On the observation of linear polarization of solar microwave bursts

It is known that mode coupling may occur in quasi-transverse magnetic field regions of the solar corona, which produces linear polarization at microwave frequencies. A microwave polarimeter measuring all 4 Stokes parameters at 8.918 GHz simultaneously at three different highfrequency bandwidths (40 kHz, 400 kHz and 5 MHz) has been developed in order to observe the linear component and its Faraday rotation. The respective minimum detectable changes of the Stokes parameters I, Q, U and V are 9, 3 and 1 solar flux unit at an integration time of 1 s. For burst intensities greater than 300 solar flux units, the minimum detectable degree of linear and circular polarization is 1 %–3 %, depending on the bandwidth. Observations of 68 bursts showed that most of the bursts were circularly polarized. No linear polarization could be found within the limits of accuracy of our polarimeter. Two possible explanations for this result are discussed. The possibility of mode coupling however cannot be excluded from these first observations.     

Suppression of the kink instability for magnetic flux ropes in the chromosphere

Energy storage in chromospheric flux ropes is discussed, in the context of solar flares. The structure is represented by a cylindrically symmetric magnetic field of finite length. The field is assumed to be approximately force-free. The stability of the field to a kink perturbation is investigated. Flux ropes are rooted in dense photospheric material. So the ends of the field lines are taken to be fixed on rigid boundaries for all perturbations. An energy perturbation method is used and the boundary conditions give a stabilizing effect. It is shown that for a moderate degree of twisting the fields are stable to a kink perturbation. Thus energy can be stored in cylindrical fields prior to release in a solar flare.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The brightness of the helium D3 line in the undisturbed chromosphere from eclipse observations

Slitless spectra of the chromosphere, observed cinematographically at the total solar eclipse of 10 July 1972, were reduced. The surface brightness distribution of the helium D₃ line in the undisturbed chromosphere was obtained in agreement with results by other observers. The available eclipse data on the D₃ absolute brightness was analysed by means of theoretical curves of growth. Intensity data by some observers were found to be certainly too high. A trend was found that the D₃ absolute brightness in the quiet chromosphere decreases with the increasing solar activity (sunspot number). This perhaps indicates a variation of the spicule number over the solar surface during a sunspot cycle.     

The He+ λ 4686 line in the low chromosphere

We report an unsuccessful search for the He⁺ 4686 line in the low chromosphere. However, at the location of this line we detect a number of other chromospheric emission lines. This leads us to the conclusion that the He⁺ 4686 identification made in the past, as well as other identifications, are probably in error. Additionally the region of the neutral helium 4713 line is also studied.On leave from Tokyo Astronomical Observatory, Mitaka, Japan.     

Measurement of linear polarization in the Hα line in solar flares

Large solar telescopes built at places with a quite excellent seeing, equipped with a sophisticated optics and control system are too expensive and unique to be used currently in hunting of sudden and short‐lasting activity events, e.g. flares and eruptive prominences. For a systematic observation of selected kinds of active phenomena it is still necessary to use smaller or medium‐sized telescopes equipped with a special setup of devices. Detection of linear polarization in the Hα line emitted in a flare seems to be just a right task and delicate matter for such a systematic observation. This kind of polarization is supposed to be generated by particle beams accelerated in thke corona and directed towards denser chromospheric layers where the particle beams deposit their kinetic energy. As the accelerated particle beams possess a preferred direction of velocity they can produce a linearly polarized light. However, the occurrence of the accelerated particle beams and the related linear polarization in the Hα line may have a tendency to appear: 1) at the early beginning of a flare 2) in pulses lasting just a few seconds or even less. To measure the linear polarization in flares regularly we have built an additional branch in the Ondřejov multichannel flare spectrograph. In this paper we describe the optical system, the detectors, the method used for data recording and reduction and we also briefly discuss the first results.     

