Frozen-in Magnetic Field Research Articles

Damping of cosmic magnetic fields

We examine the evolution of magnetic fields in an expanding fluid composed of matter and radiation with particular interest in the evolution of cosmic magnetic fields. We derive the propagation velocities and damping rates for relativistic and non-relativistic fast and slow magnetosonic and Alfv\'en waves in the presence of viscous and heat conducting processes. The analysis covers all magnetohydrodynamics modes in the radiation diffusion and the free-streaming regimes. When our results are applied to the evolution of magnetic fields in the early universe, we find that cosmic magnetic fields are damped from prior to the epoch of neutrino decoupling up to recombination. Similar to the case of sound waves propagating in a demagnetized plasma, fast magnetosonic waves are damped by radiation diffusion on all scales smaller than the radiation diffusion length. The characteristic damping scales are the horizon scale at neutrino decoupling ${(M}_{\ensuremath{\nu}}\ensuremath{\approx}{10}^{\ensuremath{-}4}{M}_{\ensuremath{\bigodot}}$ in baryons) and the Silk mass at recombination ${(M}_{\ensuremath{\gamma}}\ensuremath{\approx}{10}^{13}{M}_{\ensuremath{\bigodot}}$ in baryons). In contrast, the oscillations of slow magnetosonic and Alfv\'en waves get overdamped in the radiation diffusion regime, resulting in frozen-in magnetic field perturbations. Further damping of these perturbations is possible only if before recombination the wave enters a regime in which radiation free-streams on the scale of the perturbation. The maximum damping scale of slow magnetosonic and Alfv\'en modes is always smaller than or equal to the damping scale of fast magnetosonic waves, and depends on the magnetic field strength and its direction relative to the wave vector. Our findings have multifold implications for cosmology. The dissipation of magnetic field energy into heat during the epoch of neutrino decoupling ensures that most magnetic field configurations generated in the very early universe satisfy big bang nucleosynthesis constraints. Further dissipation before recombination constrains models in which primordial magnetic fields give rise to galactic magnetic fields or density perturbations. Finally, the survival of Alfv\'en and slow magnetosonic modes on scales well below the Silk mass may be of significance for the formation of structure on small scales.

A prominence model in a simple magnetic arcade

This paper offers an evolution scenario for a simple magnetic arcade where the frozen-in magnetic field decreases with the ascent of its arches together with the plasma. Uplift is produced by the movement of photospheric plasma with a frozen-in magnetic field, which is divergent with respect to a neutral line. A decrease in magnetic field leads to the appearance in the arcade of a height range of arches, with no high-temperature thermal equilibrium present, and to a variation of the nonequilibrium range with time. Uplift of the arcade is accompanied by the consecutive entry of new arches into this range. All arches entering the nonequilibrium range experience a transient process. Some of the earlier inquiries into the physics of this process were instrumental, in the first place, in identifying those arches which – through the production of an ascending plasma flow from the base of the arcade – are involved in the formation of a prominence (with magnetic dips appearing and evolving at the tops of these arches) and, secondly, in synthesizing a computational algorithm for the final state of the transient process, the quasi-steady-state dynamic structure of the prominence. The arcade evolution scenario, combined with the computational algorithm, constitutes a unified prominence model, a model for the transition from a simple static magnetic arcade to a quasi-steady dynamic prominence structure. The model has been used in numerical calculations of parameters of two classes of prominences: in and outside active regions. Results of the calculations are in good agreement with observations.

Interplanetary Pick-Up Ion Acceleration A Study of Anisotropic Phase-Space Diffusion

