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Author name code: martinez-gonzalez
ADS astronomy entries on 2022-09-14
author:"Martinez Gonzalez, Maria J."
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Title: Mapping the Hidden Magnetic Field of the Quiet Sun
Authors: Trelles Arjona, J. C.; Martínez González, M. J.; Ruiz
Cobo, B.
2021ApJ...915L..20T Altcode: 2021arXiv210610546T
The Sun is the only star where we can resolve the intricate magnetism
that all convective stars harbor. Yet, more than 99% of its visible
surface along the solar cycle (the so-called quiet Sun) is filled with
a tangled, unresolved magnetism. These "hidden" fields are thought
to store enough magnetic energy to play a role in the heating of
the Sun's outer atmosphere, but its field strength is still not
constrained. Previous investigations based on the Hanle effect in
atomic lines claim a strong magnetization of about 100 G, while
the same effect in molecules show a factor of 10 weaker fields. The
discrepancy disappears if the magnetic field strength of the hidden
field is not homogeneous in the solar surface. In this Letter, we
prove using magnetohydrodynamical simulations that it is possible
to infer the average field strength of the hidden quiet-Sun magnetic
fields using multiline inversions of intensity profiles in the Zeeman
regime. Using this technique with 15 spectral lines in the 1.5 μm
spectral range, we reveal that the spatial distribution of the hidden
field is strongly correlated with convection motions, and that the
average magnetization is about 46 G. Reconciling our findings with
the Hanle ones is not obvious and will require future work on both
sides, since it implies an increase of the field strength with height,
something that is physically questionable.
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Title: Empirical determination of atomic line parameters of the 1.5
μm spectral region
Authors: Trelles Arjona, J. C.; Ruiz Cobo, B.; Martínez González,
M. J.
2021A&A...648A..68T Altcode: 2021arXiv210304160T
Context. Both the quality and amount of astrophysical data are steadily
increasing over time owing to the improvement of telescopes and their
instruments. This requires corresponding evolution of the techniques
used for obtaining and analyzing the resulting data. The infrared
spectral range at 1.56 μm usually observed by the GRegor Infrared
Spectrograph (GRIS) at the GREGOR solar telescope has a width of
around 30 Å and includes at least 15 spectral lines. Normally, only a
handful of spectral lines (five at most) are used in studies using GRIS
because of the lack of atomic parameters for the others. Including more
spectral lines may alleviate some of the known ambiguities between
solar atmospheric parameters. <BR /> Aims: We used high-precision
spectropolarimetric data for the quiet Sun at 1.56 μm observed with
GRIS on the GREGOR along with the SIR inversion code in order to obtain
accurate atomic parameters for 15 spectral lines in this spectral
range. <BR /> Methods: We used inversion techniques to infer both solar
atmospheric models and the atomic parameters of spectral lines which,
under the local thermodynamic equilibrium approximation, reproduce
spectropolarimetric observations. <BR /> Results: We present accurate
atomic parameters for 15 spectral lines within the spectral range from
15 644 to 15 674 Å. This spectral range is commonly used in solar
studies because it enables the study of the low photosphere. Moreover,
the infrared spectral lines are better tracers of the magnetic fields
than the optical ones.
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Title: Photospheric magnetic topology of a north polar region
Authors: Pastor Yabar, A.; Martínez González, M. J.; Collados, M.
2020A&A...635A.210P Altcode: 2020arXiv200304267P
<BR /> Aims: We aim to characterise the magnetism of a large fraction
of the north polar region close to a maximum of activity, when the
polar regions are reversing their dominant polarity. <BR /> Methods:
We make use of full spectropolarimetric data from the CRisp Imaging
Spectro-Polarimeter installed at the Swedish Solar Telescope. The data
consist of a photospheric spectral line, which is used to infer the
various physical parameters of different quiet Sun regions by means
of the solution of the radiative transfer equation. We focus our
analysis on the properties found for the north polar region and their
comparison to the same analysis applied to data taken at disc centre
and low-latitude quiet Sun regions for reference. We also analyse
the spatial distribution of magnetic structures throughout the north
polar region. <BR /> Results: We find that the physical properties
of the polar region (line-of-sight velocity, magnetic flux, magnetic
inclination and magnetic azimuth) are compatible with those found
for the quiet Sun at disc centre and are similar to the ones found
at low latitudes close to the limb. Specifically, the polar region
magnetism presents no specific features. The structures for which
the transformation from a line-of-sight to a local reference frame
was possible harbour large magnetic fluxes (>10<SUP>17</SUP> Mx)
and are in polarity imbalance with a dominant positive polarity, the
largest ones (>10<SUP>19</SUP> Mx) being located below 73° latitude.
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Title: Science Requirement Document (SRD) for the European Solar
Telescope (EST) (2nd edition, December 2019)
Authors: Schlichenmaier, R.; Bellot Rubio, L. R.; Collados, M.;
Erdelyi, R.; Feller, A.; Fletcher, L.; Jurcak, J.; Khomenko, E.;
Leenaarts, J.; Matthews, S.; Belluzzi, L.; Carlsson, M.; Dalmasse,
K.; Danilovic, S.; Gömöry, P.; Kuckein, C.; Manso Sainz, R.;
Martinez Gonzalez, M.; Mathioudakis, M.; Ortiz, A.; Riethmüller,
T. L.; Rouppe van der Voort, L.; Simoes, P. J. A.; Trujillo Bueno,
J.; Utz, D.; Zuccarello, F.
2019arXiv191208650S Altcode:
The European Solar Telescope (EST) is a research infrastructure
for solar physics. It is planned to be an on-axis solar telescope
with an aperture of 4 m and equipped with an innovative suite of
spectro-polarimetric and imaging post-focus instrumentation. The EST
project was initiated and is driven by EAST, the European Association
for Solar Telescopes. EAST was founded in 2006 as an association
of 14 European countries. Today, as of December 2019, EAST consists
of 26 European research institutes from 18 European countries. The
Preliminary Design Phase of EST was accomplished between 2008 and
2011. During this phase, in 2010, the first version of the EST Science
Requirement Document (SRD) was published. After EST became a project
on the ESFRI roadmap 2016, the preparatory phase started. The goal
of the preparatory phase is to accomplish a final design for the
telescope and the legal governance structure of EST. A major milestone
on this path is to revisit and update the Science Requirement Document
(SRD). The EST Science Advisory Group (SAG) has been constituted by
EAST and the Board of the PRE-EST EU project in November 2017 and has
been charged with the task of providing with a final statement on the
science requirements for EST. Based on the conceptual design, the SRD
update takes into account recent technical and scientific developments,
to ensure that EST provides significant advancement beyond the current
state-of-the-art. The present update of the EST SRD has been developed
and discussed during a series of EST SAG meetings. The SRD develops
the top-level science objectives of EST into individual science
cases. Identifying critical science requirements is one of its main
goals. Those requirements will define the capabilities of EST and the
post-focus instrument suite. The technical requirements for the final
design of EST will be derived from the SRD.
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Title: High-resolution spectroscopy of Boyajian's star during optical
dimming events
Authors: Martínez González, M. J.; González-Fernández, C.; Asensio
Ramos, A.; Socas-Navarro, H.; Westendorp Plaza, C.; Boyajian, T. S.;
Wright, J. T.; Collier Cameron, A.; González Hernández, J. I.;
Holgado, G.; Kennedy, G. M.; Masseron, T.; Molinari, E.; Saario, J.;
Simón-Díaz, S.; Toledo-Padrón, B.
2019MNRAS.486..236M Altcode: 2019MNRAS.tmp..828M; 2018arXiv181206837M
Boyajian's star is an apparently normal main-sequence F-type star
with a very unusual light curve. The dipping activity of the star,
discovered during the Kepler mission, presents deep, asymmetric,
and aperiodic events. Here we present high-resolution spectroscopic
follow-up during some dimming events recorded post-Kepler observations,
from ground-based telescopes. We analyse data from the HERMES, HARPS-N,
and FIES spectrographs to characterize the stellar atmosphere and
to put some constraints on the hypotheses that have appeared in the
literature concerning the occulting elements. The star's magnetism,
if existing, is not extreme. The spots on the surface, if present,
would occupy 0.02 per cent of the area, at most. The chromosphere,
irrespective of the epoch of observation, is hotter than the values
expected from radiative equilibrium, meaning that the star has some
degree of activity. We find no clear evidence of the interstellar
medium or exocoments being responsible for the dimmings of the light
curve. However, we detect at 1-2σ level, a decrease of the radial
velocity of the star during the first dip recorded after the Kepler
observations. We claim the presence of an optically thick object with
likely inclined and high impact parameter orbits that produces the
observed Rossiter-McLaughlin effect.
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Title: Spectropolarimetric analysis of an active region
filament. II. Evidence of the limitations of a single-component model
Authors: Díaz Baso, C. J.; Martínez González, M. J.; Asensio
Ramos, A.
2019A&A...625A.129D Altcode: 2019arXiv190410688D
<BR /> Aims: Our aim is to demonstrate the limitations of using a
single-component model to study the magnetic field of an active region
filament. To do this, we analyzed the polarimetric signals of the He I
10830 Å multiplet, which were acquired with the infrared spectrograph
GRIS of the GREGOR telescope (Tenerife, Spain). <BR /> Methods: After
a first analysis of the general properties of the filament using HAZEL
under the assumption of a single-component model atmosphere, in this
second part we focus our attention on the observed Stokes profiles
and the signatures that cannot be explained with this model. <BR />
Results: We have found an optically thick filament whose blue and
red components have the same sign in the linear polarization as an
indication of radiative transfer effects. Moreover, the circular
polarization signals inside the filament show strong magnetic field
gradients. We also show that even a filament with such high absorption
still shows signatures of the circular polarization that is generated
by the magnetic field below the filament. The reason is that the
absorption of the spectral line decays very quickly toward the wings,
just where the circular polarization has a larger amplitude. In order
to separate the two contributions, we explore the possibility of a
two-component model, but the inference becomes impossible to overcome
because very many solutions are compatible with the observations.
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Title: Spectropolarimetric analysis of an active region
filament. I. Magnetic and dynamical properties from single component
inversions
Authors: Díaz Baso, C. J.; Martínez González, M. J.; Asensio
Ramos, A.
2019A&A...625A.128D Altcode: 2019arXiv190409593D
<BR /> Aims: The determination of the magnetic filed vector in solar
filaments is made possible by interpreting the Hanle and Zeeman
effects in suitable chromospheric spectral lines like those of the
He I multiplet at 10 830 Å. We study the vector magnetic field of an
active region filament (NOAA 12087). <BR /> Methods: Spectropolarimetric
data of this active region was acquired with the GRIS instrument at
the GREGOR telescope and studied simultaneously in the chromosphere
with the He I 10 830 Å multiplet and in the photosphere Si I 10 827
Å line. As has been done in previous studies, only a single-component
model was used to infer the magnetic properties of the filament. The
results are put into a solar context with the help of the Solar Dynamic
Observatory images. <BR /> Results: Some results clearly point out
that a more complex inversion had to be performed. First, the Stokes
V map of He I does not show a clear signature of the presence of the
filament. Second, the local azimuth map follows the same pattern as
Stokes V; it appears that polarity of Stokes V is conditioning the
inference to very different magnetic fields even with similar linear
polarization signals. This indication suggests that the Stokes V could
be dominated from below by the magnetic field coming from the active
region, and not from the filament itself. This evidence, and others,
will be analyzed in depth and a more complex inversion will be attempted
in the second part of this series.
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Title: Diagnostic potential of the Ca II 8542 Å line for solar
filaments
Authors: Díaz Baso, C. J.; Martínez González, M. J.; Asensio Ramos,
A.; de la Cruz Rodríguez, J.
2019A&A...623A.178D Altcode: 2019arXiv190206574D
<BR /> Aims: In this study we explore the diagnostic potential of
the chromospheric Ca II line at 8542 Å for studying the magnetic and
dynamic properties of solar filaments. We have acquired high spatial
resolution spectropolarimetric observations in the Ca II 8542 Å line
using the CRISP instrument at the Swedish 1 m Solar Telescope. <BR />
Methods: We used the NICOLE inversion code to infer physical properties
from observations of a solar filament. We discuss the validity of
the results due to the assumption of hydrostatic equilibrium. We have
used observations from other telescopes such as CHROTEL and SDO, in
order to study large scale dynamics and the long term evolution of the
filament. <BR /> Results: We show that the Ca II 8542 Å line encodes
information of the temperature, line-of-sight velocity and magnetic
field vector from the region where the filament is located. The current
noise levels only allow us to estimate an upper limit of 260 G for the
total magnetic field of the filament. Our study also reveals that if we
consider information from the aforementioned spectral line alone, the
geometric height, the temperature and the density could be degenerated
parameters outside the hydrostatic equilibrium approach.