Center-to-limb variation of Ca II line brightness oscillations in the solar chromosphere

We have investigated 15 time series of Ca II line spectrograms in quiet-Sun regions located at various distances from the disk center. Our goal is to reveal the center-to-limb variation of the brightness oscillations. The residual intensities at the centers of the Ca II K and 849.806-nm lines and the K index have been analyzed. We have considered separately two components of the chromospheric network. Our main result is that the power of the brightness oscillations in the chromosphere of the average quiet Sun decreases to the limb. This change for the boundaries of supergranulation cells (networks) is considerably larger than that for their inner parts (cells). It is mainly determined by the 5-min oscillations; the 3-min oscillations show virtually no center-to-limb variation. In addition to studying the dependence of the oscillation power on the heliocentric angle, we also consider other characteristics of the oscillatory regime of the chromosphere. For example, the low-frequency oscillations with periods longer than 700 s, which are inherent predominantly in the K line core in networks, have been separated into an isolated mode. No center-to-limb variation has been revealed for them. As a result of our discussion of the patterns found based on present-day publications on the chromosphere dynamics, we conclude that different mechanisms and sources of its heating can simultaneously make their contributions.     

Intensity and polarization line profiles in a semi-infinite Rayleigh-scattering planetary atmosphere. I. Integrated flux

Absorption and polarization line profiles as well as the curves of growth in the integrated light of a planet over the whole range of phase angles have been computed assuming a semi-infinite atmosphere scattering according to Rayleigh’s phase-matrix which takes polarization into account. The relative change in line depth and equivalent widths qualitatively agree with the observations of the CO₂ bands in Venus reported by Young, Schorn and Young (1980). It is pointed out that the bands might be formed in a part of the atmosphere which is different from that where continuum polarization originates.     

Modeling the linear polarization of optical emission from red giants

We present the results of solving the radiative transfer equation for the Stokes vector in the case of light scattering by spherical forsterite dust particles in an axisymmetric circumstellar envelope of a red giant. We have assumed that the surfaces of constant scattering-particle density are prolate or oblate spheroids, the particle density decreases with radius as N _d ∝ r ⁻², and the dust particles at the inner boundary of the envelope are in thermal equilibrium with the stellar emission at solid-phase evaporation temperature T _ev = 800 K. In the wavelength range 0.27 μm ≤ λ ≤ 1 μm, particles with radii 0.03 μm ≲ a ≲ 0.2 μm make a major contribution to the linear polarization of the stellar emission. The increase in scattering efficiency factor with decreasing wavelength λ is mainly responsible for the growth of polarization toward the short wavelengths known from observations. However, at a mean number of scatterings 1.2 ≤ N _sca ≤ 1.6, the polarization ceases to grow due to depolarization effects and decreases rapidly as the wavelength decreases further. The wavelength of the polarization maximum is determined mainly by two quantities: the particle radius and the mass loss rate. The upper limits for the degree of linear polarization in the case of light scattering in circumstellar dust envelopes with the geometries of prolate and oblate spheroids are p ≈ 3 and 5%, respectively. The polarization for light scattering by enstatite particles is higher than that for light scattering by forsterite particles approximately by 0.3%. Original Russian Text ? Yu.A. Fadeyev, 2007, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2007, Vol. 33, No. 2, pp. 123–133.     

Equator-pole differences in the solar chromosphere from Lyman-continuum data

From the analysis of OSO-4 Lyman-continuum spectroheliograms, differences in the brightness and the color temperature between the poles and the equator have been found. These differences are interpreted as resulting from a lower chromospheric density at the poles than at the equator. Two models, one for the poles and one for the equator, giving temperature and density as a function of height, explain the observations. The poles have a lower density and a smaller temperature gradient than the equator does. The differences begin in the middle chromosphere and increase toward the transition zone.     

Center-to-limb variation of low-frequency Ca II line brightness oscillations in the solar chromosphere

The goal of this paper is a detailed statistical analysis of the low-frequency Ca II line intensity oscillations containing information about the dynamics of the lower and middle chromosphere. A pixel-by-pixel analysis of the observed parameters has been performed. The following results have been obtained. (1) The low-frequency chromospheric oscillations (periods >400 s) are seen much more frequently in networks than in chromospheric network cells. (2) The relative fraction of the low-frequency chromospheric intensity oscillations increases with height. (3) The occurrence distribution of intensity oscillations as a function of the frequency is subdivided at least into two types. (4) In contrast to the low-frequency photospheric oscillations, the phase differences between the Ca II K and 849.8 nm line intensity oscillations do not give grounds to identify the low-frequency chromospheric oscillations with internal gravity waves. (5) The spectral composition of the oscillations in the network chromosphere resembles that expected in magnetic flux tubes in the nonlinear regime of conversion of transverse MHD waves at lower levels of the atmosphere into longitudinal MHD waves in its upper layer.     

