Author name code: roudier ADS astronomy entries on 2022-09-14 author:"Roudier, Thierry" ------------------------------------------------------------------------ Title: A Comparative Study of Measurements of the Suns Axisymmetric Flows: A COFFIES Effort Authors: Upton, Lisa; Jain, Kiran; Komm, Rudolf; Mahajan, Sushant; Pevtsov, Alexei; Roudier, Thierry; Tripathy, Sushanta; Ulrich, Roger; Zhao, Junwei; Basu, Sarbani; Chen, Ruizhu; DeRosa, Marc; Hess Webber, Shea; Hoeksema, J. Bibcode: 2021AGUFMSH55D1871U Altcode: Consequence Of Fields and Flows in the Interior and Exterior of the Sun (COFFIES) is a Phase-1 NASA DRIVE Science Center (DSC), with the primary objective of developing a data driven model of solar activity. One of COFFIES five primary science questions is What drives varying large-scale motions in the Sun? To address this question, we are developing a comprehensive catalog of the variable differential rotation and meridional circulation flow patterns. This catalog includes measurements of these flows as obtained by several measurement techniques: Doppler imaging, granule tracking, magnetic pattern tracking, magnetic feature tracking, as well as both time distance and ring diagram helioseismology. We show a comparison of these flows across these varied techniques, with a particular focus on the MDI/HMI/GONG/Mount Wilson overlap period (May-July 2010). We investigate the uncertainties and attempt to reconcile any discrepancies (e.g., due to flow depth or systematics associated with the different measurement techniques). This analysis will pave the way toward accurately determining the global patterns of axisymmetric flows and their regular and irregular variations during the cycle. Title: One-sided arc averaging geometries in time-distance local helioseismology Authors: Korda, David; Švanda, Michal; Roudier, Thierry Bibcode: 2021A&A...654A..84K Altcode: 2021arXiv210800872K Context. The study of solar oscillations (helioseismology) has been a very successful method of researching the Sun. Helioseismology teaches us about the structure and mean properties of the Sun. Together with mid-resolution data, the local properties were uncovered in quiet-Sun regions. However, magnetic fields affect the oscillations and prevent us from studying the properties of magnetically active regions with helioseismology.
Aims: We aim to create a new methodology to suppress the negative effects of magnetic fields on solar oscillations and measure plasma properties close to active regions.
Methods: The methodology consists of new averaging geometries, a non-linear approach to travel-time measurements, and a consistent inversion method that combines plasma flows and sound-speed perturbations.
Results: We constructed the one-sided arc averaging geometries and applied them to the non-linear approach of travel-time measurements. Using the one-sided arc travel times, we reconstructed the annulus travel times in a quiet-Sun region. We tested the methodology against the validated helioseismic inversion pipeline. We applied the new methodology for an inversion for surface horizontal flows in a region with a circular H-type sunspot. The inverted surface horizontal flows are comparable with the output of the coherent structure tracking, which is not strongly affected by the presence of the magnetic field. We show that the new methodology suppresses the negative effects of magnetic fields up to outer penumbra. We measure divergent flows with properties comparable to the moat flow.
Conclusions: The new methodology can teach us about the depth structure of active regions and physical conditions that contribute to the evolution of the active regions. Title: Changes in granulation scales over the solar cycle seen with SDO/HMI and Hinode/SOT Authors: Ballot, J.; Roudier, T.; Malherbe, J. M.; Frank, Z. Bibcode: 2021A&A...652A.103B Altcode: 2021arXiv210603556B Context. The Sun is the only star where the superficial turbulent convection can be observed at very high spatial resolution. The Solar Dynamics Observatory (SDO) has continuously observed the full Sun from space with multi-wavelength filters since July 2010. In particular, the Helioseismic and Magnetic Imager (HMI) instrument takes high-cadence frames (45 s) of continuum intensity in which solar granulation is visible.
Aims: We aimed to follow the evolution of the solar granules over an activity cycle and look for changes in their spatial properties.
Methods: We investigated the density of granules and their mean area derived directly from the segmentation of deconvolved images from SDO/HMI. To perform the segmentation, we define granules as convex elements of images.
Results: We measured an approximately 2% variation in the density and the mean area of granules over the cycle, the density of granules being greater at solar maximum with a smaller granule mean area. The maximum density appears to be delayed by about one year compared to classical activity indicators, such as the sunspot number. We complemented this study with high-spatial-resolution observations obtained with Hinode/SOTBFI (Solar Optical Telescope Broadband Filter Imager), which are consistent with our results.
Conclusions: The observed variations in solar granulation at the disc centre reveal a direct insight into the change in the physical properties that occur in the upper convective zone during a solar cycle. These variations can be due to interactions between convection and magnetic fields, either at the global scale or, locally, at the granulation scale. Title: Photospheric downflows observed with SDO/HMI, HINODE, and an MHD simulation Authors: Roudier, T.; Švanda, M.; Malherbe, J. M.; Ballot, J.; Korda, D.; Frank, Z. Bibcode: 2021A&A...647A.178R Altcode: 2021arXiv210303077R Downflows on the solar surface are suspected to play a major role in the dynamics of the convection zone, at least in its outer part. We investigate the existence of the long-lasting downflows whose effects influence the interior of the Sun but also the outer layers. We study the sets of Dopplergrams and magnetograms observed with Solar Dynamics Observatory and Hinode spacecrafts and an magnetohydrodynamic (MHD) simulation. All of the aligned sequences, which were corrected from the satellite motions and tracked with the differential rotation, were used to detect the long-lasting downflows in the quiet-Sun at the disc centre. To learn about the structure of the flows below the solar surface, the time-distance local helioseismology was used. The inspection of the 3D data cube (x, y, t) of the 24 h Doppler sequence allowed us to detect 13 persistent downflows. Their lifetimes lie in the range between 3.5 and 20 h with a sizes between 2″ and 3″ and speeds between −0.25 and −0.72 km s−1. These persistent downflows are always filled with the magnetic field with an amplitude of up to 600 Gauss. The helioseismic inversion allows us to describe the persistent downflows and compare them to the other (non-persistent) downflows in the field of view. The persistent downflows seem to penetrate much deeper and, in the case of a well-formed vortex, the vorticity keeps its integrity to the depth of about 5 Mm. In the MHD simulation, only sub-arcsecond downflows are detected with no evidence of a vortex comparable in size to observations at the surface of the Sun. The long temporal sequences from the space-borne allows us to show the existence of long-persistent downflows together with the magnetic field. They penetrate inside the Sun but are also connected with the anchoring of coronal loops in the photosphere, indicating a link between downflows and the coronal activity. A links suggests that EUV cyclones over the quiet Sun could be an effective way to heat the corona. Title: Pollux: A weak dynamo-driven dipolar magnetic field and implications for its probable planet Authors: Aurière, M.; Petit, P.; Mathias, P.; Konstantinova-Antova, R.; Charbonnel, C.; Donati, J. -F.; Espagnet, O.; Folsom, C. P.; Roudier, T.; Wade, G. A. Bibcode: 2021A&A...646A.130A Altcode: 2021arXiv210102016A Context. Pollux is considered as an archetype of a giant star hosting a planet since its radial velocity (RV) presents very stable sinusoidal variations with a period of about 590 d. We then discovered a weak magnetic field at its surface using spectropolarimetry, questioning the planetary hypothesis.
Aims: We followed up our investigations on Pollux to characterize its magnetic field and to infer the effects of magnetic activity on the RV variations.
Methods: We first used ESPaDOnS at CFHT and then Narval at TBL to obtain Stokes I and Stokes V spectra of Pollux to study their variations for a duration of 4.25 years, that is, for more than two periods of the RV variations. We used the least-squares deconvolution profiles to measure the longitudinal magnetic field and to perform a Zeeman Doppler imaging (ZDI) investigation.
Results: The longitudinal magnetic field of Pollux is found to vary with a sinusoidal behavior and a period similar to that of the RV variations. From the ZDI investigation a rotation period of Pollux is determined to be equal to 660 ± 15 days and possibly different than the period of variations of the RV. As to the magnetic topology, the poloidal component is dominant and almost purely dipolar with an inclination of 10.5° of the dipole with respect to the rotation axis. The mean strength of the surface magnetic field is 0.44 G. Pollux is found approximately as active as the Sun observed as a star and this activity could induce moderate RV variations.
Conclusions: As to the origin of the magnetic field of Pollux, we favor the hypothesis that it is maintained through contemporaneous dynamo action. Pollux appears as the representative of a class of slowly rotating and weakly magnetic G-K red giants. To explain the sinusoidal RV variations of Pollux, two scenarios are proposed. If the RV period is different from the rotation period, the observed periodic RV variations are due to the hosted planet and the contribution of Pollux magnetic activity is not significantly detected. In the peculiar case in which the two periods are equal, we cannot discard the possibility that the activity of Pollux could explain the total RV variations and that the planet hypothesis would appear unnecessary. In any case magnetic activity could contribute significantly to RV variations in some intermediate mass G-K red giants hosting planets, particularly those with small amplitude RV variations.

Based on observations obtained at the Télescope Bernard Lyot (TBL) at Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse, France, and the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council of Canada, CNRS/INSU and the University of Hawaii. Title: Evolution of exploding granules from coordinated observations by THEMIS, IRIS, SDO/HMI, and HINODE, and a simulation Authors: Roudier, T.; Malherbe, J. M.; Gelly, B.; Douet, R.; Frank, Z.; Dalmasse, K. Bibcode: 2020A&A...641A..50R Altcode: 2020arXiv200712438R Exploding granules constitute the strongest horizontal flows on the quiet Sun and contribute to the structure of the surface horizontal velocity fields which build the large-scale organization of the discrete magnetic field. In this work we explore exploding granule expansion through the observations of the ground-based THEMIS telescope, IRIS, SDO, and the Hinode space-borne instruments, and finally with the magnetohydrodynamics simulation. We evaluate the detection and the expansion of exploding granules at several wavelengths and at various spatial and temporal resolutions. To analyze the different temporal sequences, two methods of image segmentation are applied to select the granules. The first allows us to follow individually the exploding granules observed simultaneously by THEMIS, IRIS, and SDO. The second uses long time independent sequences from THEMIS, IRIS, SDO, Hinode, and a simulation. In the first method (called manual) the segmentation isolates the cell of the granules (bright granules and intergranular parts), while in the second method (called statistical) only the bright part of the granules are isolated. The results obtained with simultaneous or distinct temporal observations using the two methods of segmentation are in good agreement. The granule area evolves linearly with an expansion velocity that decreases with the radius. A rapid decrease in the velocity expansion in the first two minutes is observed. The detection and measurement of the dynamics of the explosive granules can be performed from ground- and space-based instruments. Our work reveals the usefulness of SDO data, with low spatial resolution, to study the dynamics of the exploding granules all over the solar surface. Title: Evolution of photospheric flows under an erupting filament in the quiet-Sun region Authors: Wollmann, Jiří; Švanda, Michal; Korda, David; Roudier, Thierry Bibcode: 2020A&A...636A.102W Altcode: 2020arXiv200312515W Context. We studied the dynamics of the solar atmosphere in the region of a large quiet-Sun filament, which erupted on 21 October 2010. The filament eruption started at its northern end and disappeared from the Hα line-core filtergrams line within a few hours. The very fast motions of the northern leg were recorded in ultraviolet light by the Atmospheric Imaging Assembly (AIA) imager.
Aims: We aim to study a wide range of available datasets describing the dynamics of the solar atmosphere for five days around the filament eruption. This interval covers three days of the filament evolution, one day before the filament growth and one day after the eruption. We search for possible triggers that lead to the eruption of the filament.
Methods: The surface velocity field in the region of the filament were measured by means of time-distance helioseismology and coherent structure tracking. The apparent velocities in the higher atmosphere were estimated by tracking the features in the 30.4 nm AIA observations. To capture the evolution of the magnetic field, we extrapolated the photospheric line-of-sight magnetograms and also computed the decay index of the magnetic field.
Results: We found that photospheric velocity fields showed some peculiarities. Before the filament activation, we observed a temporal increase of the converging flows towards the filament's spine. In addition, the mean squared velocity increased temporarily before the activation and peaked just before it, followed by a steep decrease. We further see an increase in the average shear of the zonal flow component in the filament's region, followed by a steep decrease. The photospheric line-of-sight magnetic field shows a persistent increase of induction eastward from the filament spine. The decay index of the magnetic field at heights around 10 Mm shows a value larger than critical one at the connecting point of the northern filament end. The value of the decay index increases monotonically there until the filament activation. Then, it decreased sharply.

