The problem of phase mixed shear Alfvén waves in the solar corona revisited

The problem of phase mixing of shear Alfvén waves is revisited taking into account dissipative phenomena specific for the solar corona. In regions of space plasmas where the dynamics is controlled by the magnetic field, transport coefficients become anisotropic with transport mechanism having different behavior and magnitude depending on the orientation with respect to the ambient magnetic field. Taking into account realistic values for dissipative coefficients we obtain that the previous results derived in context of torsional Alfvén wave phase mixing are actually heavily underestimated so phase mixing cannot be used to explain the damping of torsional Alfvén waves and heating of open coronal structures. The presented results indicate that in order for phase mixing to still be a viable mechanism to explain heating or wave damping unrealistic assumptions have to be made. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transport Effects in the Evolution of the Global Solar Magnetic Field

The axisymmetric component of the large-scale solar magnetic fields has a pronounced poleward branch at higher latitudes. In order to clarify the origin of this branch we construct an axisymmetric model of the passive transport of the mean poloidal magnetic field in the convective zone, including meridional circulation, anisotropic diffusivity, turbulent pumping and density pumping. For realistic values of the transport coefficients we find that diffusivity is prevalent, and the latitudinal distribution of the field at the surface simply reflects the conditions at the bottom of the convective zone. Pumping effects concentrate the field to the bottom of the convective zone; a significant part of this pumping occurs in a shallow subsurface layer, normally not resolved in dynamo models. The phase delay of the surface poloidal field relative to the bottom poloidal field is found to be small. These results support the double dynamo wave models, may be compatible with some form of a mixed transport scenario, and exclude the passive transport theory for the origin of the polar branch.     

The approximation of neutrino heat conduction with neutrino scattering

We have developed an algorithm for taking into account the neutrino scattering in the approximation of neutrino heat conduction. We show that in the case of incoherent neutrino scattering (e.g., by electrons), the coefficients of the temperature and chemical potential gradients are averaged over the neutrino energy using functions that can be found by numerically solving integral equations. The coherent scattering by free nucleons and atomic nuclei can be described by introducing a transport cross section. We suggest a new method for calculating the neutrino—electron scattering functions that is based on Fermi—Dirac functions of integer indices.     

Scattering properties of planetary regolith analogs

The physics of scattering of electromagnetic waves by media in which the particles are in contact, such as planetary regoliths, has been thought to be relatively well understood when the particles are larger than the wavelength. However, this is not true when the particles are comparable with or smaller than the wavelength. We have measured the scattering parameters of planetary regolith analogs consisting of suites of well-sorted abrasives whose particles ranged from larger to smaller than the wavelength. We measured the variation of reflectance as the phase angle varied from 0.05° to 140°. The following parameters of the media were then deduced: the single scattering albedo, single scattering phase function, transport mean free path, and scattering, absorption, and extinction coefficients. A scattering model based on the equation of radiative transfer was empirically able to describe quantitatively the variation of intensity with angle for each sample. Thus, such models can be used to characterize scattering from regoliths even when the particles are smaller than the wavelength. The scattering parameters were remarkably insensitive to particle size. These results are contrary to theoretical predictions, but are consistent with earlier measurements of alumina abrasives that were restricted to small phase angles. They imply that a basic assumption made by virtually all regolith scattering models, that the regolith particles are the fundamental scattering units of the medium, is incorrect. Our understanding of scattering by regoliths appears to be incomplete, even when the particles are larger than the wavelength.     

Transport phenomena and abundance anomalies in cosmic plasma

Hydrodynamical equations for a fully ionized hydrogen-helium plasma are derived by the Chapman-Enskog method. The electron and ion transport coefficients are found as the functions of electron and ion temperatures and number densities as well as of the magnetic field strength. The presented equations are needed for describing transport phenomena in laboratory and cosmic plasmas. It is shown that transport phenomena can produce abundance anomalies; e. g., a sound wave propagating through a homogeneous plasma may be accompanied by the oscillations of chemical composition. Various astrophysical consequences of the theory are discussed.     

