[en] In a number of astrophysical systems, the magnetic field, instead of varying over a scale comparable with the ''natural'' scale of the object (e.g., tens of thousands of kilometers in the case of the solar convective zone), varies over lengths that are orders of magnitude less than this (e.g., over distances down to 100 km in the case of the magnetic filaments detected in the upper part of the solar convective zone and probably present in much deeper layers). Therefore, the study of the propagation of magnetohydrodynamic (MHD) waves in plasmas with fine magnetic nonuniformities is of considerable general importance for astrophysics. We have developed a general formalism that allows one to treat the propagation of large-scale MHD waves in a finely stratified medium. We demonstrate that the presence of a fine structure of the plasma may produce considerable modifications of the modes existing in a uniform plasma, with a number of propagation modes that may even increase. We also show that the slow MHD mode may experience a collisionless damping, which causes the wave energy to be converted into the energy of the peristaltic modes of the plasma ''resonant'' layers. (c) 2000 The American Physical Society

[en] Recently, Barger et al. [Phys. Lett. B 461, 34 (1999)] computed energy losses into Kaluza-Klein modes from astrophysical plasmas in the approximation of zero density for the plasmas. We extend their work by considering the effects of finite density for two plasmon processes. Our results show that, for fixed temperature, the energy loss rate per cm3 is constant up to some critical density and then falls exponentially. This is true for transverse and longitudinal plasmons in both the direct and crossed channels over a wide range of temperature and density. A difficulty in deriving the appropriate covariant interaction energy at finite density and temperature is addressed. We find that, for the cases considered by Barger et al., the zero density approximation and the neglect of other plasmon processes is justified to better than an order of magnitude. (c) 2000 The American Physical Society

[en] Enhanced fluctuations from electromagnetic heat flux instabilities in collisionless plasmas may, through wave-particle scattering, constrain the electron heat flux q_e which flows parallel to the background magnetic field. This manuscript examines the linear Vlasov theory predictions for threshold conditions on the whistler heat flux instability at high β_e, the electron beta, a situation which may have application to both solar wind and astrophysical plasmas. Theory predicts a threshold heat flux which scales approximately as β^-0.9_e. One possible consequence of this instability is that it imposes an upper bound on q_e. We discuss possible applications of such a heat flux limit in the cooling flows of clusters of galaxies with β_e>>1. (c) (c) 2000. The American Astronomical Society

[en] Recent papers discussing advection-dominated accretion flows (ADAF) as a solution for astrophysical accretion problems should be treated with some caution because of their uncertain physical basis. The suggestions underlying ADAF involve ignoring the magnetic field reconnection in heating of the plasma flow, assuming electron heating due only to binary Coulomb collisions with ions. Here we analyze the physical processes in optically thin accretion flows at low accretion rates including the influence of an equipartition random magnetic field and heating of electrons due to magnetic field reconnection. The important role of the magnetic field pointed out by Shvartsman comes about because the magnetic energy density, E_m, increases more rapidly with decreasing distance than the kinetic energy density, E_k (or thermal energy density). Once E_m grows to a value of order E_k, further accretion to smaller distances is possible only if magnetic flux is destroyed by reconnection. For the smaller distances it is likely that there is approximate equipartition, E_m≅E_k. Dissipation of magnetic energy is associated with the destruction of magnetic flux. We discuss reasons for believing that the field annihilation leads to appreciable electron heating. Such heating significantly restricts the applicability of ADAF solutions, and it leads to a radiative efficiency of the flows of ∼25% of the standard accretion disk value. (c) (c) 2000. The American Astronomical Society

[en] The effects of finite plasma pressure and pressure anisotropy, toroidal rotation and gravity on the equilibrium, flow, and stability of plasma in dipolar magnetic configurations are considered. Dipolar equilibria are of interest for magnetic confinement experiments in the laboratory and understanding the physics of magnetospheric and astrophysical plasmas. It is demonstrated that realistic solutions of the appropriate ideal magnetohydrodynamics (MHD) equations can be found in a separable form which drastically simplifies the equations and even allows us to analytically obtain some limiting forms of the nonlinear solutions. The MHD stability of these equilibria is explicitly evaluated in some cases. (c) 2000 American Institute of Physics

