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Author name code: vannoort
ADS astronomy entries on 2022-09-14
author:"van Noort, Michiel J."
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Title: Similarities of magnetoconvection in the umbra and in the
penumbra of sunspots
Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K.
2021A&A...655A..61L Altcode: 2021arXiv211001352L
Context. It is unclear why there is a rather sharp boundary in
sunspots between the umbra and the penumbra. Both regions exhibit
magnetoconvection, which manifests in penumbral filaments in the
penumbra and in umbral dots in the umbra. <BR /> Aims: Here we compare
the physical properties of umbral dots and penumbral filaments. Our
goal is to understand how the properties of these convective features
change across the boundary between the umbra and the penumbra
and how this is related to the rapid increase in brightness at the
umbra-penumbra boundary. <BR /> Methods: We derived ensemble averages
of the physical properties of different types of convective features
based on observations of two sunspots with Hinode. <BR /> Results:
There are strong similarities between the convective features in the
outer parts of the umbra and the ones in the penumbra, with most
physical parameters being smooth and continuous functions of the
length of the features. <BR /> Conclusions: Our results indicate
that the transition in brightness from the umbra to the penumbra
is solely caused by an increased effectiveness of magnetoconvection
within individual convective cells. There is no significant difference
in the number density of convective elements between the outer umbra
and the inner penumbra. Penumbral filaments exhibit a larger area and
a higher brightness compared to umbral dots. It is still unclear how
exactly the underlying magnetic field causes the increase in the size
and brightness of convective features in the penumbra.
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Title: Correction of atmospheric stray light in restored slit spectra
Authors: Saranathan, S.; van Noort, M.; Solanki, S. K.
2021A&A...653A..17S Altcode: 2021arXiv211202648S
Context. A long-standing issue in solar ground-based observations has
been the contamination of data due to stray light, which is particularly
relevant in inversions of spectropolarimetric data. <BR /> Aims: We aim
to build on a statistical method of correcting stray-light contamination
due to residual high-order aberrations and apply it to ground-based slit
spectra. <BR /> Methods: The observations were obtained at the Swedish
Solar Telescope, and restored using the multi-frame blind deconvolution
restoration procedure. Using the statistical properties of seeing,
we created artificially degraded synthetic images generated from
magneto-hydrodynamic simulations. We then compared the synthetic data
with the observations to derive estimates of the amount of the residual
stray light in the observations. In the final step, the slit spectra
were deconvolved with a stray-light point spread function to remove
the residual stray light from the observations. <BR /> Results: The RMS
granulation contrasts of the deconvolved spectra were found to increase
to approximately 12.5%, from 9%. Spectral lines, on average, were found
to become deeper in the granules and shallower in the inter-granular
lanes, indicating systematic changes to gradients in temperature. The
deconvolution was also found to increase the redshifts and blueshifts
of spectral lines, suggesting that the velocities of granulation in
the solar photosphere are higher than had previously been observed.
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Title: How wrong are the results of inverting Fe I lines when NLTE
and 3D radiative transfer effects are ignored?
Authors: Narayanamurthy, S.; Holzreuter, R.; van Noort, M.; Solanki, S.
2021AAS...23811305N Altcode:
The Fe I 6301.5 Å and 6302.5 Å lines are widely used to probe the
solar photosphere. They are known to be affected by the non-local
thermodynamic equilibrium (NLTE) conditions due to the ultraviolet
overionisation of iron atoms in the solar atmosphere. This leads
to deviations in their level populations based on Saha-Boltzmann
statistics. When inverting their Stokes profiles to determine
atmospheric parameters, the NLTE effects are often neglected and
other quantities are tweaked to compensate for deviations from the
LTE. In this work, we discuss how the routinely employed LTE inversion
introduces errors in the derived atmospheric quantities. We show that
when the NLTE effects are neglected, these errors can be as high
as 13% in temperature, and in line-of-sight velocity and magnetic
field strength the errors can even exceed 50%. Errors are found
at the sites of granules, intergranular lanes, magnetic elements,
and basically in every region with strong vertical gradients in the
atmosphere. Similarly, strong horizontal gradients in temperature
introduce 3D effects in these lines. We find that errors due to
neglecting the 3D effects are more localised and are lower than 5%
in temperature, and lower than 20% in both velocity and magnetic field
strength. The NLTE and 3D effects are found to persist when the Stokes
profiles are spatially and spectrally degraded to the resolution of
the Swedish Solar Telescope (SST) or Daniel K. Inouye Solar Telescope
(DKIST). Our findings have wide-ranging consequences since many results
derived in solar physics are based on inversions of these Fe I lines
carried out in LTE.
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Title: Nonequilibrium Equation of State in Stellar Atmospheres
Authors: Anusha, L. S.; van Noort, M.; Cameron, R. H.
2021ApJ...911...71A Altcode: 2021arXiv210413650A
In the stellar chromospheres, radiative energy transport is dominated by
only the strongest spectral lines. For these lines, the approximation of
local thermodynamic equilibrium (LTE) is known to be very inaccurate,
and a state of equilibrium cannot be assumed in general. To calculate
the radiative energy transport under these conditions, the population
evolution equation must be evaluated explicitly, including all
time-dependent terms. We develop a numerical method to solve the
evolution equation for the atomic-level populations in a time-implicit
way, keeping all time-dependent terms to first order. We show that
the linear approximation of the time dependence of the populations
can handle very large time steps without losing accuracy. We
reproduce the benchmark solutions from earlier, well-established
works in terms of non-LTE kinetic equilibrium solutions and typical
ionization/recombination timescales in the solar chromosphere.
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Title: The influence of NLTE effects in Fe I lines on an inverted
atmosphere. II. 6301 Å and 6302 Å lines formed in 3D NLTE
Authors: Smitha, H. N.; Holzreuter, R.; van Noort, M.; Solanki, S. K.
2021A&A...647A..46S Altcode: 2021arXiv210100506S
Context. This paper forms the second part of our study of how
neglecting non-local thermodynamic equilibrium (NLTE) conditions in
the formation of Fe I 6301.5 Å and the 6302.5 Å lines affects the
atmosphere that is obtained by inverting the Stokes profiles of these
lines in LTE. The main cause of NLTE effects in these lines is the line
opacity deficit that is due to the excess ionisation of Fe I atoms by
ultraviolet (UV) photons in the Sun. <BR /> Aims: In the first paper,
these photospheric lines were assumed to have formed in 1D NLTE and the
effects of horizontal radiation transfer (RT) were neglected. In the
present paper, the iron lines are computed by solving the RT in 3D. We
investigate the effect of horizontal RT on the inverted atmosphere and
how it can enhance or reduce the errors that are due to neglecting 1D
NLTE effects. <BR /> Methods: The Stokes profiles of the iron lines
were computed in LTE, 1D NLTE, and 3D NLTE. They were all inverted
using an LTE inversion code. The atmosphere from the inversion of
LTE profiles was taken as the reference model. The atmospheres from
the inversion of 1D NLTE profiles (testmodel-1D) and 3D NLTE profiles
(testmodel-3D) were compared with it. Differences between reference and
testmodels were analysed and correspondingly attributed to NLTE and 3D
effects. <BR /> Results: The effects of horizontal RT are evident in
regions surrounded by strong horizontal temperature gradients. That is,
along the granule boundaries, regions surrounding magnetic elements,
and its boundaries with intergranular lanes. In some regions, the 3D
effects enhance the 1D NLTE effects, and in some, they weaken these
effects. In the small region analysed in this paper, the errors due to
neglecting the 3D effects are lower than 5% in temperature. In most of
the pixels, the errors are lower than 20% in both velocity and magnetic
field strength. These errors also persist when the Stokes profiles
are spatially and spectrally degraded to the resolution of the Swedish
Solar Telescope (SST) or Daniel K. Inouye Solar Telescope (DKIST). <BR
/> Conclusions: Neglecting horizontal RT introduces errors not only in
the derived temperature, but also in other atmospheric parameters. The
error sizes depend on the strength of the local horizontal temperature
gradients. Compared to the 1D NLTE effect, the 3D effects are more
localised in specific regions in the atmosphere and are weaker overall.
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Title: Non-equilibrium equation-of-state in stellar atmospheres
Authors: Lokanathapura Seetharamabhasari, Anusha; Cameron, Robert;
Van Noort, Michiel
2021cosp...43E.985L Altcode:
In the stellar atmospheres, radiative energy transport is dominated by
only the strongest spectral lines. For these lines, the approximation of
local thermo-dynamic equilibrium (LTE) is known to be very inaccurate,
and a state of equilibrium cannot be assumed in general. Therefore to
understand the structure and dynamics of stellar atmospheres through
evolving magneto-hydro-dynamic equations, one needs a non-equilibrium
equation of state. To calculate the radiative energy transport under
these conditions, the population evolution equation must be evaluated
including all time dependent terms. To this end, we have developed a new
numerical method to solve the non-LTE non-equilibrium radiative transfer
problem. We solve evolution equation for the atomic level populations
in a time-implicit way, keeping all time dependent terms to first
order. We have tested our method by reproducing earlier works, namely,
a) determining chromosperic time-scales of ionization/recombination,
b) showing that our non-equilibrium solver evolves to the statistical
equilibrium solution obtained from an independent non-LTE spectral
synthesis code. In this presentation, I will describe the method,
and discuss equilibrium solutions.
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Title: Power spectrum of turbulent convection in the solar photosphere
Authors: Yelles Chaouche, L.; Cameron, R. H.; Solanki, S. K.;
Riethmüller, T. L.; Anusha, L. S.; Witzke, V.; Shapiro, A. I.;
Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; van Noort,
M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez,
D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2020A&A...644A..44Y Altcode: 2020arXiv201009037Y
The solar photosphere provides us with a laboratory for understanding
turbulence in a layer where the fundamental processes of transport
vary rapidly and a strongly superadiabatic region lies very closely
to a subadiabatic layer. Our tools for probing the turbulence are
high-resolution spectropolarimetric observations such as have recently
been obtained with the two balloon-borne SUNRISE missions, and numerical
simulations. Our aim is to study photospheric turbulence with the
help of Fourier power spectra that we compute from observations
and simulations. We also attempt to explain some properties of the
photospheric overshooting flow with the help of its governing equations
and simulations. We find that quiet-Sun observations and smeared
simulations are consistent with each other and exhibit a power-law
behavior in the subgranular range of their Doppler velocity power
spectra with a power-law index of ≈ - 2. The unsmeared simulations
exhibit a power law that extends over the full range between the
integral and Taylor scales with a power-law index of ≈ - 2.25. The
smearing, reminiscent of observational conditions, considerably reduces
the extent of the power-law-like portion of the power spectra. This
suggests that the limited spatial resolution in some observations
might eventually result in larger uncertainties in the estimation of
the power-law indices. The simulated vertical velocity power spectra
as a function of height show a rapid change in the power-law index
(at the subgranular range) from roughly the optical depth unity layer,
that is, the solar surface, to 300 km above it. We propose that the
cause of the steepening of the power-law index is the transition from
a super- to a subadiabatic region, in which the dominant source of
motions is overshooting convection. A scale-dependent transport of
the vertical momentum occurs. At smaller scales, the vertical momentum
is more efficiently transported sideways than at larger scales. This
results in less vertical velocity power transported upward at small
scales than at larger scales and produces a progressively steeper
vertical velocity power law below 180 km. Above this height, the
gravity work progressively gains importance at all relevant scales,
making the atmosphere progressively more hydrostatic and resulting
in a gradually less steep power law. Radiative heating and cooling of
the plasma is shown to play a dominant role in the plasma energetics
in this region, which is important in terms of nonadiabatic damping
of the convective motions.
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Title: No universal connection between the vertical magnetic field
and the umbra-penumbra boundary in sunspots
Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K.
2020A&A...639A.106L Altcode: 2020arXiv200602346L
Context. It has been reported that the boundary between the umbra and
the penumbra of sunspots occurs at a canonical value of the strength
of the vertical magnetic field, independently of the size of the
spot. This critical field strength is interpreted to be the threshold
for the onset of magnetoconvection. <BR /> Aims: Here we investigate
the reasons why this criterion, also called the Jurčák criterion
in the literature, does not always identify the boundary between the
umbra and the penumbra. <BR /> Methods: We performed a statistical
analysis of 23 sunspots observed with Hinode/SOT. We compared the
properties of the continuum intensity and the vertical magnetic
field between filaments and spines and how they vary between spots of
different sizes. <BR /> Results: We find that the inner boundary of the
penumbra is not related to a universal value of the vertical magnetic
field. The properties of spines and filaments vary between spots of
different sizes. Both components are darker in larger spots and the
spines exhibit a stronger vertical magnetic field. These variations of
the properties of filaments and spines with the spot size are also the
reason for the reported invariance in the averaged vertical magnetic
field at 50% of the mean continuum intensity. <BR /> Conclusions:
The formation of filaments and the onset of magnetoconvection are not
related to a canonical value of the strength of the vertical magnetic
field. The seemingly unique magnetic field strength is rather an effect
of the filling factor of spines and penumbral filaments.
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Title: Detection of the Strongest Magnetic Field in a Sunspot
Light Bridge
Authors: Castellanos Durán, J. S.; Lagg, Andreas; Solanki, Sami K.;
van Noort, Michiel
2020ApJ...895..129C Altcode: 2020arXiv200312078C; 2020ApJ...895..129D
Traditionally, the strongest magnetic fields on the Sun have been
measured in sunspot umbrae. More recently, however, much stronger
fields have been measured at the ends of penumbral filaments carrying
the Evershed and counter-Evershed flows. Superstrong fields have
also been reported within a light bridge separating two umbrae of
opposite polarities. We aim to accurately determine the strengths of the
strongest fields in a light bridge using an advanced inversion technique
and to investigate their detailed structure. We analyze observations
from the spectropolarimeter on board the Hinode spacecraft of the
active region AR 11967. The thermodynamic and magnetic configurations
are obtained by inverting the Stokes profiles using an inversion scheme
that allows multiple height nodes. Both the traditional 1D inversion
technique and the so-called 2D coupled inversions, which take into
account the point-spread function of the Hinode telescope, are used. We
find a compact structure with an area of 32.7 arcsec<SUP>2</SUP> within
a bipolar light bridge with field strengths exceeding 5 kG, confirming
the strong fields in this light bridge reported in the literature. Two
regions associated with downflows of ∼5 km s<SUP>-1</SUP> harbor
field strengths larger than 6.5 kG, covering a total area of 2.97
arcsec<SUP>2</SUP>. The maximum field strength found is 8.2 kG, which
is the largest ever observed field in a bipolar light bridge up to now.
