Author name code: muller-richard ADS astronomy entries on 2022-09-14 author:"Muller, Richard" AND (aff:"Pic du Midi" OR aff:"Toulouse") ------------------------------------------------------------------------ Title: Five decades of solar research at the Pic du Midi Turret-Dome (1960-2010). Part 2: High spatial resolution imagery Authors: Muller, R.; Roudier, Th.; Malherbe, J. -M. Bibcode: 2021JAHH...24..921M Altcode: In the Part 2 of the series of two papers dedicated to the review on fifty years of advances in solar instrumentation and solar physics made at the Pic du Midi Turret-Dome (1960-2010) in various domains, we pre-sent the main observations and results obtained in the field of high resolution imagery, for which the instrument was originally designed. Title: Five decades of solar research at the Mic du Midi Turret-Dome (1960-2010). Part 1: Overview of instrumentation and observations Authors: Roudier, Th.; Malherbe, J. -M.; Rozelot, J. -P.; Mein, P.; Muller, R. Bibcode: 2021JAHH...24..585R Altcode: The Pic du Midi Turret-Dome, known as 'Lunette Jean Rösch'or LJR ('lunette' for refractor in French), was scientifically active during five decades between 1960 and 2010. It was dedicated to high spatial resolution observations in solar astronomy. We review fifty years of advances in solar instrumentation and solar physics made by this instrument in various domains, which took advantage of the good seeing at Pic du Midi. First, we summarize some of the LJR results in broad-band and wide-field imagery of the photosphere; as this topic was the initial goal and has been the most important contribution of the refractor, details will be provided in Part 2. Then, we present spectro-polarimetric instrumentation and observations with narrow slit and imaging spectroscopy of the photosphere and the chromosphere. The Turret-Dome also housed an original spectro-coronagraph, and observations of the high temperature coronal plasma are highlighted. Finally, we describe progress in the determination of the solar shape with the heliometer. The LJR remained active until the launch of the Hinode and Solar Dynamics Observatory satellites (in 2006 and 2010, respectively). Title: The Solar Orbiter EUI instrument: The Extreme Ultraviolet Imager Authors: Rochus, P.; Auchère, F.; Berghmans, D.; Harra, L.; Schmutz, W.; Schühle, U.; Addison, P.; Appourchaux, T.; Aznar Cuadrado, R.; Baker, D.; Barbay, J.; Bates, D.; BenMoussa, A.; Bergmann, M.; Beurthe, C.; Borgo, B.; Bonte, K.; Bouzit, M.; Bradley, L.; Büchel, V.; Buchlin, E.; Büchner, J.; Cabé, F.; Cadiergues, L.; Chaigneau, M.; Chares, B.; Choque Cortez, C.; Coker, P.; Condamin, M.; Coumar, S.; Curdt, W.; Cutler, J.; Davies, D.; Davison, G.; Defise, J. -M.; Del Zanna, G.; Delmotte, F.; Delouille, V.; Dolla, L.; Dumesnil, C.; Dürig, F.; Enge, R.; François, S.; Fourmond, J. -J.; Gillis, J. -M.; Giordanengo, B.; Gissot, S.; Green, L. M.; Guerreiro, N.; Guilbaud, A.; Gyo, M.; Haberreiter, M.; Hafiz, A.; Hailey, M.; Halain, J. -P.; Hansotte, J.; Hecquet, C.; Heerlein, K.; Hellin, M. -L.; Hemsley, S.; Hermans, A.; Hervier, V.; Hochedez, J. -F.; Houbrechts, Y.; Ihsan, K.; Jacques, L.; Jérôme, A.; Jones, J.; Kahle, M.; Kennedy, T.; Klaproth, M.; Kolleck, M.; Koller, S.; Kotsialos, E.; Kraaikamp, E.; Langer, P.; Lawrenson, A.; Le Clech', J. -C.; Lenaerts, C.; Liebecq, S.; Linder, D.; Long, D. M.; Mampaey, B.; Markiewicz-Innes, D.; Marquet, B.; Marsch, E.; Matthews, S.; Mazy, E.; Mazzoli, A.; Meining, S.; Meltchakov, E.; Mercier, R.; Meyer, S.; Monecke, M.; Monfort, F.; Morinaud, G.; Moron, F.; Mountney, L.; Müller, R.; Nicula, B.; Parenti, S.; Peter, H.; Pfiffner, D.; Philippon, A.; Phillips, I.; Plesseria, J. -Y.; Pylyser, E.; Rabecki, F.; Ravet-Krill, M. -F.; Rebellato, J.; Renotte, E.; Rodriguez, L.; Roose, S.; Rosin, J.; Rossi, L.; Roth, P.; Rouesnel, F.; Roulliay, M.; Rousseau, A.; Ruane, K.; Scanlan, J.; Schlatter, P.; Seaton, D. B.; Silliman, K.; Smit, S.; Smith, P. J.; Solanki, S. K.; Spescha, M.; Spencer, A.; Stegen, K.; Stockman, Y.; Szwec, N.; Tamiatto, C.; Tandy, J.; Teriaca, L.; Theobald, C.; Tychon, I.; van Driel-Gesztelyi, L.; Verbeeck, C.; Vial, J. -C.; Werner, S.; West, M. J.; Westwood, D.; Wiegelmann, T.; Willis, G.; Winter, B.; Zerr, A.; Zhang, X.; Zhukov, A. N. Bibcode: 2020A&A...642A...8R Altcode: Context. The Extreme Ultraviolet Imager (EUI) is part of the remote sensing instrument package of the ESA/NASA Solar Orbiter mission that will explore the inner heliosphere and observe the Sun from vantage points close to the Sun and out of the ecliptic. Solar Orbiter will advance the "connection science" between solar activity and the heliosphere.
Aims: With EUI we aim to improve our understanding of the structure and dynamics of the solar atmosphere, globally as well as at high resolution, and from high solar latitude perspectives.
Methods: The EUI consists of three telescopes, the Full Sun Imager and two High Resolution Imagers, which are optimised to image in Lyman-α and EUV (17.4 nm, 30.4 nm) to provide a coverage from chromosphere up to corona. The EUI is designed to cope with the strong constraints imposed by the Solar Orbiter mission characteristics. Limited telemetry availability is compensated by state-of-the-art image compression, onboard image processing, and event selection. The imposed power limitations and potentially harsh radiation environment necessitate the use of novel CMOS sensors. As the unobstructed field of view of the telescopes needs to protrude through the spacecraft's heat shield, the apertures have been kept as small as possible, without compromising optical performance. This led to a systematic effort to optimise the throughput of every optical element and the reduction of noise levels in the sensor.
Results: In this paper we review the design of the two elements of the EUI instrument: the Optical Bench System and the Common Electronic Box. Particular attention is also given to the onboard software, the intended operations, the ground software, and the foreseen data products.
Conclusions: The EUI will bring unique science opportunities thanks to its specific design, its viewpoint, and to the planned synergies with the other Solar Orbiter instruments. In particular, we highlight science opportunities brought by the out-of-ecliptic vantage point of the solar poles, the high-resolution imaging of the high chromosphere and corona, and the connection to the outer corona as observed by coronagraphs. Title: Does the solar granulation change with the activity cycle? Authors: Muller, R.; Hanslmeier, A.; Utz, D.; Ichimoto, K. Bibcode: 2018A&A...616A..87M Altcode: Context. Knowledge of the variation of the solar granulation properties (contrast and scale) with the 11-yr activity cycle is useful for a better understanding of the interaction between magnetic field and convection at global or local scales. A varying granulation may also contribute to irradiance variations and affect the p-mode damping rates and lifetimes.
