explanation blue bibcodes open ADS page with paths to full text
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.
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.
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.
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. <BR /> 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. <BR />
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. <BR /> 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. <BR /> 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.
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. <BR /> 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. <BR /> 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. <BR /> 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.
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. <P />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. <P />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
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.
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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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.
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. <BR />
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. <BR /> 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. <BR
/> 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. <BR /> 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.
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.
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. <BR /> 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. <BR /> 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. <BR /> 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.
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
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.
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. <BR />
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. <BR
/> 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. <BR /> 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<SUP>-1</SUP>. 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<SUP>-1</SUP>. 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
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.
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. <BR />Aims: The
size distribution of MBPs is derived for G-band images acquired by the
Hinode/SOT instrument. <BR />Methods: For identification purposes, a new
automated segmentation and identification algorithm was developed. <BR
/>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. <BR />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
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.
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. <BR
/>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. <BR
/>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.
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.
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.
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.
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<SUP>-1</SUP> 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<SUP>-1</SUP>) 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.
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.
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
Mm<SUP>2</SUP>). 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.
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.
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.
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.
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
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.
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.
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.
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 spectroheliograph<SUP>1,2</SUP>,
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 observations<SUP>3-5</SUP> to be nestling inside
the bright network. More recently<SUP>6,7</SUP> 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.
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.
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.
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.
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.
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.
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. <P />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.
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
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.
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.
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.
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.
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.
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. <P />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.
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<SUP>−1</SUP> and
0.4 km s<SUP>−1</SUP>, 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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<SUP>−1</SUP> 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.
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, I<SUP>f</SUP>/I<SUP>ph</SUP> (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 τ<SUB>5000</SUB>
= 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.
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/I<SUB>b</SUB> = 0.95 of average width
0″.36 (270 km) and which cover 43% of its surface, showing up a dark
background of brightness I/I<SUB>d</SUB> = 0.6 nearly uniform.