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Title: Solar Disk Center Shows Scattering Polarization in the Sr I
4607 Å Line
Authors: Zeuner, Franziska; Manso Sainz, Rafael; Feller, Alex; van
Noort, Michiel; Solanki, Sami K.; Iglesias, Francisco A.; Reardon,
Kevin; Martínez Pillet, Valentín
2020ApJ...893L..44Z Altcode: 2020arXiv200403679Z
Magnetic fields in turbulent, convective high-β plasma naturally
develop highly tangled and complex topologies - the solar photosphere
being the paradigmatic example. These fields are mostly undetectable by
standard diagnostic techniques with finite spatio-temporal resolution
due to cancellations of Zeeman polarization signals. Observations of
resonance scattering polarization have been considered to overcome
these problems. But up to now, observations of scattering polarization
lack the necessary combination of high sensitivity and high spatial
resolution in order to directly infer the turbulent magnetic structure
at the resolution limit of solar telescopes. Here, we report the
detection of clear spatial structuring of scattering polarization
in a magnetically quiet solar region at disk center in the Sr I
4607 Å spectral line on granular scales, confirming theoretical
expectations. We find that the linear polarization presents a
strong spatial correlation with the local quadrupole of the radiation
field. The result indicates that polarization survives the dynamic and
turbulent magnetic environment of the middle photosphere and is thereby
usable for spatially resolved Hanle observations. This is an important
step toward the long-sought goal of directly observing turbulent
solar magnetic fields at the resolution limit and investigating their
spatial structure.
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Title: Connecting the Wilson depression to the magnetic field
of sunspots
Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K.
2020A&A...635A.202L Altcode: 2020arXiv200207484L
Context. In sunspots, the geometric height of continuum optical depth
unity is depressed compared to the quiet Sun. This so-called Wilson
depression is caused by the Lorentz force of the strong magnetic
field inside the spots. However, it is not understood in detail yet
how the Wilson depression is related to the strength and geometry
of the magnetic field or to other properties of the sunspot. <BR />
Aims: We aim to study the dependence of the Wilson depression on the
properties of the magnetic field of the sunspots and how exactly the
magnetic field contributes to balancing the Wilson depression with
respect to the gas pressure of the surroundings of the spots. <BR
/> Methods: Our study is based on 24 spectropolarimetric scans of
12 individual sunspots performed with Hinode. We derived the Wilson
depression for each spot using both a recently developed method that
is based on minimizing the divergence of the magnetic field and an
approach that was developed earlier, which enforces an equilibrium
between the gas pressure and the magnetic pressure inside the spot and
the gas pressure in the quiet Sun, thus neglecting the influence of the
curvature force. We then performed a statistical analysis by comparing
the Wilson depression resulting from the two techniques with each other
and by relating them to various parameters of the sunspots, such as
their size or the strength of the magnetic field. <BR /> Results: We
find that the Wilson depression becomes larger for spots with a stronger
magnetic field, but not as much as one would expect from the increased
magnetic pressure. This suggests that the curvature integral provides
an important contribution to the Wilson depression, particularly
for spots with a weak magnetic field. Our results indicate that the
geometry of the magnetic field in the penumbra is different between
spots with different strengths of the average umbral magnetic field.
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Title: The influence of NLTE effects in Fe I lines on an inverted
atmosphere. I. 6301 Å and 6302 Å lines formed in 1D NLTE
Authors: Smitha, H. N.; Holzreuter, R.; van Noort, M.; Solanki, S. K.
2020A&A...633A.157S Altcode: 2019arXiv191207007S
Context. Ultraviolet overionisation of iron atoms in the solar
atmosphere leads to deviations in their level populations based on
Saha-Boltzmann statistics. This causes their line profiles to form in
non-local thermodynamic equilibrium (NLTE) conditions. When inverting
such profiles to determine atmospheric parameters, the NLTE effects
are often neglected and other quantities are tweaked to compensate for
deviations from the LTE. <BR /> Aims: We investigate how the routinely
employed LTE inversion of iron lines formed in NLTE underestimates
or overestimates atmospheric quantities, such as temperature (T),
line-of-sight velocity (v<SUB>LOS</SUB>), magnetic field strength (B),
and inclination (γ) while the earlier papers have focused mainly
on T. Our findings has wide-ranging consequences since many results
derived in solar physics are based on inversions of Fe I lines carried
out in LTE. <BR /> Methods: We synthesized the Stokes profiles of Fe I
6301.5 Å and 6302.5 Å lines in both LTE and NLTE using a snapshot of
a 3D magnetohydrodynamic simulation. The profiles were then inverted
in LTE. We considered the atmosphere inferred from the inversion of
LTE profiles as the fiducial model and compared it to the atmosphere
resulting from the inversion of NLTE profiles. The observed differences
have been attributed to NLTE effects. <BR /> Results: Neglecting the
NLTE effects introduces errors in the inverted atmosphere. While the
errors in T can go up to 13%, in v<SUB>LOS</SUB> and B, the errors can
go as high as 50% or above. We find these errors to be present at all
three inversion nodes. Importantly, they survive degradation from the
spatial averaging of the profiles. <BR /> Conclusions: We provide an
overview of how neglecting NLTE effects influences the values of T,
v<SUB>LOS</SUB>, B, and γ that are determined by inverting the Fe I
6300 Å line pair, as observed, for example, by Hinode/SOT/SP. Errors
are found at the sites of granules, intergranular lanes, magnetic
elements, and basically in every region susceptible to NLTE effects. For
an accurate determination of the atmospheric quantities and their
stratification, it is, therefore, important to take the NLTE effects
into account.
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Title: Superstrong photospheric magnetic fields in sunspot penumbrae
Authors: Siu-Tapia, A.; Lagg, A.; van Noort, M.; Rempel, M.; Solanki,
S. K.
2019A&A...631A..99S Altcode: 2019arXiv190913619S
Context. Recently, there have been some reports of unusually strong
photospheric magnetic fields (which can reach values of over 7 kG)
inferred from Hinode SOT/SP sunspot observations within penumbral
regions. These superstrong penumbral fields are even larger than the
strongest umbral fields on record and appear to be associated with
supersonic downflows. The finding of such fields has been controversial
since they seem to show up only when spatially coupled inversions
are performed. <BR /> Aims: Here, we investigate and discuss the
reliability of those findings by studying in detail observed spectra
associated with particularly strong magnetic fields at the inner edge
of the penumbra of active region 10930. <BR /> Methods: We applied
classical diagnostic methods and various inversions with different
model atmospheres to the observed Stokes profiles in two selected
pixels with superstrong magnetic fields, and compared the results
with a magnetohydrodynamic simulation of a sunspot whose penumbra
contains localized regions with strong fields (nearly 5 kG at τ = 1)
associated with supersonic downflows. <BR /> Results: The different
inversions provide different results: while the SPINOR 2D inversions
consider a height-dependent single-component model and return B >
7 kG and supersonic positive v<SUB>LOS</SUB> (corresponding to a
counter-Evershed flow), height-dependent two-component inversions
suggest the presence of an umbral component (almost at rest)
with field strengths ∼4 - 4.2 kG and a penumbral component with
v<SUB>LOS</SUB> ∼ 16 - 18 km s<SUP>-1</SUP> and field strengths up
to ∼5.8 kG. Likewise, height-independent two-component inversions
find a solution for an umbral component and a strongly redshifted
(v<SUB>LOS</SUB> ∼ 15 - 17 km s<SUP>-1</SUP>) penumbral component
with B ∼ 4 kG. According to a Bayesian information criterion,
the inversions providing a better balance between the quality of
the fits and the number of free parameters considered by the models
are the height-independent two-component inversions, but they lie
only slightly above the SPINOR 2D inversions. Since it is expected
that the physical parameters all display considerable gradients with
height, as supported by magnetohydrodynamic (MHD) sunspot simulations,
the SPINOR 2D inversions are the preferred ones. <BR /> Conclusions:
According to the MHD sunspot simulation analyzed here, the presence
of counter-Evershed flows in the photospheric penumbra can lead to
the necessary conditions for the observation of ∼5 kG fields at the
inner penumbra. Although a definite conclusion about the potential
existence of fields in excess of 7 kG cannot be given, their nature
could be explained (based on the simulation results) as the consequence
of the extreme dynamical effects introduced by highly supersonic
counter-Evershed flows (v<SUB>LOS</SUB> > 10 km s<SUP>-1</SUP>
and up to ∼30 km s<SUP>-1</SUP> according to SPINOR 2D). The latter
are much faster and more compressive downflows than those found in
the MHD simulations and therefore could lead to field intensification
up to considerably stronger fields. Also, a lower gas density would
lead to a deeper depression of the τ = 1 surface, making possible
the observation of deeper-lying stronger fields. The superstrong
magnetic fields are expected to be nearly force-free, meaning that
they can attain much larger strengths than expected when considering
only balance between magnetic pressure and the local gas pressure.
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Title: A comparison between solar plage and network properties
Authors: Buehler, D.; Lagg, A.; van Noort, M.; Solanki, S. K.
2019A&A...630A..86B Altcode: 2019arXiv190807464B
<BR /> Aims: We compare the properties of kG magnetic structures
in the solar network and in active region plage at high spatial
resolution. <BR /> Methods: Our analysis used six SP scans of the solar
disc centre aboard Hinode SOT and inverted the obtained spectra of the
photospheric 6302 Å line pair using the 2D SPINOR code. <BR /> Results:
Photospheric magnetic field concentrations in network and plage areas
are on average 1.5 kG strong with inclinations of 10° -20°, and have
< 400 m s<SUP>-1</SUP> internal and 2-3 km s<SUP>-1</SUP> external
downflows. At the disc centre, the continuum intensity of magnetic
field concentrations in the network are on average 10% brighter than the
mean quiet Sun, whilst their plage counterparts are 3% darker. A more
detailed analysis revealed that all sizes of individual kG patches in
the network have 150 G higher field strengths on average, 5% higher
continuum contrasts, and 800 m s<SUP>-1</SUP> faster surrounding
downflows than similarly sized patches in the plage. The speed of
the surrounding downflows also correlates with the patch area, and
patches containing pores can produce supersonic flows exceeding 11 km
s<SUP>-1</SUP> in individual pixels. Furthermore, the magnetic canopies
of kG patches are on average 9° more horizontal in the plage compared
to the network. <BR /> Conclusions: Most of the differences between the
network and plage are due to their different patch size distributions,
but the intrinsic differences between similarly sized patches likely
results from the modification of the convection photospheric convection
with increasing amounts of magnetic flux.
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Title: Response Functions for NLTE Lines
Authors: Milić, I.; van Noort, M.
2019ASPC..526..179M Altcode:
Response functions quantify the sensitivity of the emergent polarized
spectrum to perturbations in the atmospheric quantities. They are
an important diagnostics tool and an essential ingredient of the
so-called inversion codes, widely used in solar spectropolarimetry. The
computation of response functions for spectral lines formed out of
local thermodynamic equilibrium is complicated because of strong
spatial and non-linear couplings of the atomic populations. We have
recently proposed a novel, analytic approach for the computation of
NLTE response functions, and in this short contribution we discuss
the possibilities of computing response functions for scattering
polarization. We explicitly show the procedure for a two level atom
line (normal Zeeman triplet), and discuss the "formation heights"
of intensity and scattering polarization for a prototype line.
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Title: Recent advancements in the EST project
Authors: Jurčák, Jan; Collados, Manuel; Leenaarts, Jorrit; van Noort,
Michiel; Schlichenmaier, Rolf
2019AdSpR..63.1389J Altcode: 2018arXiv181100851J
The European Solar Telescope (EST) is a project of a new-generation
solar telescope. It has a large aperture of 4 m, which is necessary for
achieving high spatial and temporal resolution. The high polarimetric
sensitivity of the EST will allow to measure the magnetic field in the
solar atmosphere with unprecedented precision. Here, we summarise the
recent advancements in the realisation of the EST project regarding
the hardware development and the refinement of the science requirements.
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Title: Measuring the Wilson depression of sunspots using the
divergence-free condition of the magnetic field vector
Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K.
2018A&A...619A..42L Altcode: 2018arXiv180806867L
Context. The Wilson depression is the difference in geometric height of
unit continuum optical depth between the sunspot umbra and the quiet
Sun. Measuring the Wilson depression is important for understanding
the geometry of sunspots. Current methods suffer from systematic
effects or need to make assumptions on the geometry of the magnetic
field. This leads to large systematic uncertainties of the derived
Wilson depressions. <BR /> Aims: We aim to develop a robust method
for deriving the Wilson depression that only requires the information
about the magnetic field that is accessible from spectropolarimetry,
and that does not rely on assumptions on the geometry of sunspots
or on their magnetic field. <BR /> Methods: Our method is based on
minimizing the divergence of the magnetic field vector derived from
spectropolarimetric observations. We have focused on large spatial
scales only in order to reduce the number of free parameters. <BR />
Results: We tested the performance of our method using synthetic Hinode
data derived from two sunspot simulations. We find that the maximum and
the umbral averaged Wilson depression for both spots determined with
our method typically lies within 100 km of the true value obtained
from the simulations. In addition, we applied the method to Hinode
observations of a sunspot. The derived Wilson depression (∼600 km) is
consistent with results typically obtained from the Wilson effect. We
also find that the Wilson depression obtained from using horizontal
force balance gives 110-180 km smaller Wilson depressions than both,
what we find and what we deduce directly from the simulations. This
suggests that the magnetic pressure and the magnetic curvature force
contribute to the Wilson depression by a similar amount.
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Title: Spectropolarimetric NLTE inversion code SNAPI
Authors: Milić, I.; van Noort, M.
2018A&A...617A..24M Altcode: 2018arXiv180608134M
Context. Inversion codes are computer programs that fit a model
atmosphere to the observed Stokes spectra, thus retrieving the relevant
atmospheric parameters. The rising interest in the solar chromosphere,
where spectral lines are formed by scattering, requires developing,
testing, and comparing new non-local thermal equilibrium (NLTE)
inversion codes. <BR /> Aims: We present a new NLTE inversion code that
is based on the analytical computation of the response functions. We
named the code SNAPI, which is short for spectropolarimetic NLTE
analytically powered inversion. <BR /> Methods: SNAPI inverts full
Stokes spectrum in order to obtain a depth-dependent stratification of
the temperature, velocity, and the magnetic field vector. It is based
on the so-called node approach, where atmospheric parameters are free
to vary in several fixed points in the atmosphere, and are assumed to
behave as splines in between. We describe the inversion approach in
general and the specific choices we have made in the implementation. <BR
/> Results: We test the performance on one academic problem and on two
interesting NLTE examples, the Ca II 8542 and Na I D spectral lines. The
code is found to have excellent convergence properties and outperforms
a finite-difference based code in this specific implementation by at
least a factor of three. We invert synthetic observations of Na lines
from a small part of a simulated solar atmosphere and conclude that
the Na lines reliably retrieve the magnetic field and velocity in the
range -3 < logτ < -0.5.
