A Mechanism for the Local Production of Faculae

We offer a new viewpoint that can explain some of the recently obtained high-resolution observations of granules and faculae. Examining the data of Scharmer, Gudiksen, Kiselman et al. (2002) we observe many granules undergo an evolution that results in faculae emerging from within their boundaries, and moving towards and into intergranular lanes. These faculae have a characteristic hairpin substructure. The evolving morphology can be closely described by a fluid dynamic instability we call the “vortex/shear layer” (VSL) interaction. It occurs in all granules whose underlying structure has vorticity when they emerge into the photosphere through the sub-photospheric turbulent boundary layer (SPTBL). The VSL results in the creation of vortices from the distributed vorticity of the SPTBL. The subsequent stretching of these vortices results in high amplification of vorticity, and the concurrent high amplification of the background magnetic field. Magnetic field lines spiral around the vortices, as well as being stretched along their axis. Thus, the VSL is also the origin of a coherent local dynamo. The spiral sheathing of high magnetic flux results in a simple explanation for the “hot wall” effect. The VSL also creates the “dark lanes” observed by Lites, Scharmer, Berger et al. (2004) and groupings of bright hairpins/vortex sheet ensembles, which look like the ribbon faculae (Berger, Rouppe van der Voort, Lofdahl et al., 2004). The SPTBL results in emerging tilted granules, which when combined with the VSL create the three-dimensionality which Lites, Scharmer, Berger et al. (2004), also observed. Both the VSL and the SPTBL result, on average, in a west side bias of hairpin faculae and granular three-dimensionality. An erratum to this article is available at .     

Dieterici gas as a unified model for dark matter and dark energy

The dominance of dark energy in the universe has necessitated the introduction of a repulsive gravity source to make q₀ negative. The models for dark energy range from a simple Λ term to quintessence, Chaplygin gas, etc. We look at the possibility of how change of behaviour of missing energy density, from DM to DE, may be determined by the change in the equation of state of a background fluid instead of a form of potential. The question of cosmic acceleration can be discussed within the framework of theories which do not necessarily include scalar fields.     

The Martian Planetary Boundary Layer: Turbulent kinetic energy and fundamental similarity scales

We review selected in situ measurements and models aimed at the study of the Martian Planetary Boundary Layer (PBL). We also discuss critically the advantages and limitations of applying similarity theories to the Martian PBL and calculate the fundamental scales predicted by these theories. Finally, we obtain values of the turbulent kinetic energy (TKE) and address its budget by weighting the significance of the different terms involved in it. In situ measurements taken by the Viking and Pathfinder missions along with similarity theories conveniently adapted to Mars are used to obtain the fundamental scales, the TKE and its budget.     

Constants and Variations: From Alpha to Omega

We review some of the history and properties of theories for the variation of the gravitation and fine structure `constants'. We highlight some general features of the cosmological models that exist in these theories with reference to recent quasar data that is consistent with time-variation in alpha since a redshift of 3.5. The behaviour of a simple class of varying-alpha cosmologies is outlined in the light of all the observational constraints. We discuss the key role played by non-zero vacuum energy and curvature in turning off the variation of constants in these theories and the issue of comparing extra-galactic and local observational data. We also show why black hole thermodynamics does not enable us to distinguish between time variations of different constants. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stability of Accretion Models

A critical analysis of standard accretion models is presented. We consider the stability of models in the theories of disc accretion onto black holes and spherical/disc accretion onto a magnetosphere. We take into account realistic physics processes and geometry (inner magnetic field in the accreted plasma, finite conductivity, finite length of the field lines, finite rotation of the accreted object, and magnetic shear on the boundary between the magnetosphere and accreted plasma). The influence of these factors leads to radical changes of both the accretion as whole and the energy release in the accreting system. Strong current-sheet and Z-pinch-like structures should arise over the polar region of the accreting object. Particle acceleration in the electric fields of current discharges in these regions may be a source of efficient conversion of energy into nonthermal particles and of the emission observed from many accreting objects.     