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Title: Magnetic topology of the north solar pole
Authors: Pastor Yabar, A.; Martínez González, M. J.; Collados, M.
2018A&A...616A..46P Altcode: 2018arXiv180409075P; 2018A&A...616A..46Y
The magnetism at the poles is similar to that of the quiet Sun in the
sense that no active regions are present there. However, the polar quiet
Sun is somewhat different from that at the activity belt as it has a
global polarity that is clearly modulated by the solar cycle. We study
the polar magnetism near an activity maximum when these regions change
their polarity, from which it is expected that its magnetism should be
less affected by the global field. To fully characterise the magnetic
field vector, we use deep full Stokes polarimetric observations of the
15 648.5 and 15 652.8 Å FeI lines. We observe the north pole as well as
a quiet region at disc centre to compare their field distributions. In
order to calibrate the projection effects, we observe an additional
quiet region at the east limb. We find that the two limb datasets share
similar magnetic field vector distributions. This means that close to a
maximum, the poles look like typical limb, quiet-Sun regions. However,
the magnetic field distributions at the limbs are different from the
distribution inferred at disc centre. At the limbs, we infer a new
population of magnetic fields with relatively strong intensities (
600-800 G), inclined by 30° with respect to the line of sight, and
with an azimuth aligned with the solar disc radial direction. This
line-of-sight orientation interpreted as a single magnetic field gives
rise to non-vertical fields in the local reference frame and aligned
towards disc centre. This peculiar topology is very unlikely for such
strong fields according to theoretical considerations. We propose
that this new population at the limbs is due to the observation of
unresolved magnetic loops as seen close to the limb. These loops have
typical granular sizes as measured in the disc centre. At the limbs,
where the spatial resolution decreases, we observe them spatially
unresolved, which explains the new population of magnetic fields that is
inferred. This is the first (indirect) evidence of small-scale magnetic
loops outside the disc centre and would imply that these small-scale
structures are ubiquitous on the entire solar surface. This result has
profound implications for the energetics not only of the photosphere,
but also of the outer layers since these loops have been reported to
reach the chromosphere and the low corona.
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Title: Cosmic Magnetic Fields
Authors: Sánchez Almeida, J.; Martínez González, M. J.
2018cmf..book.....S Altcode:
Magnetic fields play an important role in many astrophysical
processes. They are difficult to detect and characterize since
often their properties have to be inferred through interpreting the
polarization of the light. Magnetic fields are also challenging to
model and understand. Magnetized plasmas behave following highly
non-linear differential equations having no general solution, so that
every astrophysical problem represents a special case to be studied
independently. Hence, magnetic fields are often an inconvenient subject
which is overlooked or simply neglected (the elephant in the room,
as they are dubbed in poster of the school). Such difficulty burdens
the research on magnetic fields, which has evolved to become a very
technical subject, with many small disconnected communities studying
specific aspects and details. The school tried to amend the situation
by providing a unifying view of the subject. The students had a chance
to understand the behavior of magnetic fields in all astrophysical
contexts, from cosmology to the Sun, and from starbursts to AGNs. The
school was planed to present a balanced yet complete review of our
knowledge, with excursions into the unknown to point out present and
future lines of research. The subject of Cosmic Magnetic Fields was
split into seven different topics: cosmic magnetic field essentials,
solar magnetic fields, stellar magnetic fields, the role of magnetic
fields on AGN feedback, magnetic fields in galaxies, magnetic fields in
galaxy clusters and at larger scales, and primordial magnetic fields
and magnetic fields in the early Universe. The corresponding lectures
were delivered by seven well known and experienced scientists that
have played key roles in the major advances of the field during the
last years: F. Cattaneo, P. Judge, O. Kochukhov, R. Keppens, R. Beck,
K. Dolag, and F. Finelli. Their lectures were recorded and are freely
available at the IAC website: http://iactalks.iac.es/talks/serie/19.
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Title: The First Post-Kepler Brightness Dips of KIC 8462852
Authors: Boyajian, Tabetha. S.; Alonso, Roi; Ammerman, Alex; Armstrong,
David; Asensio Ramos, A.; Barkaoui, K.; Beatty, Thomas G.; Benkhaldoun,
Z.; Benni, Paul; Bentley, Rory O.; Berdyugin, Andrei; Berdyugina,
Svetlana; Bergeron, Serge; Bieryla, Allyson; Blain, Michaela G.;
Capetillo Blanco, Alicia; Bodman, Eva H. L.; Boucher, Anne; Bradley,
Mark; Brincat, Stephen M.; Brink, Thomas G.; Briol, John; Brown,
David J. A.; Budaj, J.; Burdanov, A.; Cale, B.; Aznar Carbo, Miguel;
Castillo García, R.; Clark, Wendy J.; Clayton, Geoffrey C.; Clem,
James L.; Coker, Phillip H.; Cook, Evan M.; Copperwheat, Chris M.;
Curtis, J. L.; Cutri, R. M.; Cseh, B.; Cynamon, C. H.; Daniels, Alex
J.; Davenport, James R. A.; Deeg, Hans J.; De Lorenzo, Roberto; de
Jaeger, Thomas; Desrosiers, Jean-Bruno; Dolan, John; Dowhos, D. J.;
Dubois, Franky; Durkee, R.; Dvorak, Shawn; Easley, Lynn; Edwards, N.;
Ellis, Tyler G.; Erdelyi, Emery; Ertel, Steve; Farfán, Rafael. G.;
Farihi, J.; Filippenko, Alexei V.; Foxell, Emma; Gandolfi, Davide;
Garcia, Faustino; Giddens, F.; Gillon, M.; González-Carballo,
Juan-Luis; González-Fernández, C.; González Hernández, J. I.;
Graham, Keith A.; Greene, Kenton A.; Gregorio, J.; Hallakoun, Na'ama;
Hanyecz, Ottó; Harp, G. R.; Henry, Gregory W.; Herrero, E.; Hildbold,
Caleb F.; Hinzel, D.; Holgado, G.; Ignácz, Bernadett; Ilyin, Ilya;
Ivanov, Valentin D.; Jehin, E.; Jermak, Helen E.; Johnston, Steve;
Kafka, S.; Kalup, Csilla; Kardasis, Emmanuel; Kaspi, Shai; Kennedy,
Grant M.; Kiefer, F.; Kielty, C. L.; Kessler, Dennis; Kiiskinen,
H.; Killestein, T. L.; King, Ronald A.; Kollar, V.; Korhonen, H.;
Kotnik, C.; Könyves-Tóth, Réka; Kriskovics, Levente; Krumm, Nathan;
Krushinsky, Vadim; Kundra, E.; Lachapelle, Francois-Rene; LaCourse,
D.; Lake, P.; Lam, Kristine; Lamb, Gavin P.; Lane, Dave; Lau, Marie
Wingyee; Lewin, Pablo; Lintott, Chris; Lisse, Carey; Logie, Ludwig;
Longeard, Nicolas; Lopez Villanueva, M.; Whit Ludington, E.; Mainzer,
A.; Malo, Lison; Maloney, Chris; Mann, A.; Mantero, A.; Marengo,
Massimo; Marchant, Jon; Martínez González, M. J.; Masiero, Joseph R.;
Mauerhan, Jon C.; McCormac, James; McNeely, Aaron; Meng, Huan Y. A.;
Miller, Mike; Molnar, Lawrence A.; Morales, J. C.; Morris, Brett M.;
Muterspaugh, Matthew W.; Nespral, David; Nugent, C. R.; Nugent,
Katherine M.; Odasso, A.; O'Keeffe, Derek; Oksanen, A.; O'Meara,
John M.; Ordasi, András; Osborn, Hugh; Ott, John J.; Parks, J. R.;
Rodriguez Perez, Diego; Petriew, Vance; Pickard, R.; Pál, András;
Plavchan, P.; Pollacco, Don; Pozo Nuñez, F.; Pozuelos, F. J.; Rau,
Steve; Redfield, Seth; Relles, Howard; Ribas, Ignasi; Richards, Jon;
Saario, Joonas L. O.; Safron, Emily J.; Sallai, J. Martin; Sárneczky,
Krisztián; Schaefer, Bradley E.; Schumer, Clea F.; Schwartzendruber,
Madison; Siegel, Michael H.; Siemion, Andrew P. V.; Simmons, Brooke D.;
Simon, Joshua D.; Simón-Díaz, S.; Sitko, Michael L.; Socas-Navarro,
Hector; Sódor, Á.; Starkey, Donn; Steele, Iain A.; Stone, Geoff;
Strassmeier, Klaus G.; Street, R. A.; Sullivan, Tricia; Suomela, J.;
Swift, J. J.; Szabó, Gyula M.; Szabó, Róbert; Szakáts, Róbert;
Szalai, Tamás; Tanner, Angelle M.; Toledo-Padrón, B.; Tordai, Tamás;
Triaud, Amaury H. M. J.; Turner, Jake D.; Ulowetz, Joseph H.; Urbanik,
Marian; Vanaverbeke, Siegfried; Vanderburg, Andrew; Vida, Krisztián;
Vietje, Brad P.; Vinkó, József; von Braun, K.; Waagen, Elizabeth
O.; Walsh, Dan; Watson, Christopher A.; Weir, R. C.; Wenzel, Klaus;
Westendorp Plaza, C.; Williamson, Michael W.; Wright, Jason T.; Wyatt,
M. C.; Zheng, WeiKang; Zsidi, Gabriella
2018ApJ...853L...8B Altcode: 2018arXiv180100732B
We present a photometric detection of the first brightness dips of
the unique variable star KIC 8462852 since the end of the Kepler space
mission in 2013 May. Our regular photometric surveillance started in
2015 October, and a sequence of dipping began in 2017 May continuing
on through the end of 2017, when the star was no longer visible from
Earth. We distinguish four main 1%-2.5% dips, named “Elsie,”
“Celeste,” “Skara Brae,” and “Angkor,” which persist
on timescales from several days to weeks. Our main results so far
are as follows: (i) there are no apparent changes of the stellar
spectrum or polarization during the dips and (ii) the multiband
photometry of the dips shows differential reddening favoring non-gray
extinction. Therefore, our data are inconsistent with dip models that
invoke optically thick material, but rather they are in-line with
predictions for an occulter consisting primarily of ordinary dust,
where much of the material must be optically thin with a size scale
≪1 μm, and may also be consistent with models invoking variations
intrinsic to the stellar photosphere. Notably, our data do not place
constraints on the color of the longer-term “secular” dimming,
which may be caused by independent processes, or probe different
regimes of a single process.
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Title: Inference of the chromospheric magnetic field orientation in
the Ca II 8542 Å line fibrils
Authors: Asensio Ramos, A.; de la Cruz Rodríguez, J.; Martínez
González, M. J.; Socas-Navarro, H.
2017A&A...599A.133A Altcode: 2016arXiv161206088A
Context. Solar chromospheric fibrils, as observed in the core of
strong chromospheric spectral lines, extend from photospheric field
concentrations suggesting that they trace magnetic field lines. These
images have been historically used as proxies of magnetic fields
for many purposes. <BR /> Aims: Use statistical analysis to test
whether the association between fibrils and magnetic field lines is
justified. <BR /> Methods: We use a Bayesian hierarchical model to
analyze several tens of thousands of pixels in spectro-polarimetric
chromospheric images of penumbrae and chromospheric fibrils. We
compare the alignment between the field azimuth inferred from the
linear polarization signals through the transverse Zeeman effect and
the direction of the fibrils in the image. <BR /> Results: We conclude
that, in the analyzed fields of view, fibrils are often well aligned
with the magnetic field azimuth. Despite this alignment, the analysis
also shows that there is a non-negligible dispersion. In penumbral
filaments, we find a dispersion with a standard deviation of 16°,
while this dispersion goes up to 34° in less magnetized regions.
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Title: How to infer the Sun's global magnetic field using the
Hanle effect
Authors: Vieu, T.; Martínez González, M. J.; Pastor Yabar, A.;
Asensio Ramos, A.
