bluuebunny/crossref_metadata_2025 huggingface dataset:
Solar Magnetoconvection
Authors: Nordlund Å. , Stein R. F.
Published: June 2019

As a prelude to discussing the interaction of magnetic fields with convection, we first review some general properties of convection in a stratified medium. Granulation, which is the surface manifestation of the major energy carrying convection scales, is a shallow phenomenon. Below the surface, the topology changes to one of filamentary cool downdrafts, immersed in a gently ascending isentropic background. The granular downflows merge into more widely separated downdrafts, on scales of mesogranulation and super-granulation.

The local topology and time evolution of the small scale, kilo Gauss, network and facular magnetic field elements are controlled by convection on the scale of granulation. The topology and time evolution of larger scale magnetic field concentrations are controlled by the hierarchical structure of the horizontal components of the large scale velocity field. In sunspots, the small scale magnetic field structure determines the energy balance, the systematic flows and the waves. Below the surface, the small scale structure of the magnetic field may change drastically, with little observable effect at the surface. We discuss results of some recent numerical simulations of sunspot magnetic fields, and some mechanisms that may be relevant in determining the topology of the sub-surface magnetic field. Finally, we discuss the role of active region magnetic fields in the global solar dynamo.
The Observational Signature of Flux Tube Waves and an Upper Limit on the Energy Flux Transported by Them
Authors: Solanki S.K. , Roberts B.
Published: June 2019

The influence of undamped linear longitudinal tube waves on Stokes V profiles is considered. A rough upper limit is set on the energy flux transported by such waves through the photosphere. It is found that this upper limit is larger than the flux in the quiet sun. However, due to the small filling factor of the magnetic elements, the total luminosity of flux tube waves is unlikely to be larger than that of acoustic waves when averaged over the whole sun. Therefore, probably both kinds of waves contribute to chromospheric heating. However, the derived upper limit does not rule out that flux tube waves can significantly enhance the chromospheric brightness in active regions and the supergranular network where the magnetic filling factor is large.
Electric Currents in the Atmosphere of the Sun
Authors: Abramenko V.I. , Gopasyuk S.I. , Ogir M.B.
Published: June 2019

The structure of the magnetic field, proper motions of sunspots and electric currents have been studied and related to the evolution of sunspot groups. Further the height variations of the magnetic fluxes and electric currents in active regions have been explored.
Global Evolution of Photospheric Magnetic Fields
Authors: Makarov V. I. , Sivaraman K. R.
Published: June 2019

The main features concerning the evolution of the large scale photospheric magnetic fields derived from synoptic maps as well as from H-alpha synoptic charts are reviewed. The significance of a variety of observations that indicate the presence of a high latitude component as a counterpart to the sunspot phenomenon at lower latitudes is reviewed. It is argued that these two components describe the global magnetic field on the sun. It is demonstrated that this scenario is able to link many phenomena observed on the sun (coronal emission, ephemeral active regions, geomagnetic activity, torsional oscillations, polar faculae and global modes in the magnetic field pattern) with the global magnetic activity.
Study of Sun's ‘Hydromagnetic’ Oscillations Using Sunspot Data
Authors: Gokhale M.H. , Javaraiah J. , Hiremath K.M.
Published: June 2019

Spherical-harmonic-fourier (SHF) analysis of sun's ‘nominal toroidal magnetic field’, computed using sunspot data during 1874-1976, shows that sunspot activity can be considered as possibly originating in interference of sun's axi-symmetric ‘hydromagnetic’ oscillations ofodddegrees up to ℓ ≈ 21 and periods ≈ 22y. The relative amplitudes and relative phases of these oscillations remain fairly constant, leading to butterfly diagrams with stable latitude-time correlations on all latitude scales ≳9°. The amplitudes and phases do however undergo slow coherent variations. The main modes (η<11) represent an approximately standing oscillation.
Ground Based and Space Future Prospects in Solar Interferometry Applied to the Photosphere
Authors: Rozelot J.P.
Published: June 2019

This paper sets about a brief review of the current knowledge of ground based interferometry, and then, using the relevant information from this up-to-date technique, its tentative application to the study of solar granular structures. As a matter of fact, most of the fundamental processes in the photosphere take place on very small spatial scales and understanding the formation of the solar granulation, pores as well as tiny photospheric faculae, requires insight into the magnetic network where the thermal instabilities are initiated. The study of fine scale structures, typically of about 15 milliarcsec (11 km) by interferometry, i.e. to a resolution not accessible through classical optical telescopes, is certainly the key to open the door to new scenarios in solar physics. By the way, similar phenomena are likely to be accountered in other type of stars. Particular attention is paid to the role of imaging reconstruction at very high angular resolution. Some of the thus achievable solar programmes are listed, mainly with solar granulation. The new type of instrumentation involved should be in the 90's commissioned. At last, it is pointed out that solar interferometry could be also performed in space, as earlier suggested by J.L. DAME and C. AIME to ESA, for the study of flares, prominences or coronal loops, which are closely linked to the photosphere, and for the 2000's horizon. Some problems inherent to this whole prospect (ground and space) are listed, and discussion is welcomed for determining adequate programmes, and also to highlight promising directions for future investigations.
New Observational Aspects
Authors: Engvold Oddbj⊘rn
Published: June 2019

The requirements and conditions for high resolution imaging and polarimetry of the Sun are reviewed. Various methods and techniques are discussed for image stabilization and sharpening in solar observations. The new solar facilities in the Canary Islands in particular are frequently reaching diffraction limited resolution and yield new insight in the structure and dynamics of the solar atmosphere. Future ground based telescopes like THEMIS and LEST, as well as planned solar missions in space will trigger a next advance in solar physics.
Summary Lecture
Authors: Rutten Robert J.
Published: June 2019

I have found four models in the literature for doing conference summaries:

1.