AIA30.4 nm movie is available at https://www.aanda.org Title: Neural Network to Emulate Numerical Simulations of the Sun and Infer Synthetic Observations for Data Assimilation Authors: Tremblay, Benoit; Roudier, Thierry; Cossette, Jean-François; Attié, Raphaël; Rieutord, Michel; Vincent, Alain Bibcode: 2019shin.confE..30T Altcode: Satellites and ground-based observatories probe the Sun's photosphere and atmosphere and are key in studying solar activity. Meanwhile, numerical models have attempted to bridge the gap between the physics of the solar interior and such observations. However, there are physical quantities relevant to solar activity that can be modeled but that cannot be directly measured and must be inferred. For example, direct measurements of plasma motions at the photosphere are limited to the line-of-sight component. Recently, neural network computing has been used in conjunction with numerical models of the Sun to be able to recover the full velocity vector in photospheric plasma of the Quiet Sun. We used satellite observations as input in a fully convolutional neural network to generate instantaneous synthetic plasma motions, i.e. plasma motions that reflect the physics of a model but are made to look as if they were observed by a specific instrument. A parallel technique could then be invoked to eventually be able to derive the plasma velocity vector maps of the Active Sun and, by extension, other physical quantities of interest that can not yet be measured directly at the photosphere or anywhere else in the solar atmosphere. Title: Link between trees of fragmenting granules and deep downflows in MHD simulation Authors: Roudier, T.; Malherbe, J. M.; Stein, R. F.; Frank, Z. Bibcode: 2019A&A...622A.112R Altcode: 2019arXiv190103255R Context. Trees of fragmenting granules (TFG) and associated flows are suspected to play a major role in the formation of the network in the quiet Sun. We investigate the counterparts, in terms of dynamics, of surface structures detectable by high resolution observations in deeper layers up to 15 Mm, which are only available from numerical simulations.
Aims: The first aim is to demonstrate that TFG can be evidenced either from surface intensitites, vertical (Vz), or Doppler (Vdop) velocities. The second is to show that horizontal flows, which are derived from intensities or Vz/Vdop flows, are in good agreement, and that this is the case for observations and numerical simulations. The third objective is to apply this new Vz-based method to a 3D simulation to probe relationships between horizontal surface flows, TFG, and deep vertical motions.
Methods: The TFG were detected after oscillation filtering of intensities or Vz/Vdop flows, using a segmentation and labelling technique. Surface horizontal flows were derived from local correlation tracking (LCT) and from intensities or Vz/Vdop flows. These methods were applied to Hinode observations, 2D surface results of a first simulation, and 3D Vz data of a second simulation.
Results: We find that TFG and horizontal surface flows (provided by the LCT) can be detected either from intensities or Vz/Vdop component, for high resolution observations and numerical simulations. We apply this method to a 3D run providing the Vz component in depth. This reveals a close relationship between surface TFG (5 Mm mesoscale) and vertical downflows 5 Mm below the surface. We suggest that the dynamics of TFG form larger scales (the 15-20 Mm supergranulation) associated with 15 Mm downflowing cells below the surface.
Conclusions: The TFG and associated surface flows seem to be essential to understanding the formation and evolution of the network at the meso and supergranular scale.

Movies associated to Figs. 3, 11, 12, and 14 are availabe at https://www.aanda.org Title: Horizontal photospheric flows trigger a filament eruption Authors: Roudier, T.; Schmieder, B.; Filippov, B.; Chandra, R.; Malherbe, J. M. Bibcode: 2018A&A...618A..43R Altcode: 2018arXiv180802272R Context. A large filament composed principally of two sections erupted sequentially in the southern hemisphere on January 26, 2016. The central, thick part of the northern section was first lifted up and lead to the eruption of the full filament. This event was observed in Hα with the Global Oscillation Network Group (GONG) and Christian Latouche IMageur Solaire (CLIMSO), and in ultraviolet (UV) with the Atmospheric Imaging Assembly (AIA) imager on board the Solar Dynamic Observatory (SDO).
Aims: The aim of the paper is to relate the photospheric motions below the filament and its environment to the eruption of the filament.
Methods: An analysis of the photospheric motions using Solar Dynamic Observatory Helioseismic and Magnetic Imager (SDO/HMI) continuum images with the new version of the coherent structure tracking (CST) algorithm developed to track granules, as well as large-scale photospheric flows, has been performed. Following velocity vectors, corks migrate towards converging areas.
Results: The supergranule pattern is clearly visible outside the filament channel but difficult to detect inside because the modulus of the vector velocity is reduced in the filament channel, mainly in the magnetized areas. The horizontal photospheric flows are strong on the west side of the filament channel and oriented towards the filament. The ends of the filament sections are found in areas of concentration of corks. Whirled flows are found locally around the feet.
Conclusions: The strong horizontal flows with an opposite direction to the differential rotation create strong shear and convergence along the magnetic polarity inversion line (PIL) in the filament channel. The filament has been destabilized by the converging flows, which initiate an ascent of the middle section of the filament until the filament reaches the critical height of the torus instability inducing, consequently, the eruption. The n decay index indicated an altitude of 60 Mm for the critical height. It is conjectured that the convergence along the PIL is due to the large-scale size cells of convection that transport the magnetic field to their borders.

The movies associated to Figs. 1, 2, 4 and 5 are available at https://www.aanda.org. Title: Reconstruction of Horizontal Plasma Motions at the Photosphere from Intensitygrams: A Comparison Between DeepVel, LCT, FLCT, and CST Authors: Tremblay, Benoit; Roudier, Thierry; Rieutord, Michel; Vincent, Alain Bibcode: 2018SoPh..293...57T Altcode: Direct measurements of plasma motions in the photosphere are limited to the line-of-sight component of the velocity. Several algorithms have therefore been developed to reconstruct the transverse components from observed continuum images or magnetograms. We compare the space and time averages of horizontal velocity fields in the photosphere inferred from pairs of consecutive intensitygrams by the LCT, FLCT, and CST methods and the DeepVel neural network in order to identify the method that is best suited for generating synthetic observations to be used for data assimilation. The Stein and Nordlund (Astrophys. J. Lett.753, L13, 2012) magnetoconvection simulation is used to generate synthetic SDO/HMI intensitygrams and reference flows to train DeepVel. Inferred velocity fields show that DeepVel performs best at subgranular and granular scales and is second only to FLCT at mesogranular and supergranular scales. Title: Dynamics of Trees of Fragmenting Granules in the Quiet Sun: Hinode/SOT Observations Compared to Numerical Simulation Authors: Malherbe, J. -M.; Roudier, T.; Stein, R.; Frank, Z. Bibcode: 2018SoPh..293....4M Altcode: 2018arXiv180401870M We compare horizontal velocities, vertical magnetic fields, and the evolution of trees of fragmenting granules (TFG, also named families of granules) derived in the quiet Sun at disk center from observations at solar minimum and maximum of the Solar Optical Telescope (SOT on board Hinode) and results of a recent 3D numerical simulation of the magneto-convection. We used 24-hour sequences of a 2D field of view (FOV) with high spatial and temporal resolution recorded by the SOT Broad band Filter Imager (BFI) and Narrow band Filter Imager (NFI). TFG were evidenced by segmentation and labeling of continuum intensities. Horizontal velocities were obtained from local correlation tracking (LCT) of proper motions of granules. Stokes V provided a proxy of the line-of-sight magnetic field (BLOS). The MHD simulation (performed independently) produced granulation intensities, velocity, and magnetic field vectors. We discovered that TFG also form in the simulation and show that it is able to reproduce the main properties of solar TFG: lifetime and size, associated horizontal motions, corks, and diffusive index are close to observations. The largest (but not numerous) families are related in both cases to the strongest flows and could play a major role in supergranule and magnetic network formation. We found that observations do not reveal any significant variation in TFG between solar minimum and maximum. Title: Supergranulation and multiscale flows in the solar photosphere. Global observations vs. a theory of anisotropic turbulent convection Authors: Rincon, F.; Roudier, T.; Schekochihin, A. A.; Rieutord, M. Bibcode: 2017A&A...599A..69R Altcode: 2016arXiv160905785R The Sun provides us with the only spatially well-resolved astrophysical example of turbulent thermal convection. While various aspects of solar photospheric turbulence, such as granulation (one-Megameter horizontal scale), are well understood, the questions of the physical origin and dynamical organization of larger-scale flows, such as the 30-Megameters supergranulation and flows deep in the solar convection zone, remain largely open in spite of their importance for solar dynamics and magnetism. Here, we present a new critical global observational characterization of multiscale photospheric flows and subsequently formulate an anisotropic extension of the Bolgiano-Obukhov theory of hydrodynamic stratified turbulence that may explain several of their distinctive dynamical properties. Our combined analysis suggests that photospheric flows in the horizontal range of scales between supergranulation and granulation have a typical vertical correlation scale of 2.5 to 4 Megameters and operate in a strongly anisotropic, self-similar, nonlinear, buoyant dynamical regime. While the theory remains speculative at this stage, it lends itself to quantitative comparisons with future high-resolution acoustic tomography of subsurface layers and advanced numerical models. Such a validation exercise may also lead to new insights into the asymptotic dynamical regimes in which other, unresolved turbulent anisotropic astrophysical fluid systems supporting waves or instabilities operate. Title: Polar cap magnetic field reversals during solar grand minima: could pores play a role? Authors: Švanda, Michal; Brun, Allan Sacha; Roudier, Thierry; Jouve, Laurène Bibcode: 2016A&A...586A.123S Altcode: 2015arXiv151106894S We study the magnetic flux carried by pores located outside active regions with sunspots and investigate their possible contribution to the reversal of the global magnetic field of the Sun. We find that they contain a total flux of comparable amplitude to the total magnetic flux contained in polar caps. The pores located at distances of 40-100 Mm from the closest active region systematically have the correct polarity of the magnetic field to contribute to the polar cap reversal. These pores can be found predominantly in bipolar magnetic regions. We propose that during grand minima of solar activity, such a systematic polarity trend, which is akin to a weak magnetic (Babcock-Leighton-like) source term, could still be operating but was missed by the contemporary observers because of the limited resolving power of their telescopes. Title: Families of Granules, Flows, and Acoustic Events in the Solar Atmosphere from Hinode Observations Authors: Malherbe, J. -M.; Roudier, T.; Frank, Z.; Rieutord, M. Bibcode: 2015SoPh..290..321M Altcode: 2014SoPh..tmp..189M We investigate the relationship between trees of fragmenting granules (TFG), horizontal and vertical flows, and acoustic events (AE) in the photospheric network. AE are spatially concentrated and short-duration locations of acoustic energy flux. We performed observations at disk center of a 2D field of view (FOV) with high spatial and temporal resolutions provided by the Solar Optical Telescope onboard Hinode. Line profiles of Fe I 557.6 nm were recorded by the Narrow-band Filter Imager on an 80″×36″ FOV during five hours with a cadence of 22 seconds and 0.08″ pixel size. Vertical velocities were derived at two atmospheric levels allowing the determination of the energy flux at the acoustic frequency of 3.3 mHz. Families of granules and horizontal velocities were obtained from local correlation tracking (LCT) after segmentation and labeling of either continuum intensities or granular Doppler shifts. AE exhibit durations in the range 0.25 to 1 hour compatible with the lifetime of families (80 % do not last more than two hours). High-energy AE have the shortest lifetimes. We found that most AE occur in intergranular lanes located in or close to the boundaries between different families (called inter families) in regions with predominantly downward vertical motions and horizontal converging flows. In contrast, diverging flows are observed inside families, with a few AE in the intergranules. At the beginning of the sequence, when families are not yet detected, the distribution of AE is not uniform and is already organized at spatial lengths related to the mesogranular scale, with maximum contribution in the range 5″ to 10″, fully compatible with the scale of the maximum contribution of families in the TFG space. Although all sizes and durations seem to exist for families, their number decreases with increasing size and lifetime. Title: The magnetic fields at the surface of active single G-K giants Authors: Aurière, M.; Konstantinova-Antova, R.; Charbonnel, C.; Wade, G. A.; Tsvetkova, S.; Petit, P.; Dintrans, B.; Drake, N. A.; Decressin, T.; Lagarde, N.; Donati, J. -F.; Roudier, T.; Lignières, F.; Schröder, K. -P.; Landstreet, J. D.; Lèbre, A.; Weiss, W. W.; Zahn, J. -P. Bibcode: 2015A&A...574A..90A Altcode: 2014arXiv1411.6230A
Aims: We investigate the magnetic field at the surface of 48 red giants selected as promising for detection of Stokes V Zeeman signatures in their spectral lines. In our sample, 24 stars are identified from the literature as presenting moderate to strong signs of magnetic activity. An additional 7 stars are identified as those in which thermohaline mixing appears not to have occured, which could be due to hosting a strong magnetic field. Finally, we observed 17 additional very bright stars which enable a sensitive search to be performed with the spectropolarimetric technique.
Methods: We use the spectropolarimeters Narval and ESPaDOnS to detect circular polarization within the photospheric absorption lines of our targets. We treat the spectropolarimetric data using the least-squares deconvolution method to create high signal-to-noise ratio mean Stokes V profiles. We also measure the classical S-index activity indicator for the Ca ii H&K lines, and the stellar radial velocity. To infer the evolutionary status of our giants and to interpret our results, we use state-of-the-art stellar evolutionary models with predictions of convective turnover times.
Results: We unambiguously detect magnetic fields via Zeeman signatures in 29 of the 48 red giants in our sample. Zeeman signatures are found in all but one of the 24 red giants exhibiting signs of activity, as well as 6 out of 17 bright giant stars. However no detections were obtained in the 7 thermohaline deviant giants. The majority of the magnetically detected giants are either in the first dredge up phase or at the beginning of core He burning, i.e. phases when the convective turnover time is at a maximum: this corresponds to a "magnetic strip" for red giants in the Hertzsprung-Russell diagram. A close study of the 16 giants with known rotational periods shows that the measured magnetic field strength is tightly correlated with the rotational properties, namely to the rotational period and to the Rossby number Ro. Our results show that the magnetic fields of these giants are produced by a dynamo, possibly of α-ω origin since Ro is in general smaller than unity. Four stars for which the magnetic field is measured to be outstandingly strong with respect to that expected from the rotational period/magnetic field relation or their evolutionary status are interpreted as being probable descendants of magnetic Ap stars. In addition to the weak-field giant Pollux, 4 bright giants (Aldebaran, Alphard, Arcturus, η Psc) are detected with magnetic field strength at the sub-Gauss level. Besides Arcturus, these stars were not considered to be active giants before this study and are very similar in other respects to ordinary giants, with S-index indicating consistency with basal chromospheric flux.