Aims: HINODE/SOT blue continuum images taken in the frame of the synoptic program at the disk center on a daily basis between November 2006 and February 2016 are used. This period covers the minimum of activity between cycles 23 and 24 and the maximum of cycle 24.
Methods: The sharpness of a significant number of images was reduced because of instrumental aberrations or inaccurate focusing. Only the sharpest images were selected for this investigation.
Results: To be detectable with HINODE/SOT images, the variation of the granulation contrast and of the granulation scale at the disk center should have been larger than 3%. As it is not the case, it is concluded that they varied by less than 3% through the weak cycle 24. Title: Long-term trends of magnetic bright points: The evolution of MBP size Authors: Utz, D.; Van Doorsselaere, T.; Gagelmans, E.; O'Rourke, C.; Vuerinckx, A.; Muller, R.; Veronig, A. Bibcode: 2018simi.conf..179U Altcode: Magnetic Bright Points (MBPs) are small-scale, very strong, solar magnetic field concentrations visible in the lower solar atmosphere. While there is a large and ever-increasing knowledge base and understanding of large-scale solar magnetic fields, i.e., sunspots and active regions, and their involvement in the solar cycle, much less is known about small-scale fields such as MBPs. Thus, we aim on contributing to our understanding of these tiny, but, important solar features by investigating the size distribution and its variation over time.

For this purpose, we obtained the synoptic G-band data set of the Hinode mission which is now since nearly 12 years in space and operational (launched in October 2006). After careful image calibration and selection we analysed the G-band data set with an automated MBP identification algorithm to calculate in a next step the equivalent diameter of the MBPs.

The so gained size distribution follows a Gamma distribution with pronounced changes during the solar activity cycle. The MBP sizes appear to be somewhat smaller during the solar minimum and somewhat more extended during the solar maxima as expressed by the scaleparameter of the Gamma distribution. Title: Temporal relations between magnetic bright points and the solar sunspot cycle Authors: Utz, Dominik; Muller, Richard; Van Doorsselaere, Tom Bibcode: 2017PASJ...69...98U Altcode: 2017arXiv171001678U The Sun shows a global magnetic field cycle traditionally best visible in the photosphere as a changing sunspot cycle featuring roughly an 11-year period. In addition we know that our host star also harbours small-scale magnetic fields often seen as strong concentrations of magnetic flux reaching kG field strengths. These features are situated in inter-granular lanes, where they show up bright as so-called magnetic bright points (MBPs). In this short paper we wish to analyse an homogenous, nearly 10-year-long synoptic Hinode image data set recorded from 2006 November up to 2016 February in the G-band to inspect the relationship between the number of MBPs at the solar disc centre and the relative sunspot number. Our findings suggest that the number of MBPs at the solar disc centre is indeed correlated to the relative sunspot number, but with the particular feature of showing two different temporal shifts between the decreasing phase of cycle 23 including the minimum and the increasing phase of cycle 24 including the maximum. While the former is shifted by about 22 months, the latter is only shifted by less than 12 months. Moreover, we introduce and discuss an analytical model to predict the number of MBPs at the solar disc centre purely depending on the evolution of the relative sunspot number as well as the temporal change of the relative sunspot number and two background parameters describing a possibly acting surface dynamo as well as the strength of the magnetic field diffusion. Finally, we are able to confirm the plausibility of the temporal shifts by a simplistic random walk model. The main conclusion to be drawn from this work is that the injection of magnetic flux, coming from active regions as represented by sunspots, happens on faster time scales than the removal of small-scale magnetic flux elements later on. Title: Latitude dependence of the solar granulation during the minimum of activity in 2009 Authors: Muller, R.; Hanslmeier, A.; Utz, D. Bibcode: 2017A&A...598A...6M Altcode: Context. Knowledge of the latitude variation of the solar granulation properties (contrast and scale) is useful to better understand interactions between magnetic field, convection, differential rotation, and meridional circulation in the solar atmosphere.
Aims: We investigated the latitude dependence of the contrast and scale of the solar granulation, with the help of HINODE/SOT blue continuum images taken in the frame of the HOP 79 program, along the central meridian and along the equator on a monthly basis in 2009 during the last solar minimum of activity.
Methods: We selected the sharpest images in latitude and longitude intervals. The selected images in all the N-S and E-W scans taken in 2009 were combined to get statistically reliable results.
Results: The contrast of the solar granulation decreases towards the poles and the scale increases, but not regularly since a perturbation occurs at around 60° where both quantities return close to their values at the disk center.
Conclusions: Such a latitude variation in a period of minimum of activity (2009), is probably not due to magnetic field, neither the quiet magnetic field at the surface, nor the strong magnetic flux tubes associated with active regions, which could be embedded more or less deeply in the convection zone before they reach the surface. The decrease in contrast and increase in scale towards the pole seem to be related to the differential rotation and the perturbation around 60° to the meridional circulation. Title: Long-term trends of magnetic bright points. I. Number of magnetic bright points at disc centre Authors: Utz, D.; Muller, R.; Thonhofer, S.; Veronig, A.; Hanslmeier, A.; Bodnárová, M.; Bárta, M.; del Toro Iniesta, J. C. Bibcode: 2016A&A...585A..39U Altcode: 2015arXiv151107767U Context. The Sun shows an activity cycle that is caused by its varying global magnetic field. During a solar cycle, sunspots, I.e. extended regions of strong magnetic fields, occur in activity belts that are slowly migrating from middle to lower latitudes, finally arriving close to the equator during the cycle maximum phase. While this and other facts about the strong extended magnetic fields have been well known for centuries, much less is known about the solar cycle evolution of small-scale magnetic fields. Thus the question arises if similar principles exist for small-scale magnetic fields.
Aims: To address this question, we study magnetic bright points (MBPs) as proxies for such small-scale, kG solar magnetic fields. This study is based on a homogeneous data set that covers a period of eight years. The number of detected MBPs versus time is analysed to find out if there is an activity cycle for these magnetic features too and, if so, how it is related to the sunspot cycle.
Methods: An automated MBP identification algorithm was applied to the synoptic Hinode/SOT G-band data over the period November 2006 to August 2014, I.e. covering the decreasing phase of Cycle 23 and the rise, maximum, and early decrease of Cycle 24. This data set includes, at the moment of investigation, a total of 4162 images, with about 2.9 million single MBP detections.
Results: After a careful preselection and monthly median filtering of the data, the investigation revealed that the number of MBPs close to the equator is coupled to the global solar cycle but shifted in time by about 2.5 yr. Furthermore, the instantaneous number of detected MBPs depends on the hemisphere, with one hemisphere being more prominent, I.e. showing a higher number of MBPs. After the end of Cycle 23 and at the starting point of Cycle 24, the more active hemisphere changed from south to north. Clear peaks in the detected number of MBPs are found at latitudes of about ±7°, in congruence with the positions of the sunspot belts at the end of the solar cycle.