The Literal Summary.

One summarizes all t h a t has been presented, preferably interspersed with comments as “of particular interest was…”.

2.

The Historical Perspective.

One places all (or some) presentations within a historical background, preferably implying that science progresses smoothly in well-planned, orderly fashion.

3.

The Future Perspective.

One points out the way to go, preferably in overly optimistic vein.

4.

The personal Impressions.

One concedes lack of wisdom to forego balanced summarizing, prefering to discuss primarily one's own interests.

Which model to choose here? Literal summarizing seems superfluous for the oral presentations. They are printed in the preceding pages, each is effectively a summary of work published elsewhere, and many have an author's summary already. It won't be useful to summarize them here once more, but some perspective may be worthwhile.
VLBI Observations of WR Stars
Authors: Felli M. , Massi M.
Published: June 2019

We present the first VLBI radio observations of two WR stars (WR140 and WR146) and of two early type stars (HD167971 and θ1 ORI A). The derived size and brightness temperature suggest that in all cases the radio emission originates from two distinct components: a steady weak emission due to the ionized wind of the primary and a smaller diameter brighter non-thermal component, probably associated to a small region within the ionized envelope of the star.
The Hydrogen Abundances in WN Stars
Authors: Hamann W.-R. , DüNNEBEIL G. , Wessolowski U. , Schmutz W.
Published: June 2019

Hydrogen abundances in WN stars have been derived by Conti et al. (1983) from a “semiquantitative” study of the Balmer-Pickering decrement. No clear correlation between hydrogen detection and spectral subtype could be established: stars with hydrogen and stars without detectable hydrogen are found among both, the WNE (“early”) as well as the WNL (”late”).
Theory of Intrinsic Variability in Hot-Star Winds
Authors: Owocki Stanley P.
Published: June 2019

The winds of the hot, luminous, O, B, and WR stars are driven by the line-scattering of the star's continuum radiation flux. Several kinds of observational evidence indicate that such winds are highly structured and variable, and it seems likely that a root cause of this variability is the known strong instability of the line-driving mechanism. Initial dynamical models of the nonlinear evolution of this instability confirm that the wind indeed becomes highly structured, with large amplitude (~500 – 1000 km/s) shocks that separate high-speed rarefied flow from lower speed, dense shells. Remarkably, such variability can often have an intrinsic character, persisting even in the absence of explicit perturbation, and it now appears that this is a direct consequence of a degeneracy in the steady-state solutions for such models. However, recent work indicates that including scattering effects, which have so far been ignored in these pure-absorption models, might reduce or even break this steady-state solution degeneracy; through the “line-drag” effect, scattering can also reduce the strength of the instability, possibly rendering it an advective character for which wind variability now requires explicit perturbation from below. This review will examine the consequences of these ideas for understanding the likely nature of wind variability among the various kinds of early-type stars.
WR Binaries: Theoretical Aspects
Authors: De Greve J.P.
Published: June 2019

The different channels for the formation of WR-stars, as suggested in the literature, are investigated. The presently available tools, in terms of evolutionary recipies, are reviewed and the results investigated of the use of these tools, with respect to the WR-binaries.

Two of the three basic formation channels, mentioned in the literature, may serve as ways to obtain the presently observed group of WR-binaries: stellar wind mass loss in O-stars during the pre-WR phase and mass transfer in a massive close binary system. Discrepancies with observations necessitated the incorporation of stellar wind in the binary scenario.

Theoretical developments in the last years are reviewed and confronted with the observations. The necessity of mass transfer in the formation process is argued for a major fraction of the double-lined WR-binaries. Using specific evolutionary ingredients, the original parameters of the systems are calculated. We also discuss the appearance and disappearance of double-lined WR-systems as well as the existence of eccentric WR-systems after mass transfer. The evolution of a number of specific systems is discussed individually.
Decoding of the Light Changes in Wolf-Rayet Eclipsing Binaries: An Application to HD 5980 in the Small Magellanic Cloud
Authors: Breysacher J. , Perrier C.
Published: June 2019

A method of light curve analysis is described which allows the study of an eccentric partially-eclipsing system containing one component possessing an extended atmosphere. The effects of transparency as well as limb-darkening are taken into account. Preliminary results obtained for the Wolf-Rayet eclipsing binary HD 5980 in the SMC are presented.
The Origin of WR 140
Authors: De Greve J.P.
Published: June 2019

The formation of the eccentric, long period binary WR 140 (HD 193793, WC7 + 04-5V, e= 0.8, P = 7.9 yr), in terms of an eccentricity modulated late case B of mass transfer, is investigated. Using only stellar wind models, it is impossible to obtain a combination of a spectral type O4-5 together with a WC star.
Emission Line Profiles in RY Scuti: A Massive Binary in a Pre-WR Stage?
Authors: de Martino D. , Vittone A. A. , Rossi C. , Giovannelli F.
Published: June 2019

