A one-dimensional gasdynamical model of magnetospheric convection

The plasma flow in the equatorial plane of the magnetosphere is examined within the framework of a one-dimensional model in which all quantities are supposed to depend only on the distance along the Sun-Earth axis. The following models are considered: (1) the gasdynamical model in which the Ampère force is ignored, (2) the magnetohydrodynamical model in which the normal component of the Ampère force on the magnetopause is taken into account. The flow regime is calculated in the region including two regions: (1) the layer of the return flow where flow velocity is directed from the Sun, (2) the region of convection where the velocity is directed toward the Sun - on the assumption that the form of the magnetopause and the distribution of the solar wind pressure on the magnetopause are known.The following physical mechanisms are taken into account: (1) the appearance of a centrifugal force owing to the magnetopause curvature, the centrifugal force partly compensating for the solar wind pressure; (2) the existence of the critical point which is analogous to the point of transition through the local sound velocity in the Laval nozzle or in the Parker model of the solar corona. The thickness of the layer of the return flow and the velocity of convection in the magnetosphere are calculated; and the following peculiarities are found: (1) in the gasdynamical model the convection regime is only possible with high velocities corresponding to the substorm, (2) in the magnetohydrodynamic model the convection velocity and the thickness of the layer of the return flow are reduced; the reduction being connected to the fact that the pressure of the solar wind is partially compensated for by the jump of the magnetic pressure on the magnetopause.     

Solar-wind flow past a cometary ionosphere

An axisymmetric gasdynamical model of an interaction between the solar wind and comet atmospheres is formulated. Photoionization of the cometary neutral particles as well as their resonance charge-exchange are taken into account to determine the flow in the distured region containing three discontinuity surfaces (two shocks and the contact discontinuity).It is shown that these two processes are significant for many comets. The problem so formulated is solved numerically. A dependence of the obtained solutions on comets gas production, charge-exchange cross-section, photoionization time scale and other parameters is studied. The numerical solutions are in good agreement with the experimental data obtained during the spacecraft missions to the comet Halley in March 1986.     

Beam propagation and Langmuir wave generation in a plasma with κ distribution function

Using a kappa velocity distribution function for the electrons of the background plasma, the dynamics of a beam of hot electrons streaming through the plasma and the generation of Langmuir waves are investigated in the frame work of quasilinear theory. It is shown that the Langmuir waves are strongly damped by high energy tail of the Kappa distribution function. The spatial expansion of the beam is reduced and the spectral density of Langmuir waves becomes narrower. The height of the plateau in the beam distribution function increases at small velocities and the average velocity of beam is larger than that of a Maxwellian distribution. The influence of Kappa velocity distribution function on the gasdynamical parameters is investigated. It is found that, the height of plateau in the beam distribution function, and its lower velocity boundary are enhanced while, the local beam width in velocity space decreases.     

Thermal stresses in cometary nuclei

Assuming cometary nuclei composed of weakly attached cometesimals, thermal stresses due to the temperature differences between the surface and the core are calculated. Both homogeneous icy bodies and cometesimals with material inhomogeneities are considered. It is shown that spherical inclusions in water ice cause strong stresses. Even if viscoelastic effects are taken into account the stresses in the superficial regions exceed the strength of water ice and therefore cause cracks to form. The consequences of this for such irregular cometary activity as splitting and outbursts are discussed.     

An acceleration mechanism for loop transients in the outer corona

The heliocentrifugal motion of coronal loop transients is likely driven largely by the buoyant force exerted by the ambient medium. In the outer corona where the solar wind is well formed, the buoyant force results mainly from the rapid outward decrease in the ambient pressure of the solar wind. The contribution from magnetic buoyancy is not so significant as in the vicinity of the solar surface. Therefore, the pertinent features of the loop transients in the outer corona are basically gasdynamical. As a conspicuous part of coronal expansion, the motion of the compressible masses in the transient loops is largely controlled by thermal forces. The translational motion of heliocentrifugal expansion is driven by the hydrodynamic buoyant force, and the lateral motion of peripheral expansion is driven by the pressure difference between the dense plasma of the ejecta and the tenuous plasma of the ambient medium.     

