Author name code: rees ADS astronomy entries on 2022-09-14 =author:"Rees, David E." OR =author:"Rees, D.E." ------------------------------------------------------------------------ Title: A Gentle Introduction to Polarized Radiative Transfer Authors: Rees, David E. Bibcode: 2009nrt..book..213R Altcode: No abstract at ADS Title: Non-LTE Polarized Radiative Transfer in Special Lines Authors: Rees, David E.; Murphy, Graham A. Bibcode: 2009nrt..book..241R Altcode: No abstract at ADS Title: C IV Vacuum Ultraviolet Fabry-Perot Interferometers for Transition-Region Magnetography Authors: Gary, G. A.; West, E. A.; Rees, D. E.; Zukic, M.; Herman, P.; Li, J. Bibcode: 2006ASPC..358..181G Altcode: The vacuum ultraviolet region allows remote sensing of the upper levels of the solar atmosphere where the magnetic field dominates the physics. Obtaining an imaging interferometer that observes the transition region is the goal of this program. This paper gives a summary of our instrument development program (1998-2005) for a high-spectral-resolution, piezoelectric tunable Vacuum Ultraviolet Fabry-Perot Interferometer (VUV FPI) for obtaining narrow-passband images, magnetograms, and Dopplergrams of the transition region emission line of C IV (155 nm). A VUV interferometer will allow us to observe the magnetic field, flows, and heating events in the mid-transition region. The MSFC VUV FPI has measured values of

FWHM ∼ 9 pm, FSR ∼ 62 pm, finesse ∼5.3, and transmittance ∼ 50% at 157 nm. For the measurements, the University of Toronto's F2 eximer laser was used as an appropriate proxy for C IV 155 nm. This has provided the first tunable interferometer with a FWHM compatible to VUV filter magnetograph. Title: Multi-Line Spectro-Polarimetry of Stellar Magnetic Fields Using Principal Components Analysis Authors: Semel, M.; Rees, D. E.; Ramírez Vélez, J. C.; Stift, M. J.; Leone, F. Bibcode: 2006ASPC..358..355S Altcode: Zeeman-Doppler-Imaging (ZDI) is a technique for the diagnosis of stellar magnetic fields from observations of circularly polarized light, based on the weak-field approximation.

This approach has proved adequate in numerous studies of the magnetic fields of solar-type stars. To improve the signal-to-noise ratio, one can add the contributions of different lines to the polarization signature. However, this multi-line extension has been limited by a number of assumptions (e.g., the weak-field approximation). Here we present a novel approach. We created a database of synthetic polarized spectra for different combinations of magnetic-field intensities and orientations. Principal Components Analysis was used to compress the database, so that only few Principal Components are needed to represent all the calculated spectra. Title: Dynamo processes and activity cycles of the active stars AB Doradus, LQ Hydrae and HR 1099 Authors: Donati, J. -F.; Collier Cameron, A.; Semel, M.; Hussain, G. A. J.; Petit, P.; Carter, B. D.; Marsden, S. C.; Mengel, M.; López Ariste, A.; Jeffers, S. V.; Rees, D. E. Bibcode: 2003MNRAS.345.1145D Altcode: In this paper, we present new brightness and magnetic surface images of the young K0 dwarfs AB Doradus and LQ Hydrae, and of the K1 subgiant of the RS CVn system HR 1099 (=V711 Tauri), reconstructed from Zeeman-Doppler imaging spectropolarimetric observations collected at the Anglo-Australian Telescope during five observing campaigns (totalling 50 nights), from 1998 January to 2002 January. Along with the older images of the same stars (published in previous papers), our complete data set represents the first long-term series on temporal fluctuations of magnetic topologies of very active stars.

All of the magnetic images presented here indicate that large regions with predominantly azimuthal magnetic fields are continuously present at the surfaces of these stars. We take this as further evidence that the underlying dynamo processes that produce them are probably distributed throughout the entire convective zone (and not confined at its base, as in the Sun). We speculate that the radial and azimuthal field maps that we recover correspond, respectively, to the poloidal and toroidal components of the large-scale dynamo field.