---------------------------------------------------------
Title: Radiative Diagnostics in the Solar Photosphere and Chromosphere
Authors: de la Cruz Rodríguez, J.; van Noort, M.
2018smf..book..109D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Image restoration of solar spectra
Authors: van Noort, M.
2017A&A...608A..76V Altcode: 2017arXiv171109629V
Context. When recording spectra from the ground, atmospheric turbulence
causes degradation of the spatial resolution. <BR /> Aims: We present
a data reduction method that restores the spatial resolution of the
spectra to their undegraded state. <BR /> Methods: By assuming that the
point spread function (PSF) estimated from a strictly synchronized,
broadband slit-jaw camera is the same as the PSF that spatially
degraded the spectra, we can quantify what linear combination of
undegraded spectra is present in each degraded data point. <BR />
Results: The set of equations obtained in this way is found to be
generally well-conditioned and sufficiently diagonal to be solved
using an iterative linear solver. The resulting solution has regained
a spatial resolution comparable to that of the restored slit-jaw
images. <BR /> Conclusions: We have developed a new image restoration
method for the restoration of ground-based spectral data over a large
field of view. The method builds on the PSF information recovered by
the MOMFBD code and typically reaches a spatial resolution comparable
to that of the broadband slit-jaw images used to recover the PSF.
---------------------------------------------------------
Title: Normal and counter Evershed flows in the photospheric penumbra
of a sunspot. SPINOR 2D inversions of Hinode-SOT/SP observations
Authors: Siu-Tapia, A.; Lagg, A.; Solanki, S. K.; van Noort, M.;
Jurčák, J.
2017A&A...607A..36S Altcode: 2017arXiv170907386S
Context. The Evershed effect, a nearly horizontal outflow of material
seen in the penumbrae of sunspots in the photospheric layers, is a
common characteristic of well-developed penumbrae, but is still not well
understood. Even less is known about photospheric horizontal inflows in
the penumbra, also known as counter Evershed flows. <BR /> Aims: Here we
present a rare feature observed in the penumbra of the main sunspot of
AR NOAA 10930. This spot displays the normal Evershed outflow in most
of the penumbra, but harbors a fast photospheric inflow of material
over a large sector of the disk-center penumbra. We investigate the
driving forces of both, the normal and the counter Evershed flows. <BR
/> Methods: We invert the spectropolarimetric data from Hinode SOT/SP
using the spatially coupled version of the SPINOR inversion code,
which allows us to derive height-dependent maps of the relevant
physical parameters in the sunspot. These maps show considerable fine
structure. Similarities and differences between the normal Evershed
outflow and the counter Evershed flow are investigated. <BR /> Results:
In both the normal and the counter Evershed flows, the material flows
from regions with field strengths of the order of 1.5-2 kG to regions
with stronger fields. The sources and sinks of both penumbral flows
display opposite field polarities, with the sinks (tails of filaments)
harboring local enhancements in temperature, which are nonetheless
colder than their sources (heads of filaments). <BR /> Conclusions:
The anti-correlation of the gradients in the temperature and magnetic
pressure between the endpoints of the filaments from the two distinct
penumbral regions is compatible with both the convective driver and
the siphon flow scenarios. A geometrical scale of the parameters is
necessary to determine which is the dominant force driving the flows.
---------------------------------------------------------
Title: The Maximum Entropy Limit of Small-scale Magnetic Field
Fluctuations in the Quiet Sun
Authors: Gorobets, A. Y.; Berdyugina, S. V.; Riethmüller, T. L.;
Blanco Rodríguez, J.; Solanki, S. K.; Barthol, P.; Gandorfer, A.;
Gizon, L.; Hirzberger, J.; van Noort, M.; Del Toro Iniesta, J. C.;
Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..233....5G Altcode: 2017arXiv171008361G
The observed magnetic field on the solar surface is characterized by a
very complex spatial and temporal behavior. Although feature-tracking
algorithms have allowed us to deepen our understanding of this behavior,
subjectivity plays an important role in the identification and tracking
of such features. In this paper, we continue studies of the temporal
stochasticity of the magnetic field on the solar surface without relying
either on the concept of magnetic features or on subjective assumptions
about their identification and interaction. We propose a data analysis
method to quantify fluctuations of the line-of-sight magnetic field by
means of reducing the temporal field’s evolution to the regular Markov
process. We build a representative model of fluctuations converging to
the unique stationary (equilibrium) distribution in the long time limit
with maximum entropy. We obtained different rates of convergence to the
equilibrium at fixed noise cutoff for two sets of data. This indicates
a strong influence of the data spatial resolution and mixing-polarity
fluctuations on the relaxation process. The analysis is applied to
observations of magnetic fields of the relatively quiet areas around an
active region carried out during the second flight of the Sunrise/IMaX
and quiet Sun areas at the disk center from the Helioseismic and
Magnetic Imager on board the Solar Dynamics Observatory satellite.
---------------------------------------------------------
Title: Granular cells in the presence of magnetic field
Authors: Jurčák, J.; Lemmerer, B.; van Noort, M.
2017IAUS..327...34J Altcode:
We present a statistical study of the dependencies of the shapes
and sizes of the photospheric convective cells on the magnetic field
properties. This analysis is based on a 2.5 hour long SST observations
of active region NOAA 11768. We have blue continuum images taken with a
cadence of 5.6 sec that are used for segmentation of individual granules
and 270 maps of spectropolarimetric CRISP data allowing us to determine
the properties of the magnetic field along with the line-of-sight
velocities. The sizes and shapes of the granular cells are dependent
on the the magnetic field strength, where the granules tend to be
smaller in regions with stronger magnetic field. In the presence of
highly inclined magnetic fields, the eccentricity of granules is high
and we do not observe symmetric granules in these regions. The mean
up-flow velocities in granules as well as the granules intensities
decrease with increasing magnetic field strength.
---------------------------------------------------------
Title: Radiative Diagnostics in the Solar Photosphere and Chromosphere
Authors: de la Cruz Rodríguez, J.; van Noort, M.
2017SSRv..210..109D Altcode: 2016arXiv160908324D; 2016SSRv..tmp...73D
Magnetic fields on the surface of the Sun and stars in general imprint
or modify the polarization state of the electromagnetic radiation that
is leaving from the star. The inference of solar/stellar magnetic fields
is performed by detecting, studying and modeling polarized light from
the target star. In this review we present an overview of techniques
that are used to study the atmosphere of the Sun, and particularly those
that allow to infer magnetic fields. We have combined a small selection
of theory on polarized radiative transfer, inversion techniques and
we discuss a number of results from chromospheric inversions.
---------------------------------------------------------
Title: Line response functions in nonlocal thermodynamic
equilibrium. Isotropic case
Authors: Milić, I.; van Noort, M.
2017A&A...601A.100M Altcode:
Context. Response functions provide us with a quantitative measure
of sensitivity of the emergent spectrum to perturbations in the
solar atmosphere and are thus the method of choice for interpreting
spectropolarimetric observations. For the lines formed in the solar
chromosphere, it is necessary to compute these responses taking into
account nonlocal thermodynamic equilibrium (NLTE) effects. <BR />
Aims: We show how to analytically compute the response of the level
populations in NLTE to a change of a given physical quantity at a
given depth in the atmosphere. These responses are then used to compute
opacity and emissivity responses, which are then propagated to obtain
the response of the emergent intensity. <BR /> Methods: Our method is
based on the derivative of the rate equations, where we explicitly
incorporate spatial coupling in the radiative rate terms. After
considering and collecting all interdependencies, the problem reduces
to a linear system of equations with a dimension equal to the product
of the number of spatial points and the number of energy levels. <BR
/> Results: We compare analytically computed response functions with
those obtained using a finite difference approach and find very good
agreement. In addition, a more accurate way of propagating opacity
and emissivity perturbations through the numerical solution of the
radiative transfer equation was developed. <BR /> Conclusions: This
method allows for the fast evaluation of the response of the emergent
spectrum to perturbations of a given quantity at a given depth, and
thus is a significant step towards more efficient NLTE inversions.
---------------------------------------------------------
Title: Erratum: Morphological Properties of
Slender CaII H Fibrils Observed by sunrise II (<A
href="http://doi.org/10.3847/1538-4365/229/1/6">ApJS 229, 1, 6</A>)
Authors: Gafeira, R.; Lagg, A.; Solanki, S. K.; Jafarzadeh, S.;
van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta,
J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco
Suárez, D.; Riethmüller, T. L.; Schmidt, W.
2017ApJS..230...11G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Slender Ca II H Fibrils Mapping Magnetic Fields in the Low
Solar Chromosphere
Authors: Jafarzadeh, S.; Rutten, R. J.; Solanki, S. K.; Wiegelmann, T.;
Riethmüller, T. L.; van Noort, M.; Szydlarski, M.; Blanco Rodríguez,
J.; Barthol, P.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.;
Hirzberger, J.; Knölker, M.; Martínez Pillet, V.; Orozco Suárez,
D.; Schmidt, W.
2017ApJS..229...11J Altcode: 2016arXiv161003104J
A dense forest of slender bright fibrils near a small solar active
region is seen in high-quality narrowband Ca II H images from the SuFI
instrument onboard the Sunrise balloon-borne solar observatory. The
orientation of these slender Ca II H fibrils (SCF) overlaps with the
magnetic field configuration in the low solar chromosphere derived
by magnetostatic extrapolation of the photospheric field observed
with Sunrise/IMaX and SDO/HMI. In addition, many observed SCFs are
qualitatively aligned with small-scale loops computed from a novel
inversion approach based on best-fit numerical MHD simulation. Such
loops are organized in canopy-like arches over quiet areas that differ
in height depending on the field strength near their roots.
---------------------------------------------------------
Title: Magneto-static Modeling from Sunrise/IMaX: Application to an
Active Region Observed with Sunrise II
Authors: Wiegelmann, T.; Neukirch, T.; Nickeler, D. H.; Solanki, S. K.;
Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller,
T. L.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.;
Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229...18W Altcode: 2017arXiv170101458N; 2017arXiv170101458W
Magneto-static models may overcome some of the issues facing force-free
magnetic field extrapolations. So far they have seen limited use
and have faced problems when applied to quiet-Sun data. Here we
present a first application to an active region. We use solar vector
magnetic field measurements gathered by the IMaX polarimeter during
the flight of the Sunrise balloon-borne solar observatory in 2013
June as boundary conditions for a magneto-static model of the higher
solar atmosphere above an active region. The IMaX data are embedded
in active region vector magnetograms observed with SDO/HMI. This work
continues our magneto-static extrapolation approach, which was applied
earlier to a quiet-Sun region observed with Sunrise I. In an active
region the signal-to-noise-ratio in the measured Stokes parameters
is considerably higher than in the quiet-Sun and consequently the
IMaX measurements of the horizontal photospheric magnetic field allow
us to specify the free parameters of the model in a special class of
linear magneto-static equilibria. The high spatial resolution of IMaX
(110-130 km, pixel size 40 km) enables us to model the non-force-free
layer between the photosphere and the mid-chromosphere vertically
by about 50 grid points. In our approach we can incorporate some
aspects of the mixed beta layer of photosphere and chromosphere, e.g.,
taking a finite Lorentz force into account, which was not possible with
lower-resolution photospheric measurements in the past. The linear model
does not, however, permit us to model intrinsic nonlinear structures
like strongly localized electric currents.
---------------------------------------------------------
Title: The Second Flight of the Sunrise Balloon-borne Solar
Observatory: Overview of Instrument Updates, the Flight, the Data,
and First Results
Authors: Solanki, S. K.; Riethmüller, T. L.; Barthol, P.; Danilovic,
S.; Deutsch, W.; Doerr, H. -P.; Feller, A.; Gandorfer, A.; Germerott,
D.; Gizon, L.; Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.;
Lagg, A.; Meller, R.; Tomasch, G.; van Noort, M.; Blanco Rodríguez,
J.; Gasent Blesa, J. L.; Balaguer Jiménez, M.; Del Toro Iniesta,
J. C.; López Jiménez, A. C.; Orozco Suarez, D.; Berkefeld, T.;
Halbgewachs, C.; Schmidt, W.; Álvarez-Herrero, A.; Sabau-Graziati,
L.; Pérez Grande, I.; Martínez Pillet, V.; Card, G.; Centeno, R.;
Knölker, M.; Lecinski, A.
2017ApJS..229....2S Altcode: 2017arXiv170101555S
The Sunrise balloon-borne solar observatory, consisting of a 1 m
aperture telescope that provides a stabilized image to a UV filter
imager and an imaging vector polarimeter, carried out its second science
flight in 2013 June. It provided observations of parts of active regions
at high spatial resolution, including the first high-resolution images
in the Mg II k line. The obtained data are of very high quality, with
the best UV images reaching the diffraction limit of the telescope
at 3000 Å after Multi-Frame Blind Deconvolution reconstruction
accounting for phase-diversity information. Here a brief update is
given of the instruments and the data reduction techniques, which
includes an inversion of the polarimetric data. Mainly those aspects
that evolved compared with the first flight are described. A tabular
overview of the observations is given. In addition, an example time
series of a part of the emerging active region NOAA AR 11768 observed
relatively close to disk center is described and discussed in some
detail. The observations cover the pores in the trailing polarity of
the active region, as well as the polarity inversion line where flux
emergence was ongoing and a small flare-like brightening occurred in
the course of the time series. The pores are found to contain magnetic
field strengths ranging up to 2500 G, and while large pores are clearly
darker and cooler than the quiet Sun in all layers of the photosphere,
the temperature and brightness of small pores approach or even exceed
those of the quiet Sun in the upper photosphere.