A numerical comparison of theories of violent relaxation

Using N -body simulations with a large set of massless test particles, we compare the predictions of two theories of violent relaxation, the well-known Lynden-Bell theory and the more recent theory by Nakamura. We derive 'weakened' versions of both the theories in which we use the whole equilibrium coarse-grained distribution function     as a constraint instead of the total energy constraint. We use these weakened theories to construct expressions for the conditional probability   K_i (τ)  that a test particle initially at the phase-space coordinate τ would end-up in the i th macro-cell at equilibrium. We show that the logarithm of the ratio   R_ij (τ) ≡ K_i (τ)/ K_j (τ)  is directly proportional to the initial phase-space density   f ₀(τ)  for the Lynden-Bell theory and inversely proportional to   f ₀(τ)  for the Nakamura theory. We then measure   R_ij (τ)  using a set of N -body simulations of a system undergoing a gravitational collapse to check the validity of the two theories of violent relaxation. We find that both the theories are at odds with the numerical results, both qualitatively and quantitatively.     

Causal viscous universe coupled with zero-mass scalar field in higher derivative theory

Cosmological solutions in the presence of an imperfect fluid and zero-mass scalar field are obtained in higher derivative theory. We investigate both power law and exponential expansion of the universe described by full causal theories proposed by Israel and Stewart. It is observed that energy density, co-efficient of bulk viscosity decrease with time in the presence of massless scalar field and temperature increase with expansion of universe.     

On the Machian origin of inertia

We examine Sciama’s inertia theory: we generalise it, by combining rotation and expansion in one unique model, we find the angular speed of the Universe, and we stress that the theory is zero-total-energy valued. We compare with other theories of the same null energy background. We determine the numerical value of a constant which appears in the Machian inertial force expression devised by Graneau and Graneau (In the Grip of the Distant Universe—the Science of Inertia, World Scientific, Singapore, 2006), by introducing the above angular speed. We point out that this last theory is not restricted to Newtonian physics as those authors stated but is, in fact, compatible with other cosmological and gravitational theories. An argument by Berry (Principles of Cosmology and Gravitation, Adam Hilger, Bristol, 1989) is shown in order to “derive” Brans-Dicke relation in the present context.     

Revisiting the coincidence problem in f(R) gravitation

The energy densities of dark matter (DM) and dark energy (DE) are of the same order at the present epoch despite the fact that both these quantities have contrasting characteristics and are presumed to have evolved distinctively with cosmic evolution. This is a major issue in standard ΛCDM cosmology and is termed “The Coincidence Problem” which hitherto cannot be explained by any fundamental theory. In this spirit, Bisabr (2010) reported a cosmological scenario in f(R) gravity where DM and DE interact and exchange energy with each other and therefore evolve dependently. We investigate the efficiency and model independancy of the technique reported in Bisabr (2010) in addressing the Coincidence problem with the help of two f(R) gravity models with model parameters constrained from various observations. Our result confirm the idea that not all scalar-tensor gravity theories and models can circumvent the Coincidence Problem and any cosmological scenario with interacting fluids is highly model dependent and hence alternate model independent theories and ideas should be nominated to solve this mystery.     

Coincidence between Hα flare kernels and peaks of observed longitudinal electric current densities

We compare large-scale filtergrams of a hitherto neglected class 1B flare with previously published vector magnetograms and maps of photospheric longitudinal electric current density (Hagyard et al., 1985). The vector magnetic fields were mapped simultaneously with the eruption of this flare. We find a coincidence, to within the ±2″ registration accuracy of the data, between the flare kernels and the locations of maximum shear and of peak values in the longitudinal electric current density. The kernels brighten in a way which implies that the preflare heating and the main release of flare energy are spatially coincident within the limits of resolution (≈2″). A pronounced magnetic shear exists in the vertical direction at the location of the strongest flare kernels. We provide evidence that the electric currents could be maintained by the energy stored in the sheared transverse magnetic field and that the amount of energy released is proportional to the amount stored. These circumstances are consistent with theories in which flares are triggered by plasma instabilities due to surplus electric currents.     

