rezaei

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Author name code: rezaei
ADS astronomy entries on 2022-09-14
author:"Rezaei, Reza"

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Title: Observation of a small-scale magnetic vortex associated with
a chromospheric swirl: signatures of a small-scale magnetic tornado
Authors: Milena Diaz Castillo, Saida; Steiner, Oskar; Fischer,
Catherine; Berdyugina, Svetlana; Rezaei, Reza
2022cosp...44.2521M Altcode:
\newcommand{\ion}[2]{#1\,{\textsc{#2}}} High-resolution solar
observations revealed the existence of small-scale swirling vortices
in chromospheric intensity maps and velocity diagnostics. These events
are commonly localized in the quiet sun intergranular space and are
often related to small-scale magnetic flux concentrations at the solar
surface. Frequently, vortices have been observed in the vicinity of
magnetic flux concentrations, indicating a link between swirls and the
evolution of the small-scale magnetic fields. Vortices were also studied
with MHD numerical simulations of the solar atmosphere, revealing
their complexity, dynamics, and magnetic nature. In particular, it
has been suggested that the chromospheric swirling plasma motion is
due to a coherently rotating magnetic field structure, which again
is driven by a photospheric vortex flow at its footpoint. In this
contribution, we present a comprehensive description of the evolution
of an isolated small-scale magnetic element interacting with a vortex
flow, which in turn is related to a chromospheric swirl. We study
observations taken with the CRisp Imaging SpectroPolarimeter (CRISP)
instrument and the CHROMospheric Imaging Spectrometer (CHROMIS) at the
1m Swedish Solar Telescope (SST) in April 2019 as part of a SOLARNET
access program. The data were taken at quiet-Sun disk-center, recording
full Stokes photospheric maps in the \ion{Fe}{i} line at 617\,nm, full
Stokes data in the \ion{Ca}{ii} infrared triplet line at 854\,nm, and
spectroscopic maps in the H$\alpha$ 656\,nm, \ion{Ca}{ii} K 393\,nm,
and \ion{Ca}{ii} H 396\,nm lines. Utilizing the multi-wavelength
data and applying height-dependent Stokes inversion and local
correlation tracking methods, we are able to analyse the magnetic
field dynamics in the presence of vortex structures at photospheric
and chromospheric layers. The temporal evolution of the magnetic
element shows an appreciable increase in the magnetic field strength
during the interaction with the vortex flow, reaching kG values for
a few minutes. We also find a clear evidence of a Rapid Blue-shift
Excursion (RBE) associated with the magnetic field intensification
event propagating along the chromospheric vortex. In addition, we
explore the polarization signatures in the photosphere to reveal the
intrinsic structure of the magnetic element. Marginal but consistent
detection of linear polarization signals in the surroundings of the
magnetic element before intensification suggests a magnetic field
torsion. Our analysis indicates that we have observed a rotating
magnetic object reaching from the photosphere to the chromosphere,
resembling a small-scale magnetic tornado.

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Title: Photospheric Magnetic Fields of the Trailing Sunspots in
Active Region NOAA 12396
Authors: Verma, M.; Balthasar, H.; Denker, C.; Böhm, F.; Fischer,
C. E.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello
González, N.; Diercke, A.; Berkefeld, T.; Collados, M.; Feller, A.;
Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar,
A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K.; Volkmer,
R.; von der Lühe, O.; Waldmann, T.
2019ASPC..526..291V Altcode: 2018arXiv180507752V
The solar magnetic field is responsible for all aspects of solar
activity. Sunspots are the main manifestation of the ensuing solar
activity. Combining high-resolution and synoptic observations has
the ambition to provide a comprehensive description of the sunspot
growth and decay processes. Active region NOAA 12396 emerged on 2015
August 3 and was observed three days later with the 1.5-meter GREGOR
solar telescope on 2015 August 6. High-resolution spectropolarimetric
data from the GREGOR Infrared Spectrograph (GRIS) are obtained in the
photospheric lines Si I λ1082.7 nm and Ca I λ1083.9 nm, together
with the chromospheric He I λ1083.0 nm triplet. These near-infrared
spectropolarimetric observations were complemented by synoptic
line-of-sight magnetograms and continuum images of the Helioseismic
and Magnetic Imager (HMI) and EUV images of the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO).