Tables 6-8 are available in electronic form at http://www.aanda.orgBased on observations obtained at the Télescope Bernard Lyot (TBL) at Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse, France, and at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, CNRS/INSU and the University of Hawaii. Title: The Role of Subsurface Flows in Solar Surface Convection: Modeling the Spectrum of Supergranular and Larger Scale Flows Authors: Lord, J. W.; Cameron, R. H.; Rast, M. P.; Rempel, M.; Roudier, T. Bibcode: 2014ApJ...793...24L Altcode: 2014arXiv1407.2209L We model the solar horizontal velocity power spectrum at scales larger than granulation using a two-component approximation to the mass continuity equation. The model takes four times the density scale height as the integral (driving) scale of the vertical motions at each depth. Scales larger than this decay with height from the deeper layers. Those smaller are assumed to follow a Kolmogorov turbulent cascade, with the total power in the vertical convective motions matching that required to transport the solar luminosity in a mixing length formulation. These model components are validated using large-scale radiative hydrodynamic simulations. We reach two primary conclusions. (1) The model predicts significantly more power at low wavenumbers than is observed in the solar photospheric horizontal velocity spectrum. (2) Ionization plays a minor role in shaping the observed solar velocity spectrum by reducing convective amplitudes in the regions of partial helium ionization. The excess low wavenumber power is also seen in the fully nonlinear three-dimensional radiative hydrodynamic simulations employing a realistic equation of state. This adds to other recent evidence suggesting that the amplitudes of large-scale convective motions in the Sun are significantly lower than expected. Employing the same feature tracking algorithm used with observational data on the simulation output, we show that the observed low wavenumber power can be reproduced in hydrodynamic models if the amplitudes of large-scale modes in the deep layers are artificially reduced. Since the large-scale modes have reduced amplitudes, modes on the scale of supergranulation and smaller remain important to convective heat flux even in the deep layers, suggesting that small-scale convective correlations are maintained through the bulk of the solar convection zone. Title: Pollux: a stable weak dipolar magnetic field but no planet? Authors: Aurière, Michel; Konstantinova-Antova, Renada; Espagnet, Olivier; Petit, Pascal; Roudier, Thierry; Charbonnel, Corinne; Donati, Jean-François; Wade, Gregg A. Bibcode: 2014IAUS..302..359A Altcode: 2013arXiv1310.6907A Pollux is considered as an archetype of a giant star hosting a planet: its radial velocity (RV) presents sinusoidal variations with a period of about 590 d, which have been stable for more than 25 years. Using ESPaDOnS and Narval we have detected a weak (sub-gauss) magnetic field at the surface of Pollux and followed up its variations with Narval during 4.25 years, i.e. more than for two periods of the RV variations. The longitudinal magnetic field is found to vary with a sinusoidal behaviour with a period close to that of the RV variations and with a small shift in phase. We then performed a Zeeman Doppler imaging (ZDI) investigation from the Stokes V and Stokes I least-squares deconvolution (LSD) profiles. A rotational period is determined, which is consistent with the period of variations of the RV. The magnetic topology is found to be mainly poloidal and this component almost purely dipolar. The mean strength of the surface magnetic field is about 0.7 G. As an alternative to the scenario in which Pollux hosts a close-in exoplanet, we suggest that the magnetic dipole of Pollux can be associated with two temperature and macroturbulent velocity spots which could be sufficient to produce the RV variations. We finally investigate the scenarii of the origin of the magnetic field which could explain the observed properties of Pollux. Title: Proper horizontal photospheric flows in a filament channel Authors: Schmieder, B.; Roudier, T.; Mein, N.; Mein, P.; Malherbe, J. M.; Chandra, R. Bibcode: 2014A&A...564A.104S Altcode: Context. An extended filament in the central part of the active region NOAA 11106 crossed the central meridian on Sept. 17, 2010 in the southern hemisphere. It has been observed in Hα with the THEMIS telescope in the Canary Islands and in 304 Å with the EUV imager (AIA) onboard the Solar Dynamic Observatory (SDO). Counterstreaming along the Hα threads and bright moving blobs (jets) along the 304 Å filament channel were observed during 10 h before the filament erupted at 17:03 UT.
Aims: The aim of the paper is to understand the coupling between magnetic field and convection in filament channels and relate the horizontal photospheric motions to the activity of the filament.
Methods: An analysis of the proper photospheric motions using SDO/HMI continuum images with the new version of the coherent structure tracking (CST) algorithm developed to track granules, as well as the large scale photospheric flows, was performed for three hours. Using corks, we derived the passive scalar points and produced a map of the cork distribution in the filament channel. Averaging the velocity vectors in the southern hemisphere in each latitude in steps of 3.5 arcsec, we defined a profile of the differential rotation.
Results: Supergranules are clearly identified in the filament channel. Diverging flows inside the supergranules are similar in and out of the filament channel. Converging flows corresponding to the accumulation of corks are identified well around the Hα filament feet and at the edges of the EUV filament channel. At these convergence points, the horizontal photospheric velocity may reach 1 km s-1, but with a mean velocity of 0.35 km s-1. In some locations, horizontal flows crossing the channel are detected, indicating eventually large scale vorticity.
Conclusions: The coupling between convection and magnetic field in the photosphere is relatively strong. The filament experienced the convection motions through its anchorage points with the photosphere, which are magnetized areas (ends, feet, lateral extensions of the EUV filament channel). From a large scale point-of-view, the differential rotation induced a shear of 0.1 km s-1 in the filament. From a small scale point-of-view, any convective motions favored the interaction of the parasitic polarities responsible for the anchorages of the filament to the photosphere with the surrounding network and may explain the activity of the filament.

Two movies are available in electronic form at http://www.aanda.org Title: Dynamics in the filament of september 17 2010 and in its channel Authors: Mein, Nicole; Mein, Pierre; Schmieder, Brigitte; Malherbe, Jean-Marie; Roudier, Thierry Bibcode: 2014IAUS..300..451M Altcode: Dynamics of a filament is investigated in Hα. Counterstreaming flows are observed along the filament. Photospheric horizontal motions have been computed by using a Coherent Structure Tracking algorithm in the filament environment. Title: Proper horizontal photospheric flows below an eruptive filament Authors: Schmieder, Brigitte; Mein, Pierre; Mein, Nicole; Roudier, Thierry; Chandra, Ramseh Bibcode: 2014cosp...40E2926S Altcode: An analysis of the proper motions using SDO/HMI continuum images with the new version of the coherent structure tracking (CST) algorithm developed to track the granules as well as the large scale photospheric flows, was perfomed during three hours in a region containing a large filament channel on September 17, 2010. Supergranules were idenfied in the filament channel. Diverging flows inside the supergranules are similar in and out the filament channel. Using corks, we derived the passive scalar points and produced maps of cork distribution. The anchorage structures with the photosphere (feet) of the filament are located in the areas of converging flows with accumulations of corks. Averaging the velocity vectors for each latitude we defined a profile of the differential rotation. We conclude that the coupling between the convection and magnetic field in the photosphere is relatively strong. The filament experienced the convection motions through its feet. On a large scale point-of-view the differential rotation induced a shear of 0.1 km/s in the filament. On a small scale point-of-view convection motions favored the interaction/cancellation of the parasitic polarities at the base of the feet with the surrounding network explaining the brightenings,/jets and the eruption that were observed in the EUV filament. Title: Comparison of Solar Surface Flows Inferred from Time-Distance Helioseismology and Coherent Structure Tracking Using HMI/SDO Observations Authors: Švanda, Michal; Roudier, Thierry; Rieutord, Michel; Burston, Raymond; Gizon, Laurent Bibcode: 2013ApJ...771...32S Altcode: 2013arXiv1305.0875S We compare measurements of horizontal flows on the surface of the Sun using helioseismic time-distance inversions and coherent structure tracking of solar granules. Tracking provides two-dimensional horizontal flows on the solar surface, whereas the time-distance inversions estimate the full three-dimensional velocity flows in the shallow near-surface layers. Both techniques use Helioseismic and Magnetic Imager observations as input. We find good correlations between the various measurements resulting from the two techniques. Further, we find a good agreement between these measurements and the time-averaged Doppler line-of-sight velocity, and also perform sanity checks on the vertical flow that resulted from the three-dimensional time-distance inversion. Title: Magnetic Fields and Convection in the Cool Supergiant Betelgeuse Authors: Petit, P.; Aurière, M.; Konstantinova-Antova, R.; Morgenthaler, A.; Perrin, G.; Roudier, T.; Donati, J. -F. Bibcode: 2013LNP...857..231P Altcode: 2011arXiv1109.3979P We present the outcome of a highly-sensitive search for magnetic fields on the cool supergiant Betelgeuse. A time-series of six circularly polarized spectra was obtained using the NARVAL spectropolarimeter at Télescope Bernard Lyot (Pic du Midi Observatory (F)), between March and April 2010. Zeeman signatures were repeatedly detected in cross-correlation profiles, corresponding to a longitudinal component of about 1 G. The time-series unveils a smooth increase of the longitudinal field from 0.5 to 1.5 G, correlated with radial velocity fluctuations. We observe a strong asymmetry of Stokes V signatures, also varying in correlation with the radial velocity. The Stokes V line profiles are red-shifted by about 9 km s-1 with respect to the Stokes I profiles, suggesting that the observed magnetic elements may be concentrated in the sinking components of the convective flows. Title: The Power Spectrum of the Solar Surface Flows from Hinode Data and First Observations with MOF/CALAS Pic-du-Midi Authors: Roudier, T.; Rieutord, M.; Rincon, F.; Malherbe, J. -M.; Brito, D.; Berger, T.; Frank, Z.; Parés, L.; Bourrec, E.; Beigbeder, F. Bibcode: 2012ASPC..454...47R Altcode: Many features of solar surface turbulence, like the supergranulation, are still poorly understood.We use long time series of images taken by the Solar Optical Telescope on board the Hinode satellite to determine the velocity fields. The dynamics in the subgranulation range can be investigated with unprecedented precision thanks to the absence of seeing effects and the use of the MTF of SOT for correcting the spectra. The first MOF/CALAS/Pic-du-Midi results are also shown. Title: 14 Ceti: a probable Ap-star-descendant entering the Hertzsprung gap Authors: Aurière, M.; Konstantinova-Antova, R.; Petit, P.; Charbonnel, C.; Van Eck, S.; Donati, J. -F.; Lignières, F.; Roudier, T. Bibcode: 2012A&A...543A.118A Altcode: 2012arXiv1205.6962A Context. 14 Ceti is a subgiant star of F spectral class that displays variations in the S-index of its Ca ii H & K lines and an X-ray emission that is stronger than the mean observed for its spectral class, which may be due to some magnetic activity.
Aims: We attempt to Zeeman-detect and study the magnetic field of 14 Ceti and to infer its origin.
Methods: We used the spectropolarimeter Narval at the Telescope Bernard Lyot, Pic du Midi Observatory, and the least squares deconvolution method to create high signal-to-noise ratio Stokes V profiles. We derived the surface-averaged longitudinal magnetic field Bl. We also measured the S-index, and the radial velocity for each observation.
Results: 14 Ceti is Zeeman-detected for the 30 observed dates spanning from August 2007 to January 2012. The average longitudinal magnetic field does not reverse its sign, reaches about -35 G, and shows some month-long-timescale variations in our 2008 and 2011-2012 observations. The S-index follows the same long-term trend as Bl. 14 Ceti is confirmed as a single star without H-K emission cores. The strength of the observed surface magnetic field of 14 Ceti is one order of magnitude greater than the observed one for late F main-sequence stars, and is comparable to the values measured in the active late F pre-main-sequence star HR 1817. On the other hand, taking into account the post-main-sequence evolution of an Ap star, an oblique rotator model can explain the strength of the magnetic field of 14 Ceti. The variations with a timescale of months observed for both the Bl and S-index could be due to the rotation.
Conclusions: The most probable scenario to explain our observations appears to be that 14 Ceti is the descendant of a cool Ap star.