2017MNRAS.465.4414V Altcode:
We present a different approach to determine the characteristics
of the global magnetic field of the Sun based on the study of the
Hanle signals. The Hanle effect of a stellar dipole produces a surface
asymmetric pattern of linear polarization that depends on the strength
and geometry of this global field. Moreover, if the dipole is misaligned
with respect to the rotation, the Hanle signals are modulated following
the rotational period. We explore the possibility to retrieve those
characteristics by comparing the computed theoretical signatures with
actual observations. We show that this is possible, in the case of the
Sr I line of the Sun, provided that the polarimetric sensitivity is of
the order or below 10<SUP>-5</SUP>-10<SUP>-6</SUP>. The inference can
be done either using the maps of resolved signals, in particular the
spread of values obtained along different directions on the stellar
disc, or using the disc-integrated signals.
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Title: Synthetic polarimetric spectra from stellar prominences
Authors: Felipe, T.; Martínez González, M. J.; Asensio Ramos, A.
2017MNRAS.465.1654F Altcode: 2016arXiv161009282F
Stellar prominences detected in rapidly rotating stars serve as probes
of the magnetism in the corona of cool stars. We have synthesized the
temporal evolution of the Stokes profiles generated in the He I 10
830 and 5876 Å triplets during the rotation of a prominence around
a star. The synthesis was performed with the HAZEL code using a cloud
model in which the prominence is characterized by a slab located at a
fixed latitude and height. It accounts for the scattering polarization
and Zeeman and Hanle effects. Several cases with different prominence
magnetic field strengths and orientations have been analysed. The
results show an emission feature that drifts across the profile while
the prominence is out of the stellar disc. When the prominence eclipses
the star, the intensity profile shows an absorption. The scattering
induced by the prominence generates linear polarization signals
in Stokes Q and U profiles, which are modified by the Hanle effect
when a magnetic field is present. Due to the Zeeman effect, Stokes V
profiles show a signal with very low amplitude when the magnetic field
along the line of sight is different from zero. The estimated linear
polarization signals could potentially be detected with the future
spectropolarimeter Mid-resolution InfRAreD Astronomical Spectrograph,
to be attached to Gran Telescopio Canarias telescope.
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Title: Probing deep photospheric layers of the quiet Sun with high
magnetic sensitivity
Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González,
M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.;
Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.;
Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.;
Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier,
K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.;
Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.;
Schmidt, D.; Kiess, C.; Nicklas, H.
2016A&A...596A...6L Altcode: 2016arXiv160506324L
Context. Investigations of the magnetism of the quiet Sun are hindered
by extremely weak polarization signals in Fraunhofer spectral
lines. Photon noise, straylight, and the systematically different
sensitivity of the Zeeman effect to longitudinal and transversal
magnetic fields result in controversial results in terms of the strength
and angular distribution of the magnetic field vector. <BR /> Aims:
The information content of Stokes measurements close to the diffraction
limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of
spatial straylight and photon noise into account. <BR /> Methods: Highly
sensitive full Stokes measurements of a quiet-Sun region at disk center
in the deep photospheric Fe I lines in the 1.56 μm region were obtained
with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise
statistics and Stokes V asymmetries were analyzed and compared to a
similar data set of the Hinode spectropolarimeter (SOT/SP). Simple
diagnostics based directly on the shape and strength of the profiles
were applied to the GRIS data. We made use of the magnetic line ratio
technique, which was tested against realistic magneto-hydrodynamic
simulations (MURaM). <BR /> Results: About 80% of the GRIS spectra
of a very quiet solar region show polarimetric signals above a 3σ
level. Area and amplitude asymmetries agree well with small-scale
surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio
analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss
range with some concentrations of kilo-Gauss fields. <BR /> Conclusions:
The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4
are so far unique in the combination of high spatial resolution scans
and high magnetic field sensitivity. Nevertheless, the unavoidable
effect of spatial straylight and the resulting dilution of the weak
Stokes profiles means that inversion techniques still bear a high risk
of misinterpretating the data.
---------------------------------------------------------
Title: Inference of magnetic fields in the very quiet Sun
Authors: Martínez González, M. J.; Pastor Yabar, A.; Lagg, A.;
Asensio Ramos, A.; Collados, M.; Solanki, S. K.; Balthasar, H.;
Berkefeld, T.; Denker, C.; Doerr, H. P.; Feller, A.; Franz, M.;
González Manrique, S. J.; Hofmann, A.; Kneer, F.; Kuckein, C.;
Louis, R.; von der Lühe, O.; Nicklas, H.; Orozco, D.; Rezaei, R.;
Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Waldman,
T.; Volkmer, R.
2016A&A...596A...5M Altcode: 2018arXiv180410089M
Context. Over the past 20 yr, the quietest areas of the solar surface
have revealed a weak but extremely dynamic magnetism occurring
at small scales (<500 km), which may provide an important
contribution to the dynamics and energetics of the outer layers of
the atmosphere. Understanding this magnetism requires the inference
of physical quantities from high-sensitivity spectro-polarimetric
data with high spatio-temporal resolution. <BR /> Aims: We present
high-precision spectro-polarimetric data with high spatial resolution
(0.4”) of the very quiet Sun at 1.56 μm obtained with the GREGOR
telescope to shed some light on this complex magnetism. <BR /> Methods:
We used inversion techniques in two main approaches. First, we assumed
that the observed profiles can be reproduced with a constant magnetic
field atmosphere embedded in a field-free medium. Second, we assumed
that the resolution element has a substructure with either two constant
magnetic atmospheres or a single magnetic atmosphere with gradients of
the physical quantities along the optical depth, both coexisting with
a global stray-light component. <BR /> Results: Half of our observed
quiet-Sun region is better explained by magnetic substructure within
the resolution element. However, we cannot distinguish whether this
substructure comes from gradients of the physical parameters along the
line of sight or from horizontal gradients (across the surface). In
these pixels, a model with two magnetic components is preferred, and
we find two distinct magnetic field populations. The population with
the larger filling factor has very weak ( 150 G) horizontal fields
similar to those obtained in previous works. We demonstrate that the
field vector of this population is not constrained by the observations,
given the spatial resolution and polarimetric accuracy of our data. The
topology of the other component with the smaller filling factor is
constrained by the observations for field strengths above 250 G:
we infer hG fields with inclinations and azimuth values compatible
with an isotropic distribution. The filling factors are typically
below 30%. We also find that the flux of the two polarities is not
balanced. From the other half of the observed quiet-Sun area 50% are
two-lobed Stokes V profiles, meaning that 23% of the field of view
can be adequately explained with a single constant magnetic field
embedded in a non-magnetic atmosphere. The magnetic field vector and
filling factor are reliable inferred in only 50% based on the regular
profiles. Therefore, 12% of the field of view harbour hG fields with
filling factors typically below 30%. At our present spatial resolution,
70% of the pixels apparently are non-magnetised.
---------------------------------------------------------
Title: Upholding the unified model for active galactic nuclei:
VLT/FORS2 spectropolarimetry of Seyfert 2 galaxies
Authors: Ramos Almeida, C.; Martínez González, M. J.; Asensio Ramos,
A.; Acosta-Pulido, J. A.; Hönig, S. F.; Alonso-Herrero, A.; Tadhunter,
C. N.; González-Martín, O.
2016MNRAS.461.1387R Altcode: 2016arXiv160602204R
The origin of the unification model for active galactic nuclei (AGN)
was the detection of broad hydrogen recombination lines in the optical
polarized spectrum of the Seyfert 2 galaxy (Sy2) NGC 1068. Since
then, a search for the hidden broad-line region (HBLR) of nearby
Sy2s started, but polarized broad lines have only been detected
in ∼30-40 per cent of the nearby Sy2s observed to date. Here we
present new VLT/FORS2 optical spectropolarimetry of a sample of 15
Sy2s, including Compton-thin and Compton-thick sources. The sample
includes six galaxies without previously published spectropolarimetry,
some of them normally treated as non-hidden BLR (NHBLR) objects in
the literature, four classified as NHBLR, and five as HBLR based on
previous data. We report ≥4σ detections of a HBLR in 11 of these
galaxies (73 per cent of the sample) and a tentative detection in NGC
5793, which is Compton-thick according to the analysis of X-ray data
performed here. Our results confirm that at least some NHBLRs are
misclassified, bringing previous publications reporting differences
between HBLR and NHBLR objects into question. We detect broad Hα and
Hβ components in polarized light for 10 targets, and just broad Hα
for NGC 5793 and NGC 6300, with line widths ranging between 2100 and
9600 km s<SUP>-1</SUP>. High bolometric luminosities and low column
densities are associated with higher polarization degrees, but not
necessarily with the detection of the scattered broad components.
---------------------------------------------------------
Title: Upholding the unified model for AGN: VLT/FORS2
spectropolarimetry of Seyfert 2 galaxies
Authors: Ramos Almeida, Cristina; Martínez González, M.; Asensio
Ramos, A.; Acosta Pulido, J.; Hönig, S.; Alonso-Herrero, A.;
Tadhunter, C.; González-Martín, O.
2016agnw.confE..52R Altcode:
The origin of the unification model for AGN was the detection of broad
hydrogen recombination lines in the optical polarized spectrum of the
Seyfert 2 galaxy (Sy2) NGC 1068. Since then, a search for the hidden
broad-line region (HBLR) of nearby Sy2s started, but polarized broad
lines have only been detected in ~30-40% of the nearby Sy2s observed
to date. Here we present new VLT/FORS2 optical spectropolarimetry
of a sample of 15 Sy2s, including Compton-thin and Compton-thick
sources. The sample includes six galaxies without previously published
spectropolarimetry, some of them normally treated as non-hidden BLR
(NHBLR) objects in the literature, and four Sy2s classified as NHBLR
based on previous data. We report >4sigma detections of a HBLR in
11 of these galaxies (73% of the sample). Our results confirm that at
least some NHBLRs were misclassified, bringing previous publications
reporting differences between HBLR and NHBLR objects into question. We
detect broad Ha and Hb components in polarized light for 9 targets,
and just broad Ha for the other two. We do not find any correlation
between the properties of the polarized spectra and the column densities
measured from the X-rays or torus inclination, but a larger sample is
required to confirm this.
---------------------------------------------------------
Title: On the Magnetism and Dynamics of Prominence Legs Hosting
Tornadoes
Authors: Martínez González, M. J.; Asensio Ramos, A.; Arregui, I.;
Collados, M.; Beck, C.; de la Cruz Rodríguez, J.
2016ApJ...825..119M Altcode: 2016arXiv160501183M
Solar tornadoes are dark vertical filamentary structures observed
in the extreme ultraviolet associated with prominence legs and
filament barbs. Their true nature and relationship to prominences
requires an understanding of their magnetic structure and dynamic
properties. Recently, a controversy has arisen: is the magnetic field
organized forming vertical, helical structures or is it dominantly
horizontal? And concerning their dynamics, are tornadoes really rotating
or is it just a visual illusion? Here we analyze four consecutive
spectro-polarimetric scans of a prominence hosting tornadoes on its
legs, which helps us shed some light on their magnetic and dynamical
properties. We show that the magnetic field is very smooth in all the
prominence, which is probably an intrinsic property of the coronal
field. The prominence legs have vertical helical fields that show
slow temporal variation that is probably related to the motion of
the fibrils. Concerning the dynamics, we argue that (1) if rotation
exists, it is intermittent, lasting no more than one hour, and (2)
the observed velocity pattern is also consistent with an oscillatory
velocity pattern (waves).
---------------------------------------------------------
Title: Active Region Filaments Might Harbor Weak Magnetic Fields
Authors: Díaz Baso, C. J.; Martínez González, M. J.; Asensio
Ramos, A.
2016ApJ...822...50D Altcode: 2016arXiv160304645D
Recent spectropolarimetric observations of active region filaments have
revealed polarization profiles with signatures typical of the strong
field Zeeman regime. The conspicuous absence in those observations of
scattering polarization and Hanle effect signatures was then pointed
out by some authors. This was interpreted as either a signature of
mixed “turbulent” field components or as a result of optical
thickness. In this article, we present a natural scenario to explain
these Zeeman-only spectropolarimetric observations of active region
(AR) filaments. We propose a two-component model, one on top of
the other. Both components have horizontal fields, with the azimuth
difference between them being close to 90°. The component that lies
lower in the atmosphere is permeated by a strong field of the order of
600 G, while the upper component has much weaker fields, of the order of
10 G. The ensuing scattering polarization signatures of the individual
components have opposite signs, so its combination along the line of
sight reduces—and even can cancel out—the Hanle signatures, giving
rise to an apparent Zeeman-only profile. This model is also applicable
to other chromospheric structures seen in absorption above ARs.
