Diffuse Corona Research Articles

STATE TRANSITIONS IN LOW-MASS X-RAY BINARIES

We investigate the model of disk/coronal accretion into a black hole. We show that the inner regions of an accretion disk in X-ray binaries can transform from a cool standard disk to an advection-dominated flow through the known properties of Coulomb interaction in a two-temperature plasma, viscous heating, radiative processes, and thermal conduction. A hot, diffuse corona covering the disk is powered by accretion, but it exchanges mass with the underlying cold disk. If the accretion rate in the system is low enough, we show that the corona evaporates the disk away, leaving an advective flow to continue toward the hole. In the soft/hard transition commonly seen in X-ray binaries, we show that this advective flow can recondense back onto the underlying disk if the change in the system's accretion rate is slow enough due to thermal conduction. Unabsorbed spectra are produced to test against observations as well as prediction of the location of truncation radii of the accretion disk.

Turbulent and directed mass motions in various structures of the low corona

[1] A brief review on turbulent and ordered plasma motions in the solar corona is presented. Velocities in quiet coronal regions, coronal loops, coronal holes, and cavities around quiescent prominences are analyzed. Data based on interferometric observations of several total solar eclipses obtained earlier by the authors and in addition the values of turbulent velocities resulting from revising the 5303 line profiles were used. Published data based both on ground and space observations in visual and far ultraviolet (UV) spectral region are collected. The turbulent velocities in quiet loop structures and diffuse quiet corona seem to be similar, and the coronal plasma in the vicinity of the quiet protuberances is less turbulent than in the quiet corona or in the coronal holes. Velocities of directed motions for different structures in the corona are examined. It seems that the turbulent velocities are comparable by their magnitudes with the direct plasma velocities; however, at the same time they are slightly less.

Alfvén Waves in the Solar Corona

Alfvén waves, transverse incompressible magnetic oscillations, have been proposed as a possible mechanism to heat the Sun's corona to millions of degrees by transporting convective energy from the photosphere into the diffuse corona. We report the detection of Alfvén waves in intensity, line-of-sight velocity, and linear polarization images of the solar corona taken using the FeXIII 1074.7-nanometer coronal emission line with the Coronal Multi-Channel Polarimeter (CoMP) instrument at the National Solar Observatory, New Mexico. Ubiquitous upward propagating waves were seen, with phase speeds of 1 to 4 megameters per second and trajectories consistent with the direction of the magnetic field inferred from the linear polarization measurements. An estimate of the energy carried by the waves that we spatially resolved indicates that they are too weak to heat the solar corona; however, unresolved Alfvén waves may carry sufficient energy.

Water-Soluble, Unimolecular Containers Based on Amphiphilic Multiarm Star Block Copolymers

A new class of water-soluble, amphiphilic star block copolymers with a large number of arms was prepared by sequential atom transfer radical polymerization (ATRP) of n-butyl methacrylate (BMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA). As the macroinitiator for the ATRP, a 2-bromoisobutyric acid functionalized fourth-generation hyperbranched polyester (Boltorn H40) was used, which allowed the preparation of star polymers that contained on average 20 diblock copolymer arms. The synthetic concept was validated by AFM experiments, which allowed direct visualization of single molecules of the multiarm star block copolymers. DSC and SAXS experiments on bulk samples suggested a microphase-separated structure, in agreement with the core−shell architecture of the polymers. SAXS experiments on aqueous solutions indicated that the star block copolymers can be regarded as unimolecular micelles composed of a PBMA core and a diffuse PPEGMA corona. The ability of the polymers to encapsulate and releas...