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Title: Magnetic Flux Density in 3D MHD Simulations and Observations
Authors: Beck, C.; Fabbian, D.; Rezaei, R.; Puschmann, K. G.
2019ASPC..526..191B Altcode:
We compare the polarization signals induced in three-dimensional (3D)
magneto-hydrodynamical (MHD) simulations by the Zeeman effect in the
presence of photospheric magnetic fields to those in observations
at disc centre. We consider quantities determined from Stokes vector
profiles of observations of photospheric spectral lines in the visible
and near-infrared, and in corresponding synthetic spectra obtained
from numerical 3D MHD simulations with an average magnetic flux
density of 20-200 G. We match the spatial resolution of observations
by degrading the spectra of the simulations. We find that the total
unsigned vertical magnetic flux density in the simulation should
be less than 50 G to reproduce the observed polarization signals in
the quiet Sun internetwork. A value of ∼30 G best agrees with all
observations we employed.

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Title: New Insights on Penumbra Magneto-Convection
Authors: Bello González, N.; Jurčák, J.; Schlichenmaier, R.;
Rezaei, R.
2019ASPC..526..261B Altcode:
Fully-fledged penumbrae are a well characterised phenomenon from
an observational point of view. Also, MHD simulations reproduce the
observed characteristics and provide us with insights on the physical
mechanisms possibly running behind the observed processes. Yet, how this
penumbral magneto-convection sets in is still an open question. Due to
the fact that penumbra formation is a relatively fast process (of the
order of hours), it has eluded its observation with sufficient spatial
resolution by both space- and ground-based solar observatories. Only
recently, some researchers have witnessed the onset of both orphan
and sunspot penumbrae in detail. We are one of those. In July 2009, we
observed the early stages of the NOAA 11024 AR leading sunspot while
developing its penumbra. The spectro-polarimetric dataset lead us to
new observational findings. In this contribution, we put into context
our and other authors' results to draw the overall picture of sunspot
formation. Most important, the comparison on the properties of different
types of penumbrae lead us to the conclusion that the formation of
penumbrae is not just one mechanism. While the sole cause necessary
for penumbral magneto-convection is a stably inclined magnetic field,
observations show that inclined fields can be caused by flux emergence,
to form orphan penumbrae, or by field lines transported down from
upper photospheric layers, to form sunspot penumbra. This conclusion,
together with the recent findings by Jur\čák and collaborators on
a canonical value of the vertical component of the magnetic field
blocking the action of penumbral magneto-convection in umbral areas,
is a crucial step forward towards the understanding of the coupling
of solar plasmas and magnetic fields in penumbral atmospheres.

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Title: The solar chromosphere at millimetre and ultraviolet
wavelengths. I. Radiation temperatures and a detailed comparison
Authors: Jafarzadeh, S.; Wedemeyer, S.; Szydlarski, M.; De Pontieu,
B.; Rezaei, R.; Carlsson, M.
2019A&A...622A.150J Altcode: 2019arXiv190105763J
Solar observations with the Atacama Large Millimeter/submillimeter
Array (ALMA) provide us with direct measurements of the brightness
temperature in the solar chromosphere. We study the temperature
distributions obtained with ALMA Band 6 (in four sub-bands at 1.21,
1.22, 1.29, and 1.3 mm) for various areas at, and in the vicinity of,
a sunspot, comprising quasi-quiet and active regions with different
amounts of underlying magnetic fields. We compare these temperatures
with those obtained at near- and far-ultraviolet (UV) wavelengths
(and with the line-core intensities of the optically-thin far-UV
spectra), co-observed with the Interface Region Imaging Spectrograph
(IRIS) explorer. These include the emission peaks and cores of the Mg
II k 279.6 nm and Mg II h 280.4 nm lines as well as the line cores
of C II 133.4 nm, O I 135.6 nm, and Si IV 139.4 nm, sampling the
mid-to-high chromosphere and the low transition region. Splitting the
ALMA sub-bands resulted in an slight increase of spatial resolution in
individual temperature maps, thus, resolving smaller-scale structures
compared to those produced with the standard averaging routines. We
find that the radiation temperatures have different, though somewhat
overlapping, distributions in different wavelengths and in the various
magnetic regions. Comparison of the ALMA temperatures with those of
the UV diagnostics should, however, be interpreted with great caution,
the former is formed under the local thermodynamic equilibrium (LTE)
conditions, the latter under non-LTE. The mean radiation temperature
of the ALMA Band 6 is similar to that extracted from the IRIS C II
line in all areas with exception of the sunspot and pores where the C
II poses higher radiation temperatures. In all magnetic regions, the
Mg II lines associate with the lowest mean radiation temperatures in
our sample. These will provide constraints for future numerical models.

