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.
Bibcode: 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.
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.
Methods: We derived ensemble averages
of the physical properties of different types of convective features
based on observations of two sunspots with Hinode.
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.
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.
Title: Correction of atmospheric stray light in restored slit spectra
Authors: Saranathan, S.; van Noort, M.; Solanki, S. K.
Bibcode: 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.
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.
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.
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.
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.
Bibcode: 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.
Title: Nonequilibrium Equation of State in Stellar Atmospheres
Authors: Anusha, L. S.; van Noort, M.; Cameron, R. H.
Bibcode: 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.
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.
Bibcode: 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.
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.
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.
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).
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.
Title: Non-equilibrium equation-of-state in stellar atmospheres
Authors: Lokanathapura Seetharamabhasari, Anusha; Cameron, Robert;
Van Noort, Michiel
Bibcode: 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.
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.
Bibcode: 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.
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.
Bibcode: 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.
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.
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.
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.
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.
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
Bibcode: 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 arcsec2 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-1 harbor
field strengths larger than 6.5 kG, covering a total area of 2.97
arcsec2. The maximum field strength found is 8.2 kG, which
is the largest ever observed field in a bipolar light bridge up to now.
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
Bibcode: 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.
Title: Connecting the Wilson depression to the magnetic field
of sunspots
Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K.
Bibcode: 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.
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.
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.
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.
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.
Bibcode: 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.
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 (vLOS), 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.
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.
Results: Neglecting the
NLTE effects introduces errors in the inverted atmosphere. While the
errors in T can go up to 13%, in vLOS 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.
Conclusions: We provide an
overview of how neglecting NLTE effects influences the values of T,
vLOS, 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.
Title: Superstrong photospheric magnetic fields in sunspot penumbrae
Authors: Siu-Tapia, A.; Lagg, A.; van Noort, M.; Rempel, M.; Solanki,
S. K.
Bibcode: 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.
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.
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.
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 vLOS (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
vLOS ∼ 16 - 18 km s-1 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
(vLOS ∼ 15 - 17 km s-1) 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.
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 (vLOS > 10 km s-1
and up to ∼30 km s-1 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.
Title: A comparison between solar plage and network properties
Authors: Buehler, D.; Lagg, A.; van Noort, M.; Solanki, S. K.
Bibcode: 2019A&A...630A..86B
Altcode: 2019arXiv190807464B
Aims: We compare the properties of kG magnetic structures
in the solar network and in active region plage at high spatial
resolution.
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.
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-1 internal and 2-3 km s-1 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-1 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-1 in individual pixels. Furthermore, the magnetic canopies
of kG patches are on average 9° more horizontal in the plage compared
to the network.
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.
Title: Response Functions for NLTE Lines
Authors: Milić, I.; van Noort, M.
Bibcode: 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.
Title: Recent advancements in the EST project
Authors: Jurčák, Jan; Collados, Manuel; Leenaarts, Jorrit; van Noort,
Michiel; Schlichenmaier, Rolf
Bibcode: 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.
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.
Bibcode: 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.
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.
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.
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.
Title: Spectropolarimetric NLTE inversion code SNAPI
Authors: Milić, I.; van Noort, M.
Bibcode: 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.
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.
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.
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.
Bibcode: 2018smf..book..109D
Altcode:
No abstract at ADS
Title: Image restoration of solar spectra
Authors: van Noort, M.
Bibcode: 2017A&A...608A..76V
Altcode: 2017arXiv171109629V
Context. When recording spectra from the ground, atmospheric turbulence
causes degradation of the spatial resolution.
Aims: We present
a data reduction method that restores the spatial resolution of the
spectra to their undegraded state.
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.
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.
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.
Bibcode: 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.
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.
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.
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).
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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
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.
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.
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.
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 (ApJS 229, 1, 6)
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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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\prime\prime )
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.
Bibcode: 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.
Bibcode: 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-1 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-1. 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-2. 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.
Bibcode: 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
km2 s-1, 80-90 km2 s-1,
and 25-70 km2 s-1, 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.
Bibcode: 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.
Bibcode: 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-1. 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.
Bibcode: 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 }2 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.
Bibcode: 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}-11+49 km
s-1 and {15}-15+34 km s-1,
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.
Bibcode: 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
Bibcode: 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.
Bibcode: 2016A&A...594A.103D
Altcode: 2016arXiv160706242D
Context. Information on the origin of internetwork magnetic field is
hidden at the smallest spatial scales.
Aims: We try to retrieve
the power spectra with certainty to the highest spatial frequencies
allowed by current instrumentation.
Methods: To accomplish this,
we use a 2D inversion code that is able to recover information up to
the instrumental diffraction limit.
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.
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.
Bibcode: 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.
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.
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.
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.
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.
Bibcode: 2016A&A...589A..31B
Altcode: 2016arXiv160305899B
Aims: The formation and evolution of an Hα filament in active
region (AR) 10953 is described.
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.
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.
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
Bibcode: 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.
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.
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.
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-1 in the inner
penumbra and an average downflow of ~1300 ms-1 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.
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
Bibcode: 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.
Bibcode: 2015A&A...576A..27B
Altcode: 2015arXiv150101151B
Aims: The properties of magnetic fields forming an extended
plage region in AR 10953 were investigated.
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.
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-1 at log (τ)
= 0 and peak velocities of up to 10 km s-1, 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.
Bibcode: 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.
Aims: Here we introduce a new method for deducing
the inclination of magnetic elements and use it to test magnetic field
inclinations from inversions.
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.
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.
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
Bibcode: 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.
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.
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.
Results: Central hot upflows surrounded by
cooler fast downflows reaching 10 km s-1 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.
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.