Conclusions: These findings suggest that there is indeed a coupling between the activity of MBPs close to the equator with the global magnetic field. The results also indicate that a significant fraction of the magnetic flux that is visible as MBPs close to the equator originates from the sunspot activity belts. However, even during the minimum of MBP activity, a percentage as large as 60% of the maximum number of detected MBPs has been observed, which may be related to solar surface dynamo action. Title: Variations of Magnetic Bright Point Properties with Longitude and Latitude as Observed by Hinode/SOT G-band Data Authors: Utz, D.; Hanslmeier, A.; Veronig, A.; Kühner, O.; Muller, R.; Jurčák, J.; Lemmerer, B. Bibcode: 2013SoPh..284..363U Altcode: 2012arXiv1212.1310U Small-scale magnetic fields can be observed on the Sun in high-resolution G-band filtergrams as magnetic bright points (MBPs). We study Hinode/Solar Optical Telescope (SOT) longitude and latitude scans of the quiet solar surface taken in the G-band in order to characterise the centre-to-limb dependence of MBP properties (size and intensity). We find that the MBP's sizes increase and their intensities decrease from the solar centre towards the limb. The size distribution can be fitted using a log-normal function. The natural logarithm of the mean (μ parameter) of this function follows a second-order polynomial and the generalised standard deviation (σ parameter) follows a fourth-order polynomial or equally well (within statistical errors) a sine function. The brightness decrease of the features is smaller than one would expect from the normal solar centre-to-limb variation; that is to say, the ratio of a MBP's brightness to the mean intensity of the image increases towards the limb. The centre-to-limb variations of the intensities of the MBPs and the quiet-Sun field can be fitted by a second-order polynomial. The detailed physical process that results in an increase of a MBP's brightness and size from Sun centre to the limb is not yet understood and has to be studied in more detail in the future. Title: Magnetic field strength distribution of magnetic bright points inferred from filtergrams and spectro-polarimetric data Authors: Utz, D.; Jurčák, J.; Hanslmeier, A.; Muller, R.; Veronig, A.; Kühner, O. Bibcode: 2013A&A...554A..65U Altcode: 2013arXiv1304.5508U Context. Small scale magnetic fields can be observed on the Sun in G-band filtergrams as magnetic bright points (MBPs) or identified in spectro-polarimetric measurements due to enhanced signals of Stokes profiles. These magnetic fields and their dynamics play a crucial role in understanding the coronal heating problem and also in surface dynamo models. MBPs can theoretically be described to evolve out of a patch of a solar photospheric magnetic field with values below the equipartition field strength by the so-called convective collapse model. After the collapse, the magnetic field of MBPs reaches a higher stable magnetic field level.
Aims: The magnetic field strength distribution of small scale magnetic fields as seen by MBPs is inferred. Furthermore, we want to test the model of convective collapse and the theoretically predicted stable value of about 1300 G.
Methods: We used four different data sets of high-resolution Hinode/SOT observations that were recorded simultaneously with the broadband filter device (G-band, Ca II-H) and the spectro-polarimeter. To derive the magnetic field strength distribution of these small scale features, the spectropolarimeter (SP) data sets were treated by the Merlin inversion code. The four data sets comprise different solar surface types: active regions (a sunspot group and a region with pores), as well as quiet Sun.
Results: In all four cases the obtained magnetic field strength distribution of MBPs is similar and shows peaks around 1300 G. This agrees well with the theoretical prediction of the convective collapse model. The resulting magnetic field strength distribution can be fitted in each case by a model consisting of log-normal components. The important parameters, such as geometrical mean value and multiplicative standard deviation, are similar in all data sets, so only the relative weighting of the components is different. Title: Non-Varying Granulation and Photospheric Network During the Extended 2007 - 2009 Solar Minimum Authors: Muller, R.; Utz, D.; Hanslmeier, A. Bibcode: 2011SoPh..274...87M Altcode: We have analysed the wide band images taken by Hinode/SOT, in a blue continuum window and in the G-band, more or less on a daily basis in the frame of the synoptic program, to investigate the variation of the solar granulation and of the photospheric network with the activity cycle. A particular attention has been given to disentangle solar effects from instrumental ones. It appears that a substantial fraction of the images are more or less blurred and/or defocussed. During the analysed period November 2006 - July 2010, the granulation contrast of the sharpest selected images decreased steadily, the granulation scale increased and the number of MBPs decreased (they are the Bright Points of Magnetic origin which form the photospheric network in G-band images). These trends are likely of instrumental origin. Consequently, the granulation and the photospheric network have most probably not changed during the extended solar minimum 2007 - 2009. Title: The Quiet Solar Photosphere: Dynamics and Magnetism Authors: Muller, Richard Bibcode: 2011LNP...832...87M Altcode: The Sun is the only solar-type star where the dynamics and the magnetism can be studied in detail and the physical process involved understood, in particular those which occur at very small scales. This lecture is restricted to the quiet solar photosphere. The properties of the three cellular scales of motions observed at the solar surface (granulation, mesogranulation, supergranulation) are presented, as well as the numerical simulations which reproduce most granulation properties very satisfactorily. The granular convection is driven by radiative cooling through the surface. In these simulations, the mesogranulation appears to be an extension to deeper layers of the surface granular convection. The mesogranulation also appears as convective, indirectly driven by the surface radiative cooling. However, several alternative origins for both the mesogranulation and the supergranulation, have been proposed too. On the same way, the magnetic structure of the quiet photosphere is described, including the network and the fields. Their origin is discussed on the basis of the properties of the magnetic elements and of the results of numerical simulations of magneto-convection and of local dynamo. In the network, that means at the supergranular boundaries, the field is concentrated in the form of vertical flux tubes of sizes smaller than a few hundred kilometers and of magnetic field strength 1text{-}2 kG. They are visible as bright points located in the intergranular lanes. The presence of magnetic field is also ubiquitous inside the supergranules, where it is known as IntraNetwork magnetic field. This field is much different from the network field, consisting of small loops of size 1^''text{-}2^'', closely related to granules. The field strength is much lower than in the network, not exceeding a few hundreds Gauss, and of flux lower by one or two orders of magnitude. The time scale of both kinds of field is short, less than 10 min, determined by the evolution of the neighbouring granules. The origin of the intranetwork field is not yet clarified: are they fragments of magnetic flux rising from deep layers and reprocessed close to the surface by convective motions, or generated near the surface by a fast local dynamo? Title: Dynamics of isolated magnetic bright points derived from Hinode/SOT G-band observations Authors: Utz, D.; Hanslmeier, A.; Muller, R.; Veronig, A.; Rybák, J.; Muthsam, H. Bibcode: 2010A&A...511A..39U Altcode: 2009arXiv0912.1965U Context. Small-scale magnetic fields in the solar photosphere can be identified in high-resolution magnetograms or in the G-band as magnetic bright points (MBPs). Rapid motions of these fields can cause magneto-hydrodynamical waves and can also lead to nanoflares by magnetic field braiding and twisting. The MBP velocity distribution is a crucial parameter for estimating the amplitudes of those waves and the amount of energy they can contribute to coronal heating.
Aims: The velocity and lifetime distributions of MBPs are derived from solar G-band images of a quiet sun region acquired by the Hinode/SOT instrument with different temporal and spatial sampling rates.