Based on data obtained using the Télescope Bernard Lyot at Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse, France. Title: Acoustic Events in the Solar Atmosphere from Hinode/SOT NFI Observations Authors: Malherbe, J. -M.; Roudier, T.; Rieutord, M.; Berger, T.; Franck, Z. Bibcode: 2012SoPh..278..241M Altcode: 2012arXiv1207.1170M We investigate the properties of acoustic events (AEs), defined as spatially concentrated and short duration energy flux, in the quiet Sun, using observations of a 2D field of view (FOV) with high spatial and temporal resolution provided by the Solar Optical Telescope (SOT) onboard Hinode. Line profiles of Fe I 557.6 nm were recorded by the Narrow-band Filter Imager (NFI) on a 82″×82″ FOV during 75 min with a time step of 28.75 s and 0.08″ pixel size. Vertical velocities were computed at three atmospheric levels (80, 130, and 180 km) using the bisector technique, allowing the determination of energy flux to be made in the range 3 - 10 mHz using two complementary methods (Hilbert transform and Fourier power spectrum). Horizontal velocities were computed using local correlation tracking (LCT) of continuum intensities providing divergences. We found that the net energy flux is upward. In the range 3 - 10 mHz, a full FOV space and time averaged flux of 2700 W m−2 (lower layer 80 - 130 km) and 2000 W m−2 (upper layer 130 - 180 km) is concentrated in less than 1 % of the solar surface in the form of narrow (0.3″) AE. Their total duration (including rise and decay) is of the order of 103 s. Inside each AE, the mean flux is 1.6×105 W m−2 (lower layer) and 1.2×105 W m−2 (upper). Each event carries an average energy (flux integrated over space and time) of 2.5×1019 J (lower layer) to 1.9×1019 J (upper). More than 106 events could exist permanently on the Sun, with a birth and decay rate of 3500 s−1. Most events occur in intergranular lanes, downward velocity regions, and areas of converging motions. Title: MHD simulations of the solar photosphere Authors: Rieutord, M.; Rincon, F.; Roudier, T. Bibcode: 2012EAS....55....5R Altcode: 2012arXiv1206.6630R We briefly review the observations of the solar photosphere and pinpoint some open questions related to the magnetohydrodynamics of this layer of the Sun. We then discuss the current modelling efforts, addressing among other problems, that of the origin of supergranulation. Title: Some Dynamic Analysis of the Photosphere from Hinode/SOT and SDO/HMI Observations Authors: Roudier, T.; Malherbe, J.; Rieutord, M.; Berger, T.; Frank, Z.; Prat, V.; Renon, N.; Gizon, L.; Svanda, M. Bibcode: 2012ASPC..456...65R Altcode: We first present the important role played by the families of granule (or Tree of Fragmenting granules) in the formation of the photospheric network. Then, we describe the occurence and characteristics of acoustic events (AE), defined as spatially concentrated energy flux, in the quiet Sun. Finally, we present how horizontal velocities obtained from SDO/HMI data are calibrated by using Hinode/SOT observations. Title: EK Eri: Magnetic Topology and Rotational Period Authors: Aurière, M.; Konstantinova-Antova, R.; Petit, P.; Roudier, T. Bibcode: 2011ASPC..448.1065A Altcode: 2011csss...16.1065A We have observed during 3 seasons the slow rotating but active G8 giant EK Eri with the new generation spectropolarimeter NARVAL at Telescope Bernard Lyot (Observatoire du Pic du Midi, France). We got 21 observations between 20 September 2007 and 22 March 2010. For all the spectra, we detected a significant Zeeman Stokes V signature and the corresponding surface-averaged longitudinal magnetic field varies between -13 G and -98 G, without reversing its sign. We fitted the Stokes V profiles with a model of large-scale magnetic field and obtained a Zeeman Doppler image of it. Our best fitted model gives a rotational period of about 300 days. A strong magnetic spot dominates the visible surface magnetic field and is phased with the photometric spot given by the ephemeris of Dall et al. (2010). Whereas a quadripolar component dominates for i greater than 80°, for i= 60°, we get a model almost purely dipolar. In the dipolar model, the strong magnetic/photometric spot would correspond to the pole of negative polarity of the dipole, which could be the remnant of that of the Ap star progenitor of EK Eri. Title: Photospheric motions from an observational point of view: HINODE and SDO satellite observations Authors: Roudier, T. Bibcode: 2011sf2a.conf..241R Altcode: Thanks to the Solar Optical Telescope onboard HINODE, we mainly aim at the understanding of temporal evolution and spatial characterization of solar granular and supergranular features and their relation with magnetic field. We study the photospheric motions and the network magnetic field interactions using the Tree of Fragmenting Granules (TFGs). The mutual interactions of the TFGs tend to expulse corks outside of them on 3h-4h and concentrate the corks on supergranular scale in approximatively 6h-8h to form the magnetic network. Then, the long living families contribute to form the magnetic network. The measurement of the horizontal velocities by the proper motion of granules over the full Sun with SDO data, allow us to determine, for the first time, the solar differential rotation directly at the central meridian. During the starting phase of the solar cycle 24, the solar differential rotation seems to show some modification which must be confirmed. Title: A dominant magnetic dipole for the evolved Ap star candidate EK Eridani Authors: Aurière, M.; Konstantinova-Antova, R.; Petit, P.; Roudier, T.; Donati, J. -F.; Charbonnel, C.; Dintrans, B.; Lignières, F.; Wade, G. A.; Morgenthaler, A.; Tsvetkova, S. Bibcode: 2011A&A...534A.139A Altcode: 2011arXiv1109.5570A Context. EK Eri is one of the most slowly rotating active giants known, and has been proposed to be the descendant of a strongly magnetic Ap star.
Aims: We have performed a spectropolarimetric study of EK Eri over 4 photometric periods with the aim of inferring the topology of its magnetic field.
Methods: We used the NARVAL spectropolarimeter at the Bernard Lyot telescope at the Pic du Midi Observatory, along with the least-squares deconvolution method, to extract high signal-to-noise ratio Stokes V profiles from a timeseries of 28 polarisation spectra. We have derived the surface-averaged longitudinal magnetic field Bℓ. We fit the Stokes V profiles with a model of the large-scale magnetic field and obtained Zeeman Doppler images of the surface magnetic strength and geometry. We studied the classical activity indicators, the Ca ii H and K lines, the Ca ii infrared triplet, and Hα line, as well as the stellar radial velocity.
Results: Bℓ variations of up to about 80 G are observed without any reversal of its sign, and which are in phase with photometric ephemeris. The activity indicators are shown to vary smoothly on a timescale compatible with the rotational period inferred from photometry (308.8 d), however large deviations can occur from one rotation to another. The surface magnetic field variations of EK Eri appear to be dominated by a strong magnetic spot (of negative polarity) which is phased with the dark (cool) photometric spot. Our modeling shows that the large-scale magnetic field of EK Eri is strongly poloidal. For a rotational axis inclination of i = 60°, we obtain a model that is almost purely dipolar.
Conclusions: In the dipolar model, the strong magnetic/photometric spot corresponds to the negative pole of the dipole, which could be the remnant of that of an Ap star progenitor of EK Eri. Our observations and modeling conceptually support this hypothesis, suggesting an explanation of the outstanding magnetic properties of EK Eri as the result of interaction between deep convection and the remnant of an Ap star magnetic dipole. Nevertheless, the longitudinal magnetic field curve clearly shows changes from one rotation to the next, indicating that the surface magnetic topology is not static as in an Ap star.

Based on data obtained using the Télescope Bernard Lyot at Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse, France. Title: Multiwavelength Investigations of Magnetic Bright Points Authors: Kühner, O.; Utz, D.; Hanslmeier, A.; Veronig, A.; Roudier, T.; Muller, R.; Muthsam, H. Bibcode: 2011CEAB...35...29K Altcode: In this contribution we present our results regarding the study of small scale magnetic fields as seen by magnetic bright points (MBPs) in different wavelengths and hence different heights. By the determination of the size distribution of these features we are able to derive the value of the scale height parameter for the photosphere: 107 km ± 18.5 km. For the Fe I line at 630.25 nm we derived a formation height of 225 km. Title: No detection of large-scale magnetic fields at the surfaces of Am and HgMn stars Authors: Aurière, M.; Wade, G. A.; Lignières, F.; Hui-Bon-Hoa, A.; Landstreet, J. D.; Iliev, I. Kh.; Donati, J. -F.; Petit, P.; Roudier, T.; Théado, S. Bibcode: 2010A&A...523A..40A Altcode: 2010arXiv1008.3086A
Aims: We investigate the magnetic dichotomy between Ap/Bp and other A-type stars by carrying out a deep spectropolarimetric study of Am and HgMn stars.
Methods: Using the NARVAL spectropolarimeter at the Télescope Bernard Lyot (Observatoire du Pic du Midi, France), we obtained high-resolution circular polarisation spectroscopy of 12 Am stars and 3 HgMn stars.
Results: Using least squares deconvolution (LSD), no magnetic field is detected in any of the 15 observed stars. Uncertaintiies as low as 0.3 G (respectively 1 G) have been reached for surface-averaged longitudinal magnetic field measurements for Am (respectively HgMn) stars.
Conclusions: Associated with the results obtained previously for Ap/Bp stars, our study confirms the existence of a magnetic dichotomy among A-type stars. Our data demonstrate that there is at least one order of magnitude difference in field strength between Zeeman detected stars (Ap/Bp stars) and non Zeeman detected stars (Am and HgMn stars). This result confirms that the spectroscopically-defined Ap/Bp stars are the only A-type stars harbouring detectable large-scale surface magnetic fields.

Based on data obtained using the Télescope Bernard Lyot at Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse, France. Title: The magnetic field of Betelgeuse: a local dynamo from giant convection cells? Authors: Aurière, M.; Donati, J. -F.; Konstantinova-Antova, R.; Perrin, G.; Petit, P.; Roudier, T. Bibcode: 2010A&A...516L...2A Altcode: 2010arXiv1005.4845A Context. Betelgeuse is an M supergiant with a complex and extended atmosphere, which also harbors spots and giant granules at its surface. A possible magnetic field could contribute to the mass loss and to the heating of the outer atmosphere.
Aims: We observed Betelgeuse, to directly study and infer the nature of its magnetic field.
Methods: We used the new-generation spectropolarimeter NARVAL and the least square deconvolution (LSD) method to detect circular polarization within the photospheric absorption lines of Betelgeuse.
Results: We have unambiguously detected a weak Stokes V signal in the spectral lines of Betelgeuse, and measured the related surface-averaged longitudinal magnetic field B_ℓ at 6 different epochs over one month. The detected longitudinal field is about one Gauss and is apparently increasing on the time scale of our observations.
Conclusions: This work presents the first direct detection of the magnetic field of Betelgeuse. This magnetic field may be associated to the giant convection cells that could enable a “local dynamo”.

Based on observations obtained at the Télescope Bernard Lyot (TBL) at Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse, France. Title: On the power spectrum of solar surface flows Authors: Rieutord, M.; Roudier, T.; Rincon, F.; Malherbe, J. -M.; Meunier, N.; Berger, T.; Frank, Z. Bibcode: 2010A&A...512A...4R Altcode: 2009arXiv0911.3319R Context. The surface of the Sun provides us with a unique and very detailed view of turbulent stellar convection. Studying its dynamics can therefore help us make significant progress in stellar convection modelling. Many features of solar surface turbulence like the supergranulation are still poorly understood.
Aims: The aim of this work is to give new observational constraints on these flows by determining the horizontal scale dependence of the velocity and intensity fields, as represented by their power spectra, and to offer some theoretical guidelines to interpret these spectra.
Methods: We use long time-series of images taken by the Solar Optical Telescope (SOT) on board the Hinode satellite; we reconstruct both horizontal (by granule tracking) and vertical (by Doppler effect) velocity fields in a field-of-view of ~ 75 × 75 Mm2. The dynamics in the subgranulation range can be investigated with unprecedented precision thanks to the absence of seeing effects and the use of the modulation transfer function of SOT for correcting the spectra.
Results: At small subgranulation scales down to 0.4 Mm the spectral density of kinetic energy associated with vertical motions exhibits a k-10/3-like power law, while the intensity fluctuation spectrum follows either a k-17/3 or a k-3-like power law at the two continuum levels investigated (525 and 450 nm respectively). We discuss the possible physical origin of these scalings and interpret the combined presence of k-17/3 and k-10/3 power laws for the intensity and vertical velocity as a signature of buoyancy-driven turbulent dynamics in a strongly thermally diffusive regime. In the mesogranulation range and up to a scale of 25 Mm, we find that the amplitude of the vertical velocity field decreases like λ-3/2 with the horizontal scale λ. This behaviour corresponds to a k2 spectral power law. Still in the 2.5-10 Mm mesoscale range, we find that intensity fluctuations in the blue continuum also follow a k2 power law. In passing we show that granule tracking cannot sample scales below 2.5 Mm. We finally further confirm the presence of a significant supergranulation energy peak at 30 Mm in the horizontal velocity power spectrum and show that the emergence of a pore erases this spectral peak. We tentatively estimate the scale height of the vertical velocity field in the supergranulation range and find 1 Mm; this value suggests that supergranulation flows are shallow. Title: Surface structure of the CoRoT CP2 target star HD 50773 Authors: Lüftinger, T.; Fröhlich, H. -E.; Weiss, W. W.; Petit, P.; Aurière, M.; Nesvacil, N.; Gruberbauer, M.; Shulyak, D.; Alecian, E.; Baglin, A.; Baudin, F.; Catala, C.; Donati, J. -F.; Kochukhov, O.; Michel, E.; Piskunov, N.; Roudier, T.; Samadi, R. Bibcode: 2010A&A...509A..43L Altcode: 2009arXiv0911.3791L
Aims: We compare surface maps of the chemically peculiar star HD 50773 produced with a Bayesian technique and based on high quality CoRoT photometry with those derived from rotation phase resolved spectropolarimetry. The goal is to investigate the correlation of surface brightness with surface chemical abundance distribution and the stellar magnetic surface field.
Methods: The rotational period of the star was determined from a nearly 60 days long continuous light curve obtained during the initial run of CoRoT. Using a Bayesian approach to star-spot modelling, which in this work is applied for the first time for the photometric mapping of a CP star, we derived longitudes, latitudes and radii of four different spot areas. Additional parameters like stellar inclination and the spot's intensities were also determined. The CoRoT observations triggered an extensive ground-based spectroscopic and spectropolarimetric observing campaign and enabled us to obtain 19 different high resolution spectra in Stokes parameters I and V with NARVAL, ESPaDOnS, and SemelPol spectropolarimeters. Doppler and Magnetic Doppler imaging techniques allowed us to derive the magnetic field geometry of the star and the surface abundance distributions of Mg, Si, Ca, Ti, Cr, Fe, Ni, Y, and Cu.
Results: We find a dominant dipolar structure of the surface magnetic field. The CoRoT light curve variations and abundances of most elements mapped are correlated with the aforementioned geometry: Cr, Fe, and Si are enhanced around the magnetic poles and coincide with the bright regions on the surface of HD 50773 as predicted by our light curve synthesis and confirmed by photometric imaging.