The environment of dwarf spheroidal satellites; ram pressure, tides and external radiation fields

We discuss the role of environmental mechanisms in the evolution of dwarf galaxy satellites using high-resolution N-Body+SPH simulations that include simultaneously tidal forces, ram pressure and heating from ionizing radiation fields. Tidally induced bar-buckling instabilities can transform a rotating disky dwarf into a pressure supported spheroidal. Efficient gas removal requires instead a combination of tidal mass loss and ram pressure stripping in a diffuse gaseous corona around the primary system. The efficiency of ram pressure depends strongly on how extended the gas remains during the evolution. Bar driven inflows that tend to drive the gas to the bottom of the potential well can be opposed by the heating from ultraviolet radiation. We show that even fairly massive dwarfs ($V_{peak} >$ 30 km/s) would be stripped of their gas over a few Gyr if they enter the Milky Way halo at $z > 2$ thanks to the effect of the cosmic UV background. Gas mass loss can be much faster, occurring in less than 1 Gyr, if dwarf satellites have their first close approach with the primary at the epoch of bulge formation. Indeed at that time the primary galaxy should have a FUV luminosity comparable to that of major present-day starbursts, resulting in a local radiation field even more intense than the cosmic background.

Nanoassemblies formed from hydrophilic block copolymers and multivalent ions

We recently reported on the controlled formation of highly stable metal hydroxide nanoparticles. Growth control and stabilization of particles were achieved by performing hydrolysis and polycondensation reactions of polymer-complexed metal cations, the polymers being double hydrophilic block copolymers (DHBCs). The present paper focuses on the characterization of the intermediate species in the formation process, namely the mixture of multivalent metal cations plus the oppositely charged polyelectrolyte–neutral block copolymers. Complexation of metal cations by the anionic block of the DHBCs leads to the spontaneous formation of colloidal objects. Small angle neutron scattering experiments show that the aggregates exhibit a well defined core–corona architecture: the core is compact and homogeneous while the diffuse corona is constituted of polymer segments in a good solvent. The core formation is due to the water insoluble metal cation–polyacrylate complex while the corona is constituted of the water soluble neutral polymer blocks. Cryo-TEM images allowed the revelation of well-dispersed inorganic-rich domains. The presence of the longer neutral blocks prevents macroscopic phase separation of the complex and leads to the formation of hybrid micellar aggregates.

Computational simulation of initiation and implosion of circular arrays of wires in two and three dimensions

We have performed realistic two-dimensional (2-D) r-/spl theta/ resistive MHD simulations of high-current aluminum wire array initiation and implosion. These show only a moderate differentiation of wire plasma into a warm dense core and a hot diffuse corona. Wire plasmas in 28-wire array simulations implode without forming a shell; those in 56-wire simulations first merge but then separate. As both implode, thread-like plasmas settle into valleys formed across the field lines by the thread mass. Thus, shell formation does not smooth the initial wire asymmetry, because the r-/spl theta/ Rayleigh-Taylor instability amplifies it. This argues against shell-formation as the primary explanation for the observed effect of increased wire number on radiation power. We have also performed three-dimensional (3-D) ideal MHD simulations that continue those 2-D simulations; they start with a fully consistent MHD state. These simulations, perturbed between the 2-D and 3-D phases, show that azimuthally uncorrelated 3-D perturbations-appropriate for wires-grow more slowly than fully azimuthally correlated 2-D r-z perturbations. Further, the uncorrelated perturbation growth rate is smaller for 56 wires than for the 28, as the magnetic field couples more plasma threads over the same distance. These 3-D effects may explain the observed radiation power improvement with increased wire number.

Nanoparticles of a polyelectrolyte–fatty acid complex: carriers for Q 10 and triiodothyronine

Poly(ethylene imine) (PEI) was used for the complexation of dodecanoic acid (C12) resulting in a poly(ethylene imine) dodecanoate complex (PEI–C12) with a lamellar nanostructure and a repeat unit of 2.9 nm. PEI–C12 was doped with coenzyme Q 10 and the hormone triiodothyronine as typical hydrophobic and pharmacological active compounds, respectively. The PEI–C12 acts as a guest matrix that dissolves both molecules up to weight uptakes of about 20% (w/w) and 15% (w/w), respectively, both without crystallization. Agglomerate-free dispersions of core-shell type nanoparticles were developed. Ratios of PEI to C12 of 2:1 or higher were found to be suitable for this purpose. The particles exhibit hydrodynamic diameters in the range of 80–150 nm, which depend on the preparation conditions. Each particle consists of a relatively compact core surrounded by a diffuse corona. PEI–C12 forms the core, while non-complexed PEI acts as a cationic-active dispersing agent. It was found that the nanoparticles show high ζ potentials (≈+40 mV) and are stable in NaCl solutions at concentrations of up to 0.3 mol l −1. The stabilization of the nanoparticles results from a combination of ionic and steric contributions. A variation of the pH value was used to activate the dissolution of the particles. The PEI–C12 nanoparticles may have potential as carriers for hydrophobic drugs.