---------------------------------------------------------
Title: Inversion of Stokes profiles with systematic effects
Authors: Asensio Ramos, A.; de la Cruz Rodríguez, J.; Martínez
González, M. J.; Pastor Yabar, A.
2016A&A...590A..87A Altcode: 2016arXiv160405470A
Quantitative thermodynamical, dynamical and magnetic properties of the
solar and stellar plasmas are obtained by interpreting their emergent
non-polarized and polarized spectrum. This inference requires the
selection of a set of spectral lines that are particularly sensitive to
the physical conditions in the plasma and a suitable parametric model
of the solar/stellar atmosphere. Nonlinear inversion codes are then
used to fit the model to the observations. However, the presence of
systematic effects, like nearby or blended spectral lines, telluric
absorption, or incorrect correction of the continuum, among others,
can strongly affect the results. We present an extension to current
inversion codes that can deal with these effects in a transparent
way. The resulting algorithm is very simple and can be applied to any
existing inversion code with the addition of a few lines of code as
an extra step in each iteration.
---------------------------------------------------------
Title: Where are the solar magnetic poles?
Authors: Pastor Yabar, A.; Martinez Gonzalez, M. J.; Collados, M.
2015MNRAS.453L..69P Altcode: 2018arXiv180409748P
Regardless of the physical origin of stellar magnetic fields -
fossil or dynamo induced - an inclination angle between the magnetic
and rotation axes is very often observed. Absence of observational
evidence in this direction in the solar case has led to generally assume
that its global magnetic field and rotation axes are well aligned. We
present the detection of a monthly periodic signal of the photospheric
solar magnetic field at all latitudes, and especially near the poles,
revealing that the main axis of the Sun's magnetic field is not aligned
with the surface rotation axis. This result reinforces the view of our
Sun as a common intermediate-mass star. Furthermore, this detection
challenges and imposes a strong observational constraint to modern
solar dynamo theories.
---------------------------------------------------------
Title: Estimating the magnetic field strength from magnetograms
Authors: Asensio Ramos, A.; Martínez González, M. J.; Manso Sainz, R.
2015A&A...577A.125A Altcode: 2015arXiv150307670A
A properly calibrated longitudinal magnetograph is an instrument
that measures circular polarization and gives an estimation of the
magnetic flux density in each observed resolution element. This usually
constitutes a lower bound of the field strength in the resolution
element, given that it can be made arbitrarily large as long as it
occupies a proportionally smaller area of the resolution element
and/or becomes more transversal to the observer while still produce
the same magnetic signal. However, we know that arbitrarily stronger
fields are less likely - hG fields are more probable than kG fields,
with fields above several kG virtually absent - and we may even have
partial information about their angular distribution. Based on a set of
sensible considerations, we derive simple formulae based on a Bayesian
analysis to give an improved estimation of the magnetic field strength
for magnetographs.
---------------------------------------------------------
Title: Spectro-Polarimetric Imaging Reveals Helical Magnetic Fields
in Solar Prominence Feet
Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos,
A.; Beck, C.; de la Cruz Rodríguez, J.; Díaz, A. J.
2015ApJ...802....3M Altcode: 2015arXiv150103295M
Solar prominences are clouds of cool plasma levitating above
the solar surface and insulated from the million-degree corona by
magnetic fields. They form in regions of complex magnetic topology,
characterized by non-potential fields, which can evolve abruptly,
disintegrating the prominence and ejecting magnetized material
into the heliosphere. However, their physics is not yet fully
understood because mapping such complex magnetic configurations
and their evolution is extremely challenging, and must often be
guessed by proxy from photometric observations. Using state-of-the-art
spectro-polarimetric data, we reconstruct the structure of the magnetic
field in a prominence. We find that prominence feet harbor helical
magnetic fields connecting the prominence to the solar surface below.
---------------------------------------------------------
Title: Constraining the shaping mechanism of the Red Rectangle
through the spectro-polarimetry of its central star
Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz,
R.; Corradi, R. L. M.; Leone, F.
2015A&A...574A..16M Altcode: 2014arXiv1410.6365M
We carried out high-sensitivity spectro-polarimetric observations of
the central star of the Red Rectangle protoplanetary nebula with the
aim of constraining the mechanism that gives its biconical shape. The
stellar light of the central binary system is linearly polarised since
it is scattered on the dust particles of the nebula. Surprisingly,
the linear polarisation in the continuum is aligned with one of the
spikes of the biconical outflow. Also, the observed Balmer lines,
as well as the Ca ii K lines, are polarised. These observational
constraints are used to confirm or reject current theoretical models
for the shaping mechanism of the Red Rectangle. We propose that the
observed polarisation is not very likely to be generated by a uniform
biconical stellar wind. Also, the hypothesis of a precessing jet does
not completely match observations since it requires a larger aperture
jet than for the nebula.
---------------------------------------------------------
Title: Hierarchical analysis of the quiet-Sun magnetism
Authors: Asensio Ramos, A.; Martínez González, M. J.
2014A&A...572A..98A Altcode: 2014arXiv1410.5953A
Standard statistical analysis of the magnetic properties of the
quiet Sun rely on simple histograms of quantities inferred from
maximum-likelihood estimations. Because of the inherent degeneracies,
either intrinsic or induced by the noise, this approach is not optimal
and can lead to highly biased results. We carried out a meta-analysis
of the magnetism of the quiet Sun from Hinode observations using a
hierarchical probabilistic method. This method allowed us to infer the
statistical properties of the magnetic field vector over the observed
field-of-view, consistently taking into account the uncertainties
in each pixel that are due to noise and degeneracies. Our results
imply that the magnetic fields are very weak, below 275 G with 95%
probability, with a slight preference for horizontal fields, although
the distribution is not far from a quasi-isotropic distribution.
---------------------------------------------------------
Title: Upper Limits to the Magnetic Field in Central Stars of
Planetary Nebulae
Authors: Asensio Ramos, A.; Martínez González, M. J.; Manso Sainz,
R.; Corradi, R. L. M.; Leone, F.
2014ApJ...787..111A Altcode: 2014arXiv1404.2718A
More than about 20 central stars of planetary nebulae (CSPNs) have
been observed spectropolarimetrically, yet no clear, unambiguous
signal of the presence of a magnetic field in these objects has
been found. We perform a statistical (Bayesian) analysis of all the
available spectropolarimetric observations of CSPN to constrain the
magnetic fields in these objects. Assuming that the stellar field is
dipolar and that the dipole axis of the objects is oriented randomly
(isotropically), we find that the dipole magnetic field strength
is smaller than 400 G with 95% probability using all available
observations. The analysis introduced allows integration of future
observations to further constrain the parameters of the distribution,
and it is general, so that it can be easily applied to other classes of
magnetic objects. We propose several ways to improve the upper limits
found here.
---------------------------------------------------------
Title: A search for magnetic fields on central stars in planetary
nebulae
Authors: Leone, F.; Corradi, R. L. M.; Martínez González, M. J.;
Asensio Ramos, A.; Manso Sainz, R.
2014A&A...563A..43L Altcode: 2014arXiv1401.6282L
Context. One of the possible mechanisms responsible for the panoply
of shapes in planetary nebulae is the presence of magnetic fields
that drive the ejection of ionized material during the proto-planetary
nebula phase. <BR /> Aims: Therefore, detecting magnetic fields in such
objects is of key importance for understanding their dynamics. Still,
magnetic fields have not been detected using polarimetry in the central
stars of planetary nebulae. <BR /> Methods: Circularly polarized light
spectra have been obtained with the Focal Reducer and Low Dispersion
Spectrograph at the Very Large Telescope of the European Southern
Observatory and the Intermediate dispersion Spectrograph and Imaging
System at the William Herschel Telescope. Twentythree planetary nebulae
that span very different morphology and evolutionary stages have been
selected. Most of central stars have been observed at different rotation
phases to point out evidence of magnetic variability. <BR /> Results:
In this paper, we present the result of two observational campaigns
aimed to detect and measure the magnetic field in the central stars of
planetary nebulae on the basis of low resolution spectropolarimetry. In
the limit of the adopted method, we can state that large scale fields
of kG order are not hosted on the central star of planetary nebulae.
---------------------------------------------------------
Title: Granular-sized magnetic flux emergence
Authors: Martínez González, María Jesús
2014cosp...40E2017M Altcode:
It is believed that solar magnetic fields are created in the
tachocline, the interface between the convection zone and the radiative
interior. Due to buoyancy instabilities, they move upward and emerge
into the solar atmosphere in the form of Ω-shaped flux tubes. The
largest emerging active regions produce sunspots or pores, lasting
from days or weeks to moths, and the ephemeral regions emerge to
feed the quiet Sun at supergranular scales (the network), lasting
from hours to days. A surprising finding in solar physics in the last
few years was the discovery that, despite their stochastic character,
the quietest areas of the quiet Sun are formed, at granular scales, by
small dipoles that live for several minutes. Are these dipoles just the
end tail of the same emergence phenomenon than active regions? or are
they the result of a surface phenomenon? In this talk I review all the
present knowledge of the smallest scale emergence phenomenon observed
in the Sun and I put in in context on the solar global magnetism.
---------------------------------------------------------
Title: Polarisation and magnetic fields in cool stars and the Sun. A
session in memory of Meir Semel
Authors: Martínez González, M. J.; Marsden, S. C.
2013AN....334..164M Altcode: 2013csss...17..164M
Astrophysical plasmas and magnetic fields are inextricably coupled
to each other. But magnetic fields are elusive; they cannot be
fully characterized with standard (spectroscopic) observational
techniques. Fortunately, polarisation, the third fundamental property
of light, encodes all the quantitative information on the magnetism
of the objects we observe. This splinter session brought together the
solar and stellar spectropolarimetric communities to discuss some of
the fundamental unsolved questions of solar and stellar magnetism and
the future of spectropolarimetric observations.
---------------------------------------------------------
Title: Hanle Effect for Stellar Dipoles and Quadrupoles
Authors: Manso Sainz, R.; Martínez González, M. J.
2012ApJ...760....7M Altcode: 2012arXiv1209.6187M
We derive exact expressions for the degree of lineal polarization over
a resolved or integrated stellar disc due to resonance scattering
and the Hanle effect from a dipolar or quadrupolar distribution of
magnetic fields. We apply the theory of scattering polarization within
the formalism of the spherical tensors representation for the density
matrix and radiation field. The distribution of linear polarization
over the stellar disk for different configurations of the magnetic
field is studied and its topology discussed. For an unresolved dipole,
the resulting polarization can be expressed in terms of just three
functions (of the inclination angle and effective dipole strength)
that are calculated numerically and their behavior discussed. Dipolar
and (aligned) quadrupoles are considered in some detail, but the
techniques here—in particular, the extensive use of the spherical
tensor formalism for polarization—can easily be applied to more
general field configurations.
---------------------------------------------------------
Title: Anomalous Circular Polarization Profiles in the He I 1083.0
nm Multiplet from Solar Spicules
Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz,
R.; Beck, C.; Belluzzi, L.
2012ApJ...759...16M Altcode: 2012arXiv1209.2589M
We report Stokes vector observations of solar spicules and a prominence
in the He I 1083 nm multiplet carried out with the Tenerife Infrared
Polarimeter. The observations show linear polarization profiles that
are produced by scattering processes in the presence of a magnetic
field. After a careful data reduction, we demonstrate the existence of
extremely asymmetric Stokes V profiles in the spicular material that
we are able to model with two magnetic components along the line of
sight, and under the presence of atomic orientation in the energy
levels that give rise to the multiplet. We discuss some possible
scenarios that can generate the atomic orientation in spicules. We
stress the importance of spectropolarimetric observations across the
limb to distinguish such signals from observational artifacts.
---------------------------------------------------------
Title: Resolving the Internal Magnetic Structure of the Solar Network
Authors: Martínez González, M. J.; Bellot Rubio, L. R.; Solanki,
S. K.; Martínez Pillet, V.; Del Toro Iniesta, J. C.; Barthol, P.;
Schmidt, W.
