explanation blue bibcodes open ADS page with paths to full text
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.
2009nrt..book..213R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Non-LTE Polarized Radiative Transfer in Special Lines
Authors: Rees, David E.; Murphy, Graham A.
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.
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 <P />FWHM ∼ 9 pm,
FSR ∼ 62 pm, finesse ∼5.3, and transmittance ∼ 50% at 157
nm. For the measurements, the University of Toronto's F<SUB>2</SUB>
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.
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. <P />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.
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. <P />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. <P />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.
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.
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.
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.
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.
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,
sigma<SUP>2</SUP> CrB, II Peg and UX Ari.
---------------------------------------------------------
Title: Circularly polarized spectroscopic observations of RS CVn
systems.
Authors: Donati, J. -F.; Semel, M.; Rees, D. E.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1987BAAS...19..938M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Gentle Introduction to Polarized Radiative Transfer
Authors: Rees, David E.
1987nrt..book..213R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Non-LTE Polarized Radiative Transfer in Special Lines
Authors: Rees, David E.; Murphy, Graham A.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 D<SUB>1</SUB>
and D<SUB>2</SUB>, 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 D<SUB>1</SUB> and D<SUB>2</SUB> 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.
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.
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.
1977ROLun..12...25R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On Facular Models
Authors: Rees, D. E.
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.
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 K<SUB>3</SUB> emission evolves into a single K<SUB>2</SUB> (red)
peak with no K<SUB>3</SUB> 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 K<SUB>2</SUB> emission then fades completely although the
continuum threads are still strong. Finally a strong K<SUB>2</SUB>
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.
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.
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.
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.
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.
