Author name code: semel ADS astronomy entries on 2022-09-14 author:"Semel, Meir" ------------------------------------------------------------------------ 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. Bibcode: 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, Ωeq, was found to be 4.54 ± 0.01 rad d-1, with a photospheric shear δΩ of 0.21+0.02-0.03 rad d-1. This equates to an equatorial rotation period of ∼1.39 ± 0.01 d, with the equatorial region lapping the poles every ∼ 30+5-3 d. Using the magnetic features, the equatorial rotation rate, Ωeq, was found to be 4.51 ± 0.01 rad d-1, with a photospheric shear δΩ of 0.24 ± 0.03 rad d-1. This differential rotation is approximately four times that observed on the Sun. 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. Bibcode: 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.

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. 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. Bibcode: 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.

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. Title: Eigenpolarimetry: An Algebraic Approach to Polarization-free Telescopes and Instruments Authors: López Ariste, A.; Semel, M. Bibcode: 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. 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. Bibcode: 2010A&A...512A...6R Altcode: 2010arXiv1001.1003R
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.
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.
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. 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. Bibcode: 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.
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.
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.
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.
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. 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. Bibcode: 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.

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.

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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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.
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.
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. Title: Strength distribution of solar magnetic fields in photospheric quiet Sun regions Authors: Ramírez Vélez, J. C.; López Ariste, A.; Semel, M. Bibcode: 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.
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.
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.
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.
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. Title: First polarimetric observations and modeling of the FeH F4Δ-X4Δ system Authors: Afram, N.; Berdyugina, S. V.; Fluri, D. M.; Semel, M.; Bianda, M.; Ramelli, R. Bibcode: 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.
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.
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.
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.
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. 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. Bibcode: 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. 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. Bibcode: 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. Title: Multi-Line Spectro-Polarimetry of Stellar Magnetic Fields Using Principal Components Analysis Authors: Semel, M.; Rees, D. E.; Ramírez Vélez, J. C.; Stift, M. J.; Leone, F. Bibcode: 2006ASPC..358..355S Altcode: Zeeman-Doppler-Imaging (ZDI) is a technique for the diagnosis of stellar magnetic fields from observations of circularly polarized light, based on the weak-field approximation.

This approach has proved adequate in numerous studies of the magnetic fields of solar-type stars. To improve the signal-to-noise ratio, one can add the contributions of different lines to the polarization signature. However, this multi-line extension has been limited by a number of assumptions (e.g., the weak-field approximation). Here we present a novel approach. We created a database of synthetic polarized spectra for different combinations of magnetic-field intensities and orientations. Principal Components Analysis was used to compress the database, so that only few Principal Components are needed to represent all the calculated spectra. Title: 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. Bibcode: 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-1) and the late F-dwarf HR 1817 (v sin i = 52 km s-1). 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. 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. Bibcode: 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 Teff 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. 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. Bibcode: 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. Title: Inversion of solar magnetic fields in photospheric quiet regions. Authors: Ramírez Vélez, J. C.; López Ariste, A.; Semel, M. Bibcode: 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. 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. Bibcode: 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.

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-1. 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. 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. Bibcode: 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 Msolar, while the secondary is a dwarf of mass 1.0+/-0.1 Msolar. 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. Bibcode: 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 Bibcode: 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. Bibcode: 2003MNRAS.345.1145D Altcode: In this paper, we present new brightness and magnetic surface images of the young K0 dwarfs AB Doradus and LQ Hydrae, and of the K1 subgiant of the RS CVn system HR 1099 (=V711 Tauri), reconstructed from Zeeman-Doppler imaging spectropolarimetric observations collected at the Anglo-Australian Telescope during five observing campaigns (totalling 50 nights), from 1998 January to 2002 January. Along with the older images of the same stars (published in previous papers), our complete data set represents the first long-term series on temporal fluctuations of magnetic topologies of very active stars.

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

We find, in particular, that some signatures, for instance the relative fraction of magnetic energy stored in the large-scale poloidal and toroidal field components, and the polarity of the axisymmetric component of the field, are variable with time, and provide potentially fruitful diagnostics for investigating magnetic cycles in active stars other than the Sun. We report here the detection of partial polarity switches in some of the axisymmetric field components of two of our programme stars (AB Dor and LQ Hya), suggesting that the dynamo operating in these stars may be cyclic. Title: Spectropolarimetry and polarization-dependent fringes Authors: Semel, M. Bibcode: 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 10-3 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 10-5, 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.