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Title: High-resolution imaging and near-infrared spectroscopy of
penumbral decay
Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; Rezaei,
R.; Sobotka, M.; Deng, N.; Wang, H.; Tritschler, A.; Collados, M.;
Diercke, A.; González Manrique, S. J.
2018A&A...614A...2V Altcode: 2018arXiv180103686V
 Aims: Combining high-resolution spectropolarimetric and imaging
data is key to understanding the decay process of sunspots as it
allows us to scrutinize the velocity and magnetic fields of sunspots
and their surroundings. Methods: Active region NOAA 12597
was observed on 2016 September 24 with the 1.5-meter GREGOR solar
telescope using high-spatial-resolution imaging as well as imaging
spectroscopy and near-infrared (NIR) spectropolarimetry. Horizontal
proper motions were estimated with local correlation tracking, whereas
line-of-sight (LOS) velocities were computed with spectral line fitting
methods. The magnetic field properties were inferred with the "Stokes
Inversions based on Response functions" (SIR) code for the Si I and Ca
I NIR lines. Results: At the time of the GREGOR observations,
the leading sunspot had two light bridges indicating the onset of
its decay. One of the light bridges disappeared, and an elongated,
dark umbral core at its edge appeared in a decaying penumbral sector
facing the newly emerging flux. The flow and magnetic field properties
of this penumbral sector exhibited weak Evershed flow, moat flow, and
horizontal magnetic field. The penumbral gap adjacent to the elongated
umbral core and the penumbra in that penumbral sector displayed LOS
velocities similar to granulation. The separating polarities of a new
flux system interacted with the leading and central part of the already
established active region. As a consequence, the leading spot rotated
55° clockwise over 12 h. Conclusions: In the high-resolution
observations of a decaying sunspot, the penumbral filaments facing the
flux emergence site contained a darkened area resembling an umbral core
filled with umbral dots. This umbral core had velocity and magnetic
field properties similar to the sunspot umbra. This implies that the
horizontal magnetic fields in the decaying penumbra became vertical
as observed in flare-induced rapid penumbral decay, but on a very
different time-scale.

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Title: The magnetic nature of umbra-penumbra boundary in sunspots
Authors: Jurčák, J.; Rezaei, R.; González, N. Bello; Schlichenmaier,
R.; Vomlel, J.
2018A&A...611L...4J Altcode: 2018arXiv180108983J
Context. Sunspots are the longest-known manifestation of solar
activity, and their magnetic nature has been known for more than a
century. Despite this, the boundary between umbrae and penumbrae,
the two fundamental sunspot regions, has hitherto been solely
defined by an intensity threshold. Aim. Here, we aim at studying the
magnetic nature of umbra-penumbra boundaries in sunspots of different
sizes, morphologies, evolutionary stages, and phases of the solar
cycle. Methods: We used a sample of 88 scans of the Hinode/SOT
spectropolarimeter to infer the magnetic field properties in at the
umbral boundaries. We defined these umbra-penumbra boundaries by
an intensity threshold and performed a statistical analysis of the
magnetic field properties on these boundaries. Results: We
statistically prove that the umbra-penumbra boundary in stable sunspots
is characterised by an invariant value of the vertical magnetic field
component: the vertical component of the magnetic field strength
does not depend on the umbra size, its morphology, and phase of the
solar cycle. With the statistical Bayesian inference, we find that the
strength of the vertical magnetic field component is, with a likelihood
of 99%, in the range of 1849-1885 G with the most probable value
of 1867 G. In contrast, the magnetic field strength and inclination
averaged along individual boundaries are found to be dependent on the
umbral size: the larger the umbra, the stronger and more horizontal
the magnetic field at its boundary. Conclusions: The umbra and
penumbra of sunspots are separated by a boundary that has hitherto been
defined by an intensity threshold. We now unveil the empirical law of
the magnetic nature of the umbra-penumbra boundary in stable sunspots:
it is an invariant vertical component of the magnetic field.