Energy storage and deposition in a solar flare

High temporal and spatial resolution solar X-ray pictures of a flare at 1827 UT on 5 September 1973 were taken with the S-056 Aerospace Corporation/Marshall Space Flight Center telescope on the Apollo Telescope Mount. Photographs taken at 9 s intervals allow detailed information to be obtained about the site of the energy release, as well as about the evolution of the flare itself. Observations show that the flare occured in an entire arcade of loops rather than in any single loop. Sequential brightening of different X-ray features indicates that some excitation moved perpendicular to the magnetic field of the arcade at velocities of 180–280 km/s. The most intense X-ray features were located in places where the magnetic field composing the arcade had a small radius of curvature with horizontal field gradients higher than the surrounding region and where the axis of the arcade changed direction. We feel that the arcade geometry strongly influenced the propagation of the triggering disturbance, as well as the storage and site of the subsequent deposition of energy. A magnetosonic wave is suggested as the propagating mechanism triggering instabilities that may have existed in the preflare structure. This event demonstrates that all energy emitted during a flare need not be released immediately nor in the same location, thereby eliminating some problems encountered in many flare theories. Conditions for energy release are discussed.     

Cosmology of f(T) gravity in a holographic dark energy and nonisotropic background

It is shown that the acceleration of the universe can be understood by considering a f(T) gravity models. Modified teleparallel gravity theory with the torsion scalar has recently gained a lot of attention as a possible explanation of dark energy. For these f(T) gravity models, a variant of the accelerating cosmology reconstruction program is developed. We consider spatially homogenous and anisotropic Bianchi type I universe in the context of f(T) gravity. The de Sitter, power-law and general exponential solutions are assumed for the scale factor in each spatial direction and the corresponding cosmological models are reconstructed. We reconstruct f(T) theories from two different holographic dark energy models in different time durations. For the holographic dark energy model, the dark energy dominated era with new setting up is chosen for reconstruction, and the Ricci dark energy model, radiation, matter and dark energy dominated time durations are all investigated. Finally we have obtained a modified gravity action consistent with the holographic dark energy scenario.     

White Dwarf Constraints On Exotic Physics

Nonstandard theories of fundamental interactions typically predict the existence of new kinds of weakly interacting particles. These can escape freely from stellar interiors and act as additional source of cooling. Considerable agreement of a variety of astrophysical observations with standard physics can serve as a source of constraints on non-standard ideas. In this paper we consider G117-B15A pulsating white dwarf for which the secular rate, at which the period of its fundamental mode increases, has been accurately measured. This star has been claimed the most stable oscillator ever recorded in the optical band. Because an additional channel of energy loss would speedup the cooling rate, one is able to use this stability to derive a bound on axion mass and on theories with large extra dimensions. We also point to the possibility of using similar arguments to constrain supersymmetric paticles. This revised version was published online in July 2006 with corrections to the Cover Date.     

Lorentz violation from gamma-ray bursts

The constancy of light speed is a basic assumption in Einstein’s special relativity, and consequently the Lorentz invariance is a fundamental symmetry of space–time in modern physics. However, it is speculated that the speed of light becomes energy-dependent due to the Lorentz invariance violation (LV) in various new physics theories. We analyse the data of the energetic photons from the gamma-ray bursts (GRBs) by the Fermi Gamma-Ray Space Telescope, and find more events to support the energy dependence in the light speed with both linear and quadratic form corrections. We provide two scenarios to understand all the new-released Pass 8 data of bright GRBs by the Fermi-LAT Collaboration, with predictions from such scenarios being testable by future detected GRBs.     

Fission and geosynchronous release of the Moon

The problem of the origin of the Moon has led to various hypotheses: simultaneous accretion, fission, capture, etc. These theories were based primarily on global mechanical considerations. New geological data (Turcotteet al., 1974; Kahn and Pompea, 1978) have led to fresh approaches and new versions of these theories.As suggested by Wise (1969) and O'Keefe (1972), the initial Earth may have taken unstable forms when radial segregation sped up the rotation. The Moon may have been created as the small part of the pyroid of Poincaré.Fission theory was mainly discarded, in the past, on the basis of energy considerations. We are now arriving at the conclusion that these considerations are void if the fission was followed by a very long period of geostationary rotation of the Moon at a distance of about 3 Earth radius (i.e., out of the Roche limit). Indeed the large amount of energy of the initial system could have been released slowly and therefore evacuated by losses of material and radiation.The accretion of the Earth and the radial segregation of heavy chemicals toward the center has led to a differential rotation of the different layers with a faster rotation at the center. During the geostationary period the Moon was synchronous with respect to the surface layer. That Earth-Moon system has both a correct angular momentum and a large stability provided that the viscosity of intermediate layers was small enough, which is in concordance with its high temperature.Even with a very hot system, a superficial cold layer appears because of its low conductivity and the radiation equilibrium with outer space. This implies a slow loss of energy: the geosynchronous Moon receded extremely slowly.During the geostationary period lithophile elements were extracted with water by the radial segregation and were deposited in the area facing the Moon. One massive continent was formed, as suggested by Grjebine (1978).As the continent became thicker and sank into the mantle, convection currents appeared and speeded up the cooling of the Earth. The viscosity increased and the synchronization between the Moon and the surface of the Earth became more difficult to maintain. When synchronism was broken important lunar tides transferred energy and momentum from the Earth to the Moon which receded toward its present position and the modification of its equilibrium shape explains the formation of lunar maria in the near side.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.     