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. Bibcode: 2003SPIE.4853..235L Altcode: We report on a new spectropolarimeter for the measurement of vector magnetic fields on prominences using the HeD3 line in the Evans Solar Facility at the Sacramento Peak Observatory. Title: The magnetic field and wind confinement of θ1 Orionis C Authors: Donati, J. -F.; Babel, J.; Harries, T. J.; Howarth, I. D.; Petit, P.; Semel, M. Bibcode: 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 θ1 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 θ1 Ori C in the context of the magnetically confined wind-shock model of Babel & Montmerle. In the case of θ1 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 1010-1011cm-3) 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 θ1 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 θ1 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. Bibcode: 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 & 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 χ2 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. Bibcode: 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 D3 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. Bibcode: 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. Bibcode: 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 Bibcode: 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<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 < 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. Bibcode: 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. Bibcode: 2000A&A...355..759R Altcode: This paper presents PCA inversion, a novel application of Principal Component Analysis to the problem of spectral line inversion, ie. solar/stellar atmospheric model parameter estimation from spectral lines. For a given type of spectral line we compute a database of synthetic spectral profiles using a large number of models. Inversion of an observed profile to obtain an atmospheric model is equivalent to a problem in pattern recognition, finding the nearest profile in the synthetic profile database. To reduce dimensionality we use the synthetic data as a PCA training set to decompose each synthetic (and observed) profile into a sum of a small number of principal components, or eigenprofiles. The coefficients of this decomposition can be regarded as elements of a low-dimensional eigenfeature vector. The eigenfeatures are smooth functions of model parameters, indicating that eigenfeatures for parameters not in the training set could be easily estimated by interpolation. Search for the nearest profile is fast because it is done in the eigenfeature vector space. We illustrate the method using several types of synthetic spectra: unpolarised intensity profiles of a line formed in a Milne-Eddington model atmosphere; unpolarised Hα flux profiles of a line formed in non-Local Thermodynamic Equilibrium in the chromosphere of a cool star; and polarised Stokes parameter profiles of a line split by the Zeeman effect in the presence of a magnetic field. We also apply PCA to a set of Stokes data observed in a sunspot region by the High Altitude Observatory Advanced Stokes Polarimeter. PCA inversion is proposed as a fast alternative to non-linear least squares inversion commonly used for solar magnetic field measurements based on such Stokes data. Title: First results from THEMIS spectropolarimetric mode Authors: López Ariste, A.; Rayrole, J.; Semel, M. Bibcode: 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. Bibcode: 1999A&AS..139..417L Altcode: In a previous paper \cite[(Semel & 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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 & 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 1997MNRAS.291..658D Altcode: This paper reports the results of five years (five runs, 23 nights) of spectropolarimetric observations of active stars with the UCL Echelle Spectrograph of the Anglo-Australian Telescope. 225 circularly (and four linearly) polarized spectra were recorded on 28 objects (21 active stars and seven calibration standards) using the new technique of Zeeman-Doppler imaging. To extract polarization echelle spectra from raw frames, we developed a new dedicated automatic software package (called ESprit, which utilizes optimal extraction techniques) whose detailed description is given in the paper. For each recorded spectrum, we extract `mean' polarized and unpolarized profiles using `least-squares deconvolution', a technique similar to cross-correlation, which can enhance enormously the sensitivity of Zeeman-Doppler imaging, by up to 7.5 mag in flux with respect to a single average line analysis or by 4.5 mag compared with the older technique of Donati et al. in the particular case of a K1 star. Magnetic field is detected unambiguously on 14 objects, namely the weak-line T Tauri star V410 Tau, the pre-main-sequence binary HD 155555, the ZAMS stars AB Dor and LQ Hya, the dwarf flare star CC Eri, the RS CVn systems HR 1099, EI Eri, TY Pyx, CF Tuc, SZ Psc, II Peg, IM Peg and IL Hya, and the FK Com star YY Men. Marginal field detections are also obtained for the weak-line T Tauri star HD 283572 and the Herbig Ae star HD 104237. Except on HR 1099 and II Peg, our results represent the first direct field detections ever reported on these objects, and in particular the first direct field detection on as young a star as V410 Tau. Most of the magnetic signatures we detect on cool stars show several sign reversals throughout the line profile, indicating that the parent field structure is rather complex and must feature (as expected) many small-scale magnetic regions of different polarities. For all stars on which Zeeman detections are recorded with sufficient accuracy (namely LQ Hya, CC Eri, HR 1099, EI Eri, II Peg, IL Hya and YY Men), differential least-squares deconvolution from both the blue and the red parts of the spectral domain indicates that the magnetic regions we detect are mostly 500 to 1000 K cooler than, and sometimes at the same temperature as, but never warmer than the surrounding photosphere. Serendipitous results include the first detection (i) of small-amplitude radial velocity variations (1.3kms^-1 peak to peak) of the Herbig Ae star HD 104237 with small enough a period (37.5+/-1 min) that they must be due to stellar pulsations and (ii) of the solar-like secondary component of the RS CVn system IL Hya. Title: Differential rotation and magnetic polarity patterns on AB Dor. Authors: Collier Cameron, A.; Donati, J. -F.; Semel, M. Bibcode: 1997AAONw..83....8C Altcode: No abstract at ADS Title: On The Detection of Currents and Parker's Objection Authors: Skumanich, A.; Semel, M. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 × 10−4. Title: The magnetohydrodynamic equilibrium of coronal helmet streamers. Authors: Cuperman, S.; Bruma, C.; Dryer, M.; Semel, M. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 1993A&A...278..231S Altcode: An account is given of the instrumental setup and observational procedure that led to the qualifying of Zeeman-Doppler imaging and to new detections of magnetic fields on four stars HR 1099, sigma2 CrB, II Peg and UX Ari. Title: Identification and elimination of the residual ambiguity in the sign of observed photospheric magnetic fields Authors: Cuperman, S.; Li, J.; Semel, M. Bibcode: 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 Bperpendicular 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. Bibcode: 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. Bibcode: 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. Bibcode: 1992A&A...265..669D Altcode: We applied the new technique of Zeeman-Doppler Imaging on nine active RS CVn binary systems. In four systems (HR 1099, sigma 2 CrB, II Peg and UX Ari), we detected a magnetic field on the cool active component; in each of them, the corresponding Stokes V signatures is found to vary with rotational phase. Moreover, on HR 1099, we have observational evidence that the magnetic structure varied significantly within a period of 1.3 yr. By contrast, magnetic field is repeatedly not detected on other stars, such as the hotter component of sigma 2 CrB, although these are also chromospherically very active. Our results suggest that the horizontal size scale of magnetic topologies may differ significantly among the observed stars. Large monopolar regions are likely present on those for which Zeeman signatures are detected; a more complex ordering of field lines with smaller monopolar regions and close bipolar groups may define the others. Title: 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. Bibcode: 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. Bibcode: 1992A&A...265..682D Altcode: Spectropolarimetric, spectroscopic, and photometric observations of the RS CVn binary system HR 1099 = V711 Tau (K1IV+G5V) were made from Oct. 1988 to Jan. 1991. From Doppler imaging of the unpolarized spectra and the corresponding data, two maximum entropy images of the temperature distribution of the active K1 subgiant at epochs 1988.9 and 1990.9 were derived. Zeeman-Doppler imaging of the circularly polarized spectra has led to the first magnetic maps of a star other than the sun. The fragmentary observations made at epoch 1989.6 suggest that the star's magnetic field lines were emerging radially and/or poloidally from an equatorial warm region at that time, quite likely one of these reconstructed in the 1988.9 temperature image. Title: 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. Bibcode: 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. Bibcode: 1992ASPC...26..353D Altcode: 1992csss....7..353D No abstract at ADS Title: Zeeman-Doppler imaging of solar-type and AP stars. IV. Maximum entropy reconstruction of 2D magnetic topologies. Authors: Brown, S. F.; Donati, J. -F.; Rees, D. E.; Semel, M. Bibcode: 1991A&A...250..463B Altcode: Recently Semel (1989) introduced the technique of Zeeman-Doppler Imaging (ZDI) which essentially consists of Doppler Imaging using the Stokes V (circular polarization) profile. A code which uses maximum entropy principles to recover the distribution of brightness, abundance, and magnetic fields across the surface of stars is presented. For brightness and abundance mappings, the code is found to produce results comparable to those found in the literature. The first reconstructions of magnetic test structures from the Stokes V parameter alone show that this quantity contains enough spatial information to recover spotted images but not dipolar ones. According to the magnetic topologies that may be expected in solar-type or Ap stars, it is concluded that ZDI may be well adapted in deriving magnetic maps of active RS CVn systems, while it is anticipated that the use of the two other Stokes parameters (linear polarizations) are needed to derive reliable results for Ap stars. Title: Erratum - Detection of a Magnetic Region on HR:1099 Authors: Donati, J. F.; Semel, M.; Rees, D. E.; Taylor, K.; Robinson, R. D. Bibcode: 1991A&A...248..337D Altcode: No abstract at ADS Title: Removal of singularities in the Cauchy problem for the extrapolation of solar force-free magnetic fields Authors: Cuperman, S.; Demoulin, P.; Semel, M. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1990A&A...232L...1D Altcode: The paper reports results of a magnetic field measuring campaign on the active K component of the RS CVn system HR 1099, using Zeeman-Doppler imaging. A localized magnetic field is detected near quadrature (phase 0.85). Assuming this magnetic region has a circular shape, its longitude and latitude are, respectively, estimated to be 86 + or - 4 deg and 5 + or - 5 deg. This equatorial region is largely monopolar, has a mean field strength of 985 + or - 270 G, covers 18 + or - 3 percent of the total stellar surface and may be colocalized with a bright photospheric spot. Title: 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. Bibcode: 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. Bibcode: 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.