The CoRoT space mission was developed and is operated by the French space agency CNES, with participation of ESA's RSSD and Science Pograms, Austria, Belgium, Brazil, Germany, and Spain. Title: Multiwavelength alignment of Hinode/SOT Data Authors: Kuehner, O.; Utz, D.; Hanslmeier, A.; Veronig, A.; Roudier, T.; Muller, R.; Muthsam, H. Bibcode: 2010CEAB...34...31K Altcode: First results regarding the spatial alignment of observations taken at different wavelengths are presented. An exceptionally long time series (48 hours) of data, obtained by the Solar Optical Telescope (SOT) of the Hinode satellite, has been analysed. Hinode delivers (among other data) high resolution seeing free filtergrams in the magnetic sensitive G-band, blue continuum, chromospheric Ca II H line and Fe I line. For the study of the position and dynamics of small scale fields (MBPs; Magnetic Bright Points) at different wavelengths (at different heights in the photosphere and chromosphere) the alignment of these data at pixel and subpixel level will be essential. Results concerning the height dependence of MBPs will also be of vital importance for questions regarding the coronal heating. Title: Hinode - Synoptic observations of convection dynamics Authors: Hanslmeier, A.; Muller, R.; Utz, D.; Roudier, T. Bibcode: 2010CEAB...34...81H Altcode: The variation of solar convection over the solar activity cycle is still discussed with controversial results. In this paper we study the solar granulation and its variation over the unusual long lasting solar minimum between cycle 23 and 24. Spatially highly resolved Hinode images were segmented and the mean value of the segmented granules as well as their number was found to be practically constant. Title: Brightness profiles and size distributions of MBPs observed in different heights by HINODE/SOT data Authors: Kühner, Otmar; Veronig, Astrid; Utz, Dominik; Hanslmeier, Arnold; Muthsam, Herbert; Muller, Richard; Roudier, Thierry Bibcode: 2010cosp...38.2948K Altcode: 2010cosp.meet.2948K We study the characteristics of Magnetic Bright Points (MBPs) observed at different wave-lenghts and hence different heights of the photosphere and chromosphere. The data sets were obtained with the Solar Optical Telescope (SOT) of the Hinode satellite. Hinode delivers (among other data) high resolution seeing free filtergrams in the blue continuum, the Fe I line, the magnetic sensitive G-band and the chromospheric Ca II H line. Due to the small scale structure of MBPs it was essential for our study to develop an algorithm for the image co-alignment at subpixel level for the images taken at these four wavelenghts. We studied an exceptionally long time series (48h) and analyzed the brightness profiles and size distributions of MBPs at different heights. The mean size of the features increases with increasing height and shows an exponential behavior. We obtained the scale height parameter of the photosphere to be 110 km. Title: Supergranulation, Network Formation, and TFGs Evolution from Hinode Observations Authors: Roudier, T.; Rincon, F.; Rieutord, M.; Brito, D.; Beigbeder, F.; Parès, L.; Malherbe, J. -M.; Meunier, N.; Berger, T.; Frank, Z. Bibcode: 2009ASPC..415..203R Altcode: In this paper, we analyse a a 48h high-resolution time sequence of the quiet Sun photosphere obtained with the Solar Optical Telescope onboard Hinode. Using floating corks advected by velocity fields inferred from photometry measurements, we show that long-living Trees of Fragmenting Granules play a crucial role in the advection of small-scale magnetic fields and in the build-up of the magnetic network. Title: Discovery of a weak magnetic field in the photosphere of the single giant Pollux Authors: Aurière, M.; Wade, G. A.; Konstantinova-Antova, R.; Charbonnel, C.; Catala, C.; Weiss, W. W.; Roudier, T.; Petit, P.; Donati, J. -F.; Alecian, E.; Cabanac, R.; van Eck, S.; Folsom, C. P.; Power, J. Bibcode: 2009A&A...504..231A Altcode: 2009arXiv0907.1423A Aims: We observe the nearby, weakly-active single giant, Pollux, in order to directly study and infer the nature of its magnetic field.
Methods: We used the new generation spectropolarimeters ESPaDOnS and NARVAL to observe and detect circular polarization within the photospheric absorption lines of Pollux. Our observations span 18 months from 2007-2009. We treated the spectropolarimetric data using the Least-Squares Deconvolution method to create high signal-to-noise ratio mean Stokes V profiles. We also measured the classical activity indicator S-index for the Ca ii H and K lines, and the stellar radial velocity (RV).
Results: We have unambiguously detected a weak Stokes V signal in the spectral lines of Pollux, and measured the related surface-averaged longitudinal magnetic field B_l. The longitudinal field averaged over the span of the observations is below one gauss. Our data suggest variations of the longitudinal magnetic field, but no significant variation of the S-index. We observe variations of RV which are qualitatively consistent with the published ephemeris for a proposed exoplanet orbiting Pollux. The observed variations of Bl appear to mimic those of RV, but additional data for this relationship to be established. Using evolutionary models including the effects of rotation, we derive the mass of Pollux and we discuss its evolutionary status and the origin of its magnetic field.
Conclusions: This work presents the first direct detection of the magnetic field of Pollux, and demonstrates that ESPaDOnS and NARVAL are capable of obtaining sub-G measurements of the surface-averaged longitudinal magnetic field of giant stars, and of directly studying the relationships between magnetic activity, stellar evolution and planet hosting of these stars.

Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, CNRS/INSU and the University of Hawaii, and the Télescope Bernard Lyot (TBL) at Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse, France. Title: Direct detection of a magnetic field on the surface of slowly rotating giant stars Authors: Aurière, Michel; Konstantinova-Antova, R.; Petit, P.; Wade, G.; Roudier, T. Bibcode: 2009IAUS..259..431A Altcode: We present first results of the magnetic survey of a sample of slow rotating giant stars for which an X-ray emission or variations of CaII H & K lines have been already detected. Title: Development of large and fast cmos aps cameras at latt Authors: Beigbeder, F.; Bourrec, E.; Dupieux, M.; Delaigue, G.; Rondi, S.; Rieutord, M.; Meunier, N.; Roudier, T. Bibcode: 2009EAS....37..301B Altcode: Since 2004, at the Laboratoire d'Astrophysique de Toulouse Tarbes (LATT), we work with CMOS APS detectors, firstly to develop a large-field,high-resolution camera for the observation of the solar supergranulation, secondly to develop a fast camera for an adaptive optics test bench. In these two projects, we use detectors from FillFactory, now continued by Cypress Semiconductor Corporation: IBIS4-14000, IBIS-16000, LUPA-4000, 14 Mpixels, 16 Mpixels, 4 Mpixels respectively. The last one just reads in a 240 × 240 pixels window to obtain readout rate of 1000 Image/s. For these purposes we developed dedicated controllers to follow the high pixel rate and multi-output readout of this type of detectors. We also studied the characterization methods and measured the main parameters of these CMOS detectors to know their behaviour. Using these kinds of APS detectors in these two particular projects proves that we can already find niches to use CMOS detectors in astronomy taking advantage of their present specificities. Recent improvements like back illumination, noise reduction, should rapidly open news possibilities. Title: EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars Authors: Aurière, M.; Konstantinova-Antova, R.; Petit, P.; Charbonnel, C.; Dintrans, B.; Lignières, F.; Roudier, T.; Alecian, E.; Donati, J. F.; Landstreet, J. D.; Wade, G. A. Bibcode: 2008A&A...491..499A Altcode: 2008arXiv0810.2228A Aims: We observe the slowly-rotating, active, single giant, EK Eri, to study and infer the nature of its magnetic field directly.
Methods: We used the spectropolarimeter NARVAL at the Telescope Bernard Lyot, Pic du Midi Observatory, and the Least Square Deconvolution method to create high signal-to-noise ratio Stokes V profiles. We fitted the Stokes V profiles with a model of the large-scale magnetic field. We studied the classical activity indicators, the Ca ii H and K lines, the Ca ii infrared triplet, and Hα line.
Results: We detected the Stokes V signal of EK Eri securely and measured the longitudinal magnetic field Bl for seven individual dates spanning 60% of the rotational period. The measured longitudinal magnetic field of EK Eri reached about 100 G and was as strong as fields observed in RSCVn or FK Com type stars: this was found to be extraordinary when compared with the weak fields observed at the surfaces of slowly-rotating MS stars or any single red giant previously observed with NARVAL. From our modeling, we infer that the mean surface magnetic field is about 270 G, and that the large scale magnetic field is dominated by a poloidal component. This is compatible with expectations for the descendant of a strongly magnetic Ap star.

Based on data obtained using the Télescope Bernard Lyot at Observatoire du Pic du Midi, CNRS/INSU and Université de Toulouse, France. Title: Supergranules over the solar cycle Authors: Meunier, N.; Roudier, T.; Rieutord, M. Bibcode: 2008A&A...488.1109M Altcode: Context: The origin of supergranulation has not been understood yet. Contradictory results have been obtained in the past concerning the relation between supergranule properties (mostly cell size) and the solar cycle.
Aims: We propose to study the variation in supergranule cell sizes and velocity fields over the solar cycle, as well as the intensity variation inside supergranules.
Methods: We define supergranule cells from maps of horizontal velocity field divergences. The flow fields are derived from MDI/SOHO intensity maps. The intensity variation in supergranules are compared to the variation inside granules resulting from a numerical simulation. The variation in these profiles with the cell size and over the solar cycle is also analysed.
Results: We find that cell sizes are smaller on average at cycle maximum. We also find that the slope between Log (V_rms) and Log (R) is weakly correlated with the spot number (i.e. the global activity level) but anti-correlated with the local magnetic field. We also confirm the decrease in the intensity variation from cell centre to the boundary, which puts a lower limit on the temperature variation of 0.57 ± 0.06 K. This temperature difference is of 1.03 ± 0.05 K when considering the areas of strongest divergence and strongest convergence. We observe a strong similarity in the intensity variation inside supergranules and granules. A significant variation with the cell size is observed, also similar to that in granules, but the variation over the solar cycle is not significant.
Conclusions: The sign of the variation in supergranule cell sizes over the solar cycle is in agreement with what can be expected from the influence of the magnetic field. The observed intensity variations show that a common process could be the origin of both granules and supergranules. Title: Dynamics of the Solar Surface Reavealed by Granule Tracking Authors: Rieutord, M.; Roudier, T. Bibcode: 2008ESPM...12..2.9R Altcode: We present the recent results obtained by the Toulouse-Tarbes group on the dynamics of the solar surface at meso and supergranular scale, and their interactions with the magnetic field.

First, using the 14-Mpixel CALAS camera at the Pic-du-Midi observatory, we obtained a 7.5h-long sequence of high resolution images with unprecedented field size (524"x350"). Tracking granules, we have determined the velocity field at the Sun's surface in great detail from a scale of 2.5 Mm up to 250 Mm. The kinetic energy density spectrum shows that supergranulation peaks at 36Mm and spans on scales ranging between 20Mm and 75Mm. The decrease of supergranular flows in the small scales is close to a 1/k^2-power law, steeper than the equipartition Kolmogorov one. The probability distribution function of the divergence field shows the signature of intermittency of the supergranulation and thus its turbulent nature.

