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Author name code: semel
ADS astronomy entries on 2022-09-14
author:"Semel, Meir" 

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Title: Magnetic fields and differential rotation on the pre-main
    sequence - III. The early-G star HD 106506
Authors: Waite, I. A.; Marsden, S. C.; Carter, B. D.; Hart, R.;
   Donati, J. -F.; Ramírez Vélez, J. C.; Semel, M.; Dunstone, N.
2011MNRAS.413.1949W    Altcode: 2011arXiv1102.3475W; 2011MNRAS.tmp..486W
  We present the photometry and spectropolarimetry of the
  pre-main-sequence star HD 106506. A photometric rotational period of
  ∼1.416 ± 0.133 d has been derived using observations at Mount Kent
  Observatory (MKO). Spectropolarimetric data obtained with the 3.9-m
  Anglo-Australian Telescope (AAT) were used to derive spot occupancy
  and magnetic maps of the star through the technique of Zeeman Doppler
  imaging (ZDI). The resulting brightness maps indicate that HD 106506
  displays photospheric spots at all latitudes including a predominant
  polar spot. Azimuthal and radial magnetic images of this star have
  been derived, and a significant azimuthal magnetic field is indicated,
  in line with other active young stars. A solar-like differential
  rotation law was incorporated into the imaging process. Using Stokes I
  information the equatorial rotation rate, Ω<SUB>eq</SUB>, was found to
  be 4.54 ± 0.01 rad d<SUP>-1</SUP>, with a photospheric shear δΩ of
  0.21<SUP>+0.02</SUP><SUB>-0.03</SUB> rad d<SUP>-1</SUP>. This equates to
  an equatorial rotation period of ∼1.39 ± 0.01 d, with the equatorial
  region lapping the poles every ∼ 30<SUP>+5</SUP><SUB>-3</SUB> d. Using
  the magnetic features, the equatorial rotation rate, Ω<SUB>eq</SUB>,
  was found to be 4.51 ± 0.01 rad d<SUP>-1</SUP>, with a photospheric
  shear δΩ of 0.24 ± 0.03 rad d<SUP>-1</SUP>. This differential
  rotation is approximately four times that observed on the Sun.

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Title: Magnetic fields and differential rotation on the pre-main
    sequence - I. The early-G star HD 141943 - brightness and magnetic
    topologies
Authors: Marsden, S. C.; Jardine, M. M.; Ramírez Vélez, J. C.;
   Alecian, E.; Brown, C. J.; Carter, B. D.; Donati, J. -F.; Dunstone,
   N.; Hart, R.; Semel, M.; Waite, I. A.
2011MNRAS.413.1922M    Altcode: 2011arXiv1101.5859M; 2011MNRAS.tmp..487M
  Spectroscopic and spectropolarimetric observations of the pre-main
  sequence early-G star HD 141943 were obtained at four observing
  epochs (in 2006, 2007, 2009 and 2010). The observations were
  undertaken at the 3.9-m Anglo-Australian Telescope using the UCLES
  echelle spectrograph and the SEMPOL spectropolarimeter visitor
  instrument. Brightness and surface magnetic field topologies were
  reconstructed for the star using the technique of least-squares
  deconvolution to increase the signal-to-noise ratio of the data. <P
  />The reconstructed brightness maps show that HD 141943 had a weak
  polar spot and a significant amount of low-latitude features, with
  little change in the latitude distribution of the spots over the 4 yr
  of observations. The surface magnetic field was reconstructed at three
  of the epochs from a high-order (l≤ 30) spherical harmonic expansion
  of the spectropolarimetric observations. The reconstructed magnetic
  topologies show that in 2007 and 2010 the surface magnetic field was
  reasonably balanced between poloidal and toroidal components. However,
  we find tentative evidence of a change in the poloidal/toroidal ratio
  in 2009 with the poloidal component becoming more dominant. At all
  epochs the radial magnetic field is predominantly non-axisymmetric
  while the azimuthal field is predominantly axisymmetric with a ring
  of positive azimuthal field around the pole similar to that seen on
  other active stars.

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Title: Magnetic fields and differential rotation on the pre-main
    sequence - II. The early-G star HD 141943 - coronal magnetic field,
    Hα emission and differential rotation
Authors: Marsden, S. C.; Jardine, M. M.; Ramírez Vélez, J. C.;
   Alecian, E.; Brown, C. J.; Carter, B. D.; Donati, J. -F.; Dunstone,
   N.; Hart, R.; Semel, M.; Waite, I. A.
2011MNRAS.413.1939M    Altcode: 2011arXiv1101.5866M; 2011MNRAS.tmp..485M
  Spectropolarimetric observations of the pre-main sequence early-G star
  HD 141943 were made at three observing epochs (2007, 2009 and 2010). The
  observations were made using the 3.9-m Anglo-Australian Telescope with
  the UCLES echelle spectrograph and the SEMPOL spectropolarimeter visitor
  instrument. The brightness and surface magnetic field topologies (given
  in Paper I) were used to determine the star's surface differential
  rotation and reconstruct the coronal magnetic field of the star. <P
  />The coronal magnetic field at the three epochs shows on the largest
  scales that the field structure is dominated by the dipole component
  with possible evidence for the tilt of the dipole axis shifting between
  observations. We find very high levels of differential rotation on HD
  141943 (∼8 times the solar value for the magnetic features and ∼5
  times solar for the brightness features), similar to that evidenced by
  another young early-G star, HD 171488. These results indicate that a
  significant increase in the level of differential rotation occurs for
  young stars around a spectral type of early-G. We also find for the
  2010 observations that there is a large difference in the differential
  rotation measured from the brightness and magnetic features, similar
  to that seen on early-K stars, but with the difference being much
  larger. We find only tentative evidence for temporal evolution in the
  differential rotation of HD 141943.

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Title: Eigenpolarimetry: An Algebraic Approach to Polarization-free
    Telescopes and Instruments
Authors: López Ariste, A.; Semel, M.
2011ASPC..437..403L    Altcode:
  We have developed new algebraic tools based upon the symmetries of
  Mueller matrices that allow us to circumvent those problems while
  keeping the adjective polarization-free for our telescopes and not
  requiring expensive and delicate calibrations. The basic concept is
  to modulate entrance polarization into the eigenvectors of the system
  so that polarization information arrives unscathed at the analyzing
  beamsplitters, placed at the instrument or detector level. I will
  describe the basic concept, its application to THEMIS and the
  propositions for EST.

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Title: Spectropolarimetric multi line analysis of stellar magnetic
    fields
Authors: Ramírez Vélez, J. C.; Semel, M.; Stift, M.; Martínez
   González, M. J.; Petit, P.; Dunstone, N.
2010A&A...512A...6R    Altcode: 2010arXiv1001.1003R
  <BR /> Aims: In this paper we study the feasibility of inferring the
  magnetic field from polarized multi line spectra using two methods:
  The pseudo line approach and The PCA-ZDI approach. <BR /> Methods:
  We use multi line techniques, meaning that all the lines of a stellar
  spectrum contribute to obtain a polarization signature. The use of
  multiple lines dramatically increases the signal-to-noise-ratio of
  these polarizations signatures. Using one technique, the pseudo line
  approach, we construct the pseudo line as the mean profile of all the
  individual lines. The other technique, the PCA-ZDI approach proposed
  recently by Semel et al. (2006, ASPC, 358, 355) for the detection
  of polarized signals, combines principle components analysis (PCA)
  and the Zeeman Doppler imaging technique (ZDI). This new method has
  a main advantage: the polarized signature is extracted using cross
  correlations between the stellar spectra and functions containing
  the polarization properties of each line. is extracted using cross
  correlations between the stellar spectra and functions containing the
  polarization properties of each line." These functions are the principal
  components of a database of synthetic spectra. The synthesis of the
  spectra of the database are obtained using the radiative transfer
  equations in LTE. The profiles built with the PCA-ZDI technique are
  called multi Zeeman signatures. <BR /> Results: The construction of the
  pseudo line as well as the multi Zeeman signatures is a powerful tool
  in the study of stellar and solar magnetic fields. The information of
  the physical parameters that governs the line formation is contained
  in the final polarized profiles. We have shown in particular using
  inversion codes that the magnetic field vector can be properly inferred
  with both approaches despite the magnetic field regime.

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Title: Multiline Zeeman signatures through line addition
Authors: Semel, M.; Ramírez Vélez, J. C.; Martínez González,
   M. J.; Asensio Ramos, A.; Stift, M. J.; López Ariste, A.; Leone, F.
2009A&A...504.1003S    Altcode: 2009arXiv0906.4226S
  Context: To obtain a significant Zeeman signature in the polarised
  spectra of a magnetic star, we usually “add” the contributions
  of numerous spectral lines; the ultimate goal is to recover the
  spectropolarimetric prints of the magnetic field in these line
  additions. <BR />Aims: Here we want to clarify the meaning of these
  techniques of line addition; in particular, we try to interpret the
  meaning of the “pseudo-line” formed during this process and to
  find out why and how its Zeeman signature is still meaningful. <BR
  />Methods: We create a synthetic case of line addition and apply well
  tested standard solar methods routinely used in research on magnetism
  in the Sun. <BR />Results: The results are convincing and the Zeeman
  signatures well detected; Solar methods are found to be quite efficient
  for stellar observations. We statistically compare line addition
  with least-squares deconvolution and demonstrate that they both give
  very similar results, as a consequence of the special statistical
  properties of the weights. <BR />Conclusions: The Zeeman signatures
  are unequivocally detected in this multiline approach. We suggest
  that magnetic field detection is reliable well beyond the weak-field
  approximation. Linear polarisation in the spectra of solar type stars
  can be detected when the spectral resolution is sufficiently high.

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Title: Surface magnetic fields on two accreting TTauri stars: CVCha
    and CRCha
Authors: Hussain, G. A. J.; Collier Cameron, A.; Jardine, M. M.;
   Dunstone, N.; Ramirez Velez, J.; Stempels, H. C.; Donati, J. -F.;
   Semel, M.; Aulanier, G.; Harries, T.; Bouvier, J.; Dougados, C.;
   Ferreira, J.; Carter, B. D.; Lawson, W. A.
2009MNRAS.398..189H    Altcode: 2009arXiv0905.0914H; 2009MNRAS.tmp..997H
  We have produced brightness and magnetic field maps of the surfaces of
  CVCha and CRCha: two actively accreting G- and K-type TTauri stars in
  the ChamaeleonI star-forming cloud with ages of 3-5Myr. Our magnetic
  field maps show evidence for strong, complex multipolar fields
  similar to those obtained for young rapidly rotating main-sequence
  stars. Brightness maps indicate the presence of dark polar caps and
  low-latitude spots - these brightness maps are very similar to those
  obtained for other pre-main-sequence and rapidly rotating main-sequence
  stars. <P />Only two other classical TTauri stars have been studied
  using similar techniques so far: V2129Oph and BPTau. CVCha and CRCha
  show magnetic field patterns that are significantly more complex than
  those recovered for BPTau, a fully convective TTauri star. <P />We
  discuss possible reasons for this difference and suggest that the
  complexity of the stellar magnetic field is related to the convection
  zone; with more complex fields being found in TTauri stars with
  radiative cores (V2129Oph, CVCha and CRCha). However, it is clearly
  necessary to conduct magnetic field studies of TTauri star systems,
  exploring a wide range of stellar parameters in order to establish how
  they affect magnetic field generation, and thus how these magnetic
  fields are likely to affect the evolution of TTauri star systems as
  they approach the main sequence.

