A self-consistent model of a thermally balanced quiescent prominence in magnetostatic equilibrium in a uniform gravitational field

We present a theoretical model of quiescent prominences in the form of an infinite vertical sheet. Self-consistent solutions are obtained by integrating simultaneously the set of nonlinear equations of magnetostatic equilibrium and thermal balance. The basic features of the models are: (1) The prominence matter is confined to a sheet and supported against gravity by a bowed magnetic field. (2) The thermal flux is channelled along magnetic field lines. (3) The thermal flux is everywhere balanced by Low's (1975b) hypothetical heat sink which is proportional to the local density. (4) A constant component of the magnetic field along the length of the prominence shields the cool plasma from the hot surrounding. We assume that the prominence plasma emits more radiation than it absorbs from the radiation fields of the photosphere, chromosphere and corona, and we interpret the above hypothetical heat sink to represent the amount of radiative loss that must be balanced by a nonradiative energy input. Using a central density and temperature of 10¹¹ particles cm^–3 and 5000 K respectively, a magnetic field strength between 2 to 10 gauss and a thermal conductivity that varies linearly with temperature, we discuss the physical properties implied by the model. The analytic treatment can also be carried out for a class of more complex thermal conductivities. These models provide a useful starting point for investigating the combined requirements of magnetostatic equilibrium and thermal balance in the quiescent prominence.     

Magnetic fields of extragalactic radio sources: Testing cosmological models

Based on the synchrotron radiation mechanism with self-absorption, we estimate the magnetic fields of compact (～0 _. ^″ 001) structures of radio galaxies. Using a model radial dependence of the magnetic field, we estimate the field strength near the event horizon of a supermassive black hole. The latter turns out to be higher than that followed from the popular Blandford-Znajek mechanism. The magnetic fields are determined by taking into account evolution, which allows the redshift dependence of the magnetic field of the plasma surrounding a supermassive black hole to be derived. We show that various cosmological models can be tested in principle using magnetic field measurements of compact radio sources. We estimate the magnetic field of the farthest radio-loud quasar SDSS J0836+0054.     

The influence of fluctuation fields on the force-free field

In Paper I (Hu, 1982), we discussed the the influence of fluctuation fields on the force-free field for the case of conventional turbulence and demonstrated the general relationships. In the present paper, by using the approach of local expansion, the equation of average force-free field is obtained as (1+b)?×B ₀=(α#x002B;a)B ₀#x002B;a ⁽¹⁾×B ₀#x002B;K. The average coefficientsa,a ⁽¹⁾,b, andK show the influence of the fluctuation fields in small scale on the configurations of magnetic field in large scale. As the average magnetic field is no longer parallel to the average electric current, the average configurations of force-free fields are more general and complex than the usual ones. From the view point of physics, the energy and momentum of the turbulent structures should have influence on the equilibrium of the average fields. Several examples are discussed, and they show the basic features of the fluctuation fields and the influence of fluctuation fields on the average configurations of magnetic fields. The astrophysical environments are often in the turbulent state, the results of the present paper may be applied to the turbulent plasma where the magnetic field is strong.     

Impact of magnetic field on the gas mass fraction of galaxy clusters

Magnetic fields have been observed in galaxy clusters with strengths of the order of  ~ μG. The non-thermal pressure exerted by magnetic fields also contributes to the total pressure in galaxy clusters and can in turn affect the estimates of the gas mass fraction, f_gas. In this paper, we have considered a central magnetic field strength of 5μG, motivated by observations and simulations of galaxy clusters. The profile of the magnetic field has also been taken from the results obtained from simulations and observations. The role of magnetic field has been taken into account in inferring the gas density distribution through the hydrostatic equilibrium condition (HSE) by including the magnetic pressure. We have found that the resultant gas mass fraction is smaller with magnetic field as compared to that without magnetic field. However, this decrease is dependent on the strength and the profile of the magnetic field. We have also determined the total mass using the NFW profile to check for the dependency of f_gas estimates on total mass estimators. From our analysis, we conclude that for the magnetic field strength that galaxy clusters seem to possess, the non-thermal pressure from magnetic fields has an impact of  ≈ 1 % on the gas mass fraction of galaxy clusters. However, with upcoming facilities like Square Kilometre Array (SKA), it can be further expected to improve with more precise observations of the magnetic field strength and profile in galaxy clusters, particularly in the interior region.     