Methods: We developed an automatic segmentation, identification and tracking algorithm to analyse G-Band image sequences to obtain the lifetime and velocity distributions of MBPs. The influence of temporal/spatial sampling rates on these distributions is studied and used to correct the obtained lifetimes and velocity distributions for these digitalisation effects.
Results: After the correction of algorithm effects, we obtained a mean MBP lifetime of (2.50 ± 0.05) min and mean MBP velocities, depending on smoothing processes, in the range of (1-2) km~s-1. Corrected for temporal sampling effects, we obtained for the effective velocity distribution a Rayleigh function with a coefficient of (1.62 ± 0.05) km~s-1. The x- and y-components of the velocity distributions are Gaussians. The lifetime distribution can be fitted by an exponential function. 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: The size distribution of magnetic bright points derived from Hinode/SOT observations Authors: Utz, D.; Hanslmeier, A.; Möstl, C.; Muller, R.; Veronig, A.; Muthsam, H. Bibcode: 2009A&A...498..289U Altcode: 2009arXiv0912.2637U Context: Magnetic bright points (MBPs) are small-scale magnetic features in the solar photosphere. They may be a possible source of coronal heating by rapid footpoint motions that cause magnetohydrodynamical waves. The number and size distribution are of vital importance in estimating the small scale-magnetic-field energy.
Aims: The size distribution of MBPs is derived for G-band images acquired by the Hinode/SOT instrument.
Methods: For identification purposes, a new automated segmentation and identification algorithm was developed.
Results: For a sampling of 0.108 arcsec/pixel, we derived a mean diameter of (218 ± 48) km for the MBPs. For the full resolved data set with a sampling of 0.054 arcsec/pixel, the size distribution shifted to a mean diameter of (166 ± 31) km. The determined diameters are consistent with earlier published values. The shift is most probably due to the different spatial sampling.
Conclusions: We conclude that the smallest magnetic elements in the solar photosphere cannot yet be resolved by G-band observations. The influence of discretisation effects (sampling) has also not yet been investigated sufficiently. Title: Umbral Fine Structures in Sunspots Observed with Hinode Solar Optical Telescope Authors: Kitai, Reizaburo; Watanabe, Hiroko; Nakamura, Tahei; Otsuji, Ken-ichi; Matsumoto, Takuma; UeNo, Satoru; Nagata, Shin'ichi; Shibata, Kazunari; Muller, Richard; Ichimoto, Kiyoshi; Tsuneta, Saku; Suematsu, Yoshinori; Katsukawa, Yukio; Shimizu, Toshifumi; Tarbell, Theodore D.; Shine, Richard A.; Title, Alan M.; Lites, Bruce Bibcode: 2007PASJ...59S.585K Altcode: 2007arXiv0711.3266K A high resolution imaging observation of a sunspot umbra was made with the Hinode Solar Optical Telescope. Filtergrams at wavelengths of the blue and green continua were taken during three consecutive days. The umbra consisted of a dark core region, several diffuse components, and numerous umbral dots. We derived basic properties of umbral dots (UDs), especially their temperatures, lifetimes, proper motions, spatial distribution, and morphological evolution. The brightness of UDs is confirmed to depend on the brightness of their surrounding background. Several UDs show fission and fusion. Thanks to the stable condition of the space observation, we could for the first time follow the temporal behavior of these events. The derived properties of the internal structure of the umbra are discussed from the viewpoint of magnetoconvection in a strong magnetic field. Title: Variations of the granulation related to the solar cycle and with respect to its position on the solar disk Authors: Muller, R.; Hanslmeier, A.; Saldaña-Muñoz, M. Bibcode: 2007A&A...475..717M Altcode: Aims:We investigate variations in the scale and contrast of the solar granulation related to the solar cycle during the period 1978-1993. Furthermore, as a by-product, we have detected a variation with respect to the solar longitude, along the solar equator.
Methods: All images were taken on film with the 50 cm refractor of the Pic du Midi Observatory, under excellent seeing conditions. Scale and contrast were derived from power spectra computed with digitized images. This simple and robust statistical method allowed us to get reliable results, independent of any image-processing parameter.
Results: The contrast of the solar granulation varies nearly in phase with the solar cycle, being smaller at the periods of solar maximum. But we detected no corresponding variation in the scale; if there is one, it must be of low amplitude and masked by a spatial variation in the scale with respect to the position on the solar equator, which amounts to 3% rms. The contrast also varies with the position on the equator, in phase with the scale: where the scale is larger, the contrast is higher too. The amplitude of this spatial variation in the granulation scale is 9%; large photometric uncertainties did not allow us to quantify the amplitude of the contrast variation. Title: Space and time variation of the solar granulation . Authors: Muller, R.; Hanslmeier, A.; Saldaña-Muñoz, M. Bibcode: 2007MmSAI..78...71M Altcode: Granulation images, taken on film with the 50 cm refractor of the Pic du Midi Observatory between 1978 and 1993, have been analysed. The solar granulation appears to vary, both in space, along the solar equator, and in time, in phase with the activity cycle. Around the equator, the granulation scale varies with an amplitude as large as 15%; the contrast varies similarly: where the scale increases, the contrast increases too. Concerning the time variation, only a variation of the contrast is detected: it is smaller at periods of solar maxima. We cannot quantify the amplitude of the variation because of photometic uncertainties. We do not detect any variation of the scale of the granulation: it must be of much lower amplitude than the contrast variation. Title: Spectro polarimetry with liquid crystals . Authors: Malherbe, J. -M.; Roudier, Th.; Moity, J.; Mein, P.; Arnaud, J.; Muller, R. Bibcode: 2007MmSAI..78..203M Altcode: We report spectro polarimetric observations made with the spectrograph of the Lunette Jean Rösch at Pic du Midi, France. We have tested Ferroelectric (FLC) and Nematic (NLC) Liquid Crystals. The instrument setup is briefly decribed, together with first observations of magnetic fields obtained with the Multichannel Subtractive Double Pass (MSDP). Polarization analysis of various spectral lines performed with the single pass (SP) spectrograph in active regions or at the limb is also presented. Title: Standards for Space Solar Cells and Arrays Authors: Bailey, S.; Snyder, D.; Scheiman, D.; Mueller, R.; Pichetto, V.; Emery, K.; Baur, C.; Messenger, S.; Goodbody, C. Bibcode: 2005ESASP.589E..95B Altcode: Calibration standards for space solar cells are discussed for three extra-terrestrial measurement facilities, the CNES balloon, the JPL balloon, and the NASA GRC aircraft. Results are presented for the short circuit current and open circuit voltage measurements that reflect less than a percent variation among the three facilities. A discussion of future ISO activities for panels, cells and arrays is presented. Title: Phase diversity restoration of sunspot images. II. Dynamics around a decaying sunspot Authors: Bonet, J. A.; Márquez, I.; Muller, R.; Sobotka, M.; Roudier, Th. Bibcode: 2005A&A...430.1089B Altcode: Two time series, taken simultaneously in the G-band and in white-light, and corrected for telescope aberrations and turbulence perturbations using the method of phase diversity, are employed to study the motions of granules and G-band bright points (GBPs) in the moat of an old regular sunspot. Local correlation tracking and feature tracking have been utilized for this purpose. A large-scale radial outflow with a mean velocity of 0.51 km s-1 has been measured in the sunspot moat. Centres of diverging horizontal motions, identified with families of granules formed by repeatedly splitting granules, move away from the sunspot. Most of the GBPs in the moat also move outwards through radially orientated ``channels'' (confined between the borders of adjacent families) with velocities comparable to those of the adjacent granules. However, 