2012ApJ...758L..40M Altcode: 2012arXiv1209.2584M
We analyze the spectral asymmetry of Stokes V (circularly polarized)
profiles of an individual network patch in the quiet Sun observed
by Sunrise/IMaX. At a spatial resolution of 0farcs15-0farcs18, the
network elements contain substructure which is revealed by the spatial
distribution of Stokes V asymmetries. The area asymmetry between the
red and blue lobes of Stokes V increases from nearly zero at the core
of the structure to values close to unity at its edges (single-lobed
profiles). Such a distribution of the area asymmetry is consistent
with magnetic fields expanding with height, i.e., an expanding
magnetic canopy (which is required to fulfill pressure balance and
flux conservation in the solar atmosphere). Inversion of the Stokes
I and V profiles of the patch confirms this picture, revealing a
decreasing field strength and increasing height of the canopy base
from the core to the periphery of the network patch. However, the
non-roundish shape of the structure and the presence of negative area
and amplitude asymmetries reveal that the scenario is more complex than
a canonical flux tube expanding with height surrounded by downflows.
---------------------------------------------------------
Title: Analytical Calculation of Stokes Profiles of Rotating Stellar
Magnetic Dipole
Authors: Martínez González, M. J.; Asensio Ramos, A.
2012ApJ...755...96M Altcode: 2012arXiv1206.2502M
The observation of the polarization emerging from a rotating star
at different phases opens up the possibility to map the magnetic
field in the stellar surface thanks to the well-known Zeeman-Doppler
imaging. When the magnetic field is sufficiently weak, the circular
and linear polarization profiles locally in each point of the star are
proportional to the first and second derivatives of the unperturbed
intensity profile, respectively. We show that the weak-field
approximation (for weak lines in the case of linear polarization)
can be generalized to the case of a rotating star including the
Doppler effect and taking into account the integration on the stellar
surface. The Stokes profiles are written as a linear combination of
wavelength-dependent terms expressed as series expansions in terms of
Hermite polynomials. These terms contain the surface-integrated magnetic
field and velocity components. The direct numerical evaluation of these
quantities is limited to rotation velocities not larger than eight
times the Doppler width of the local absorption profiles. Additionally,
we demonstrate that in a rotating star, the circular polarization
flux depends on the derivative of the intensity flux with respect to
the wavelength and also on the profile itself. Likewise, the linear
polarization depends on the profile and on its first and second
derivatives with respect to the wavelength. We particularize the
general expressions to a rotating dipole.
---------------------------------------------------------
Title: Dead Calm Areas in the Very Quiet Sun
Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos,
A.; Hijano, E.
2012ApJ...755..175M Altcode: 2012arXiv1206.4545M
We analyze two regions of the quiet Sun (35.6 × 35.6 Mm<SUP>2</SUP>)
observed at high spatial resolution (lsim100 km) in polarized light by
the IMaX spectropolarimeter on board the SUNRISE balloon. We identify
497 small-scale (~400 km) magnetic loops, appearing at an effective
rate of 0.25 loop h <SUP>-1</SUP> arcsec<SUP>-2</SUP> further, we
argue that this number and rate are underestimated by ~30%. However,
we find that these small dipoles do not appear uniformly on the solar
surface: their spatial distribution is rather filamentary and clumpy,
creating dead calm areas, characterized by a very low magnetic signal
and a lack of organized loop-like structures at the detection level
of our instruments, which cannot be explained as just statistical
fluctuations of a Poisson spatial process. We argue that this is an
intrinsic characteristic of the mechanism that generates the magnetic
fields in the very quiet Sun. The spatio-temporal coherences and the
clumpy structure of the phenomenon suggest a recurrent, intermittent
mechanism for the generation of magnetic fields in the quietest areas
of the Sun.
---------------------------------------------------------
Title: Model Selection for Spectropolarimetric Inversions
Authors: Asensio Ramos, A.; Manso Sainz, R.; Martínez González,
M. J.; Viticchié, B.; Orozco Suárez, D.; Socas-Navarro, H.
2012ApJ...748...83A Altcode: 2012arXiv1201.5063A
Inferring magnetic and thermodynamic information from
spectropolarimetric observations relies on the assumption of
a parameterized model atmosphere whose parameters are tuned by
comparison with observations. Often, the choice of the underlying
atmospheric model is based on subjective reasons. In other cases,
complex models are chosen based on objective reasons (for instance,
the necessity to explain asymmetries in the Stokes profiles) but it
is not clear what degree of complexity is needed. The lack of an
objective way of comparing models has, sometimes, led to opposing
views of the solar magnetism because the inferred physical scenarios
are essentially different. We present the first quantitative model
comparison based on the computation of the Bayesian evidence ratios for
spectropolarimetric observations. Our results show that there is not
a single model appropriate for all profiles simultaneously. Data with
moderate signal-to-noise ratios (S/Ns) favor models without gradients
along the line of sight. If the observations show clear circular and
linear polarization signals above the noise level, models with gradients
along the line are preferred. As a general rule, observations with large
S/Ns favor more complex models. We demonstrate that the evidence ratios
correlate well with simple proxies. Therefore, we propose to calculate
these proxies when carrying out standard least-squares inversions to
allow for model comparison in the future.
---------------------------------------------------------
Title: Influence of phase-diversity image reconstruction techniques
on circular polarization asymmetries
Authors: Asensio Ramos, A.; Martínez González, M. J.; Khomenko,
E.; Martínez Pillet, V.
2012A&A...539A..42A Altcode: 2011arXiv1111.2496A
Context. Full Stokes filter-polarimeters are key instruments for
investigating the rapid evolution of magnetic structures on the solar
surface. To this end, the image quality is routinely improved using
a-posteriori image reconstruction methods. <BR /> Aims: We analyze
the robustness of circular polarization asymmetries to phase-diversity
image reconstruction techniques. <BR /> Methods: We used snapshots of
magneto-hydrodynamical simulations carried out with different initial
conditions to synthesize spectra of the magnetically sensitive Fe
i line at 5250.2 Å. We degraded the synthetic profiles spatially
and spectrally to simulate observations with the IMaX full Stokes
filter-polarimeter. We also simulated the focused/defocused pairs of
images used by the phase-diversity algorithm for reconstruction and the
polarimetric modulation scheme. We assume that standard optimization
methods are able to infer the projection of the wavefront on the Zernike
polynomials with 10% precision. We also consider the less favorable case
of 25% precision. We obtain reconstructed monochromatic modulated images
that are later demodulated and compared with the original maps. <BR />
Results: Although asymmetries are often difficult to define in the
quiet Sun due to the complexity of the Stokes V profiles, we show
how asymmetries are degraded with spatial and spectral smearing. The
results indicate that, although image reconstruction techniques reduce
the spatial smearing, they can modify the asymmetries of the profiles,
which are mainly caused by the appearance of spatially-correlated noise.
---------------------------------------------------------
Title: Analytical maximum likelihood estimation of stellar magnetic
fields
Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos,
A.; Belluzzi, L.
2012MNRAS.419..153M Altcode: 2011MNRAS.tmp.1583M; 2011arXiv1108.4366M
The polarized spectrum of stellar radiation encodes valuable information
on the conditions of stellar atmospheres and the magnetic fields
that permeate them. In this paper, we give explicit expressions to
estimate the magnetic field vector and its associated error from the
observed Stokes parameters. We study the solar case where specific
intensities are observed and then the stellar case, where we receive
the polarized flux. In the second case, we concentrate on the explicit
expression for the case of a slow rotator with a dipolar magnetic
field geometry. Moreover, we also give explicit formulae to retrieve
the magnetic field vector from the least-squares deconvolution (LSD)
profiles without assuming mean values for the LSD artificial spectral
line. The formulae have been obtained assuming that the spectral lines
can be described in the weak-field regime and using a maximum likelihood
approach. The errors are recovered by means of the Hermitian matrix. The
bias of the estimators is analysed in depth.
---------------------------------------------------------
Title: Advection and dispersal of small magnetic elements in the
very quiet Sun
Authors: Manso Sainz, R.; Martínez González, M. J.; Asensio Ramos, A.
2011A&A...531L...9M Altcode:
We track small magnetic structures on very quiet regions (internetwork)
of the Sun. We follow the footpoints of small-scale magnetic loops that
appear on the photosphere at granular scales using spectropolarimetric
and magnetographic data obtained with Hinode. We find two different
regimes for their wanderings. Within granules (where they appear),
they seem to be passively advected by the plasma - which is
justified by their relatively low magnetic flux (~10<SUP>16</SUP>
Mx), and magnetic field strength (~200 G). The plasma flow thus
traced is roughly laminar with a characteristic mean velocity of 2
km s<SUP>-1</SUP> and very low vorticity. Once the magnetic markers
reach intergranular lanes, they remain there and are buffeted by the
random flows of neighbouring granules and turbulent intergranules,
follow random walks, and disperse across the solar surface with a
diffusion constant of 195 km<SUP>2</SUP> s<SUP>-1</SUP>. While on
their intergranular random walking, they may fall close to whirlpools
(on scales ≲400 km) associated with convective downdrafts, similar
to the events recently reported in mesogranular and supergranular cell
boundaries tracking magnetic bright points, which provides additional
evidence that these events are ubiquitous on the solar surface.
---------------------------------------------------------
Title: Non-detection of Magnetic Fields in the Central Stars of the
Planetary Nebulae NGC 1360 and LSS 1362
Authors: Leone, Francesco; Martínez González, María J.; Corradi,
Romano L. M.; Privitera, Giovanni; Manso Sainz, Rafael
2011ApJ...731L..33L Altcode: 2011arXiv1104.1042L
The presence of magnetic fields is an attractive hypothesis for shaping
planetary nebulae (PNe). We report on observations of the central star
of the two PNe NGC 1360 and LSS 1326. We performed spectroscopy on
circularly polarized light with the Focal Reducer and Low Dispersion
Spectrograph at the Very Large Telescope of the European Southern
Observatory. Contrary to previous reports, we find that the effective
magnetic field, which is the average over the visible stellar disk of
longitudinal components of the magnetic fields, is null within errors
for both stars. We conclude that direct evidence of magnetic fields on
the central stars of PNe is still missing—either the magnetic field
is much weaker (<600 G) than previously reported, or more complex
(thus leading to cancellations), or both. Certainly, indirect evidence
(e.g., MASER emission) fully justify further efforts to point out the
strength and morphology of such magnetic fields.
---------------------------------------------------------
Title: The Magnetic Fields of the Quiet Sun
Authors: Sánchez Almeida, J.; Martínez González, M.
2011ASPC..437..451S Altcode: 2011arXiv1105.0387S
This work reviews our understanding of the magnetic fields observed
in the quiet Sun. The subject has undergone a major change during the
last decade (quiet revolution), and it will remain changing since the
techniques of diagnostic employed so far are known to be severely
biased. Keeping these caveats in mind, our work covers the main
observational properties of the quiet Sun magnetic fields: magnetic
field strengths, unsigned magnetic flux densities, magnetic field
inclinations, as well as the temporal evolution on short time-scales
(loop emergence), and long time-scales (solar cycle). We also summarize
the main theoretical ideas put forward to explain the origin of the
quiet Sun magnetism. A final prospective section points out various
areas of solar physics where the quiet Sun magnetism may have an
important physical role to play (chromospheric and coronal structure,
solar wind acceleration, and solar elemental abundances).
---------------------------------------------------------
Title: Unnoticed Magnetic Field Oscillations in the Very Quiet Sun
Revealed by SUNRISE/IMaX
Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz,
R.; Khomenko, E.; Martínez Pillet, V.; Solanki, S. K.; López Ariste,
A.; Schmidt, W.; Barthol, P.; Gandorfer, A.
2011ApJ...730L..37M Altcode: 2011arXiv1103.0145M
We present observational evidence for oscillations of magnetic flux
density in the quiet areas of the Sun. The majority of magnetic
fields on the solar surface have strengths of the order of or lower
than the equipartition field (300-500 G). This results in a myriad of
magnetic fields whose evolution is largely determined by the turbulent
plasma motions. When granules evolve they squash the magnetic field
lines together or pull them apart. Here, we report on the periodic
deformation of the shapes of features in circular polarization observed
at high resolution with SUNRISE. In particular, we note that the
area of patches with a constant magnetic flux oscillates with time,
which implies that the apparent magnetic field intensity oscillates
in antiphase. The periods associated with this oscillatory pattern
are compatible with the granular lifetime and change abruptly, which
suggests that these oscillations might not correspond to characteristic
oscillatory modes of magnetic structures, but to the forcing by granular
motions. In one particular case, we find three patches around the same
granule oscillating in phase, which means that the spatial coherence
of these oscillations can reach 1600 km. Interestingly, the same kind
of oscillatory phenomenon is also found in the upper photosphere.
