Force and energy balance in the transition region network

Two-dimensional numerical models of the solar transition region are calculated using an inverse coordinates method which attains pressure equilibrium between the network magnetic field and the external comparatively field-free gas. If A(y, z) is the magnetic potential (a scalar in 2D), which is constant on field lines, the method involves interchanging dependent and independent variables to obtain a quasi-linear PDE for y(A, z), which is solved iteratively. The advantage of this approach is that magnetic field lines, including any magnetic interface, become coordinate lines, thereby simplifying the energy equation and free boundary problem. In order to examine the effects of self-consistent geometry on the thermal structure of the transition region network, we calculate four models. The energy balance includes the effects of radiation, conduction, and enthalpy flux. It is confirmed that the lower branch of the emission measure curve cannot be explained within the single fluxtube model if the classical Spitzer thermal conductivity is used. However, by including a turbulent thermal conductivity as proposed by Cally (1990a), transition region models are obtained for which the resulting emission measure curves exhibit the correct behaviour, including the observed turn-up below about 200 000 K. In summary, the broad conclusions of previous non-turbulent 2D models are confirmed, but most importantly, the turbulent conductivity hypothesis tested in 1D by Cally is shown to produce excellent agreement with observations in the more realistic geometry.     

Accretion in Strong Gravity: from Galactic to Supermassive Black Holes

The galactic black hole binary systems give an observational template showing how the accretion flow changes as a function of increasing mass accretion rate, or L/L_Edd. These data can be synthesised with theoretical models of the accretion flow to give a coherent picture of accretion in strong gravity, in which the major hard-soft spectral transition is triggered by a change in the nature and geometry of the inner accretion flow from a hot, optically thin plasma to a cool, optically thick accretion disc. However, a straightforward application of these models to AGN gives clear discrepancies in overall spectral shape. Either the underlying accretion model is wrong, despite its success in describing the Galactic systems and/or there is additional physics which breaks the simple scaling from stellar to supermassive black holes.     

Cosmological scalar fields that mimic the ΛCDM cosmological model

We look for cosmologies with a scalar field (dark energy without cosmological constant), which mimic the standard ΛCDM cosmological model yielding exactly the same large-scale geometry described by the evolution of the Hubble parameter (i.e. photometric distance and angular diameter distance as functions on z). Asymptotic behavior of the field solutions is studied in the case of spatially flat Universe with pressureless matter and separable scalar field Lagrangians; the cases of power-law kinetic term and power-law potential are considered. Exact analytic solutions are found in some special cases. A number of models have the field solutions with infinite behavior in the past or even singular behavior at finite redshifts. We point out that introduction of the cosmological scalar field involves some degeneracy leading to lower precision in determination of Ω _m . To remove this degeneracy additional information is needed besides the data on large-scale geometry. The article is published in the original.     

Plane symmetric inhomogeneous perfect fluid universe with electromagnetic field in Lyra geometry

A new class of plane-symmetric inhomogeneous cosmological models of perfect fluid distribution with electro-magnetic field based on Lyra’s geometry is obtained by considering a time dependent displacement field. The source of the magnetic field is due to an electric current produced along the z-axis. Only F ₁₂ is a non-vanishing component of electromagnetic field tensor. To get the deterministic solutions, the free gravitational field is assumed to be of Petrov type-II non-degenerate. It has been found that the displacement vector β(t) behaves like cosmological term Λ which is consistent with the recent observations of type Ia supernovae. It is also observed that β(t) affects entropy. Some geometric and physical behaviour of the models are also discussed in presence of magnetic field.      

Pulsar Wind Nebulae in Egret Error Boxes

A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM.     

Bianchi type-VI0 cosmological models in certain theories of gravitation

Exact Bianchi type-VI₀ cosmological solutions to Einstein's equations are presented in vacuum and for stiff-matter in the normal gauge for Lyra's geometry and in scalar-tensor theories developed by Saez and Ballester (1985) and Lau and Prokhovnik (1986). Also cosmological solutions are obtained for pure massive strings (p-strings) and for pure geometric strings. The dynamical behaviour of the models have been discussed.     