6% of the GBPs move faster (>1.4 km s-1) than the neighbouring granules. GBPs in the moat are not regularly distributed but they are less frequent on its solar centre side. Title: Phase diversity restoration of sunspot images. I. Relations between penumbral and photospheric features Authors: Bonet, J. A.; Márquez, I.; Muller, R.; Sobotka, M.; Tritschler, A. Bibcode: 2004A&A...423..737B Altcode: 2004astro.ph..5542B We investigate the dynamics of and the relations between small-scale penumbral and photospheric features near the outer penumbral boundary: penumbral grains (PGs), dark penumbral fibrils, granules, and photospheric G-band bright points. The analysis is based on a 2 h time sequence of a sunspot close to disc center, taken simultaneously in the G-band and in the blue continuum at 450.7 nm. Observations were performed at the Swedish Vacuum Solar Telescope (La Palma) in July 1999. A total of 2564 images (46 arcsec × 75 arcsec) were corrected for telescope aberrations and turbulence perturbations by applying the inversion method of phase diversity. Our findings can by summarized as follows: (a) one third of the outward-moving PGs pass through the outer penumbral boundary and then either continue moving as small bright features or expand and develop into granules. (b) Former PGs and G-band bright points next to the spot reveal a different nature. The latter have not been identified as a continuation of PGs escaping from the penumbra. The G-band bright points are mostly born close to dark penumbral fibrils where the magnetic field is strong, whereas PGs stem from the less-magnetized penumbral component and evolve presumably to non-magnetic granules or small bright features. Title: Bright Points in the Internetwork Quiet Sun Authors: Sánchez Almeida, J.; Márquez, I.; Bonet, J. A.; Domínguez Cerdeña, I.; Muller, R. Bibcode: 2004ApJ...609L..91S Altcode: 2004astro.ph..5515S; 2004astro.ph..5515A High-resolution G-band images of the interior of a supergranulation cell show ubiquitous bright points (BPs; some 0.3 BPs per Mm2). They are located in intergranular lanes and often form chains of elongated blobs whose smallest dimension is at the resolution limit (135 km on the Sun). Most of them live for a few minutes, having peak intensities from 0.8 to 1.8 times the mean photospheric intensity. These BPs are probably tracing intense magnetic concentrations, whose existence has been inferred in spectropolarimetric measurements. Our finding provides a new convenient tool for the study of the internetwork magnetism, so far restricted to the interpretation of weak polarimetric signals. Title: Relation between families of granules, mesogranules and photospheric network Authors: Roudier, Th.; Muller, R. Bibcode: 2004A&A...419..757R Altcode: The analysis of a solar granulation 3-h time sequence obtained at the Pic du Midi Observatory confirms the existence of Trees of Fragmenting Granules (TFG). The TFG lifetime histogram is fitted by a power law with an exponent equal to -1.68. The positive divergences (i.e. mesogranules) come from several families that are visible at different stages of the mesogranule evolution. We observe a good coincidence between the cork (passive scalars) distribution and the TFG boundaries. Thus, TFGs seem to play a role in the diffusion of the magnetic elements on the Sun surface. Title: High spatial resolution capabilities of Doppler measurements with the Pic du Midi MSDP spectrograph Authors: Roudier, Th.; Malherbe, J. M.; Mein, P.; Muller, R.; Coutard, C.; Lafon, M.; Grimaud, F. Bibcode: 2003A&A...409..793R Altcode: We analyse observations in the NaD1 line (lambda 5896 Å) obtained with the MSDP spectrograph of the Turret Dome of the Pic du Midi Observatory. Individual images reveal high spatial resolution and the data reduction shows the high capabilities of the spectrograph to get doppler measurements up to the limit of the resolution of the refractor (0\farcs3). Dopplershifts are obtained in the middle and high photosphere. The smallest granules 0\farcs4 show upward motions in the middle photosphere. Title: Spatial and temporal relations between magnetic elements and bright points in the photospheric network Authors: Muller, R.; Dollfus, A.; Montagne, M.; Moity, J.; Vigneau, J. Bibcode: 2000A&A...359..373M Altcode: 28 min time series of high resolution magnetograms, continuum filtergrams and line core filtergrams have been obtained in the line FeI 6173, with the 50 cm refractor of the Pic du Midi Observatory, in a quiet area near the solar disk center. Their comparison shows that every bright point of the photospheric network coincides better than 0"5 with a magnetic element of small size (smaller than 1"). The maximum of magnetic field strength of a magnetic element does not necessarily occur at the same time as the maximum of brightness. Such spatial and temporal relations cannot be explained by static models of flux tubes, but they can be by a dynamic flux tube model, as computed by Steiner et al. (1996). The observations also show that a magnetic element present in the network becomes bright and forms a bright point when it is compressed by the surrounding granules as they converge. This is an indication that the dynamics of granulation plays a major role in the concentration of the magnetic field into thin flux tubes, near the surface of the Sun. Title: The Solar Granulation Authors: Muller, R. Bibcode: 1999ASSL..239...35M Altcode: 1999msa..proc...35M The series of lectures I gave at the Kanzelhöhe summer-school, are presented here in the form of a wide overview of our current knowledge on the solar granulation, including observational results as well as theoretical modelling. A brief description of techniques of observation and image processing are included. Observational results are compared to theoretical expectations -- and vice-versa -- as often as possible. One can notice a rather good agreement between observations and theoretical expectations, which means that the solar granulation is fairly well understood. The last sections concern the excitation of 5-min oscillations in the convective pattern, the interaction between granules and magnetic flux tubes, and the variation of granulation over the solar cycle. Title: White-Light Enhancements and Small-Scale Chromospheric Activities in an Active Region Authors: Kitai, Reizaburo; Muller, Richard Bibcode: 1996SoPh..165..155K Altcode: A microflare or a group of Ellerman bombs was found to be associated with several points of white-light enhancements. These points had similar sizes as facular points (d ≃ 0.3 arc sec). Temporal evolution of these activities is described. Origins of these activities are discussed to be deeply seated excess heating in chromospheric and photospheric levels. Title: The proper motion of network bright points and the heating of the solar corona. Authors: Muller, R.; Roudier, Th.; Vigneau, J.; Auffret, H. Bibcode: 1994A&A...283..232M Altcode: Since it has been proved that acoustic waves are inefficient, it has been widely accepted that the solar corona is heated by waves which propagate along magnetic flux tubes and are generated by the turbulent granular motions. Such motions, however, may have yet been measured nor the involved energy evaluated. We present in this paper the first measurements of the proper motion of the footprints of magnetic flux tubes, at the photospheric level, outside active regions. For this purpose, we have used a high resolution granulation movie obtained at the Pic du Midi Observatory, in which we were able to identify many network bright points (NBPs). As they are associated to the magnetic flux tubes, we have used these tiny bright points as tracers of magnetic fields. Velocity histograms have been derived. They show that NBPs proper motion is turbulent in nature and that its mean speed is 1.4 km/s. Velocities as large as 3 km/s have been measured. Using then the number density of NBPs derived by Muller & Roudier (1984) 200/100 sec x 100 sec, it has been possible to calculate the energy flux which can be carried toward the corona, using an expression recently published by Choudhuri et al. (1993a). This flux appears to be about one order of magnitude larger than is necessary. Actually, the wave flux should even be much larger than that, if we keep in mind that the magnetic flux visible as