On the dust and gas associated with Sharpless 252

The stellar cluster and the massive molecular cloud associated with theHii region Sharpless 252 have been studied by means of multicolour polarization and molecular line measurements. The average wavelength of the maximum polarization _max and polarization efficiency for the cluster stars are similar to the values observed for the nearby field stars. Two local maxima lying only 2 apart were found in the molecular cloud core in CO, NH₃, and HCO⁺. The excitation conditions and radial velocities associated with the maxima are different.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

SPECTROSCOPOLARIMETRIC SENSING OF ENERGY DEPOSITION INTO THE CHROMOSPHERE DURING SOLAR FLARES – I. Observations

Measurements of linear polarization in hydrogen H and H lines, made with the Large Solar Vacuum Telescope of Baikal Astrophysical Observatory and Automated Solar Telescope of Sayan Solar Observatory, affiliated with the Russian Institute of Solar and Terrestrial Physics, are reported in this paper. Short-term polarization associated with solar flares is found to be present in active regions. There is a significant tendency for the H polarization vector to be radial, i.e., in the flare-to-disk-center direction. This polarization may be due to atmospheric bombardment by hecta keV protons. On the other hand, the polarization vector is found to be perpendicular to the radial direction at some locations where the line profile has a typical mustache shape suggesting a bombardment by energetic electrons. The H line is also linearly polarized. However, no preferential direction of polarization is found in this line, which is formed more deeply in the solar atmosphere.     

On the fine structure of polarized elements in solar flares and moustaches

H spectral observations of flares and moustaches using linear polarization analyser have been carried out. It is found that some flare knots and moustaches show strong polarization.     

Interpretation of type I- and IV mB-bursts and noise storms by mode coupling in the warm plasma

The circular polarization of radiation emitted from the solar type I- and IV mB-bursts and noise storms is not understood very well. For an attempted new explanation the dispersion equations for the ordinary (left-handed) and the extraordinary (right-handed) wave are derived from the well-known tensor conductivity of warm plasma proposing a very small angle between magnetic field and propagation direction, and the plasma parametersX1,Y1. Taking into account a plasma temperature different from zero, conditions of a coupling point (Budden, 1961) are attained very nearly even if the very small collision frequency is neglected. It is shown, that the observed ordinary (left-handed) polarization may be explained by a process of mode-coupling between the originally emitted extraordinary (right-handed) wave and the resulting ordinary wave. The source of the right-handed radiation may be gyro-radiation or erenkov-radiation. The proposed mechanism is in accordance with the main observational facts. It remains open, whether the known magnetic asymmetry of active regions accounts for the prevailing left-handed polarization.     

The Australian radio-telescope

A large synthesis radio telescope is under construction in Australia at a cost of $A 30.7million. In comprises a 6 km-long compact array of 22 m antenna at Culgoora and a long baseline array of 319 km, achieved by adding another 22 m antenna near Coonabarabran and the existing 64 m antennae at Parkes. On occasion it will be possible to use this arry with one of the deep space network antennas at Tidbinbilla to achieve a baseline of 580 km.The arrays have been designed to provide high-resolution radio images over a wide field with high-dynamic range plus good polarization and spectral line performance. The operating frequency range is from about 327 MHz to over 50 GHz.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

A nonlinear tensor field and the second metric tensor

In our preceding paper {see [L. Sh. Grigorian and S. Gottlöber, Astrofizika (in press)]} we investigated a self-gravitating system consisting of a scalar field and a linear tensor field _ik= _ki with minimal coupling and with allowance for the action of vacuum polarization effects. In the present paper we investigate the case of a nonlinear tensor field _ik. The action S ⁽⁾ of the field _ik is determined by the difference R_ik — _ik, where R_ik is the space-time Ricci tensor and Rik is the analogous quantity constructed using the metric _ik=g_ik+_ik induced by _ik ( is a free parameter). Here S ⁽⁾ coincides with the previously known expression for the action of a linear field _ik. Equations of motion are derived for _ik in curved space-time. The energy-momentum metric tensor, determining the contribution of _ik to the gravitational field equations, is calculated.Translated from Astrofizika, Vol. 39, No. 1, pp. 135–144, January-March, 1996.     

Observations of resonance polarization in Cai λ4227

The resonance polarization in Ca i 4226.73 has been measured close to the solar limb for three values of = cos. The polarization has maxima in the wings at about –500 and + 400 mÅ from the line center, the plane of polarization (electric vector) being parallel to the solar limb. The blue wing is more polarized than the red wing, contrary to earlier results of Brückner (1963). Local minima in the polarization curves occur at the position of blends. The polarization in the wings increases rapidly with decreasing . At = 0.15 the maximum polarization in the blue wing is about 2.4%.The observations are in good qualitative agreement with the recent theory of Dumont et al. (1973), but the observed polarization degrees are significantly smaller than the computed values. This may be explained by the inadequate formula for van der Waals broadening used in the theoretical calculations. Much of the discrepancies between observed and computed values disappear if the recent value of Lites (1974) for the collisional damping constant is used. Further improvements in theory and observation are discussed.     

