Vector magnetic fields in prominences

A preliminary discussion is presented of measurements of the polarization of the He i D₃ multiplet in a quiescent prominence, observed with a wavelength-scanning Stokes polarimeter. For a series of 43 observations in the same prominence, the linear polarization of the major component of D₃ lies primarily in the range 1 to 2% and of the wing component, the range 2 to 5%; the polarization vector angle lies primarily in the range 10–25° for the major component, and 25–35° for the other component. From a more limited data set, the polarization of both components is found to first increase as a function of height in the prominence, and then to decrease; the polarization angles of the major component vary in a random-like way with height, while the wing component shows a systematic change. The amount of polarization and the angle of polarization are governed by the Hanle effect. The collective effect of the group of lines at the peak of D₃ evidently has a different sensitivity to the Hanle effect than does the wing component, thus yielding at least four independent measurements - two polarizations and two angles. With some redundancy, the vector magnetic field can then be established using the detailed theory of the Hanle effect. Since the wing component of D₃ is a simple triplet, an initial estimate of the magnetic field strength and its horizontal orientation, 0, relative to the line of sight, is simply obtained. Examples of such calculations are presented.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 78-17292 with the National Science Foundation.     

Magnetic fields in quiescent prominences

We discuss the longitudinal component of the magnetic field, B , based on data from about 135 quiescent prominences observed at Climax during the period 1968–1969. The measurements are obtained with the magnetograph which records the Zeeman effect on hydrogen, helium and metal lines. Use of the following lines, H; Hei, D₃, Hei, 4471 Å; Nai, Di and D₂, leads to the same value for the observed magnetic field component in these prominences. For more than half of the prominences their mean field, B , satisfy the inequalities 3 G B 8 G, and the overall mean value for all the prominences is 7.3 G. As a rule, the magnetic field enters the prominence on one side and exits on the other, but in traversing the prominence material, the field tends to run along the long axis of the prominence.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The determination of vector magnetic fields in prominences from the observations of the Stokes profiles in the D3 line of helium

A method is presented to measure the magnetic field vector in prominences by means of the polarimetric observations in the D₃ line of He obtained with the High Altitude Observatory Stokes polarimeter. The characteristics of the observed Stokes profiles are discussed. The theory of the Hanle effect is reformulated in the representation of the irreducible tensors of the density matrix, and is generalized to derive the circular polarization profiles across the spectral line in terms of the intensity and direction of the prominence magnetic field. The circular polarization profile so deduced can be employed to obtain useful information which adds to that carried by the linear polarization observations. A non-linear least-squares algorithm is proposed to derive the measurement of the magnetic field from the observations, and a consistency check is suggested to test the adequacy of the theoretical model to describe the physics of the He I atomic excitation in prominences.On leave from: Astrophysical Observatory of Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The Imaging Vector Magnetograph at Haleakalā IV: Stokes Polarization Spectra in the Sodium D<Subscript>1</Subscript> 589.6 nm Spectral Line

The Imaging Vector Magnetograph (IVM) at the Mees Solar Observatory, Haleakalā, Maui, Hawai’i, obtained many years of vector magnetic-field data in the photospheric Fe i 630.25 nm line. In the latter period of its operation, the IVM was modified to allow routine observations in the chromospheric Na i D₁ line, as well as the Fe i line. We describe the sodium observational data in detail, including the data-reduction steps that differ from those employed for the Fe i 630.25 nm line, to obtain calibrated Stokes polarization spectra. We have performed a systematic comparison between the observational data and synthetic NLTE Na i D₁ Stokes spectra derived for a variety of solar-appropriate atmospheric and magnetic configurations. While the Na i D₁ Stokes polarization signals from the solar atmosphere are expected to be weak, they should generally be within the IVM capability. A comparison between synthetic spectra and observational data indicates that this is indeed the case.     