NBPs, represents only a fraction of the total magnetic flux really present in the photosphere, outside active regions. We conclude that the solar corona may really be heated by waves which propagate along magnetic flux tubes rooted in the photosphere, and shaken by granular motions. The propagation of these waves along flux tubes needed however to be understood better. Title: Formation of Network Bright Points by Granule Compression Authors: Muller, R.; Roudier, Th. Bibcode: 1992SoPh..141...27M Altcode: Network Bright Points (NBPs) are tiny, subarcsecond, bright features, visible in high-resolution filtergrams taken in white light as well as in photospheric and chromospheric absorption lines. They form the photospheric network and are associated with kilogauss, concentrated magnetic fields. Their behaviour is studied in a 3-hour, high-resolution granulation movie recorded at the Pic-du-Midi Observatory and processed at Lockheed Palo Alto Research Laboratory. The movie shows the important role played by granules. It appears that NBPs are formed in dark spaces when surrounding granules converge to fill this space. The formation is a fast phenomenon which lasts only 4 min. The lifetime of NBPs is 18 min on the average. About 15% of them split when they are squeezed between two expanding granules. Some consequences concerning the strength of the magnetic field during the formation of NBPs are discussed. 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: Fine Structure of Umbrae and Penumbrae Authors: Muller, R. Bibcode: 1992ASIC..375..175M Altcode: 1992sto..work..175M The fine structure of a well developed sunspot, as observed with a spatial resolution of 0.3 or 0.25 arcsec, is described. In addition to the morphological properties, the small scale features of umbrae and penumbrae are examined with emphasis on their association with the velocity and magnetic field patterns. The main classes of models are described. Finally, some recommendations are given concerning future observations of sunspots. 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: Center-to-limb variation of the network bright points in the solar photosphere Authors: Auffret, H.; Muller, R. Bibcode: 1991A&A...246..264A Altcode: A set of very high resolution photographs is used to analyze in detail the center-to-limb variation (CLV) of the contrast of the network bright points (NBPs) which form the photospheric network. The observations were made along the equator, away from active regions. On an average, the contrast increases from 8 percent at the disk center to 27 percent at mu = 0.3 and then decreases closer to the limb. However, because most NBPs disappear selectively toward the limb, this CLV is not representative of a characteristic point. A significant CLV of the contrast can be obtained only for the brightest, medium size NBPs (0.4-0.7 arcsec), their contrast measured with a 0.23-arcsec resolution increases from 22 percent at the center of the disk to 40 percent at mu = 0.4 and then decreases closer to the limb. Title: A solar interferometric mission for ultrahigh resolution imaging and spectroscopy: SIMURIS Authors: Damé, L.; Acton, L.; Bruner, M.; Connes, P.; Cornwell, T.; Foing, B. H.; Heyvaerts, J.; Lemaire, P.; Martić, M.; Muller, R.; Porteneuve, J.; Roca Cortés, T.; Riehl, J.; Rutten, R.; Séchaud, M.; Smith, P.; Thorne, A. P.; Title, A. M.; Vial, J. -C.; Visser, H.; Weigelt, G. Bibcode: 1991AdSpR..11a.383D Altcode: 1991AdSpR..11..383D SIMURIS is an interferometric investigation of the very fine structure of the solar atmosphere from the photosphere to the corona. It was proposed to ESA /1/, November 30 1989, for the Next Medium Size Mission - M2, and accepted in February 1990 for an Assessment Study in the context of the Space Station. The main scientific objectives will be outlined, and the ambitious model payload featuring the Solar Ultraviolet Network (SUN), a 2 m long monolithic array of 4 telescopes of Ø20 cm, and the Imaging Fourier Transform Spectrometer (IFTS), an UV and Visible Imaging Fourier Transform Spectrometer coupled to a Ø40 cm Gregory, described. Title: Results from high resolution solar images and spectra obtained at the Pic du Midi Observatory (1986-1990) Authors: Roudier, Th.; Muller, R.; Vigneau, J.; Auffret, H.; Espagnet, O.; Simon, G. W.; Title, A. M.; Frank, Z.; Shine, R. A.; Tarbell, T. D.; Mein, P.; Malherbe, J. M. Bibcode: 1991AdSpR..11e.205R Altcode: 1991AdSpR..11..205R We present an overview of our recent results about solar granulation and mesogranulation, obtained with Pic du Midi observations. These results were obtained during 1986-1990 using image and spectrographic analysis of high spatial resolution data. The study of the solar granulation, with 2 Dim. ``Multichannel Subtractive Double Pass'' (M.S.D.P.) spectra, shows a clear change of the dynamical regime at 3'' (⋍ 2200 km) of the photospheric velocity field when oscillatory components are filtered out.

A three hour movie obtained on film at Pic du Midi Observatory and analyzed at the Lockheed Research Laboratory and the National Solar Observatory (Sacramento Peak) was used to calculate the horizontal flow pattern. The mean lifetime of the diverging areas related to mesogranulation is estimated at 3 hours; these diverging areas are swept by the supergranulation flow towards the supergranule boundary with a mean speed of 0.4 km/s. Title: The large-scale pattern formed by the spatial distribution of granules Authors: Muller, R.; Roudier, Th.; Vigneau, J. Bibcode: 1990SoPh..126...53M Altcode: The spatial distribution of granule sizes at the surface of the sun is investigated. Granules have been separated into two classes: those larger than 1″.37 and those smaller, where 1″.37 is the critical scale defined by Roudier and Muller (1986). It is found that granules are not distributed at random: large granules appear to be clustered, forming a cellular pattern with a characteristic scale of 7″; small granules form a similar and complementary pattern. These patterns are probably related to the mesogranulation. Title: Solar Ultraviolet Network: an interferometric investigation of the fundamental solar astrophysical scales Authors: Dame, Luc; Moreau, Bernard G.; Cornwell, Timothy J.; Visser, H.; Title, Alan M.; Acton, Loren W.; Aime, Claude; Braam, Bart M.; Bruner, Marilyn E.; Connes, Pierre; Faucherre, Michel; Foing, B. H.; Haisch, Bernhard M.; Hoekstra, Roel; Heyvaerts, Jean; Jalin, Rene; Lemaire, Philippe; Martic, Milena; Muller, R.; Noens, J. C.; Porteneuve, Jacques; Schulz-Luepertz, E.; von der Luehe, Oskar Bibcode: 1989SPIE.1130..126D Altcode: The Solar UV Network (SUN) presently proposed is an interferometric system, based on the principles of stabilized interferometry, which will be capable of solar observations with spatial resolutions better than 0.013 arcsec. SUN will consist of four 20-cm diameter telescopes aligned nonredundantly on a 2-m baseline. SUN is judged to be ideally deployable by the NASA Space Station, if implemented on a pointing platform whose performance is of the order of the Instrument Pointing System flown on Spacelab 2. The compact, nonredundant configuration of SUN's telescopes will allow high-resolution imaging of a 2 x 2 arcsec field on the solar disk. Title: Perturbation of the Granular Pattern by the Presence of Magnetic Flux Tubes Authors: Muller, R.; Roudier, Th.; Hulot, J. C. Bibcode: 1989SoPh..119..229M Altcode: The presence of flux tubes, visible as Network Bright Points (NBP) at the surface of the Sun outside active regions, disturbs the granular pattern in a similar way as it can be observed in laboratory convection experiments. Around an NBP, granules are smaller, more numerous than around a normal intergranular space without NBP; they are elongated and pointing towards it. Such a perturbation is visible several minutes before the appearance of the NBP, indicating that the magnetic flux, although not yet visible as a bright point, is already interacting with the convection pattern. The perturbation reaches its maximum within one minute after the NBP appearance; and the granular pattern returns to normal just after the NBP disappearance. Title: The Variation of the Mean Size of the Photospheric Granules Close to and Away from a Sunspot Authors: Macris, C.; Prokakis, Th.; Dialetis, D.; Muller, R. Bibcode: 1989SoPh..122..209M