Precise determination of the orientation of the plane of polarization in the solar corona

It has been shown by Molodensky (1973), that precise measurements of the position of the plane of polarization in the corona may allow us to observe overthermal electrons in the solar corona. For such measurements during the eclipse of 10 July 1972, a method based on the photographic recordings of the corona by means of a cineset and with an automatically rotating polaroid has been developed. A technique has also been developed for determining the position of the plane of polarization by means of isophotes obtained with polarization filters. This technique uses the photometric data for determining phase shifts between the apparent intensity variation curve and a similar curve expressing the rotation phase of the polaroid. The results of the measurements for h/r _⊙ =0.5 to 0.9 allow us to conclude that:

1. The plane of polarization (E-vector position) coincides very exactly with the tangential direction in the region of N-W limb. The maximum deviations of this plane amounts to 1–1.5°, and the mean-square deviations in this region amount to ～0,3° at h/R _⊙ ≈1. This coronal region was the least active one and there were no spots there.
2. The corona near the E limb consisted of two ‘fans’ divided by a thin beam. In that region some deviations of the plane of polarizarion from the tangential direction were revealed. Those deviations were of the order of 3°. During the time of the eclipse there were some groups of spots behind the E limb (but close to this limb). The observed deviations were apparently connected with those groups.
3. Calculations have been made of the turn of the plane of polarization caused by an inhomogeneity in the radiation field from the photosphere and due to the presence of spots. The effect qualitatively coincided with that shown by the measurements.

     

Magnetic fields in galaxies as revealed by the polarization of light

Some new results on optical polarization in galaxies are reported. These results as well as some other available data indicate the presence of large-scale magnetic fields in galaxies. Spiral galaxies seen nearly edge-on show polarization of light in the dark band across the center, indicating that the large-scale magnetic fields in these galaxies are mainly parallel to the symmetry plane of each galaxy, as has also been observed in the Milky Way. In more tilted galaxies, where the spiral structure can be seen more clearly, the observed polarization effects indicate large-scale magnetic fields along the arms. Observed polarization effects also indicate the presence of ordered magnetic fields in some peculiar galaxies. In NGC 2685 the fields seem to be parallel to the helical or arc-like filaments. In NGC 3718 the dark band which might be a dust-bar through the nucleus shows strong optical polarization in the direction of the band, indicating a large-scale magnetic field in the same direction.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May, 1978.     

OJ 287: Polarization and photometric behaviour during 1971–76

The results are given of polarimetric and photometric observations of BL Lacertae-type object OJ 287 for 1972–76. These, and all other data available from the literature, are used in a comparative analysis of polarimetric and photometric properties of the object. The variations of time-scales from several years to several hours are noted. The variability is caused by the flaring up and fading of separate sources (hot spots) of polarized (synchrotron) radiation. The existence of a preferable direction of polarization (₀=80°) is an indication of a stable magnetic field. It may be used as an argument in favour of the single-body hypothesis of Lacertids. The behaviour of OJ 287 during a 6-day interval may be explained by intensity changes of a single source with constant parameters of polarization. It is found that, for this source,p=42.8%, ₀=101°. The night variations of brightnes and parameters of polarization limit the dimensions of the source responsible for this variability (R<-5×10¹⁴ cm) and enable us to estimate its degree of polarization (p50%).     

The relation between brightness fluctuations and polarization of solar metre-wave emission

Solar 80 MHz emission recorded by the Culgoora radioheliograph over a number of years, no matter what its spectral type, has followed a persistent pattern. Emission with average polarization < 40%, even if bursty, shows constant polarization: the polarization remains weak during both lulls and bursts of brightness. By contrast, if emission with average polarization > 40% is bursty, it shows fluctuating polarization: the polarization decreases during the bursts. The effect is discussed in terms of streaming versus gyrating electrons.     