Limb observations of He I 1083 nm

Imaging spectroscopic data of the He i 1083 nm limb emission were taken on several dates in October and November 1995 with the NASA/NSO spectromagnetograph at the NSO/Kitt Peak vacuum telescope and on 9 December, 1993 with the Michigan infrared camera at the NSO/Sacramento Peak vacuum tower telescope. Emission line profiles were observed in quiet-Sun and coronal hole locations on the northern and southern solar poles and on the east solar limb. The height of the He i 1083 nm shell above the continuum limb at 1083 nm was measured to be 2.11 ± 0.12 Mm with the Kitt Peak data, and 1.74 ± 0.05 Mm with the Sacramento Peak data. The Kitt Peak data show (1) within the measurement error there is no significant difference in the height or thickness of the emission shell in coronal holes compared with the quiet Sun, (2) the 1083 nm emission intensity drops by 50% in coronal holes, (3) the line width decreases by about 2 km s^-1 in coronal holes (suggesting less inclined spicules), (4) the line width of the He i 1083 nm line jumps significantly as the line of sight crosses the solar limb (consistent with a higher temperature upper shell), (5) a quiescent prominence shows a smaller spectral line width (consistent with a cooler temperature or less velocity broadening), and (6) the entire emission shell and the prominence show a He i spectral component ratio of about 8 (suggesting optically thin emission).Operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation.     

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 Å) line and the coronagraph observations are recorded in the Fexiv (5303 Å) 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.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 78-17292 with the National Science Foundation.     

Removing instrumental polarization from infrared solar polarimetric observations

Full Stokes polarimetry is obtained using the National Solar Observatory Vacuum Tower Telescope at Sacramento Peak while observing the magnetically sensitive infrared Fei line at wavelength of 1.56. A technique is described which makes use of the high magnetic resolution in this spectral range to remove instrumental polarization from observed StokesQ, U, andV line profiles.Supported under a USAF/AFOSR research initiative.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.     

Visible coronal emission associated with a quiescent prominence

Emission-line coronagraph images of a high-latitude, nominally quiescent prominence, recorded at wavelengths of H, 6374 Å (Fex) and 5303 Å (Fe xiv), are analyzed. Over a two-day period, the coronal images, which are found to arise predominantly from coronal emission, evolve such that the emission becomes concentrated at locations corresponding to the outer regions of the prominence. This edge enhancement has similar characteristics to results inferred from EUV prominence observations. It is postulated that this coronal emission associated with the prominence results from MHD wave dissipation. Dissipation lengths for slow-mode, fast-mode and Alfvén waves are estimated for different prominence conditions. Of these, fast-mode waves appear to be the most physically realistic heating source if the prominence magnetic field is along the length of the prominence.Operated by the Association of Universities for Research in Astronomy, Inc., under contract AST 78-17292 with the National Science Foundation.     

Steady flows in active regions observed with the HeI 10830 Å line

We show that the He i 10830 A line gives reliable Doppler shift measurements in the upper chromosphere above active regions. Persistent flow patterns in active regions observed near the solar limb show features previously noted in Dopplergrams using the Civ transition region ultraviolet emission line. Unlike the Civ measurements, however, the He i absorption shows a strong correlation with the line-of-sight velocity images in certain regions of some active regions.Operated by the Association of Universities for Research in Astronomy, Inc., under contract AST 78-17292 with the United States National Science Foundation.The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation.Visiting Astronomers, Sacramento Peak Observatory.     

Vector magnetic fields in sunspots

Observations of a round, unipolar sunspot in the Zeeman triplet Fe i 6302.5 with the High Altitude Observatory Stokes Polarimeter are used to derive the vector magnetic field in the spot. The behavior of the magnitude, inclination, and azimuth of the field vector B across the spot is discussed. A linear relation is found between the continuum intensity I _c and the field magnitude B. Time series obtained in the umbra show significant power in the magnitude of the field at a period of t 180 s but the other components of the field vector do not display this behavior.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Complete determination of the magnetic field vector and of the electron density in 14 prominences from linear polarizaton measurements in the HeI D3 and Hα lines