Altcode: We study the mean size of granules as a function of distance from the boundaries of the sunspot penumbra. We use for the determination of the mean size two different methods, a visual and a photometric. In all cases the mean diameter of the granules away from the spot was greater than the mean diameter of the granules in the neighbourhood of the penumbra. Our study is based on an excellent sequence of photos, taken at the Pic-du-Midi Observatory on May 11, 1979. Title: Center to limb variation of solar granulation Authors: Jain, R.; Muller, R. Bibcode: 1989SoPh..123..185J Altcode: From high resolution white light photographs it is determined that the surface density of solar granulation decreases towards limb. A schematic model is proposed to interpret both decrease of surface density and size of granules in the upper photosphere. Title: A possible relation between lifetime and location of solar granules Authors: Dialetis, D.; Macris, C.; Prokakis, T.; Muller, R. Bibcode: 1988A&A...204..275D Altcode: The authors have studied the spatial distribution of the long-lived granules from a population of 346 granules located in a photospheric region 37arcsec×37arcsec. The study is based on an exceptional sequence of pictures taken at the Pic-du-Midi Observatory on May 16, 1979. The authors consider as long-lived granules all the granules with lifetime greater than the mean lifetime of the population (≈15 min). They have found that they are not randomly distributed, but they are located in cellular regions that cover ≈35% of the whole area of the region under study. The possible relation of the pattern composed by the long-lived granules with the "mesogranulation" is discussed. Title: Variability of solar granulation Authors: Muller, R. Bibcode: 1988AdSpR...8g.159M Altcode: 1988AdSpR...8..159M The temporal variation of the mean size of granules recently discovered (Macris et al., 1983; Macris et al. 1983; Muller and Roudier, 1984) is confirmed using a new set of photographs of higher quality and homogeneity : granules are smaller at sunspot maximum. However the amplitude of the variation is found to be much smaller : 5 % instead of 10 to 20 %. Moreover it is found that the number of granules is highly dependent of the image quality; this has to be taken into account when counting granules.

In this paper, as well as in the previous ones, granules were identified visually, making it difficult to maintain the same granule definition for every analysed frame. Two computer image processings are presented and discussed, namely an auto-correlation analysis and an isophote countour display in order to try to overcome this difficulty. However both processing methods appear to be inappropriate. The first process fails because at very high spatial resolution (0.25), the granulation has no longer a characteristic size (its size histogram showing an increasing number of small granules, Roudier and Muller, 1986). Granules appear to have more and more fine structure and they are not homogeneously distributed at the surface of the Sun : the larger (> 1.4, the critical size), as well as the smaller granules form a pattern of mesogranulation scale. The variation of the granule number density (or the granule average size) is well correlated not only with the variation of the annual mean number of sunspots, but also with the number of Network Bright Points. NBPs are the tiny bright points forming the photospheric network and associated with magnetic flux tubes. Their number, which is proportional to the magnetic flux outside of active regions, varies nearly in antiphase with the sunspot number by a factor of 3 or so. The variation of the size of granules probably results from the interaction between the convection zone and the magnetic field at the global scale of the Sun. However, from the correlation with the NBP variation, a local interaction between granules and flux tubes cannot be excluded. Title: Motions around a Decaying Sunspot Authors: Muller, R.; Mena, B. Bibcode: 1987SoPh..112..295M Altcode: We have measured the motion of facular points and granules in the same region near a decaying sunspot. It is found that both features move away across the moat surrounding the sunspot. The mean speed of facular points is larger than that of granules: 0.65 km s−1 and 0.4 km s−1, respectively. These results are consistent with previous measurements of the speed of bright network features and moving magnetic fields, as well as of non-magnetic photospherical material. They support models in which a decaying sunspot is at the center of a supergranule, whose horizontal motions sweep out granules and magnetic flux tubes associated to the facular points. It is also found that granules are dragged by supergranular motions away of the moat. Title: Dynamics of the solar granulation. Authors: Muller, R.; Roudier, Th.; Malherbe, J. M.; Mein, P. Bibcode: 1987PAICz..66..175M Altcode: 1987eram....1..175M In order to understand the origin of the solar granulation, convective or turbulent, in this paper the authors report results about the intensity-velocity correlation and the energy power spectrum derived from the Doppler shifts measurement on two-dimensional high resolution spectrograms. Title: Fine structure of active regions. Authors: Muller, R. Bibcode: 1987PAICz..66...15M Altcode: 1987eram....1...15M Properties of the fine structure of a well developped active region, as visible at the photospheric level, including umbral dots and penumbral filaments of sunspots, facular points, disturbed granules, are described. Properties of a few chromospheric features are also described. Empirical and theoretical models which can explain the observed properties are discussed. Title: Supergranular motions around a decaying sunspot. Authors: Muller, R.; Mena, B. Bibcode: 1987PAICz..66..181M Altcode: 1987eram....1..181M The authors have measured the motion of facular points and granules in the same region near a decaying sunspot. It is found that both solar features move away across the moat surrounding the sunspot. The results support models in which a decaying sunspot is at the center of a supergranule, whose horizontal motions sweep out granules and magnetic flux tubes associated to the facular points. Title: Structure of the solar granulation. Authors: Roudier, Th.; Muller, R. Bibcode: 1986SoPh..107...11R Altcode: 1987SoPh..107...11R The structure of the solar granulation has been analysed using computer-processed images of two very high resolution (0″.25) white-light pictures obtained at the Pic-du-Midi Observatory. Title: The Fine Structure of the Quiet Sun Authors: Muller, R. Bibcode: 1985SoPh..100..237M Altcode: The observed properties of the small-scale features visible in the quiet photosphere — the granulation, of convective origin, and the network bright points, associated with kG magnetic fields — are described. The known properties of the magnetic flux tubes associated with network bright points are also presented. Empirical models derived from the observations are discussed, as well as a few theoretical models of particular importance for the understanding of the origin of the small-scale features of the quiet photosphere. Finally, the observational evidences showing that the structure of the granulation and of the photospheric network are varying over the solar cycle are reported. Title: On the Structure of Sunspot Penumbra Authors: Muller, R. Bibcode: 1985SoPh...98...51M Altcode: The structure of sunspot penumbrae is discussed in terms of bright filaments on a dark background, as opposed to dark filaments above a bright granular background. 