An equatorially symmetric rotator model for magnetic stars. II. Inhomogeneous element distribution

For the construction of more realistic models of magnetic stars the influence of large inhomogeneities in the distribution of line absorbing elements over the stellar surface on the determination of the mean longitudinal magnetic field component, Beff, is discussed. In this paper we assume the same magnetic field geometry as in paper I, i.e. the superposition of a dipole and an axissymmetrical quadrupole both lying in the equatorial plane with their axes parallel to one another. The models for the inhomogeneties can be classified by the concentration of the line absorbing elements: (1) at the polar caps, (2) in rings surrounding the polar caps, (3) in a belt in the neighbourhood of the “magnetic” latitude δ = 90°. The result of the computations shows that the amount of compensations of both polarities by integration over the visible hemisphere may be remarkably reduced, yielding values of Beff not so far away from the polar field strength as in the homogeneous models. Moreover, the large quadrupole contribution necessary in some cases to represent the observations in the homogeneous models may be considerably smaller with inhomogeneites taken into account. First comparisons between computed and observed values of Beff(t) are made for the stars α²CVn, 53 Cam and HD 215441.     

Faint field galaxies: an explanation of the faint blue excessusing number evolution models

Pure luminosity evolution models for galaxies provide an unacceptable fit to the redshifts and colors of faint galaxies. In this paper we demonstrate, using HST morphological number counts derived both from the I ₈₁₄-band of WFPC2 in the Medium Deep Survey (MDS) and the Hubble Deep Field (HDF) and from the H _1.6-band of NICMOS, and ground-based spectroscopic data of the Hawaii Deep Field and the Canada-France Redshift Survey, that number evolution is necessary for galaxies, regardless of whether the cosmic geometry is flat, open, or Λ-dominated. Furthermore, we show that the number evolution is small at redshifts of z<1, but large at z>1, and that this conclusion is valid for all the three cosmological models under consideration. If the universe is open or Λ-dominated, the models, which are subject to the constraint of the conservation of the comoving mass density of galaxies, naturally predict a population of star-forming galaxies with the redshift distribution peaking at z=2∼ 3, which seems to be consistent with the recent findings from Lyman-break photometric selection techniques. If the cosmological model is flat, however, the conservation of the comoving mass density is invalid. Hence, in order to account for the steep slope of B-band number counts at faint magnitudes in the flat universe, such a star-forming galaxy population has to be introduced ad hoc into the modelling alongside the merger assumption. This revised version was published online in July 2006 with corrections to the Cover Date.     

Note on the stability of parallel magnetic fields

The stability characteristics of parallel magnetic fields when fluid motions are present along the lines of force is studied. The stability criterion for both symmetric (m=0) and asymmetric (m=1) modes are discussed and the results obtained by Trehan and Singh (1978) are amended in the present study. The results obtained for the cylindrical geometry are shown to play an important role forka<4, wherek is the wave number,a is the radius of the cylinder, compared to the results obtained by Geronicolas (1977) for the slab geometry.     

Magnetic Energy Storage and Current Density Distributions for Different Force-Free Models

In the last decades, force-free-field modelling has been used extensively to describe the coronal magnetic field and to better understand the physics of solar eruptions at different scales. Especially the evolution of active regions has been studied by successive equilibria in which each computed magnetic configuration is subject to an evolving photospheric distribution of magnetic field and/or electric-current density. This technique of successive equilibria has been successful in describing the rate of change of the energetics for observed active regions. Nevertheless the change in magnetic configuration due to the increase/decrease of electric current for different force-free models (potential, linear and nonlinear force-free fields) has never been studied in detail before. Here we focus especially on the evolution of the free magnetic energy, the location of the excess of energy, and the distribution of electric currents in the corona. For this purpose, we use an idealised active region characterised by four main polarities and a satellite polarity, allowing us to specify a complex topology and sheared arcades to the coronal magnetic field but no twisted flux bundles. We investigate the changes in the geometry and connectivity of field lines, the magnetic energy and current-density content as well as the evolution of null points. Increasing the photospheric current density in the magnetic configuration does not dramatically change the energy-storage processes within the active region even if the magnetic topology is slightly modified. We conclude that for reasonable values of the photospheric current density (the force-free parameter α<0.25 Mm⁻¹), the magnetic configurations studied do change but not dramatically: i) the original null point stays nearly at the same location, ii) the field-line geometry and connectivity are slightly modified, iii) even if the free magnetic energy is significantly increased, the energy storage happens at the same location. This extensive study of different force-free models for a simple magnetic configuration shows that some topological elements of an observed active region, such as null points, can be reproduced with confidence only by considering the potential-field approximation. This study is a preliminary work aiming at understanding the effects of electric currents generated by characteristic photospheric motions on the structure and evolution of the coronal magnetic field.     