The present paper is devoted to the interpretation of linear polarization data obtained in 14 quiescent prominences with the Pic-du-Midi coronagraph-polarimeter by J. L. Leroy, in the two lines Hei D₃ andH quasi-simultaneously. The linear polarization of the lines is due to scattering of the anisotropic photospheric radiation, modified by the Hanle effect due to the local magnetic field. The interpretation of the polarization data in the two lines is able to provide the 3 components of the magnetic field vector, and one extra parameter, namely the electron density, because the linear polarization of H is also sensitive to the depolarizing effect of collisions with the electrons and protons of the medium. Moreover, by using two lines with different optical thicknesses, namely Hei D₃, which is optically thin, and H, which is optically thick ( = 1), it is possible to solve the fundamental ambiguity, each line providing two field vector solutions that are symmetrical in direction with respect to the line of sight in the case of the optically thin line, and which have a different symmetry in the case of the optically thick line.It is then possible to determine without ambiguity the polarity of the prominence magnetic field with respect to that of the photospheric field: 12 prominences are found to be Inverse polarity prominences, whereas 2 prominences are found to be Normal polarity prominences. It must be noticed that in 12 of the 14 cases, the line-of-sight component of the magnetic field vector has a Normal polarity (to the extent that the notion of polarity of a vector component is meaningful; no polarity can be derived in the 2 remaining cases); this may explain the controversy between the results obtained with methods based on the Hanle effect with results obtained through the Zeeman effect. A dip of the magnetic field lines across the prominence has been assumed, to which the optically thick H line is sensitive, and the optically thin Hei D₃ line is insensitive.For the Inverse prominences, the average field strength is 7.5±1.2 G, the average angle,, between the field vector and the prominence long axis is 36° ± 15°, the average angle, , between the outgoing field lines and the solar surface at the prominence boundary is 29° ± 20°, and the average electron density is 2.1 × 10¹⁰ ± 0.7 × 10¹⁰ cm^–3. For the Normal prominences, the average field strength is 13.2±2.0 G, the average angle,, between the field vector and the prominence long axis is 53° ± 15°, the average angle, , between the outgoing field lines and the solar surface at the prominence boundary is 0° ± 20° (horizontal field), and the average electron density is 8.7 × 10⁹ ± 3.0 × 10⁹ cm^–3.     

The Ca ii emission lines in quiescent prominences

Observations of the Ca ii H, K, and infrared triplet lines are compared with theoretical predictions from the slab models of Heasley and Milkey (1976). While the theoretical models describe the hydrogen and helium emission spectra of quiescent prominences satisfactorily the predicted Ca ii lines are systematically too bright. The most likely reason for the discrepancy is the inapplicability of the symmetric slab prominence model for lines which become even moderately optically thick in prominences.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Visiting scientist at Kitt Peak National Observatory.     

Energy balance in cool quiescent prominences

The energy balance for cool quiescent prominences is examined using a 6000 km, 6000 K isothermal slab model prominence with a density gradient dictated by a modified Kippenhahn-Schlüter model. The model is irradiated from both sides by the coronal, chromospheric, and photospheric radiation fields. The radiative transfer problem is solved in detail for the Lyman continuum and H to determine the net radiative energy loss for hydrogen. An estimate of the energy loss for Ca ii H and K indicates that this source of energy loss is unimportant when compared with the hydrogen radiation. The radiative energy loss is easily balanced by the conductive energy gain from the corona.The only difficulty with our model is that the total hydrogen density must be of the order of 3 × 10¹²/ cm³ to match the n = 2 population density of 5 × 10⁴/cm³ obtained from observation. To support a prominence of this density and a thickness of 6000 km against gravity requires magnetic fields of the order of 20 G which is much higher than the average magnetic field in quiescent prominences deduced from limb observations. Two possible explanations for this discrepancy are given.Currently at the Max-Planck-Institut für Physik und Astrophysik, München, Germany.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A relation between magnetic field strength and temperature in sunspots

We present Stokes I Zeeman splitting measurements of sunspots using the highly sensitive (g = 3) Fe i line at = 1.5649 m. The splittings are compared with simultaneous intensity measurements in the adjacent continuum. The relation between magnetic field strength and temperature has a characteristic, nonlinear shape in all the spots studied. In the umbra, there is an approximately linear relation between B ² and T _b, consistent with magnetohydrostatic equilibrium in a nearly vertical field. A distinct flattening of the B ² vs T _brelationship in the inner penumbra may be due to changes in the lateral pressure balance as the magnetic field becomes more horizontal; spatially unresolved intensity inhomogeneities may also influence the observed relation.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.     

A proposed correction to the solar abundances of carbon and oxygen utilizing new and accurate theoretical forbidden transition probabilities

Previously published solar abundances of oxygen and carbon can be corrected to be logN(O) = 8.93 and logN(C) = 8.60 on the hydrogen log-scale when new accurate forbidden electric quadrupole transition probabilities A _Q(s^–1) are used. Such A _Q's, based on the new atomic structure and electron correlation theory, developed recently by Sinanolu and coworkers, are reported for the (¹ S ₀-¹ D ₂) lines of [C i], [N ii, [O i] and [O iii] and the (² P-² D) lines of [N i] and [O ii]. The available experimental values are also given for comparison.Work supported by Grant No. GP-29471 from the U.S. National Science Foundation.     