RY Scuti (HD 169515) is a massive 9thmag eclipsing binary (Porb = 11.12d) surrounded by a peculiar nebula. High resolution spectroscopic observations in Hα, He I (λ5876), [N II] (λλ6548, 6584), [A III] (λ7136) and [S III] (λλ9069, 9532) are presented. All emission lines show a complex profile with two main structures: a sharp and strong red component and a very broad fainter complex blue one in which at least three sub-components can be detected. We have analysed these profiles in order to derive information on the velocity field in the nebula. Permitted and forbidden lines show the same velocity field indicating a common line forming region. The presence of a multi-structured blue component in all profiles gives evidence that velocity gradients are present within the nebula. Systemic corrected expansion velocities of + 30 and −45,−30,−9 Km/s are found for the red and the three blue components respectively. Weaker emission structures, reflecting the same asymmetries of the strong emissions, are observed close to the permitted lines at larger velocities (Vb = −189 Km/s and Vr = 158 K m/s). The observed velocity field indicates an asymmetric mass outflow from the system very likely through the second Lagrange point rather than via stellar wind. The system should loose mass during the mass exchange phase. Using the masses (M1 = 39M⊙, M2 = 49M⊙) and radii (R1 = 37R⊙, R2 = 41R⊙) of the two components (Milano et al., 1981), and locating RY Scuti in the [log R, log M] diagram for early type contact binaries from Leung and Schneider (1979), we find that this system has just evolved-off the Terminal Age Main Sequence. Moreover, placing RY Scuti in the [log P, log M1/M2] diagram for O+O and WR+OB binaries of Massey (1981) we find that it is in an evolutionary phase just preceeding a WR+OB stage.
Deceleration Zones in the Winds of WR and P Cygni Type Stars
Authors: Nugis Tiit
Published: June 2019

Specific features both of continua and of line spectra of WR and P Cygni type stars can probably be explained if an extensive deceleration zone is proposed to exist in their stellar winds. The outflowing matter is first accelerated near the stellar surface, then follows the deceleration of the flow and after that the final acceleration of the outflowing matter takes place (i.e. the wind has an ADA-structure: acceleration-deceleration-acceleration). Such a structure of the wind probably arises due to the multiscattering of photons in the envelope having two detached shells, which are optically thick in resonance lines (these shells can form if ionization stratification is present in the envelope).
Massive Stars and Giant HII Regions: The High-Energy Picture
Authors: Montmerle Thierry
Published: June 2019

Giant HII regions contain highly energetic objects: luminous, massive stars (including Wolf-Rayet stars) generating powerful winds, as well as, often, supernova remnants. These objects interact with the surrounding gas by creating shock waves. Part of the energy input is radiated away in the form of X-rays; also, protons and electrons may be accelerated in situ and generate γ-rays by collisions with the ionized gas. In addition, the stars themselves (including the accompanying low-mass PMS stars) are sources of X-rays, and W-R stars may emit continuum y-rays and are associated with nuclear γ-ray lines seen in the interstellar medium. Therefore, both through the stars they contain and through interactions within the gas, giant HII regions are, in addition to their more traditional properties and over nearly 7 decades in energy, important sources of high-energy radiation.
Are Wolf-Rayet Stars the Progenitors of Type IB/IC Supernovae?
Authors: Filippenko Alexei V.
Published: June 2019

I discuss evidence for and against the hypothesis that Type Ib and Type Ic supernovae (SNe) are produced by core collapse in massive, evolved progenitors. A key object is SN 1987K, whose spectroscopic classification changed from Type II to Type Ib/Ic as it aged. The progenitor of SN 1987K may well have been a massive star which experienced incomplete mass loss, leaving a thin outer envelope of hydrogen. However, several arguments are used to conclude that in most SNe Ib/Ic, the pre-supernova mass loss cannot be caused entirely by strong winds as in Wolf-Rayet stars. Mass transfer in close binary systems is probably important, but in such cases the supernova progenitor is not necessarily a Wolf-Rayet star; instead, it may be a relatively quiescent, hot, low-mass helium star that explodes via core collapse. For example, the rapid decline of the light curve of the Type Ic SN 1987M, and its seemingly low ejected mass, are consistent with this idea. It is also possible that some, but not all, SNe Ib/Ic arise from deflagrations or detonations of white dwarfs.
On the Luminosity Function of Open Clusters Containing Wolf-Rayet Stars
Authors: Admiranto A.G. , Hidayat B.
Published: June 2019

It is generally accepted that mass loss and overshooting are the two main processes that play important roles in the formation of Wolf-Rayet stars (Chiosi & Maeder, 1986). Such mechanisms modify the interior of a star. We cannot, however, disregard the possibility that there are external factors, which can influence the formation and evolution of Wolf-Rayet stars.
Calibration of MK Types by Fitting the HR Diagrams of Three Moving Clusters
Authors: Garrison R. F.
Published: June 2019

A composite HR diagram constructed from the main sequences of three moving clusters (Hyades, α Persei, and the Inner Region of Upper Scorpius) using MK classifications, is presented and discussed.
Absolute Magnitudes of O Stars from a Study of Visual Multiple Systems
Authors: Burnichon M. L.
Published: June 2019