Interaction, critical, percolation and kinetic glass transitions in pluronic L-64 micellar solutions

We briefly discuss results of analyses of an extensive set of small angle neutron scattering (SANS) intensity distributions from a class of Pluronic tri-block copolymer micelles in aqueous solutions. This class of Pluronic has a symmetric structure (PEO MPPO NPEO M), with PPO/PEO molecular weight ratio of 60/40. It is shown that the micelles are spherical, each consisting of a hydrophobic core and a diffuse hydrophilic corona region having substantial hydration. We use a previously developed ‘cap-and-gown’ model for the microstructure of the micelle, taking into account the polymer segmental distribution and water penetration profile in the core and corona regions. We treat the inter-micellar correlations using a sticky hard sphere model. With this combination, we are able to fit all SANS intensities satisfactorily for micellar solutions within the range of disordered micellar phase in absolute scale. The structure and interaction of micelle stay is essentially the same as the concentration increases. But the aggregation number and surface stickiness increases, and the micelle becomes less hydrated with increasing temperature. Micellar core is not completely dry but contains up to 20% (volume fraction) of solvent molecules at lower temperatures. We then discuss, in some details, Pluronic L64 (PEO 13PPO 30PEO 13) micellar system. This system shows an inverted bi-nodal line with a lower critical consolute point and a percolation line. We investigated the structure and interaction between these micelles as temperature approaches the bi-nodal line along iso-concentration lines. The model developed above is able to describe SANS data in critical region also satisfactorily. As one approaches the bi-nodal line at constant weight fraction of the copolymer, the aggregation number and the stickiness parameter increase. In particular, at weight fraction of 5%, the stickiness parameter approaches the critical value 10.2 at T=330.9 K. We investigated the effect of hydrophobic impurities in the commercial polymer on the critical phenomenon. We conclude that in both the pure and impure systems, the micellar solution shows a critical demixing point where micelles stay spherical but interact strongly with each other by a short range temperature dependent attraction. Furthermore, from this fitting procedure we find relationships between the stickiness parameter and temperature, the volume fraction of micelles and the polymer concentration. Using these two relations, we are able to map the phase diagram of the sticky sphere model onto that of the micellar solution. The agreement between the theory and experimental phase behavior is satisfactory. Finally, we briefly describe the recently found kinetic glass transition line in this system using a scaling plot of SANS data below and above the transition.

Transition of a multipin negative corona in atmospheric air to a glow discharge

It is commonly accepted that, as the current increases, a diffuse negative corona inevitably goes over to a strongly nonuniform and nonsteady spark discharge. In this paper, a new effect—the transition of a negative corona to a diffuse glow discharge at atmospheric pressure—is studied experimentally and numerically. The evolution of the corona parameters during the transition to the regime of a glow discharge is traced.

The Off‐Limb Behavior of the First Ionization Potential Effect in \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $T> 5\times 10^{5}$ \end{document} K Solar Plasmas

We investigate the behavior of the solar first ionization potential (FIP) effect (the abundance enhancement of elements with first ionization potential of less than 10 eV in the corona with respect to photospheric values) with height above the limb in a region of diffuse quiet corona observed by the SUMER instrument on SOHO, with emphasis on so-called upper transition region lines. Previous disk observations have shown different abundance patterns in emission from lines at temperatures above and below ~8×105 K, with an FIP effect clearly visible at the higher temperatures and less so at the lower temperatures. Our initial aim is to determine whether such a difference is also visible in off-limb observations. We find a low-FIP element enhancement of a factor of 3-4 indicated in all line ratios. The Mg VII 868.11/Ne VII 895.17 ratio is also seen to decrease toward a photospheric value when tracked down from the corona to the limb. This is markedly different from the behavior of higher temperature line ratios and may be related to the differing heating and mass supply mechanisms for plasmas at temperatures above and below ~8×105 K. An additional unexpected feature of our observations is that in low-FIP/high-FIP line ratios formed at temperatures close to the freeze-in temperature of the fast solar wind (~106 K), there is also a small diminution of the FIP enhancement at the highest altitudes observed in this study. We discuss the possible relevance of this to the origin of the fast solar wind.