The projection of fractal objects

Motivated by several recent studies on the geometry of molecular cloud structures, we perform two experiments that consider the projection of fractal images onto a two-dimensional screen. Evidence is presented to support the proposition that the dimensionD _P of the contours surrounding the projected images are given byD _s–1, whereD _s>2 is the fractal dimension of the object itself. A brief survey of the geometry of astrophysical cloud structures is also presented.     

The Geminga fraction

Radio-quiet γ-ray pulsars like Geminga may account for a number of the unidentified EGRET sources in the Galaxy. The number of Geminga-like pulsars is very sensitive to the geometry of both the γ-ray and radio beams. Recent studies of the shape and polarization of pulse profiles of young radio pulsars have provided evidence that their radio emission originates in wide cone beams at altitudes that are a significant fraction (1–10%) of their light cylinder radius. Such wide radio emission beams will be visible at a much larger range of observer angles than the narrow core components thought to originate at lower altitude. Using 3D geometrical modeling that includes relativistic effects from pulsar rotation, we study the visibility of such radio cone beams as well as that of the γ-ray beams predicted by slot gap and outer gap models. From the results of this study, one can obtain revised predictions for the fraction of Geminga-like, radio quiet pulsars present in the γ-ray pulsar population.      

Multi‐epoch time‐resolved photometry of the eclipsing polar CSS081231:071126+440405

The eclipsing polar CSS081231:071126+440405 turned bright (V_max ∼ 14.5) in late 2008 and was subsequently observed intensively with small and medium‐sized telescopes. A homogeneous analysis of this comprehensive dataset comprising 109 eclipse epochs is presented and a linear ephemeris covering the five years of observations, about 24000 orbital cycles, is derived. Formally this sets rather tight constraints on the mass of a hypothetical circumbinary planet, M_pl ≤ 2 M_Jup. This preliminary result needs consolidation by long‐term monitoring of the source. The eclipse lasts 433.08 ± 0.65 s, and the orbital inclination is found to be i = 79.3°–83.7°. The centre of the bright phase displays accretion‐rate dependent azimuthal shifts. No accretion geometry is found that explains all observational constraints, suggesting a complex accretion geometry with possible pole switches and a likely non‐dipolar field geometry. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the possibility of detecting solar flare effects in the zodiacal light

To evaluate possible effects of solar flares on the brightness of the inner zodiacal light, it is necessary to consider the brightness contribution along the line of sight and as a function of Sun-particle distance. For this purpose, models of the brightness contribution along the line of sight are presented for both dielectric and metallic particles with a spatial distribution of the form r^?ν, ν = 0, 1, 2. These models are discussed in terms of the geometry of shock front interaction. A reported zodiacal light enhancement following a solar flare (Blackwell and Ingham, 1961) is analyzed on the basis of the shock front geometry.     

Solution of an inverse problem of the dynamics of a particle

The three-dimensional inverse problem of particle dynamics is studied here. The potentialU and the corresponding energyh are determined by the given family of possible trajectories. The classification of the solutions due to the geometry of the given family is obtained.     