Envelope dynamics of iron-core supernova models

Wilson has found that the neutrino transport mechanisms is unable to generate a supernova explosion in stars with collapsing iron cores. We have utilized Wilson’s analysis to investigate the behavior of the overlying potentially explosive layers which Wilson omitted. The outer boundary of the core of Wilson’s models moves in such a, manner as to deliver a shock to the base of the envelope. We have numerically followed the progress of such shocks into the envelope of a realistic model obtained from evolutionary calculations. We find that only shocks so strong as to be inconsistent with our treatment are capable of ejecting material. For reasonable shocks the nuclear burning does not proceed rapidly at densities below ?～10⁶g cm^?3, and the nuclear energy released is less than the shock energy in all models that come near to ejecting matter. The initial model adopted here, which is based on a particular set of evolutionary calculations and which neglects rotation and magnetic fields, seems destined to generate a black hole. The creation of a black hole in such a way is probably not attended by a supernova explosion.     

Electrical conductivity in sunspots and the quiet photosphere

The electrical conductivity, thermal conductivity and viscosity of models of the quiet photosphere and the umbra of a sunspot have been calculated using LTE ionization equilibria and the Chapman-Enskog theory of transport coefficients. The results are presented in tabular form, and compared with previous calculations.     

Time-dependent transport of energetic particles in magnetic turbulence: computer simulations versus analytical theory

We explore numerically the transport of energetic particles in a turbulent magnetic field configuration. A test-particle code is employed to compute running diffusion coefficients as well as particle distribution functions in the different directions of space. Our numerical findings are compared with models commonly used in diffusion theory such as Gaussian distribution functions and solutions of the cosmic ray Fokker–Planck equation. Furthermore, we compare the running diffusion coefficients across the mean magnetic field with solutions obtained from the time-dependent version of the unified non-linear transport theory. In most cases we find that particle distribution functions are indeed of Gaussian form as long as a two-component turbulence model is employed. For turbulence setups with reduced dimensionality, however, the Gaussian distribution can no longer be obtained. It is also shown that the unified non-linear transport theory agrees with simulated perpendicular diffusion coefficients as long as the pure two-dimensional model is excluded.     

Organic synthesis in the coma of Comet Hale–Bopp?

Several organic molecules have now been detected in the coma of Hale–Bopp. These species may either emanate from the nucleus, or, as has been suggested by Bockelée–Morvan et al., could be synthesized in the coma. We have modelled the gas phase chemistry which occurred in the coma of Hale–Bopp, concentrating on the observed organic molecules HCOOH, HCOOCH₃, HC₃N and CH₃CN. We find that gas phase chemical reactions are unable to synthesize the observed abundances of these molecules, so all these species are most probably present in the nuclear ice. We briefly discuss the implications of this result for the connection between cometary and interstellar ices.     

The Mechanism involved in the Reversals of the Sun's Polar Magnetic Fields

Models of the polarity reversals of the Sun's polar magnetic fields based on the surface transport of flux are discussed and are tested using observations of the polar fields during Cycle 23 obtained by the National Solar Observatory at Kitt Peak. We have extended earlier measurements of the net radial flux polewards of ±60° and confirm that, despite fluctuations of 20%, there is a steady decline in the old polarity polar flux which begins shortly after sunspot minimum (although not at the same time in each hemisphere), crosses the zero level near sunspot maximum, and increases, with reversed polarity during the remainder of the cycle. We have also measured the net transport of the radial field by both meridional flow and diffusion across several latitude zones at various phases of the Cycle. We can confirm that there was a net transport of leader flux across the solar equator during Cycle 23 and have used statistical tests to show that it began during the rising phase of this cycle rather than after sunspot maximum. This may explain the early decrease of the mean polar flux after sunspot minimum. We also found an outward flow of net flux across latitudes ±60° which is consistent with the onset of the decline of the old polarity flux. Thus the polar polarity reversals during Cycle 23 are not inconsistent with the surface flux-transport models but the large empirical values required for the magnetic diffusivity require further investigation.     