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: The Structure of the Solar Granulation Authors: Muller, R.; Roudier, Th. Bibcode: 1985LNP...233..242M Altcode: 1985hrsp.proc..242M The structure of the solar granulation has been analysed using computer-processed images of two very high resolution (0.25) white light pictures obtained at Pic du Midi Observatory. The narrow range of sizes of granules is not confirmed: on the contrary, it is found that the number of granules increases continuously toward the smaller scales; this means that the solar granulation has no characteristic scale. Nevertheless the solar granules appear to have a vertical size of 1.37, for which drastic changes in the properties of granules occur; in particular the fractal dimension changes at the critical size, which is also the size of the granules providing the largest contribution to the total granule area and radiation. The granules smaller than the critical size could be of turbulent origin. Title: Simulated Correlation Tracking on Solar Granulation Authors: Andreassen, O.; Engvold, O.; Muller, R. Bibcode: 1985LNP...233...91A Altcode: 1985hrsp.proc...91A The applicability of low contrast granulation images of the sun to telescope tracking over an extended period of time is evaluated. The reliability of correlation tracking in solar telescopes is tested by simulating correlation tracking in a time series of high resolution images of solar granulation. The cross correlation power for images recorded on May 16, 1979 at the Pic-du-Midi Observatory, France is calculated. The relationships between the cross-correlation peak value and time, and the peak value and the overlap area of the two images are analyzed. The data reveal that the granulation structure observed during good seeing shows identifiable peak values of the cross-correlation for time differences less than 8-9 minutes. Examples of the correlation scheme applied to areas of 21 arcsec x 21 arcsec are presented. It is noted that the cross-correlation technique is useful for recognizing an area of the sun for tracking over periods that are several times the lifetime of granules. Title: High Resolution in Solar Physics Authors: Muller, R. Bibcode: 1985LNP...233.....M Altcode: 1985hrsp.proc.....M Papers are presented on the Large European Solar Telescope, the European Observatory at the Canary Islands, high resolution solar observations, and adaptive image stabilization of solar observations. Topics discussed include data reduction techniques for high resolution observations, the fine structure of the solar photosphere, and procedures for magnetic field strength determination. Consideration is given to the theoretical analysis of fine photospheric structures and the use of surface small-scale features in the study of the solar interior. 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: Variability of the Quiet Photospheric Network Authors: Muller, R.; Roudier, Th. Bibcode: 1984SoPh...94...33M Altcode: High-resolution photographs of the photospheric network taken in the CaII K 3933 Å line and at λ4308 Å are analysed in order to study the variation, in latitude and over the sunspot cycle, of its density (the density is defined as the number of network elements - also called facular points - per surface unity). It appears that the density of the photospheric network is not distributed uniformly at the surface of the Sun: on September 1983, during the declining phase of the current activity cycle, it was weakened at both the low (equatorial) and high (polar) active latitudes, while it was tremendously enhanced toward the pole. The density at the equator is varying in antiphase to the sunspot number: it increases by a factor 3 or more from maximum to minimum of activity. As a quantum of magnetic flux is associated to each network element, density variations of the photospheric network express in fact variations of the quiet Sun magnetic flux. It thus results that the quiet Sun magnetic flux is not uniformly distributed in latitude and not constant over the solar cycle: it probably varies in antiphase to the flux in active regions. Title: On the Relation Between Chromospheric and Photospheric Fine Structure in an Active Region Authors: Kitai, R.; Muller, R. Bibcode: 1984SoPh...90..303K Altcode: A comparative study was done on the bright fine structure in the upper photosphere and in the lower chromosphere of an active region. The results are shown in the following: (a) The bright points in the Hα wing are cospatial to the facular points, which confirms the result of Wilson (1981). (b) Some points bright in the Hα wing are associated with the facular granules which have larger sizes than the facular points, (c) The brightness enhancement in the Ha wing is positively correlated to the enhancement in the blue continuum. However, the correlation is not so strong. (d) The moustache points are also cospatial to the facular features. (e) The geometrical shape of a moustache point is like a funnel and diverging upward in the upper photospheric and the lower chromospheric levels. Title: The fine structure of light-bridges in sunspots Authors: Muller, R. Bibcode: 1979SoPh...61..297M Altcode: 1978SoPh...61..297M High resolution photographs obtained at the Pic du Midi Observatory show that there are three types of sunspot light bridges according to their morphological structures: the photospheric ones, the penumbral ones and the umbral ones. Consequently there are no specific structures in light bridges; it results that they should not be due to specific physical properties. Properties of the fine structure of a penumbral light bridge are described. Title: Morphological properties and origin of the photospheric facular granules. Authors: Muller, R. Bibcode: 1977SoPh...52..249M Altcode: From time series of high resolution photographs, morphological properties of the photospheric facular granules were derived. The facular granules are cells of the common granular pattern, brighter than the normal granules when seen between cos θ = 0.6 and the limb. Their apparent diameter, which decreases towards the limb, is smaller than that of the normal granules: 0″.65 and 1″.25 respectively at cos θ = 0.55; their lifetime is 25 min but their `bright stage' lifetime is only 15 min; they are visible closer to the limb than the normal granules: 1″.2 compared to 2″-5″; the brightening of the facular granules occurs at a faster rate than their fading. From the great similitude of both morphological properties and temperature models of facular and normal granules, it appears possible that the photospheric facular granules are convective cells modified by the presence of a magnetic field of some hundreds Gauss. Title: Characteristics of the Displacement of the Penumbral Bright Grains of Sunspots Authors: Muller, R. Bibcode: 1976SoPh...48..101M Altcode: It is confirmed that the penumbral bright grains are moving towards the sunspots umbra. At the umbra-penumbra boundary their horizontal velocity is about 0.5 km s−1 and their displacement is inclined downwards with an angle of 5° to 20°. Title: A Model of Photospheric Faculae Deduced from White Light High Resolution Pictures Authors: Muller, R. Bibcode: 1975SoPh...45..105M Altcode: High resolution pictures (about 0″.3) of photospheric faculae near the solar limb have been obtained with the Pic du Midi 50 cm refractor; their granular structure then clearly appears. The microphotometric study of these facular granules shows that the ratio of their intensity to the photospheric intensity, If/Iph (cosθ) reaches a maximum near cosθ = 0.3 and then decreases towards the limb. The values of this ratio have been corrected with a most likely spread function. Then a temperature model of a facular granule is obtained: with respect to the neighbouring photosphere, this granule appears as a photospheric `hot cloud' which does not extend high in the solar atmosphere (thickness 100 km above τ5000 = 1). The temperature excess is 750K at maximum. This hot region is located over a layer which is cooler than the normal photosphere at the same level. Another hot region might extend above the photospheric `hot cloud', possibly up to the chromosphere. This photospheric facula model which is confined to the lower photosphere seems to indicate that this phenomenon is different from the photospheric network which is visible up to the lower chromosphere. Title: Étude photométrique des structures fines de la pénombre d'une tache solaire Authors: Muller, R. Bibcode: 1973SoPh...32..409M Altcode: The microphotometric analysis of the fine structure of a sunspot penumbra, photographed in white light with the 38 cm refractor of the Pic du Midi Observatory with a resolution very close to 0''.3, allows to give from it, at λ5280, the following picture: the penumbra appears to consist of bright grains, lined up in the form of filaments, with an average brightness I/Ib = 0.95 of average width 0″.36 (270 km) and which cover 43% of its surface, showing up a dark background of brightness I/Id = 0.6 nearly uniform.