Second, using a 48hours-long sequence of white light images from the HINODE satellite, we determined the evolution of granule families, which give birth to correlated motion at mesogranular scale. Using passive cork and magnetic field observations, we show that the surface velocity field via family organization is sufficient to explain the magnetic field concentrations at the Sun's surface, namely the magnetic network. We also find robust scaling laws for the life time of granule families. Title: Weak magnetic fields in CP stars Authors: Aurière, M.; Wade, G. A.; Lignières, F.; Landstreet, J. D.; Donati, J. -F.; Hui Bon Hoa, A.; Iliev, I.; Petit, P.; Roudier, T.; Silvester, J.; Theado, S. Bibcode: 2008CoSka..38..211A Altcode: We present the first results of a magnetic survey of main sequence A and late B stars with NARVAL, the new generation spectropolarimeter available at Telescope Bernard Lyot at Pic du Midi Observatory. We observed 3 HgMn stars, 12 Am stars and the only Ap star for which no Zeeman detection was obtained with MuSiCoS. If we combine our results with those of our previous survey of weak magnetic fields in spectroscopically selected Ap/Bp stars we get the following situation:

- Ap/Bp stars: all these stars appear to host a detectable magnetic field. A threshold dipolar magnetic field of 300 G appears to exist, below which fields are very rare and perhaps altogether absent.