Interplanetary pick-up ions originate from ionizations of neutral interstellar atoms in the heliosphere. Over the past periods it was generally expected that after pick-up by the frozen-in solar wind magnetic fields these ions quickly isotropize in velocity space by strong pitch- angle scattering, they do, however, not assimilate to the ambient solar wind ions. Meanwhile careful investigations of pick-up ion data obtained with the plasma analyzers on AMPTE and ULYSSES could clearly reveal that, especially at periods of flow-aligned fields, noticeably anisotropic distributions must prevail. To better understand the evolutionary tracks of pick-up ions in interplanetary phase-space we carried out an injection study which takes into account all relevant convection and diffusion processes, i.e. describing pitch angle scattering, adiabatic cooling, drifts and energy diffusion. As demonstrated here particles injected at 1 AU establish a distribution function with substantial anisotropies up to distances beyond 6 AU. Only under the action of fairly strong isotropic turbulence levels a trend towards isotropy can be recognized. The bulk velocity of the injected pick-up ions turns out to be remarkably smaller than the solar wind velocity. It also is obvious that pick-ups are strongly spread out from that solar wind plasma parcel into which they were originally implanted. As one consequence it must be concluded that the derivation of interstellar He gas parameters, using He pick-up ion flux data, require appreciable caution. Due to anisotropic spatial diffusion the location of the LISM helium cone axis, i.e. the LISM wind vector, and the LISM helium temperature are hidden in the associated He+pick-up ion flux patterns.

Thermal Instability in Magnetized Interstellar Clouds

Abstract The effect of a magnetic field on the ionizational-thermal acoustic instability in the cold diffuse ISM is discussed in this paper. It is found that the frozen-in magnetic field leads to new types of instabilities called “magnetoacoustic” instabilities. These instabilities fall into two classes: slow and fast magnetoacouistic instabilities, corresponding to two types of waves in an MHD plasma. The fast modes are found to be effectively suppressed in comparison with the slow modes at the mean Galactic intensities of the magnetic field. Some implications of those results are discussed.

Toroidal Magnetic Fields and the Evolution of Wind-driven Nebulae

Recent analytical work on magnetic shaping of wind-driven nebulae suggests that diverging stellar winds with frozen-in toroidal magnetic fields (occurring naturally when the wind originates from a rotating source) may evolve into collimated outflows. We report numerical simulations which show that the MHD collimation of stellar winds is indeed possible provided that the field carried by the wind is sufficiently strong. Our collimation mechanism operates in the shocked wind region of the nebula, leading to the formation of an astrophysical plasma gun recently invoked to accelerate jets associated with accretion disks.

Interaction of Wide-Angle MHD Winds with Flared Disks

view Abstract Citations (42) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interaction of Wide-Angle MHD Winds with Flared Disks Li, Zhi-Yun ; Shu, Frank H. Abstract A magnetocentrifugal mechanism probably underlies the narrow, high-speed, optical jets associated with young stellar objects. Recent theoretical calculations of jet formation in the X-wind model suggest the co-existence of a wide-angle component surrounding the well-collimated jet. We consider whether the predicted wide-angle component can reveal itself through interaction with an underlying flared accretion disk. We use the method of characteristics to calculate numerically the properties of the resulting shockwave. Because the frozen-in magnetic field reduces the effective Mach number of the flow, the shock is too weak at a distance of 10-100 AU to produce [O I] λ6300 or [S II] λ6717/6731 emission. Unless disks have obstructing features more extreme than conventional flaring (e.g., their own wind), the uncollimated component of an X-wind will remain optically hidden until it impacts material well off the disk plane at larger distances from the central star (e.g., an infalling envelope or the ambient molecular cloud). Publication: The Astrophysical Journal Pub Date: September 1996 DOI: 10.1086/177688 Bibcode: 1996ApJ...468..261L Keywords: ISM: JETS AND OUTFLOWS; MAGNETOHYDRODYNAMICS: MHD; SHOCK WAVES; STARS: MASS LOSS; STARS: PRE-MAIN-SEQUENCE full text sources ADS |

Plasma Conductivity in a Cometary Ionosphere

view Abstract Citations (3) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Plasma Conductivity in a Cometary Ionosphere Ershkovich, A. I. ; Israelevich, P. L. Abstract The one-dimensional spherically symmetric model of Buti & Eviatar results in very low plasma conductivity value in the inner coma of comet Halley. Anomalous resistivity cannot account for this result, as the electric current in the comet Halley ionosphere is shown to be too small. On the other hand, replacement of the one-dimensional model by the axisymmetric model using the same (Giotto) data does not result in contradiction with the frozen-in magnetic field approximation. Thus, we arrive at the conclusion that three-dimensional character of the plasma flow around a comet is crucial, and the one-dimensional model of Buti & Eviatar becomes defective on approaching the diamagnetic cavity. The conductivity σ0 (which governs the magnetic field diffusion) in the magnetic barrier of comet Halley is almost coincident with that given by the classical Spitzer-Härm formula for a binary gas: σ0 ≍ 2 x 1011 esu. With this value, the magnetic Reynolds number Rem ≫ 1, which means that the frozen-in magnetic field approximation is justified almost throughout the cometary ionosphere (cf. the results of Cravens). Publication: The Astrophysical Journal Pub Date: June 1996 DOI: 10.1086/177383 Bibcode: 1996ApJ...464..959E Keywords: COMETS: GENERAL; COMETS: INDIVIDUAL NAME: HALLEY; PLASMAS full text sources ADS |