---------------------------------------------------------
Title: Small Magnetic Loops Connecting the Quiet Surface and the
Hot Outer Atmosphere of the Sun
Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos,
A.; Bellot Rubio, L. R.
2010ApJ...714L..94M Altcode: 2010arXiv1003.1255M
Sunspots are the most spectacular manifestation of solar magnetism,
yet 99% of the solar surface remains "quiet" at any time of the solar
cycle. The quiet sun is not void of magnetic fields, though; they
are organized at smaller spatial scales and evolve relatively fast,
which makes them difficult to detect. Thus, although extensive quiet
Sun magnetism would be a natural driver to a uniform, steady heating of
the outer solar atmosphere, it is not clear what the physical processes
involved would be, due to lack of observational evidence. We report on
the topology and dynamics of the magnetic field in very quiet regions of
the Sun from spectropolarimetric observations of the Hinode satellite,
showing a continuous injection of magnetic flux with a well-organized
topology of Ω-loop from below the solar surface into the upper
layers. At first stages, when the loop travels across the photosphere,
it has a flattened (staple-like) geometry and a mean velocity ascent
of ~3 km s<SUP>-1</SUP>. When the loop crosses the minimum temperature
region, the magnetic fields at the footpoints become almost vertical and
the loop topology resembles a potential field. The mean ascent velocity
at chromospheric height is ~12 km s<SUP>-1</SUP>. The energy input rate
of these small-scale loops in the lower boundary of the chromosphere
is (at least) of 1.4 × 10<SUP>6</SUP>-2.2 × 10<SUP>7</SUP> erg
cm<SUP>-2</SUP> s<SUP>-1</SUP>. Our findings provide empirical evidence
for solar magnetism as a multi-scale system, in which small-scale
low-flux magnetism plays a crucial role, at least as important as active
regions, coupling different layers of the solar atmosphere and being
an important ingredient for chromospheric and coronal heating models.
---------------------------------------------------------
Title: The stochastic, intermittent nature of quiet Sun magnetism
Authors: Martinez González, M. J.; Manso Sainz, R.; López Ariste,
A.; Bellot Rubio, L. R.; Bianda, M.
2010iac..talk....1M Altcode: 2010iac..talk..153M
No abstract at ADS
---------------------------------------------------------
Title: Statistical Analysis of the very Quiet Sun Magnetism
Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos,
A.; López Ariste, A.; Bianda, M.
2010ApJ...711L..57M Altcode: 2010arXiv1001.4551M
The behavior of the observed polarization amplitudes with spatial
resolution is a strong constraint on the nature and organization
of solar magnetic fields below the resolution limit. We study the
polarization of the very quiet Sun at different spatial resolutions
using ground- and space-based observations. It is shown that 80% of the
observed polarization signals do not change with spatial resolution,
suggesting that, observationally, the very quiet Sun magnetism
remains the same despite the high spatial resolution of space-based
observations. Our analysis also reveals a cascade of spatial scales
for the magnetic field within the resolution element. It is manifest
that the Zeeman effect is sensitive to the microturbulent field usually
associated with Hanle diagnostics. This demonstrates that Zeeman and
Hanle studies show complementary perspectives of the same magnetism.
---------------------------------------------------------
Title: Spectropolarimetric multi line analysis of stellar magnetic
fields
Authors: Ramírez Vélez, J. C.; Semel, M.; Stift, M.; Martínez
González, M. J.; Petit, P.; Dunstone, N.
2010A&A...512A...6R Altcode: 2010arXiv1001.1003R
<BR /> Aims: In this paper we study the feasibility of inferring the
magnetic field from polarized multi line spectra using two methods:
The pseudo line approach and The PCA-ZDI approach. <BR /> Methods:
We use multi line techniques, meaning that all the lines of a stellar
spectrum contribute to obtain a polarization signature. The use of
multiple lines dramatically increases the signal-to-noise-ratio of
these polarizations signatures. Using one technique, the pseudo line
approach, we construct the pseudo line as the mean profile of all the
individual lines. The other technique, the PCA-ZDI approach proposed
recently by Semel et al. (2006, ASPC, 358, 355) for the detection
of polarized signals, combines principle components analysis (PCA)
and the Zeeman Doppler imaging technique (ZDI). This new method has
a main advantage: the polarized signature is extracted using cross
correlations between the stellar spectra and functions containing
the polarization properties of each line. is extracted using cross
correlations between the stellar spectra and functions containing the
polarization properties of each line." These functions are the principal
components of a database of synthetic spectra. The synthesis of the
spectra of the database are obtained using the radiative transfer
equations in LTE. The profiles built with the PCA-ZDI technique are
called multi Zeeman signatures. <BR /> Results: The construction of the
pseudo line as well as the multi Zeeman signatures is a powerful tool
in the study of stellar and solar magnetic fields. The information of
the physical parameters that governs the line formation is contained
in the final polarized profiles. We have shown in particular using
inversion codes that the magnetic field vector can be properly inferred
with both approaches despite the magnetic field regime.
---------------------------------------------------------
Title: The quiet Sun magnetic field observed with ZIMPOL on
THEMIS. I. The probability density function
Authors: Bommier, V.; Martínez González, M.; Bianda, M.; Frisch,
H.; Asensio Ramos, A.; Gelly, B.; Landi Degl'Innocenti, E.
2009A&A...506.1415B Altcode:
Context: The quiet Sun magnetic field probability density function (PDF)
remains poorly known. Modeling this field also introduces a magnetic
filling factor that is also poorly known. With these two quantities,
PDF and filling factor, the statistical description of the quiet Sun
magnetic field is complex and needs to be clarified. <BR />Aims:
In the present paper, we propose a procedure that combines direct
determinations and inversion results to derive the magnetic field
vector and filling factor, and their PDFs. <BR />Methods: We used
spectro-polarimetric observations taken with the ZIMPOL polarimeter
mounted on the THEMIS telescope. The target was a quiet region at disk
center. We analyzed the data by means of the UNNOFIT inversion code,
with which we inferred the distribution of the mean magnetic field α
B, α being the magnetic filling factor. The distribution of α was
derived by an independent method, directly from the spectro-polarimetric
data. The magnetic field PDF p(B) could then be inferred. By introducing
a joint PDF for the filling factor and the magnetic field strength, we
have clarified the definition of the PDF of the quiet Sun magnetic field
when the latter is assumed not to be volume-filling. <BR />Results: The
most frequent local average magnetic field strength is found to be 13
G. We find that the magnetic filling factor is related to the magnetic
field strength by the simple law α = B_1/B with B<SUB>1</SUB> = 15
G. This result is compatible with the Hanle weak-field determinations,
as well as with the stronger field determinations from the Zeeman effect
(kGauss field filling 1-2% of space). From linear fits, we obtain the
analytical dependence of the magnetic field PDF. Our analysis has also
revealed that the magnetic field in the quiet Sun is isotropically
distributed in direction. <BR />Conclusions: We conclude that the
quiet Sun is a complex medium where magnetic fields having different
field strengths and filling factors coexist. Further observations
with a better polarimetric accuracy are, however, needed to confirm
the results obtained in the present work. <P />Based on observations
made with the French-Italian telescope THEMIS operated by the CNRS
and CNR on the island of Tenerife in the Spanish Observatorio del
Teide of the Instituto de Astrofísica de Canarias. Present address:
Instituto de Astrofísica de Canarias, vía Láctea s/n, 38205 La
Laguna, Tenerife, Spain.
---------------------------------------------------------
Title: Multiline Zeeman signatures through line addition
Authors: Semel, M.; Ramírez Vélez, J. C.; Martínez González,
M. J.; Asensio Ramos, A.; Stift, M. J.; López Ariste, A.; Leone, F.
2009A&A...504.1003S Altcode: 2009arXiv0906.4226S
Context: To obtain a significant Zeeman signature in the polarised
spectra of a magnetic star, we usually “add” the contributions
of numerous spectral lines; the ultimate goal is to recover the
spectropolarimetric prints of the magnetic field in these line
additions. <BR />Aims: Here we want to clarify the meaning of these
techniques of line addition; in particular, we try to interpret the
meaning of the “pseudo-line” formed during this process and to
find out why and how its Zeeman signature is still meaningful. <BR
/>Methods: We create a synthetic case of line addition and apply well
tested standard solar methods routinely used in research on magnetism
in the Sun. <BR />Results: The results are convincing and the Zeeman
signatures well detected; Solar methods are found to be quite efficient
for stellar observations. We statistically compare line addition
with least-squares deconvolution and demonstrate that they both give
very similar results, as a consequence of the special statistical
properties of the weights. <BR />Conclusions: The Zeeman signatures
are unequivocally detected in this multiline approach. We suggest
that magnetic field detection is reliable well beyond the weak-field
approximation. Linear polarisation in the spectra of solar type stars
can be detected when the spectral resolution is sufficiently high.
---------------------------------------------------------
Title: Emergence of Small-scale Magnetic Loops Through the Quiet
Solar Atmosphere
Authors: Martínez González, M. J.; Bellot Rubio, L. R.
2009ApJ...700.1391M Altcode: 2009arXiv0905.2691M
We investigate the emergence of magnetic flux in the quiet Sun
at very small spatial scales, focusing on the magnetic connection
between the photosphere and chromosphere. The observational data
consist of spectropolarimetric measurements and filtergrams taken
with the Hinode satellite and the Dutch Open Telescope. We find that
a significant fraction of the magnetic flux present in internetwork
regions appears in the form of Ω-shaped loops. The emergence rate
is 0.02 loops per hour and arcsec<SUP>-2</SUP>, which brings 1.1
× 10<SUP>12</SUP> Mx s<SUP>-1</SUP> arcsec<SUP>-2</SUP> of new
flux to the solar surface. Initially, the loops are observed as
small patches of linear polarization above a granular cell. Shortly
afterward, two footpoints of opposite polarity become visible in
circular polarization within or at the edges of the granule and start
moving toward the adjacent intergranular space. The orientation of the
footpoints does not seem to obey Hale's polarity rules. The loops are
continuously buffeted by convective motions, but they always retain a
high degree of coherence. Interestingly, 23% of the loops that emerge
in the photosphere reach the chromosphere (16 cases out of 69). They
are first detected in Fe I 630 nm magnetograms and 5 minutes later
in Mg I b 517.3 nm magnetograms. After about 8 minutes, some of them
are also observed in Ca II H line-core images, where the footpoints
produce small brightness enhancements.
---------------------------------------------------------
Title: Centre to Limb Variation of the Internetwork Magnetism
Authors: Martínez González, M. J.; Asensio Ramos, A.; López
Ariste, A.
2009ASPC..405..219M Altcode:
We analyse 1.5 μm spectropolarimetric observations of quiet
regions at different positions on the solar surface. Neither the
circular and linear polarization signals nor the area and amplitude
asymmetries present a clear trend with the position on the Sun. This
fact discards a network-like scenario for the internetwork magnetic
fields. An isotropically distributed magnetic field would reproduce the
observations, whatever its organisation (myriads of low-lying loops,
turbulent field, etc.).
---------------------------------------------------------
Title: Bayesian Inversion of Stokes Profiles
Authors: Asensio Ramos, A.; Martínez González, M. J.;
Rubiño-Martín, J. A.
2009ASPC..405..315A Altcode:
Inversion techniques are the most powerful methods to obtain information
about the thermodynamical and magnetic properties of solar and stellar
atmospheres. In the last years, we have witnessed the development of
highly sophisticated inversion codes that are now widely applied to
spectro-polarimetric observations. However, no realistic confidence
intervals are obtained for the inferred parameters. Additionally, it
is fundamental to apply efficient techniques to assess the ability
of models to reproduce the observations and to know to what extent
the models have to be refined or can be simplified. We propose the
application of Bayesian techniques. The posterior distribution, that
takes into account both the a-priori knowledge and the information
given by the data, is efficiently sampled using a Markov Chain Monte
Carlo method. We demonstrate the capabilities of the code with the
aid of an interesting example.