- Am and HgMn stars: no magnetic field is detected up to now for the 15 observed stars. An accuracy down to 0.3 G (respectively 1 G) is reached on longitudinal magnetic field measurements for Am (respectively HgMn) stars. We propose a possible interpretation of the observed threshold field which naturally explains the magnetic dichotomy: there exists a critical field strength above which stable magnetic configurations exist and below which any large scale field configuration is destroyed by some instability. Title: Topological Departures from Translational Invariance along a Filament Observed by THEMIS Authors: Dudík, J.; Aulanier, G.; Schmieder, B.; Bommier, V.; Roudier, T. Bibcode: 2008SoPh..248...29D Altcode: 2008SoPh..tmp...53D We study the topology of the 3D magnetic field in a filament channel to address the following questions: Is a filament always formed in a single flux tube? How does the photospheric magnetic field lead to filament interruptions and to feet formation? What is the relation between feet-related field lines and the parasitic polarities? What can topological analyses teach us about EUV filament channels? To do so, we consider a filament observed on 6 October 2004 with THEMIS/MTR, in Hα with the full line profile simultaneously and cospatially with its photospheric vector magnetic field. The coronal magnetic field was calculated from a "linear magnetohydrostatic" extrapolation of a composite THEMIS-MDI magnetogram. Its free parameters were adjusted to get the best match possible between the distribution of modeled plasma-supporting dips and the Hα filament morphology. The model results in moderate plasma β≤1 at low altitudes in the filament, in conjunction with non-negligible departures from force-freeness measured by various metrics. The filament here is formed by a split flux tube. One part of the flux tube is rooted in the photosphere aside an observed interruption in the filament. This splitted topology is due to strong network polarities on the edge of the filament channel, not to flux concentrations closer to the filament. We focus our study to the northwest portion of the filament. The related flux tube is highly fragmented at low altitudes. This fragmentation is due to small flux concentrations of two types. First, some locally distort the tube, leading to noticeable thickness variations along the filament body. Second, parasitic polarities, associated with filament feet, result in secondary dips above the related local inversion line. These dips belong to long field lines that pass below the flux tube. Many of these field lines are not rooted near the related foot. Finally, the present model shows that the coronal void interpretation cannot be ruled out to interpret the wideness of EUV filament channels. Title: Large-scale horizontal flows in the solar photosphere. III. Effects on filament destabilization Authors: Roudier, T.; Švanda, M.; Meunier, N.; Keil, S.; Rieutord, M.; Malherbe, J. M.; Rondi, S.; Molodij, G.; Bommier, V.; Schmieder, B. Bibcode: 2008A&A...480..255R Altcode: 2007arXiv0711.3112R Aims:We study the influence of large-scale photospheric motions on the destabilization of an eruptive filament, observed on October 6, 7, and 8, 2004, as part of an international observing campaign (JOP 178).
Methods: Large-scale horizontal flows were investigated from a series of MDI full-disc Dopplergrams and magnetograms. From the Dopplergrams, we tracked supergranular flow patterns using the local correlation tracking (LCT) technique. We used both LCT and manual tracking of isolated magnetic elements to obtain horizontal velocities from magnetograms.
Results: We find that the measured flow fields obtained by the different methods are well-correlated on large scales. The topology of the flow field changed significantly during the filament eruptive phase, suggesting a possible coupling between the surface flow field and the coronal magnetic field. We measured an increase in the shear below the point where the eruption starts and a decrease in shear after the eruption. We find a pattern in the large-scale horizontal flows at the solar surface that interact with differential rotation.
Conclusions: We conclude that there is probably a link between changes in surface flow and the disappearance of the eruptive filament. Title: Solar supergranulation revealed by granule tracking Authors: Rieutord, M.; Meunier, N.; Roudier, T.; Rondi, S.; Beigbeder, F.; Parès, L. Bibcode: 2008A&A...479L..17R Altcode: 2008arXiv0801.1369R Context: Supergranulation is a pattern of the velocity field at the surface of the Sun, which has been known about for more than fifty years, however, no satisfactory explanation of its origin has been proposed.
Aims: New observational constraints are therefore needed to guide theoretical approaches which hesitate between scenarios that either invoke a large-scale instability of the surface turbulent convection or a direct forcing by buoyancy.
Methods: Using the 14-Mpixel CALAS camera at the Pic-du-Midi observatory, we obtained a 7.5 h-long sequence of high resolution images with unprecedented field size. Tracking granules, we have determined the velocity field at the Sun's surface in great detail from a scale of 2.5 Mm up to 250 Mm.
Results: The kinetic energy density spectrum shows that supergranulation peaks at 36 Mm and spans on scales ranging between 20 Mm and 75 Mm. The decrease of supergranular flows in the small scales is close to a k-2-power law, steeper than the equipartition Kolmogorov one. The probability distribution function of the divergence field shows the signature of intermittency of the supergranulation and thus its turbulent nature.
Conclusions: Title: Properties of sunspot moats derived from horizontal motions Authors: Sobotka, M.; Roudier, T. Bibcode: 2007A&A...472..277S Altcode: Context: Sunspots in late phases of evolution are usually surrounded by annular moats, regions where systematic horizontal flows are observed to be directed radially away from the spot. These flows are considered to be a manifestation of the sub-photospheric convection.
Aims: The characteristics of moats are derived at two different heights in the solar atmosphere from horizontal motions around sunspots of different sizes, shapes, and phases of evolution. We also study the temporal evolution of moats.
Methods: Local correlation tracking is applied to approximately 70-min long time series of white-light and 1600 Å images, acquired by the satellite TRACE, to analyse the horizontal motions of photospheric granules and C IV emission structures in the vicinity of 32 sunspots. Moat regions are defined by means of radially-oriented, outward velocities.
Results: Relations between sunspot types and the occurrence, areas, and horizontal velocities of moats in the photosphere and transition region are described. Moats do not show substantial changes during the period of about 12 h. Observed asymmetries in moat shapes and velocities are related to the height in the atmosphere, to sunspot age, and to proper motion. It is suggested that the sub-photospheric convective flows around sunspots may be influenced by the spots' proper motion through the convection zone. Title: Tracking granules on the Sun's surface and reconstructing velocity fields. I. The CST algorithm Authors: Rieutord, M.; Roudier, T.; Roques, S.; Ducottet, C. Bibcode: 2007A&A...471..687R Altcode: 2007arXiv0707.1991R Aims:Determination of horizontal velocity fields on the solar surface is crucial for understanding the dynamics of structures like mesogranulation or supergranulation or simply the distribution of magnetic fields.
Methods: We pursue here the development of a method called CST for coherent structure tracking, which determines the horizontal motion of granules in the field of view.
Results: We first devise a generalization of Strous method for the segmentation of images and show that when segmentation follows the shape of granules more closely, granule tracking is less effective for large granules because of increased sensitivity to granule fragmentation. We then introduce the multi-resolution analysis on the velocity field, based on Daubechies wavelets, which provides a view of this field on different scales. An algorithm for computing the field derivatives, like the horizontal divergence and the vertical vorticity, is also devised. The effects from the lack of data or from terrestrial atmospheric distortion of the images are also briefly discussed. Title: Tracking granules on the Sun's surface and reconstructing velocity fields. II. Error analysis Authors: Tkaczuk, R.; Rieutord, M.; Meunier, N.; Roudier, T. Bibcode: 2007A&A...471..695T Altcode: 2007arXiv0707.1994T Context: The determination of horizontal velocity fields at the solar surface is crucial to understanding the dynamics and magnetism of the convection zone of the sun. These measurements can be done by tracking granules.
Aims: Tracking granules from ground-based observations, however, suffers from the Earth's atmospheric turbulence, which induces image distortion. The focus of this paper is to evaluate the influence of this noise on the maps of velocity fields.
Methods: We use the coherent structure tracking algorithm developed recently and apply it to two independent series of images that contain the same solar signal.
Results: We first show that a k-ω filtering of the times series of images is highly recommended as a pre-processing to decrease the noise, while, in contrast, using destretching should be avoided. We also demonstrate that the lifetime of granules has a strong influence on the error bars of velocities and that a threshold on the lifetime should be imposed to minimize errors. Finally, although solar flow patterns are easily recognizable and image quality is very good, it turns out that a time sampling of two images every 21 s is not frequent enough, since image distortion still pollutes velocity fields at a 30% level on the 2500 km scale, i.e. the scale on which granules start to behave like passive scalars.
Conclusions: The coherent structure tracking algorithm is a useful tool for noise control on the measurement of surface horizontal solar velocity fields when at least two independent series are available. Title: Are supergranule sizes anti-correlated with magnetic activity? Authors: Meunier, N.; Roudier, T.; Tkaczuk, R. Bibcode: 2007A&A...466.1123M Altcode: Context: The variation of supergranule cell sizes with the magnetic environment is still controversial.
Aims: We study this relation in detail to understand the discrepancies observed between previous results.
Methods: We determine the cell size using divergence of horizontal flows derived from local correlation tracking of intensity maps (MDI/SOHO). We study the variation of the cell size as a function of the magnetic field inside the cell. We also consider which component of the magnetic field most influences the cell size.
Results: Our main conclusion is that there are no large cells when the magnetic field (in absolute value) averaged over the cell is large. This is mostly due to the magnetic field inside the cell (intranetwork fields), while strong network magnetic fields (at the cell boundary) are associated with larger cells. Further studies of the evolution of the cells and of the flux imbalance suggest that a high level of weak fields may prevent the formation of large cells. This is compatible with the expectation that strong magnetic fields should prevent large-scale flows.
Conclusions: The relation between the local activity level determined by the average magnetic field inside the cells and the supergranule size is not linear. Furthermore, it strongly depends on the definition of the activity level (magnetic field inside the cell or magnetic network) and on the magnetic sensitivity of the data. This last point probably explains at least partially the conflicting results obtained up to now. Title: The superrotation of solar supergranules Authors: Meunier, N.; Roudier, T. Bibcode: 2007A&A...466..691M Altcode: Context: Supergranules have long been believed to rotate faster than plasma and than magnetic structures. However, it has recently been shown that this could be due to a bias appearing when using Dopplergrams, which are strongly affected by projection effets.
Aims: Our purpose is to perform new measurements of supergranule angular velocities using a technique that would not be significantly influenced by projection effets.
Methods: We are therefore tracking horizontal divergence maps smoothed on the supergranular scale, either globally or on the scale of cells. Tracking the magnetic field using the same technique allows them to be used as a reference. Doppler tracking is performed for a direct comparison.
Results: We confirm that the tracking of Doppler features is not reliable when computing the dynamics on the supergranular scale. However, we find that divergence features are still rotating faster than the magnetic field, when using two independent time series.
Conclusions: We conclude that the long-standing puzzle of supergranular superrotation persists, and that interpreting it in terms of anchorage depth inside the convective zone may not be sufficient to explain it. Title: Horizontal Motions in the Vicinity of Sunspots Authors: Sobotka, M.; Roudier, T. Bibcode: 2007ASPC..368..259S Altcode: Many sunspots in late phases of evolution are surrounded by an annular ``moat'', where systematic horizontal flows radially away from the sunspot are observed. Applying the local correlation tracking technique to series of white-light and 1600 Å images acquired by the satellite TRACE, we use photospheric granules and C IV emission structures to track horizontal motions in the vicinity of sunspots of different sizes, shapes, and phases of evolution. We define the moat regions on the basis of horizontal motions. We present preliminary results about the relations between the sunspot types and the occurrence, areas, and velocities of the moats in the photosphere and chromosphere. Further we analyze observed asymmetries in moat shapes and velocities and their relation to the sunspot age and proper motion. Title: Solar Spectro-Polarimetry at Pic-du-Midi/LJR Authors: Arnaud, J.; Roudier, T.; Malherbe, M.; Moity, J. Bibcode: 2006ASPC..358..167A Altcode: A modern polarization analysis package was installed in 2004 at the 50 cm Pic-du-Midi solar telescope (Lunette Jean Rösch-LJR) by one of us (JMM) to complement LJR instrumentation which already included a spectrograph built by Z. Mouradian (LESIA), an MSDP device built by P. Mein (LESIA), and one La Vision CCD camera bought by R. Muller (LATT) and installed by one of us (JMM). We describe hereafter characteristics of the telescope and of its instrumentation and present some of the first Zeeman and Hanle effects measurements obtained using LJR new polarimeter. Title: The DynaMICS perspective Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies, S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.; Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.; Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat, E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.; Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes, I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.; Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.; Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.; Thompson, M.; Vauclair, S.; Zahn, J. P. Bibcode: 2006ESASP.624E..24T Altcode: 2006soho...18E..24T No abstract at ADS Title: Scientific Objectives of the Novel Formation Flying Mission Aspiics Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies, S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.; Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.; Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat, E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.; Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes, I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.; Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.; Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.; Thompson, M.; Vauclair, S.; Zahn, J. P. Bibcode: 2006ESASP.617E.164L Altcode: 2006soho...17E.164L No abstract at ADS Title: The EUV Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO): Science Plan and Instrument Overview Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies, S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.; Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.; Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat, E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.; Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes, I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.; Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.; Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.; Thompson, M.; Vauclair, S.; Zahn, J. P. Bibcode: 2006ESASP.617E.165W Altcode: 2006soho...17E.165W No abstract at ADS Title: The Dynamics Project Authors: Turck-Chièze, S.; Schmutz, W.; Thuillier, G.; Jefferies, S.; Pallé; Dewitt, S.; Ballot, J.; Berthomieu, G.; Bonanno, A.; Brun, A. S.; Christensen-Dalsgaard, J.; Corbard, T.; Couvidat, S.; Darwich, A. M.; Dintrans, B.; Domingo, V.; Finsterle, W.; Fossat, E.; Garcia, R. A.; Gelly, B.; Gough, D.; Guzik, J.; Jiménez, A. J.; Jiménez-Reyes, S.; Kosovichev, A.; Lambert, P.; Lefebvre, S.; Lopes, I.; Martic, M.; Mathis, S.; Mathur, S.; Nghiem, P. A. P.; Piau, L.; Provost, J.; Rieutord, M.; Robillot, J. M.; Rogers, T.; Roudier, T.; Roxburgh, I.; Rozelot, J. P.; Straka, C.; Talon, S.; Théado, S.; Thompson, M.; Vauclair, S.; Zahn, J. P. Bibcode: 2006ESASP.617E.162T Altcode: 2006soho...17E.162T No abstract at ADS Title: Photospheric flows measured with TRACE II. Network formation Authors: Krijger, J. M.; Roudier, T. Bibcode: 2003A&A...403..715K Altcode: We analyse a 7 d (167 h) sequence of TRACE white-light images with 1 arcsec angular resolution taken at 1 min cadence. The TRACE resolution and the fast cadence allows us to produce maps of the horizontal flow fields with high angular (1 arcsec) and temporal resolution (5 min). The field of view of 128arcsec x 128arcsec (~93 Mm x 93 Mm) covers approximately an area of 10 to 30 supergranules. This area was followed during solar rotation. Magnetic flux was artificially inserted into the successive flow maps in the form of ephemeral regions with positive and negative polarity. The emergence rate of 2 x 1022 Mx h-1 with an average flux per region of about 1.1 x 1019 Mx produces a good reproduction of the chromospheric network as observed in images taken simultaneously at 1600 Å. In addition, we show that the quiet network can be maintained only if field elements of both polarities are inserted into the flow fields. Our analysis suggests that the network is fully replenished on a time scale of a day and the lifetimes of the magnetic elements are of a similar duration. Title: A simulation of solar convection at supergranulation scale Authors: Rieutord, M.; Ludwig, H. -G.; Roudier, T.; Nordlund, .; Stein, R. Bibcode: 2002NCimC..25..523R Altcode: 2001astro.ph.10208R We present here numerical simulations of surface solar convection which cover a box of 30$\times30\times$3.2 Mm$^3$ with a resolution of 315$\times315\times$82, which is used to investigate the dynamics of scales larger than granulation. No structure resembling supergranulation is present; possibly higher Reynolds numbers (i.e. higher numerical resolution), or magnetic fields, or greater depth are necessary. The results also show interesting aspects of granular dynamics which are briefly presented, like extensive p-mode ridges in the k-$\omega$ diagram and a ringlike distribution of horizontal vorticity around granules. At large scales, the horizontal velocity is much larger than the vertical velocity and the vertical motion is dominated by p-mode oscillations. Title: Photospheric flows measured with TRACE Authors: Krijger, J. M.; Roudier, T.; Rieutord, M. Bibcode: 2002A&A...387..672K Altcode: We analyse white-light image sequences taken with the Transition Region and Coronal Explorer (TRACE) using an optimised local correlation tracking (LCT) method to measure the horizontal flows in the quiet solar photosphere with high spatial (1 arcsec) and temporal (5 min) resolution. Simultaneously taken near-ultraviolet images from TRACE confirm that our LCT-determined flows recover the actual supergranulation pattern, thus proving that the topology of the horizontal flow distribution and network assembly may be studied from long-duration TRACE white-light sequences with our method. Title: Are granules good tracers of solar surface velocity fields? Authors: Rieutord, M.; Roudier, T.; Ludwig, H. -G.; Nordlund, Å.; Stein, R. Bibcode: 2001A&A...377L..14R Altcode: 2001astro.ph..8284R Using a numerical simulation of compressible convection with radiative transfer mimicking the solar photosphere, we compare the velocity field derived from granule motions to the actual velocity field of the plasma. We thus test the idea that granules may be used to trace large-scale velocity fields at the sun's surface. Our results show that this is indeed the case provided the scale separation is sufficient. We thus estimate that neither velocity fields at scales less than 2500 km nor time evolution at scales shorter than 0.5 hr can be faithfully described by granules. At larger scales the granular motions correlate linearly with the underlying fluid motions with a slope of ≲2 reaching correlation coefficients up to ~ 0.9. Title: On mesogranulation, network formation and supergranulation Authors: Rieutord, M.; Roudier, T.; Malherbe, J. M.; Rincon, F. Bibcode: 2000A&A...357.1063R Altcode: We present arguments which show that in all likelihood mesogranulation is not a true scale of solar convection but the combination of the effects of both highly energetic granules, which give birth to strong positive divergences (SPDs) among which we find exploders, and averaging effects of data processing. The important role played by SPDs in horizontal velocity fields appears in the spectra of these fields where the scale ~ 4 Mm is most energetic; we illustrate the effect of averaging with a one-dimensional toy model which shows how two independent non-moving (but evolving) structures can be transformed into a single moving structure when time and space resolution are degraded. The role of SPDs in the formation of the photospheric network is shown by computing the advection of floating corks by the granular flow. The coincidence of the network bright points distribution and that of the corks is remarkable. We conclude with the possibility that supergranulation is not a proper scale of convection but the result of a large-scale instability of the granular flow, which manifests itself through a correlation of the flows generated by SPDs. Title: Internal intermittency on the Sun surface? Authors: Roudier, T.; Rieutord, M.; Malherbe, J. M.; Vigneau, J. Bibcode: 1999joso.proc..197R Altcode: An improved version of LCT and FT is presented and used to determine high spatial and temporal resolution horizontal flow fields at the Sun's surface. The granule diplacements and group evolution can be followed. The horizontal flow fields calculated by these methods show a structured flow at mean scale (4 to 8 arcsec) by a group of granules which is probably related to the local intermittency. Title: Magnetic reconnection driven by emergence of sheared magnetic field. Authors: Schmieder, B.; Aulanier, G.; Demoulin, P.; van Driel-Gesztelyi, L.; Roudier, T.; Nitta, N.; Cauzzi, G. Bibcode: 1997A&A...325.1213S Altcode: Recurrent subflares (Class C) were observed in the NOAA 7608 active region on 27 October 1993. From multi-wavelength observations (white-light, magnetic field, H-alpha, X-ray), obtained during a coordinated campaign between Pic du Midi and Yohkoh, it appears that these flares were double ribbon flares caused by new flux emergence. As the flare begins, the X-ray emission observed with Yohkoh/SXT is loop-shaped with the axis almost parallel to the magnetic inversion line, while during the flare development, X-ray loops appear at the location of the emerging flux. The extrapolation of the photospheric magnetic field in a linear force-free field configuration allows identification of the magnetic configuration given by the flares. The Hα flare ribbons are located at the intersections of the computed quasi-separatrice layers (QSLs) with the chromosphere. We show that the initial loop-shaped X-ray emission region is in fact formed by several smaller loops directed in a nearly orthogonal direction with their feet anchored close to or in the Hα ribbons. During the flare development there are X-ray loops which represent only one foot of open or largescale magnetic loops. For the studied flares the puzzling soft X-rays observations could only be understood with the help of Hα and magnetic data combined with a modeling of the coronal magnetic field. Further, from the deduced magnetic field topology, the width of the QSLs and our present knowledge of 3-D magnetic reconnection, we conclude that the flare was due to magnetic reconnection driven by emergence of sheared magnetic field impacting in the pre-existing coronal field. Title: Intergranular plumes and formation of network bright points. Authors: Roudier, T.; Malherbe, J. M.; November, L.; Vigneau, J.; Coupinot, G.; Lafon, M.; Muller, R. Bibcode: 1997A&A...320..605R Altcode: We discuss the temporal evolution of the photospheric intergranular lanes using a 1 hour time sequence of white-light images of solar granulation. The time series was obtained with the 50 cm refractor at the turret dome of the Pic du Midi Observatory. Analysis reveals the existence of singularities in the intergranular lanes that we call ``intergranular holes''. Intergranular holes, which have diameters between 0.24arcsec and 0.45arcsec, are continuously visible for more than 45 minutes. The holes appear to be systematically distributed at the periphery of mesogranular and supergranular cells. Our study reveals the formation of bright points (BPs) in 4 out of 14 cases studied very close to the intergranular holes, suggesting that intergranular holes may be the locations where magnetic flux tubes are formed. Title: 3-D reconnection related to new emerging flux Authors: Schmeider, B.; Démoulin, P.; Aulanier, G.; Malherbe, J. M.; van Driel-Gesztelyi, L.; Mandrini, C. H.; Roudier, T.; Nitta, N.; Harra-Murnion, L. K. Bibcode: 1997AdSpR..19.1871S Altcode: We present evidences that emergence of new flux in the lower atmosphere leads to magnetic reconnection of field lines. In a first phase the phenomenon is observed in the chromosphere by the formation of dark filaments (arch filament system) which are overlaid by bright loops visible in soft X-rays. Different types of event appear according to the magnetic field configuration and the amount of energy involved. 3-D modelling of the photospheric magnetic field provides a new tool for understanding reconnection in real configurations. The observed chromospheric and coronal loops are good diagnostics for the modelling. We document our statement by examples obtained during coordinated campaigns with the Hα Multichannel Subtractive Double Pass spectrographs-MSDP (Pic du Midi and Tenerife) and the Yohkoh instruments. Title: Magnetic reconnection driven by an emerging flux. Authors: Aulanier, G.; Démoulin, P.; Schmieder, B.; Malherbe, J. M.; van Driel-Gesztelyi, L.; Roudier, T. Bibcode: 1997joso.proc...51A Altcode: No abstract at ADS Title: Spatial relation between the 5-minute oscillations and granulation patterns. Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, P.; Mein, N.; Malherbe, J. M. Bibcode: 1996A&A...313..297E Altcode: A white light, high resolution movie of the quiet photosphere has been used to study the spatial relation between the solar granulation and the 5-min oscillations of high degree mode (small spatial scale). The granular and oscillatory brightness components have been separated with the help of a specific filter, in the k-ω diagram. The surface of the Sun appears to be covered with adjacent oscillating cells of apparent size 2" (1500km) to 3" (2000km). Some cells are noticeable by their amplitude which is much larger than average. These strong oscillations usually appear in expanding intergranular spaces. Our observations suggest that the 5-min oscillations are stochastically generated by turbulent convection. However, the most energetic oscillations occur in downflows only, well separated of each other in time and space. Title: Post-Flare Loops of 26 June 1992, III Authors: Wiik, J. E.; Schmieder, B.; Heinzel, P.; Roudier, T. Bibcode: 1996SoPh..166...89W Altcode: Post-flare loops were observed on June 26, 1992 in the Hα line with the Multichannel Subtractive Double-Pass spectrograph (MSDP) on Pic-du-Midi and with the Swedish telescope on La Palma. The highly dynamic loops are inhomogeneous (blobs). The cool loops were observed 10-12 hours after the X 3.9 class flare which had a maximum on June 25 at 20∶11 UT. From 2D images obtained with the MSDP on June 26 we derive Hα intensities and Doppler velocities of the loop plasma. Using a geometrical reconstruction technique we show that these loops are mainly perpendicular to the solar surface and have the shape of a dipole magnetic configuration. We derive the bulk-flow velocities along the loop as a function of height using the Doppler velocities and the results from the loop reconstruction. Where the Doppler velocities are too small, we derive the bulk-flow velocities from the displacements of the falling blobs. We discuss existing deviations from free-fall velocity in the lower parts of the loops. Title: Signatures of New Emerging Flux in the Solar Atmosphere Authors: Schmieder, B.; Malherbe, J. M.; Mein, P.; Mein, N.; van Driel-Gesztelyi, L.; Roudier, T.; Nitta, N.; Harra-Murnion, L. K. Bibcode: 1996ASPC..111...43S Altcode: 1997ASPC..111...43S The emergence of new flux in the low atmosphere leads to magnetic reconnection of field lines. In a stable phase the phenomenon is observed in the chromosphere by the formation of dark filaments (arch filament system, AFS). The authors show how bright loops visible in soft X-rays are co-aligned with the AFS. Different types of events appear as the released energy increases. With less energetic phenomena than flares one observes surges, jets or X-ray bright points, according to the configuration of the field lines (open/closed). A low-level reconnection process is detectable as an X-ray bright point. If the energy is ≡1028ergs cm-3, one observes subflares. The authors document their statement by showing examples observed in coordinated observations obtained with the MSDP (Pic du Midi and Tenerife) and Yohkoh/SXT and BCS for the events occurring on Oct 5, 1994, Oct 27, 1993, and May 1, 1993. Title: Penetration of the solar granulation into the photosphere: height dependence of intensity and velocity fluctuations. Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N.; Mein, P. Bibcode: 1995A&AS..109...79E Altcode: A 16-min time series of two-dimensional "Multichanel Subtractive Double Pass" spectrograms, recorded in the NaD_2_ 5690 line, has been used to study the vertical structure of the photosphere and, in particular, the penetration of granules. A statistical and a morphological investigations have been made. The 5-min oscillations have been carefully filtered in the k-ω space. Intensity fluctuations associated to granules disappear in a very short distance, about 60 to 90km above the continuum level. The intensity features in the upper photosphere are not related to the granules. The velocity fluctuations associated to the granulation, cross the whole thickness of the photosphere. The photosphere can thus be separated into two distinct layers, but only for the intensity features, not for the velocity features which are the same from the bottom to the top of the stable atmosphere. In the lower photosphere, below 90km, only granules are detected. In the upper photosphere, our observations suggest that the weak pattern visible there, is turbulent in origin. In addition to this weak pattern, bright sinking plumes have been discovered; they are associated with strong downflows, often adjacent to strong upflows. The size of most plumes is 1" to 2"; however the most outstanding of them may be as large as 4". Their origin, which may be photospheric or chromospheric, is still unknown. Title: THEMIS IR Capabilities Authors: Roudier, T.; Mein, P.; Rayrole, J. Bibcode: 1995itsa.conf..195R Altcode: No abstract at ADS Title: BASS 2000: Base solaire 'sol' 2000 (solar 'ground-based' database 2000) Authors: Roudier, T. Bibcode: 1995VA.....39..115R Altcode: No abstract at ADS Title: Dynamics of post-flare loops observed on June 26, 1992 Authors: Wiik, J. E.; Schmieder, B.; Heinzel, P.; Roudier, T. Bibcode: 1994ESASP.373..207W Altcode: 1994soho....3..207W No abstract at ADS Title: Peculiar interactions between granules and network bright points in the solar photosphere Authors: Roudier, T.; Espagnet, O.; Muller, R.; Vigneau, J. Bibcode: 1994A&A...287..982R Altcode: The magnetic elements which form the photospheric network, outside active regions, are visible as tiny bright features, smaller than 0.5", called network bright points (NBPs). 15 to 20% of them elongate during their life, when they are squeezed between two expanding granules, or simply pushed by an expanding or a moving granule. Some statistical properties are reported. The most interesting property is that their size and brightness increases simultaneously indicating a strong increase of radiative flux as they grow. They can become as large as 1000 km x 400 km and as bright as 1.25 the brightness of the average photosphere. As theoretical models predict that magnetic flux tubes of size larger than 300 km should be darker than the mean photosphere, our observations suggest that during the elongation, some mechanical energy should contribute in the heating of the tubes. This additional energy may heat some features in the upper photosphere like chromospheric spicules and transition zone jets or microflares, whose production rates and lifetimes are about the same as those of elongation of NBPS. Elongated NBPs are shaked by the turbulent granules, similarly to the normal NBPs and thus should contribute to the heating of the solar corona too. Title: Turbulent power spectra of solar granulation. Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N. Bibcode: 1993A&A...271..589E Altcode: High resolution "Multichannel Subtractive Double Pass" spectrograms and white light photographs have been used to compute power spectra of the solar granulation. The 5 min oscillations have been carefully filtered, and the influence of the variation of the mean size of granules at the mesogranulation scale has been taken into account.