Compressive fluctuations in the solar wind and their polytropic index

Abstract. Magnetohydrodynamic compressive fluctuations of the interplanetary plasma in the region from 0.3 to 1 AU have been characterized in terms of their polytropic index. Following Chandrasekhar's approach to polytropic fluids, this index has been determined through a fit of the observed variations of density and temperature. At least three different classes of fluctuations have been identified: (1) variations at constant thermal pressure, in low-speed solar wind and without a significant dependence on distance, (2) adiabatic variations, mainly close to 1 AU and without a relevant dependence on wind speed, and (3) variations at nearly constant density, in fast wind close to 0.3 AU. Variations at constant thermal pressure are probably a subset of the ensemble of total-pressure balanced structures, corresponding to cases in which the magnetic field magnitude does not vary appreciably throughout the structure. In this case the pressure equilibrium has to be assured by its thermal component only. The variations may be related to small flow-tubes with approximately the same magnetic-field intensity, convected by the wind in conditions of pressure equilibrium. This feature is mainly observed in low-velocity solar wind, in agreement with the magnetic topology (small open flow-tubes emerging through an ensemble of closed structures) expected for the source region of slow wind. Variations of adiabatic type may be related to magnetosonic waves excited by pressure imbalances between contiguous flow-tubes. Such imbalances are probably built up by interactions between wind flows with different speeds in the spiral geometry induced by the solar rotation. This may account for the fact that they are mainly found at a large distance from the sun. Temperature variations at almost constant density are mostly found in fast flows close to the sun. These are the solar wind regions with the best examples of incompressible behaviour. They are characterized by very stable values for particle density and magnetic intensity, and by fluctuations of Alfvénic type. It is likely that temperature fluctuations in these regions are a remnant of thermal features in the low solar atmosphere. In conclusion, the polytropic index appears to be a useful tool to understand the nature of the compressive turbulence in the interplanetary plasma, as far as the frozen-in magnetic field does not play a crucial role.

Cosmic rays from accreting isolated neutron stars

Interstellar matter that is accreted onto isolated magnetic neutron stars in the Galaxy ( ~ 109 by number) is accelerated and reflected back by (MHD) shocks, which envelope the stars. The integrated power in the Galaxy, Lcr,ns, is ≳ 1040 erg s–1, the energy distribution is a power law of spectral index > 2, and the particle energy can be raised to 106 GeV, consistent with the power and spectrum of primary cosmic rays in the Galaxy. The major contribution to Lcr,ns comes from a minority of ~ 107 isolated neutron stars which are located within dense clouds. Sources in these clouds, that are generally spread within the Galactic disc, can explain the concentration of high-energy cosmic rays in the Galactic plane, as deduced from pion decay spectra in gamma-ray observations. The soft X-ray luminosity from these neutron stars is consistent with the Galactic X-ray background. The accretion may be associated with ion-neutral bias, that is further enhanced by ion confinement in frozen-in magnetic fields, which can raise the relative abundance of first ionization potential (FIP) elements in the cosmic rays.

On general transformations and variational principles for the magnetohydrodynamics of ideal fluids. Part 1. Fundamental principles

A new frozen-in field w (generalizing vorticity) is constructed for ideal magnetohydrodynamic flow. In conjunction with the frozen-in magnetic field h, this is used to obtain a generalized Weber transformation of the MHD equations, expressing the velocity as a bilinear form in generalized Weber variables. This expression is also obtained from Hamilton's principle of least action, and the canonically conjugate Hamiltonian variables for MHD flow are identified. Two alternative energy-type variational principles for three-dimensional steady MHD flow are established. Both involve a functional R which is the sum of the total energy and another conserved functional, the volume integral of a function Φ of Lagrangian coordinates. It is shown that the first variation δ1R vanishes if Φ is suitably chosen (as minus a generalized Bernoulli integral). Expressions for the second variation δ2R are presented.