We find, in particular, that some signatures, for instance the relative fraction of magnetic energy stored in the large-scale poloidal and toroidal field components, and the polarity of the axisymmetric component of the field, are variable with time, and provide potentially fruitful diagnostics for investigating magnetic cycles in active stars other than the Sun. We report here the detection of partial polarity switches in some of the axisymmetric field components of two of our programme stars (AB Dor and LQ Hya), suggesting that the dynamo operating in these stars may be cyclic. Title: Pattern recognition techniques and the measurement of solar magnetic fields Authors: Lopez Ariste, Arturo; Rees, David E.; Socas-Navarro, Hector; Lites, Bruce W. Bibcode: 2001SPIE.4477...96L Altcode: Measuring vector magnetic fields in the solar atmosphere using the profiles of the Stokes parameters of polarized spectral lines split by the Zeeman effect is known as Stokes Inversion. This inverse problem is usually solved by least-squares fitting of the Stokes profiles. However least-squares inversion is too slow for the new generation of solar instruments (THEMIS, SOLIS, Solar-B, ...) which will produce an ever-growing flood of spectral data. The solar community urgently requires a new approach capable of handling this information explosion, preferably in real-time. We have successfully applied pattern recognition and machine learning techniques to tackle this problem. For example, we have developed PCA-inversion, a database search technique based on Principal Component Analysis of the Stokes profiles. Search is fast because it is carried out in low dimensional PCA feature space, rather than the high dimensional space of the spectral signals. Such a data compression approach has been widely used for search and retrieval in many areas of data mining. PCA-inversion is the basis of a new inversion code called FATIMA (Fast Analysis Technique for the Inversion of Magnetic Atmospheres). Tests on data from HAO's Advanced Stokes Polarimeter show that FATIMA isover two orders of magnitude faster than least squares inversion. Initial tests on an alternative code (DIANNE - Direct Inversion based on Artificial Neural NEtworks) show great promise of achieving real-time performance. In this paper we present the latest achievements of FATIMA and DIANNE, two powerful examples of how pattern recognition techniques can revolutionize data analysis in astronomy. Title: Fast inversion of spectral lines using principal component analysis. I. Fundamentals Authors: Rees, D. E.; López Ariste, A.; Thatcher, J.; Semel, M. Bibcode: 2000A&A...355..759R Altcode: This paper presents PCA inversion, a novel application of Principal Component Analysis to the problem of spectral line inversion, ie. solar/stellar atmospheric model parameter estimation from spectral lines. For a given type of spectral line we compute a database of synthetic spectral profiles using a large number of models. Inversion of an observed profile to obtain an atmospheric model is equivalent to a problem in pattern recognition, finding the nearest profile in the synthetic profile database. To reduce dimensionality we use the synthetic data as a PCA training set to decompose each synthetic (and observed) profile into a sum of a small number of principal components, or eigenprofiles. The coefficients of this decomposition can be regarded as elements of a low-dimensional eigenfeature vector. The eigenfeatures are smooth functions of model parameters, indicating that eigenfeatures for parameters not in the training set could be easily estimated by interpolation. Search for the nearest profile is fast because it is done in the eigenfeature vector space. We illustrate the method using several types of synthetic spectra: unpolarised intensity profiles of a line formed in a Milne-Eddington model atmosphere; unpolarised Hα flux profiles of a line formed in non-Local Thermodynamic Equilibrium in the chromosphere of a cool star; and polarised Stokes parameter profiles of a line split by the Zeeman effect in the presence of a magnetic field. We also apply PCA to a set of Stokes data observed in a sunspot region by the High Altitude Observatory Advanced Stokes Polarimeter. PCA inversion is proposed as a fast alternative to non-linear least squares inversion commonly used for solar magnetic field measurements based on such Stokes data. Title: Spectropolarimetric observations of active stars Authors: Donati, J. -F.; Semel, M.; Carter, B. D.; Rees, D. E.; Collier Cameron, A. Bibcode: 1997MNRAS.291..658D Altcode: This paper reports the results of five years (five runs, 23 nights) of spectropolarimetric observations of active stars with the UCL Echelle Spectrograph of the Anglo-Australian Telescope. 