---------------------------------------------------------
Title: A Tale of Two Emergences: Sunrise II Observations of Emergence
Sites in a Solar Active Region
Authors: Centeno, R.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.;
Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger,
J.; Riethmüller, T. L.; van Noort, M.; Orozco Suárez, D.; Berkefeld,
T.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229....3C Altcode: 2016arXiv161003531C
In 2013 June, the two scientific instruments on board the second Sunrise
mission witnessed, in detail, a small-scale magnetic flux emergence
event as part of the birth of an active region. The Imaging Magnetograph
Experiment (IMaX) recorded two small (∼ 5<SUP>\prime\prime</SUP> )
emerging flux patches in the polarized filtergrams of a photospheric Fe
I spectral line. Meanwhile, the Sunrise Filter Imager (SuFI) captured
the highly dynamic chromospheric response to the magnetic fields pushing
their way through the lower solar atmosphere. The serendipitous capture
of this event offers a closer look at the inner workings of active
region emergence sites. In particular, it reveals in meticulous detail
how the rising magnetic fields interact with the granulation as they
push through the Sun’s surface, dragging photospheric plasma in
their upward travel. The plasma that is burdening the rising field
slides along the field lines, creating fast downflowing channels at
the footpoints. The weight of this material anchors this field to the
surface at semi-regular spatial intervals, shaping it in an undulatory
fashion. Finally, magnetic reconnection enables the field to release
itself from its photospheric anchors, allowing it to continue its
voyage up to higher layers. This process releases energy that lights
up the arch-filament systems and heats the surrounding chromosphere.
---------------------------------------------------------
Title: Photospheric Response to an Ellerman Bomb-like Event—An
Analogy of Sunrise/IMaX Observations and MHD Simulations
Authors: Danilovic, S.; Solanki, S. K.; Barthol, P.; Gandorfer,
A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.;
Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.;
Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229....5D Altcode: 2016arXiv160903817D
Ellerman Bombs are signatures of magnetic reconnection, which is an
important physical process in the solar atmosphere. How and where they
occur is a subject of debate. In this paper, we analyze Sunrise/IMaX
data, along with 3D MHD simulations that aim to reproduce the exact
scenario proposed for the formation of these features. Although
the observed event seems to be more dynamic and violent than the
simulated one, simulations clearly confirm the basic scenario for the
production of EBs. The simulations also reveal the full complexity of
the underlying process. The simulated observations show that the Fe I
525.02 nm line gives no information on the height where reconnection
takes place. It can only give clues about the heating in the aftermath
of the reconnection. However, the information on the magnetic field
vector and velocity at this spatial resolution is extremely valuable
because it shows what numerical models miss and how they can be
improved.
---------------------------------------------------------
Title: Transverse Oscillations in Slender Ca II H Fibrils Observed
with Sunrise/SuFI
Authors: Jafarzadeh, S.; Solanki, S. K.; Gafeira, R.; van Noort, M.;
Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer,
A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco Suárez, D.;
Riethmüller, T. L.; Schmidt, W.
2017ApJS..229....9J Altcode: 2016arXiv161007449J
We present observations of transverse oscillations in slender Ca II
H fibrils (SCFs) in the lower solar chromosphere. We use a 1 hr long
time series of high- (spatial and temporal-) resolution seeing-free
observations in a 1.1 Å wide passband covering the line core of Ca
II H 3969 Å from the second flight of the Sunrise balloon-borne solar
observatory. The entire field of view, spanning the polarity inversion
line of an active region close to the solar disk center, is covered with
bright, thin, and very dynamic fine structures. Our analysis reveals
the prevalence of transverse waves in SCFs with median amplitudes and
periods on the order of 2.4 ± 0.8 km s<SUP>-1</SUP> and 83 ± 29 s,
respectively (with standard deviations given as uncertainties). We
find that the transverse waves often propagate along (parts of) the
SCFs with median phase speeds of 9 ± 14 km s<SUP>-1</SUP>. While the
propagation is only in one direction along the axis in some of the
SCFs, propagating waves in both directions, as well as standing waves
are also observed. The transverse oscillations are likely Alfvénic
and are thought to be representative of magnetohydrodynamic kink
waves. The wave propagation suggests that the rapid high-frequency
transverse waves, often produced in the lower photosphere, can
penetrate into the chromosphere with an estimated energy flux of ≈15
kW m<SUP>-2</SUP>. Characteristics of these waves differ from those
reported for other fibrillar structures, which, however, were observed
mainly in the upper solar chromosphere.
---------------------------------------------------------
Title: Kinematics of Magnetic Bright Features in the Solar Photosphere
Authors: Jafarzadeh, S.; Solanki, S. K.; Cameron, R. H.; Barthol, P.;
Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon,
L.; Hirzberger, J.; Knölker, M.; Martínez Pillet, V.; Orozco Suárez,
D.; Riethmüller, T. L.; Schmidt, W.; van Noort, M.
2017ApJS..229....8J Altcode: 2016arXiv161007634J
Convective flows are known as the prime means of transporting magnetic
fields on the solar surface. Thus, small magnetic structures are good
tracers of turbulent flows. We study the migration and dispersal
of magnetic bright features (MBFs) in intergranular areas observed
at high spatial resolution with Sunrise/IMaX. We describe the flux
dispersal of individual MBFs as a diffusion process whose parameters are
computed for various areas in the quiet-Sun and the vicinity of active
regions from seeing-free data. We find that magnetic concentrations
are best described as random walkers close to network areas (diffusion
index, γ =1.0), travelers with constant speeds over a supergranule
(γ =1.9{--}2.0), and decelerating movers in the vicinity of flux
emergence and/or within active regions (γ =1.4{--}1.5). The three
types of regions host MBFs with mean diffusion coefficients of 130
km<SUP>2</SUP> s<SUP>-1</SUP>, 80-90 km<SUP>2</SUP> s<SUP>-1</SUP>,
and 25-70 km<SUP>2</SUP> s<SUP>-1</SUP>, respectively. The MBFs in
these three types of regions are found to display a distinct kinematic
behavior at a confidence level in excess of 95%.
---------------------------------------------------------
Title: Spectropolarimetric Evidence for a Siphon Flow along an
Emerging Magnetic Flux Tube
Authors: Requerey, Iker S.; Ruiz Cobo, B.; Del Toro Iniesta, J. C.;
Orozco Suárez, D.; Blanco Rodríguez, J.; Solanki, S. K.; Barthol,
P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.;
van Noort, M.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229...15R Altcode: 2016arXiv161106732R
We study the dynamics and topology of an emerging magnetic flux
concentration using high spatial resolution spectropolarimetric data
acquired with the Imaging Magnetograph eXperiment on board the sunrise
balloon-borne solar observatory. We obtain the full vector magnetic
field and the line of sight (LOS) velocity through inversions of
the Fe I line at 525.02 nm with the SPINOR code. The derived vector
magnetic field is used to trace magnetic field lines. Two magnetic flux
concentrations with different polarities and LOS velocities are found
to be connected by a group of arch-shaped magnetic field lines. The
positive polarity footpoint is weaker (1100 G) and displays an upflow,
while the negative polarity footpoint is stronger (2200 G) and shows
a downflow. This configuration is naturally interpreted as a siphon
flow along an arched magnetic flux tube.
---------------------------------------------------------
Title: Morphological Properties of Slender Ca II H Fibrils Observed
by SUNRISE II
Authors: Gafeira, R.; Lagg, A.; Solanki, S. K.; Jafarzadeh, S.;
van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta,
J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco
Suárez, D.; Riethmüller, T. L.; Schmidt, W.
2017ApJS..229....6G Altcode: 2016arXiv161200319G
We use seeing-free high spatial resolution Ca II H data obtained by
the SUNRISE observatory to determine properties of slender fibrils
in the lower solar chromosphere. In this work we use intensity images
taken with the SuFI instrument in the Ca II H line during the second
scientific flight of the SUNRISE observatory to identify and track
elongated bright structures. After identification, we analyze theses
structures to extract their morphological properties. We identify
598 slender Ca II H fibrils (SCFs) with an average width of around
180 km, length between 500 and 4000 km, average lifetime of ≈400
s, and average curvature of 0.002 arcsec<SUP>-1</SUP>. The maximum
lifetime of the SCFs within our time series of 57 minutes is ≈2000
s. We discuss similarities and differences of the SCFs with other
small-scale, chromospheric structures such as spicules of type I and
II, or Ca II K fibrils.
---------------------------------------------------------
Title: A New MHD-assisted Stokes Inversion Technique
Authors: Riethmüller, T. L.; Solanki, S. K.; Barthol, P.; Gandorfer,
A.; Gizon, L.; Hirzberger, J.; van Noort, M.; Blanco Rodríguez, J.;
Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez
Pillet, V.; Knölker, M.
2017ApJS..229...16R Altcode: 2016arXiv161105175R
We present a new method of Stokes inversion of spectropolarimetric
data and evaluate it by taking the example of a Sunrise/IMaX
observation. An archive of synthetic Stokes profiles is obtained
by the spectral synthesis of state-of-the-art magnetohydrodynamics
(MHD) simulations and a realistic degradation to the level of the
observed data. The definition of a merit function allows the archive
to be searched for the synthetic Stokes profiles that best match the
observed profiles. In contrast to traditional Stokes inversion codes,
which solve the Unno-Rachkovsky equations for the polarized radiative
transfer numerically and fit the Stokes profiles iteratively, the new
technique provides the full set of atmospheric parameters. This gives
us the ability to start an MHD simulation that takes the inversion
result as an initial condition. After a relaxation process of half an
hour solar time we obtain physically consistent MHD data sets with
a target similar to the observation. The new MHD simulation is used
to repeat the method in a second iteration, which further improves
the match between observation and simulation, resulting in a factor
of 2.2 lower mean {χ }<SUP>2</SUP> value. One advantage of the new
technique is that it provides the physical parameters on a geometrical
height scale. It constitutes a first step toward inversions that give
results consistent with the MHD equations.
---------------------------------------------------------
Title: Oscillations on Width and Intensity of Slender Ca II H Fibrils
from Sunrise/SuFI
Authors: Gafeira, R.; Jafarzadeh, S.; Solanki, S. K.; Lagg, A.;
van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta,
J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco
Suárez, D.; Riethmüller, T. L.; Schmidt, W.
2017ApJS..229....7G Altcode: 2017arXiv170102801G
We report the detection of oscillations in slender Ca II H fibrils
(SCFs) from high-resolution observations acquired with the Sunrise
balloon-borne solar observatory. The SCFs show obvious oscillations in
their intensity, but also their width. The oscillatory behaviors are
investigated at several positions along the axes of the SCFs. A large
majority of fibrils show signs of oscillations in intensity. Their
periods and phase speeds are analyzed using a wavelet analysis. The
width and intensity perturbations have overlapping distributions
of the wave period. The obtained distributions have median values
of the period of 32 ± 17 s and 36 ± 25 s, respectively. We
find that the fluctuations of both parameters propagate in
the SCFs with speeds of {11}<SUB>-11</SUB><SUP>+49</SUP> km
s<SUP>-1</SUP> and {15}<SUB>-15</SUB><SUP>+34</SUP> km s<SUP>-1</SUP>,
respectively. Furthermore, the width and intensity oscillations have a
strong tendency to be either in anti-phase or, to a smaller extent, in
phase. This suggests that the oscillations of both parameters are caused
by the same wave mode and that the waves are likely propagating. Taking
all the evidence together, the most likely wave mode to explain all
measurements and criteria is the fast sausage mode.
---------------------------------------------------------
Title: Solar Coronal Loops Associated with Small-scale Mixed Polarity
Surface Magnetic Fields
Authors: Chitta, L. P.; Peter, H.; Solanki, S. K.; Barthol, P.;
Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van
Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco
Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229....4C Altcode: 2016arXiv161007484C
How and where are coronal loops rooted in the solar lower
atmosphere? The details of the magnetic environment and its evolution
at the footpoints of coronal loops are crucial to understanding the
processes of mass and energy supply to the solar corona. To address
the above question, we use high-resolution line-of-sight magnetic
field data from the Imaging Magnetograph eXperiment instrument on the
Sunrise balloon-borne observatory and coronal observations from the
Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory
of an emerging active region. We find that the coronal loops are
often rooted at the locations with minor small-scale but persistent
opposite-polarity magnetic elements very close to the larger dominant
polarity. These opposite-polarity small-scale elements continually
interact with the dominant polarity underlying the coronal loop through
flux cancellation. At these locations we detect small inverse Y-shaped
jets in chromospheric Ca II H images obtained from the Sunrise Filter
Imager during the flux cancellation. Our results indicate that magnetic
flux cancellation and reconnection at the base of coronal loops due
to mixed polarity fields might be a crucial feature for the supply of
mass and energy into the corona.
---------------------------------------------------------
Title: Image restoration of polarimetric slit spectra
Authors: van Noort, Michiel
2017psio.confE..90V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observed and simulated power spectra of kinetic and magnetic
energy retrieved with 2D inversions
Authors: Danilovic, S.; Rempel, M.; van Noort, M.; Cameron, R.
2016A&A...594A.103D Altcode: 2016arXiv160706242D
Context. Information on the origin of internetwork magnetic field is
hidden at the smallest spatial scales. <BR /> Aims: We try to retrieve
the power spectra with certainty to the highest spatial frequencies
allowed by current instrumentation. <BR /> Methods: To accomplish this,
we use a 2D inversion code that is able to recover information up to
the instrumental diffraction limit. <BR /> Results: The retrieved power
spectra have shallow slopes that extend further down to much smaller
scales than has been found before. They do not seem to show any power
law. The observed slopes at subgranular scales agree with those obtained
from recent local dynamo simulations. Small differences are found for
the vertical component of kinetic energy that suggest that observations
suffer from an instrumental effect that is not taken into account. <BR
/> Conclusions: Local dynamo simulations quantitatively reproduce the
observed magnetic energy power spectra on the scales of granulation
down to the resolution limit of Hinode/SP, within the error bars
inflicted by the method used and the instrumental effects replicated.
---------------------------------------------------------
Title: Internetwork magnetic field as revealed by two-dimensional
inversions
Authors: Danilovic, S.; van Noort, M.; Rempel, M.
2016A&A...593A..93D Altcode: 2016arXiv160700772D
Context. Properties of magnetic field in the internetwork regions
are still fairly unknown because of rather weak spectropolarimetric
signals. <BR /> Aims: We address the matter by using the two-dimensional
(2D) inversion code, which is able to retrieve the information on
smallest spatial scales up to the diffraction limit, while being less
susceptible to noise than most of the previous methods used. <BR />
Methods: Performance of the code and the impact of various effects
on the retrieved field distribution is tested first on the realistic
magneto-hydrodynamic (MHD) simulations. The best inversion scenario
is then applied to the real data obtained by Spectropolarimeter (SP)
on board Hinode. <BR /> Results: Tests on simulations show that: (1)
the best choice of node position ensures a decent retrieval of all
parameters; (2) the code performs well for different configurations
of magnetic field; (3) slightly different noise levels or slightly
different defocus included in the spatial point spread function
(PSF) produces no significant effect on the results; and (4)
temporal integration shifts the field distribution to a stronger,
more horizontally inclined field. <BR /> Conclusions: Although the
contribution of the weak field is slightly overestimated owing to noise,
2D inversions are able to recover well the overall distribution of the
magnetic field strength. Application of the 2D inversion code on the
Hinode SP internetwork observations reveals a monotonic field strength
distribution. The mean field strength at optical depth unity is ~
130 G. At higher layers, field strength drops as the field becomes
more horizontal. Regarding the distribution of the field inclination,
tests show that we cannot directly retrieve it with the observations
and tools at hand, however, the obtained distributions are consistent
with those expected from simulations with a quasi-isotropic field
inclination after accounting for observational effects.