---------------------------------------------------------
Title: Flux Cancellation in the Solar Photosphere: a near-IR Line
of Mn I as a Diagnostic Tool
Authors: Asensio Ramos, A.; Martínez González, M. J.; López Ariste,
A.; Trujillo Bueno, J.; Collados, M.
2009ASPC..405..215A Altcode:
Recently, \cite{asensio_2_asensio_mn07} pointed out that the near-IR
line of Mn I at 15262.702 Å provides a new diagnostic window for
exploring the magnetism of the quiet Sun. In contrast with previously
considered Mn I lines located at visible wavelengths this near-IR
line has the advantage that the shape of its intensity profile is very
sensitive to the presence of magnetic fields. This enhanced magnetic
sensitivity is produced by the coincidence of two favorable facts:
the enhanced Zeeman sensitivity of near-IR lines and because this line
is subjected to particularly strong Paschen-Back perturbations due to
the hyperfine structure of manganese. Of great diagnostic interest is
that the intensity profile itself give us information on the unsigned
magnetic flux, while the polarization profiles are sensitive to the
net flux. An application to spectropolarimetric observations with the
Tenerife Infrared Polarimeter has allowed us to obtain the first flux
cancellation map in an enhanced network region.
---------------------------------------------------------
Title: Multiline Zeeman signatures as demonstrated through the
Pseudo-line
Authors: Semel, M.; Ramirez Velez, J. C.; Stift, M. J.; Martinez
Gonzalez, M. J.; Lopez Ariste, A.; Leone, F.
2008arXiv0810.3543S Altcode:
In order to get a significant Zeeman signature in the polarised spectra
of a magnetic star, we usually 'add' the contributions of numerous
spectral lines; the ultimate goal is to recover the spectropolarimetric
prints of the magnetic field in these line additions. Here we want to
clarify the meaning of these techniques of line addition; in particular,
we try to interpret the meaning of the 'pseudo-line' formed during
this process and to find out why and how its Zeeman signature is still
meaningful. We create a synthetic case of lines addition and apply
well tested standard solar methods routinely used in the research
on magnetism in our nearest star. The results are convincing and the
Zeeman signatures well detected; Solar methods are found to be quite
efficient also for stellar observations. The Zeeman signatures are
unequivocally detected in this multiline approach. We may anticipate
the outcome magnetic fields to be reliable well beyond the weak-field
approximation. Linear polarisation in the spectra of solar type stars
can be detected when the spectral resolution is sufficiently high.
---------------------------------------------------------
Title: PCA detection and denoising of Zeeman signatures in polarised
stellar spectra
Authors: Martínez González, M. J.; Asensio Ramos, A.; Carroll,
T. A.; Kopf, M.; Ramírez Vélez, J. C.; Semel, M.
2008A&A...486..637M Altcode: 2008arXiv0804.4692M
Aims: Our main objective is to develop a denoising strategy to
increase the signal to noise ratio of individual spectral lines
of stellar spectropolarimetric observations. <BR />Methods: We
use a multivariate statistics technique called Principal Component
Analysis. The cross-product matrix of the observations is diagonalized
to obtain the eigenvectors in which the original observations can be
developed. This basis is such that the first eigenvectors contain
the greatest variance. Assuming that the noise is uncorrelated a
denoising is possible by reconstructing the data with a truncated
basis. We propose a method to identify the number of eigenvectors for
an efficient noise filtering. <BR />Results: Numerical simulations are
used to demonstrate that an important increase of the signal to noise
ratio per spectral line is possible using PCA denoising techniques. It
can be also applied for detection of magnetic fields in stellar
atmospheres. We analyze the relation between PCA and commonly used
techniques like line addition and least-squares deconvolution. Moreover,
PCA is very robust and easy to compute.
---------------------------------------------------------
Title: Multiline Spectropolarimetry of the Quiet Sun at 5250 and
6302 Å
Authors: Socas-Navarro, H.; Borrero, J. M.; Asensio Ramos, A.;
Collados, M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez
González, M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez
Almeida, J.
2008ApJ...674..596S Altcode:
The reliability of quiet-Sun magnetic field diagnostics based on the
Fe I lines at 6302 Å has been questioned by recent work. Here we
present the results of a thorough study of high-resolution multiline
observations taken with the new spectropolarimeter SPINOR, comprising
the 5250 and 6302 Å spectral domains. The observations were analyzed
using several inversion algorithms, including Milne-Eddington,
LTE with 1 and 2 components, and MISMA codes. We find that the
line-ratio technique applied to the 5250 Å lines is not sufficiently
reliable to provide a direct magnetic diagnostic in the presence
of thermal fluctuations and variable line broadening. In general,
one needs to resort to inversion algorithms, ideally with realistic
magnetohydrodynamic constrains. When this is done, the 5250 Å lines
do not seem to provide any significant advantage over those at 6302
Å. In fact, our results point toward a better performance with the
latter (in the presence of turbulent line broadening). In any case,
for very weak flux concentrations, neither spectral region alone
provides sufficient constraints to fully disentangle the intrinsic
field strengths. Instead, we advocate for a combined analysis of both
spectral ranges, which yields a better determination of the quiet-Sun
magnetic properties. Finally, we propose the use of two other Fe I
lines (at 4122 and 9000 Å) with identical line opacities that seem
to work much better than the others.
---------------------------------------------------------
Title: Near-IR internetwork spectro-polarimetry at different
heliocentric angles
Authors: Martínez González, M. J.; Asensio Ramos, A.; López Ariste,
A.; Manso Sainz, R.
2008A&A...479..229M Altcode: 2007arXiv0710.5219M
Aims:The analysis of near infrared spectropolarimetric data at the
internetwork in different regions on the solar surface could offer
constraints that reject current modeling of these quiet areas. <BR
/>Methods: We present spectro-polarimetric observations of very
quiet regions for different values of the heliocentric angle for the
Fe i lines at 1.56 μm, from disc centre to positions close to the
limb. The spatial resolution of the data is 0.7{-}1”. We analyse
direct observable properties of the Stokes profiles as the amplitude
of circular and linear polarisation, as well as the total degree of
polarisation. The area and amplitude asymmetries are also studied. <BR
/>Results: We do not find any significant variation in the properties of
the polarimetric signals with the heliocentric angle. This means that
the magnetism of the solar internetwork remains the same regardless
of the position on the solar disc. This observational fact discards
the possibility of modeling the internetwork as a network-like
scenario. The magnetic elements of internetwork areas seem to be
isotropically distributed when observed at our spatial resolution.
---------------------------------------------------------
Title: Internetwork magnetic field distribution from simultaneous
1.56 μm and 630 nm observations
Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.;
Beck, C.
2008A&A...477..953M Altcode: 2007arXiv0711.0267M
Aims:We study the contradictory magnetic field strength distributions
retrieved from independent analyses of spectropolarimetric observations
in the near-infrared (1.56 μm) and in the visible (630 nm) spectral
ranges in internetwork regions. <BR />Methods: To solve this apparent
controversy, we present simultaneous and co-spatial 1.56 μm and 630 nm
observations of an internetwork area. The properties of the circular
and linear polarization signals, as well as the Stokes V area and
amplitude asymmetries, are discussed. As a complement, we also used
inversion techniques to infer the physical parameters of the solar
atmosphere. As a first step, the infrared and visible observations
are analysed separately to check their compatibility. Finally, the
simultaneous inversion of the two data sets is performed. <BR />Results:
The magnetic flux densities retrieved from the individual analysis
of the infrared and visible data sets are strongly correlated. The
polarity of the Stokes V profiles is the same at co-spatial pixels
in both wavelength ranges. This indicates that both 1.56 μm and
630 nm observations trace the same magnetic structures on the solar
surface. The simultaneous inversion of the two pairs of lines reveals
an internetwork full of sub-kG structures that fill only 2% of the
resolution element. A correlation is found between the magnetic field
strength and the continuum intensity: equipartition fields (B∼ 500
G) tend to be located in dark intergranular lanes, whereas weaker
field structures are found inside granules. The most probable unsigned
magnetic flux density is 10 Mx/cm^2. The net magnetic flux density in
the whole field of view is nearly zero. This means that both polarities
cancel out almost exactly in our observed internetwork area.
---------------------------------------------------------
Title: Bayesian inversion of Stokes profiles
Authors: Asensio Ramos, A.; Martínez González, M. J.;
Rubiño-Martín, J. A.
2007A&A...476..959A Altcode: 2007arXiv0709.0596A
Context: Inversion techniques are the most powerful methods to obtain
information about the thermodynamical and magnetic properties of
solar and stellar atmospheres. In the recent years, we have witnessed
the development of highly sophisticated inversion codes that are now
widely applied to spectro-polarimetric observations. The majority of
these inversion codes are based on the optimization of a complicated
non-linear merit function. The experience gained has facilitated the
recovery of the model that best fits a given observation. However,
and except for the recently developed inversion codes based
on database search algorithms together with the application
of Principal Component Analysis, no reliable and statistically
well-defined confidence intervals can be obtained for the parameters
inferred from the inversions. <BR />Aims: A correct estimation of the
confidence intervals for all the parameters that describe the model is
mandatory. Additionally, it is fundamental to apply efficient techniques
to assess the ability of models to reproduce the observations and
to determine to what extent the models have to be refined or can
be simplified. <BR />Methods: Bayesian techniques are applied to
analyze the performance of the model to fit a given observed Stokes
vector. The posterior distribution, that takes into account both the
information about the priors and the likelihood, is efficiently sampled
using a Markov chain Monte Carlo method. For simplicity, we focus on
the Milne-Eddington approximate solution of the radiative transfer
equation and we only take into account the generation of polarization
through the Zeeman effect. However, the method is extremely general
and other more complex forward models can be applied, even allowing
for the presence of atomic polarization. <BR />Results: We illustrate
the method with different problems, from academic to more realistic
examples. We show that the information provided by the posterior
distribution is fundamental to understand and determine the amount
of information available in the Stokes profiles in these particular
cases. <P />Appendix A and B are only available in electronic form
at http://www.aanda.org.
---------------------------------------------------------
Title: Multi-Line Quiet Sun Spectro-Polarimetry at 5250 and 6302 Å
Authors: Socas-Navarro, H.; Borrero, J.; Asensio Ramos, A.; Collados,
M.; Domínguez Cerdeña, I.; Khomenko, E. V.; Martínez González,
M. J.; Martínez Pillet, V.; Ruiz Cobo, B.; Sánchez Almeida, J.
2007arXiv0710.1099S Altcode:
The reliability of quiet Sun magnetic field diagnostics based on the
\ion{Fe}{1} lines at 6302 Åhas been questioned by recent work. We
present here the results of a thorough study of high-resolution
multi-line observations taken with the new spectro-polarimeter SPINOR,
comprising the 5250 and 6302 Åspectral domains. The observations were
analyzed using several inversion algorithms, including Milne-Eddington,
LTE with 1 and 2 components, and MISMA codes. We find that the
line-ratio technique applied to the 5250 Ålines is not sufficiently
reliable to provide a direct magnetic diagnostic in the presence
of thermal fluctuations and variable line broadening. In general,
one needs to resort to inversion algorithms, ideally with realistic
magneto-hydrodynamical constrains. When this is done, the 5250 Ålines
do not seem to provide any significant advantage over those at 6302
Å. In fact, our results point towards a better performance with the
latter (in the presence of turbulent line broadening). In any case,
for very weak flux concentrations, neither spectral region alone
provides sufficient constraints to fully disentangle the intrinsic field
strengths. Instead, we advocate for a combined analysis of both spectral
ranges, which yields a better determination of the quiet Sun magnetic
properties. Finally, we propose the use of two other \ion{Fe}{1} lines
(at 4122 and 9000 Å) with identical line opacities that seem to work
much better than the others.
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Title: Low-lying magnetic loops in the solar internetwork
Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.;
Solanki, S. K.
2007A&A...469L..39M Altcode: 2007arXiv0705.1319M
Aims:We study the structure of the magnetic field vector in the
internetwork and search for the presence of small-scale loops. <BR
/>Methods: We invert 1.56 μm spectropolarimetric observations of
internetwork regions at disc centre by applying the SIR code. This
allows us to recover the atmospheric parameters that play a role in
the formation of these spectral lines. We are mainly interested in the
structure of the magnetic field vector. <BR />Results: We find that
many opposite polarity elements of the internetwork are connected by
short (2-6´´), low-lying (photospheric) loops. These loops connect
at least the 10-20% of the internetwork flux visible in our data. Also
we have some evidence that points towards a dynamic scenario that can
be produced by the emergence of internetwork magnetic flux.