Gyro-synchrotron emission in a magnetic dipole field for the application to the center-to-limb variation of microwave impulsive bursts

In order to interpret the observed center to limb variations of spectrum and polarization of microwave impulsive bursts, gyro-synchrotron emission from nonthermal electrons trapped in a magnetic dipole field is computed. The theoretical spectrum and polarization are consistent with observed ones if we put an outer boundary of the radio source at a layer of 100-60 G or (7–9) × 10⁴ km in height. Rather small observed center-limb variations in intensity and polarization are attributed to the distribution of , an angle between the magnetic field and the direction of observer, in the radio source emitting the burst, though the intensity and polarization depend strongly on especially at small values of .     

The Zeeman effect for weak magnetic fields

The polarization of a normal Zeeman triplet is discussed for the case in which the lifetime of the excited state of the atom is comparable to or shorter than the period of Larmor precession.     

Properties of theβ pictoris disc deduced from optical imaging polarimetry

The asymmetries in the brightness structure and levels of polarization between the two sides of the Pic disc are discussed. The possibility of these differences being caused by a planet, or planets, within the circumstellar disc is investigated.     

The λ 747 coronal line at the 1966 eclipse

Observations are presented of emission line resonance polarization in Fe xiii 10747 at the total solar eclipse of 12 November 1966. Useful data, with angular resolution 15, describe three quadrants of the corona from 1.08 R to a maximum of 1.6 R . The direction of the electric vector of observed polarization is perpendicular to the solar limb, to the limits of accuracy of measurement, in at least 74% of all cases. Departures in the other points are consistent with the magnetic depolarization expected from the non-radial fields of streamers. Polarizations observed range from near zero at the limb to 80 % and higher at 1.6 R . Averaged polarization is highest in non-streamer regions, where above 1.2 R it suggests pure radiative excitation of the 10747 line. Below 1.2 R , and in a dense streamer, the polarization is significantly depressed, indicating dominant collisional excitation of the line wherever the electron density exceeds 50 × 10⁶ cm^–3.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Delay between the circularly polarized components in fine structures during solar type IV events

We analyzed intermediately polarized (20–80%) fine structures (pulsations, sudden reductions, fiber bursts and zebras) that were recorded in type IV events. The mean polarization degree was practically the same for all the fine structures recorded in an interval lasting a few minutes and it was similar to the polarization of the continuum. A detailed analysis during the evolution of single structures reveals changes in polarization (in particular an undulation at flux density minima) even stronger than 20%. They were caused by a delay, up to 0.1 s, between the two circularly polarized components. The weaker polarimetric component was delayed in 2 sets and the stronger one in 1 set. In the event of April 24, 1985 different types of fine structures were sporadically detected in more than one hour long time interval. Short delays of the stronger or of the weaker component were sometimes observed.The events characterized by fine structures are generally totally polarized in the ordinary mode. We assume that this holds also for the phenomena studied here. The observed intermediate polarization therefore requires a depolarization due to propagation effects. We discuss the mode coupling and the reflection of the original radio signal that could also generate the delay of the weaker and the stronger component respectively. The possibility of polarization variation due to the change of the angle between the direction of the propagation and the magnetic field in a quasi-transversal region and in a low intensity magnetic field in a current sheet is also given.Presented at the CESRA-Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994.     

Polarimetry of comets: A review

Some attempts of polarimetric sounding of Comet Halley will be undertaken from the flyby probes. In order to facilitate the final planning and the future interpretation of these experiments we have done a thorough analysis of practically all available polarimetric observations. An emphasis is made on interpretation attempts and their discussion. The results of the phase dependence of polarization investigations are presented covering a wide range of phase angles . The chief peculiarities of this dependence are: maximum polarization at = 90, diminishing through zero at 20, negative values up to several per cent and a final growth to zero at zero . A division into gaseous and dusty comets on polarimetric basis is revealed. The wavelength dependence of polarization is discussed. The numerous results of detailed polarimetry are compared to the negative results of attempts to detect the elliptical polarization. New observational problems arising from the evidence given by the negative polarization at small phase angles and by the opposition effect recently discovered are discussed.     

Sensitivity of γ-ray detectors to polarization

Previous studies have shown that the largest -ray detector to date, EGRET, does not have useful polarization sensitivity. We have explored here some improved approaches to analyzing -ray pair production events, leading to important gains in sensitivity to polarization. The performance of the next generation -ray instrument GLAST is investigated using a detailed Monte Carlo simulation of the complete detector.     