225 circularly (and four linearly) polarized spectra were recorded on 28 objects (21 active stars and seven calibration standards) using the new technique of Zeeman-Doppler imaging. To extract polarization echelle spectra from raw frames, we developed a new dedicated automatic software package (called ESprit, which utilizes optimal extraction techniques) whose detailed description is given in the paper. For each recorded spectrum, we extract `mean' polarized and unpolarized profiles using `least-squares deconvolution', a technique similar to cross-correlation, which can enhance enormously the sensitivity of Zeeman-Doppler imaging, by up to 7.5 mag in flux with respect to a single average line analysis or by 4.5 mag compared with the older technique of Donati et al. in the particular case of a K1 star. Magnetic field is detected unambiguously on 14 objects, namely the weak-line T Tauri star V410 Tau, the pre-main-sequence binary HD 155555, the ZAMS stars AB Dor and LQ Hya, the dwarf flare star CC Eri, the RS CVn systems HR 1099, EI Eri, TY Pyx, CF Tuc, SZ Psc, II Peg, IM Peg and IL Hya, and the FK Com star YY Men. Marginal field detections are also obtained for the weak-line T Tauri star HD 283572 and the Herbig Ae star HD 104237. Except on HR 1099 and II Peg, our results represent the first direct field detections ever reported on these objects, and in particular the first direct field detection on as young a star as V410 Tau. Most of the magnetic signatures we detect on cool stars show several sign reversals throughout the line profile, indicating that the parent field structure is rather complex and must feature (as expected) many small-scale magnetic regions of different polarities. For all stars on which Zeeman detections are recorded with sufficient accuracy (namely LQ Hya, CC Eri, HR 1099, EI Eri, II Peg, IL Hya and YY Men), differential least-squares deconvolution from both the blue and the red parts of the spectral domain indicates that the magnetic regions we detect are mostly 500 to 1000 K cooler than, and sometimes at the same temperature as, but never warmer than the surrounding photosphere. Serendipitous results include the first detection (i) of small-amplitude radial velocity variations (1.3kms^-1 peak to peak) of the Herbig Ae star HD 104237 with small enough a period (37.5+/-1 min) that they must be due to stellar pulsations and (ii) of the solar-like secondary component of the RS CVn system IL Hya. Title: Numerical Methods in Polarized Radiative Transfer Authors: Rees, D. E.; Geers, G. Bibcode: 1996SoPh..164..103R Altcode: This paper looks at three aspects of numerical methods for solving polarized radiative transfer problems associated with spectral line formation in the presence of a magnetic field. First we prove "Murphy's law for Stokes evolution operators" which is the basis of the efficient algorithm used in the SPSR software package to compute the Stokes line depression contribution functions. Then we use a two-stream model to explain the efficacy of the field-free method in which the non-LTE line source function in a uniform magnetic field is approximated by the source function neglecting the magnetic field. Finally we introduce a totally new and computationally efficient approach to solving non-LTE problems based on a method of sparsely representing integral operators using wavelets. As an illustration, the wavelet method is used to solve the source function integral equation for a two-level atomic model in a finite atmosphere with coherent scattering, ignoring polarization. Title: Zeeman-Doppler imaging of active stars. III. Instrumental and technical considerations. Authors: Semel, M.; Donati, J. -F.; Rees, D. E. Bibcode: 1993A&A...278..231S Altcode: An account is given of the instrumental setup and observational procedure that led to the qualifying of Zeeman-Doppler imaging and to new detections of magnetic fields on four stars HR 1099, sigma2 CrB, II Peg and UX Ari. Title: Circularly polarized spectroscopic observations of RS CVn systems. Authors: Donati, J. -F.; Semel, M.; Rees, D. E. Bibcode: 1992A&A...265..669D Altcode: We applied the new technique of Zeeman-Doppler Imaging on nine active RS CVn binary systems. In four systems (HR 1099, sigma 2 CrB, II Peg and UX Ari), we detected a magnetic field on the cool active