Strong magnetic field helical Čerenkov effect with some astrophysica implications

It is at very strong magnetic fields that the helical Čerenkov effect, originating from the electron guiding center for an electron in helical motion in a magnetic field which is superimposed on a dielectric medium, resembles most closely the ordinary Čerenkov effect. In the absence of extremely strong magnetic fields in the laboratory, we turn our attention to the neutron stars (pulsar) and supernovae which can have magnetic fields whose values can easily be in the range of 10⁵ — 10⁹ T. The medium in which these magnetic fields reside is likely to be an ionized medium; that is, a plasma, which, as usual, may be assumed to be dominated by electrons. Here we wish to argue that in such a strong magnetic field dominated medium, at least on a classical level, radiation process associated with the helical Čerenkov radiation could be rather important.     

Propagation of spiral density waves in a magneto-active disk

The propagation of spiral density waves in a differentially rotating, self-gravitating, magnetoactive and highly flattened disk is investigated by using the asymptotic theory for tightly wound spirals developed by Lin and his collaborators. We adopt the continuum fluid model as the primary basis, and our treatment will be largely analytical. The disk plasma is studied in the frozen field approximation and inhomogenceous magnetic fields in the plane of the disk are considered in detail.In a differentially rotating disk with strong magnetic fields, the field lines will be considerably distorted and the mutual influence of magnetic fields and differential rotation is by no means obvious.In this paper we present a new asymptotic dispersion relation for tightly wound spiral density waves with magnetic fields along the spiral armsB (r). The effects of the magnetic fields lead to such terms likek ²(a ² +V _A ² ), wherek is the wave number,a represents the speed of sound,V _A = (B ²/4)^1/2 is the Alfvén speed,B denotes the field strength, and is the plasma density. These terms depict the well-known magnetoacoustic waves and could have been anticipated without a detailed computation. However the interaction of magnetic fields and differential rotation may give rise to other significant terms which are not so easy to foresee.We also present a more exact local dispersion relation by using the WKB approximation and study the effects of magnetic fields on the growth rate through the parametersQ andJ defined in the literature.Although the effects of the magnetic fields are rather insignificant for applications to Galactic dynamics, the effects of the magnetic fields are important for applications to star formation and problems related to the solar nebula.     

Magnetostatic atmospheres: A family of isothermal solutions

Most models of large scale solar magnetic fields assume either that the fields are potential or that they are force free. We present a new, analytic, two parameter family of magnetic fields in equilibrium with isothermal plasma in a gravitational field. We discuss these models from the viewpoint of the insight into the balance of magnetic pressure gradient, and gravitational forces that they provide. We show that substantial deviations from the potential field configuration are obtained for plasma of order unity, and we emphasize the variety of possible relationships between isobars and magnetic fieldlines.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Laboratory Simulation of Magnetospheric Plasma Shocks

An experimental simulation of planetary magnetospheres is being developed to investigate the formation of collisionless shocks and their effects. Two experimental situations are considered. In both, the solar wind is simulated by laser ablation plasmas. In one case, the “solar wind” flows across the magnetic field of a high-current discharge. In the other, a transverse magnetic field is embedded in the plasma flow, which interacts with a conductive obstacle. The ablation plasma is created using the “Tomcat” laser, currently emitting 5 J in a 6 ns pulse at 1 μm wavelength and irradiance above 10¹³ W/cm². The “Zebra” z-pinch generator, with load current up to 1 MA and voltage up to 3.5 MV produces the magnetic fields. Hydrodynamic modeling is used to estimate the plasma parameters achievable at the front of the plasma flow and to optimize the experiment design. Particle-in-cell simulations reveal details of the interaction of the “solar wind” with an external magnetic field, including flow collimation and heating effects at the stopping point. Hybrid simulations show the formation of a bow shock at the interaction of a magnetized plasma flow with a conductor. The plasma density and the embedded field have characteristic spatial modulations in the shock region, with abrupt jumps and fine structure on the skin depth scale.     