---------------------------------------------------------
Title: Formation of a solar Hα filament from orphan penumbrae
Authors: Buehler, D.; Lagg, A.; van Noort, M.; Solanki, S. K.
2016A&A...589A..31B Altcode: 2016arXiv160305899B
<BR /> Aims: The formation and evolution of an Hα filament in active
region (AR) 10953 is described. <BR /> Methods: Observations from the
Solar Optical Telescope (SOT) aboard the Hinode satellite starting
from UT 18:09 on 27th April 2007 until UT 06:08 on 1st May 2007 were
analysed. 20 scans of the 6302 Å Fe I line pair recorded by SOT/SP were
inverted using the spatially coupled version of the SPINOR code. The
inversions were analysed together with co-spatial SOT/BFI G-band and
Ca II H and SOT/NFI Hα observations. <BR /> Results: Following the
disappearance of an initial Hα filament aligned along the polarity
inversion line (PIL) of the AR, a new Hα filament formed in its place
some 20 h later, which remained stable for, at least, another 1.5
days. The creation of the new Hα filament was driven by the ascent of
horizontal magnetic fields from the photosphere into the chromosphere
at three separate locations along the PIL. The magnetic fields at
two of these locations were situated directly underneath the initial
Hα filament and formed orphan penumbrae already aligned along the Hα
filament channel. The 700 G orphan penumbrae were stable and trapped in
the photosphere until the disappearance of the overlying initial Hα
filament, after which they started to ascend into the chromosphere at
10 ± 5 m/s. Each ascent was associated with a simultaneous magnetic
flux reduction of up to 50% in the photosphere. The ascended orphan
penumbrae formed dark seed structures in Hα in parallel with the PIL,
which elongated and merged to form an Hα filament. The filament channel
featured horizontal magnetic fields of on average 260 G at log (τ) =
-2 suspended above the nearly field-free lower photosphere. The fields
took on an overall inverse configuration at log (τ) = -2 suggesting
a flux rope topology for the new Hα filament. The destruction of the
initial Hα filament was likely caused by the flux emergence at the
third location along the PIL. <BR /> Conclusions: We present a new
interpretation of the Hα filament formation in AR 10953 whereby the
mainly horizontal fields of orphan penumbrae, aligned along the Hα
filament channel, ascend into the chromosphere, forming seed fragments
for a new, second Hα filament. The orphan penumbral fields ascend
into the chromosphere ~9-24 h before the Hα filament is fully formed.
---------------------------------------------------------
Title: Depth-dependent global properties of a sunspot observed by
Hinode using the Solar Optical Telescope/Spectropolarimeter
Authors: Tiwari, Sanjiv K.; van Noort, Michiel; Solanki, Sami K.;
Lagg, Andreas
2015A&A...583A.119T Altcode: 2015arXiv150804830T
Context. For the past two decades, the three-dimensional structure
of sunspots has been studied extensively. A recent improvement in the
Stokes inversion technique prompts us to revisit the depth-dependent
properties of sunspots. <BR /> Aims: In the present work, we aim to
investigate the global depth-dependent thermal, velocity, and magnetic
properties of a sunspot, as well as the interconnection between
various local properties. <BR /> Methods: We analysed high-quality
Stokes profiles of the disk-centred, regular, leading sunspot of NOAA
AR 10933, acquired by the Solar Optical Telescope/Spectropolarimeter
(SOT/SP) on board the Hinode spacecraft. To obtain depth-dependent
stratification of the physical parameters, we used the recently
developed, spatially coupled version of the SPINOR inversion code. <BR
/> Results: First, we study the azimuthally averaged physical parameters
of the sunspot. We find that the vertical temperature gradient in the
lower- to mid-photosphere is at its weakest in the umbra, while it is
considerably stronger in the penumbra, and stronger still in the spot's
surroundings. The azimuthally averaged field becomes more horizontal
with radial distance from the centre of the spot, but more vertical
with height. At continuum optical depth unity, the line-of-sight
velocity shows an average upflow of ~300 ms<SUP>-1</SUP> in the inner
penumbra and an average downflow of ~1300 ms<SUP>-1</SUP> in the
outer penumbra. The downflow continues outside the visible penumbral
boundary. The sunspot shows, at most, a moderate negative twist of
<5° at log (τ) = 0, which increases with height. The sunspot umbra
and the spines of the penumbra show considerable similarity with regard
to their physical properties, albeit with some quantitative differences
(weaker, somewhat more horizontal fields in spines, commensurate
with their location being further away from the sunspot's core). The
temperature shows a general anti-correlation with the field strength,
with the exception of the heads of penumbral filaments, where a weak
positive correlation is found. The dependence of the physical parameters
on each other over the full sunspot shows a qualitative similarity to
that of a standard penumbral filament and its surrounding spines. <BR />
Conclusions: The large-scale variation in the physical parameters of
a sunspot at various optical depths is presented. Our results suggest
that the spines in the penumbra are basically the outward extension
of the umbra. The spines and the penumbral filaments, together, are
the basic elements that form a sunspot penumbra.
---------------------------------------------------------
Title: Statistical analysis of supersonic downflows in sunspot
penumbrae.
Authors: Kim, Hyunnam; Lagg, Andreas; Solanki, Sami K.; Narayan,
Gautam; van Noort, Michiel; Kim, Kap-Sung
2015IAUGA..2254868K Altcode:
Supersonic downflow patches was found in the outer edge of sunspot
penumbra. These patches are believed to be the return channels of the
Evershed flow. There was previous study to investigate their structure
in detail using Hinode SOT/SP observations (M. van Noort et al. 2013)
but their data sample was only two sunspots. To make general description
it needs to check more sunspot data sample.We selected 242 downflow
patches of 16 sunspots using Hinode SOT/SP observations from 2006 to
2012. Height-dependent maps of atmospheric parameters of these downflows
was produced by using HeLix which was height dependent LTE inversion
code of Stokes profiles.Statistical analysis of magnetic field strength,
inclination angle of field line, temperature and line-of-sight velocity
are presented. The recovered atmospheric data tell us that downflow
patches have different physical signatures comparing normal penumbra
properties. Furthermore, our results of three height-dependent layer
support that heating process should occur on the downflow patches in
the middle of layer.
---------------------------------------------------------
Title: Properties of solar plage from a spatially coupled inversion
of Hinode SP data
Authors: Buehler, D.; Lagg, A.; Solanki, S. K.; van Noort, M.
2015A&A...576A..27B Altcode: 2015arXiv150101151B
<BR /> Aims: The properties of magnetic fields forming an extended
plage region in AR 10953 were investigated. <BR /> Methods:
Stokes spectra of the Fe I line pair at 6302 Å recorded by the
spectropolarimeter aboard the Hinode satellite were inverted using
the SPINOR code. The code performed a 2D spatially coupled inversion
on the Stokes spectra, allowing the retrieval of gradients in optical
depth within the atmosphere of each pixel, whilst accounting for the
effects of the instrument's PSF. Consequently, no magnetic filling
factor was needed. <BR /> Results: The inversion results reveal that
plage is composed of magnetic flux concentrations (MFCs) with typical
field strengths of 1520 G at log (τ) = -0.9 and inclinations of
10°-15°. The MFCs expand by forming magnetic canopies composed of
weaker and more inclined magnetic fields. The expansion and average
temperature stratification of isolated MFCs can be approximated well
with an empirical plage thin flux tube model. The highest temperatures
of MFCs are located at their edges in all log (τ) layers. Whilst
the plasma inside MFCs is nearly at rest, each is surrounded by a
ring of downflows of on average 2.4 km s<SUP>-1</SUP> at log (τ)
= 0 and peak velocities of up to 10 km s<SUP>-1</SUP>, which are
supersonic. The downflow ring of an MFC weakens and shifts outwards
with height, tracing the MFC's expansion. Such downflow rings often
harbour magnetic patches of opposite polarity to that of the main MFC
with typical field strengths below 300 G at log (τ) = 0. These opposite
polarity patches are situated beneath the canopy of their main MFC. We
found evidence of a strong broadening of the Stokes profiles in MFCs
and particularly in the downflow rings surrounding MFCs (expressed
by a microturbulence in the inversion). This indicates the presence
of strong unresolved velocities. Larger magnetic structures such as
sunspots cause the field of nearby MFCs to be more inclined.
---------------------------------------------------------
Title: Inclinations of small quiet-Sun magnetic features based on
a new geometric approach
Authors: Jafarzadeh, S.; Solanki, S. K.; Lagg, A.; Bellot Rubio,
L. R.; van Noort, M.; Feller, A.; Danilovic, S.
2014A&A...569A.105J Altcode: 2014arXiv1408.2443J
Context. High levels of horizontal magnetic flux have been reported
in the quiet-Sun internetwork, often based on Stokes profile
inversions. <BR /> Aims: Here we introduce a new method for deducing
the inclination of magnetic elements and use it to test magnetic field
inclinations from inversions. <BR /> Methods: We determine accurate
positions of a set of small, bright magnetic elements in high spatial
resolution images sampling different photospheric heights obtained by
the Sunrise balloon-borne solar observatory. Together with estimates
of the formation heights of the employed spectral bands, these provide
us with the inclinations of the magnetic features. We also compute
the magnetic inclination angle of the same magnetic features from the
inversion of simultaneously recorded Stokes parameters. <BR /> Results:
Our new, geometric method returns nearly vertical fields (average
inclination of around 14° with a relatively narrow distribution
having a standard deviation of 6°). In strong contrast to this, the
traditionally used inversions give almost horizontal fields (average
inclination of 75 ± 8°) for the same small magnetic features,
whose linearly polarised Stokes profiles are adversely affected by
noise. We show that for such magnetic features inversions overestimate
the flux in horizontal magnetic fields by an order of magnitude. <BR />
Conclusions: The almost vertical field of bright magnetic features from
our geometric method is clearly incompatible with the nearly horizontal
magnetic fields obtained from the inversions. This indicates that the
amount of magnetic flux in horizontal fields deduced from inversions is
overestimated in the presence of weak Stokes signals, in particular if
Stokes Q and U are close to or under the noise level. Inversions should
be used with great caution when applied to data with no clear Stokes Q
and no U signal. By combining the proposed method with inversions we are
not just improving the inclination, but also the field strength. This
technique allows us to analyse features that are not reliably treated
by inversions, thus greatly extending our capability to study the
complete magnetic field of the quiet Sun.
---------------------------------------------------------
Title: Vigorous convection in a sunspot granular light bridge
Authors: Lagg, Andreas; Solanki, Sami K.; van Noort, Michiel;
Danilovic, Sanja
2014A&A...568A..60L Altcode: 2014arXiv1407.1202L
Context. Light bridges are the most prominent manifestation of
convection in sunspots. The brightest representatives are granular
light bridges composed of features that appear to be similar to
granules. <BR /> Aims: An in-depth study of the convective motions,
temperature stratification, and magnetic field vector in and around
light bridge granules is presented with the aim of identifying
similarities and differences to typical quiet-Sun granules. <BR />
Methods: Spectropolarimetric data from the Hinode Solar Optical
Telescope were analyzed using a spatially coupled inversion technique
to retrieve the stratified atmospheric parameters of light bridge and
quiet-Sun granules. <BR /> Results: Central hot upflows surrounded by
cooler fast downflows reaching 10 km s<SUP>-1</SUP> clearly establish
the convective nature of the light bridge granules. The inner part
of these granules in the near surface layers is field free and is
covered by a cusp-like magnetic field configuration. We observe
hints of field reversals at the location of the fast downflows. The
quiet-Sun granules in the vicinity of the sunspot are covered by a
low-lying canopy field extending radially outward from the spot. <BR
/> Conclusions: The similarities between quiet-Sun and light bridge
granules point to the deep anchoring of granular light bridges in
the underlying convection zone. The fast, supersonic downflows are
most likely a result of a combination of invigorated convection
in the light bridge granule due to radiative cooling into the
neighboring umbra and the fact that we sample deeper layers, since the
downflows are immediately adjacent to the slanted walls of the Wilson
depression. <P />The two movies are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424071/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Analysis of magnetic fields in a plage region using a spatially
coupled 2D inversion technique
Authors: Buehler, David; Lagg, Andreas; Solanki, Sami K.; Van Noort,
Michiel
2014cosp...40E.427B Altcode:
The properties of magnetic features (MFCs) within a plage region
in the vicinity of a sunspot were investigated at high spatial
resolution. Stokes spectra of the 630nm line pair recorded by the
spectropolarimeter aboard Hinode were inverted using an extended
version of the SPINOR code. The code preformed a spatially coupled
inversion of the Stokes spectra using three log(tau) nodes in optical
depth. No magnetic filling factors was employed. The analysis of the
inversion results reveals that the MFCs have typical field strengths
of 1500G at log(tau)=-0.9 and inclinations between 10-15 degrees in
all three log(tau) nodes. The MFCs expand by forming magnetic canopies
composed of weaker and more inclined magnetic fields. The expansion of
the magnetic field and temperature stratification of MFCs with optical
depth is in good agreement with a thin flux tube model. Whilst the gas
inside magnetic flux concentrations is typically at rest, the majority
of MFCs were surrounded by a ring of downflows with an average value
of 2.5km/s at log(tau)=0. The ring gradually shifts outwards following
the expansion of the MFC. Within the downflow rings of MFCs small
magnetic patches of opposite polarity to that of the main MFC were
identified, which are predominantly situated beneath the canopy of
its main MFC. We found evidence for a strong broadening of the Stokes
profiles within MFCs and their surrounding downflow rings (expressed
by a microturbulence in the inversion). This indicates the presence
of strong unresolved velocities.
---------------------------------------------------------
Title: Peripheral downflows in sunspot penumbrae
Authors: van Noort, M.; Lagg, A.; Tiwari, S. K.; Solanki, S. K.