The method consists of two steps, namely: (a) Expression of force-free field equations in terms of the observed quantities Bz, B2x, B2y and BxBy 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 ES(z) = ∫ (B2x + B2y + B2x)ds, and (iii) retaining the sign that leads to a decrease with height (z) of Es(z).

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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1989musi.work...37D Altcode: No abstract at ADS Title: Méthode d'imagerie Zeeman-Doppler. Authors: Donati, J. -F.; Semel, M.; Praderie, F. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1985spit.conf..758H Altcode: No abstract at ADS Title: Measurement of Magnetic Fields - Sun and Stars Authors: Semel, M. Bibcode: 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. Bibcode: 1984cms..conf...79S Altcode: No abstract at ADS Title: Détermination des champs magnétiques dans les structures solaires non résolues. Authors: Semel, M. Bibcode: 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. Bibcode: 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. Bibcode: 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°.

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. Bibcode: 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°.

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. Bibcode: 1982PASJ...34..299M Altcode: Emerging Flux Regions (EFR's) on the Sun observed in the Meudon-Mitaka Collaborative Solar Observation Program (1979) are discussed.

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.

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. Bibcode: 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. Bibcode: 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.

(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.

(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. Bibcode: 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. Bibcode: 1980jfss.conf..149S Altcode: No abstract at ADS Title: Small Scale Unresolved Solar Magnetic Fields Authors: Ribes, E.; Semel, M. Bibcode: 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. Bibcode: 1979A&A....74....1R Altcode: The problem of multidimensional LTE line formation in a magnetic field is solved in a model of an unresolved magnetic element imbedded in the solar photosphere. Horizontal and vertical variations of the physical parameters are included. Calculations show that, in circular polarization analysis, the centre of gravity displacement of the line profile gives a very good measurement of the longitudinal field averaged over the area observed. For small line weakenings the accuracy is probably better than 10%. Also the method is not affected by Zeeman saturation. A method based on centre of gravity measurements is proposed for inferring a 'model-independent' value of the average longitudinal field inside an unresolved magnetic element. Title: Polarimetry with the coronograph of the Norikura Observatory. Authors: Shimizu, I.; Semel, M. Bibcode: 1979TokRe..18..696S Altcode: No abstract at ADS Title: The Treatment of the Stokes Parameters and Meaurement of Magnetic Field Authors: Semel, M. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1968IAUS...35..134S Altcode: No abstract at ADS Title: Contribution à létude des champs magnétiques dans les régions actives solaires Authors: Semel, M. Bibcode: 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. Bibcode: 1961AnAp...24...54M Altcode: No abstract at ADS