Estimation of Solar Prominence Magnetic Fields Based on the Reconstructed 3D Trajectories of Prominence Knots

We present an estimation of the lower limits of local magnetic field strengths in quiescent, activated, and active (surges) prominences, based on reconstructed three-dimensional (3D) trajectories of individual prominence knots. The 3D trajectories, velocities, tangential and centripetal accelerations of the knots were reconstructed using observational data collected with a single ground-based telescope equipped with a Multi-channel Subtractive Double Pass imaging spectrograph. Lower limits of magnetic fields channeling observed plasma flows were estimated under assumption of the equipartition principle. Assuming approximate electron densities of the plasma n _e=5×10¹¹?cm^?3 in surges and n _e=5×10¹⁰?cm^?3 in quiescent/activated prominences, we found that the magnetic fields channeling two observed surges range from 16 to 40?Gauss, while in quiescent and activated prominences they were less than 10?Gauss. Our results are consistent with previous detections of weak local magnetic fields in the solar prominences.     

The instability of a horizontal magnetic field in an atmosphere stable against convection

The theoretical problem posed by the buoyant escape of a magnetic field from the interior of a stably stratified body bears directly on the question of the present existence of primordial magnetic fields in stars. This paper treats the onset of the Rayleigh-Taylor instability of the upper boundary of a uniform horizontal magnetic field in a stably stratified atmosphere. The calculations are carried out in the Boussinesq approximation and show the rapid growth of the initial infinitesimal perturbation of the boundary. This result is in contrast to the extremely slow buoyant rise of a separate flux tube in the same atmosphere. Thus for instance, at a depth of 1/3R beneath the surface of the Sun, a field of 10² G develops ripples over a scale of 10³ km in a characteristic time of 50 years, whereas the characteristic rise time of the same field in separate flux tubes with the same dimensions is 10¹⁰ years. Thus, the development of irregularities proceeds quickly, soon slowing, however, to a very slow pace when the amplitude of the irregularities becomes significant. Altogether the calculations show the complexity of the question of the existence of remnant primordial magnetic fields in stellar interiors.This work was supported in part by the National Aeronautics and Space Administration under Grant NGL 14-001-001     

Non,thermal Cosmic Gravitational Radiation and Redshift Fluctuations

Intergalactic gravitational radiation fields with wavelengths in the range of 1 to 10³ Megaparsecs have remarkable observational effects if the energy density of the fields becomes comparable to the equivalent “critical” cosmological matter density χo = 3H₀². In this case the perturbation in the space-time metric caused by the radiation field is high enough to change the frequency and to shift the path of light rays of extragalactic objects in a random manner by observable amounts. The present paper discusses the wave spectrum as a function of cosmic time and gives a statistical treatment of the redshift fluctuations.     

3-D Rpic Simulations of Relativistic Jets: Particle Acceleration, Magnetic Field Generation, and Emission