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Title: Stellar Magnetic Fields in Swollen Convection Zones
Authors: Marsden, S. C.; Donati, J. -F.; Petit, P.; Dunstone, N. J.;
   Jardine, M.; Carter, B. D.; Waite, I. A.; Semel, M.; Ramirez Velez, J.
2009ASPC..405..531M    Altcode:
  Solar magnetic activity is generated through dynamo action operating at
  the base of the solar convection zone. However, for rapidly rotating
  solar-type stars this might not be the case with magnetic images
  showing regions of near-surface azimuthal field indicating that
  the operation of dynamo may in fact be distributed throughout the
  entire convection zone. Here we present the first magnetic images of
  a pre-main sequence star with both components having swollen outer
  convection zones. These results are part of an international study
  to understand how the generation of magnetic fields is affected by
  basic stellar parameters such as mass, rotation rate, the depth of
  the stellar convection zone, and binarity. The magnetic images were
  obtained by observing the star in circularly polarised light and using
  the technique of Zeeman Doppler imaging.

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Title: Multiline Zeeman signatures as demonstrated through the
    Pseudo-line
Authors: Semel, M.; Ramirez Velez, J. C.; Stift, M. J.; Martinez
   Gonzalez, M. J.; Lopez Ariste, A.; Leone, F.
2008arXiv0810.3543S    Altcode:
  In order to get a significant Zeeman signature in the polarised spectra
  of a magnetic star, we usually 'add' the contributions of numerous
  spectral lines; the ultimate goal is to recover the spectropolarimetric
  prints of the magnetic field in these line additions. Here we want to
  clarify the meaning of these techniques of line addition; in particular,
  we try to interpret the meaning of the 'pseudo-line' formed during
  this process and to find out why and how its Zeeman signature is still
  meaningful. We create a synthetic case of lines addition and apply
  well tested standard solar methods routinely used in the research
  on magnetism in our nearest star. The results are convincing and the
  Zeeman signatures well detected; Solar methods are found to be quite
  efficient also for stellar observations. The Zeeman signatures are
  unequivocally detected in this multiline approach. We may anticipate
  the outcome magnetic fields to be reliable well beyond the weak-field
  approximation. Linear polarisation in the spectra of solar type stars
  can be detected when the spectral resolution is sufficiently high.

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Title: PCA detection and denoising of Zeeman signatures in polarised
    stellar spectra
Authors: Martínez González, M. J.; Asensio Ramos, A.; Carroll,
   T. A.; Kopf, M.; Ramírez Vélez, J. C.; Semel, M.
2008A&A...486..637M    Altcode: 2008arXiv0804.4692M
  Aims: Our main objective is to develop a denoising strategy to
  increase the signal to noise ratio of individual spectral lines
  of stellar spectropolarimetric observations. <BR />Methods: We
  use a multivariate statistics technique called Principal Component
  Analysis. The cross-product matrix of the observations is diagonalized
  to obtain the eigenvectors in which the original observations can be
  developed. This basis is such that the first eigenvectors contain
  the greatest variance. Assuming that the noise is uncorrelated a
  denoising is possible by reconstructing the data with a truncated
  basis. We propose a method to identify the number of eigenvectors for
  an efficient noise filtering. <BR />Results: Numerical simulations are
  used to demonstrate that an important increase of the signal to noise
  ratio per spectral line is possible using PCA denoising techniques. It
  can be also applied for detection of magnetic fields in stellar
  atmospheres. We analyze the relation between PCA and commonly used
  techniques like line addition and least-squares deconvolution. Moreover,
  PCA is very robust and easy to compute.

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Title: Strength distribution of solar magnetic fields in photospheric
    quiet Sun regions
Authors: Ramírez Vélez, J. C.; López Ariste, A.; Semel, M.
2008A&A...487..731R    Altcode: 2008arXiv0806.1326R
  Context: The magnetic topology of the solar photosphere in its quietest
  regions is hidden by the difficulties to disentangle magnetic flux
  through the resolution element from the field strength of unresolved
  structures. The observation of spectral lines with strong coupling with
  hyperfine structure, like the observed Mn i line at 553.7 nm, allows
  such differentiation. <BR />Aims: To analyse the distribution of field
  strengths in the network and intranetwork of the solar photosphere
  through inversion of the Mn i line at 553.7 nm. <BR />Methods: An
  inversion code for the magnetic field using the principal component
  analysis (PCA) has been developed. Statistical tests are run on
  the code to validate it. The code has to draw information from the
  small-amplitude spectral feature appearing in the core of the Stokes
  V profile of the observed line for field strengths below a certain
  threshold, coinciding with lower limit of the Paschen-Back effect in
  the fine structure of the involved atomic levels. <BR />Results: The
  inversion of the observed profiles, using the circular polarisation (V)
  and the intensity (I), shows the presence of magnetic fields strengths
  in a range from 0 to 2 kG, with predominant weak strength values. Mixed
  regions with mean strength field values of 1130 and 435 Gauss are
  found associated with the network and intranetwork, respectively. <BR
  />Conclusions: The Mn i line at 553 nm probes the field strength
  distribution in the quiet sun and shows the predominance of weak,
  hectoGauss fields in the intranetwork, and strong, kiloGauss fields in
  the network. It also shows that both network and intranetwork are to be
  understood at our present spatial resolutions as field distributions,
  of which we hint at the mean properties.

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Title: First polarimetric observations and modeling of the FeH
    F<SUP>4</SUP>Δ-X<SUP>4</SUP>Δ system
Authors: Afram, N.; Berdyugina, S. V.; Fluri, D. M.; Semel, M.;
   Bianda, M.; Ramelli, R.
2007A&A...473L...1A    Altcode: 2007arXiv0708.0298A
  Context: Lines of diatomic molecules are typically much more
  temperature and pressure sensitive than atomic lines, which makes
  them ideal, complementary tools for studying cool stellar atmospheres
  as well as the internal structure of sunspots and starspots. The
  FeH F^4Δ-X^4Δ system represents such an example that exhibits in
  addition a large magnetic field sensitivity. However, the current
  theoretical descriptions of these transitions including the molecular
  constants involved are only based on intensity measurements because
  polarimetric observations have not been available so far, which limits
  their diagnostic value. Furthermore, the theory was optimized to
  reproduce energy levels and line strengths without taking the magnetic
  sensitivities into account. <BR />Aims: We present for the first
  time spectropolarimetric observations of the FeH F^4Δ-X^4Δ system
  measured in sunspots to investigate their diagnostic capabilities for
  probing solar and stellar magnetic fields. In particular, we investigate
  whether the current theoretical model of FeH can reproduce the observed
  Stokes profiles including their magnetic properties. <BR />Methods: The
  polarimetric observations of the FeH F^4Δ-X^4Δ system in Stokes I and
  V are compared with synthetic Stokes profiles modeled with radiative
  transfer calculations. This allows us to infer the temperature and
  the magnetic field strength of the observed sunspots. <BR />Results:
  We find that the current theory successfully reproduces the magnetic
  properties of a large number of lines in the FeH F^4Δ-X^4Δ
  system. In a few cases the observations indicate a larger Zeeman
  splitting than predicted by the theory. There, our observations have
  provided additional constraints, which allowed us to determine empirical
  molecular constants. <BR />Conclusions: The FeH F^4Δ-X^4Δ system is
  found to be a very sensitive magnetic diagnostic tool. Polarimetric
  data of these lines, in contrast to intensity measurements, provide us
  with more direct and detailed information to study the coolest parts
  of sunspot and starspot umbrae, and cool active dwarfs.

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Title: The photospheric magnetic field and coronal structure of
    HD 171488
Authors: Marsden, S. C.; Jardine, M.; Donati, J. -F.; Petit, P.;
   Semel, M.; Carter, B. D.
2007MmSAI..78..307M    Altcode:
  From spectropolarimetric observations of the young, single early
  G-dwarf HD 171488, we have used Zeeman Doppler imaging to reconstruct
  brightness and magnetic maps of the star. The inclusion of a solar-like
  differential rotation law into the imaging process has enabled the
  measurement of the star's surface differential rotation from the
  brightness features. In addition, we have created a coronal structure
  for HD 171488 based on the surface magnetic field distribution.

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Title: Quiet-Sun Magnetism Seen with a Mn Line: Km-Sized Magnetic
    Structures
Authors: López Ariste, A.; Ramírez Vélez, J. C.; Tomczyk, S.;
   Casini, R.; Semel, M.
2006ASPC..358...54L    Altcode:
  We observed Manganese lines with large hyperfine structure and used them
  to disentangle strength from flux in the measurement of photospheric
  magnetic fields. In observations of the quiet sun with both ASP and
  THEMIS, we measure flux from the amplitude of Stokes V in Fe lines, and
  the Mn line, crudely analyzed, places the field strength either above
  or below a threshold of 600 G, which is set by the atomic structure. In
  the case of THEMIS observations, having determined magnetic flux and
  field strength for every pixel, one can estimate filling factors of
  the magnetic field and determine characteristic scales. Structures at
  scales smaller than 50 km are revealed.

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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.

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Title: The Magnetic Activity of Young Suns: Magnetic Topologies and
    Differential Rotation of F- and G-Dwarfs
Authors: Marsden, S. C.; Mengel, M. W.; Donati, F.; Carter, B. D.;
   Semel, M.; Petit, P.
2006ASPC..358..401M    Altcode:
  Maps of magnetic field topologies for rapidly rotating, solar-type stars
  have been produced for the past decade, and offer a unique insight into
  the operation of the stellar dynamo. The creation of these maps uses
  the combined techniques of Least-Squares Deconvolution (LSD) to increase
  the signal-to-noise of the Stokes-V profiles, and Zeeman Doppler Imaging
  (ZDI) to create the magnetic maps from the Stokes-V profiles. However,
  for solar-type stars almost all of the targets imaged to date have
  been lower-mass (early-K) stars, limiting the range of operating
  parameters of the stellar dynamo. We present magnetic maps of two
  higher-mass, solar-type stars, the early G-dwarf HD 171488 (v sin i =
  38 km s<SUP>-1</SUP>) and the late F-dwarf HR 1817 (v sin i = 52 km
  s<SUP>-1</SUP>). Both stars have been observed in left- and right-hand
  circularly polarized light using the SEMPOL spectro-polarimeter with
  the Anglo-Australian telescope. Both LSD and ZDI have then been used to
  infer topologies of the photospheric magnetic fields of the two stars,
  thus expanding the mass range of solar-type stars imaged using these
  techniques. In addition, the surface differential rotation of the
  two stars has been determined from the data, with both stars showing
  significantly higher differential rotation than that of lower-mass
  stars.