[en] We propose a mechanism for the fast dissipation of magnetic fields which is effective in a stratified medium where ion motions can be neglected. In such a medium, the field is frozen into the electrons, and Hall currents prevail. Although Hall currents conserve magnetic energy, in the presence of density gradients they are able to create current sheets which can be sites for efficient dissipation of magnetic fields. We recover the frequency ω_MH for Hall oscillations modified by the presence of density gradients. We show that these oscillations can lead to an exchange of energy between different components of the field. We calculate the time evolution, and show that magnetic fields can dissipate on a time scale of order 1/ω_MH. This mechanism can play an important role in magnetic dissipation in systems with very steep density gradients, where the ions are static such as those found in the solid crust of neutron stars. (c) 2000 The American Physical Society

[en] Thanks to helioseismology, the equation of state of the plasma of the solar interior can be diagnosed. Although the gas in the solar interior is only weakly coupled and weakly degenerate, the great observational accuracy of the helioseismic measurements puts strong constraints on the nonideal part of the equation of state. The helioseismic verification of major nonideal effects in the equation of state of solar matter has become well established. The dominant contribution is the Coulomb pressure, conventionally described in the Debye-Hueckel approximation. However, recently the increased precision of the helioseismic diagnosis has brought significant observational progress beyond the Debye-Hueckel approximation. This is illustrated with the subtle effect of excited states in bound systems, in particular hydrogen. (c) 2000 The American Astronomical Society

[en] The Mihalas-Hummer-Daeppen (MHD) equation of state is a part of the Opacity Project (OP), where it mainly provides ionization equilibria and level populations of a large number of astrophysically relevant species. Its basic concept is the idea of perturbed atomic and ionic states. At high densities, when many-body effects become dominant, the concept of perturbed atoms loses its sense. For that reason, the MHD equation of state was originally restricted to the plasma of stellar envelopes, that is, to relatively moderate densities, which should not exceed ρ<10^-2 g cm-3. However, helioseismological analysis has demonstrated that this restriction is much too conservative. The principal feature of the original Hummer and Mihalas paper is an expression for the destruction probability of a bound state (ground state or excited) of a species (atomic or ionic), linked to the mean electric microfield of the plasma. Hummer and Mihalas assumed, for convenience, a simplified form of the Holtsmark microfield for randomly distributed ions. An improved MHD equation of state (Q-MHD) is introduced. It is based on a more realistic microfield distribution that includes plasma correlations. Comparison with an alternative post-Holtsmark formalism (APEX) is made, and good agreement is shown. There is a clear signature of the choice of the microfield distribution in the adiabatic index γ1, which makes it accessible to present-day helioseismological analysis. However, since these thermodynamic effects of the microfield distribution are quite small, it also follows that the approximations chosen in the original MHD equation of state were reasonable. A particular feature of the original MHD papers was an explicit list of the adopted free energy and its first- and second-order analytical derivatives. The corresponding Q-MHD quantities are given in the Appendix. (c) (c) 1999. The American Astronomical Society

[en] An experiment using a large laser facility to simulate young supernova remnants (SNRs) is discussed. By analogy to the SNR, the laboratory system includes dense matter that explodes, expansion and cooling to produce energetic, flowing plasma, and the production of shock waves in lower-density surrounding matter. The scaling to SNRs in general and to SN1987A in particular is reviewed. The methods and results of x-ray radiography, by which the system in diagnosed, are discussed. The data show that the hohlraum used to provide the energy for explosion does so in two ways--first, through its radiation pulse, and second, through an additional impulse that is attributed to stagnation pressure. Attempts to model these dynamics are discussed. (c) 2000 American Institute of Physics

[en] In this paper we discuss the dynamics of an ion interacting with large-amplitude Alfvacute en waves. The objective of the present analysis is to attain an in-depth understanding of the ion-pickup process which has been extensively studied in the literature by means of both quasilinear theory and numerical simulations. In general, results from self-consistent simulations provide a more complete picture of the ion pickup process, but details of the pickup process are not easily comprehended on the basis of these results. For this reason, the present study is carried out in which a test particle approach is used. It is found that for moderately large-amplitude Alfvacute en waves, an approximate analytical solution for the ion equation of motion can be obtained. This solution clarifies a number of basic issues such as (1) whether the cyclotron resonance is a necessary condition for the pickup to occur, (2) what is the role of initial ion phase space position on subsequent pitch angle scattering, and (3) how the wave amplitude affects the maximum velocity that an ion can gain along the direction of the ambient magnetic field during the pickup process. copyright 1997 American Institute of Physics