We have applied numerical simulations and modeling to the particle acceleration, magnetic field generation, and emission from relativistic shocks. We investigate the nonlinear stage of theWeibel instability and compare our simulations with the observed gamma-ray burst emission. In collisionless shocks, plasma waves and their associated instabilities (e.g., the Weibel, Buneman and other two-stream instabilities) are responsible for particle (electron, positron, and ion) acceleration and magnetic field generation. 3-D relativistic electromagnetic particle (REMP) simulations with three different electron-positron jet velocity distributions and also with an electron-ion plasma have been performed and show shock processes including spatial and temporal evolution of shocks in unmagnetized ambient plasmas. The growth time and nonlinear saturation levels depend on the initial jet parallel velocity distributions. Simulations show that the Weibel instability created in the collisionless shocks accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The nonlinear fluctuation amplitude of densities, currents, electric, and magnetic fields in the electron-positron shocks are larger for smaller jet Lorentz factor. This comes from the fact that the growth time of the Weibel instability is proportional to the square of the jet Lorentz factor. We have performed simulations with broad Lorentz factor distribution of jet electrons and positrons, which is assumed to be created by photon annihilation. Simulation results with this broad distribution show that the Weibel instability is excited continuously by the wide-range of jet Lorentz factor from lower to higher values. In all simulations the Weibel instability is responsible for generating and amplifying magnetic fields perpendicular to the jet propagation direction, and contributes to the electron’s (positron’s) transverse deflection behind the jet head. This small scale magnetic field structure contributes to the generation of “jitter” radiation from deflected electrons (positrons), which is different from synchrotron radiation in uniform magnetic fields. The jitter radiation resulting from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks. The detailed studies of shock microscopic process evolution may provide some insights into early and later GRB afterglows.     

Interaction of particles with ion cyclotron waves and magnetosonic waves. Observations from GEOS 1 and GEOS 2

Coordinated observations involving ion composition, thermal plasma, energetic particle, and ULF magnetic field data from GEOS 1 and 2 often reveal the presence of electromagnetic ion cyclotron and magnetosonic waves, which are distinguished by their respective polarization characteristics and frequency spectra. The ion cyclotron waves are identified by a magnetic field perturbation that lies in a plane perpendicular to the Earth's magnetic field B₀ and propagate along B₀. They are associated with the abundance of cold He⁺ in the presence of anisotropic pitch angle distributions of ions having energies E > 20 keV, and were observed at frequencies near the He⁺ gyrofrequency. The magnetosonic waves are characterized by a magnetic field perturbation parallel to B₀ and thus seem to be propagating perpendicular to the Earth's magnetic field. They often occur at harmonics (not always including the fundamental) at the proton gyrofrequency and are associated with phase-space-density distributions that peak at energies E ～ 5–30 keV and at a pitch angle of 90°. Such a ring-like distribution is shown to excite instability in the magnetosonic mode near harmonics of the proton gyrofrequency. Magnetosonic waves are associated in other cases with sharp spatial gradients in energetic ion intensity. Such gradients are encountered in the early afternoon sector (as a consequence of the drift shell distortion caused by the convection electric field) and could likewise constitute a source of free energy for plasma instabilities.     

Electron-positron plasma generation in a magnetar magnetosphere

We consider the electron—positron plasma generation processes in the magnetospheres of magnetars—neutron stars with strong surface magnetic fields, B ? 10¹⁴–10¹⁵ G. We show that the photon splitting in a magnetic field, which is effective at large field strengths, does not lead to the suppression of plasma multiplication, but manifests itself in a high polarization of γ-ray photons. A high magnetic field strength does not give rise to the second generation of particles produced by synchrotron photons. However, the density of the first-generation particles produced by curvature photons in the magnetospheres of magnetars can exceed the density of the same particles in the magnetospheres of ordinary radio pulsars. The plasma generation inefficiency can be attributed only to slow magnetar rotation, which causes the energy range of the produced particles to narrow. We have found a boundary in the \(P - \dot P\) diagram that defines the plasma generation threshold in a magnetar magnetosphere.     