Corrections to the lunisolar precession and the motion of the equinox from proper motions of Cepheids

Corrections to Newcomb's lunisolar precession and to motion of the equinox are evaluated from proper motions of 77 Cepheids, employing recently determined values of their distances. Results are diverse, depending on the weighting method. If the effect of the cosmic dispersion is neglected and ₀ is imposed to be 25 km/s/kpc, thenp ₁=1.1±0.2 and +e = 1.0 ± 0.1 per century in the FK4 system.Presented at IAU Colloquium No. 9, The IAU System of Astronomical Constants, Heidelberg, Germany, August 12–14, 1970.     

Behaviour of the polarization at dm-m wavelengths during the evolution of five two-ribbon flares

The general behaviour of the circular polarization at dm-m wavelengths during the evolution of five two-ribbon flares is investigated. The changes of polarization, if present, occurred 10 to 20 min after the impulsive phases. Increases of the radio and X-ray fluxes occurred at the moments when the H ribbons started to extend over spot umbrae.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste). Italy, 27–31 May, 1985     

A search for polarization in Ellerman bombs

Ellerman bombs, also called moustaches, are transient brightenings at tiny (< 1 arc sec) points in the lower chromosphere whose spectra are characterized by elongated emission wings on the hydrogen Balmer lines. Babin and Koval recently found linear polarization as high as 20% in bombs, but no physical process that could produce such a high degree of polarization was suggested. A new observational study of polarization in Ellerman bombs is reported here. Images of 32 bombs were obtained with a digital video system viewing the Sun through a 6 Å filter at the Sacramento Peak Vacuum Tower Telescope. A novel polarizing beamsplitter divided each image into two interleaved polarized components which passed simultaneously through a single set of optics and were separated only during data analysis. The sensitivity threshold of the measurements was 1%. In 4 cases out of 32, linear polarization above 2% was detected. The higher incidence of > 2% polarization reported by Babin and Koval is not confirmed.     

Polarization of solar active regions at 9.5 mm wavelength

A study of the circular polarization structure of solar active regions has been made from data obtained at 9.5 mm wavelength, using the 85 ft reflector and polarimeter at the Naval Research Laboratory Maryland Point Observatory. The angular resolution of the telescope at this wavelength is 1.6. All important active regions observed at 9.5 mm are bipolar in nature, the degree of polarization is about the same for both right and left circular components and it ranges up to about 4%. These oppositely polarized components correspond with the Mt. Wilson magnetic regions of opposite polarity; the line of zero polarization delineates clearly the neutral line between the regions of opposite polarity on magnetograms. Unipolar regions in magnetograms also show up as unipolar regions at 9.5 mm. Magnetic fields as low as 5–10 G on magnetograms manifest as distinctly polarized regions on 9.5 mm maps. A line of zero polarization seems to delineate the extent of absorption features observed at 9.5 mm in coincidence with H dark filaments.     

PROBLEMS AND PROGRESS IN COMPUTING THREE-DIMENSIONAL CORONAL ACTIVE REGION MAGNETIC FIELDS FROM BOUNDARY DATA

An overview of the whole process of reconstructing the coronal magnetic field from boundary data measured at the photosphere is presented. We discuss the errors and uncertainties in the data and in the data reduction process. The problems include noise in the magnetograph measurements, uncertainties in the interpretation of polarization signals, the 180° ambiguity in the transverse field, and the fact that the photosphere is not force-free. Methods for computing the three-dimensional structure of coronal active region magnetic fields, under the force-free assumption, from these boundary data, are then discussed. The methods fall into three classes: the extrapolation technique, which seeks to integrate upwards from the photosphere using only local values at the boundary; the current-field iteration technique, which propagates currents measured at the boundary along field lines, then iteratively recomputes the magnetic field due to this current distribution; and the evolutionary technique, which simulates the evolution of the coronal field, under quasi-physical resistive magnetohydrodynamic equations, as currents injected at the boundary are driven towards the observed values. The extrapolation method is mathematically ill-posed, and must be heavily smoothed to avoid exponential divergence. It may be useful for tracing low-lying field lines, but appears incapable of reconstructing the magnetic field higher in the corona. The original formulation of the current-field iteration method had problems achieving convergence, but a recent reformulation appears promising. Evolutionary methods have been applied to several real datasets, with apparent success.     

A solar flare X-ray polarimeter for the space shuttle

We have recently built and tested an instrument designed to measure the polarization of the hard (5–30 keV) X-ray emission from solar flares, and thereby to investigate the energy release mechanism and constrain flare models. In particular, these measurements will help to determine whether hard X-ray bursts are produced by nonthermal or by thermal electrons. The polarimeter makes use of the angular dependence of Thomson scattering from targets of metallic lithium. It has an energy resolution of a few keV, a time resolution of 5 s, and sufficient sensitivity to measure polarization levels (3) of a few percent in about 10 s for a moderate strength solar flare. The instrumental polarization has been directly measured and found to be within the design goal of 1%. This polarimeter is scheduled to be flown as part of the OSS-1 pallet on an early Space Shuttle mission.