Nature of the heating mechanism for the diffuse solar corona

The temperature of the Sun's outer atmosphere (the corona) exceeds that of the solar surface by about two orders of magnitude, but the nature of the coronal heating mechanisms has long been a mystery1. The corona is a magnetically dominated environment, consisting of a variety of plasma structures including X-ray bright points, coronal holes and coronal loops. The latter are closed magnetic structures that occur over a range of scales and are anchored at each end in the solar surface. Large-scale regions of diffuse emission are made up of many long coronal loops2. Here we present X-ray observations of the diffuse corona from which we deduce its likely heating mechanism. We find that the observed variation in temperature along a loop is highly sensitive to the spatial distribution of the heating. From a comparison of the observations and models we conclude that uniform heating gives the best fit to the loop temperature distribution, enabling us to eliminate previously suggested mechanisms of low-lying heating near the footpoints of a loop. Our findings favour turbulent breaking and reconnection of magnetic field lines as the heating mechanism of the diffuse solar corona.

Heating the Solar Corona by Magnetic Reconnection

Here I review briefly the theory of magnetohydrodynamic reconnection and ask what observational evidence is there that it is heating the corona. In particular, the new directions in which three-dimensional theory for reconnection is heading are outlined. Part of the coronal heating problem has been solved with the identification of reconnection driven by converging flux motions as the key for x-ray bright points. Furthermore, it has been shown that the large-scale diffuse corona is heated rather uniformly, so that turbulent reconnection by braiding or ion-cyclotron waves driven by network micro-flares are prime candidates. Finally, reconnection is the natural explanation for a wide variety of phenomena discovered by SOHO including explosive events, blinkers, the magnetic carpet and even possibly tornadoes.

Coronal Heating and the Vertical Temperature Structure of the Quiet Corona

The radial variation of temperature in the inner corona is examined using long-exposure Yohkoh images of two regions of diffuse (quiet) corona. The results indicate a steady radial increase of temperature for both regions, out to 0.7 and 0.95 solar radii above the limb. We find that the filter-ratio data for the two regions is well fitted in each case by integration over the line of sight of a spherically symmetric model atmosphere in hydrostatic equilibrium and with a temperature profile due to a conserved inward heat flux. An Abel inversion process is also applied to the data, and this gives results consistent with the spherically symmetric, conserved-heat flux model. These results imply that the nonthermal energy responsible for heating these regions of the quiet corona is being deposited beyond the observed range of heights. However, the diffuse regions we examined are believed to be partly closed-field regions, and so, the radial models require careful interpretation. We discuss the implications for coronal heating in these regions.

Melting of Ordered Arrays and Shape Transitions in Highly Concentrated Diblock Copolymer Solutions

Polystyrene/polyisoprene (PS/PI) diblocks suspended in decane serve as a model system for the investigation of highly concentrated diblock copolymer solutions. Bulk melts of PS/PI with no solvent exhibit ordered morphologies including lamellae, close-packed cylinders, etc. that depend on the block asymmetry. These same diblocks self-assemble in decane at low concentrations to form monodisperse, spherical micelles with a dense polystyrene core and a diffuse polyisoprene corona. Strongly interacting polymeric micelles, observed at modest polymer concentrations, order into cubic arrays that include both face-centered and body-centered cubic crystals depending on the length scale of the repulsion relative to the core dimension. These ordered morphologies of the melt and micellar crystals provide limiting reference states for the poorly understood high-concentration regime studied in this work. As we increase the polymer concentration, we observe a curious melting of the micellar crystals before the onset of anisotropy. Since the melting of the micellar crystal is not predicated upon shape transitions, we return to tethered-chain models of our spherical polymeric micelles to qualitatively describe the disordering process in terms of a loss of the osmotic pressure gradient between micelles. One system exhibits a reentrant disorder−order−disorder−order phase transition. Finally, the development of anisotropy in the scattering pattern is linked to shape transitions that develop as melt conditions are approached. We monitor the degree of anisotropy to estimate the concentration for the onset of these shape transitions.