Jovian plasma torus interaction with Europa: 3D hybrid kinetic simulation. First results

The hybrid kinetic model supports comprehensive simulation of the interaction between different spatial and energetic elements of the Europa moon-magnetosphere system with respect to variable upstream magnetic field and flux or density distributions of plasma and energetic ions, electrons, and neutral atoms. This capability is critical for improving the interpretation of the existing Europa flyby measurements from the Galileo orbiter mission, and for planning flyby and orbital measurements (including the surface and atmospheric compositions) for future missions. The simulations are based on recent models of the atmosphere of Europa ( [Cassidy et al., 2007] and [Shematovich et al., 2005]). In contrast to previous approaches with MHD simulations, the hybrid model allows us to fully take into account the finite gyroradius effect and electron pressure, and to correctly estimate the ion velocity distribution and the fluxes along the magnetic field (assuming an initial Maxwellian velocity distribution for upstream background ions). Non-thermal distributions of upstream plasma will be addressed in future work. Photoionization, electron-impact ionization, charge exchange and collisions between the ions and neutrals are also included in our model. We consider two models for background plasma: (a) with O⁺⁺ ions; (b) with O⁺⁺ and S⁺⁺ ions. The majority of O₂ atmosphere is thermal with an extended cold population (Cassidy et al., 2007). A few first simulations already include an induced magnetic dipole; however, several important effects of induced magnetic fields arising from oceanic shell conductivity will be addressed in later work.     

Heat blanketing envelopes and thermal radiation of strongly magnetized neutron stars

Strong (B?10⁹ G) and superstrong (B?10¹⁴ G) magnetic fields profoundly affect many thermodynamic and kinetic characteristics of dense plasmas in neutron star envelopes. In particular, they produce strongly anisotropic thermal conductivity in the neutron star crust and modify the equation of state and radiative opacities in the atmosphere, which are major ingredients of the cooling theory and spectral atmosphere models. As a result, both the radiation spectrum and the thermal luminosity of a neutron star can be affected by the magnetic field. We briefly review these effects and demonstrate the influence of magnetic field strength on the thermal structure of an isolated neutron star, putting emphasis on the differences brought about by the superstrong fields and high temperatures of magnetars. For the latter objects, it is important to take proper account of a combined effect of the magnetic field on thermal conduction and neutrino emission at densities ρ?10¹⁰ g?cm^?3. We show that the neutrino emission puts a B-dependent upper limit on the effective surface temperature of a cooling neutron star.     

Magnetic fields in protoplanetary disks

Magnetic fields likely play a key role in the dynamics and evolution of protoplanetary disks. They have the potential to efficiently transport angular momentum by MHD turbulence or via the magnetocentrifugal acceleration of outflows from the disk surface. Magnetically-driven mixing has implications for disk chemistry and evolution of the grain population, and the effective viscous response of the disk determines whether planets migrate inwards or outwards. However, the weak ionisation of protoplanetary disks means that magnetic fields may not be able to effectively couple to the matter. I examine the magnetic diffusivity in a minimum solar nebula model and present calculations of the ionisation equilibrium and magnetic diffusivity as a function of height from the disk midplane at radii of 1 and 5 AU. Dust grains tend to suppress magnetic coupling by soaking up electrons and ions from the gas phase and reducing the conductivity of the gas by many orders of magnitude. However, once grains have grown to a few microns in size their effect starts to wane and magnetic fields can begin to couple to the gas even at the disk midplane. Because ions are generally decoupled from the magnetic field by neutral collisions while electrons are not, the Hall effect tends to dominate the diffusion of the magnetic field when it is able to partially couple to the gas, except at the disk surfaces where the low density of neutrals permits the ions to remain attached to the field lines. For a standard population of 0.1 μm grains the active surface layers have a combined column Σ_active≈2 g cm⁻² at 1 AU; by the time grains have aggregated to 3 μm, Σ_active≈80 g cm⁻². Ionisation in the active layers is dominated by stellar X-rays. In the absence of grains, X-rays maintain magnetic coupling to 10% of the disk material at 1 AU (i.e. Σ_active≈150 g cm⁻²). At 5 AU the Σ_active≈Σ_total once grains have aggregated to 1 μm in size.     