component; in each of them, the corresponding Stokes V signatures is found to vary with rotational phase. Moreover, on HR 1099, we have observational evidence that the magnetic structure varied significantly within a period of 1.3 yr. By contrast, magnetic field is repeatedly not detected on other stars, such as the hotter component of sigma 2 CrB, although these are also chromospherically very active. Our results suggest that the horizontal size scale of magnetic topologies may differ significantly among the observed stars. Large monopolar regions are likely present on those for which Zeeman signatures are detected; a more complex ordering of field lines with smaller monopolar regions and close bipolar groups may define the others. Title: Photospheric imaging of the RS CVn system HR 1099. Authors: Donati, J. -F.; Brown, S. F.; Semel, M.; Rees, D. E.; Dempsey, R. C.; Matthews, J. M.; Henry, G. W.; Hall, D. S. Bibcode: 1992A&A...265..682D Altcode: Spectropolarimetric, spectroscopic, and photometric observations of the RS CVn binary system HR 1099 = V711 Tau (K1IV+G5V) were made from Oct. 1988 to Jan. 1991. From Doppler imaging of the unpolarized spectra and the corresponding data, two maximum entropy images of the temperature distribution of the active K1 subgiant at epochs 1988.9 and 1990.9 were derived. Zeeman-Doppler imaging of the circularly polarized spectra has led to the first magnetic maps of a star other than the sun. The fragmentary observations made at epoch 1989.6 suggest that the star's magnetic field lines were emerging radially and/or poloidally from an equatorial warm region at that time, quite likely one of these reconstructed in the 1988.9 temperature image. Title: Mapping Magnetic Fields on Rapidly Rotating Stars: Application to the RS CVn System HR 1099 Authors: Donati, J. -F.; Brown, S. F.; Semel, M.; Rees, D. E. Bibcode: 1992ASPC...26..353D Altcode: 1992csss....7..353D No abstract at ADS Title: Zeeman-Doppler imaging of solar-type and AP stars. IV. Maximum entropy reconstruction of 2D magnetic topologies. Authors: Brown, S. F.; Donati, J. -F.; Rees, D. E.; Semel, M. Bibcode: 1991A&A...250..463B Altcode: Recently Semel (1989) introduced the technique of Zeeman-Doppler Imaging (ZDI) which essentially consists of Doppler Imaging using the Stokes V (circular polarization) profile. A code which uses maximum entropy principles to recover the distribution of brightness, abundance, and magnetic fields across the surface of stars is presented. For brightness and abundance mappings, the code is found to produce results comparable to those found in the literature. The first reconstructions of magnetic test structures from the Stokes V parameter alone show that this quantity contains enough spatial information to recover spotted images but not dipolar ones. According to the magnetic topologies that may be expected in solar-type or Ap stars, it is concluded that ZDI may be well adapted in deriving magnetic maps of active RS CVn systems, while it is anticipated that the use of the two other Stokes parameters (linear polarizations) are needed to derive reliable results for Ap stars. Title: Erratum - Detection of a Magnetic Region on HR:1099 Authors: Donati, J. F.; Semel, M.; Rees, D. E.; Taylor, K.; Robinson, R. D. Bibcode: 1991A&A...248..337D Altcode: No abstract at ADS Title: The chromospheres of late-type stars. I. EPS Eridani as a test case of multiline modelling. Authors: Thatcher, John D.; Robinson, Richard D.; Rees, David E. Bibcode: 1991MNRAS.250...14T Altcode: A new model of the lower chromosphere of the dwarf K2 star Epsilon Eridani is derived by matching flux profiles of the Ca IR triplet lines 8498 and 8542 A H-alpha and H-beta lines and the Na D lines (all observed simultaneously at the AAT), and the Ca II K line. The coupled non-LTE equations of statistical equilibrium and radiative transfer are solved under the constraint of hydrostatic equilibrium using the Carlsson (1986) code. Within the framework of the model, the Na D lines are an important photospheric diagnostic, and the Ca IR triplet lines can be used to locate the temperature minimum. The computed H-alpha and H-beta depths are highly sensitive constraints on the transition zone gradients and base pressures allowing us to derive a pressure at the base of the transition zone of 0.9 dyn/cm. Title: Detection of a magnetic region of HR 1099. Authors: Donati, J. -F.; Semel, M.; Rees, D. E.; Taylor, K.; Robinson, R. D. Bibcode: 1990A&A...232L...1D Altcode: The paper reports results of a magnetic field measuring