From a careful study of physical multiple systems, it has been possible to derive differentially absolute magnitudes and intrinsic colours of O stars from the corresponding quantities, relatively well-known, of companions of these stars.
Ground Based and Extraterrestrial Observations of Stellar Flux
Authors: Code A. D.
Published: June 2019

The significance of recent improvements made in the absolute monochromatic flux measurements of the Sun and Vega are discussed with a special emphasis on the absolute calibration of the V-magnitude of stars. The difficulties relating to the determination of vacuum ultraviolet fluxes are outlined and the improvements that can be achieved using synchroton radiation as the fundamental laboratory source are presented. Colour indices derived from OAO-2 observation are given for about 80 early type stars and the (1700 – V) colour is discussed in terms of interstellar reddening and sensitivity to effective temperature. Basic problems requiring further investigation are finally outlined.
Precision Differential CCD Photometry
Authors: Balona L. A.
Published: June 2019

In this paper we describe the technique and reduction procedure in use at the SAAO for differential CCD photometry. Much of this work involves the detection of microvariables in galactic and Magellanic Cloud open clusters. The large number of frames requires automated cleaning and reduction software (principally the DoPhot package) which is briefly described. The process of selecting comparison stars and the technique of detecting low-amplitude variables is discussed. The software is well suited to be used for on-line reductions at the telescope and a start has been made in this direction. Finally, some astrophysical results involving precision differential CCD photometry are described.
The Modernization of the Pulkovo Photographic (Photoelectric) Vertical Circle by a CCD Array
Authors: Goncharov G. A. , Bagildinsky B. K. , Kornilov E. V. , Polojentsev D. D. , Rumyantsev K. V. , Shkutov V. D.
Published: June 2019

The Zverev photographic vertical circle (PVC) of the Pulkovo observatory is in the process of modernization. The features of the vertical circle are:

a)

Maksutov mirror-lens optical system with small aberrations and wide passband: aperture: 20 cm, focal length: 200 cm, focal scale: 103 arcsec/mm.

b)

Very compact instrument: 140 cm total length, 60 cm — tube.

c)

Wide field: 25 × 25 mm = 40′ × 40′. Wide-field imaging can be combined with meridian observations.

d)

Easily-reversible instrument: reversing takes less than 30 seconds.

e)

Two divided vertical circles of glass. Photoelectric circle reading microscopes.

f)

Photographic micrometer in focal plane. This will be changed with a CCD micrometer.
A New Ocular Micrometer for the MAHIS
Authors: Kirian T. R. , Korepanov V. S. , Grozdilov V. M.
Published: June 2019

A working model of an ocular micrometer has been designed for the MAHIS (Meridian Automated Horizontal Instrument by Sukcharev). An optical scheme of the micrometer includes the following devices:

a)

Artificial light marks in the focal plane of the objective. The marks have an increased sharpness and have a stable scale factor under defocusing. An amplitude-phase grid illuminated by coherent light from the second main point of the objective is used to make the marks.

b)

The observed stars and artificial marks are imaged on the CCD chip by means of an additional objective. This objective also corrects chromatic aberations of the main objective.

c)

A concentric meniscus is used to compensate for the chromatic refraction of the atmosphere. The meniscus center of curvature coincides with the center of the system's pupil image. In this case the compensation is equal at all points of the field of view.

d)

The possibility of measuring the normal attitude of the flat mirror relative to the main instrumental plane during each observation is discussed. For these purposes there needs to be a holographic grid on the mirror surface, an artificial zenith or nadir horizon and an autocollimated source of light in the focal plane.

e)

Laboratory investigations of a working model of a CCD camera are being carried out and the basic software is being developed. This work is planned to be finished in the autumn of 1994. The CCD matrix ISD 011A (NPO “Electron”, St. Petersburg, Russia) 512 × 512 pixels, with pixel size of 16 × 16 mkm is used. The noise is 5 electrons/pixel/sec when the temperature is −40° C (thermoelectric cooler).
PbS and CCD Array Autocollimation Micrometers for the Infrared Meridian Circle
Authors: Yershov V. N.
Published: June 2019

A new infrared meridian instrument is being developed at Pulkovo Observatory. The main purpose of the instrument is to extend the fundamental coordinate system to the K-infrared waveband and to faint stars at visual and I-wavebands. The instrument has a 30-cm primary mirror made from astrositall. An intermediate focal plane is used to introduce luminous reference marks. One can obtain autocollimated images of the marks at the intermediate focal plane with the use of a polished chamber located around the central hole of the primary mirror. The secondary mirror of the telescope forms images of the marks and of their autocollimated counterparts and passes them to the plane of a photodetector (Fig. 1.). The luminous marks give a reference frame for the measurements. These measurements are not affected by displacements of any optical unit placed after the intermediate focal plane or by displacements of the detector. The measurements are done relative to the coordinates of the average between positions of the luminous mark and its autocollimated image. Any small constant difference between the center of curvature and the optical axis position can be determined in the laboratory.
On the Accuracy of CCD and Photographic Observations of Asteroids and Their Current Orbit Determinations
Authors: Bykov O. P.
Published: June 2019