2013A&A...557A..24V Altcode: 2013arXiv1308.0466V
Context. Sunspot penumbrae show high-velocity patches along
the periphery. <BR /> Aims: The high-velocity downflow patches
are believed to be the return channels of the Evershed flow. We
aim to investigate their structure in detail using Hinode SOT/SP
observations. <BR /> Methods: We employ Fourier interpolation in
combination with spatially coupled height dependent LTE inversions
of Stokes profiles to produce high-resolution, height-dependent maps
of atmospheric parameters of these downflows and investigate their
properties. <BR /> Results: High-speed downflows are observed over
a wide range of viewing angles. They have supersonic line-of-sight
velocities, some in excess of 20 km s<SUP>-1</SUP>, and very high
magnetic field strengths, reaching values of over 7 kG. A relation
between the downflow velocities and the magnetic field strength is
found, in good agreement with MHD simulations. <BR /> Conclusions:
The coupled inversion at high resolution allows for the accurate
determination of small-scale structures. The recovered atmospheric
structure indicates that regions with very high downflow velocities
contain some of the strongest magnetic fields that have ever been
measured on the Sun. <P />Two movies are available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Structure of sunspot penumbral filaments: a remarkable
uniformity of properties
Authors: Tiwari, Sanjiv Kumar; van Noort, Michiel; Lagg, Andreas;
Solanki, Sami K.
2013A&A...557A..25T Altcode: 2013arXiv1307.3668T
Context. The sunspot penumbra comprises numerous thin, radially
elongated filaments that are central for heat transport within
the penumbra, but whose structure is still not clear. <BR /> Aims:
We aim to investigate the fine-scale structure of these penumbral
filaments. <BR /> Methods: We perform a depth-dependent inversion of
spectropolarimetric data of a sunspot very close to solar disk center
obtained by Solar Optical Telescope/Spectropolarimeter onboard the
Hinode spacecraft. We have used a recently developed, spatially coupled
2D inversion scheme, which allows us to analyze the fine structure
of individual penumbral filaments up to the diffraction limit of
the telescope. <BR /> Results: Filaments of different sizes in all
parts of the penumbra display very similar magnetic field strengths,
inclinations, and velocity patterns. The temperature structure is also
similar, although the filaments in the inner penumbra have cooler
tails than those in the outer penumbra. The similarities allowed
us to average all these filaments and to subsequently extract the
physical properties common to all of them. This average filament
shows upflows associated with an upward-pointing field at its inner,
umbral end (head) and along its axis, as well as downflows along the
lateral edge and strong downflows in the outer end (tail) associated
with a nearly vertical, strong, and downward-pointing field. The
upflowing plasma is significantly, i.e., up to 800 K, hotter than the
downflowing plasma. The hot, tear-shaped head of the averaged filament
can be associated with a penumbral grain. The central part of the
filament shows nearly horizontal fields with strengths in the range
of 1 kG. The field above the filament converges, whereas a diverging
trend is seen in the deepest layers near the head of the filament. The
fluctuations in the physical parameters along and across the filament
increase rapidly with depth. <BR /> Conclusions: We put forward a
unified observational picture of a sunspot penumbral filament. It
is consistent with such a filament being a magneto-convective cell,
in line with recent magnetohydrodynamic simulations. The uniformity
of its properties over the penumbra sets constraints on penumbral
models and simulations. The complex and inhomogeneous structure of the
filament provides a natural explanation for a number of long-running
controversies in the literature.
---------------------------------------------------------
Title: Physical properties of a sunspot chromosphere with umbral
flashes
Authors: de la Cruz Rodríguez, J.; Rouppe van der Voort, L.;
Socas-Navarro, H.; van Noort, M.
2013A&A...556A.115D Altcode: 2013arXiv1304.0752D
We present new high-resolution spectro-polarimetric Ca IIλ8542
observations of umbral flashes in sunspots. At nearly 0.18 arcsec,
and spanning about one hour of continuous observation, this is the
most detailed dataset published thus far. Our study involves both LTE
and non-LTE inversions (but includes also a weak field analysis as a
sanity check) to quantify temperatures, mass flows and the full magnetic
field vector geometry. We confirm earlier reports that UFs have very
fine structure with hot and cool material intermixed at sub-arcsecond
scales. The shock front is roughly 1000 K hotter than the surrounding
material. We do not observe significant fluctuations of the field in the
umbra. In the penumbra, however, the passage of the running penumbral
waves alter the magnetic field strength by some 200 G (peak-to-peak
amplitude) but it does not change the field orientation (at least not
significantly within our sensitivity of a few degrees). From a fast
Fourier transform analysis, we find a trend of decreasing power at high
temporal frequencies at those locations with more horizontal magnetic
fields, for the line-of-sight velocity and magnetic field strength. In
the outer penumbra we find an absence of high frequency power while
there is increasingly more power at high frequencies towards the
umbra. <P />Movie and Appendices A and B are available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Vertical flows and mass flux balance of sunspot umbral dots
Authors: Riethmüller, T. L.; Solanki, S. K.; van Noort, M.; Tiwari,
S. K.
2013A&A...554A..53R Altcode: 2013arXiv1305.1164R
A new Stokes inversion technique that greatly reduces the effect of the
spatial point spread function of the telescope is used to constrain
the physical properties of umbral dots (UDs). The depth-dependent
inversion of the Stokes parameters from a sunspot umbra recorded
with Hinode SOT/SP revealed significant temperature enhancements and
magnetic field weakenings in the core of the UDs in deep photospheric
layers. Additionally, we found upflows of around 960 m/s in peripheral
UDs (i.e., UDs close to the penumbra) and ≈600 m/s in central UDs. For
the first time, we also detected systematic downflows for distances
larger than 200 km from the UD center that balance the upflowing mass
flux. In the upper photosphere, we found almost no difference between
the UDs and their diffuse umbral background.
---------------------------------------------------------
Title: Spatially coupled inversion of spectro-polarimetric image
data. I. Method and first results
Authors: van Noort, M.
2012A&A...548A...5V Altcode: 2012arXiv1210.4636V
Context. When inverting solar spectra, image degradation effects that
are present in the data are usually approximated or not considered. <BR
/> Aims: We develop a data reduction method that takes these issues
into account and minimizes the resulting errors. <BR /> Methods:
By accounting for the diffraction PSF of the telescope during the
inversions, we can produce a self-consistent solution that best fits the
observed data, while simultaneously requiring fewer free parameters than
conventional approaches. <BR /> Results: Simulations using realistic MHD
data indicate that the method is stable for all resolutions, including
those with pixel scales well beyond those that can be resolved with a
0.5 m telescope, such as the Hinode SOT. Application of the presented
method to reduce full Stokes data from the Hinode spectro-polarimeter
results in dramatically increased image contrast and an increase in
the resolution of the data to the diffraction limit of the telescope
in almost all Stokes and fit parameters. The resulting data allow for
detecting and interpreting solar features that have so far only been
observed with 1m class ground-based telescopes. <BR /> Conclusions: A
new inversion method was developed that allows for accurate fitting of
solar spectro-polarimetric imaging data over a large field of view,
while simultaneously improving the noise statistics and spatial
resolution of the results significantly.
---------------------------------------------------------
Title: 2D Inversions
Authors: van Noort, Michiel
2012decs.confE..37V Altcode:
A new approach to inversion of high-resolution spectro-polarimetric
solar image data is presented that explicitly takes the effects of
telescope diffraction and other optical abberations in the observed
data into account. The 2-dimensional solution can reliably reproduce
the atmospheres from simulated data cubes and significantly improves
the accuracy of profiles fitted to Hinode-SP data, compared to an
equivalent 1-dimensional solution, without needing a more complex
atmospheric model.
---------------------------------------------------------
Title: Stokes imaging polarimetry using image restoration: a
calibration strategy for Fabry-Pérot based instruments
Authors: Schnerr, R. S.; de La Cruz Rodríguez, J.; van Noort, M.
2011A&A...534A..45S Altcode: 2010arXiv1012.1225S
Context. The combination of image restoration and a Fabry-Pérot
interferometer (FPI) based instrument in solar observations results
in specific calibration issues. FPIs generally show variations over
the field-of-view, while in the image restoration process, the 1-to-1
relation between pixel space and image space is lost, thus complicating
any correcting for such variations. <BR /> Aims: We develop a data
reduction method that takes these issues into account and minimizes the
resulting errors. <BR /> Methods: By accounting for the time variations
in the telescope's Mueller matrix and using separate calibration data
optimized for the wavefront sensing in the MOMFBD image restoration
process and for the final deconvolution of the data, we have removed
most of the calibration artifacts from the resulting data. <BR />
Results: Using this method to reduce full Stokes data from CRISP at
the SST, we find that it drastically reduces the instrumental and
image restoration artifacts resulting from cavity errors, reflectivity
variations, and the polarization dependence of flatfields. The results
allow for useful scientific interpretation. Inversions of restored
data from the δ sunspot AR11029 using the Nicole inversion code,
reveal strong (~10 km s<SUP>-1</SUP>) downflows near the disk center
side of the umbra. <BR /> Conclusions: The use of image restoration
in combination with an FPI-based instrument leads to complications in
the calibrations and intrinsic limitations to the accuracy that can
be achieved. We find that for CRISP, the resulting errors can be kept
mostly below the polarimetric accuracy of ~10<SUP>-3</SUP>. Similar
instruments aiming for higher polarimetric and high spectroscopic
accuracy, will, however, need to take these problems into account.
---------------------------------------------------------
Title: Observation and analysis of chromospheric magnetic fields .
Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; van Noort,
M.; Rouppe van der Voort, L.
2010MmSAI..81..716D Altcode: 2010arXiv1004.0698D
The solar chromosphere is a vigorously dynamic region of the sun,
where waves and magnetic fields play an important role. To improve
chromospheric diagnostics, we present new observations in Ca II 8542
carried out with the SST/CRISP on La Palma, working in full-Stokes
mode. We measured Stokes line profiles in active regions. The line
profiles observed close to the solar limb show signals in all four
Stokes parameters, while profiles observed close to disk center only
show signals above the noise level in Stokes I and V. We used the NLTE
inversion code 'NICOLE' to derive atmospheric parameters in umbral
flashes present in a small round sunspot without penumbra.
---------------------------------------------------------
Title: Photospheric Temperatures from Ca IIH
Authors: Henriques, V. M. J.; Kiselman, D.; van Noort, M.
2010ASSP...19..511H Altcode: 2010mcia.conf..511H
The temperature stratification in the upper photosphere can be extracted
from Ca II H&K spectrograms following Shine and Linsky (1974)
by assuming LTE, the Eddington-Barbier approximation, hydrostatic
equilibrium, and that Ca II is mostly in the ground state. Rouppe van
der Voort (2002) confirmed that these assumptions were solid for a wide
range in the Ca IIK wings and further developed the method including
forward computation using MULTI (Carlsson 1986).
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Title: Spectropolarimetric Diagnostics at the Solar Photosphere near
the Limb
Authors: Yelles Chaouche, L.; Solanki, S. K.; Rouppe van der Voort,
L.; van Noort, M.
2009ASPC..405..189Y Altcode: 2009arXiv0910.2885Y
In the present work, we investigate the formation of Stokes profiles
and spectro-polarimetric diagnostics in an active region plage near
the limb. We use 3-D radiation-MHD simulations with unipolar fields
of an average strength of 400 G, which is largely concentrated in
flux tubes in which the field reaches typical kilo-Gauss values. We
generate synthetic Stokes spectra by radiative transfer calculations,
then we degrade the simulated Stokes signal to account for observational
conditions. The synthetic data treated in this manner are compared with
and found to roughly reproduce spectro-polarimetric high-resolution
observations at μ=0.39 obtained by the SOUP instrument with the
Swedish 1-m Solar Telescope at the beginning of 2006.
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Title: CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
Authors: Scharmer, G. B.; Narayan, G.; Hillberg, T.; de la Cruz
Rodriguez, J.; Löfdahl, M. G.; Kiselman, D.; Sütterlin, P.; van
Noort, M.; Lagg, A.
2008ApJ...689L..69S Altcode: 2008arXiv0806.1638S
We discuss penumbral fine structure in a small part of a pore,
observed with the CRISP imaging spectropolarimeter at the Swedish
1-m Solar Telescope (SST), close to its diffraction limit of
0.16”. Milne-Eddington inversions applied to these Stokes data
reveal large variations of field strength and inclination angle over
dark-cored penumbral intrusions and a dark-cored light bridge. The
mid-outer part of this penumbra structure shows ~0.3” wide spines,
separated by ~1.6” (1200 km) and associated with 30° inclination
variations. Between these spines, there are no small-scale magnetic
structures that easily can be identified with individual flux tubes. A
structure with nearly 10° more vertical and weaker magnetic field is
seen midway between two spines. This structure is cospatial with the
brightest penumbral filament, possibly indicating the location of a
convective upflow from below.
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Title: Stokes imaging polarimetry using image restoration at the
Swedish 1-m solar telescope
Authors: van Noort, M. J.; Rouppe van der Voort, L. H. M.
2008A&A...489..429V Altcode: 2008arXiv0805.4296V
Aims: We aim to achieve both high spatial resolution and high
polarimetric sensitivity, using an earth-based 1m-class solar
telescope, for the study of magnetic fine structure on the surface of
the Sun. <BR />Methods: We use a setup with 3 high-speed, low-noise
cameras to construct datasets with interleaved polarimetric states,
particularly suitable for Multi-Object Multi-Frame Blind Deconvolution
image restorations. We discuss the polarimetric calibration routine
and various potential sources of error in the results. <BR />Results:
We obtained near diffraction limited images, which have a noise level
of ≈ 10<SUP>-3</SUP> I_cont. We confirm that dark cores have a weaker
magnetic field and a lower inclination angle with respect to the solar
surface than the edges of the penumbral filament. We demonstrate that
the magnetic field strength in faculae-striations is significantly
lower than in other nearby parts of the faculae.
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Title: Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution
Authors: Scharmer, G.; Henriques, V.; Hillberg, T.; Kiselman, D.;
Löfdahl, M.; Narayan, G.; Sütterlin, P.; van Noort, M.; de la Cruz
Rodríguez, J.
2008ESPM...12..2.5S Altcode:
We present first observations of sunspots with the imaging
spectropolarimeter CRISP, recently installed at the Swedish 1-m
Solar Telescope (SST) on La Palma. This spectropolarimeter is based
on a high-fidelity dual Fabry-Perot filter system. <P />Two liquid
crystals and a polarizing beam splitter are used to reduce seeing
induced I,Q,U,V crosstalk by simultaneously recording images with
two 1kx1k back-illuminated Sarnoff CCD's. A third CCD simultaneously
records broadband images through the pre-filter of the FPI filter
system, allowing image reconstruction and co-alignment of images of
different polarization states and at different wavelengths in Zeeman
sensitive spectral lines. <P />The first data, recorded in April 2008,
demonstrate the capability of this system to record high cadence,
high S/N polarimetric data with a spatial resolution at or close to
the diffraction limit of the SST at 630 nm, 0.16 arcsec. We discuss
the analysis of first spectropolarimetric data for sunspots, based on
Milne-Eddington inversion techniques.