campaign on the active K component of the RS CVn system HR 1099, using Zeeman-Doppler imaging. A localized magnetic field is detected near quadrature (phase 0.85). Assuming this magnetic region has a circular shape, its longitude and latitude are, respectively, estimated to be 86 + or - 4 deg and 5 + or - 5 deg. This equatorial region is largely monopolar, has a mean field strength of 985 + or - 270 G, covers 18 + or - 3 percent of the total stellar surface and may be colocalized with a bright photospheric spot. Title: Stokes Profile Analysis and Vector Magnetic Fields. II. Formal Numerical Solutions of the Stokes Transfer Equations Authors: Rees, D. E.; Murphy, G. A.; Durrant, C. J. Bibcode: 1989ApJ...339.1093R Altcode: Two numerical methods for formal integration of the Stokes transfer equations for line formation in a strong magnetic field were tested by computing Stokes profiles for a Zeeman triplet in a Milne-Eddington model atmosphere, and for the anomalously split Ca II K line in a realistic solar model. The first method is a Feautrier (1964) type method, in which the equations are written in second-order form and solved by finite-differences. The second method is a new solution called DELO, in which an integral equation for the Stokes vector is formulated in terms of the lambda operator (LO) associated with the diagonal elements (DE) of the absorption matrix. It is shown that the DELO method is faster and more accurate than the Feautrier method, and that both methods are more efficient than the Runge-Kutta integration method. Title: Transfer and Redistribution of Polarized Light in Resonance Lines. I. Quantum Formulation with Collisions Authors: Streater, A.; Cooper, J.; Rees, D. E. Bibcode: 1988ApJ...335..503S Altcode: Equations are derived quantum mechanically for the radiative transfer of polarized light and statistical equilibrium governing the formation of resonance lines in nonhydrogenic species. Redistribution due to collisions is specifically included, but magnetic fields are neglected. The formulation accounts for excited level interference and optical pumping and is applicable to solar resonance lines such as Ca II H and K and Mg II h and k. Title: Stokes Profile Analysis and Vector Magnetic Fields. IV. Synthesis and Inversion of the Chromospheric MG i B Lines Authors: Lites, B. W.; Skumanich, A.; Rees, D. E.; Murphy, G. A. Bibcode: 1988ApJ...330..493L Altcode: An analytic solution to the transfer equations for polarized radiation which allows for departures from LTE in the overall excitation of a chromospheric line is discussed, and a method of least-squares inversion is applied to profiles of Mg I b lines at 5172.7 and 5183.6 A observed in December 1978. An analytic form expressing the source function dependence on line-center optical depth leads to a generalization of an existing least-squares procedure which is found to be effective in recovering the magnetic field from simulated Stokes profiles under specified conditions. Reasonable magnetic field values are obtained if a macroturbulent profile smearing of 1-2 km/s is used and if the profiles are corrected for scattered light. Title: Stokes Profile Analysis and Vector Magnetic Fields. III. Extended Temperature Minima of Sunspot Umbrae as Inferred from Stokes Profiles of MG i lambda 4571 Authors: Lites, B. W.; Skumanich, A.; Rees, D. E.; Murphy, G. A.; Carlsson, M. Bibcode: 1987ApJ...318..930L Altcode: Observed Stokes profiles of Mg I 4571 A are analyzed as a diagnostic of the magnetic field and thermal structure at the temperature minimum of sunspot umbrae. Multilevel non-LTE transfer calculations of the Mg I-II-III excitation and ionization balance in model umbral atmospheres show: (1) Mg I to be far less ionized in sunspot umbrae than in the quiet sun, leading to greatly enhanced opacity in 4571 A, and (2) LTE excitation of 4571 A. Existing umbral models predict emission cores of the Stokes I profile due to the chromospheric temperature rise. This feature is not present in observed umbral profiles. Moreover, such an emission reversal causes similar anomalous features in the Stokes Q, U, V profiles, which are also not observed. Umbral atmospheres with extended temperature minima are suggested. Implications for chromospheric heating mechanisms and the utility of this line for solar vector magnetic field measurements are discussed. Title: Synthesis and Inversion of the Chromospheric Mg I ????