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Title: PCA Technique Applied to the Detection of ZDI Circular
    Polarization in Solar-Type Stars
Authors: Rámirez Vélez, J. C.; Semel, M.; Stift, M. J.; Leone, F.
2006ASPC..358..405R    Altcode:
  We present a new detection method of ZDI circular polarization applied
  to observations of solar-type stars. To extract the polarization signal
  we analyze the observed spectra over intervals of about 2500 Å, and
  we take into account the contribution of all spectral lines to the
  resulting polarization signal. We employ a PCA technique to analyze the
  synthetic database of profiles. We apply singular value decomposition
  to the synthetic data over the same spectral range as for the observed
  data. The principal parameters entering the magnetic spectral synthesis
  code COSSAM are the effective temperature T<SUB>eff</SUB> and the
  magnetic field strength B. We find that the first 10 eigenvectors
  are sufficient to approximate the contribution of all the lines to
  the resultant averaged circular polarization signal. The principle of
  this new method can also be employed to detect linear polarization,
  and it is of course not restricted to solar type stars.

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Title: Multisite, Multiwavelength Studies of the Active Cool Binary
    CC Eri
Authors: Budding, E.; Bembrick, C.; Carter, B. D.; Erkan, N.; Jardine,
   M.; Marsden, S. C.; Osten, R.; Petit, P.; Semel, M.; Slee, O. B.;
   Waite, I.
2006Ap&SS.304...13B    Altcode: 2006Ap&SS.tmp...94B
  New data acquired on the active, cool binary CC Eri ranged
  across the spectrum from Chandra X-ray to broadband photometry and
  microwave observations using the VLA and ATCA. Also, high-dispersion
  spectropolarimetry using the AAT enabled Zeeman-Doppler imaging to be
  performed. Our interpretations infer strong localised concentrations
  of the stellar magnetic field, manifested by surface activity and
  related large coronal plasma structures. Comprehensive matching of the
  modelling parameters awaits more detailed investigation. This brief
  interim review includes consideration of the ATCA data. Microwave radio
  emission is usually low level (‘quiescent’), but occasionally flares
  of several mJy peak intensity are observed. We associate the emission,
  generally, with wave-like mechanisms, expanding through the outer
  atmosphere. Related characteristics of this emission are discussed.

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Title: Inversion of solar magnetic fields in photospheric quiet
    regions.
Authors: Ramírez Vélez, J. C.; López Ariste, A.; Semel, M.
2006IAUJD...3E..13R    Altcode:
  Spectropolarymetric observational data and the correspondent
  magnetic field inversion of the solar photosphere in quiet regions are
  presented. The presence of atomic coupling between the nuclei momentum
  (I) and total electron momentum ( J[LS]), known as hyperfine structure
  atomic regime where the total momentum F = I + J, was founded in the
  observed data for the atom MnI at 553.7 nm. The signature of these
  hyperfine structure is clearly detected in circular polarisation in form
  of protuberance in the central part of the profile. We have developed
  an inversion code of the magnetic field using the Principal Component
  Analysis (PCA). An statistical test is applied to the code obtaining
  satisfactory inversion results. The inversion of the observed profiles,
  using the circular polarisation (V) and the intensity (I), shows mixed
  regions with values of |B| higher than 800 Gauss and lower than 600
  Gauss associated with the network and internetwork respectively.

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Title: Surface differential rotation and photospheric magnetic field
    of the young solar-type star HD 171488 (V889 Her)
Authors: Marsden, S. C.; Donati, J. -F.; Semel, M.; Petit, P.; Carter,
   B. D.
2006MNRAS.370..468M    Altcode: 2006MNRAS.tmp..643M
  We present spectropolarimetric observations of the young, single early
  G-dwarf HD 171488. These observations were obtained over a five-night
  period in 2004 September at the 3.9-m Anglo-Australian Telescope using
  the SEMPOL spectropolarimeter visitor instrument. Using the technique
  of least-squares deconvolution to increase the signal-to-noise ratio
  of the data, we have applied Zeeman Doppler imaging to reconstruct
  brightness and magnetic surface topologies of the star. The brightness
  image shows a large polar spot with weaker low- to mid-latitude
  features, confirming an earlier Doppler imaging observation. The
  reconstruction of the surface magnetic field shows regions of radial
  field at all latitudes (except near the pole) and regions of azimuthal
  field predominantly at high latitudes (60°-70°), with the azimuthal
  field almost forming a ring around the polar regions. <P />We have
  incorporated a solar-like differential rotation law into the imaging
  process to determine the surface differential rotation of cool spots on
  HD 171488. This gives an equatorial rotation rate of 1.313 +/- 0.004 d
  and a surface shear of dΩ = 0.402 +/- 0.044 radd<SUP>-1</SUP>. This
  means that the equator of HD 171488 laps the poles every ~16 +/-
  2 d and that HD 171488 has a photospheric shear approximately seven
  times the solar value. This is the largest measurement of surface
  differential rotation yet obtained using the Doppler imaging method
  and is over twice the value of previously observed early G-dwarfs.

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Title: A Sun in the Spectroscopic Binary IM Pegasi, the Guide Star
    for the Gravity Probe B Mission
Authors: Marsden, S. C.; Berdyugina, S. V.; Donati, J. -F.; Eaton,
   J. A.; Williamson, M. H.; Ilyin, I.; Fischer, D. A.; Muñoz, M.;
   Isaacson, H.; Ratner, M. I.; Semel, M.; Petit, P.; Carter, B. D.
2005ApJ...634L.173M    Altcode:
  We present the first detection of the secondary of the spectroscopic
  binary system IM Pegasi (HR 8703), the guide star for the NASA-Stanford
  relativity gyroscope mission Gravity Probe B. In support of this
  mission, high-resolution echelle spectra of IM Peg have been obtained
  on an almost nightly basis. Applying the technique of least-squares
  deconvolution, we achieve very high signal-to-noise ratio line profiles
  and detect the orbit of the secondary of the system. Combining almost
  700 new radial velocity measurements of both the primary and secondary
  of the system with previous measurements, we derive improved orbital
  parameters of the IM Peg system. Using these estimates along with
  the previously determined range of orbital inclination angles for
  the system, we find that the primary of IM Peg is a giant of mass
  1.8+/-0.2 M<SUB>solar</SUB>, while the secondary is a dwarf of mass
  1.0+/-0.1 M<SUB>solar</SUB>.

---------------------------------------------------------
Title: Full Stokes Spectropolarimetry of Hα in Prominences
Authors: López Ariste, A.; Casini, R.; Paletou, F.; Tomczyk, S.;
   Lites, B. W.; Semel, M.; Landi Degl'Innocenti, E.; Trujillo Bueno,
   J.; Balasubramaniam, K. S.
2005ApJ...621L.145L    Altcode:
  We report on spectropolarimetric observations of Hα in prominences
  made with the Télescope Héliographique pour l'Etude du Magnétisme et
  des Instabilités Solaires and the High Altitude Observatory/Advanced
  Stokes Polarimeter. Stokes Q and U show the expected profile shape from
  resonance scattering polarization and the Hanle effect. In contrast,
  most of the time, Stokes V does not show the antisymmetric profile shape
  typical of the Zeeman effect but a profile that indicates the presence
  of strong atomic orientation in the hydrogen levels, to an extent that
  cannot be explained by invoking the alignment-to-orientation transfer
  mechanism induced by the prominence magnetic field. We found that the
  largest signal amplitudes of Stokes V (comparable to that of Stokes
  Q and U) could be produced by a process of selective absorption of
  circularly polarized radiation from the photosphere, which requires
  that the prominence be in the vicinity of an active region. Although
  recent observations of active region filaments indicate such a
  selective absorption mechanism as a plausible explanation of the
  anomalous signals observed, the particular set of conditions that
  must be met suggest that a different explanation may be required
  to explain the almost ubiquitous symmetric V signal observed in Hα
  prominences. Therefore, we speculate that an alternative mechanism
  inducing strong atomic orientation at the observed level could be due
  to the presence of electric fields inducing an electric Hanle effect
  on Hα. Although we are still working toward a careful modeling of
  this effect, including both electric and magnetic fields, we present
  some preliminary considerations that seem to support this possibility.

---------------------------------------------------------
Title: τ Boo b: Not so bright, but just as heavy
Authors: Collier-Cameron, Andrew; Horne, Keith; James, David; Penny,
   Alan; Semel, Meir
2004IAUS..202...75C    Altcode:
  No abstract at ADS

---------------------------------------------------------
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: Spectropolarimetry and polarization-dependent fringes
Authors: Semel, M.
2003A&A...401....1S    Altcode:
  An account is given of the formation of polarization-dependent
  fringes, the “enemies" of spectropolarimetry, present practically
  in all polarization devices. Typically, multiple reflections
  give rise to secondary beams coherent with the main beam, but with
  wavelength-dependent phase differences. Polarized fringes may appear in
  any particular Stokes parameter or their combinations, and may (or may
  not) be seen in the intensity. The mechanism of formation of fringes in
  polarization is demonstrated. Fringes of analyzers, beam-splitter and
  retarders are evaluated. Flat-fielding techniques are discussed and
  ways for the removal of polarized fringes are suggested. Techniques
  that are efficient in removing fringes in intensity may be inadequate
  or insufficient for polarization. Anti-reflection coating, for
  instance, may reduce fringes, but not below 10<SUP>-3</SUP> of
  the intensity. This level would often be acceptable for intensity
  measurements but not necessarily for polarization, where one may often
  need to evaluate signals down to the level of 10<SUP>-5</SUP>, as in
  the observation of resonance line polarization and the second solar
  spectrum. Flat-fielding, by observing disk center, may be satisfactory
  to remove fringes for solar observations but this is not appropriate
  for stellar observations. The understanding of how these fringes are
  formed is essential both for the design of polarimeters and for their
  proper use. <P />Appendix C is only available in electronic form at
  http://www.edpsciences.org

---------------------------------------------------------
Title: Polarimeter for the study of magnetic fields in prominences
Authors: Lopez Ariste, Arturo; Tomczyk, Steven; Semel, Meir;
   Darnell, A.
2003SPIE.4853..235L    Altcode:
  We report on a new spectropolarimeter for the measurement of vector
  magnetic fields on prominences using the HeD<SUB>3</SUB> line in the
  Evans Solar Facility at the Sacramento Peak Observatory.