Observational evidence for expansion in the SSS spectra of novae

For several novae, a bright X‐ray source with a spectrum resembling the class of Super Soft X‐ray Sources (SSS) has been observed a few weeks to months after outburst. Novae are powered by explosive nuclear burning on the surface of a white dwarf, and enough energy is produced to power a radiatively driven wind. Owing to the evolution of the opacity of the ejecta, the observable spectrum gradually shifts from optical to soft X‐rays (SSS phase). It has sometimes been assumed that at the beginning of the SSS phase no more mass loss occurs. However, high‐resolution X‐ray spectra of some novae have shown highly blue‐shifted absorption lines, indicating a significant expansion. In this paper, I show that all novae that have been observed with X‐ray gratings during their SSS phase show significant blue shifts. I argue that all models that attempt to explain the X‐ray bright SSS phase have to accommodate the continued expansion of the ejecta (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the stability of the phase problem

Phase retrieval of a signal given its intensity is considered as a problem of statistically estimating a set of unknown parameters, the Zernike coefficients. Specifically, the phase problem is presented in the context of classical wave optics in the Fresnel approximation. Investigating the stability in this case suggests first learning if the Zernike coefficients can be restored in principle. If this is indeed the case, it then suggests determining the accuracy of their estimation. The stability of a solution to the phase problem depends, as it does for the other inverse problems, on the spectrum of the Fisher information matrix. An explicit representation of the Fisher matrix is given, and its spectrum is calculated for in-focus and out-of-focus images of a pointlike source. Simulations show that the solutions in the latter case are generally stable, so the coefficients of the Zernike series can be determined with an acceptable accuracy. The principal components, the mutually independent combinations of aberrations that are a generalization of the coefficients of the well-known Karhunen-Loeve decomposition, are calculated. As an example of this approach, the maximum-likelihood method is used to determine the aberrations of the optical system.     

Five-color photometry of Saturn and its rings

Analysis of 206 high-quality plates from three recent apparitions taken in five colors has yielded several photometric parameters for Saturn and its A and B rings. Phase curves and geometric albedos are derived for two regions of Saturn and for each ring. The phase coefficients of the rings are found to be independent of the ring-plane inclination angle. A comparison of the phase curves shows that the particles of ring A exhibit a larger phase coefficient than do those of ring B. When examined with a multiple-scattering model using Henyey-Greenstein phase functions, the observations of the ring tilt effect indicate that the particles of ring A may also have lower single-scattering and geometric albedos. The color dependence of the geometric albedo of the particles in ring B is shown to be very similar to that of Europa (J II). We find for ring A an optical thickness of 0.50 (0.45 ≤ τ_A ≤ 0.57) and for the Cassini division, 0.018 ± 0.004.     

Secular brightness increases of Titan,Uranus, and Neptune, 1972–1976

The brightnesses of Titan, Uranus, and Neptune in b (4718 ÅA) and y (5508 ÅA) have increased linearly since 1972 at rates ranging from 0.005 to 0.025 mag yr^?1. The observations were made differentially on a number of nights each season with respect to a network of comparison stars whose relative magnitudes were determined by independent measurements. Solar phase coefficients were derived for each object, and all observations have been normalized to zero solar phase angle and mean heliocentric distances. No explanation for the changes has been found, but a possible influence of solar activity upon planetary albedo is suggested by the fact that all of the objects observed have brightened during the declining half of the solar cycle.     

Helioseismic solar cycle changes and splitting coefficients

Using the GONG data for a period over four years, we have studied the variation of frequencies and splitting coefficients with solar cycle. Frequencies and even-order coefficients are found to change significantly with rising phase of the solar cycle. We also find temporal variations in the rotation rate near the solar surface.     

Rigidly rotating modes of the solar magnetic field

Discrete rigidly rotating components (modes) of the large-scale solar magnetic field have been investigated. We have used a specially calculated basic set of functions to resolve the observed magnetic field into discrete components. This adaptive set of functions, as well as the expansion coefficients, have been found by processing a series of digitized synoptic maps of the background magnetic field over a 20-year period. As a result, dependences have been obtained which describe the spatial structure and the temporal evolution of the 27-day and 28-day rigidly rotating modes of the Sun's magnetic field.The spatial structure of the modes has been compared with simulations based on the known flux-transport equation. In the simulations, the rigidly rotating modes were regarded as stationary states of the magnetic field whose rigid rotation and stability were maintained by a balance between the emergence of magnetic flux from stationary sources located at low latitudes and the horizontal transport of flux by turbulent diffusion and poleward directed meridional flow. Under these assumptions, the structure of the modes is determined solely by the horizontal velocity field of the plasma, except for the low-latitude zone where sources of magnetic flux concentrate. We have found a detailed agreement between the simulations and the results of the data analysis, provided that the amplitude of the meridional flow velocity and the diffusion constant are equal to 9.5 m s^–1 and 600 km² s^–1, respectively.The analysis of the expansion coefficients has shown that the rigidly rotating modes undergo rapid step-like variations which occur quasi-periodically with a period of about two years. These variations are caused by separate surges of magnetic flux in the photosphere, so that each new surge gives rise to a rapid replacement of old large-scale magnetic structures by newly arisen ones.     