Physics and Gasdynamics of the Heliospheric Interface

During 30 years, a big theoretical effort to understand the physical processes in the heliospheric interface has followed the pioneer papers by Parker (1961) and Baranov et al. (1971). The heliospheric interface is a shell formed by the solar wind interaction with the ionized component of the circumsolar local interstellar medium (LISM). For fully ionized supersonic interstellar plasma two-shocks (the termination shock and the bow shock) and a contact discontinuity (the heliopause) are formed in the solar wind/LISM interaction. However, LISM consists of at least of three components additional to plasma: H-atoms, galactic cosmic rays and magnetic field. The interstellar atoms that penetrate into the solar wind, are ionized there and form pickup ions. A part of the pickup ions is accelerated to high energies of anomalous cosmic rays (ACRs). ACRs may modify the plasma flow upstream the termination shock and in the heliosheath. In this short review I summarize current understanding of the physical and gasdynamical processes in the heliospheric interface, outline unresolved problems and future perspectives. This revised version was published online in July 2006 with corrections to the Cover Date.     

CALCULATION OF VARIABLE DRAG AND HEAT-TRANSFER COEFFICIENTS IN METEORIC PHYSICS EQUATIONS

The conservation of the ablation parameter is one of the main assumptions of the classical physical theory of meteors (PTM). But this value does vary dramatically on meteoroids trajectories for the most part of them. The drag and heat-transfer coefficients in the basic equations of PTM are considered from the gasdynamical point of view and mathematically modeled in the present paper. Analytical approximations of the drag and heat transfer coefficients valid for any flow regimes and separate approximation of the drag coefficient for free-molecule regime are offered and discussed. They use the Reynolds number as the main parameter characterizing flow regimes. The variations of coefficients result in the variation of the ablation parameter by 100% for meteoroids, passing all the flow regimes. The importance of correct calculation of the coefficients for meteoroids motion modeling was demonstrated by the numerical examples.     

The Splitting of Comet Halley

In combination with the authors' previous observation about the splitting of Comet Halley in March 1986,the events involving the sharp,straight feature in the antisolar direction observed in the head of Cornet halley in 1910 (such as those occurring on May 14,25 and 31,and June 2) are redis-cussed.The analysis leads to the following scenario;When comet Halley explodes and splits,a fragment jettisoned or thrown off from the nucleus will,after moving in the direction of its tail,develop into a mini-comet.Although not weel developed or permanent,it has its own plasma tail and,sometimes,a dust tail.If Bobrovnikoff's definition of a secondary nucleus is assumed,then the fragment should be considered as a real secondary nucleus.It seems that the current idea of a tailward jet suggested by Sekanina and Larson is a wrong explanation for the plasma tail of a mini-comet and hence the rotation period of 52-53h for Comet Halley is doubtful.     

Bipolar outflows and the evolution of stars

Hypersonic bipolar outflows are a ubiquitous phenomena associated with both young and highly evolved stars. Observations of Planetary Nebulae, the nebulae surrounding Luminous Blue Variables such as η Carinae, Wolf Rayet bubbles, the circumstellar environment of SN 1987A and Young Stellar Objects all reveal high velocity outflows with a wide range of shapes. In this paper the current state of our theoretical understanding of these outflows is reviewed. Beginning with Planetary Nebulae considerable progress has been made in understanding bipolar outflows as the result of stellar winds interacting with the circumstellar environment. In what has been called the “Generalized Wind Blown Bubble” (GWBB) scenario, a fast tenuous wind from the central star expands into a ambient medium with an aspherical (toroidal) density distribution. Inertial gradients due to the gaseous torus quickly lead to an expanding prolate or bipolar shell of swept-up gas bounded by strong shock waves. Numerical simulations of the GWBB scenario show a surprisingly rich variety of gasdynamical behavior, allowing models to recover many of the observed properties of stellar bipolar outflows including the development of collimated supersonic jets. In this paper the physics behind the GWBB scenario is reviewed in detail and its strengths and weakness are considered. Alternative models involving MHD processes are also examined. Applications of these models to each of the principle classes of stellar bipolar outflow (YSO, PNe, LBV, SN87A) are then reviewed. Outstanding issues in the study of bipolar outflows are considered as are those questions which arise when the outflows are viewed as a single class of phenomena occurring across the HR diagram.     