Resonant Transition Radiation and Solar Radio Bursts

This paper presents general relations for the intensity of the resonant transition radiation (RTR) and their detailed analysis. This analysis shows that the spectrum amplitude of the x-mode at some frequencies for high-energy electrons can grow with the magnetic field increase in some interval from zero value; it can even dominate over that for the o-mode. With further magnetic field increase, the intensity of the RTR x-mode decreases in comparison with the intensity of the o-mode and this decrease is higher for higher velocities of energetic electrons. The polarization of the RTR depends on the velocity of energetic electrons, too. For velocities lower than some velocity limit v<v _i the RTR emission is unpolarized in a broad interval of magnetic field intensities in the radio source. For reasonable values of indices of the power-law distribution functions of energetic electrons, the RTR is broadband in frequencies (df/f≈0.2−0.4). Furthermore, we show various dependencies of the RTR and its spectral characteristics. Assuming the same radio flux of the transition radiation and the gyro-synchrotron one at the Razin frequency, we estimate the limit magnetic field in the radio source of the transition radiation. Then, we analyze possible sources of small-scale inhomogeneities (thermal density fluctuations, Langmuir and ion-sound waves), which are necessary for the transition radiation. Although the small-scale inhomogeneities connected with the Langmuir waves lead to the plasma radiation, which is essentially stronger than RTR, the inhomogeneities of the ion-sound waves are suitable for the RTR without any other radiation. We present the relations describing the RTR for anisotropic distribution functions of fast electrons. We consider the distribution functions of fast electrons in the form of the Legendre polynomials which depend on the pitch-angle. We analyze the influence of the degree of the anisotropy (an increase of the number of terms in the Legendre polynomial) on spectral characteristics of the RTR. A comparison with previous studies is made. As an example of the use of the derived formulas for the RTR, the 24 December 1991 event is studied. It is shown that the observed decimetric burst can be generated by the RTR in the plasma with the density inhomogeneities at the level 〈ΔN ²〉/N ²=2.5⋅10⁻⁵.     

Modeling Magnetic Tower Jets in the Laboratory

The twisting of magnetic fields threading an accretion system can lead to the generation on axis of toroidal field loops. As the magnetic pressure increases, the toroidal field inflates, producing a flow. Collimation is due to a background corona, which radially confines this axially growing “magnetic tower”. We investigate the possibility of studying in the laboratory the dynamics, confinement and stability of magnetic tower jets. We present two-dimensional resistive magnetohydrodynamic simulations of radial arrays, which consist of two concentric electrodes connected radially by thin metallic wires. In the laboratory, a radial wire array is driven by a 1 MA current which produces a hot, low density background plasma. During the current discharge a low plasma beta (β < 1), magnetic cavity develops in the background plasma (β is the ratio of thermal to magnetic pressure). This laboratory magnetic tower is driven by the magnetic pressure of the toroidal field and it is surrounded by a shock envelope. On axis, a high density column is produced by the pinch effect. The background plasma has >rsim1, and in the radial direction the magnetic tower is confined mostly by the thermal pressure. In contrast, in the axial direction the pressure rapidly decays and an elongated, well collimated magnetic-jet develops. This is later disrupted by the development of m = 0 instabilities arising in the axial column.     

Magnetic field inclination angle at geosynchronous orbit

Simultaneous magnetic field data from the geostationary satellites GOES 2 and GOES 3 have shown significant differences between the inclination angles measured at these two satellites. The relationship between the two inclination angles is examined for three magnetic activity groups and eight local time sectors by using data from GOES 2 and GOES 3 for the 12-month period from March 1979 to February 1980. During this time GOES 2 was 1.09 R_E off the geomagnetic equator in the northern geomagnetic hemisphere, while GOES 3 was separated from GOES 2 by 2 h L.T. and 0.47 R_E off the geomagnetic equator, also in the northern geomagnetic hemisphere. The average inclination angle is found to differ substantially from the predictions based on the previously published magnetic field models. The relationship between the two inclination angles is fitted by a simple model of magnetic field perturbation that varies as a power of distance from the Equator. During magnetic disturbances, the spatial variation of magnetic field perturbation on the nightside suggests that the ring current density increases with distance from the geomagnetic equator.