Analysis of Polarimetric,Photometric, and Spectroscopic Observations of Comet C/1996 Q1 (Tabur)

We present the imaging polarimetry and photometry of Comet C/1996 Q1 (Tabur) obtained on October 10, 1996, with a two-channel focal reducer attached to the 2-m Pik Terskol Observatory telescope through blue (4430/44 Å) and red (6420/26 Å) continuum filters and through a 6620/59 Å filter that isolated the NH₂(0.7.0) band. We analyze the 3600–9300 Å long-slit spectrograms of the comet taken on October 5–6, 1996, with the 2.6-m Crimean Astrophysical Observatory telescope. The NH₂(0.8.0) 6408 Å emission and an unidentified 6428 Å emission were found to fall within the pass band of the red filter. The blue filter transmits weak unidentified emissions at 4424–4444 Å and partially C₂(4360 Å). Correction for the depolarizing effect of molecular emissions resulted in an increase of the dust polarization by 2–4% in the near-nucleus region and by almost a factor of 2 in the outer coma regions. However, the polarization and color differences between different coma regions remained even after correction for the contribution of emissions. We found no dust polarization difference between the gas comet Tabur and the dust comet C/1988 A1 (Liller), which are believed to be fragments of a common parent comet. The NH₂coma was found to be elongated perpendicular to the comet radius vector. The causes of the spatial asymmetry in the NH₂molecular distribution are yet to be established. We study the evolution of activity and the spatial distribution of dust brightness, polarization, and color in the comet. We consider a taxonomic classification of gas and dust comets according to dust polarization properties. The polarization differences between dust and gas comets at large phase angles are most likely related both to the actual differences in dust and to the effect of molecular emissions, nuclear gas- and dust-production rates and to the evolution of grain properties with distance from the nucleus.     

Kinematics of solar prominences

Two sequences of OSO-4 spectroheliograms in Mg x and Si xii obtained during October–November 1967 and covering the intervals of 83 and 22 hr, respectively, have been analyzed to reveal quasi-periodic oscillations of EUV flux from solar sources with a periodicity of 5–14 hr. The oscillation periods of the emission flux from local sources over sunspots and magnetic field enhancements in plages without spots have been investigated in correlation with characteristics of the respective AR and plages. The greatest periods (> 8 hr) are shown to be peculiar of small sunspots or sunspot groups at the initial or final stage of their development, whereas the smallest periods ( 5–6 hr) are observed in the case of large well-developed groups at the maximum stage of development. In quiet regions on the Sun and plages without spots, the oscillation periods are 6–8 hr. The surface areas in which the oscillations are synchronous and coincide in phase have typical dimensions of 1 in quiet and 1 to 5 in active regions. These areas form a spatial structure similar to the chromospheric network and supergranules. The characteristic lifetime of the structure elements is 1.5–2 days.     

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.     

Spotless flares and type II radio bursts

Flares accompanied by type II meter radio bursts that occurred in plages with no visible spots are examined in this paper. There have been found 12 such spotless flares observed in the period of January 1981–August, 1990. Six out of all the flares may be said to have not been associated with any filament activation or disruption. A few of these flares have shown features of major events. The study suggests that a filament activation seems not to be the crucial factor for the occurrence of major flares in regions with no visible spots.     

Electron impact polarization of Lα-radiations from hydrogen-like ions

The Oppenheimer-Penny theory, as developed by Percival and Seaton (1958), is applied to calculate the polarization of L radiations from hydrogen-like ions when the impact electrons are distributed such that their probability is more in the regions close to the magnetic field. The degrees of polarization are averaged over the energy of non-thermal electrons generated during the initial phase of solar flares. It is found that the degrees of polarization do not depend on the atomic number of the ion.     

The imaging vector magnetograph at Haleakala

We describe an instrument we have built and installed at Mees Solar Observatory on Haleakala, Maui, to measure polarization in narrow-band solar images. Observations in Zeemansensitive photospheric lines have been made for nearly all solar active regions since the instrument began operations in 1992. The magnetograph includes a 28-cm aperture telescope, a polarization modulator, a tunable Fabry-Pérot filter, CCD cameras and control electronics. Stokes spectra of a photospheric line are obtained with 7 pm spectral resolution, 1 arc sec spatial resolution over a field 4.7 arc min square, and polarimetric precision of 0.1%. A complete vector magnetogram observation can be made every eight minutes. The flexibility of the instrument encourages diverse observations: besides active region magnetograms we have made, for example, composite vector magnetograms of the full solar disk, and H polarization movies of flaring regions.     

An investigation of the linear polarization in emission features of active regions

A study has been made of linear polarization in moustaches using the H and H lines. A Wollaston prism and an achromatic /2-plate were employed in the study. In the course of the observations, the major axis of the plate was placed at 0° and 22°.5 with respect to the horizontal axis of the installation. The Stokes parameters J, Q, and U were recorded, thus enabling the degree of polarization and the field azimuth to be determined. During the first three minutes of the existence of the moustaches, the two lines revealed a linear polarization of about 7% located at the center of the lines. No polarization was found during the later stage of the moustaches' lifetimes. Whenever the linear polarization is present, there is a tendency for the polarization to decrease rapidly, in accompaniment of a turning of the azimuth. Several mechanisms for interpreting the polarization observed in the moustaches are examined. Some suggestions are made in support of the excitation of hydrogen atoms in the moustaches initiated by a high-energy electron flux or vertical heat conduction.     

