Coronal Polarization Research Articles

Polarization of the solar corona in the green line: Contradiction between theory and observations

The main results of polarization observations in the 530.3-nm line and their role in studying the physical conditions, structure, and magnetic field in the solar corona are discussed. A serious discrepancy between the observations and widely-accepted theoretical concepts was revealed: the theory predicts that the orientation of the polarization electric vector should be nearly radial, in contradiction with the observational results. In particular, the polarization vectors for both the green line and white-light corona in high-latitude streamers were tangential during the eclipse of July 11, 1991. The dependence of the degree of polarization on the angle between the radial direction and the magnetic-field vector was calculated without any a priori assumptions about the configuration of coronal fields. This theoretical analysis of the polarization-vector orientation for magnetic-dipole emission in the green line are in agreement with results obtained previously in other studies. Some ways to resolve the observed discrepancies are discussed.

Dynamics of Polar Plumes Observed at the 1998 February 26 Eclipse

This paper presents first observations of dynamics of the white-light solar corona detected during the few minutes of totality of a solar eclipse. Perturbations of a polar plume associated with an embedded ‘jet’ structure observed simultaneously at 195 A with the EUV Imaging Telescope (EIT) aboard the SOHO spacecraft lead to estimates of the electron density fluctuations accompanying the jet: ±15%. The morphological behavior of the jet, its apparent upward propagation speed of ≈ 200 km s−1, and the inferred density perturbations suggest that the jet is led by a weak, outward-propagating shock resulting from the injection of material at high velocity at the base of the corona. Smaller perturbations of the white-light corona are apparent at many other locations, sustaining hope that propagating Alfven waves may be measurable in the solar corona. Density perturbations associated with the jet follow from empirical electron density models of the polar inter-plume and plume regions, as derived from the ground-based eclipse measurements of coronal polarization brightness. These models indicate polar plume densities 4–6 times that of the interplume low corona.

Possible east side predominance of the optical emissions of the solar corona

A long term analysis of the green and the red line intensities of the emitting solar corona as well as the polarization of the white corona, which have been compiled by the Pic-du-Midi, Kislovodsk, Irkutsk and Lomnisky Styt observatories, has led to some very interesting results. A prominent East-West asymmetry is obvious in most of the data while a very characteristic seasonal variation of this asymmetry with maxima close to December and minima in July–August is also present. All the errors involved in coronal optical measurements have been examined in a previous paper but none of them have been underlined as the possible cause of the east-west asymmetry. In such a case, the presence of this asymmetry should not be ignored while the reason for its existence should be studied, extensively. Two approximations to a possible explanation of the solar E-W asymmetries have been reported in the discussion section of this article.

White-Light Polarization and Large-Scale Coronal Structures

The results of the white-light polarization measurements performed during three solar eclipses (1973, 1980, 1991) are presented. The eclipse images were processed and analysed by the same technique and method and, consequently, the distributions of the polarization and coronal intensity around the Sun were obtained in unified form for all three solar eclipses. The mutual comparisons of our results, and their comparison with the distributions found by other authors, allowed the real accuracy of the current measurements of the white-light corona polarization, which is not worse than ±5%, to be estimated. We have investigated the behaviour of the polarization in dependence on heliocentric distance in helmet streamers and coronal holes. Simultaneous interpretation of the data on polarization and intensity in white-light helmet streamers is only possible if a considerable concentration of coronal matter (plasma) towards the plane of the sky is assumed. The values obtained for the coronal hole regions can be understood within the framework of a spherically symmetrical model of the low density solar atmosphere. A tendency towards increasing polarization in coronal holes, connected with the decrease of the hole's size and with the transition from the minimum to the maximum of the solar cycle, was noticed. The problem of how the peculiarities of the large-scale coronal structures are related to the orientation of the global (dipole) solar magnetic field and to the degree of the goffer character of the coronal and interplanetary current sheet is discussed briefly.