---------------------------------------------------------
Title: Turbulent fields in the quiet sun from Hanle and Zeeman
effects with THEMIS
Authors: López Ariste, A.; Malherbe, J. M.; Manso Sainz, R.; Asensio
Ramos, A.; Ramírez Vélez, J. C.; Martínez González, M.
2007sf2a.conf..596L Altcode:
The measurement of the Sr I line with full polarimetry and spatial
resolution with THEMIS has provided the unexpected result of an apparent
correlation between the Hanle effect signals and the Zeeman effect
signals on this line. Traditionnally, Hanle effect signals in linear
polarisation have been interpreted as the signature of unorganized,
turbulent fields, while Zeeman effect signals -mostly in circular
polarisation- were interpreted as structured fields. The correlation
between both observed by THEMIS requires a change of mind respect to the
picture of the quiet sun, with fewer structured fields and a turbulent
field visible also in deep magnetograms. This picture is supported also
by recent results with Mn lines with strong coupling with its hyperfine
structure and of center-to-limb histograms of Zeeman amplitudes.
---------------------------------------------------------
Title: The Intrinsic Dimensionality of Spectropolarimetric Data
Authors: Asensio Ramos, A.; Socas-Navarro, H.; López Ariste, A.;
Martínez González, M. J.
2007ApJ...660.1690A Altcode: 2007astro.ph..1604A
The amount of information available in spectropolarimetric data is
estimated. To this end, the intrinsic dimensionality of the data is
inferred with the aid of a recently derived estimator based on nearest
neighbor considerations and obtained applying the principle of maximum
likelihood. We show in detail that the estimator correctly captures the
intrinsic dimension of artificial data sets with known dimension. The
effect of noise in the estimated dimension is analyzed thoroughly,
and we conclude that it introduces a positive bias that needs to be
accounted for. Real simultaneous spectropolarimetric observations
in the visible 630 nm and the near-infrared 1.5 μm spectral regions
are also investigated in detail, showing that the near-infrared data
set provides more information of the physical conditions in the solar
atmosphere than the visible data set. Finally, we demonstrate that the
amount of information present in an observed data set is a monotonically
increasing function of the number of available spectral lines.
---------------------------------------------------------
Title: A Near-Infrared Line of Mn I as a Diagnostic Tool of the
Average Magnetic Energy in the Solar Photosphere
Authors: Asensio Ramos, A.; Martínez González, M. J.; López Ariste,
A.; Trujillo Bueno, J.; Collados, M.
2007ApJ...659..829A Altcode: 2006astro.ph.12389A
We report on spectropolarimetric observations of a near-IR line of Mn I
located at 15262.702 Å whose intensity and polarization profiles are
very sensitive to the presence of hyperfine structure. A theoretical
investigation of the magnetic sensitivity of this line uncovers several
interesting properties. The most important one is that the presence
of strong Paschen-Back perturbations due to the hyperfine structure
produces an intensity line profile whose shape changes according to the
absolute value of the magnetic field strength. A line ratio technique is
developed from the intrinsic variations of the line profile. This line
ratio technique is applied to spectropolarimetric observations of the
quiet solar photosphere in order to explore the probability distribution
function of the magnetic field strength. Particular attention is given
to the quietest area of the observed field of view, which was encircled
by an enhanced network region. A detailed theoretical investigation
shows that the inferred distribution yields information on the average
magnetic field strength and on the spatial scale at which the magnetic
field is organized. A first estimation gives ~250 G for the mean field
strength and a tentative value of ~0.4" for the spatial scale at which
the observed magnetic field is horizontally organized.
---------------------------------------------------------
Title: Determination of field strengths in the quiet Sun
Authors: López Ariste, A.; Martínez González, M. J.; Ramírez
Vélez, J. C.
2007A&A...464..351L Altcode:
The magnetism in the quiet regions of the solar photosphere carries
information on the dynamo processes and its interaction with the
convection of the outer layers of the sun. Unfortunately, the scales
of the magnetic structures on these regions are mostly unresolved. It
is therefore instrumental to tell apart the intrinsic field strengths
in those regions from the flux through the resolution element. This
disentanglement has been far from obvious, leading to opposing views
of the magnetic topology in the unresolved structures of the quiet
Sun. Our study contributes to the disentanglement of field strength from
flux in the quiet Sun, at least, through the use of new observational
constrains in the form of spectropolarimetry of Mn i lines observed
in the solar spectrum.The chosen Mn lines present a strong coupling
with hyperfine structure resulting in spectral features, present or
absent as a function of field strength alone. We observe one of those
lines simultaneously and co-spatially with the Fe i lines at 630 nm,
at the core of the previous measurements.The inversion of the observed
Fe lines results in either strong or weak fields depending on the
initializations of the inversion algorithm. All the solutions show
nevertheless equally good values for the σ parameter and are therefore
equally valuable as solutions. The Mn however selects unambiguously
strong or weak fields, sometimes agreeing with the inversions of the Fe
lines, but half the time disagreeing with them.The Fe i lines at 630 nm,
in the conditions found in the quiet Sun, carry no binding information
on field strength. A proper analysis of quiet Sun magnetism should
necessarily pass through its simultaneous and co-spatial observation
with other lines imposing constraints on field strength, as the Mn i
lines here analyzed. Ultimately, the magnetic topology of the quiet Sun
shall arise from the coherent analysis of all these lines, sensitive
to the Zeeman effect.
---------------------------------------------------------
Title: Internetwork magnetic fields observed in the visible.
Authors: Martínez González, M. J.
2007MmSAI..78...59M Altcode:
We present internetwork quiet Sun spectropolarimetric observations taken
simultaneously at the Fe I lines at 630 and 525 nm. The magnetometry
at 630 nm in these low flux regions of the Sun's photosphere has been
put in doubt by a recent work. The 525 nm lines are supposed to give
reliable results concerning the magnetic field strength in regions where
the magnetic structures are not resolved. We study the compatibility
of the results obtained in both spectral ranges to confirm or deny the
affirmation that the 630 nm lines are not valid for recovering the
magnetic field strength at the internetwork quiet Sun. We conclude
that both the 630 nm and the 525 nm pair of lines give no trustable
results concerning the magnetic field strength. However, the magnetic
flux density is correctly recovered from both spectral ranges.
---------------------------------------------------------
Title: Internetwork magnetic fields
Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.
2007msfa.conf..157M Altcode:
Spectropolarimetric observations of the internetwork obtained in the
1.56 [mμ]m Fe I doublet are used to calculate the distributions of
magnetic field strength, filling factor and magnetic flux density
at different positions on the solar surface. We go one step further
and describe what the observations show about the three-dimensional
structure of the magnetic fields at the internetwork quiet Sun.
---------------------------------------------------------
Title: Internetwork Magnetic Field Distribution from Simultaneous
Fe I 1.5 µ and Fe I 630 nm Observations
Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.
2006ASPC..358...36M Altcode:
We took simultaneous observations in Fe I 1.5 µ and Fe I 630 nm of
an internetwork region. We discuss the compatibility of the data in
the two spectral ranges, and present results for the magnetic field
distribution. We show that the larger contribution comes from hG
field strengths.
---------------------------------------------------------
Title: On the validity of the 630 nm Fe I lines for magnetometry of
the internetwork quiet Sun
Authors: Martínez González, M. J.; Collados, M.; Ruiz Cobo, B.
2006A&A...456.1159M Altcode: 2006astro.ph..5446M
Aims.The purpose of this work is to analyze the reliability of the
magnetic field strengths inferred from the 630 nm pair of Fe i lines
in internetwork quiet Sun regions.<BR /> Methods: .Some numerical
experiments have been performed that demonstrate the inability
of these lines to recover the magnetic field strength in such low
flux solar regions.<BR /> Results: .It is shown how different model
atmospheres, with magnetic field strengths ranging from a few hundred
Gauss to kiloGauss, give rise to Stokes profiles that cannot be
distinguished. The reasons for this degeneracy are discussed.<BR />
---------------------------------------------------------
Title: Campos magnéticos en el Sol en calma
---------------------------------------------------------
Title: Campos magnéticos
en el Sol en calma
---------------------------------------------------------
Title: Magnetic fields in the quiet sun;
Authors: Martínez González, María Jesús
2006PhDT.......335M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic flux in the internetwork quiet Sun
Authors: Khomenko, E. V.; Martínez González, M. J.; Collados, M.;
Vögler, A.; Solanki, S. K.; Ruiz Cobo, B.; Beck, C.
2005A&A...436L..27K Altcode:
We report a direct comparison of the amplitudes of Stokes spectra of the
Fe i 630 nm and 1.56 μm lines produced by realistic MHD simulations
with simultaneous observations in the same spectral regions. The
Stokes spectra were synthesized in snapshots with a mixed polarity
magnetic field having a spatially averaged strength, < B >,
between 10 and 30 G. The distribution of Stokes V amplitudes depends
sensitively on < B >. A quiet inter-network region was observed
at the German VTT simultaneously with TIP (1.56 μm) and POLIS (630
nm). We find that the Stokes V amplitudes of both infrared and visible
observations are best reproduced by the simulation snapshot with <
B > = 20 G. In observations with 1 resolution, up to 2/3 of the
magnetic flux can remain undetected.
---------------------------------------------------------
Title: Modeling the photometric and dynamical behavior of
Super-Schmidt meteors in the Earth's atmosphere
Authors: Bellot Rubio, L. R.; Martínez González, M. J.; Ruiz
Herrera, L.; Licandro, J.; Martínez-Delgado, D.; Rodríguez-Gil,
P.; Serra-Ricart, M.
2002A&A...389..680B Altcode:
Faint meteors observed with Super-Schmidt cameras are re-examined
in order to assess whether their dynamical and photometric behavior
can be described by means of the single body theory. Velocities,
decelerations and magnitudes are fitted simultaneously to synthetic
curves resulting from integration of the appropriate set of differential
equations. The parameters determined by this procedure are the ablation
coefficient, the shape-density coefficient and the preatmospheric
mass of each individual meteoroid. It turns out that 73% of the
meteors analyzed here (with magnitudes in the range from +2.5 to -5)
are reasonably well described by this theory, suggesting that they
did not undergo significant fragmentation during their atmospheric
flight. Nevertheless, we identify some systematic differences between
observed and theoretical light curves of meteors for which the fit
is good. Meteoroid bulk densities are estimated from the retrieved
shape-density coefficients. The distributions of individual values
are broad, indicating that objects of different densities coexist
within the same meteoroid population. The average density is found to
be 2400, 1400, and 400 kg m<SUP>-3</SUP> for A-type, B-type and C-type
meteoroids, respectively. These results do not confirm the large values
determined from quasicontinuous fragmentation models.
---------------------------------------------------------
Title: Modeling the dynamical and photometric behavior of faint
meteors in the Earth's atmosphere
Authors: Bellot Rubio, Luis R.; Martínez González, María Jesús;
Ruiz Herrera, Lola; Licandro, Javier; Martínez Delgado, David;
Rodríguez Gil, Pablo; Serra-Ricart, Miquel
2001ESASP.495..525B Altcode: 2001mete.conf..525B
Faint meteors observed with Super-Schmidt cameras are re-examined
in order to assess whether their dynamical and photometric behavior
can be described by means of the single body theory. Velocities,
decelerations and magnitudes are fitted simultaneously to synthetic
curves resulting from integration of the appropriate set of differential
equations. The parameters determined by this procedure are the ablation
coefficient, the shape-density coefficient and the preatmospheric mass
of the meteoroids. It turns out that 64% of the meteors analyzed here
are reasonably well described by this theory, suggesting that they
did not undergo significant fragmentation during their atmospheric
flight. We identify some systematic differences between observed and
theoretical light curves of meteors for which the fit is good. From
the retrieved shape-density coefficients we derive meteoroid bulk
densities. The average density turns out to be 1.0 g cm<SUP>-3</SUP>,
with individual values ranging from 0.2 g cm<SUP>-3</SUP> to 1.4 g
cm<SUP>-3</SUP>. This work represent a first step toward the analysis
of low-light level video observations of very small meteoroids.