-Line Stokes Profiles Authors: Murphy, G. A.; Rees, D. E.; Lites, B. W.; Skumanich, A. Bibcode: 1987BAAS...19..938M Altcode: No abstract at ADS Title: A Gentle Introduction to Polarized Radiative Transfer Authors: Rees, David E. Bibcode: 1987nrt..book..213R Altcode: No abstract at ADS Title: Non-LTE Polarized Radiative Transfer in Special Lines Authors: Rees, David E.; Murphy, Graham A. Bibcode: 1987nrt..book..241R Altcode: No abstract at ADS Title: Extended Temperature Minima of Sunspot Umbrae Authors: Lites, B. W.; Skumanich, A.; Rees, D. E.; Murphy, G. A.; Carlsson, M. Bibcode: 1986BAAS...18..662L Altcode: No abstract at ADS Title: Observational diagnostics for models of magnetic flux tubes Authors: Ribes, E.; Rees, D. E.; Cheng, F. Bibcode: 1985ApJ...296..268R Altcode: Dynamic models of flux tubes were used to interpret the presence of mass motions with large velocity gradients associated with spatially unresolved magnetic elements in the solar photosphere. A downflow was necessary in order to concentrate the magnetic field and provide a heating mechanism to brighten the flux tubes. There are two types of steady flow models: one has a moderate field strength (less than 1 kG) and a downflow decelerating with increasing depth and the second has a strong field and a downflow accelerating with increasing depth. To distinguish between these two field strengths, observed line profiles were compared to calculated line profiles. The comparison revealed that the main properties of the intensity line profiles and asymmetries of Stokes V-profiles was best explained by magnetic elements with moderate field strength. Further study is necessary to determine the reason for the redshift of the V-zero crossing point. Title: Dynamic models of flux tubes in the interpretation of polarization measurements Authors: Ribes-Nesme, E.; Rees, D. E.; Cheng, F. Bibcode: 1985svmf.nasa..300R Altcode: Recent observations of Stokes parameter profiles indicate the presence of mass motions with large velocity gradients associated with small-scale magnetic elements. Dynamic models of flux tubes were used in order to interpret observations of unresolved elements. It is clear that the physical picture of the dynamic models will be quite different from the hydrostatic ones since there is a strong coupling between the magnetic and the velocity field. Polarization measurements have to be interpreted in terms of dynamic models. Two-D steady flow solutions in slender magnetic tubes have been worked out. It was found that the main properties of the intensity line profiles as well as the asymmetries of the V Stokes profiles can be explained best in terms of magnetic elements with moderate field strength. Title: Least squares inversion of Stokes profiles in the presence of velocity gradients Authors: Skumanich, A.; Rees, D. E.; Lites, B. W. Bibcode: 1985svmf.nasa..306S Altcode: The Auer, Heasley and House Stokes inversion procedure in use at High Altitude Observatory is based on the analytic solution of the equation of transfer for polarized light where the representation of the thermodynamic and magnetic structure of the atmosphere is assumed to have a high degree of invariance, namely, a Milne-Eddington (ME) structure with a constant magnetic field. In the presence of invariance breaking gradients the resultant Stokes profiles are represented only approximately, if at all, by analytic forms. The accuracy of the inversion parameters and their significance as measures of actual structure are explored for the ME and the Landman-Finn sunspot models under the effects of velocity gradients. The resulting field parameters are good to a few percent and prove to be insensitive to the errors committed by the use of a ME-representation, but the resulting ME parameters yield a less precise measure of thermal structure. Title: Least squares inversion of Stokes profiles in the presence of velocity gradients. Authors: Skumanich, A.; Rees, D. E.; Lites, B. W. Bibcode: 1985NASCP2374..306S Altcode: The authors study the performance of the least squares fit when applied to theoretical profiles calculated with a realistic model of a sunspot. The model has gradients in all physical parameters including magnetic and velocity fields. This numerical experiment allows one to assess the physical significance of the Milne-Eddington and mean field parameters provided by the inversion. Observed Stokes profiles invariably have asymmetric distortions due to velocity gradients in the solar atmosphere. The inversion method symmetrizes the profiles prior to fitting and it is important to know how this affects the inference on the magnetic vector. Title: Solar CA II