It results that both the power spectra of velocity and intensity fluctuations, displayed in a log P - log k scale, are characteristic of a turbulent atmosphere: they present a discontinuity at 3" = 2000 km (which corresponds to the size of a large granules) and an energy which decreases according to the Kolmogorov -5/3 power law in the granulation range; another discontinuity, at 1".4 = 1000 km (which corresponds approximately to a Peclet number Pe = 1, for which thermal diffusion and advection are of equal importance) is present in the intensity power spectrum, which decreases with a -17/3 power law, in the range of sizes smaller than 1".4.

These results indicate that granules are turbulent eddies. They have some convective characters (like the correlation between velocities and brightness) because they are formed in a strongly superadiabatic atmosphere. Title: Turbulence et granulation solaire. Authors: Espagnet, O.; Muller, R.; Roudier, T. Bibcode: 1992JAF....43...35E Altcode: No abstract at ADS Title: Dynamique de la photosphère solaire: granulation, mésogranulation, oscillations. Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N.; Mein, P. Bibcode: 1992JAF....43...36E Altcode: No abstract at ADS Title: Evolution and advection of solar mesogranulation Authors: Muller, Richard; Auffret, Herve; Roudier, Thierry; Vigneau, Jean; Simon, George W.; Frank, Zoe; Shine, Richard A.; Title, Alan M. Bibcode: 1992Natur.356..322M Altcode: GRANULAR structure on the Sun's surface, with a typical scale of 1-2 Mm, has been known since 1800, and one hundred years ago, with the first observations by spectroheliograph1,2, a mesh-like bright network was found with a characteristic scale of 30 Mm (40''). This pattern was found, thirty years ago, to be coincident with close-packed convective cells ('supergranulation') revealed by Doppler observations3-5 to be nestling inside the bright network. More recently6,7 an intermediate 'mesogranular' structure was found, with a characteristic scale of 3-10 Mm. We have obtained a three-hour sequence of observations at the Pic du Midi observatory which shows the evolution of mesogranules from appearance to disappearance with unprecedented clarity. We see that the supergranules, which are known to carry along (advect) the granules with their convective motion, also advect the mesogranules to their boundaries. This process controls the evolution and disappearance of mesogranules. Title: La fondation de l'observatoire et les débuts de l'astronomie au Pic du Midi (1852 - 1947). Dernière partie: Le règne de la coupole Baillaud. Authors: Malherbe, J. -M.; Coutard, C.; Roudier, T. Bibcode: 1991LAstr.105R...1M Altcode: No abstract at ADS Title: La fondation de l'observatoire et les débuts de l'astronomie au Pic du Midi (1852 - 1947). Deuxième partie: L'introduction de l'astronomie. Authors: Malherbe, J. -M.; Coutard, C.; Roudier, T. Bibcode: 1991LAstr.105Q...1M Altcode: No abstract at ADS Title: La fondation de l'observatoire et les débuts de l'astronomie au Pic du Midi (1852 - 1947). Introduction. Première partie: La fondation de l'observatoire. Authors: Malherbe, J. -M.; Coutard, C.; Roudier, T. Bibcode: 1991LAstr.105....6M Altcode: No abstract at ADS Title: Dynamics of the solar granulation. II - Statistical analysis: Power spectra, coherence, phase Authors: Roudier, T.; Vigneau, J.; Espagnet, O.; Muller, R.; Mein, P.; Malherbe, J. M. Bibcode: 1991A&A...248..245R Altcode: The solar granulation is analyzed statistically by the computation of coherence, phase, and power spectra using data provided by the Multichannel Subtractive Double Pass spectrograph of the Pic du Midi Observatory. The main result of the analysis is that the velocity power spectrum of the granulation changes in shape at 3 arcsec: the raw spectrum features a slope discontinuity; the corrected spectra have a power maximum at 3 arcsec. Between 3 arcsec and the resolution limit (0.8 arcsec), the power decreases almost linearly, with a slope close to -5/3, consistent with the Kolmogorov power law for a turbulent energy cascade. Title: Dynamics of solar granulation. I - Processing of MSDP spectra Authors: Roudier, T.; Vigneau, J.; Mein, P.; Malherbe, J. M.; Coutard, C.; Hellier, R.; Muller, R. Bibcode: 1991A&A...248..237R Altcode: High resolution (about 0.5 arcsec) 2D spectra have been obtained with the Multichannel Subtractive Double Pass spectrograph of the Pic du Midi Observatory in a quiet region at the solar disk center. The adapted image processing which provides 2D intensity and velocity maps of the solar granulation, at two intensity levels of the line NaD2, is described. A quantitative evaluation of the spatial resolution is determined both for the intensity and velocity granulation fields. Title: La granulation solaire dans les régions calmes et actives. Authors: Roudier, Thierry Bibcode: 1990LAstr.104..345R Altcode: The current understanding of solar granulation in quiet and active regions is examined. Recent observations are discussed in relation to current theory. The origin of granulation is considered with reference to turbulent convection processes which concentrate and disperse the magnetic field in the photosphere. It is concluded that, despite recent hypotheses on the turbulent orgin of small granules (fractal analysis), convective penetration into stable layers of the photosphere is still the most likely scenario. Title: Proper Motion and Lifetime of Mesogranules Authors: Frank, Z.; Muller, R.; Roudier, T.; Vigneau, J.; Shine, R.; Tarbell, T.; Title, A.; Topka, K.; Simon, G. Bibcode: 1989BAAS...21..841F Altcode: No abstract at ADS Title: Amateur/professional Cooperation in 2 Solar Studies Authors: Roudier, T.; Muller, R.; Hulot, J. C.; Vaissière, F. Bibcode: 1988scaa.conf..168R Altcode: 1988IAUCo..98..168R No abstract at ADS Title: Structures fines de la photosphère solaire. Authors: Roudier, T. Bibcode: 1987JAF....29...14R Altcode: No abstract at ADS Title: Digital filtering of low-contrast images with the possibility of structural analysis by segmentation in connected domains Authors: Roudier, T.; Coupinot, G.; Hecquet, J.; Muller, R. Bibcode: 1985JOpt...16..107R Altcode: A digital filtering technique is developed by analogy to the optical blurred-mask procedure and applied to astronomical images, demonstrating its usefulness in cases where the image is to be segmented by intensity levels into connected domains after processing. Images of granules on the solar surface, globular clusters, Venus and its atmosphere, and a solar prominence are treated; and the physiological implications of the blurred-mask filter are discussed. Title: Variability of the quiet photospheric network. Authors: Muller, R.; Roudier, T. Bibcode: 1984ESASP.220..239M Altcode: 1984ESPM....4..239M High resolution photographs of the photospheric network taken in the Ca II K 3933 Å line and λ4308 Å are analysed in order to study the variation, in latitude and over the solar cycle, of its density. Title: Variability of the structure of the granulation over the solar activity cycle. Authors: Muller, R.; Roudier, T. Bibcode: 1984ESASP.220...51M Altcode: 1984ESPM....4...51M High resolution photographs of solar granulation, obtained between 1976 and 1983, have been computer processed, in order to study the structure of the granulation and its variation over the solar cycle. Title: Variation of the Mesh of the Granular Network Along the Solar Cycle Authors: Macris, C. J.; Mueller, R.; Rosch, J.; Roudier, T. Bibcode: 1984ssdp.conf..265M Altcode: Using high resolution photographs of the granular field in the center of the disk obtained since 1966 at the Pic-du-Midi Observatory, it is shown, either by measuring the center-to-center distances or by counting granules, that a substantial change in these parameters does occur along the undecennal cycle, the number of granules per 10×10 arc seconds being around 35 at minimum and 45 at maximum. Automatic counting of features displaying closed isophotal curves is under way. Title: Lists of minima of eclipsing binaries. Authors: Carnevali, P.; Diethelm, R.; Figer, A.; Germann, R.; Le Borgne, J. -F.; Locher, K.; Mauron, N.; Marot, A.; Peter, H.; Ralincourt, P.; Remis, J.; Sanchez, C.; Domec, C.; Hevesi, Z.; Rolland, R.; Roudier, T.; Behagle, M.; Royer, A.; Dumarchi, G. Bibcode: 1975BBSAG..19....1C Altcode: No abstract at ADS