An application of the Classical Laplacian Method and new Pulkovo AMP-method for current asteroid orbit determinations is given. The CERES software package created at the Institute of Theoretical Astronomy (Russia) was applied to calculate (O-C)-differences for 200 numbered minor planets observed irregularly and quasisimultaneously in 1993 by CCD as well as by photographic techniques at 25 observatories (ESO, SERGA, Kitt Peak etc.). The accuracy of the observations was estimated by means of the standard error of the average (O-C) differences for each type of observation obtained by each telescope. As a whole the CCD-observations of the numbered minor planets are considerably more precise in comparison to the photographic ones. Some results are given in Table 1.
Precision CCD Photometry of the Horizontal Branch
Authors: Davis Philip A. G.
Published: June 2019

The Strömgren four-color system is well suited to the measure and analysis of horizontal-branch stars. The increased accuracy of the CCD photometric system and the ability to measure fainter stars in very crowded regions make the combination of the CCD system and four-color photometry an excellent one to study horizontal-branch stars in globular clusters.
Stellar Photometry by a Perfect CCD
Authors: Thomsen Bjarne , Jensen Frank Grundahl
Published: June 2019

We have developed algorithms for the construction of a simulated globular cluster, as it would appear on a frame obtained with a perfect CCD. This has allowed us to compare the absolute errors in the photometry of two different programs, DAOPHOT II (Stetson 1991, 1994) and SPS (Janes and Heasley 1993), with a special emphasis on criteria for selecting stars with the best photometry.
Applications of a Realistic Model for CCD Imaging
Authors: Howell Steve B. , Merline William J.
Published: June 2019

We have constructed a computer model for simulation of point sources imaged on CCDs. An attempt has been made to ensure that the model produces “data” that mimic real data taken with two-D detectors. To be realistic, such simulations must include randomly generated noise of the appropriate type from all sources. The synthetic data are output as simple one-D integrations, as two-D radial slices, and as three-D intensity plots. Each noise source can be turned on or off so they can be studied independently as well as in combination to provide insight into the image components.
Energy Dissipation Mechanisms in the Solar Corona
Authors: Heyvaerts J.
Published: June 2019

Present ideas concerning the electric heating of the solar corona are reviewed. We consider in some more detail the dissipation of MHD waves in strong horizontal gradients of the Alfvén velocity. Then we consider the evolution of DC currents in the solar corona. Some theories aiming at the evaluation of the net rate of energy dissipation by such mechanisms will be described. A short account will be given of a recent analytical study based on a generalization of Taylor's hypothesis concerning the evolution of magnetic helicity in plasma with a large magnetic Reynolds number.
Quasi–Static Evolution of Force–Free Magnetic Fields and a Model for Two–Ribbon Solar Flares
Authors: Aly J. J.
Published: June 2019

We show that a sheared 2–D force–free field can evolve in a quasi–static way towards an open configuration, and apply this result to a qualitative theory of two–ribbon solar flares.
Current Sheets in Solar Flares
Authors: Priest E. R.
Published: June 2019

Until recently magnetic reconnection in solar flares was discussed simplistically in terms of either a spontaneous tearing mode instability or a driven Petschek mode. Now the subtle relationship between these two extremes is much better understood. Current sheets may form and reconnection may be initiated in many different ways. There are also a variety of nonlinear pathways from a reconnection instability and several types of driven reconnection.

In solar flares current sheets may be important as new flux emerges from below the photosphere and also as a magnetic arcade closes down after being blown open by an eruptive instability. Numerical simulations of these sheets will be described, including new features such as the presence of a fast shock in Petschek's mechanism and impulsive bursty reconnection due to secondary tearing and coalescence.
An Interpretation of the Millisecond Time Variation in Hard X-Ray Solar Flares
Authors: Krishan V. , Kundu M. R.
Published: June 2019

Recent observations of the fast time variability in the hard X-ray emission from solar flares are interpreted. The fast spikes are assumed to be superimposed on the thermal X-ray emission. The rise and fall of a spike are caused by disruptions in the plasma. The rise time represents the impulsive heating time and the decay or fall time represents a quick cooling of the plasma due to the accelerating growth rate of the m=1 tearing mode. The estimated characteristic time durations of the spike are found to be in good agreement with the observed ones.
Plasma Instabilities Generated by Streaming Particles
Authors: André Mats
Published: June 2019

It is well known that particles streaming along the ambient magnetic field in space plasmas may generate waves with frequencies of the order of the local ion gyrofrequency (ion waves), (Kindel and Kennel 1971). In this study we analyze the dispersion relation of these waves numerically and discuss mechanisms for damping and instability. All numerical results in this report are obtained with the computer code WHAMP (Rönnmark 1982a,b), which solves the dispersion relation of linear waves in a homogeneous plasma for a complex frequency as a function of a real wavevector. As a specific example we consider the S3-3 satellite observations of banded electrostatic ion waves, associated with streaming particles (Kintner et. al. 1979, Cattell 1981).
Anomalous Transport in Current Sheets
Authors: Huba J. D.
Published: June 2019

A review of several microinstabilities that have been suggested as possible anomalous transport mechanisms in current sheets is presented. The specific application is to a ‘field reversed plasma’ which is relevant to the so-called ‘diffusion region’ of a reconnection process. The linear and nonlinear properties of the modes are discussed, and each mode is assessed as to its importance in reconnection processes based upon these properties. It is concluded that the two most relevant instabilities are the ion acoustic instability and the lower-hybrid-drift instability. However, each instability has limitations as far as reconnection is concerned, and more research is needed in this area.
Kelvin-Helmholtz Instabilities in a Magnetised Compressible Plasma (Sheared Flow)
Authors: Ray T. P. , Ershkovich A. I.
Published: June 2019