The structure and physical properties of the quiet corona as inferred from the July 11, 1991 solar eclipse data

Abstract We were successful in observing the linear polarization of the solar corona in the white light and in the light of the green emission line 530.3 nm during the July 11, 1991 total solar eclipse (La Paz, Mexico). Preliminary analysis reveals remarkable differences in the degree of polarization for both sets of data, particularly for the distribution of polarization around the sun's limb. The upper theoretical limits of the degree of polarization are compared with our results. The real structure of the corona is compared with the calculated coronal magnetic field. The rather unusual shape and structure of the eclipse corona in a given of the solar cycle are discussed in connection with a definition of coronal flattening.

FeXIV Line Emission Polarization of the July 11, 1991 Solar Corona

AbstractHigh resolution images, obtained during July 11, 1991 total solar eclipse, allowed us to estimate the degree of solar corona polarization in the light of FeXIV 530.3 nm emission line and in the white light, as well. Very preliminary analysis reveals remarkable differences in the degree of polarization for both sets of data, particularly as for level of polarization and its distribution around the Sun’s limb.

Thermal pulse study of the polarization distributions produced in polyvinylidene fluoride by corona poling at constant current

A thermal pulse study of the polarization profiles in samples of 12-μm-thick, biaxially oriented polyvinylidene fluoride after corona poling under approximately constant-current conditions, using a modified corona triode in atmospheric air, is reported. An electrical characterization of the corona triode is also reported to show how it may be operated in the constant-current mode. Samples poled without electrode on the corona-exposed surface show polarization distributions sensitive to the corona polarity, with polarization depletion on the corona side of the samples when the corona is positive. Polarization-reversal experiments show switching inhomogeneities with a pronounced dependence on the initial corona polarity. The above observations are consistent with a simple model in which positive charges from the positive corona partially penetrate the sample during poling and cause an inhomogeneous reduction of the poling field.

Polarization of the white-light corona and its large-scale structure in the period of solar cycle maximum

Distributions of brightness and polarization,p, were obtained for the February 16, 1980 solar corona. Isophotes have a circular shape, typical for the period of the solar cycle maximum. A variety of structural features are distinctly seen in the distribution ofp. The polarization reaches 55%, and thep values are comparatively high, not only in the well-defined streamers, but in the overlapping faint rays and small streamers, as well. A theoretical interpretation of the observed high degrees of polarization, taking into account the data on coronal brightness, is very difficult. This cannot be done within the scope of spherically symmetric models of the corona; the assumption of a high concentration of coronal matter into the plane of the sky is needed. With the most extreme densities in coronal structures, it is not, however, possible to exceed the observed valuep = 55%. Taking into account the accuracy of the polarization measurements, there are no reasons to reject the Thomson scattering as a basic mechanism to explain the origin of the white-light corona.

Electric-field-induced phase changes in polyvinylidene fluoride: Effects from corona polarity and moisture

The constant-current corona triode has been used to charge and monitor the surface potential buildup of polyvinylidene fluoride samples in α form. Under dry air and positive polarity it is possible to pole the samples to sufficiently high electric fields so as to cause a phase change from the nonpolar to the polar δ form. This phase transition is confirmed by x-ray diffractograms. Surface potential measurements on δ samples display a ferroelectric behavior in which a plateau appears in the surface potential buildup curve due to the dipole reorientation. The values of the remanent polarization and the coercive field were estimated to be ∼50 mC/m2 and ∼65 MV/m, respectively.

Ozone generation in DC-energized electrostatic precipitators

Ozone emissions from a short wire-plate precipitator and three commercial electronic air cleaners were measured. Ozone generation was most strongly affected by the corona current and polarity of the discharge electrode. To a lesser extent, the type of corona (i.e. whether tuft or glow) was also important. Water vapor seems to reduce the ozone emission rate for negative corona, but ozone emissions from positive coronas are relatively unaffected. Ozone emission rate was also affected by the radius of the discharge electrode.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Polarization of the corona during the maximum of the solar cycle

Abstract The preliminary results of measuring the polarization of the white-light corona, carried out during the 16 Feb 1980 solar eclipse, are presented. The degree of polarization was observed to be relatively high all round the sun, with the exception of a coronal hole near the south pole. Together with well-expressed streams, high degrees of polarization were observed in the narrow ray at P = 210° and also in the broad region P = 310°–350°. A possible non-homogeneous model of the white-light corona for the period of the solar maximum is discussed.