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Title: High-resolution Observations of Hα Spectra with a Subtractive
Double Pass
Authors: Beck, C.; Rezaei, R.; Choudhary, D. P.; Gosain, S.;
Tritschler, A.; Louis, R. E.
2018SoPh..293...36B Altcode: 2017arXiv171207077B
High-resolution imaging spectroscopy in solar physics has relied on
Fabry-Pérot interferometers (FPIs) in recent years. FPI systems,
however, become technically challenging and expensive for telescopes
larger than the 1 m class. A conventional slit spectrograph with a
diffraction-limited performance over a large field of view (FOV) can
be built at much lower cost and effort. It can be converted into an
imaging spectro(polari)meter using the concept of a subtractive double
pass (SDP). We demonstrate that an SDP system can reach a similar
performance as FPI-based systems with a high spatial and moderate
spectral resolution across a FOV of 100″×100″
with a spectral coverage of 1 nm. We use Hα spectra taken with an SDP
system at the Dunn Solar Telescope and complementary full-disc data to
infer the properties of small-scale superpenumbral filaments. We find
that the majority of all filaments end in patches of opposite-polarity
fields. The internal fine-structure in the line-core intensity of Hα
at spatial scales of about 0.″5 exceeds that in other parameters
such as the line width, indicating small-scale opacity effects in a
larger-scale structure with common properties. We conclude that SDP
systems in combination with (multi-conjugate) adaptive optics are a
valid alternative to FPI systems when high spatial resolution and a
large FOV are required. They can also reach a cadence that is comparable
to that of FPI systems, while providing a much larger spectral range
and a simultaneous multi-line capability.

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Title: Structure of sunspot light bridges in the chromosphere and
transition region
Authors: Rezaei, R.
2018A&A...609A..73R Altcode: 2017arXiv171110229R
Context. Light bridges (LBs) are elongated structures with enhanced
intensity embedded in sunspot umbra and pores. Aims: We studied
the properties of a sample of 60 LBs observed with the Interface
Region Imaging Spectrograph (IRIS). Methods: Using IRIS near-
and far-ultraviolet spectra, we measured the line intensity, width,
and Doppler shift; followed traces of LBs in the chromosphere and
transition region (TR); and compared LB parameters with umbra and quiet
Sun. Results: There is a systematic emission enhancement in
LBs compared to nearby umbra from the photosphere up to the TR. Light
bridges are systematically displaced toward the solar limb at higher
layers: the amount of the displacement at one solar radius compares
well with the typical height of the chromosphere and TR. The intensity
of the LB sample compared to the umbra sample peaks at the middle/upper
chromosphere where they are almost permanently bright. Spectral lines
emerging from the LBs are broader than the nearby umbra. The systematic
redshift of the Si IV line in the LB sample is reduced compared to
the quiet Sun sample. We found a significant correlation between the
line width of ions arising at temperatures from 3 × 104
to 1.5 × 105 K as there is also a strong spatial
correlation among the line and continuum intensities. In addition,
the intensity-line width relation holds for all spectral lines in this
study. The correlations indicate that the cool and hot plasma in LBs are
coupled. Conclusions: Light bridges comprise multi-temperature
and multi-disciplinary structures extending up to the TR. Diverse
heating sources supply the energy and momentum to different layers,
resulting in distinct dynamics in the photosphere, chromosphere, and TR.

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Title: The Temperature - Magnetic Field Relation in Observed and
Simulated Sunspots
Authors: Sobotka, Michal; Rezaei, Reza
2017SoPh..292..188S Altcode: 2017arXiv171109821S
Observations of the relation between continuum intensity and magnetic
field strength in sunspots have been made for nearly five decades. This
work presents full-Stokes measurements of the full-split (g =3 ) line
Fe I 1564.85 nm with a spatial resolution of 0.5″ obtained
with the GREGOR Infrared Spectrograph in three large sunspots. The
continuum intensity is corrected for instrumental scattered light, and
the brightness temperature is calculated. Magnetic field strength and
inclination are derived directly from the line split and the ratio of
Stokes components. The continuum intensity (temperature) relations to
the field strength are studied separately in the umbra, light bridges,
and penumbra. The results are consistent with previous studies, and it
was found that the scatter of values in the relations increases with
increasing spatial resolution thanks to resolved fine structures. The
observed relations show trends common for the umbra, light bridges, and
the inner penumbra, while the outer penumbra has a weaker magnetic field
than the inner penumbra at equal continuum intensities. This fact can
be interpreted in terms of the interlocking comb magnetic structure of
the penumbra. A comparison with data obtained from numerical simulations
was made. The simulated data generally have a stronger magnetic field
and a weaker continuum intensity than the observations, which may
be explained by stray light and limited spatial resolution of the
observations, and also by photometric inaccuracies of the simulations.