We have investigated Kelvin-Helmholtz (K-H) instabilities for a homogeneous compressible plasma containing a uniform magnetic field and a linear velocity shear. A derivation of the relevant K-H dispersion equation and details regarding method of solution are given elsewhere (submitted to Mon. Not. R. Astr. Soc.). We present here an outline of our results.
MHD Analysis of the Evolution of Solar Magnetic Fields and Currents in an Active Region
Authors: Wu S. T. , Wang J. F. , Tandberg-Hanssen E.
Published: June 2019

We have used a self-consistent magnetohydrodynamics model to study the evolution of solar magnetic fields in an active region. The problem has been cast as an initial boundary-value problem based on explicit mathematical formalism (i.e., method of projected characteristics), whereby a variety of horizontal photospheric motions can be treated. In this paper we deal specifically with photospheric shear motions in the active region. Our results show the evolution of the magnetic energy, the electric current systems, including the distributions of J⊥ (current perpendicular to the magnetic field) J| (current parallel to the magnetic field), the magnetic field configuration and a comparison between the build-up of magnetic energy in a pre-twisted field and in an initial untwisted field due to photospheric shearing motions. From these results we conclude that the energy build-up is confined within a certain region near the neutral line at the photospheric level. The possible location of the particle acceleration also can be studied.
Spatio-Temporal Features of the Development of Microwave Emission of Active Regions and Flares
Authors: Smolkov G. Ya.
Published: June 2019

At SibIZMIR, a stepwise commissioning of the Siberian Solar Radio Telescope is being under way (Smolkov, 1982a; Smolkov et al., 1983a). This special-purpose instrument is designed to: 1) survey and monitor during daylight hours the state of solar activity (SA) with high two-dimensional resolution on a real time basis in a wave band permitting maximum possible detectability of active regions (AR's) and flares (F's); 2) study structures and development of AR's by day and the time of their observation on the solar disk; 3) study F's; 4) study the three-dimensional pattern of development of AR's and F's jointly with the Sayan Mountain and Baikal Astrophysical Observatories of SibIZMIR (Smolkov, 1982b), and 5) to make a synoptic study of SA during one or several solar rotations in the interests of solving challenging problems of Solar Physics and STP.
Concerning the Dynamics of Energetic Protons in Coronal Magnetic Loops: Dispersion Effects of Alfven Waves
Authors: Mazur V. A. , Stepanov A. V.
Published: June 2019

It is shown that the existence of plasma density inhomogeneities (ducts) elongated along the magnetic field in coronal loops, and of Alfven wave dispersion, associated with the taking into account of gyrotropy U ≡ ω/ωi ≪ 1 (Leonovich et al., 1983), leads to the possibility of a quasi-longitudinal k⊥ < √U k‖ propagation (wave guiding) of Alfven waves. Here ω is the frequency of Alfven waves, ωi is the proton gyrofrequency, and k is the wave number. It is found that with the parameter ξ = ω2 R/ωi A > 1, where R is the inhomogeneity scale of a loop across the magnetic field, and A is the Alfven wave velocity, refraction of Alfven waves does not lead, as contrasted to Wentzel's inference (1976), to the waves going out of the regime of quasi-longitudinal propagation. As the result, the amplification of Alfven waves in solar coronal loops can be important. A study is made of the cyclotron instability of Alfven waves under solar coronal conditions.
The Observations of the Earth Rotation and the Stellar System
Authors: Sato Koichi , Yokoyama Koichi , Tanikawa Kiyotaka , Goto Yukio
Published: June 2019

The fundamental reference of the Earth rotation observation by the method of optical astrometry, such as VZT, PZT, astrolabe, transit instrument and so on, relies upon the stellar system. Hence the stellar positions and proper motions, and the celestial reference coordinate system are essential to preserve the system of the Earth orientation parameters determined by the optical astrometry.
A Photographic Astrometric Telescope for Brazil
Authors: Machado L. E. da S.
Published: June 2019

This paper describes a short focus Astrometric Telescope (D=40cm; F=200cm) at the astronomical station of Campinas, in the State of Sao Paulo, which is a joint operation including the Federal University of Rio de Janeiro, the State University of Campinas, the Catholic University of Campinas and the Municipal Government of Campinas City.
Current Work with the Photoelectric Transit Instrument at the Observatory of Torino
Authors: Chiumiento G. , Fracastoro M. G. , Sarasso M.
Published: June 2019

Anderlucci et al. (1983) describe a multislit photoelectric micrometer attached to the transit instrument of the Observatory of Torino and report some preliminary results.
Automation and Software of the Wanschaff Vertical Circle at Goloseevo
Authors: Kharin A.S. , Kukharskij L.A. , Lazorenko P.F. , Minyajlo N.F. , Tsesis M.L.
Published: June 2019