---------------------------------------------------------
Title: The magnetic field and wind confinement of θ<SUP>1</SUP>
    Orionis C
Authors: Donati, J. -F.; Babel, J.; Harries, T. J.; Howarth, I. D.;
   Petit, P.; Semel, M.
2002MNRAS.333...55D    Altcode:
  We report the detection, through spectropolarimetric observations,
  of a strong dipolar magnetic field of presumably fossil origin at
  the surface of the very young O star θ<SUP>1</SUP> Ori C. The Stokes
  V signatures we detect are variable with time, the variations being
  consistent with rotational modulation. A detailed modelling of our
  observations indicates that this dipole field has an intensity of
  1.1+/-0.1kG and is inclined at 42°+/-6° with respect to the rotation
  axis (assumed to be inclined at 45° to the line of sight). We find,
  in particular, that the positive magnetic pole comes closest to the
  observer when the variable Hα emission component observed on this star
  reaches maximum strength. This discovery represents the first definite
  detection of a magnetic field in an O star, as well as the first
  detection of a fossil field in a very young star. We also investigate
  in this paper the magnetic confinement of the radiatively driven wind
  of θ<SUP>1</SUP> Ori C in the context of the magnetically confined
  wind-shock model of Babel &amp; Montmerle. In the case of θ<SUP>1</SUP>
  Ori C, this model predicts the formation of a large magnetosphere
  (extending as far as 2-3R*), consisting of a very hot post-shock
  region (with temperatures in excess of 10MK and densities of about
  10<SUP>10</SUP>-10<SUP>11</SUP>cm<SUP>-3</SUP>) generated by the strong
  collision of the wind streams from both stellar magnetic hemispheres, as
  well as a dense cooling disc forming in the magnetospheric equator. We
  find that this model includes most of the physics required to obtain a
  satisfactory level of agreement with the extensive data sets available
  for θ<SUP>1</SUP> Ori C in the literature (and, in particular, with
  the recent X-ray data and the phase-resolved spectroscopic observations
  of ultraviolet and optical wind lines) provided that the mass-loss
  rate of θ<SUP>1</SUP> Ori C is at least 5 times smaller than that
  predicted by radiatively driven wind models. We finally show how new
  observations with the XMM or Chandra spacecraft could help us constrain
  this model much more tightly and thus obtain a clear picture of how
  magnetic fields can influence the winds of hot stars.

---------------------------------------------------------
Title: Stellar differential rotation from direct star-spot tracking
Authors: Collier Cameron, A.; Donati, J. -F.; Semel, M.
2002MNRAS.330..699C    Altcode: 2001astro.ph.11050C
  On the Sun, the rotation periods of individual sunspots not only trace
  the latitude-dependence of the surface rotation rate, but also provide
  clues as to the amount of subsurface fluid shear. In this paper we
  present the first measurements of stellar differential rotation made by
  tracking the rotation of individual star-spots with sizes comparable
  to the largest sunspots. To achieve this we re-analyse four sequences
  of densely sampled, high signal-to-noise ratio echelle spectra of AB
  Doradus spanning several stellar rotations in 1996 December. Using
  spectral subtraction, least-squares deconvolution and matched-filter
  analysis, we demonstrate that it is possible to measure directly
  the velocity amplitudes and rotation periods of large numbers of
  individual star-spots at low to intermediate latitude. We derive values
  for the equatorial rotation rate and the magnitude of the surface
  differential rotation, both of which are in excellent agreement with
  those obtained by Donati &amp; Collier Cameron from cross-correlation
  of Doppler images derived a year earlier in 1995 December, and with a
  re-analysis of the 1996 data by the χ<SUP>2</SUP> landscape method. The
  differences between the rotation rates of individual spots and the
  fitted differential rotation law are substantially greater than the
  observational errors. The smaller spots show a greater scatter about
  the mean relation than the larger ones, which suggests that buffeting
  by turbulent supergranular flows could be responsible.

---------------------------------------------------------
Title: Full-Stokes spectropolarimetry of solar prominences
Authors: Paletou, F.; López Ariste, A.; Bommier, V.; Semel, M.
2001A&A...375L..39P    Altcode:
  We report on first spectropolarimetric observations of solar prominences
  made at THEMIS. Hereafter we shall emphasis on the preliminary analysis
  of He I D<SUB>3</SUB> full-Stokes observations. Our measurements
  clearly show manifestations of both the Hanle and Zeeman effects. More
  generally, we demonstrate the very favourable performances of the THEMIS
  multi-line spectropolarimetric mode for further new investigations of
  the magnetic field in solar prominences. Based on observations made at
  THEMIS, operated on the Island of Tenerife by CNRS-CNR in the Spanish
  Observatorio del Teide of the Instituto de Astrofísica de Canarias.

---------------------------------------------------------
Title: Current Ground-Based Stellar Spectropolarimeters and their
    Use for Zeeman-Doppler Imaging
Authors: Semel, M.; López Ariste, A.
2001ASPC..248..575S    Altcode: 2001mfah.conf..575S
  No abstract at ADS

---------------------------------------------------------
Title: Scattering Polarization Observations with the Tenerife Gregory
    Coudé Telescope
Authors: Dittmann, O.; Trujillo Bueno, J.; Semel, M.; López Ariste, A.
2001ASPC..236..125D    Altcode: 2001aspt.conf..125D
  No abstract at ADS

---------------------------------------------------------
Title: tau Boo b: Not so bright, but just as heavy
Authors: Collier Cameron, Andrew; Horne, Keith; James, David; Penny,
   Alan; Semel, Meir
2000astro.ph.12186C    Altcode:
  We present new results derived from high-resolution optical spectra
  of the tau Boo system, secured in March-May 2000. The results do
  not show the same feature reported by Cameron et al (1999) as a
  candidate reflected-light signature from the planet. Together with
  earlier results from the 1998 and 1999 seasons, the new data yield
  a 99.9 percent upper limit on the opposition planet/star flux ratio
  epsilon&lt;3.5E-5 between 387.4 and 586.3 nm, a factor 3 deeper than the
  upper limit of Charbonneau et al (1999). For an assumed planet radius
  Rp= 1.2 RJup, the upper limit on the mean geometric albedo is p &lt;
  0.22, 40 percent that of Jupiter. We find new evidence that the star's
  rotation is synchronised with the planet's orbital motion. Using a
  Monte Carlo analysis we infer that the planet's mass must lie in the
  range 5.5 to 10 times the mass of Jupiter.

---------------------------------------------------------
Title: Fibre Feeding the UES
Authors: Walton, N. A.; Collier-Cameron, A.; Semel, M.
2000INGN....3...29W    Altcode:
  This report summarises the recent successful deployment of a fibre
  feed to the Utrecht Echelle Spectrograph (UES). The main advantage
  of this feed, as opposed to the conventional Nasmyth direct feed, is
  that the starlight is spread along the slit, redu cing sensitivity to
  flat field errors, and allowing increased integration times on bright
  objects. The prototype setup, which uses a 33-m fibre and is optimised
  for wavelengths around 5500 Å, gives throughput at least as good as
  conventional slit spectrosc opy with the more efficient of the UES
  de-rotators between 5000 and 6000 Å.

---------------------------------------------------------
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: First results from THEMIS spectropolarimetric mode
Authors: López Ariste, A.; Rayrole, J.; Semel, M.
2000A&AS..142..137L    Altcode:
  We present here the very first spectropolarimetric results obtained with
  the multiline spectroscopy mode (MTR) of THEMIS. The principal problems
  found during the data analysis are exposed. A first characterisation
  of the quality of this observing mode is given. Based on observations
  made with THEMIS operated on the island of Tenerife by CNRS-CNR in
  the Spanish Observatorio del Teide of the Instituto de Astrofísica
  de Canarias.

---------------------------------------------------------
Title: DIAGONAL: A numerical solution of the Stokes transfer equation
Authors: López Ariste, A.; Semel, M.
1999A&AS..139..417L    Altcode:
  In a previous paper \cite[(Semel &amp; López Ariste 1999]{art1},
  hereafter referred to as Paper I) the authors found a new solution for
  the integration of the radiative transfer equation for polarized light
  (RTE). The present paper is a continuation of that work, and shows how
  this solution has been incorporated into a new numerical code for the
  integration of the RTE. This code, called DIAGONAL, is characterized
  by an analytical handling of most model atmospheres, differing in that
  from other codes. It gives exact solutions in new, non-trivial cases. In
  the general case when no complete analytical solution is available,
  the code resorts to a multi-layer technique, thus providing a solution
  that combines an approximated analytical model and a minimized residual
  treated linearly. The algorithm is shown both to be very stable and
  to reach a high precision even for a small number of integration
  layers. This results in a very fast code. A comparison with DELO has
  been carried out. DIAGONAL is available, upon request to the authors.

---------------------------------------------------------
Title: Analytical solution of the radiative transfer equation for
    polarized light
Authors: López Ariste, A.; Semel, M.
1999A&A...350.1089L    Altcode: 1999astro.ph..9232A; 1999astro.ph..9232L
  A new formalism is introduced for the transfer of polarized
  radiation. Stokes parameters are shown to be four-vectors in a
  Minkowski-like space and, most strikingly, the radiative transfer
  equation (RTE) turns out to be an infinitesimal transformation under
  the Poincaré (plus dilatations) group. A solution to the transfer
  equation as a finite element of this group is proposed.

---------------------------------------------------------
Title: Integration of the radiative transfer equation for polarized
light: the exponential solution
Authors: Semel, M.; López Ariste, A.
1999A&A...342..201S    Altcode: 1998astro.ph.11153S
  The radiative transfer equation (RTE) for polarized light accepts a
  convenient exponential solution when the absorption matrix commutes
  with its integral. We characterize some of the matrix depth variations
  which are compatible with the commutation condition. Eventually
  the vector solution may be diagonalized and one may obtain four
  independent scalar solutions with four optical depths, complex in
  general. When the commutation condition is not satisfied, one must
  resort to a determination of an appropriate evolution operator, which
  is shown to be well determined mathematically, but whose explicit
  form is, in general, not easy to apply in a numerical code. However,
  we propose here an approach to solve a general case not satisfying
  the commutation condition.

---------------------------------------------------------
Title: Exponential solutions to the radiative transfer equation for
    polarized light
Authors: López Ariste, A.; Semel, M.
1999ASSL..243..157L    Altcode: 1999sopo.conf..157L
  No abstract at ADS

---------------------------------------------------------
Title: Magnetic topology and prominence patterns on AB Doradus
Authors: Donati, J. -F.; Collier Cameron, A.; Hussain, G. A. J.;
   Semel, M.
1999MNRAS.302..437D    Altcode:
  We report new Zeeman--Doppler imaging observations of the rapidly
  rotating young K0 dwarf AB Doradus, obtained with the Anglo-Australian
  Telescope in 1996 December. From such observations, simultaneous
  brightness and magnetic images of the stellar photosphere of AB Dor
  were reconstructed at three different epochs. The magnetic topology
  of AB Dor is found to be very complex, with at least 12 different
  radial field regions of opposite polarities located all around the
  star. Significant azimuthal field fluxes are also detected in the
  form of one negative polarity region close to the equator, a series
  of positive polarity patches at intermediate latitudes and an almost
  complete ring of negative polarity encircling the rotational pole at
  high latitudes. In particular, the azimuthal polarities we reconstruct
  are in very good agreement with those obtained by Donati &amp;
  Cameron, confirming that this field component is directly related
  to the dynamo-generated large-scale toroidal magnetic structure. The
  triple polarity latitudinal pattern observed for this structure in the
  upper hemisphere of AB Dor indicates that the degree of the underlying
  large-scale poloidal structure in an axisymmetric spherical harmonics
  expansion is equal to or greater than five. It also strengthens the
  idea that the dynamo processes operating in AB Dor feature a non-solar
  component distributed throughout the convective zone. From the subtle
  distortion of successive brightness images, we can also confirm the
  surface differential rotation first measured on this star by Donati
  &amp; Cameron in both sense and magnitude, with a pole rotating more
  slowly than the equator by about one part in 220. Finally, the rotation
  periods we measure for four prominences (from the recurrence rate of
  their spectral signatures in Balmer lines) confirm the presumption
  that such clouds are anchored at intermediate to high latitudes. The
  intrinsic variability of these prominences is not associated with any
  abrupt changes in the photospheric brightness or magnetic distributions,
  implying that they essentially result from the reorganization of
  coronal field lines.