Wave systems in the chromosphere

Observations are reported of two, possibly three, distinct wave systems in the Hα chromosphere.

1. Velocity films show waves propagating predominantly outwards along mottles and fibrils from as close as 2000 km to the network axis at velocities of the order of 70 km s^-1. The line-of-sight component of the velocity amplitude is estimated to be typically 5 km s^-1. The velocities are accompanied by propagating intensity fluctuations. The system is interpreted as one of basically Alfvén waves. Similar waves are observed propagating predominantly outwards along superpenumbral fibrils radiating from a small sunspot.
2. The velocities in the chromospheric granulation undergo fluctuations of an oscillatory character but without any observable horizontal propagation. The intensities show a close correlation with the velocities, maximum intensity occurring about T/4 after maximum downward velocity. The period is variable across the surface (2.5 min upwards). The intensity-velocity correlation is characteristic of a standing compressional wave.
3. Intensity cinefilms at Hα line centre show in places a horizontal drift of the chromospheric granulation pattern at about 12 km s^-1 without any accompanying vertical velocity fluctuations. It is not known whether this is due to a gas stream at sonic velocities, or to a horizontally propagating sound wave.

The Alfvén wave system is shown to make a significant contribution to coronal heating. Whether the velocity fluctuations in the chromospheric granulation also make an important contribution depends on whether there are upwardly propagating or standing waves; this is not yet established despite the intensity-velocity correlation.     

Investigation of the linear polarization in infrared absorption bands

We consider the effects of the grain size, shape, structure, and chemical composition as well as the angle between the grain rotation axis and the incident ray on the full widths at half maximum (FWHM) of the polarization bands in the two deepest infrared absorption bands observed in the spectra of protostars, the water-ice band centered at 3.1 μm and the silicate band centered at 9.7 μm, using a core—mantle confocal spheroid model with various axial ratios a/b and relative volumes of the core material. We have found that the observed polarization bands with FWHM_p < 0.3 μm in the water-ice absorption band can be explained only by oblate and prolate particles with r _v ≤ 0.35 μm and the polarization bands with FWHM_p ≈ 0.3 μm can be explained only by particles with r _v ≈ 0.35 μm. Broad silicate absorption bands (FWHM ≈ 3 μm) with broad polarization bands (FWHM_p ≈ 2.7 μm) can be explained by particles with r _v ≈ 0.35 μm. Narrow silicate absorption bands (FWHM ≤ 3 μm) with any FWHM of the polarization bands can be explained by a mixture of particles of two types of olivine. Narrow polarization bands (FWHM_p ≈ 2 μm) with broad absorption bands can be explained only by very small particles, r _v ≤ 0.1 μm. We have found the relationships between the effective polarization and extinction cross sections and estimated the ranges of observed polarizabilities that can be explained by particles of given shape and orientation in each of the bands independently. Independent studies of the observational data for each of the bands are shown to give a wider choice of particle model parameters.     

Inhomogeneous structure of the solar chromosphere from Lyman-continuum data

We describe a new model of the chromosphere based on Lyman-continuum observations by Harvard spectrometers aboard the satellites OSO 4 and OSO 6. The model assumes (a) that a random distribution of optically thick inhomogeneities overlies a plane-parallel homogeneous atmosphere, and (b) that the Lyman continuum in the chromosphere is optically thick and the only significant opacity source between 600 and 912 Å.The temperature, gas pressure, electron pressure, particle densities, and b ₁ (the hydrogen ground-state departure coefficient) are calculated as a function of height in the chromosphere.The model reproduces the observed quiet-region intensities in the Lyman continuum. The inhomogeneous structures, which we believe to be spicules, are inferred to be optically thick in the Lyman continuum and to have a source function below that of the mean chromosphere. If they are also optically thick in the free-free (millimeter) continuum and cooler than about 5000K, they could produce the observed limb darkening at 1 and 3 mm. Such low temperatures are at odds with current spicule models, but could exist in the cores of spicules.The Lyman-continuum emission shortward of 750 Å shows an excess emission over that predicted by the above model. This is found to be consistent with the existence of a temperature plateau with T 22000K in the very high chromosphere.     