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Title: Chromospheric impact of an exploding solar granule
Authors: Fischer, C. E.; Bello González, N.; Rezaei, R.
2017A&A...602L..12F Altcode: 2017arXiv170600770F
Context. Observations of multi-wavelength and therefore height-dependent
information following events throughout the solar atmosphere and
unambiguously assigning a relation between these rapidly evolving
layers are rare and difficult to obtain. Yet, they are crucial for our
understanding of the physical processes that couple the different
regimes in the solar atmosphere. Aims: We characterize
the exploding granule event with simultaneous observations of
Hinode spectroplarimetric data in the solar photosphere and Hinode
broadband Ca II H images combined with Interface Region Imaging
Spectrograph (IRIS) slit spectra. We follow the evolution of an
exploding granule and its connectivity throughout the atmosphere and
analyze the dynamics of a magnetic element that has been affected
by the abnormal granule. Methods: In addition to magnetic
flux maps we use a local correlation tracking method to infer the
horizontal velocity flows in the photosphere and apply a wavelet
analysis on several IRIS chromospheric emission features such as
Mg II k2v and Mg II k3 to detect oscillatory phenomena indicating
wave propagation. Results: During the vigorous expansion of
the abnormal granule we detect radially outward horizontal flows,
causing, together with the horizontal flows from the surrounding
granules, the magnetic elements in the bordering intergranular lanes
to be squeezed and elongated. In reaction to the squeezing, we detect
a chromospheric intensity and velocity oscillation pulse which we
identify as an upward traveling hot shock front propagating clearly
through the IRIS spectral line diagnostics of Mg II h&amp;k. 
Conclusions: Exploding granules can trigger upward-propagating shock
fronts that dissipate in the chromosphere. Movies associated
to Figs. A.1 and A.2 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201731120/olm">http://www.aanda.org</A>

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Title: The Polarization Signature of Photospheric Magnetic Fields
in 3D MHD Simulations and Observations at Disk Center
Authors: Beck, C.; Fabbian, D.; Rezaei, R.; Puschmann, K. G.
2017ApJ...842...37B Altcode: 2017arXiv170506812B
Before using three-dimensional (3D) magnetohydrodynamical (MHD)
simulations of the solar photosphere in the determination of elemental
abundances, one has to ensure that the correct amount of magnetic
flux is present in the simulations. The presence of magnetic flux
modifies the thermal structure of the solar photosphere, which affects
abundance determinations and the solar spectral irradiance. The amount
of magnetic flux in the solar photosphere also constrains any possible
heating in the outer solar atmosphere through magnetic reconnection. We
compare the polarization signals in disk-center observations of the
solar photosphere in quiet-Sun regions with those in Stokes spectra
computed on the basis of 3D MHD simulations having average magnetic
flux densities of about 20, 56, 112, and 224 G. This approach allows
us to find the simulation run that best matches the observations. The
observations were taken with the Hinode SpectroPolarimeter (SP),
the Tenerife Infrared Polarimeter (TIP), the Polarimetric Littrow
Spectrograph (POLIS), and the GREGOR Fabry-Pèrot Interferometer
(GFPI), respectively. We determine characteristic quantities of full
Stokes profiles in a few photospheric spectral lines in the visible
(630 nm) and near-infrared (1083 and 1565 nm). We find that the
appearance of abnormal granulation in intensity maps of degraded
simulations can be traced back to an initially regular granulation
pattern with numerous bright points in the intergranular lanes
before the spatial degradation. The linear polarization signals in
the simulations are almost exclusively related to canopies of strong
magnetic flux concentrations and not to transient events of magnetic
flux emergence. We find that the average vertical magnetic flux density
in the simulation should be less than 50 G to reproduce the observed
polarization signals in the quiet-Sun internetwork. A value of about 35
G gives the best match across the SP, TIP, POLIS, and GFPI observations.

