be observable as hot spots produced locally at the sites of coalescence. Cataclysmic variables, and in particular dwarf novae, provide a natural interpretation for these calculations.

Two ribbon flares on the sun are sometimes preceded by luminous arcade like structures in the corona with fillament activity. The arcades which are associated with the coronal magnetic field are sometimes without these fillaments when they are found to be relatively long lived. The reasons underlying the stability and instability of these structures may be relevant in understanding the basic mechanism of two ribbon flares. Here we present results on the equilibrium structure of coronal arcades and their hydromagnetic stability taking into account the effects of magnetic field line-tying on the photosphere.

We have used a self-consistent magnetohydrodynamics model to study the evolution of solar magnetic fields in an active region. The problem has been cast as an initial boundary-value problem based on explicit mathematical formalism (i.e., method of projected characteristics), whereby a variety of horizontal photospheric motions can be treated. In this paper we deal specifically with photospheric shear motions in the active region. Our results show the evolution of the magnetic energy, the electric current systems, including the distributions of J⊥ (current perpendicular to the magnetic field) J| (current parallel to the magnetic field), the magnetic field configuration and a comparison between the build-up of magnetic energy in a pre-twisted field and in an initial untwisted field due to photospheric shearing motions. From these results we conclude that the energy build-up is confined within a certain region near the neutral line at the photospheric level. The possible location of the particle acceleration also can be studied.

Current sheets are found to be subject to bending waves described by a dispersion relation indicating that these are, essentially, modified surface Alfvén waves. Applications to the observed magnetic polarity sectors in the solar wind and to other astrophysical environments, such as planetary magnetospheres, are suggested.

One of the major discoveries in solar physics over the past decade has been the association of coronal holes with high-speed solar wind streams (Zirker 1977 and references therein). On the other hand, advances in X-ray and radio instrumentation (e.g., Einstein, VLA, VLBI, etc.) in the past few years have allowed detailed observations of collimated outflows from rather more distant objects, such as young stars and active galaxies (Beer 1981, Lada 1982, Ferrari and Pacholczyk 1983 and references therein). The remarkable structural similarities between jets of magnetized gas from our Sun, other active stars, and active galactic nuclei suggest that these phenomena may be manifestations of similar hydrodynamic processes operating on both small and large scales. In this article, we shall use the experience gained by studying the nearest known astrophysical jet - high-speed solar wind streams - to address some of the problems of astrophysical jet acceleration and collimation associated with objects as diverse as SS 433, star-forming molecular clouds and, in particular, jets associated with galaxies and quasars.

It is commonly anticipated that high-energy electrons play an important role for the wave emission in flare bursts. For instance, electrons with >100 KeV are considered to create microwave emissions through gyro-synchrotron process and hard x-rays may be due to bremstrahlung with >25 KeV electrons. However, electron acceleration mechanism itself is still in speculations.

Up until about five years ago all models for hard X-ray bursts consisted of streaming nonthermal electrons interacting with an ambient plasma (Brown 1975). Even in its most efficient form of thick-target emission in which electrons are stopped in the ambient plasma, this type of model is very inefficient because the electrons lose about 105 times more energy in Coulomb collisions with the ambient plasma than in X-rays resulting from bremsstrahlung. As a result, according to the latest estimates, at least 20% of the dissipated flare energy must go into accelerated electrons at the peak of the impulsive phase (Duijveman et al. 1982). Stimulated by observations of hard X-rays with thermal spectra (Crannel et al. 1978; Elcan 1978), analysis of a thermal model in which all the electrons in a given volume are heated to a temperature Te = 108K was begun (Brown et al. 1979; Smith and Lilliequist 1979; Vlahos and Papadopoulos 1979). It was recognized from the beginning that some electrons in the tail of the distribution would escape through the conduction fronts formed and mimic nonthermal streaming electrons. This thermal model with loss of electrons or dissipation became known as the dissipative thermal model (Emslie and Vlahos 1980). If the escaping electrons are not replenished, they will cease to make a contribution after a fraction of a second and the source will become a pure thermal source. It will be shown below that collisional replenishment (Smith and Brown 1980) is too slow.