Dynamics of cosmic current layers with localized lower-hybrid-drift turbulence

A two-dimensional magnetohydrodynamic model of the dynamics of tail-like current layers caused by anomalous electrical resistivity in a plasma with lower-hybrid-drift (LHD) turbulence is considered. Additionally to the LHD-resistivity, a resistivity pulse in the magnetic neutral sheet is given initiating a magnetic reconnection process. Then the temporal and spatial evolution of the magnetic and electric fields, the plasma convection and the anomalous resistivity are obtained numerically. Taking into account more exact expressions for the LHD-resistivity in the current layer as done in former works, the LHD-turbulence is found to be excited farther from the neutral sheet, and thus, with the time, secondary current sheets are obtained in the plasma-magnetic field system. It is shown that the inductive electric field moving from the magnetic neutral sheet to the current layer periphery during the reconnection process may be considered as indicator of the plasma disturbances.     

Acceleration of particles by electron plasma waves in a moderate magnetic field

A general scheme is established to examine any magnetohydrodynamic (MHD) configuration for its acceleration potential including the effects of various types of plasma waves. The analysis is restricted to plasma waves in a magnetic field with electron cyclotron frequency less than, but comparable to, the electron plasma frequency (moderate field). The general role of electron plasma waves is examined in this paper independent of a specific MHD configuration or generating mechanism in the weak turbulence limit. The evolution of arbitrary wave spectra in a non-relativistic plasma is examined, and it is shown that the nonlinear, process of induced scattering on the polarization clouds of ions leads to the collapse of the waves to an almost one-dimensional spectrum directed along the magnetic field. The subsequent acceleration of non-relativistic and relativistic particles is considered. It is shown for non-relativistic particles that when the wave distribution has a negative slope the acceleration is retarded for lower velocities and enhanced for higher velocities compared to acceleration by an isotropic distribution of electron plasma waves in a magnetic field. This change in behavior is expected to affect the development of wave spectra and the subsequent acceleration spectrum.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Loss-Cone Instability and Formation of Zebra Patterns in Type IV Solar Radio Bursts

The loss-cone instability of energetic electrons at double plasma resonance is considered. Conditions required for the formation of a zebra pattern in type IV solar radio bursts are determined. It is shown that electrons with a power-law energetic spectrum can effectively excite upper-hybrid waves at double plasma resonance. Stripes of a zebra pattern become more pronounced with an increase of the loss-cone opening angle and the power-law spectral index. The growth rate at the resonance frequencies decreases with an increase of the cyclotron harmonic number. Interpretation of observations and diagnostics of plasma for the April 21, 2002, event are performed. Conclusions about the impulsive mode of injection of energetic electrons into a coronal arc are made.     

A theory of the origin of the split pair burst emission from the solar corona

The radio emissions caused by electron streams in a non-isothermal plasma are studied quantitatively. It is proposed that conversion of the stream-excited plasma waves into electromagnetic waves by scattering on the thermal fluctuations at nonisothermal sonic oscillation frequency is the origin of the emission of the split-pair burst near the plasma frequency. The occurrence of the split-pair bursts near the second harmonic of the plasma frequency can be due to combination scattering of the stream-excited plasma waves by electron density fluctuations which are produced by the scattered plasma waves. With a streamer model in which the electron densities are two times those in Newkirk's model, both the observed frequency splitting and the rate of drift of the split pair can be explained as the result of plasma radiation caused by a stream of 10 keV electrons. A tentative model for the split-pair emission is suggested.     

First phase acceleration mechanisms and implications for hard X-ray burst models in solar flares

Requirements for the number of nonthermal electrons which must be accelerated in the impulsive phase of a flare are reviewed. These are uncertain by two orders of magnitude depending on whether hard X-rays above 25 keV are produced primarily by hot thermal electrons which contain a small fraction of the flare energy or by nonthermal streaming electrons which contain > 50% of the flare energy. Possible acceleration mechanisms are considered to see to what extent either X-ray production scenario can be considered viable. Direct electric field acceleration is shown to involve significant heating. In addition, candidate primary energy release mechanisms to convert stored magnetic energy into flare energy, steady reconnection and the tearing mode instability, transfer at least half of the stored energy into heat and most of the remaining energy to ions. Acceleration by electron plasma waves requires that the waves be driven to large amplitude by electrons with large streaming velocities or by anisotropic ion-acoustic waves which also require streaming electrons for their production. These in turn can only come from direct electric field acceleration since it is shown that ion-acoustic waves excited by the primary current cannot amplify electron plasma waves. Thus, wave acceleration is subject to the same limitations as direct electric field acceleration. It is concluded that at most 0.1% of the flare energy can be deposited into nonthermal streaming electrons with the energy conversion mechanisms as they have been proposed and known acceleration mechanisms. Thus, hard X-ray production above 10 keV primarily by hot thermal electrons is the only choice compatible with models for the primary energy release as they presently exist.     