---------------------------------------------------------
Title: On small active region filaments, fibrils and surges
Authors: Lin, Y.; Martin, S. F.; Engvold, O.; Rouppe van der Voort,
L. H. M.; van Noort, M.
2008AdSpR..42..803L Altcode:
High resolution Hα images and magnetograms (0.2 arc s) of an active
region were obtained in alternating time series at 42 s cadences
using the Swedish 1-m Solar Telescope on 2004 August 21. The Hα
filtergrams reveal an active region filament and surges consisting
of thread-like structures which have widths similar to the widths of
chromospheric fibrils, both recorded down to the resolution limit in
the best images. All observed structures in the active region appear
highly dynamic. Fibrils show counterstreaming strongly resembling the
counterstreaming threads in filaments. Streaming, along the threads
of surges extending more than 10 arc s, is higher in speed (∼20
km s<SUP>-1</SUP>) than in the filament and fibrils and appears to
flow independently over and above the chromospheric fibrils. Blue
shifts seen in the Hα Dopplergrams confirm the outward mass motion
of the surges. However, in at least one case, we also see simultaneous
downflows from the same site but in the opposite direction and downward
toward the chromosphere. We suggest that the site between these
two outward and downward flows identifies the place where magnetic
reconnection could occur and thereby cause of the surge. This appears
to imply that the reconnection site is in the high chromosphere or
low corona.
---------------------------------------------------------
Title: SST/CRISP Magnetometry with Fe I 630.2 nm
Authors: Narayan, G.; Scharmer, G. B.; Hillberg, T.; Lofdahl, M.;
van Noort, M.; Sutterlin, P.; Lagg, A.
2008ESPM...122.120N Altcode:
We present recent full Stokes observations in the Fe I 630.2 nm
line with CRISP, an imaging spectropolarimeter at the Swedish 1-m
Solar Telescope (SST). The observations reach a spatial resolution
of 0".16, close to the diffraction limit of the SST, representing
a major improvement over any past ground based or space based
spectropolarimetric data. We describe the data acquisition and reduction
methods and present results of Milne-Eddington(ME) inversions applied
on observations of plage.
---------------------------------------------------------
Title: SST/CRISP observations of Ca II 854.2 nm
Authors: de la Cruz Rodríguez, J.; van Noort, M.
2008ESPM...12.2.77D Altcode:
We present observations of the Ca II 854.2 nm line using the new
Crisp Imaging Spectropolarimeter (CRISP) at the SST. CRISP allows
high-cadence, high-spatial resolution scans through the line profile
with full Stokes polarimetry. We discuss the performance of the
instrument and the diagnostic potential of the Ca II IR triplet lines.
---------------------------------------------------------
Title: Moat Flow in the Vicinity of Sunspots for Various Penumbral
Configurations
Authors: Vargas Domínguez, S.; Rouppe van der Voort, L.; Bonet,
J. A.; Martínez Pillet, V.; Van Noort, M.; Katsukawa, Y.
2008ApJ...679..900V Altcode: 2008arXiv0802.1457V
High-resolution time series of sunspots have been obtained with
the Swedish 1 m Solar Telescope between 2003 and 2006 at different
locations on the solar disk. Proper motions in seven different active
regions have been studied. The analysis was performed by applying local
correlation tracking to every series of sunspots, each of them more than
40 minutes long. The sunspots' shapes include a different variety of
penumbral configurations. We report on the systematic behavior of the
large-scale outflows surrounding the sunspots, commonly known as moat
flows, that are essentially present only when preceded by a penumbra
not tangential but perpendicular to the sunspot border. We present
one case for which this rule appears not to be confirmed. We speculate
that the magnetic neutral line, which is located in the vicinity of the
anomalous region, might be responsible for blocking the outflow. These
new results confirm the systematic and strong relation between the
moat flows and the existence of penumbrae. A comparative statistical
study between moats and standard granulation is also performed.
---------------------------------------------------------
Title: Evidence of Traveling Waves in Filament Threads
Authors: Lin, Y.; Engvold, O.; Rouppe van der Voort, L. H. M.; van
Noort, M.
2007SoPh..246...65L Altcode:
High-resolution Hα filtergrams (0.2″) obtained with the Swedish 1-m
Solar Telescope resolve numerous very thin, thread-like structures in
solar filaments. The threads are believed to represent thin magnetic
flux tubes that must be longer than the observable threads. We report
on evidence for small-amplitude (1 - 2 km s<SUP>−1</SUP>) waves
propagating along a number of threads with an average phase velocity of
12 km s<SUP>−1</SUP> and a wavelength of 4″. The oscillatory period
of individual threads vary from 3 to 9 minutes. Temporal variation
of the Doppler velocities averaged over a small area containing a
number of individual threads shows a short-period (3.6 minutes) wave
pattern. These short-period oscillations could possibly represent fast
modes in accordance with numerical fibril models proposed by Díaz et
al. (Astron. Astrophys.379, 1083, 2001). In some cases, it is clear
that the propagating waves are moving in the same direction as the
mass flows.
---------------------------------------------------------
Title: High-Resolution Observations and Numerical Simulations of
Chromospheric Fibrils and Mottles
Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2007ASPC..368...65D Altcode:
With the recent advent of the Swedish 1-m Solar Telescope (SST),
advanced image processing techniques, as well as numerical simulations
that provide a more realistic view of the chromosphere, a comprehensive
understanding of chromospheric jets such as spicules, mottles and
fibrils is now within reach. In this paper, we briefly summarize results
from a recent analysis of dynamic fibrils, short-lived jet-like features
that dominate the chromosphere (as imaged in Hα) above and about active
region plage. Using extremely high-resolution observations obtained
at the SST, and advanced numerical 2D radiative MHD simulations, we
show that fibrils are most likely formed by chromospheric shock waves
that occur when convective flows and global oscillations leak into the
chromosphere along the field lines of magnetic flux concentrations. <P
/>In addition, we present some preliminary observations of quiet Sun
jets or mottles. We find that the mechanism that produces fibrils
in active regions is most likely also at work in quiet Sun regions,
although it is modified by the weaker magnetic field and the presence of
more mixed-polarity. A comparison with numerical simulations suggests
that the weaker magnetic field in quiet Sun allows for significantly
stronger (than in active regions) transverse motions that are
superposed on the field-aligned, shock-driven motions. This leads
to a more dynamic, and much more complex environment than in active
region plage. In addition, our observations of the mixed polarity
environment in quiet Sun regions suggest that other mechanisms, such
as reconnection, may well play a significant role in the formation of
some quiet Sun jets. Simultaneous high-resolution magnetograms (such
as those provided by Hinode), as well as numerical simulations that
take into account a whole variety of different magnetic configurations,
will be necessary to determine the relative importance in quiet Sun of,
respectively, the fibril-mechanism and reconnection.
---------------------------------------------------------
Title: Magnetoacoustic Shocks as a Driver of Quiet-Sun Mottles
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Hansteen,
V. H.; Carlsson, M.; van Noort, M.
2007ApJ...660L.169R Altcode: 2007astro.ph..3535R
We present high spatial and high temporal resolution observations of
the quiet Sun in Hα obtained with the Swedish 1-m Solar Telescope on
La Palma. We observe that many mottles, jetlike features in the quiet
Sun, display clear up- and downward motions along their main axis. In
addition, many mottles show vigorous transverse displacements. Unique
identification of the mottles throughout their lifetime is much harder
than for their active region counterpart, dynamic fibrils. This is
because many seem to lack a sharply defined edge at their top, and
significant fading often occurs throughout their lifetime. For those
mottles that can be reliably tracked, we find that the mottle tops
often undergo parabolic paths. We find a linear correlation between
the deceleration these mottles undergo and the maximum velocity they
reach, similar to what was found earlier for dynamic fibrils. Combined
with an analysis of oscillatory properties, we conclude that at least
part of the quiet-Sun mottles are driven by magnetoacoustic shocks. In
addition, the mixed-polarity environment and vigorous dynamics suggest
that reconnection may play a significant role in the formation of some
quiet-Sun jets.
---------------------------------------------------------
Title: Observations and Simulations of Fibrils and Mottles
Authors: De Pontieu, Bart; Hansteen, Viggo H.; Rouppe van der Voort,
Luc; van Noort, Michiel; Carlsson, Mats
2007astro.ph..2081D Altcode:
With the recent advent of the Swedish 1-m Solar Telescope (SST),
advanced image processing techniques, as well as numerical simulations
that provide a more realistic view of the chromosphere, a comprehensive
understanding of chromospheric jets such as spicules, mottles and
fibrils is now within reach. In this paper, we briefly summarize
results from a recent analysis of dynamic fibrils, short-lived
jet-like features that dominate the chromosphere (as imaged in
H-alpha) above and about active region plage. Using extremely
high-resolution observations obtained at the SST, and advanced
numerical 2D radiative MHD simulations, we show that fibrils are most
likely formed by chromospheric shock waves that occur when convective
flows and global oscillations leak into the chromosphere along the
field lines of magnetic flux concentrations. In addition, we present
some preliminary observations of quiet Sun jets or mottles. We find
that the mechanism that produces fibrils in active regions is most
likely also at work in quiet Sun regions, although it is modified by
the weaker magnetic field and the presence of more mixed-polarity. A
comparison with numerical simulations suggests that the weaker magnetic
field in quiet Sun allows for significantly stronger (than in active
regions) transverse motions that are superposed on the field-aligned,
shock-driven motions. This leads to a more dynamic, and much more
complex environment than in active region plage. In addition, our
observations of the mixed polarity environment in quiet Sun regions
suggest that other mechanisms, such as reconnection, may well play a
significant role in the formation of some quiet Sun jets.
---------------------------------------------------------
Title: Ca IIH line wing images of sunspot penumbrae recorded with
the Swedish 1-m Solar Telescope
Authors: Narayan, G.; van Noort, M. J.; Scharmer, G.
2007msfa.conf..213N Altcode:
We present recent Ca IIH images of sunspot penumbrae taken with the
Swedish 1m Solar Telescope (SST) during June-July 2006 and restored to
a resolution close to 0".1. Images were recorded at different Ca IIH
line positions using one tunable filter, one fixed wing filter and
a wide band quasi-continuum filter. Apart from the images recorded
at line center (and formed under non-LTE conditions), this provides
temperature information from approximately the first 200-300 km above
the photosphere.
---------------------------------------------------------
Title: Solar image restoration
Authors: Löfdahl, M. G.; van Noort, M. J.; Denker, C.
2007msfa.conf..119L Altcode:
Image restoration is used to repair solar images degraded by the
turbulence in Earth's atmosphere. Restoration algorithms are based on
models of the optical system that produce the images from the solar
source of radiation, through Earth's atmosphere and telescope/instrument
optics, to the detectors recording the data. In this review, these
model components are discussed in the context of two very different
classes of image restoration methods, i.e., Speckle Imaging and Phase
Diversity/Multi-Frame Blind Deconvolution, which have been successfully
used during the last two decades. The strengths and weaknesses of
these two approaches are discussed, as well as some variants and
recent progress.
---------------------------------------------------------
Title: High-Resolution Observations and Modeling of Dynamic Fibrils
Authors: De Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2007ApJ...655..624D Altcode: 2007astro.ph..1786D
We present unprecedented high-resolution Hα observations, obtained
with the Swedish 1 m Solar Telescope, that, for the first time,
spatially and temporally resolve dynamic fibrils in active regions on
the Sun. These jetlike features are similar to mottles or spicules in
quiet Sun. We find that most of these fibrils follow almost perfect
parabolic paths in their ascent and descent. We measure the properties
of the parabolic paths taken by 257 fibrils and present an overview
of the deceleration, maximum velocity, maximum length, and duration,
as well as their widths and the thickness of a bright ring that often
occurs above dynamic fibrils. We find that the observed deceleration
of the projected path is typically only a fraction of solar gravity
and incompatible with a ballistic path at solar gravity. We report on
significant differences of fibril properties between those occurring
above a dense plage region and those above a less dense plage region
where the magnetic field seems more inclined from the vertical. We
compare these findings to advanced numerical two-dimensional radiative
MHD simulations and find that fibrils are most likely formed by
chromospheric shock waves that occur when convective flows and global
oscillations leak into the chromosphere along the field lines of
magnetic flux concentrations. Detailed comparison of observed and
simulated fibril properties shows striking similarities of the values
for deceleration, maximum velocity, maximum length, and duration. We
compare our results with observations of mottles and find that a
similar mechanism is most likely at work in the quiet Sun.
---------------------------------------------------------
Title: High Spatial Resolution Observations of Solar Magnetic
Structures
Authors: Rouppe van der Voort, L.; van Noort, M.; Carlsson, M.;
Hansteen, V.
2006ASPC..354...37R Altcode:
We present observations of the dynamic evolution of photospheric
magnetic structures in the G-band, continuum, magnetograms and
Dopplergrams. The observations were obtained with the Swedish one-m
Solar Telescope on La Palma. Using adaptive optics and the Multi-Object
Multi-Frame Blind Deconvolution image restoration technique, we obtained
several datasets at close to the diffraction limit of the telescope
(0.1 arcsec) over long periods of time. We show examples of the
dynamical evolution of different magnetic structures: the advection
of individual bright points by the granular flow, the formation and
fragmentation of flux sheets, and the continuous transition between
micro-pores, elongated ribbons and more circular “flowers”. Narrow
sheets with downdrafts are found right at the edges of magnetic field
concentrations.