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Title: A distinct magnetic property of the inner penumbral
boundary. II. Formation of a penumbra at the expense of a pore
Authors: Jurčák, J.; Bello González, N.; Schlichenmaier, R.;
Rezaei, R.
2017A&A...597A..60J Altcode: 2016arXiv161201745J; 2016A&A...597A..60J
Context. We recently presented evidence that stable
umbra-penumbra boundaries are characterised by a distinct
canonical value of the vertical component of the magnetic
field, Bstablever. In order to trigger
the formation of a penumbra, large inclinations in the magnetic
field are necessary. In sunspots, the penumbra develops and
establishes by colonising both umbral areas and granulation, that
is, penumbral magneto-convection takes over in umbral regions with
Bver&lt;Bstablever, as well as
in granular convective areas. Eventually, a stable umbra-penumbra
boundary settles at Bstablever. Aims:
Here, we aim to study the development of a penumbra initiated at
the boundary of a pore, where the penumbra colonises the entire pore
ultimately. Methods: We have used Hinode/SOT G-band images to
study the evolution of the penumbra. Hinode/SOT spectropolarimetric
data were used to infer the magnetic field properties in the studied
region. Results: The penumbra forms at the boundary of a pore
located close to the polarity inversion line of NOAA 10960. As the
penumbral bright grains protrude into the pore, the magnetic flux in
the forming penumbra increases at the expense of the pore magnetic
flux. Consequently, the pore disappears completely giving rise to an
orphan penumbra. At all times, the vertical component of the magnetic
field in the pore is smaller than Bstablever
≈ 1.8 kG. Conclusions: Our findings are in an agreement
with the need of Bstablever for establishing
a stable umbra-penumbra boundary: while Bver in
the pore is smaller than Bstablever, the
protrusion of penumbral grains into the pore area is not blocked,
a stable pore-penumbra boundary does not establish, and the pore
is fully overtaken by the penumbral magneto-convective mode. This
scenario could also be one of the mechanisms giving rise to orphan
penumbrae. The movie associated to Fig. 1 is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201628547/olm">http://www.aanda.org</A>

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Title: MOSiC: an analysis tool for IRIS spectral data
Authors: Rezaei, Reza
2017arXiv170104421R Altcode:
This is a manual for the MOSiC package. MOSiC is a collection of IDL
programs for profile analysis and Gaussian fitting of the Mg II h/k
lines along with Gaussian fitting of the C II 133.5 nm line pair, the O
I 135.6, the Cl I 135.2, the Si IV 139.7 and 140.3 and the O IV 140.0
nm lines observed with the IRIS near UV and far UV spectrograph. It
was tested by analyzing over a hundred different IRIS data sets (quiet
Sun, sunspot, ...). It works for off limb data, although it is still
experimental. MOSiC analyzes different spectral lines separately and
returns line intensity, width, and velocity for each line. A few sample
profiles and maps are included in this manual.

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Title: Canonical Bver value on umbra/penumbra boundaries
Authors: Jurcak, Jan; Bello González, Nazaret; Schlichenmaier, Rolf;
Rezaei, Reza
2017psio.confE.112J Altcode:
No abstract at ADS

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Title: Deep probing of the photospheric sunspot penumbra: no evidence
of field-free gaps
Authors: Borrero, J. M.; Asensio Ramos, A.; Collados, M.;
Schlichenmaier, R.; Balthasar, H.; Franz, M.; Rezaei, R.; Kiess, C.;
Orozco Suárez, D.; Pastor Yabar, A.; Berkefeld, T.; von der Lühe,
O.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Soltau, D.; Volkmer, R.;
Waldmann, T.; Denker, C.; Hofmann, A.; Staude, J.; Strassmeier, K. G.;
Feller, A.; Lagg, A.; Solanki, S. K.; Sobotka, M.; Nicklas, H.
2016A&A...596A...2B Altcode: 2016arXiv160708165B
Context. Some models for the topology of the magnetic field in
sunspot penumbrae predict regions free of magnetic fields or with
only dynamically weak fields in the deep photosphere. Aims:
We aim to confirm or refute the existence of weak-field regions in
the deepest photospheric layers of the penumbra. Methods:
We investigated the magnetic field at log τ5 = 0 is
by inverting spectropolarimetric data of two different sunspots
located very close to disk center with a spatial resolution of
approximately 0.4-0.45”. The data have been recorded using the GRIS
instrument attached to the 1.5-m solar telescope GREGOR at the El
Teide observatory. The data include three Fe I lines around 1565 nm,
whose sensitivity to the magnetic field peaks half a pressure scale
height deeper than the sensitivity of the widely used Fe I spectral
line pair at 630 nm. Before the inversion, the data were corrected
for the effects of scattered light using a deconvolution method with
several point spread functions. Results: At log τ5
= 0 we find no evidence of regions with dynamically weak (B&lt;
500 Gauss) magnetic fields in sunspot penumbrae. This result is much
more reliable than previous investigations made on Fe I lines at 630
nm. Moreover, the result is independent of the number of nodes employed
in the inversion, is independent of the point spread function used to
deconvolve the data, and does not depend on the amount of stray light
(I.e., wide-angle scattered light) considered.