Albedo perturbation models: General formalism and applications to LAGEOS

The force due to radiation pressure on a satellite of arbitrary shape is written in a general form within a formalism similar to that used in the theory of radiative transfer in atmospheres. Then the corresponding integrals are evaluated for the simple case of a spherically symmetric satellite, and applied to model the perturbation due to the Earth-reflected radiation flux on LAGEOS. For this purpose, the optical behaviour of the Earth's surface and atmosphere is described as a combination of Lambertian diffusion (continents), partial specular reflection consistent with Fresnel law (oceans) and anisotropic diffusion according to Chandrasekhar's radiative transfer theory (clouds). The in-plane Gauss componentsT andS vs. mean anomaly are computed for a simple orbital geometry and for different models of the Earth's optical properties. A sensitive dependence is found on the assumed cloud distribution, with significant perturbations possibly arising from oceanic specular reflection when the satellite is close to the Earth's shadow boundaries.On leave from Astronomical Institute, Charles University, védská 8, 15000 Prague 5, Czechoslovakia     

Fracton Excitations as a Driving Mechanism for the Self-Organized Dynamical Structuring in the Solar Wind

Structural properties of the interplanetary magnetic field (IMF) are discussed. Our main interest is concentrated on the dynamical structuring mechanisms associated with the dominant role of the wave processes in the solar wind. We argue that the IMF possibly reveals the self-organized clustering driven by the low-frequency magnetosonic waves. It is shown that the self-organized geometry of the IMF is a fractal, a specific object having a number of unusual topological features; this fractal geometry is self-consistently generated by the allowed magnetosonic modes. To give an accurate treatment of waves on fractals, we propose an unconventional approach based on the wave equation with the generalized, fractional time derivative. The allowed magnetosonic modes are then defined as the generalized "resonance" solutions to the fractional wave equation and termed "fractons", vibrational excitations of fractal objects. We found that the self-organized fractal geometry of the IMF as maintained by the fractons could be described by the value of the Hausdorff fractal dimension D≈ 4/3. Convection of the IMF fractal structures by a spacecraft observer is shown to result in the power-law behavior of the Fourier energy density spectrum of the in situobserved IMF turbulence, P(f) ∝ f ^−α, with the characteristic slope α ≈ 5/3. This revised version was published online in July 2006 with corrections to the Cover Date.     

Exact solutions of some cosmological models in Lyra geometry

Bianchi type-V anisotropic cosmological models have been studied in the theory based on Lyra’s geometry in normal gauge. The physical and kinematical behaviors of the models have been discussed in the presence and absence of the magnetic field.      

Comparison of Five Numerical Codes for Automated Tracing of Coronal Loops

The three-dimensional (3D) modeling of coronal loops and filaments requires algorithms that automatically trace curvilinear features in solar EUV or soft X-ray images. We compare five existing algorithms that have been developed and customized to trace curvilinear features in solar images: i) the oriented-connectivity method (OCM), which is an extension of the Strous pixel-labeling algorithm (developed by Lee, Newman, and Gary); ii) the dynamic aperture-based loop-segmentation method (developed by Lee, Newman, and Gary); iii) unbiased detection of curvilinear structures (developed by Steger, Raghupathy, and Smith); iv) the oriented-direction method (developed by Aschwanden); and v) ridge detection by automated scaling (developed by Inhester). We test the five existing numerical codes with a TRACE image that shows a bipolar active region and contains over 100 discernable loops. We evaluate the performance of the five codes by comparing the cumulative distribution of loop lengths, the median and maximum loop length, the completeness or detection efficiency, the accuracy, and flux sensitivity. These algorithms are useful for the reconstruction of the 3D geometry of coronal loops from stereoscopic observations with the STEREO spacecraft, or for quantitative comparisons of observed EUV loop geometries with (nonlinear force-free) magnetic field extrapolation models.     

Energy balance of the corona and the origin of quasi-stationary high-speed solar wind streams

The energy balance of open-field regions of the corona and solar wind and the influence of the flow geometry in the corona upon the density and temperature, are analyzed. It is found that the energy flux arriving at the corona is constant for the corona's open regions with different flow geometries. For the waves heating the corona and solar wind, the dependence of the absorption coefficient on the corona's plasma density is found to be within the range of distances r=1.05–1.5R . It is shown that the wave absorption is more dependent on electron density than the coronal emission. It is this difference that causes lower-density coronal holes to be colder than quiet regions. It is found that the additional energy flux necessary for providing energy balance of the corona and for producing solar wind is a flux of Alfvén waves, which can provide the energy needed for producing quasi-stationary high-speed solar wind streams. Theoretical models of coronal holes and the question of why the high-speed solar wind streams are precisely flowing out of coronal holes, are discussed.