K resonance line polarization Authors: Rees, D. E.; Saliba, G. J. Bibcode: 1983PASA....5..186R Altcode: 1983PASAu...5..186R The linear polarization profiles formed by resonance scattering are calculated for the Ca II K line using various empirical solar atmosphere models in order to assess the potential value of resonance polarization observations for solar atmosphere diagnostics. In particular, intensity and polarization profiles are shown for three solar models: HSRA (Ginerich et al., 1971) and VALA and VALC (Vernazza et al., 1981). It is found that there are significant differences between the polarization profiles for the three models, especially near the core and wing maxima. These differences are encouraging evidence that linear polarization data may be valuable in delineating temperature and density models of the solar atmosphere. Title: Non-LTE resonance line polarization with partial redistribution effects Authors: Rees, D. E.; Saliba, G. J. Bibcode: 1982A&A...115....1R Altcode: The physics of formation of non-LTE resonance line polarization is studied for a two-level atom. Partial redistribution effects are included via an approximate treatment of the case of coherent scattering in an atomic rest frame in which the upper atomic level is naturally broadened. The transfer equations are solved in finite and semi-infinite model atmospheres and the relevance of the solutions to solar observations is discussed. In particular, qualitative agreement is obtained with the linear polarization profiles of Ca I 4227 A, NaD2, and Ca II K observed by Stenflo et al. (1980). These profiles have maxima in the line core and the line wings, contrary to Wiehr's (1981) NaD2 data in which the core maximum is not observed. Title: Coronal emission-line polarization from the statistical equilibrium of magnetic sublevels. II - Fe XIV 5303 A Authors: House, L. L.; Querfeld, C. W.; Rees, D. E. Bibcode: 1982ApJ...255..753H Altcode: Coronal magnetic fields influence the intensity and linear polarization of light scattered by coronal Fe XIV ions. To interpret polarization measurements of Fe XIV 5303 A coronal emission requires a detailed understanding of the dependence of the emitted Stokes vector on coronal magnetic field direction, electron density, and temperature and on height of origin. The required dependence is included in the solutions of statistical equilibrium for the ion which are solved explicitly for 34 magnetic sublevels in both the ground and four excited terms. The full solutions are reduced to equivalent simple analytic forms which clearly show the required dependence on coronal conditions. The analytic forms of the reduced solutions are suitable for routine analysis of 5303 green line polarimetric data obtained at Pic du Midi and from the Solar Maximum Mission Coronagraph/Polarimeter. Title: Solar polarimetry - Observations and theories Authors: Rees, D. E. Bibcode: 1982PASA....4..335R Altcode: 1982PASAu...4..335R Some recent observations of polarization in solar spectral lines are reviewed, emphasizing their theoretical interpretation. Classical scattering theory is used to study the diagnostic problems which arise in the zero, weak, and strong field regimes. The interpretation of photospheric, prominence, coronal, and sunspot magnetic fields is addressed. Studies of the Hanle effect, measurement of the forbidden coronal emission line polarization, and analysis of the complete wavelength profiles of the four Stokes parameters of a Zeeman split line are emphasized. Title: Resonance-Line Polarization - Part Six - Line Wing Transfer Calculations Including Excited State Interference Authors: Auer, L. H.; Rees, D. E.; Stenflo, J. O. Bibcode: 1980A&A....88..302A Altcode: A heuristic theory of polarized radiative transfer is developed for the wings of solar resonance lines. Magnetic fields are neglected. The theory includes quantum mechanical interference between j = ½ and 3/2 excited states of line transitions sharing a common j = ½ ground state. Examples of such lines are Ca II and K, Na I D1 and D2, and Mg II h and k. Calculations are made with the HSRA solar model for these lines as well as the dipole-type transition Ca I 4227 which is not affected by interference. The results for Ca I 4227, Ca II H and K and Na I D1 and D2 compare very well with recent observations, lending support to our theory. The polarization predicted in the Mg H h and k lines is the largest of all indicating these lines to be prime candidates for linear polarization observations in the UV spectrum. Title: Line