---------------------------------------------------------
Title: An ambiguity-free determination of J_Z in solar active regions
Authors: Semel, M.; Skumanich, A.
1998A&A...331..383S    Altcode:
  We propose a way to derive a radial electric current density from
  vector magnetograms that is free of any particular choice of the two
  possible azimuths for the plane of the sky field. The method is applied
  to active region data obtained with the Advanced Stokes Polarimeter
  (ASP). A comparison of this current density with that derived for
  two particular sets of possible azimuth assignments allows one to
  evaluate the appropriateness of each set. In addition one may verify
  the detection of currents in general and the occurrence of current
  sheets, i.e. field discontinuities, in particular.

---------------------------------------------------------
Title: Differential Rotation and Magnetic Polarity Patterns on
    AB Doradus
Authors: Donati, J. -F.; Collier Cameron, A.; Hussain, G. A. J.;
   Semel, M.
1998ASPC..154.1966D    Altcode: 1998csss...10.1966D
  We report new Zeeman-Doppler imaging observations of the rapidly
  rotating young K0 dwarf AB Doradus, obtained with the Anglo-Australian
  Telescope in 1996 December. We show simultaneous brightness and
  magnetic images of the stellar photosphere, reconstructed at
  three different epochs over the course of a seven-night observing
  run. Latitude-by-latitude cross-correlation of the resulting images
  confirms the form and amplitude of the surface differential pattern
  found in the previous year's observations by Donati &amp; Cameron
  (1997), with the pole rotating slower than the equator by about one
  part in 220. We also present dynamic spectra showing the distribution
  of H alpha-absorbing clouds in the stellar corona at the same epoch.

---------------------------------------------------------
Title: Boundary Conditions and the Extrapolation of Magnetic Fields
    (Invited Review)
Authors: Semel, M.
1998ASPC..155..423S    Altcode: 1998sasp.conf..423S
  No abstract at ADS

---------------------------------------------------------
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: Differential rotation and magnetic polarity patterns on AB Dor.
Authors: Collier Cameron, A.; Donati, J. -F.; Semel, M.
1997AAONw..83....8C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On The Detection of Currents and Parker's Objection
Authors: Skumanich, A.; Semel, M.
1997SPD....28.1605S    Altcode: 1997BAAS...29..920S
  We propose a way to derive a radial electric current density
  (hereafter current) from vector magnetograms that is free of any
  particular choice of the two possible azimuths for the plane of
  the sky (POS) field. The method is applied to active region data
  obtained with the Advanced Stokes Polarimeter. A comparison of this
  current density with that derived for two particular sets of possible
  azimuth assignments allows one to evaluate the appropriateness of
  each set and to detect the occurrence of current sheets, i.e. field
  discontinuities. A minimum current constructed from ambiguity-free
  POS and line-of-sight currents allows one to verify the detection of
  currents in general. Parker's scenario leads to a current distribution
  that is statistically distributed about zero, looks like noise and is
  also discontinuous. We observe such noise as well as continuous and
  significant currents. We conclude that the unresolved magnetic field
  is not necessarily always in the form of individual flux tubes.

---------------------------------------------------------
Title: Azimuthal structures in the wind and chromosphere of the
    Herbig AE star AB Aurigae. Results from the MUSICOS 1992 campaign.
Authors: Boehm, T.; Catala, C.; Donati, J. -F.; Welty, A.; Baudrand,
   J.; Butler, C. J.; Carter, B.; Collier-Cameron, A.; Czarny, J.; Foing,
   B.; Ghosh, K.; Hao, J.; Houdebine, E.; Huang, L.; Jiang, S.; Neff,
   J. E.; Rees, D.; Semel, M.; Simon, T.; Talavera, A.; Zhai, D.; Zhao, F.
1996A&AS..120..431B    Altcode:
  The observations of the MUSICOS 1992 campaign concerned three scientific
  programs, among which was the study of azimuthal structures in the wind
  and chromosphere of the pre-main sequence Herbig Ae star AB Aur. The He
  I 5876A line of AB Aur, which is formed in the expanding chromosphere
  of this star, most probably in the innermost parts of its wind, was
  continuously monitored at a spectral resolution of 30000 for about 4
  days. The line was discovered to be variable in a spectacular way, the
  profile changing from pure emission to a composite profile including a
  deep absorption component in the course of a few hours. In this paper we
  present strong clues that suggest a rotational modulation of the line,
  with a period corresponding to the stars rotation period of 32hrs. We
  confirmed the covariation of several non-photospheric spectral lines,
  formed at very different radial distances of the star, which strengthens
  the idea that the variability is the signature of azimuthal structures
  in the wind of AB Aur, most probably due to magnetically confined
  streams or loops emanating from the stellar surface. We present the
  data collected during the MUSICOS 1992 campaign, but also a data set of
  the He I D3 line obtained with FOE at KPNO during the years 1991-1994
  and a series of IUE spectra containing the Mg II h &amp; k UV lines
  obtained one month prior to the campaign. A thorough discussion of
  the possible interpretations of the spectacular variations of the He
  I 5876A line is presented.

---------------------------------------------------------
Title: Zeeman Doppler imaging of stars with the AAT
Authors: Carter, B.; Brown, S.; Donati, J. -F.; Rees, D.; Semel, M.
1996PASA...13..150C    Altcode:
  Zeeman Doppler Imaging (ZDI) is a recent technique for measuring
  magnetic fields on rapidly rotating, active stars. ZDI employs
  spectropolarimetry taken at different rotational phases, to derive
  information on the magnetic field distribution over the stellar
  surface. The Zeeman effect is used to identify the presence of a
  magnetic field, and variations in Doppler wavelength shifts across
  the rapidly rotating star allow fields to be resolved on different
  parts of the visible disc. Analysis of the spectra can be used
  to produce both thermal and surface magnetic images. ZDI requires
  very high S/N spectra to be acquired, within a time interval short
  compared to the stellar rotation period. As a result, a large aperture
  telescope is needed. Since an initial successful test in 1989, the
  3.9m Anglo-Australian Telescope has been used to obtain ZDI spectra
  of active stars of different evolutionary stages. The observations
  have concentrated on the K subgiant in the RS CVn system HR 1099,
  to monitor changes on this bright and active star. With the advent
  in 1991 of ZDI spectropolarimetry with the AAT 'echelle spectrograph,
  it has become possible to co-add the polarisation signature from the
  many magnetically sensitive lines recorded simultaneously. As a result,
  stellar magnetic field detections of unprecedented quality have been
  obtained. The aims of this paper are to briefly outline the principles
  of ZDI, describe the instrumental setup at the AAT and present some
  preliminary results from recent observations.

---------------------------------------------------------
Title: Field Azimuth Disambiguation Using Ambiguity-Free Currents
Authors: Skumanich, A.; Semel, M.
1996SoPh..164..291S    Altcode:
  Using the ambiguity-free vertical current defined by Semel and Skumanich
  (1995) we derive a minimum-current azimuth disambiguation for the
  observed magnetic field in the active region NOAA 7201. A comparison of
  such a minimum-current azimuth resolution with those from other extant
  methods indicates that the resulting resolution, even though found
  to be limited by noise, is a useful first approximation. A comparison
  of our minimum current distribution with the currents we derive from
  an extant disambiguation (Lites et al., 1995) indicates the presence
  of current discontinuities in the form of linear features near the
  magnetic neutral line of the associated δ-spot.

---------------------------------------------------------
Title: Zeeman-Doppler Imaging of Solar-Type Stars: Multi Line
    Technique
Authors: Semel, M.; Li, J.
1996SoPh..164..417S    Altcode:
  In this work, a multi-line spectropolarimetric detection using an
  Echelle spectrograph is described. The polarization of Zeeman effect
  is detected by the use of more than 200 lines observed in the solar
  type star, HR1099. Using the statistics analysis in a sample of 200
  lines, we found on the average a polarization signal of about 3 ×
  10<SUP>−4</SUP>.

---------------------------------------------------------
Title: The magnetohydrodynamic equilibrium of coronal helmet
    streamers.
Authors: Cuperman, S.; Bruma, C.; Dryer, M.; Semel, M.
1995A&A...299..389C    Altcode:
  The equilibrium of coronal helmet streamers is investigated within
  the framework of an axisymmetric magnetohydrodynamic polytropic
  model. The main features of this work are: (i) The eight MHD vector
  equations are reduced - via analytical manipulations - to three scalar
  equations involving the magnetic flux function, the current density
  and temperature; (ii) The reduced system of equations is solved by
  the aid of a computational algorithm including both relaxation and
  iterative techniques, and is continued until the maximum local relative
  change with time becomes smaller than 10^-4^; (iii) At the Sun, only
  three physical quantities are fixed, namely, the radial component of
  the magnetic field, the temperature and the particle density. The
  other quantities - the θ-magnetic field, radial and θ-components
  of the streaming velocity - are determined in a self-consistent way;
  (iv) The coronal helmet streamer structure is obtained by: specifying
  the (observed) latitude extension on the sun of the region of closed
  magnetic field lines, requiring the field lines in the meridian plane
  to become purely radial at 10 solar radii, and imposing the continuity
  condition on the field line separating the region of closed field
  lines from that of open field lines. The algorithm used in this work
  enables the separate analysis of the following problems of interest:
  (i) the linear problem - plasma acceleration in a prescribed helmet
  streamer-like configuration; (ii) The quasi-linear problem - the
  feedback effect of the plasma (solar wind) flow on the vacuum magnetic
  configuration; and (iii) the non-linear problem - the consistent
  equilibrium state of the magnetic field and plasma flow in coronal
  helmet streamers. The quantitative results obtained in these studies
  are presented, compared and discussed.