Influence of electron-positron pair production on a pulsar magnetospheric structure

A self-consistent pulsar magnetospheric model with electron-positron pair production is considered. Unlike conventional models, the primary particles (electrons) are accelerated towards the neutron star and their curvature radiation towards a star generates electron-positron plasma near the neutron star. Inside an outflow channel, the generated plasma flows away from the pulsar magnetosphere. A part of the plasma electrons returns and, being accelerated towards the star, regenerate the plasma by their curvature radiation. It is shown that plasma production near the star causes an appearance of positron and electron equatorial belts. The plasma concentration and the flux of the returning electrons are estimated. The portion of the energy entering into the pulsar magnetosphere and its dependence on pulsar parameters are estimated.     

Viscous damping of surface magnetohydrodynamic waves on magnetic interface in cold plasmas

Surface magnetohydrodynamic wave propagation on a magnetic interface in a cold plasma is studied. The anisotropic ion viscosity is taken into account. Only long waves damping weakly in a wave period are considered. The dispersion equation is obtained. This equation is shown always to have exactly one root if there is no viscosity. The dependences of phase velocity, penetration depth and damping decrement of waves on the parameters of undisturbed plasma and wave propagation direction are investigated. The resulting application for describing of surface wave damping in the solar corona is discussed.     

Vorontsov-Vel’yaminov’s rows in giant spiral galaxies: Geometrical properties and physical interpretation

A phenomenon discovered by Vorontsov-Vel’yaminov — straight segments in the spiral structure of giant galaxies — is studied. The main geometrical and physical characteristics of these structures are demonstrated using the example of the galaxies M101 and especially M51. It is shown that rows alternate with regular spiral arms over almost their entire extent in M101, M51, and a number of other galaxies. The relationship between rows and straight warm dust lanes at the inner edges of spiral arms is traced. Assuming these dust lanes to be gas layers compressed by a spiral shock wave, the possibility that rows are formed in local segments of the spiral shock front that are being flattened is discussed. New observational facts about rows support the quantitative consequences of this gasdynamical picture. Translated from Astrofizika, Vol. 41. No. 4, pp. 609–622, October–December, 1998.     

Effect of Induced Toroidal Rotation on Poloidal Rotation and Ion Heat Conductivity of Tokamak Edge Plasmas

The transport processes in edge (collisional) plasmas of tokamaks with smooth profiles of macroscopic plasma parameters and induced poloidal and toroidal plasma flows, are considered. The toroidal and poloidal velocities of particles, the radial electric field and the ion heat flux are derived. It is shown that forces, induced by radio frequency waves, plasma turbulence or neutral beam injection, can be used to control the poloidal and toroidal plasma velocities, as well as ion heat conductivity, in a wide range of these values.     

Numerical Treatment of Radiative Transfer

We present a numerical treatment of radiative transfer in three dimensions. The radiation is modeled by the grey moment M ₁ system. The introduced scheme is able to compute accurate numerical solutions over a broad class of regimes from the transport to the diffusive limit. We discuss numerical issues concerning the resolution and the parallelization of this scheme for multi-dimensional simulations. Several numerical results are then presented, which show that this approach is robust and have the correct behavior in both the diffusive and free-streaming limits. We also present a comparison in two dimensions of our code with a Monte-Carlo transfer code.     

Collective neutrino-plasma interactions

Nonlinear processes describing the interaction of neutrinos with collective plasma oscillations and the excitation of plasma turbulence by a large neutrino flux is discussed. The excitation considered is the inverse processes of neutrino emission by plasma waves first considered by Tsytovich (V.N. Tsytovich, Soviet Fiz. Dokl. 9 (1965) 1114). The process is similar to a beam plasma instability considered as inverse Landau damping in which the usual electromagnetic interactions are important. In the neutrino beam relaxation the weak interaction can play a similar role. We emphasize here the possibility of another process namely the interaction of an intense neutrino flux with a strongly turbulent plasma. The turbulence can also be assumed to be due to the shock produced at the early stages of a type II supernova (SN) explosion. The scattering of the neutrinos in the turbulent plasma is shown to be sufficient for transferring momentum and energy from the neutrino flux to the plasma causing the shock to continue moving outward and eventually creating the blow-off of the mantle of the star producing type II SN.