The Wanschaff vertical circle, set up at Goloseevo in 1949 was built in 1914. The instrument was originally equiped with four visual microscopes for reading the circle, with a visual eyepiece micrometer, and with two levels for the determination of the inclination of the vertical axis. After the instrument had been installed in a new pavilion in 1972, the visual microscopes were replaced by photographic devices, and a semi-automatic machine for measuring the circle readings with output on punched tape was developed. In 1973,the eyepiece micrometer was equiped with a precision potentiometer of the type which together with an automatic voltmeter of the type the digitizer micrometer readings punches them on tape. The other information is punched on cards by the operator at the data desk.
Determination of the Division Corrections of the Bordeaux Declination Circle by the Benevides-Boczko Method
Authors: Requième Y. , Rapaport M.
Published: June 2019

The automatic declination reading system implemented on the Bordeaux meridian circle with a new divided circle is shortly presented. The determination of the division errors by the Benevides-Boczko method was carried out in December 1982 and in March 1983: the standard deviation between the two sets of corrections is about 0.015″.
The Use of Space Telescope to Tie the HIPPARCOS Reference Frame to an Extragalactic Reference Frame
Authors: Hemenway Paul D. , Duncombe Raynor L.
Published: June 2019

Because of the nature of observations with the HIPPARCOS, the observations will be reduced to a set of positions, motions, and parallaxes with respect to a rigid reference frame (HIPPARCOS Instrumental System). This reference frame will have an unknown a priori rotation with respect to both an inertial reference frame and an extragalactic reference frame.
Tycho Astrometry and Photometry
Authors: Høg E.
Published: June 2019

The HIPPARCOS Star Mappers were designed to determine the satellite attitude by observation of stars with known positions. ESA has now enhanced them so that the continuous photon counts from two photo multipliers with B and V response will be recorded. The records will be used to derive an astrometric and photometric catalogue, the TYCHO Catalogue, complete to about B=11 mag and thus containing about 500 000 stars. The positions, annual proper motions and parallaxes will have an accuracy of about 0.03 arcsec and the magnitudes one of 0.03 mag.

The instrument is described and some aspects of the data reduction are discussed, namely the evaluation of a photon record by means of numerical filters, and the use of a TYCHO Input Catalogue.
On the Reference Frame of the Planetary Ephemerides
Authors: Standish E. Myles
Published: June 2019

The planetary and lunar ephemerides have their own inherent reference frame which is one of the key frames to be considered in the unification of the many present day frames, both celestial and terrestrial. The use of this frame requires an understanding of its own accuracies, both internal (i.e., relative) and external(e.g., w.r.t. the FK4 system or w.r.t. the dynamical equinox). Such accuracies are discussed in this paper. Also noted are the expected improvements in the future which will be obtained from newer data types.
Relating the JPL VLBI Reference Frame and the Planetary Ephemerides
Authors: Newhall X. X. , Preston R. A. , Esposito P. B.
Published: June 2019

A series of VLBI observations of planet-orbiting spacecraft and angularly nearby quasars has provided an estimate of the relative orientation of the JPL VLBI reference frame and the ephemeris frame of the inner planets. We find the two frames to be coincident in both right ascension and declination to less than 20 milliarcseconds (mas).
A Fully-Automated System of Astrometric Data Collection and Processing
Authors: Sergienko V.I. , Radchuk A.G. , Pavlov B.A. , Kudeeva V.S.
Published: June 2019

The automated astronomical system “Astrometrist” for the determination of the parameters of terrestrial rotation (DTRP) is intended for astrometric and meteorological data collection and on-line processing and feedback to the user by a computer.
Steep Spectrum Radio Cores and Far-Sightedness
Authors: van Breugel Wil
Published: June 2019

Quite naturally so, VLBI'ers tend to be somewhat far-sighted: because of the very high resolutions, and poor UV-coverages, obtained with current VLBI networks only distant objects can be suitably studied. Unfortunately one thus tends to oversee some basic properties of a very popular class of objects, namely the compact, steep-spectrum radio cores. I would like to put these objects in their proper perspective.
The Infrared, Optical and Ultraviolet Properties of Active Nuclei
Authors: Ulrich Marie-Helene
Published: June 2019

We review the important properties of active nuclei, in particular (i) the optical polarization and its relation to the jets found by VLBI (ii) the energy distribution and the temporal variations of the continuum spectrum and (iii) the distribution of the matter in the broad line region.
Superluminal Effects and Bulk Relativistic Motion
Authors: Cohen M. H. , Unwin S. C.
Published: June 2019

The observational status of superluminal radio sources is reviewed, and the results for one well-studied source (3C 345) are discussed in detail. We discuss the relativistic-beam model of superluminal sources, and concentrate on one aspect, namely the geometric constraints provided by combined X-ray and proper-motion measurements.
Multi-Epoch Observations of Survey Sources
Authors: Readhead A.C.S. , Pearson T. J. , Unwin S. C.
Published: June 2019

Five objects mapped as part of a VLBI survey have been re-observed at 5 GHz, and four of them have also been observed at 10 GHz. Three of the objects show no substantial structural variations: an upper limit of 2c can be placed on the apparent relative velocities of the components. One object (0711+356) shows structural variations which are mostly simply described in terms of a superluminal contraction. The remaining object (3C371, 1807+698) shows substantial structural variations which suggest that it probably is a superluminal source. The source 0710+439 is especially interesting as it consists of a central flat-spectrum core component straddled by two compact steep-spectrum components.
Superluminal Motion in Weak Quasar Cores
Authors: Porcas R. W.
Published: June 2019