formation in an unresolved magnetic element: a test of the centre of gravity method. Authors: Rees, D. E.; Semel, M. D. Bibcode: 1979A&A....74....1R Altcode: The problem of multidimensional LTE line formation in a magnetic field is solved in a model of an unresolved magnetic element imbedded in the solar photosphere. Horizontal and vertical variations of the physical parameters are included. Calculations show that, in circular polarization analysis, the centre of gravity displacement of the line profile gives a very good measurement of the longitudinal field averaged over the area observed. For small line weakenings the accuracy is probably better than 10%. Also the method is not affected by Zeeman saturation. A method based on centre of gravity measurements is proposed for inferring a 'model-independent' value of the average longitudinal field inside an unresolved magnetic element. Title: Non-LTE resonance line polarization in the absence of magnetic fields. Authors: Rees, D. E. Bibcode: 1978PASJ...30..455R Altcode: The transfer equations for non-LTE resonance line formation with polarization are solved by an integral equation technique. Magnetic fields and collisional depolarization are neglected and complete redistribution in frequency is assumed for scattering. Calculations are made for both finite and semi-infinite plane-parallel atmospheres. A detailed discussion is given of the physical effects produced in a variety of models. The results should provide a convenient basis for interpreting calculations in more realistic solar models. Key words: Non-LTE line formation; Resonance line; Solar models. Title: Resonance line polarization in finite atmospheres. Authors: Rees, D. E. Bibcode: 1977ROLun..12...25R Altcode: No abstract at ADS Title: On Facular Models Authors: Rees, D. E. Bibcode: 1974IAUS...56..177R Altcode: No abstract at ADS Title: High Resolution Spectroscopy of the Disk Chromosphere. II. Time Sequence Observations of Ca II H and K Emissions Authors: Wilson, P. R.; Rees, D. E.; Beckers, J. M.; Brown, D. R. Bibcode: 1972SoPh...25...86W Altcode: Two independent sets of high resolution time series spectra of the CaII H and K emission obtained at the Solar Tower and at the Big Dome of the Sacramento Peak Observatory on September 11th, 1971 are reported. The evolutionary behaviour of the emission first reported by Wilson and Evans is confirmed but the detail of the evolution is found to be more complex. In one case, a doubly peaked feature showing some K3 emission evolves into a single K2 (red) peak with no K3 emission. Coincidentally, a neighbouring doubly peaked feature evolves to a very strong blue peak. In an entirely independent sequence a doubly peaked feature evolves into a single red peak. The K2 emission then fades completely although the continuum threads are still strong. Finally a strong K2 blue peak appears. These developments are confirmed by intensity profiles obtained from the spectra. Title: Line Transfer in the Presence of Two-Dimensional Velocity Gradients Authors: Cannon, C. J.; Rees, D. E. Bibcode: 1971ApJ...169..157C Altcode: Feautrier's (1964) method is generalized to solve the equation of radiative transfer for a spectral line in an atmosphere exhibiting horizontal fluctuations in all physical parameters together with multidimensional velocity gradients. odel two-dimensional velocity problems are discussed. Title: On the Absorption Matrix in a Non-Uniform Magnetic Field Authors: Rees, David E. Bibcode: 1971SoPh...16...67R Altcode: Beckers (1969) and Staude (1969) give contradictory expressions for the absorption matrix for a normal Zeeman triplet in a non-uniform magnetic field. Beckers' result is shown to be the correct one. Title: Velocity-Dependent Source Functions Authors: Rees, David E. Bibcode: 1970PASA....1..384R Altcode: 1970PASAu...1..384R We consider the effect of a velocity gradient on the source function for a spectral line formed in an atmosphere in non local thermodynamic equilibrium, a problem examined previously by Kulander and Hummer and Rybicki. A numerical method is used which extends the Feautrier technique. Title: Line Formation in a Magnetic Field Authors: Rees, David E. Bibcode: 1969SoPh...10..268R Altcode: The effect of noncoherent scattering is examined for an absorption line formed in a uniform magnetic field. It is shown that the Stokes parameters of the line radiation may be computed by using the line source function in the absence of a magnetic field as a first approximation for that in the presence of a magnetic field.