Mass motion in the solar chromosphere

A high resolution profile of the solar Oi 1304.9 Å line has been measured from rocket spectrograms. The profile is nearly flat-topped, showing only a slight solar reversal after instrument effects and absorption due to atomic oxygen in the earth's atmosphere have been allowed for. A theoretical analysis of this line, under the assumption of non-LTE conditions and a homogeneous, spherically symmetric chromosphere, predicts a rather deep solar reversal. The theoretical profile may be made consistent with the observed profile if mass motion is present in the chromospheric region where the line is formed. A Gaussian distribution of up and down velocities with a root mean square velocity of about 7 km/sec gives best agreement between the predicted and observed profile. This result is consistent with the conclusion made from a study of high resolution profiles of solar lines in the visible spectrum that mass vertical velocities increase with height above the photosphere.     

Studies of the solar chromosphere from millimetre and sub-millimetre observations

A compilation of brightness temperature data in the millimetre and sub-millimetre wavelength ranges is used to obtain an empirical relation T _B = C ⁿ between brightness temperature and wavelength, valid between 1 mm and 1 cm. An analytically soluble model, giving electron temperatures and densities between 1500 km and 4000 km above the photosphere is derived from this relationship, and eclipse data of Thomas and Athay, of the emission at the head of the Balmer continuum. Although this over-simplified model lacks precision in the height co-ordinate, it allows a scale height of around 1000 km for electrons to be deduced, and supplies a convincing test for the absence of hydrostatic equilibrium throughout the region. A more comprehensive and reliable though still simplified numerical model is then presented, being a modification of previous models accounting only for millimetre data. It shows marked departure from UV derived models in this region, and an explanation for the discrepancy is proposed in terms of thermal inhomogeneities on the scale of the chromospheric supergranulation. The stratified model is then geometrically modified to account for observed centrelimb profiles of the Sun measured throughout the sub-millimetre and millimetre region. The scale of any roughness thus introduced is related to the notable lack of millimetric limb brightening, and observational tests for that scale are suggested here. A qualitative picture of this part of the chromosphere is proposed, consistent with existing observations in the millimetre, visible and UV regions of the spectrum.     

The compton effect in the chromosphere

We assume that the lines of the solar spectrum, formed in the reversing layer, when travelling for an optical thickness τ in the chromosphere are subjected to a red shift Δλ(τ) due to multiple Compton scattering. We limit our investigation to the difference of the red shift at the limb—the red shift at the centre of the disc giving a good agreement between the theoretical results and observations.     

Weakening of the solar extreme-ultraviolet line emission by lyman continuum absorption as derived from line ratios

The intensity ratios of Niii, Oiii, Oiv, and Ov lines observed by the HCO experiment on Skylab are compared with the results of recent multilevel calculations. It is found that solar transition-region spectra require Lyman continuum absorption. The equivalent optical thickness of the absorbers causing the weakening is found to be 1.6–1.7 for a quiet area, 1.4–1.9 for a coronal hole, and 2.4–2.5 for active regions. These values are consistent with previous estimates from different methods.     

Helium emission in the middle chromosphere

Slitless spectrograms obtained during the eclipse of 10 June 1972 have been analyzed to determine the height distribution of the D₃ He line intensity.For undisturbed regions the maximum of D₃ line intensity is confirmed to exist at about 1700km above the limb. Besides the above mentioned maximum, in plages a considerable intensity may be observed at low heights (h < 1000 km).An analysis of these observations for h > 1000 km has been carried out within the low temperature mechanism of triplet helium emission taking into account the helium ionization by XUV radiation. The density dependence of the 2³ S level population at different XUV flux values has been calculated. Our observations give N _e 2 × 10¹⁰ cm^–3 in the chromosphere at h = 2000 km. The probable coincidence of the H and He emission small filaments in the middle chromosphere is discussed.