Spectral models of AM Her stars

Most of the optical light from AM Her systems is cyclotron radiation. Simple models for the spectrum and polarization are successful for some systems, but in others the spectrum rolls over more gently at high frequencies and is polarized over a wider range of frequencies than the models predict. This paper considers the emission from both the sides and top of the accretion column and the effects of oscillations in the shock height. These features lead to a slower roll over at high frequencies than is found in the simple models, but it is still not as flat as in some of the AM Her systems.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

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.     

Stellar polarization and the structure of the magnetic field of our galaxy

The stellar polarization data have been examined using a new catalogue containing accurate stellar distances. On the assumption of a magnetic alignment hypothesis, correlations on the larger distance scale indicate the existence of a dominant regular magnetic field, although its characteristics are difficult to determine. Within 500 pc its direction is towardsl45° and beyond this towardsl60°, though it is clear that such a longitudinal model is too simple. There is also some evidence for an inclination of this field to the galactic plane. The distribution of the polarization vectors away from the galactic plane has been examined and it is proposed that the two largest loop structures, previously identified as Supernova remnants, are linked by the regular field. Incremental polarization maps have been produced but they show little correlation with the spiral structure. The polarization appears to be saturated at about 1 kpc from the Sun, which is explained as the result of an observational selection effect. On the smaller distance scales an autocorrelation analysis in different directions has revealed no obvious coherence in the irregular component on scales greater than 50 pc.     

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.     

Broad-band circular polarimetry of sunspots, 0.4–1.7 microns: Spatial scans with a 3.4 arc sec diameter aperture

Spatial scans with a resolution of 3.4 arc sec of the broad-band circular polarization of several sunspots have been made in five filter bands over the wavelength range 0.4–1.7µ with a sensitivity of 1 × 10^–6 fractional polarization. The scans, across a spot through the penumbra and umbra center, revealed two important features: (1) The broad-band circularly polarized fluxV reverses in sign, or diminishes to near zero, at the center of the umbral region relative to the outer penumbra. This effect was wavelength dependent and was most clearly detected as a definite reversal in a band at 1.2µ, although a reversal was also detected in a very broad band extending from 0.8 to 1.6µ. (2) There is a marked asymmetry: in all cases the limbward penumbral region exhibited strongerV values than did the disk-center (inward) side of the spot, at all observed wavelengths. Such previously unreported structure in the magnetic circular polarization of sunspots provides new clues for understanding the anomalous large broad-band polarization at short wavelengths and at the same time imposes new constraints on sunspot models. For example, the polarization reversal in the umbra relative to the penumbra can be naively explained by return-flux sunspot models; but this is not the only interpretation. Alternatively, it can relate to reversals in mass-flow velocities and/or vertical velocity gradients, as between the umbra and penumbra.     

Contribution to interstellar extinction from an astrophysical microsoot?

The optical constants of a carbon polymorph, calculated using the Drude dispersion theory, are shown to lead to close agreements with the mean interstellar extinction curve over the waveband 0.3 ^-1 5.5 m^-1 for particles of Rayleigh scattering sizes. Astrophysical microsoot grains of radii 50Å may be related to microdiamond grains of similar sizes that have recently been discovered in carbonaceous chondrites. It is postulated that such microsoot particles could contribute to the nearly invariable interstellar extinction law observed over the near IR, visible and near to mid UV spectral regions. Admixtures of microsoot with micron-sized microsoot clumps, microdiamond and hollow organic / biologic grains provide an explanation for the full range of extinction and polarization data.     

Polarization changes with time during solar microwave impulsive bursts

The evolution with time of circular polarization (t) from solar bursts at 7 GHz presents, in the majority of cases, a polarization degree peak before the maximum flux time. The subsequent evolution of (t) is continuous and usually increasing. The changes could be caused by superimposed polarization effects, due to the fast emissive electrons (dominant in the first phase), and to the propagation effects caused by the coronal condensation where the event occurred (dominant in the second phase). In an approximate approach, (t) is connected to the movement of the source in the second phase, being qualitatively sound, but limited to the lack of knowledge on acceleration processes and on magnetic field topology in the active region where the flares take place.     

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.