---------------------------------------------------------
Title: Zeeman Doppler Imaging
Authors: Semel, M.
1995ASPC...71..340S    Altcode: 1995tosm.conf..340S; 1995IAUCo.149..340S
  No abstract at ADS

---------------------------------------------------------
Title: Azimuthal Structures in the Wind and Chromosphere of the
    Herbig Ae-Star Ab-Aurigae - Preliminary Results from the MUSICOS-1992
    Campaign
Authors: Catala, C.; Bohm, T.; Donati, J. F.; Simon, T.; Welty, A.;
   Houdebine, E.; Huang, L.; Jiang, S.; Zhai, D.; Neff, J.; Foing, B. H.;
   Ghosh, K.; Butler, J.; Collier-Cameron, A.; Baudrand, J.; Czarny, J.;
   Zhao, F.; Talavera, A.; Kennelly, T.; Carter, B.; Rees, D.; Semel,
   M.; Cutispoto, G.; Rodono, M.
1994SoPh..155..185C    Altcode:
  MUSICOS (for MUlti-SIte COntinuous Spectroscopy) is an international
  project to facilitate and organize world-wide multi-site campaigns
  in high resolution spectroscopy, in view of obtaining a complete time
  coverage of various types of variable stellar phenomena.

---------------------------------------------------------
Title: THEMIS polarimetry
Authors: Semel, M.
1994ASIC..433..509S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Polarization analyzer installed at the Pic du Midi Observatory.
Authors: Mouradian, Z.; Scholiers, W.; Semel, M.
1994imfm.conf..110M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On the removal of the 180 deg sign ambiguity in vector
magnetograph measurements: the divergence-free method (divergence
    of B identical to 0)
Authors: Li, J.; Cuperman, S.; Semel, M.
1993A&A...279..214L    Altcode:
  A systematic numerical study concerning the applicability of a
  new criterion for the removal of the 180 deg ambiguity in sign of
  vector magnetograph measurements (Wu &amp; Ai 1990) is presented. This
  criterion based on the condition divergence of B identical to 0 alone,
  is free of any assumption concerning the divergence (convergence) of the
  magnetic field with the height. Some twenty analytical (force-free field
  and potential field) models covering a variety of possible magnetic
  configurations are used as test cases for our study. The limitations
  of the method are also indicated.

---------------------------------------------------------
Title: Circular polarization and variability in the spectra of
    herbig Ae/Be stars. I. The Fe II 5018 A and He I 5876 A lines of AV
    OnB Aurigae.
Authors: Catala, C.; Bohm, T.; Donati, J. -F.; Semel, M.
1993A&A...278..187C    Altcode:
  We present high resolution spectroscopic and spectro-polarimetric
  observations of the Fe II 5018 and He I 5876 A lines in the pre-main
  sequence Herbig Ae star AB Aur. No signal was detected in the circular
  polarization (Stokes V) profile of the magnetically sensitive Fe
  II 5018 A line, yielding upper limits of the order of 1 kG for the
  photospheric magnetic field. This upper limit is still much higher than
  the equipartition magnetic field at the photosphere, estimated to be 60
  G. Both lines are spectacularly variable on a night-to-night basis. In
  particular, the Fe II 5018 A line appears as a photospheric absorption
  line in one of our spectra, but as a variable emission line, usually
  composed of a flat-topped broad emission and a roughly triangular
  emission, in the other spectra. The He I 5876 A line often includes a
  red absorption component and a blue emission component, but also often
  appears entirely in emission with a varible asymmetry. We estimate the
  regions of formation of the two lines, with a very simplified treatment
  of ionization and excitation processes, and within the framework
  of existing models of the wind and chromosphere of AB Aur. We find
  that the Fe II 5018 A line is formed in the bulk of the expanding
  chromosphere, while the He I 5876 A line is formed at the very base
  of the wind/chromosphere complex. Although the data presented here
  are not sufficient to fully interpret such a complex variability,
  we suggest the idea that it could be linked to the co-rotation of
  structures in the wind and chromosphere of AB Aur. The photospheric
  appearance of the Fe II 5018 A line in one of our spectra may be
  due to a temporary change in the chromospheric physical conditions,
  like an increase of the temperature, followed by a return to 'normal'
  conditions, with a time scale shorter than one day.

---------------------------------------------------------
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: Identification and elimination of the residual ambiguity in
    the sign of observed photospheric magnetic fields
Authors: Cuperman, S.; Li, J.; Semel, M.
1993A&A...278..279C    Altcode:
  The existing methods for the removal of the 180 degree ambiguity in
  the sign of the observed transverse photospheric magnetic field present
  some inherent-computational and/or intrinsic limitations. In this work
  we propose and demonstrate suitable procedures for the identification
  and elimination of the residual 180 degree ambiguity in the sign of
  B<SUB>perpendicular</SUB> observed fields, that is of the ambiguity
  which is not removed by the various methods in use. These procedures
  are tested on the case of force-free magnetic field configurations
  for which exact analytical solutions to compare with exist.

---------------------------------------------------------
Title: Reconstruction of coronal magnetic configurations - The case
    of strongly nonlinear force-free fields
Authors: Cuperman, S.; Bruma, C.; Zoler, D.; Semel, M.
1993A&A...270..480C    Altcode:
  A method for the reconstruction of force-free field (FFF) coronal
  magnetic configurations above active regions is proposed. It is based on
  the mapping of the entire space surrounding the sun onto a rectangular
  box of dimensions (1,2,27 pi), inside which the FFF Maxwell's equations
  are solved. The boundary conditions used are the field components
  observed at the photosphere, their vanishing at infinity, and finiteness
  of the magnetic potential components along the poles. The solution is
  based on relaxation techniques and is illustrated for the 2 1/2D case,
  that is, the case in which all three field components B(r), B(theta),
  and B(phi) are finite and depend only on the polar coordinates r and
  theta. The equilibrium state is considered to be reached when the mean
  relative change in any of the field components, from one time step to
  another, is smaller than a prescribed infinitesimal number.

---------------------------------------------------------
Title: Alternative method for the removal of the 180 ambiguity in
    the sign of the observed transverse photospheric magnetic field
Authors: Cuperman, S.; Li, J.; Semel, M.
1993A&A...268..749C    Altcode:
  A relatively simple method for the removal of the 180 deg ambiguity
  in the azimuth of the observed transverse photospheric magnetic
  field is proposed; it applies to both center of disk and off-disk
  center observations. The method consists in selecting that sign which
  corresponds to a decrease with height of the heliographic magnetic
  field. For this, the observed field components at a point have
  to be first transformed into heliographic components and then the
  selection rule must be applied. After application of the method to
  simulated observations, heliographic analytical solutions were used
  as a benchmark test. Both center-of-disk and off-disk-center cases
  were considered; in the last case, all latitude angles between 0 deg
  and 90 deg were treated. The results were expressed as percentage of
  success in determining the correct sign of the observed transverse
  component as a function of the complexity of the heliographic magnetic
  field configuration as well as of the value of the latitude angle. In
  the center of disk case, the percentage of success varies between
  100 percent and a minimum of about 90 percent, as depending on the
  complexity of the magnetic configuration; the performance of the method
  can be further improved by increasing the number of grid points. In
  the off disk-center case, the method is limited to relatively low
  latitude angles.

---------------------------------------------------------
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: Magnetic shear-based removal of the 180-deg ambiguity in
    the observed transverse photospheric magnetic field - Feasibility
    and limitations
Authors: Cuperman, S.; Li, J.; Semel, M.
1992A&A...265..296C    Altcode:
  Results of a systematic investigation of the reliability of the magnetic
  shear-based method for the removal of the 180-deg ambiguity in the
  observed transverse photospheric magnetic field are presented. The
  method consists of the evaluation of the azimuth angle between the
  observed transverse field component and that of a potential field,
  calculated by using the line-of-sight component as the boundary
  condition for the field normal to the image plane. A variety of the
  force-free field (FFF) magnetic configurations is considered. This
  includes the cases of relatively weakly and strongly space-dependent
  fields, relatively small and large maximal values of the FFF-function
  alpha(r), and same-sign and space-changing sign alpha(r) functions. The
  'logical' and 'global' success of the method in removing the 180-deg
  ambiguity for the 20 cases studied is presented and discussed.

---------------------------------------------------------
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: Determination of force-free magnetic fields above the
    photosphere using three-component boundary conditions. II - Analysis
    and minimization of scale-related growing modes and of computational
    induced singularities
Authors: Demoulin, P.; Cuperman, S.; Semel, M.
1992A&A...263..351D    Altcode:
  The problem of the extrapolation of the observed three-component
  photospheric magnetic fields within is studied within the framework
  of the nonlinear force-free field model. The possible vertically
  growing modes and their relationship to the horizontal and vertical
  chosen scales is discussed, and the mathematical singularities arising
  along neutral field lines and leading to nondefinite alpha values are
  examined. A search is made for simple procedures aimed at minimizing
  these algorithm-dependent computational errors.

---------------------------------------------------------
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: Removal of singularities in the Cauchy problem for the
    extrapolation of solar force-free magnetic fields
Authors: Cuperman, S.; Demoulin, P.; Semel, M.
1991A&A...245..285C    Altcode:
  The singularities occurring in the Cauchy problem for the
  extrapolation of solar nonlinear force-free magnetic fields at
  positions of vanishing normal component, B(z), are removed. This is
  based on the observation that the constancy of the quantity alpha(r),
  characterizing the force-free magnetic fields, along a given magnetic
  field line, implies that the singularity in Maxwell's equation is of
  mathematical rather than of physical origin. Thus, requiring also the
  vanishing of the numerator at P(0) leads to an undetermined form for
  alpha. By using Taylor's expansions in two variables (x and y) about
  P(0) for both numerator and denominator, the actual value for alpha,
  namely alpha (P(0), is obtained. The procedure is tested on the case
  of the analytical model proposed by Low (1982).

---------------------------------------------------------
Title: Active regions, sunspots and their magnetic fields.
Authors: Semel, Meir; Mouradian, Zadig; Soru-Escaut, Irina; Maltby,
   Per; Rees, David; Makita, Mitsugu; Sakurai, Takashi
1991sia..book..844S    Altcode:
  Surface magnetism is the progenitor of active regions, sunspots, and
  all related phenomena. This cause and effect is reversible so that,
  using well-established empirical laws, the presence and morphology of
  photospheric magnetic fields can be deduced from active-region light
  emission structure. In the (simplifying) case of sunspots, MHD and
  thermodynamic theory find some success in the interpretation of the
  interaction of magnetic fields and solar plasma. Coronal magnetic fields
  also appear to be predictable by extrapolation techniques starting
  from the photospheric conditions. Alternatively, surface magnetism can
  be observed "directly" by means of the spectroscopic Zeeman effect
  and Stokes polarimetry. Eventually these empirical, theoretical and
  direct-measurement techniques must converge to identical results as
  we better understand the physics of active regions.

---------------------------------------------------------
Title: Towards Magnetic Images of Rapidly Rotating Late-Type Stars
Authors: Donati, J. -F.; Semel, M.
1991LNP...380..326D    Altcode: 1991sacs.coll..326D; 1991IAUCo.130..326D
  We present the first magnetic detections using Zeeman-Doppler Imaging
  (ZDI) in the two bright rapidly rotating RS CVn systems HR 1099 and
  2 CrB, and discuss their compatibility with various recent results on
  magnetic activity in cool stars.