Impulsive brightenings and velocity transients in prominences

Observations of quiescent prominences with the Zeiss Universal Birefringent Filter at Sacramento Peak show short-lived brightenings and velocity transients in H and D₃. The larger events range in area from 25 to 170 square arc sec, have lifetimes of approximately 30 min, velocities of 30 km s^–1, and total energy excesses up to 7 × 10²⁷ ergs. These events do not disrupt the stable structure of the prominence, and are interpreted as either condensation events or low energy flares.Visiting astronomer, Sacramento Peak Observatory, operated by the Association of Universities for Research in Astronomy Inc. under contrast AST-78-17292 with the National Science Foundation.Visiting student, Sacramento Peak Observatory, operated by the Association of Universities for Research in Astronomy Inc. under contract AST-78-17292 with the National Science Foundation.     

Motions in solar magnetic tubes

The line-centre-magnetogram technique has been used to measure the average velocity in magnetic elements in plages and isolated magnetic elements (including dipoles) in Ca ii 8542, Mg i 5183, Fe i 8688 and C i 9111. The velocities vary from 0.6 km s^–1 downflow in the line of deepest origin to zero in the highest. The smooth curve obtained by combining these with the results of other investigators is in conformity with Giovanelli's (1977) theory of inflow in the neighborhood of the temperature minimum.This material is based upon research supported by the National Science Foundation under its contract No. AST 74-04129 with the Association of Universities for Research in Astronomy, Inc., for management, operation and maintenance of the Kitt Peak National Observatory.Visiting Astronomer, Kitt Peak National Observatory.     

Spectral Characteristics of the He <Emphasis Type="SmallCaps">i</Emphasis> D<Subscript>3</Subscript> Line in a Quiescent Prominence Observed by THEMIS

We analyze the observations of a quiescent prominence acquired by the Téléscope Heliographique pour l’Étude du Magnetisme et des Instabilités Solaires (THEMIS) in the He?i 5876 Å (He?i D₃) multiplet aiming to measure the spectral characteristics of the He?i D₃ profiles and to find for them an adequate fitting model. The component characteristics of the He?i D₃ Stokes I profiles are measured by the fitting system by approximating them with a double Gaussian. This model yields an He?i D₃ component peak intensity ratio of \(5.5\pm0.4\), which differs from the value of 8 expected in the optically thin limit. Most of the measured Doppler velocities lie in the interval ±?5 km?s^?1, with a standard deviation of ±?1.7 km?s^?1 around the peak value of 0.4 km?s^?1. The wide distribution of the full-width at half maximum has two maxima at 0.25 Å and 0.30 Å for the He?i D₃ blue component and two maxima at 0.22 Å and 0.31 Å for the red component. The width ratio of the components is \(1.04\pm0.18\). We show that the double-Gaussian model systematically underestimates the blue wing intensities. To solve this problem, we invoke a two-temperature multi-Gaussian model, consisting of two double-Gaussians, which provides a better representation of He?i D₃ that is free of the wing intensity deficit. This model suggests temperatures of 11.5 kK and 91 kK, respectively, for the cool and the hot component of the target prominence. The cool and hot components of a typical He?i D₃ profile have component peak intensity ratios of 6.6 and 8, implying a prominence geometrical width of 17 Mm and an optical thickness of 0.3 for the cool component, while the optical thickness of the hot component is negligible. These prominence parameters seem to be realistic, suggesting the physical adequacy of the multi-Gaussian model with important implications for interpreting He?i D₃ spectropolarimetry by current inversion codes.     

Vertical velocities and oscillations in quiescent filaments

Analysis of He i 10 830 Å spectral observations of a large, quiescent filament reveals a pronounced oscillatory behaviour of the vertical mass motion. The filament is situated in a quiet region more than 15° away from the nearest active region.It is concluded that the magnetic field of the quiescent filament, which occurs in the form of long thin flux ropes, moves with the gas and that there is no net mass flow perpendicular to the most frequently observed horizontal field lines. The oscillatory motion is accompanied by phase dependent variation of the He i line intensity which could possibly imply wave induced compression of the plasma.     

The center-to-limb wavelength shift of a number of Fraunhofer lines

The center-to-limb wavelength shifts of the cores of faint, medium and strong Fraunhofer lines are presented: Fei 3767, Caii 3968, Ceii 4222, Cai 4227, Nai 5896, Baii 6497, and Ki 7699.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.