Glassy states with frozen-in currents and paramagnetism of granular high temperature superconductors

Interaction between spontaneous current loops (orbital magnetic moments) induced by weak links with negative coupling energies (π-junctions) can cause paramagnetism of granular high-temperature superconductors. For a not very small concentration of π-junctions p interaction between current loops yields new types of long-range order, such as superconductivity with frozen-in currents, normal and superconducting spin-glass-like states. At certain values of p the system consists of percolating superconducting clusters and normal regions with frozen-in magnetic field and ordered orbital magnetic moments.

The heliosphere and the Ulysses mission

The interplanetary medium consists primarily of the supersonic solar wind, carrying the frozen-in magnetic field extending from the solar corona. The properties of this medium are controlled by the state of the corona and by dynamic processes occurring in the medium itself. As a result, there are significant variations in those properties as a function of heliolatitude. In situ observations over the past three decades have been largely confined to the neighbourhood of the solar equatorial plane. While many of the important processes have been identified and studied extensively, observations are required as a function of heliolatitude to define large-scale structures and their dependence on processes in the solar corona. The Ulysses mission, launched in October 1990, is the first space probe dedicated to the exploration of the heliosphere out of the ecliptic plane. By January 1994, the spacecraft had reached a heliolatitude of 50° south. The first results of the mission are summarized here, including the evolution and disappearance of the interplanetary magnetic sector structure; the onset of the dominance of the high-speed solar wind stream originating in the expanding southern coronal hole; observations of the signatures of complex coronal mass ejections; the high-latitude structure of the heliospheric magnetic field, and the evolution of corotating interaction regions as a function of heliolatitude. In particular, the abrupt change in the rotation rate of the sector structure in mid-1992, followed by the equatorward extension of the southern polar coronal hole, represent new observations related to the evolution of large-scale coronal structures and solar magnetic fields and to processes controlling the solar activity cycle.

Ion distribution function in the magnetosheath: Fine structure

The previous results on the magnetosheath plasma investigation show that the magnetosheath ion population often consists of a shocked solar wind and a magnetospheric component with a higher energy. Both these components are usually considered as having a Maxwellian energy ion distribution function (DF), whereas measurements with sufficient temporal and angular resolution reveal a “beam” structure of DF in a large region downstream of the bow shock. In this contribution we present experimental data demonstrating such structure in the low-energy part (up to 20 keV) of the ion DF in that region. We suggest some processes likely to be responsible for the creation of a smooth magnetosheath DF from the narrow beams which originated in the shock. Our results give a high degree of correlation between these processes and the magnetic field fluctuations. We are led to the conclusion that either the low-frequenccy waves with high amplitudes are generated during the relaxation process or the assumption of frozen-in magnetic field does not hold with sufficient accuracy.

Self‐consistent steady state model of the low‐latitude boundary layer

Results are presented from a steady state numerical model of the low‐latitude boundary layer (LLBL) on closed field lines and its coupling to the dayside auroral ionosphere. In the model the boundary layer approximation is used, the result being that inertial forces are taken into account only in the main flow direction (−x) where they are balanced by pressure forces, j × B forces, and viscous forces. Motion in the transverse directions (y and z) is treated kinematically, the force balances in these two directions being purely static. Computationally, the model is two dimensional, describing the motion of plasma and frozen‐in magnetic field in the equatorial (xy) plane but allowing for lowest‐order polynomial variation of some quantities with the coordinate (z) perpendicular to that plane. The plasma expands and compresses isentropically; the magnetic field is calculated self‐consistently, which leads to approximately parabolic field line shape in planes parallel to the magnetopause (the xz plane), with maximum field curvature near the magnetopause edge of the LLBL. Coupling to the ionosphere via region 1 field‐aligned currents is included. The effects of the ionosphere are represented by two parallel resistive plates at fixed height above and below the equatorial plane. The model can be used to investigate the influence of various physical parameters, for example, viscous and magnetic Reynolds numbers, and of boundary conditions at the magnetopause and in the magnetosphere on the LLBL development in the −x direction. Special attention is given to viscous effects which, under suitable circumstances, lead to a region 1 current that first increases and then reduces with increasing longitude away from local noon. Asymptotic matching of the antisunward motion of the cool LLBL plasma to sunward convection of hot plasma in the magnetosphere is illustrated along with the entrainment of magnetospheric plasma by the antisunward LLBL flow.