Corona Torch enhanced N2 laser output energy

An experimental investigation is conducted to enhance the output energy of a N2 laser using a flow enhanced corona discharge phenomena (Corona Torch). The results show that the output energy increases using that type of corona by more than three-fold. Moreover, the corona polarity shows a significant effect on the output energy of the N2 laser.

Reduction of Nitric Oxide in Flue Gases by Point to Plane Corona Discharge with Catalytical Coatings on the Plane Electrode

Abstract A corona discharge with catalytical coatings on its plane electrodes has been studied as a technique for NOx reduction in gas mixtures. The underlying concept is that the combination of a gas discharge with calalytically active electrode coatings may allow the activation of the thermodynamically favourable NO-decay, e.g. into the 2NO → N2 + 02 reaction channel. Widely differing materials such as metal oxide catalysts, conductive polymers, Si3N4-passivation and nitro enamon were tested as electrode coatings. With given corona polarity the different coatings gave widely differing DeNOx-efficiencies, the values of which are distributed over an interval of about two orders of magnitude. The largest energy yield this far obtained is 4mg of reduced NO per dissipated kilojoule of electrical energy. Further experiments will aim at improving this value by a factor of five at least, so that a typical coal power plant‚s flue gas could be cleaned of its NO from, for example, 1200 ppm to 200 ppm by employing ...

Laboratory analyses of corona discharges

An experimental research program was conducted to characterize corona generation from different eléctrode geometries in a range of conditions comparable to those found in electrostatic precipitators. A wire-parallel plate device and a wire-cylinder device were used to monitor the spatial distribution and temporal stability of corona discharges. Visual observations of corona phenomena were made with a sensitive video camera. Conditions that were varied in the experiments included temperature, corona electrode design, corona polarity, and method of energization. Measurements of the distributions of corona current and statistical analyses permit an assessment of the impact of corona electrode design on electrostatic precipitator performance. A description of the laboratory apparatus and a summary of experimental results is included.

Laboratory analysis of back-corona discharge

An experimental research program was designed and performed to characterize back-corona generation and behaviour in a range of environments and geometries common to electrostatic precipitators (ESP's). A wire-parallel plate device was utilized to monitor the intensity and distribution of back-corona discharges. Back-corona was generated by subjecting a deposited layer of fly ash to corona current. Visual observations of back-corona emissions were made with a sensitive video camera. These observations provided new insight into the characteristics of back-corona phenomena. Conditions that were varied in the experiments included ash resistivity, corona electrode design, corona polarity, and ash-layer thickness and surface condition. The effect of pulsed energization on back-corona was also studied. Analyses of visual observations and measured distributions of corona- and back-corona current allowed an assessment of the effects of back-corona emissions on ESP-operating parameters.

Comparison of coronal emission-line structure and polarization

Simultaneous observations obtained with the HAO/SPO coronal emission-line polarimeter and the new SPO emission-line coronagraph are compared. The polarimeter data are measured in the Fexiii (10747 A) line and the coronagraph observations are recorded in the Fexiv (5303 A) line. The polarimeter field-of-view is relatively coarse compared with the resolution limit of the coronagraph. Therefore, the observed coronal polarization cannot be related directly to the detailed loop structures that characterize the localized coronal distribution, but it is found that large-scale emission features have corresponding polarization signatures. Since the measured linear polarization vectors describe the projected magnetic field direction, as shown theoretically, it follows that the form of the large-scale coronal distribution describes corresponding magnetic field structures. Measured polarization values are consistent with those predicted theoretically. Interpretation of these data to obtain corresponding magnetic field directions suggests that the value usually accepted for iron abundance in the corona might be too large.