---------------------------------------------------------
Title: The magnetic field of the AP star epsilon UMa.
Authors: Donati, J. -F.; Semel, M.; del Toro Iniestia, J. C.
1990A&A...233L..17D    Altcode:
  The magnetic field of the Ap star Epsilon UMa was measured using the
  Zeeman-Doppler Imaging method. Ultrahigh S/N polarized profiles of the
  4923 A magnetically sensitive Fe II line were recorded. Unambiguous
  Zeeman signatures were found at phases here S/N was greater than or
  equal to 1750.

---------------------------------------------------------
Title: Zeeman-Doppler Imaging - a New Option for Magnetic Field
    Study of Ap-Stars and Solar Type Stars
Authors: Donati, J. -F.; Semel, M.
1990SoPh..128..227D    Altcode: 1990IAUCo.121P.227D
  In the task of studying stellar magnetic fields, polarimetric methods
  have been intensively used in Ap stars. But the observational material
  classically used to reconstruct stellar magnetic structures (average
  longitudinal magnetic field as a function of rotational phase) is not
  rich enough in spatial information to derive geometries more complex
  than centered or decentered dipoles.

---------------------------------------------------------
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: Determination of force-free magnetic fields above the
    photosphere using three-component boundary conditions - Moderately
    non-linear case
Authors: Cuperman, S.; Ofman, L.; Semel, M.
1990A&A...230..193C    Altcode:
  The calculation of the magnetic field components and the tracing
  of the magnetic field lines above the photosphere are considered
  within the framework of the nonlinear force-free field model, upon
  using three-component magnetic fields as boundary conditions. This
  vertical integration represents an extrapolation in the small and is
  free of any implicit or explicit assumption in the large. As a study
  case, an analytical model providing magnetic field components at a
  surface (representing the photosphere) and above it is used. Magnetic
  field components and magnetic field lines at z greater than 0 are
  obtained. Their comparison with the analytical ones provides a measure
  of the calculational accuracy. The method is demonstrated for the case
  of moderately nonlinear force-free forces.

---------------------------------------------------------
Title: The absolute value and sign of the function alpha(r) in the
    force-free magnetic field modelling of photospheric observations
Authors: Cuperman, S.; Ofman, L.; Semel, M.
1990A&A...227..227C    Altcode:
  A relatively simple method for the determination of the quantity α =
  J/B characterizing the force-free magnetic fields based on photospheric
  vector magnetic fields is presented. Magnetic configurations
  for which the sign of α does not change are considered. <P
  />The method consists of two steps, namely: (a) Expression of
  force-free field equations in terms of the observed quantities
  B<SUB>z</SUB>, B<SUP>2</SUP><SUB>x</SUB>, B<SUP>2</SUP><SUB>y</SUB>
  and B<SUB>x</SUB>B<SUB>y</SUB> and determination of |α|; (b) Selection
  of the proper sign of α (corresponding to the magnetic configuration
  under consideration) by (i) solving the FFF equations under the
  assumptions α = - |α| and α = +|α|, (ii) using the two types of
  solutions to calculate the surface energy integral E<SUB>S</SUB>(z)
  = ∫ (B<SUP>2</SUP><SUB>x</SUB> + B<SUP>2</SUP><SUB>y</SUB>
  + B<SUP>2</SUP><SUB>x</SUB>)ds, and (iii) retaining the sign that
  leads to a decrease with height (z) of E<SUB>s</SUB>(z). <P />The
  proposed method is tested by considering three different FFF magnetic
  configurations for which analytical solutions exist.

---------------------------------------------------------
Title: Extrapolation of photospheric potential magnetic fields using
    oblique boundary values - A simplified approach
Authors: Cuperman, S.; Ofman, L.; Semel, M.
1990A&A...227..583C    Altcode:
  The problem of extrapolating photospheric potential magnetic fields
  is addressed using the oblique line-of-sight component B1(rs) as a
  boundary condition and reducing it to that for the normal line-of-sight
  component Bz(rs). The observed B1(rs) component is used along with
  the direction cosines of the line-of-sight alpha, beta, and gamma
  to calculate semianalytically the photospheric components Bx(rs)
  and By(rs). All these values are used to find the normal photospheric
  component Bx(rs). Finally, using the distribution Bz(rs) as boundary
  values, the relatively simpler problem corresponding to the case
  in which the normal components are known is solved. The method is
  tested on the case of an analytical model configuration for which
  exact solutions in the half-space above the photosphere exist.

---------------------------------------------------------
Title: Spectropolarimetry of solar faculae - High spatial resolution
    results
Authors: del Toro Iniesta, J. C.; Collados, M.; Sanchez Almeida, J.;
   Semel, M.
1990A&A...227..591D    Altcode:
  A new method to measure the magnetic field strength of small-scale
  solar magnetic concentrations is presented. It is based on the center
  of gravity method (Semel, 1967), is independent of radiative transfer
  calculations and only observable parameters are needed. This method
  also provides parameters like filling factor (area fraction occupied by
  the tubes), continuum intensity contrast between flux tubes and their
  surroundings, in a two-component model scheme. The method is applied
  to spectropolarimetric high spatial resolution data. Local variations
  of the above parameters inside single faculae are found. This result
  suggests some indications about flux tube evolution. A comparison with
  low spatial resolution results is also made.

---------------------------------------------------------
Title: Zeeman-Doppler imaging of active stars. II. Numerical
    simulation and first observational results.
Authors: Donati, J. -F.; Semel, M.; Praderie, F.
1989A&A...225..467D    Altcode:
  A numerical study is proposed for testing the sensitivity and resolving
  ability of the Zeeman-Doppler Imaging (DZI) of active stars. It is
  shown that, in a star with v sin i = 32 km/s, a line at 6000 A with
  a Lande factor of two can be used to detect a 1000 G single magnetic
  spot covering 10 percent of the visible stellar surface, provided
  S/N = 350 or greater per 80 mA pixel. It is also shown that this 1000
  G magnetic threshold is lowered by a factor of two in the case of a
  Doppler-resolved bipolar group, each spot of which covers 10 percent
  of the visible stellar hemisphere.

---------------------------------------------------------
Title: Zeeman-Doppler imaging of active stars. I - Basic principles.
Authors: Semel, M.
1989A&A...225..456S    Altcode:
  Methods are proposed for the analysis of polarimetric measurements in
  the spectra of active stars. The Doppler effect of rapidly rotating
  stars may help to disclose magnetic field distributions, which otherwise
  may not be seen due to even distribution of positive and negative
  fields. The methods developed allow one to average the measurements
  of many lines.

---------------------------------------------------------
Title: Determination of constant-alpha force-free magnetic fields
    above the photosphere using three-component boundary conditions
Authors: Cuperman, S.; Ofman, L.; Semel, M.
1989A&A...216..265C    Altcode:
  The constant-α, force-free magnetic field equations are
  numerically integrated for the case in which all three field
  components are specified at the photo sphere and used as boundary
  conditions. Test-cases successfully compare the numerical results with
  exact analytical values.

---------------------------------------------------------
Title: Zeeman-Doppler imaging method.
Authors: Donati, J. F.; Semel, M.; Praderie, F.
1989musi.work...37D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Méthode d'imagerie Zeeman-Doppler.
Authors: Donati, J. -F.; Semel, M.; Praderie, F.
1989JAF....34R...6D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Extrapolation functions for constant-alpha force-free fields -
    Green's method for the oblique boundary value
Authors: Semel, M.
1988A&A...198..293S    Altcode:
  The Green's method approach is used to generalize the existing
  solutions for the constant-alpha force-free fields to the oblique
  case, in which the observed longitudinal magnetic field component is
  not normal to the surface. Consideration is given to the existence
  and uniqueness of the solutions. If a bounded solution exists, it is
  uniquely determined in terms of one component of the magnetic field
  on the observed plane surface.

---------------------------------------------------------
Title: Polarimetry and imagery through uniaxial crystals - Application
    to solar observations with high spatial resolution
Authors: Semel, M.
1987A&A...178..257S    Altcode:
  Huygens's principle is applied to the general case of plane waves
  incident on uniaxial crystals with plane surface and the corresponding
  laws of refraction in analytical form are derived. Optical aberrations
  are calculated and their effects on solar observations with high
  spatial resolution are discussed. Solutions for beam-splitters free
  of aberrations are proposed.

---------------------------------------------------------
Title: Observations of the Magnetic Fine Structure of a Facula
Authors: del Toro Iniesta, J. C.; Semel, M.; Collados, M.
1987rfsm.conf..122D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Continuum intensity and magnetic flux of solar fluxtubes.
Authors: Del Toro Iniesta, J. C.; Semel, M.; Collados, M.; Sánchez
   Almeida, J.
1987PAICz..66..265D    Altcode: 1987eram....1..265D
  The continuum contrast between fluxtubes and their quiet background,
  and the magnetic flux carried by these magnetic elements, have been
  determined at different points of a solar facula, in the frame of a
  two-component model from spectropolarimetric observations of 1arcsec
  spatial resolution. Local spatial variations of these two parameters
  have been obtained.

---------------------------------------------------------
Title: Observations of the magnetic fine structure of a facula.
Authors: Del Toro Iniesta, J. C.; Semel, M.; Collados, M.
1987rfsm.conf..127D    Altcode:
  Simultaneous spectropolarimetric observations of a facula have
  been carried out in 10 spectral lines with a spatial resolution of
  1arcsec. Local variations of the magnetic field strength and the filling
  factor of fluxtubes were obtained. The analysis of the velocities inside
  fluxtubes shows that positive and negative Doppler shifts are present,
  at the same time, at different points of the facula.

---------------------------------------------------------
Title: Unresolved Magnetic Structures in the Sun: Oberservational
    Results from Analysis of Faculae and Network Spectra
Authors: Semel, M.
1986ssmf.conf...39S    Altcode:
  There are various methods to determine the field strength in the
  unresolved flux tube. They vary according to the choice of spectral
  lines, the weight given to the observables (intensity, profiles,
  polarization...), model assumptions and physical considerations. The
  author summarizes some methods.

---------------------------------------------------------
Title: Determination of Magnetic Fields in Unresolved Features
Authors: Semel, M.
1985LNP...233..178S    Altcode: 1985hrsp.proc..178S
  Observations of solar faculae and networks reveal the unresolved nature
  of the magnetic field configurations, with present achieved spatial
  resolutions. While the apparent field is weak (from a few gauss to
  several hundreds) the "true" field is likely to be much stronger
  (up to kilogauss fields). There are various methods to determine
  the field strength in the unresolved flux tube. They vary according
  to the choice of spectral lines, the weight given to the observables
  (intensity, profiles, polarisation ...), model assumptions and physical
  considerations. The purpose of this paper is to give a review of these
  methods and assessment of their capacities and limits.