Probing Statistical Isotropy of Cosmological Radio Sources using Square Kilometre Array

There currently exist many observations which are not consistent with the cosmological principle. We review these observations with a particular emphasis on those relevant for the Square Kilometre Array (SKA). In particular, several different data sets indicate a preferred direction pointing approximately towards the Virgo cluster. We also observe a hemispherical anisotropy in the Cosmic Microwave Background radiation (CMB) temperature fluctuations. Although these inconsistencies may be attributed to systematic effects, there remains the possibility that they indicate new physics and various theories have been proposed to explain them. One possibility, which we discuss in this review, is the generation of perturbation modes during the early pre-inflationary epoch, when the Universe may not obey the cosmological principle. Better measurements will provide better constraints on these theories. In particular, we propose measurement of the dipole in number counts, sky brightness, polarized flux and polarization orientations of radio sources. We also suggest test of alignment of linear polarizations of sources as a function of their relative separation. Finally we propose measurement of hemispherical anisotropy or equivalently dipole modulation in radio sources.     

Presence of dark energy and dark matter: does cosmic acceleration signifies a weak gravitational collapse?

In this work the collapsing process of a spherically symmetric star, made of dust cloud, in the background of dark energy is studied for two different gravity theories separately, i.e., DGP Brane gravity and Loop Quantum gravity. Two types of dark energy fluids, namely, Modified Chaplygin gas and Generalised Cosmic Chaplygin gas are considered for each model. Graphs are drawn to characterize the nature and the probable outcome of gravitational collapse. A comparative study is done between the collapsing process in the two different gravity theories. It is found that in case of dark matter, there is a great possibility of collapse and consequent formation of Black hole. In case of dark energy possibility of collapse is far lesser compared to the other cases, due to the large negative pressure of dark energy component. There is an increase in mass of the cloud in case of dark matter collapse due to matter accumulation. The mass decreases considerably in case of dark energy due to dark energy accretion on the cloud. In case of collapse with a combination of dark energy and dark matter, it is found that in the absence of interaction there is a far better possibility of formation of black hole in DGP brane model compared to Loop quantum cosmology model.     

Statistical Properties of the Highest Pulses in Gamma-Ray Bursts

We study the statistical properties of the highest pulses within individual gamma-ray bursts (GRBs). A wavelet package analysis technique and a developed pulse-finding algorithm have been applied to identify the highest pulses from burst profiles observed by BATSE on board CGRO from 1991 April 21 to 1999 January 26. The statistical light curves of the highest pulses in four energy channels have been derived by an aligning method, which illustrate the temporal evolution of the pulse emission. Our result that narrower pulses go with higher energies is consistent with previous findings. By normalizing both the pulse durations and counts to unity, “characteristic” profiles of the highest pulses in the four channels are also derived. The four characteristic profiles are turned out to be almost the same, thus strongly support the previous conclusion that the temporal profiles in different energy channels are self-similar and the previous conjecture on GRB pulses, implying that the emission process is similar at different energies. The cosmological time dilation effect is examined by investigating the relationship between the pulse flux and pulse duration. An anti-correlation between the two was found, which agrees with the expectation of the cosmological time dilation effect. Also, the evolution of the pulse duration with the observational epoch is studied. The result shows that the pulse duration tends to be shorter in later epochs. This trend cannot be explained by the present theoretical models, and may represent a great challenge to current theories.