The importance of studying the statistics of the occurrence of superluminal motion in an unbiased sample of radio sources is stressed, and methods for selecting such samples are reviewed. The weak radio cores of quasars exhibiting extended, double-lobed emission can be used for this purpose. Recent studies of the superluminal motion in the core of 3C 179, a quasar selected from such a sample, are described.
An Outburst of OJ 287
Authors: Kikuchi Sen , Inoue Makoto
Published: June 2019

Optical and radio behaviours of OJ 287 in the recent outburst are presented. The light curve is similar to that in the previous outburst in 1970–1971, although the maximum brightness is about 1 mag fainter. The intraday variations are found in both optical and radio regions. We find no periodicity of time scales less than 0.2 days.
Evidence of Relativistic Beaming in BL Lac-Type Objects
Authors: Worrall Diana M.
Published: June 2019

Multifrequency observations of a variable extragalactic object, when all acquired within the inferred variability time scale of the source, can provide clues to the source's energy mechanisms. A guest observer program with the International Ultraviolet Explorer satellite has provided the focus for such measurements of a few BL Lac-type and related objects at frequencies in the radio, mm, IR, visual and UV. Earlier-epoch X-ray measurements are included in subsequent model fitting. This paper summarizes some of the observations in this program. Synchrotron self-Compton (SSC) models are then applied to the data, leading to the conclusions that the objects are relativistically beamed and that radio emission, at least below a frequency of ~20 GHz, is from a separate source region.
A Nuclear Regula-Tori Model for Quasars and Seyferts
Authors: Begelman Mitchell C.
Published: June 2019

Although VLBI reveals structure in active galaxies on scales as small as a few parsecs, this is still likely to be several orders of magnitude larger than the scales on which jets are accelerated. We must therefore rely on theory, bolstered by indirect evidence, to deduce the behavior of plasma immediately surrounding the “central engine”. One hypothesis which has received much attention recently postulates that jets emerge from thick tori of gas which surround massive black holes. The low optical and X-ray luminosities of radio galaxies imply that tori in these objects would have to be supported by ion pressure, while tori in quasars and Seyferts may be supported by radiation pressure. Rees et al. have proposed that two parameters, α(the ratio of viscous stress to pressure) and (the ratio of the accretion rate to the value associated with the Eddington limit), are sufficient to determine the general characteristics of a torus, independent of the mass of the black hole. While existing observations of radio jets do not contradict the existence of such a homology, systematics involving the broad emission lines in quasars and Seyferts suggest that the homology may fail in these objects, at least insofar as conditions at ≲1 parsec reflect conditions in the torus. I report here on how the onset of nuclear reactions in a radiation-supported torus may break the homology, and regulate the properties of the broad line regions in quasars and Seyferts.
Jets and Broad Emission Line Regions
Authors: Barthel Peter , Norman Colin , Miley George
Published: June 2019

In recent years an elegant hypothesis for a unified scheme for compact and extended radio sources, and also radio quiet QSOs and radio loud Quasars has been developed (Scheuer & Readhead 1979, Orr & Brown 1982). In this scheme the relative orientation of (relativistic) beam and observer is thought to explain the observed source morphology. We feel that observations suggest examining compact and extended radio sources from still another “angle”.
1038+528 A, B: Phase-Reference and Spectral-Index Maps
Authors: Marcaide J. , Shapiro I. , Cohen N. , Corey B. , Cotton W. , Gorenstein M. , Rogers A. , Romney J. , Rönnäng B.
Published: June 2019

On 1981 March 17–18 we undertook MkIII VLBI observations of the quasars 1038+528 A, B (Owen et al. 1979; Owen et al. 1980) with an array of 7 telescopes operating simultaneously at λ3.6 and λ13 cm with right circular polarization reception at each wavelength. Because the sources are ~33″ apart they could be observed simultaneously at every telescope. Thus the corrupting contributions of the propagation medium and the instrumentation were approximately the same for each of the quasars, hence allowing us to calibrate the structure phase of B with respect to a reference point chosen in the map of A using the expression

where φB and φA are the observed fringe phases, φGB and φGA are the geometric contribution with respect to the reference points chosen in each map and φSA is the structure phase contribution with respect to the reference point chosen in the A map.
Radio Flares from Circinus X-1
Authors: Nicolson G. D.
Published: June 2019

The galactic x-ray source Circinus X-1 is notable for its periodic flares at x-ray, optical, infrared and radio wavelengths. The flares recur at 16.6 day intervals, but there is considerable variability from one cycle to another. (See Dower et al, 1982, and references therein for a detailed review of the properties of Circinus X-1).
Nearly Microarcsecond Precision Differential Astrometry
Authors: Marcaide J. , Shapiro I. , Ball J. , Bartel N. , Clark T. , Corey B. , Gorenstein M. , Preston R. , Ratner M. , Rogers A. , Whitney A.
Published: June 2019

In studies of extragalactic radio sources with multiple compact components the determination of which components, if any, are stationary and which moving is of importance. In order to learn about the radio properties of the individual components it is also relevant to be able to register maps made at several wavelengths. Both tasks are usually not possible with VLBI because of the irrecoverable corruption of the fringe phase introduced by the propagation medium and the instrumentation. However, when two or more compact radio sources are separated by only a small angle from each other difference techniques can be used to help tackle both questions.