Galactic cosmic ray modulation from 1965–1970

Numerical solutions of the cosmic-ray equation of transport within the solar cavity and including the effects of diffusion, convection, and energy losses due to adiabatic deceleration, have been used to reproduce the modulation of galactic electrons, protons and helium nuclei observed during the period 1965–1970. Kinetic energies between 10 and 10⁴ MeV/nucleon are considered. Computed and observed spectra (where data is available) are given for the years 1965, 1968, 1969 and 1970 together with the diffusion coefficients. These diffusion coefficients are assumed to be of separable form in rigidity and radial dependence, and are consistent with the available magneticfield power spectra. The force-field solutions are given for these diffusion coefficients and galactic spectra and compared with the numerical solutions. For each of the above years we have (i) determined the radial density gradients near Earth; (ii) found the mean energy losses suffered by galactic particles as they diffuse to the vicinity of the Earth's orbit; (iii) shown quantitatively the exclusion of low-energy galactic protons and helium nuclei from near Earth by convective effects; and (iv), for nuclei of a given energy near Earth, obtained their distribution in energy before entering the solar cavity. It is shown that the energy losses and convection lead to near-Earth nuclei spectra at kinetic energies ≤100 MeV/nucleon in which the differential intensity is proportional to the kinetic energy with little dependence on the form of the galactic spectrum. This dependence is in agreement with the observed spectra of all species of atomic nuclei and we argue that this provides strong observational evidence for the presence of energy losses in the propagation process; and for the exclusion of low energy galactic nuclei from near Earth.     

Atomic oxygen transport in the thermosphere

The photodissociation of oxygen in the lower thermosphere is evaluated to obtain its global average value and the hemispheric imbalance. The observed concentrations of atomic oxygen do not reflect this imbalance in production due to the effect of seasonal wind patterns redistributing the atomic oxygen. The wind system necessary to compensate for the imbalance in solar thermal input into the lower thermosphere is found to transport an amount of atomic oxygen sufficient to compensate for the hemispheric imbalance in production. Ionospheric data indicate a winter enhancement in atomic oxygen concentration; to produce this, a higher degree of oxygen dissociation than that normally accepted (i.e. higher than an atomic to molecular oxygen ratio of unity at 120 km) is needed. The concept that the concentrations of atomic oxygen observed over the winter polar region are maintained by transport from lower latitudes requires that eddy diffusion coefficients derived from vertical transport at low latitudes (ignoring horizontal transport) be reduced by about 25 per cent.     

Modes of sediment transport in channelized water flows with ramifications to the erosion of the Martian outflow channels

Depending on their grain sizes (settling velocities), sediments are transported in rivers as bed load, in suspension, or as wash load. The coarsest material rolls or bounces along the bottom as bed load whereas finer material is placed into suspension by the water turbulence. The finest sediments are transported as wash load, evenly distributed through the water depth and effectively moving at the same rate as the water. The criteria for quantitatively determining which grain-size ranges are being transported in terrestrial rivers as bed load, suspended load and wash load are applied to an analysis of sediment transport in the large Martian outflow channels, assuming their origin to have been from water flow. Of importance is the balance of the effects of the reduced Martian gravity on the water flow velocity versus the reduction in grain settling velocities. Analyses were performed using grain densities ranging from 2.90 g/cm³ (basalt) to 1.20 g/cm³ (volcanic ash). The results show that the Martian flows could have transported cobbles in suspension and that nearly all sand-size material and finer would have been transported as wash load. Wash-load transport requires little or no net expenditure of the water-flow power, so the sands and finer could have been carried in nearly unlimited quantities. A comparison with terrestrial rivers indicates that concentrations as high as 60–70% by weight of wash-load sediment could have prevailed in the Martian flows, resulting in the very rapid erosion of the channels.