Measurement of Hydrogen Velocity Distributions in the Extended Solar Corona

H I Lyα spectral line profiles have been measured in polar regions of the solar corona at projected heliocentric heights of 1.8-3.5 R☉. Observations were made with the Ultraviolet Coronal Spectrometer on the Spartan 201 satellite from 16:52 to 04:04 UT on 1993 April 11-12 (south pole) and from 12:28 to 22:09 UT on 1993 April 12 (north pole). In general, the coronal profiles cannot be accurately curve-fitted with a single-Gaussian function. The fits with two Gaussians yield most probable velocities of 158 and 322 km s-1 (south) and 98 and 266 km s-1 (north). These parameters vary by less than 10% (1 σ) over the observed heights. The observations are consistent with a line-of-sight model that attributes the narrow component to background streamers and to sites within polar coronal holes or a surrounding diffuse corona, and the broad component to the polar coronal holes and/or plumes. This interpretation suggests that there are regions within the observed coronal holes that have hydrogen and proton kinetic temperatures of (4-6) × 106 K, which is 4-10 times higher than the expected electron temperatures at the same heights. However, other models with, for example, local non-Maxwellian velocity distributions are also consistent with the observations.

EUVE Observations of the Seyfert Galaxy MRK 279

We report EUVE spectral and photometric data of the Seyfert 1 galaxy MRK 279. The photometric data show large amplitude variations over time scales less than 10,000 s. The spectrum is characterized by several features between 80 and 100 Å. We compare the observed data with several models. We can rule out the possibility that the EUV emission is from a diffuse corona or intercloud medium. Models that assume the soft X-ray/EUV emission results from reprocessing in an optical BLR region are also inconsistent with the data. A collisional excitation model is consistent with the observations but requires a cloud density ≥ 1011 cm−3.

Energetics and dynamics of solar activity

The activity of the Sun occurs in the solar atmosphere and is driven and confined by the Sun's magnetic field. The plasma atmosphere comprises the cool (≲ 104K) and dense chromosphere, an intermediate transition region, and the hot (Tc ≳ 106K) and diffuse (nc 108 cm−3) corona; the solar atmosphere has been well observed from the ultraviolet through hard X rays, by the Skylab and Solar Maximum Mission spacecraft, among others. The atmospheric magnetic field has its source in the solar interior and is driven and energized by the global differential rotation and local turbulent motions at the visible photospheric surface; this field is mostly known from Zeeman‐effect and microwave measurements. The magnetic field lines return to the surface in active regions and are open in coronal holes that provide the source for the solar wind.

Measurements of Pulsed Corona in Coaxial Electrode Structures

A low-inductance pulse generator is used to deliver a high-voltage short-duration pulse to an air-filled coaxial electrode configuration. When the voltage is below the sparkover value, the pulse energy is delivered to a diffuse corona which visually appears to fill the Interelectrode region. Energy transfer, current distribution, and pulse shape are measured for variations in electrode diameters, air temperature, and voltage. Over a range of voltages up to sparkover, the entire stored energy is delivered to the corona. Pulse rise times are approximately 0.2 μs, the maximum peak voltage is 92 kV, peak currents equal approximately 400 A, and pulse energies range up to 1.6 J.

Development of Spark Discharges

IN a recent communication1 concerning the mechanism of growth of the leader strokes preceding spark discharges, Allibone has stated that the leader stroke develops behind a voluminous corona discharge and that the concentrated bright streamer, which is readily photographed, is in the nature of a ‘core' to this corona. The same view has been expressed by ourselves on various occasions and in earlier reports2, and not only have we succeeded in demonstrating beyond doubt the existence of this diffuse corona discharge forming the head of the leader stroke, but we have also been making extensive investigations of its characteristics during the past eighteen months.