Comparison of heliospheric current sheet structure obtained from potential magnetic field computations and from observed polarization coronal brightness

We compare the structure of the heliospheric current sheet early in sunspot cycle 21 as computed from the observed photospheric magnetic field with a potential field approximation and as inferred from synoptic maps of the observed coronal polarization brightness. On most of the solar rotations compared, the two methods give essentially the same results; the basic shape of the warped current sheet and the amplitude (in solar latitude) of the displacements of the sheet from the solar equator are similar. On one rotation the current sheet computed with the potential field approximation appears to be distorted by a large photospheric region of unbalanced magnetic flux.

Motions and mass changes of a persistent coronal streamer

A coronal streamer was observed by the white light coronagraph on Skylab during 5 successive limb passages between 1 June, 1973 and 6 August, 1973. The Skylab data give independent measures of coronal brightness and polarization, as functions of time. These permit the distinction between changes in the coronal streamer's appearance due to solar rotation and actual structural changes. The streamer's visual appearance changed slightly between successive limb passages indicating that it was not a steady state feature. Measurements of the streamer's latitude, brightness, and polarization during 3 east limb passages show that: (1) the streamer's axis migrated southward from 25° N at first east limb passage to 11° N at second east limb passage to 8° N latitude at third east limb passage; (2) the streamer's mass (and mass gradient with height), varied by between 20 and 50% from one east limb passage to the next; (3) the streamer's longitudinal extent was also observed to be less on successive east limb passages; and (4) mass changes (distinct from coronal transients) occurring over hours were detected during at least two limb passages. Comparison of the outer coronal observations with observations from lower in the solar atmosphere indicate that the streamer was associated with a complex of solar activity consisting of active regions and filaments. This complex of activity shifted southward by the same amount as the streamer. The variations detected in the streamer preclude the detailed determination of its three-dimensional structure.

The polarization of the inner solar corona at the eclipse of 10 July 1972

Photographs of the corona in the continuum spectrum (580–700 nm) have been obtained with a doublet camera (F=4m) through rotating sector polarizers with the vibrational directions oriented radially and tangentially to the solar limb. Isophotes of the total emission and its polarized component, as well as diagrams giving the degree of polarization for the (K+F)-corona and the electron corona proper (PK), have been plotted up to the distance of 1R⊗ from the limb. In some inner regions the polarizationPK exceeds the highest value possible for Thomson scattering under the given conditions. The reality of this anomally is dubious; systematic photometrical errors may account for it. Three-dimensional forms of 15 different coronal rays have been ascertained by comparing the course of polarization along the rays to the family of Baumach curves. The rays are found to deflect substantially from the radial directions out of the plane of the sky. Mean values of the coronal brightness and polarization versus the distance from the Sun have been determined. Contrary to well-known models (Van de Hulst) the mean polarization of the electron corona (PK) decreases with distance after reaching the maximum (≈50%) at (1.4–1.6)R⊗ from the Sun's centre. This decrease can be explained by deflection of the streamers from radial directions.

Observations of coronal polarization at the solar eclipse of 7 March, 1970

Polarigraphic observations of the 7 March, 1970 eclipse were made at Miahuatlan (Mexico) with a camera of 120 cm focal length. A polarizing filter in front of the objective could be adjusted at 8 different positions, 22.5° apart. Reduction of eight photographs of the white light corona yields polarization at 72 position angles and from r = 1.1 up to 2.1. High polarization which cannot be explained with Thomson scattering was not observed. An analysation of the measuring accuracy shows, that it is not possible to determine exactly the direction of polarization in the outer corona with the classical method of measuring polarization with a small number of photographs. The coronal hole in the south-west quadrant is analysed. The low intensity and polarization can be explained by a hole with an extent in longitude between 1 and 2 times its extent in latitude and with a minimum electron density between 0 and 0.3 of that outside the hole.