Magnetic reconnection in the disc and halo

This paper presents some new ideas about magnetic fields in the interstellar medium. Movements due to the galactic fountain are analysed and interpreted in terms of their implications for a frozen-in interstellar magnetic field. We find that magnetic field lines will be drawn up into the halo (or thick disc) by expanding hot gas and, in falling back to a different position, there will be tangling and reconnecting effects. Also, under certain circumstances, a new sort of field amplification may occur in parts of the interstellar medium

Calculation of the global system of the field-aligned currents on the dayside of the magnetosphere

Using the well-known equation for the normal component of the current which exist near the tangential discontinuity in the plasma in the case of the frozen-in magnetic field, and supposing that the current closes in the ionosphere in the auroral oval in the region 1, one calculates and compares with the data of observations the dependence of the density of the field-aligned current at the level of the ionosphere on the local time.

The use of cosmic rays for continuous monitoring and prediction of some dangerous phenomena for the earth's civilization

The main idea of the International Cosmic Ray Service (ICRS) is to combine satellite and spaceprobe cosmic rays, magnetic and plasma data with groundbased cosmic ray data (exchanged in real time) for obtaining continuous information on the electromagnetic and radiation situation in the interplanetary space and Earth's magnetosphere: prediction of great geomagnetic storms, big increases of radiation hazards and other dangerous phenomena in space and on the Earth for people and technology. ICRS can predict not only geomagnetic storms and unfavorable days in the environment (especially important for old people and people with some diseases), but, in combination with astrophysical methods, can predict big increases of radiation hazards very dangerous for the Earth's civilization and big changes in the environment due to extremely powerful solar flares and local supernova explosions. We hope that, after some additional investigation of high energy cosmic-ray distribution function outside the heliosphere, it could be possible to solve by ICRS more complicated problems: to determine in combination with astrophysical methods the location and velocity of nearest dust-molecular galactic clouds with frozen-in magnetic fields and predict the expected time of the Sun capturing by some clouds with possible changes of Earth's global climate. The foundation of ICRS could bring a new possibility of development to the cosmic ray observatories, release scientists from a lot of routine work and increase the fundamental and applied research efficiency.

Importance of synchrotron cooling in self-consistent spherical accretion models

We present a self-consistent model of stationary, spherical accretion onto black holes which generalizes the results of Nobili et al. (1991) to include the radiative effects of a frozen-in magnetic field. Cyclosynchrotron cooling is found to be important in the inner region of the flow, lowering the temperature and producing an increase of the emitted luminosity with respect to unmagnetized solutions. Preheating effects, due to strong Comptonization, limit, however, the existence of magnetic accretion models to very low values of the magnetic field, typically a few percent of the equipartition value, and luminosity never exceed 0.01 L Edd

SPIN-(FLAVOR) PRECESSION AND SHORT WAVELENGTH VACUUM OSCILLATION AS A SOLUTION FOR THE SOLAR NEUTRINO PUZZLE

We investigate the possibility of spin-(flavor) precession combined with short wavelength vacuum oscillation as a solution for the solar neutrino puzzle. A large frozen-in magnetic field inside the sun with a neutrino magnetic moment of the order of 10-10 Bohr magneton can completely depolarize the νeL resulting in a factor of half of the emitted number. With a short wavelength vacuum oscillation and maximal mixing, the number of νeL's reaching the earth is reduced by another factor of half; this explains the Homestake chlorine experiment. The difference between the Homestake and the Kamiokande-II experiments can be attributed to the contribution to the Cherenkov radiation in the latter through the neutral current and electromagnetic interactions of the components which are inert in the former.

Dynamical evolution and molecular abundances of interstellar clouds

Dynamical models are presented that start with interstellar gas in an initial diffuse state and consider their gravitational collapse and the formation of dense cores. Frozen-in tangled magnetic fields are included to mimic forces that might oppose gravitational contraction and whose effectiveness may increase with increasing core densities. Results suggest the possibility that dense cloud cores may be dynamically evolving ephemeral objects, such that their lifespan at a given core density decreases as that density increases.