---------------------------------------------------------
Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks
Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2006AGUFMSH23B0359D Altcode:
With the recent advent of the Swedish 1-m Solar Telescope (SST),
advanced image processing techniques, as well as numerical simulations
that provide a more realistic view of the chromosphere, a comprehensive
understanding of chromospheric jets such as spicules, mottles and
fibrils is now within reach. We will present results from a recent
analysis of dynamic fibrils, short-lived jet-like features that
dominate the chromosphere (as imaged in Hα) above and about active
region plage. These jets are similar to mottles and spicules in quiet
Sun. Our analysis is based on a time series of extremely high-resolution
(120 km) images taken in Hα linecenter at 1 second cadence, obtained by
the Oslo group at the SST in October 2005. The 78 min long time series
for the first time, spatially and temporally resolves dynamic fibrils
in active regions. Our analysis shows that most of the fibrils follow
almost perfect parabolic paths in their ascent and descent. We measure
the properties of the parabolic paths taken by 257 different dynamic
fibrils, and find that the observed deceleration of the projected
path is typically only a fraction of solar gravity, and incompatible
with a ballistic path at solar gravity. We report on significant
differences of measured fibril properties between those occurring in
association with a dense plage region, and those above a less dense
plage region where the magnetic field seems more inclined away from
the vertical. We compare these observational findings to advanced
numerical 2D radiative MHD simulations, and find that fibrils are most
likely formed by chromospheric shock waves that occur when convective
flows and global oscillations leak into the chromosphere along the
field lines of magnetic flux concentrations. Detailed comparison
of the properties of fibrils found in our observations and those in
our numerical simulations shows striking similarities of the values
for deceleration, maximum velocity, maximum length and duration. The
numerical simulations also reproduce the correlations we observe between
various fibrils properties, as well as the regional differences, taking
into account the different magnetic configuration for the various
regions. We compare our results with observations of mottles and find
that a similar mechanism is most likely at work in the quiet Sun.
---------------------------------------------------------
Title: Solar Image Restoration by use of Multi-Object Multi-Frame
Blind Deconvolution
Authors: van Noort, M.; Rouppe van der Voort, L.; Löfdahl, M.
2006ASPC..354...55V Altcode:
We present examples of the application of the image restoration
method of Multi-Object Multi-Frame Blind Deconvolution to observations
obtained with the Swedish one-meter Solar Telescope on La Palma. This
restoration method is an extension of Joint Phase Diverse Speckle
image restoration. Multiple realizations of multiple objects
can now be restored jointly, facilitating near-perfect alignment
between different objects. This greatly reduces false signals in the
determination of derived quantities, such as magnetograms, Dopplergrams
and G-band-continuum difference images.
---------------------------------------------------------
Title: High-Resolution Observations of Fast Events in the Solar
Chromosphere
Authors: van Noort, M. J.; Rouppe van der Voort, L. H. M.
2006ApJ...648L..67V Altcode:
We present new, high spatial and high temporal resolution observations
of the Sun in Hα obtained with the Swedish 1 m Solar Telescope on
La Palma. The combined use of adaptive optics and image restoration
techniques yielded a near-diffraction-limited time series with a cadence
of 3 frames s<SUP>-1</SUP> of two different active regions. The unique
combination of high temporal and spatial resolution reveals to us the
existence of highly dynamic structures, moving at velocities of up to
240 km s<SUP>-1</SUP>, and high-velocity waves in the chromosphere. The
rapid motions appear to be common, as they are observed in two data sets
recorded in succession at different locations on the solar disk. The
dynamic events are probably associated with reconfigurations of the
magnetic field.
---------------------------------------------------------
Title: Rapid Temporal Variability of Faculae: High-Resolution
Observations and Modeling
Authors: De Pontieu, B.; Carlsson, M.; Stein, R.; Rouppe van der Voort,
L.; Löfdahl, M.; van Noort, M.; Nordlund, Å.; Scharmer, G.
2006ApJ...646.1405D Altcode:
We present high-resolution G-band observations (obtained with the
Swedish 1 m Solar Telescope) of the rapid temporal variability of
faculae, which occurs on granular timescales. By combining these
observations with magnetoconvection simulations of a plage region, we
show that much of this variability is not intrinsic to the magnetic
field concentrations that are associated with faculae, but rather
a phenomenon associated with the normal evolution and splitting of
granules. We also show examples of facular variability caused by
changes in the magnetic field, with movies of dynamic behavior of
the striations that dominate much of the facular appearance at 0.1"
resolution. Examples of these dynamics include merging, splitting,
rapid motion, apparent fluting, and possibly swaying.
---------------------------------------------------------
Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks
Authors: Hansteen, V. H.; De Pontieu, B.; Rouppe van der Voort, L.;
van Noort, M.; Carlsson, M.
2006ApJ...647L..73H Altcode: 2006astro.ph..7332H
The formation of jets such as dynamic fibrils, mottles, and spicules
in the solar chromosphere is one of the most important, but also
most poorly understood, phenomena of the Sun's magnetized outer
atmosphere. We use extremely high resolution observations from the
Swedish 1 m Solar Telescope combined with advanced numerical modeling
to show that in active regions these jets are a natural consequence of
upwardly propagating slow-mode magnetoacoustic shocks. These shocks
form when waves generated by convective flows and global p-mode
oscillations in the lower lying photosphere leak upward into the
magnetized chromosphere. We find excellent agreement between observed
and simulated jet velocities, decelerations, lifetimes, and lengths. Our
findings suggest that previous observations of quiet-Sun spicules and
mottles may also be interpreted in light of a shock-driven mechanism.
---------------------------------------------------------
Title: High Resolution Spectropolarimetry of Penumbral Formation
with IBIS
Authors: Reardon, Kevin; Casini, R.; Cavallini, F.; Tomczyk, S.;
Rouppe van der Voort, L.; Van Noort, M.; Woeger, F.; Socas Navarro,
H.; IBIS Team
2006SPD....37.3503R Altcode: 2006BAAS...38..260R
We present the results of first spectropolarimetric observations
made with the Interferometric Bidimensional Spectrometer (IBIS)
at the NSO/Dunn Solar Telescope. The use of narrowband imaging and
post-facto reconstruction techniques allows for observations close
to the diffraction limit of the vector magnetic field. We will show
observations of the the formation of an individual penumbral filament
around a small pore. We measure the magnetic field and velocity field
of the forming penumbral filament. The spectropolarimetric mode of
IBIS will be available to the community in the fall of 2006.
---------------------------------------------------------
Title: Dynamics of an active region filament, fibrils and surges in
high resolution
Authors: Lin, Y.; Martin, S. F.; Engvold, O.; Rouppe van der Voort,
L. H. M.; van Noort, M.
2006cosp...36.3193L Altcode: 2006cosp.meet.3193L
High resolution H alpha and magnetograms 0 2 arc sec of an active region
were obtained in alternating time series at 42 sec cadences using the
Swedish 1-m Solar Telescope on 2004 August 21 The 79 times 52 arcsec
field of view was centered at N11 W5 The H alpha filtergrams reveal an
active region filament and surges consisting of thread-like structures
which have widths similar to the widths of chromospheric fibrils both
recorded down to the resolution limit in the best images All observed
structures in the active region are highly dynamic The flow speeds in
some active filament threads 25 km s -1 are higher than typical speeds
in quiescent filament threads Fibrils show counterstreaming strongly
resembling the counterstreaming threads in filaments The transverse
speeds of mass within fibrils are comparable to the typical speeds
of mass flows in quiescent filament threads sim 10 km s -1 Streaming
along the threads of surges extending more than 10 arc sec is higher
in speed sim 20 km s -1 than in the filament and fibrils and appears
to flow over the chromospheric fibrils Small surges near one end of
the filament appear to flow into the filament thereby mass seems to
be added to the filament Blue shifts seen in the H alpha Dopplergrams
confirm the outward mass motion of the surges However in at least one
case we also see simultaneous red shifts from the same site in the
opposite direction toward the chromosphere We suggest that the site
between these two opposite motions identifies the place where magnetic
---------------------------------------------------------
Title: Solar magnetic elements at 0.1 arcsec resolution. II. Dynamical
evolution
Authors: Rouppe van der Voort, L. H. M.; Hansteen, V. H.; Carlsson,
M.; Fossum, A.; Marthinussen, E.; van Noort, M. J.; Berger, T. E.
2005A&A...435..327R Altcode:
Small magnetic structures can be seen in G-band filtergrams as isolated
bright points, strings of bright points and dark micro-pores. At a
resolution of 0.1 arcsec, new forms of magnetic structures are found
in strong field areas: elongated “ribbons” and more circular
“flowers”. In this paper we study the temporal evolution of
these small scale magnetic structures. In strong-field regions the
time-evolution is more that of a magnetic fluid than that of collections
of flux-tubes that keep their identity. We find that the granular flow
concentrates the magnetic field into flux sheets that are visible as
thin bright features in the filtergrams. Weak upflows are found in
the flux sheets and downflows in the immediate surroundings. The flux
sheets often become unstable to a fluting instability and the edges
buckle. The sheets tend to break up into strings of bright points,
still with weak upflows in the magnetic elements and zero velocity or
downflows between them. Where there are larger flux concentrations
we find ribbons, flowers and micro-pores. There is a continuous
transition between these forms and they evolve from one form to
another. The appearance is mostly determined by the horizontal size
- larger structures are dark (micro-pores), narrower structures are
ribbon shaped and the flowers are the smallest in extent. All these
structures have darker inner parts and a bright edge. The plasma is
found to be at rest in the ribbons, with small concentrations of weak
upflow sites. Narrow sheets with downdrafts are found right at the
edges of the magnetic field concentrations.
---------------------------------------------------------
Title: Solar Image Restoration By Use Of Multi-frame Blind
De-convolution With Multiple Objects And Phase Diversity
Authors: Van Noort, Michiel; Der Voort, Luc Rouppe Van; Löfdahl,
Mats G.
2005SoPh..228..191V Altcode: 2005SoPh..228..191N
An extension of Joint Phase Diverse Speckle image restoration is
presented. Multiple realizations of multiple objects having known
wavefront relations with each other can now be restored jointly. As
the alignment of the imaging setup does not change, near-perfect
alignment can be achieved between different objects, thus greatly
reducing false signals in the determination of derived quantities,
such as magnetograms, Dopplergrams, etc. The method was implemented
in C++ as an image restoration server, to which worker clients can
connect and disconnect randomly, so that a large number of CPUs can be
used to speed up the restorations. We present a number of examples of
applications of the restoration method to observations obtained with
the Swedish 1-m Solar Telescope on La Palma.
---------------------------------------------------------
Title: Multidimensional ALI Radiative Transfer in Cartesian,
Cylindrical and Spherical Coordinate Systems
Authors: van Noort, M.; Hubeny, I.; Lanz, T.
2003ASPC..288..445V Altcode: 2003sam..conf..445V
A new Radiative Transfer code that can calculate the non-LTE line
transfer problem in a two-level atom formulation in Cartesian,
cylindrical and spherical coordinate systems is presented. The transfer
equation is solved using the ALI and the short characteristics methods,
while allowing for an arbitrary 3-dimensional velocity field. <P />The
code is modularised so that changing geometry can be accomplished by
simply setting a switch, and parallelised for use on a networked PC
cluster to increase computational speed. The spatial parallelization
method is employed. It is found to be robust and efficient, while not
relying heavily on fast communication. <P />The internal accuracy of
the code is tested extensively in all three geometries and is shown
to be in good agreement with appropriate 1-D solutions.
---------------------------------------------------------
Title: Multidimensional radiative transfer
Authors: van Noort, Michiel Jan
2003PhDT........54V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Multidimensional Non-LTE Radiative Transfer. I. A Universal
Two-dimensional Short-Characteristics Scheme for Cartesian, Spherical,
and Cylindrical Coordinate Systems
Authors: van Noort, Michiel; Hubeny, Ivan; Lanz, Thierry
2002ApJ...568.1066V Altcode:
We have developed an efficient and robust two-dimensional non-LTE
radiation transfer solver appropriate for line transfer in the
equivalent two-level atom formalism. The numerical method applies
the accelerated lambda iteration technique together with the
short-characteristics scheme. The code presented in this paper
incorporates all three standard geometries (Cartesian, cylindrical,
and spherical) in a transparent way while allowing for arbitrary
(three-dimensional) velocity fields. The geometry-specific parts of
the radiative transfer solver are modularized so that the change of
geometry is accomplished by simply setting the appropriate switch. We
have also developed a parallel version of the code, in which we
use a parallelization in spatial subdomains, and showed that such a
scheme is sufficiently robust. We have performed a number of tests
of the performance of the solver in all three geometries. Finally,
we discuss the internal accuracy of the transfer solutions depending
on the number of spatial, angular, and frequency grid points.
---------------------------------------------------------
Title: Multidimensional, non-LTE Radiation Transport in Astrophysical
Media
Authors: Hubeny, I.; van Noort, M.; Lanz, T.
1998AAS...193.6918H Altcode: 1998BAAS...30.1352H
We present first results of our long-term program whose ultimate goal
is to develop multidimensional, self-consistent, non-LTE radiation
hydrodynamic and magnetohydrodynamic spectroscopic diagnostics of
astronomical objects. As the first step, we have developed an efficient
and robust 2-D and 3-D radiation transfer solver, based on the short
characteristics method. We assume the source function either to be
fully specified (i.e., the so-called formal solution of the transfer
equation), or to be given in terms of the equivalent-two-level
atom formalism. In the latter case, we have adopted the idea of
the Accelerated Lambda Iteration technique to treat iteratively the
non-LTE coupling of radiation and matter. We compare the efficiency and
numerical accuracy of several variants of the short-characteristics
scheme. Finally, we present some illustrative examples of radiative
transfer for 2-D and 3-D media with the specified temperature, density,
and velocity structure.
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Title: Fe II emission lines in the UV spectrum of Sirius-A and VEGA
Authors: van Noort, M.; Lanz, T.; Lamers, H. J. G. L. M.; Kurucz,
R. L.; Ferlet, R.; Hebrard, G.; Vidal-Madjar, A.
1998A&A...334..633V Altcode: 1998astro.ph..3127V
We present high-quality HST/GHRS spectra in the Hydrogen Lalpha
spectral region of Vega and Sirius-A. Thanks to the signal-to-noise
ratio achieved in these observations and to the similarity of the two
spectra, we found clear evidence of emission features in the low flux
region, lambda lambda 1190-1222 Angstroms. These emission lines can
be attributed unambiguously to Fe Ii and Cr Ii transitions. In this
spectral range, silicon lines are observed in absorption. We built
a series of non-LTE model atmospheres with different, prescribed
temperature stratification in the upper atmosphere and treating Fe
Ii with various degrees of sophistication in non-LTE. Emission lines
are produced by the combined effect of the Schuster mechanism and
radiative interlocking, and can be explained without the presence of
a chromosphere. Silicon absorption lines and the Lalpha profile set
constraints on the presence of a chromosphere, excluding a strong
temperature rise in layers deeper than tau_R ~ 10(-4) . Based on
observations with the NASA/ESA Hubble Space Telescope obtained at the
Space Telescope Science Institute, which is operated by AURA, Inc.,
under NASA contract NAS 5-26555.