---------------------------------------------------------
Title: Energy Release and Energy Transport Below the Transition Zone
    in Solar Flares
Authors: Hénoux, J. C.; Chambe, G.; Heristchi, D.; Semel, M.;
   Woodgate, B.; Shine, R.; Beckers, J.
1985spit.conf..758H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Measurement of Magnetic Fields - Sun and Stars
Authors: Semel, M.
1984apoa.conf...37S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Transfert de rayonnement en lumière polarisée. Application
    à la mesure des champs magnétiques par effet Zeeman.
Authors: Semel, M.
1984cms..conf...79S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Détermination des champs magnétiques dans les structures
    solaires non résolues.
Authors: Semel, M.
1984cms..conf..375S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Conductive Heat Flux in the Chromosphere Derived from Line
    Linear Polarization Observation
Authors: Henoux, J. -C.; Chambe, G.; Heristchi, D.; Semel, M.;
   Woodgate, B.; Shine, D.; Beckers, J.
1983SoPh...86..115H    Altcode:
  Linear polarization in two chromospheric lines (Hα and SI 1437 A) was
  observed in the gradual phase of solar flares. The polarized electric
  vector is directed towards disk center.

---------------------------------------------------------
Title: Impact linear polarization observed in a UV chromospheric
    line during a solar flare
Authors: Henoux, J. C.; Chambe, G.; Sahal, S.; Semel, M.; Woodgate,
   B.; Shine, D.; Beckers, J.; Machado, M.
1983ApJ...265.1066H    Altcode:
  Linear polarization was observed in the S 11437 A line in bright
  flaring points during the soft X-ray emission from a flare on 1980
  July 15. The degree of polarization is about 25% and is detected at a
  signal-to-noise ratio of 2.9. The polarized electric vector is directed
  toward disk center to within 3°. <P />This polarization could be due
  to collisional excitation of S I by energetic electrons beamed in the
  vertical direction. Direct excitation by a highly energetic beam of
  electrons of order 10-100 keV is doubtful. We suggest that the heat flux
  in the region connecting the transition zone to the high chromosphere
  during the gradual phase of a flare could lead to an anisotropic
  excitation. Then the observed polarization would be due to vertical
  motions of the transition zone sweeping the preexisting chromosphere.

---------------------------------------------------------
Title: Impact linear polarization observed in a UV chromospheric
    line during a solar flare
Authors: Henoux, J. C.; Chambe, G.; Semel, M.; Woodgate, B.; Shine,
   R.; Beckers, J.
1982AdSpR...2k.155H    Altcode: 1982AdSpR...2..155H
  Linear polarization was observed in the S I 1437Å line in bright
  flaring points during the soft X-ray emission. The degree of
  polarization is about 25% and is detected at a signal to noise ratio
  of 2.9. The polarized electric vector is directed towards disk center
  to within 3°. <P />This polarization could be due to collisional
  excitationm of S I by energetic electrons beamed in the vertical
  direction. We suggest that the heat flux in the region interconnecting
  the transition zone to the high chromosphere during the gradual phase
  of a flare could lead to an anisotropic excitation. Then the observed
  polarization would be due to vertical motions of the transition zone
  sweeping the preexisting chromosphere.

---------------------------------------------------------
Title: Magnetic and Velocity Fields of Emerging Flux Regions on
    the Sun
Authors: Martres, M. J.; Rayrole, J.; Semel, M.; Soru-Escaut, I.;
   Tanaka, K.; Makita, M.; Moriyama, F.; Unno, W.
1982PASJ...34..299M    Altcode:
  Emerging Flux Regions (EFR's) on the Sun observed in the Meudon-Mitaka
  Collaborative Solar Observation Program (1979) are discussed. <P />The
  rule found earlier (Martres et al. 1973) for the growth (or decay) of
  the N polarity or the decay (or growth) of the S polarity magnetic field
  in relation to the photospheric clockwise (or counterclockwise) vortex
  motion is reconfirmed for the EFR in the present solar cycle. The rule
  is emphasized as an indication of active generation of an electric
  current system by a photospheric motion. <P />The chromospheric
  manifestations of EFR's seen in Hα as arch filament systems (AFS's) and
  the enhanced brightness are examined with respect to the photospheric
  magnetic field structure derived from the white light and magnetic
  observations. An isolated EFR is associated with an AFS which is
  well coaligned with the photospheric transverse field. The typical
  EFR configuration may be distorted if the EFR appears in the region
  of the pre-existing strong field. The observational characteristics
  of such EFR's can also be interpreted in terms of the emergence of a
  nearly potential field associated with the dissipation of an excess
  magnetic energy. An EFR born in an older EFR shows that the flux loop
  is low-lying and tends to relax into a potential field configuration.

---------------------------------------------------------
Title: Magnetic fields observed in a sunspot and faculae using 12
    lines simultaneously
Authors: Semel, M.
1981A&A....97...75S    Altcode:
  In a sunspot the measured magnetic fields are in agreement for all
  12 lines observed, indicating the reliability of the observational
  method used. On the other hand, the discrepancies between the fields
  observed in 12 different lines are quite large for faculae, which is
  a characteristic of unresolved structures. There is, however, a good
  correlation between the measured magnetic field and the equivalent width
  of the corresponding line. An analysis in terms of 'relative magnetic
  curve of growth' is proposed. Application to star spots is discussed.

---------------------------------------------------------
Title: Hydrodynamical and Electrodynamical Interactions Between
    Magnetic Features in the Active Region Photosphere
Authors: Unno, W.; Tanaka, K.; Semel, M.
1981PASJ...33..495U    Altcode:
  Theoretical interpretation is attempted for hydrodynamical and
  electrodynamical phenomena in an active region observed by various means
  in the Meudon-Mitaka Solar Collaboration Observations in 1979. The
  theoretical treatment assumes the vertical motions to be localized
  in small regions so that these regions should behave as singular
  points in a two-dimensional (horizontal) irrotational flow. <P />(a)
  Some pieces of observational evidence are quantitatively examined,
  showing that hydrodynamical interactions are likely to be effective
  between magnetic features in an active region. <P />(b) There is
  also evidence which shows the existence of the dynamo action in the
  active-region photosphere. The law found by Martres et al. (1973)
  on the growth and decay of different magnetic polarities relative to
  the sense of rotation of the associated vortex motions is interpreted
  also in terms of the photospheric electromotoric mechanism.

---------------------------------------------------------
Title: A precise optical polarization analyzer
Authors: Semel, M.
1980A&A....91..369S    Altcode:
  An optical analyzer system suitable for the precise measurement of
  polarization (down to 0.0001 or less) is described. Two states of
  polarization in the solar spectrum for a wide spectral range and for
  many solar points can be observed simultaneously. The analysis of
  polarization is obtained by using a pair of quartz crystals ensuring
  the same optical path for the two beams, whose separation is very small
  (200 or 400 microns in the present experiment). Consequently, all the
  effects such as optical aberrations and turbulence are practically
  the same for the two beams when passing through an optical system. An
  application to magnetic measurements is discussed.

---------------------------------------------------------
Title: Observational Test for Hydrodynamical Models of Solar Faculae
Authors: Semel, M.; Ribes, E.; Rees, D.
1980jfss.conf..149S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Small Scale Unresolved Solar Magnetic Fields
Authors: Ribes, E.; Semel, M.
1980jfss.conf..129R    Altcode:
  No abstract at ADS

---------------------------------------------------------
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: Polarimetry with the coronograph of the Norikura Observatory.
Authors: Shimizu, I.; Semel, M.
1979TokRe..18..696S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Treatment of the Stokes Parameters and Meaurement of
    Magnetic Field
Authors: Semel, M.
1972lfpm.conf...89S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On the Broadening of the Balmer Lines during a Flare
Authors: Semel, M.; Soru-Escaut, I.
1971A&A....12..340S    Altcode:
  The hypothesis involved in the interpretation of flare spectra were
  chosen in this work as follows: 1) the total number of ab sorb ants
  per cm2, N2, in the emission layer should be practically the same
  for all the members of the Balmer series; 2) the maximum intensity
  in each line approximates the source function. With the help of
  these hypotheses, the analysis of the observational data showed that
  independently of the broadening mecanism, the first members of the
  series are saturated. Consequently it was necessary to study the
  line wings and to introduce a hypothesis concerning the broadening
  mecanism: either Stark or Doppler. As a criterion of choice between
  these two effects we have examined the ratios of the optical depths r,
  corresponding to different members of the series. This method led us
  to draw the conclusion that the Stark effect predominates. A coherent
  solution for the whole series is proposed. Finally we discussed the
  problem concerning the determination of the electron density The and
  of N2 - We have suggested the possible effects of inhomogeneities of
  the electron density. Key words: flare spectra - Stark broadening

---------------------------------------------------------
Title: Measurement of Magnetic Fields
Authors: Semel, M.
1971IAUS...43...37S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A Proposed Method of Calibrating Measurements of Linearly
    Polarized Light for the Obser- vation and Analysis of Zeeman Effects
    in Solar Spectral Lines
Authors: Semel, M.
1970A&A.....9..356S    Altcode:
  Th the proposed method the calibration depends only on the broadening
  mechanism of Fraunhofer lines. Because this mechanism is expected to
  be common to many lines, the use of several lines with different Land
  factors may lead to reliable measurements of the magnetic field and
  to the determination of the broadening parameters.

---------------------------------------------------------
Title: The Use of the Magnetograph for the Study of the Unresolved
    Solar Fine Structure (Answer to Harvey and Livingston)
Authors: Semel, M.
1970A&A.....9..152S    Altcode:
  The influence of the unresolved solar fine structure on magnetic
  measurements is discussed. A method for the determination of such
  effects by the use of a magnetograph is advanced.

---------------------------------------------------------
Title: Evaluation of the Electric Current in a Sunspot by the Study
    of the Observed Transverse Component of the Magnetic Field
Authors: Rayrole, J.; Semel, M.
1970A&A.....6..288R    Altcode:
  The method employed for the evaluation of electric current seems
  to give significant values. The analysis of these results leads to
  arguments unfavourable for a force free model. At a certain point,
  a direct estimation of the Lorentz force is made. Finally we discuss
  the difficulties and errors in the reduction of the data.

---------------------------------------------------------
Title: A Proposal for a Solar Magnetograph
Authors: Semel, M.
1970A&A.....5..330S    Altcode:
  A proposal for a magnetograph which should solve the whole problem
  of calibration is advanced. In fact calibration becomes almost banal,
  and is almost independent of line formation. A standard magnetograph
  could be modified for use of this method with very little difficulty.

---------------------------------------------------------
Title: Étude Critique d'un Champ Current-Free dans l'Atmosphère
    Solaire
Authors: Semel, M.; Rayrole, J.
1968IAUS...35..134S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Contribution à létude des champs magnétiques dans les
    régions actives solaires
Authors: Semel, M.
1967AnAp...30..513S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Champs magnétiques dans un centre d'activité solaire avant
    et pendant une éruption
Authors: Michard, R.; Mouradian, Z.; Semel, M.
1961AnAp...24...54M    Altcode:
  No abstract at ADS