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Title: Spectropolarimetric observations of an arch filament system
with the GREGOR solar telescope
Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.;
Kuckein, C.; Kavka, J.; Kučera, A.; Schwartz, P.; Vašková, R.;
Berkefeld, T.; Collados Vera, M.; Denker, C.; Feller, A.; Hofmann,
A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.;
Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2016AN....337.1050B Altcode: 2016arXiv160901514B
Arch filament systems occur in active sunspot groups, where a fibril
structure connects areas of opposite magnetic polarity, in contrast to
active region filaments that follow the polarity inversion line. We
used the GREGOR Infrared Spectrograph (GRIS) to obtain the full
Stokes vector in the spectral lines Si I λ1082.7 nm, He I λ1083.0
nm, and Ca I λ1083.9 nm. We focus on the near-infrared calcium line
to investigate the photospheric magnetic field and velocities, and
use the line core intensities and velocities of the helium line to
study the chromospheric plasma. The individual fibrils of the arch
filament system connect the sunspot with patches of magnetic polarity
opposite to that of the spot. These patches do not necessarily coincide
with pores, where the magnetic field is strongest. Instead, areas are
preferred not far from the polarity inversion line. These areas exhibit
photospheric downflows of moderate velocity, but significantly higher
downflows of up to 30 km s-1 in the chromospheric helium
line. Our findings can be explained with new emerging flux where the
matter flows downward along the field lines of rising flux tubes,
in agreement with earlier results.

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Title: Magnetic fields of opposite polarity in sunspot penumbrae
Authors: Franz, M.; Collados, M.; Bethge, C.; Schlichenmaier, R.;
Borrero, J. M.; Schmidt, W.; Lagg, A.; Solanki, S. K.; Berkefeld,
T.; Kiess, C.; Rezaei, R.; Schmidt, D.; Sigwarth, M.; Soltau, D.;
Volkmer, R.; von der Luhe, O.; Waldmann, T.; Orozco, D.; Pastor Yabar,
A.; Denker, C.; Balthasar, H.; Staude, J.; Hofmann, A.; Strassmeier,
K.; Feller, A.; Nicklas, H.; Kneer, F.; Sobotka, M.
2016A&A...596A...4F Altcode: 2016arXiv160800513F
Context. A significant part of the penumbral magnetic field returns
below the surface in the very deep photosphere. For lines in the
visible, a large portion of this return field can only be detected
indirectly by studying its imprints on strongly asymmetric and
three-lobed Stokes V profiles. Infrared lines probe a narrow layer
in the very deep photosphere, providing the possibility of directly
measuring the orientation of magnetic fields close to the solar
surface. Aims: We study the topology of the penumbral magnetic
field in the lower photosphere, focusing on regions where it returns
below the surface. Methods: We analyzed 71 spectropolarimetric
datasets from Hinode and from the GREGOR infrared spectrograph. We
inferred the quality and polarimetric accuracy of the infrared data
after applying several reduction steps. Techniques of spectral
inversion and forward synthesis were used to test the detection
algorithm. We compared the morphology and the fractional penumbral
area covered by reversed-polarity and three-lobed Stokes V profiles for
sunspots at disk center. We determined the amount of reversed-polarity
and three-lobed Stokes V profiles in visible and infrared data of
sunspots at various heliocentric angles. From the results, we computed
center-to-limb variation curves, which were interpreted in the context
of existing penumbral models. Results: Observations in visible
and near-infrared spectral lines yield a significant difference in the
penumbral area covered by magnetic fields of opposite polarity. In
the infrared, the number of reversed-polarity Stokes V profiles is
smaller by a factor of two than in the visible. For three-lobed Stokes
V profiles the numbers differ by up to an order of magnitude.

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Title: Upper chromospheric magnetic field of a sunspot penumbra:
observations of fine structure
Authors: Joshi, J.; Lagg, A.; Solanki, S. K.; Feller, A.; Collados,
M.; Orozco Suárez, D.; Schlichenmaier, R.; Franz, M.; Balthasar,
H.; Denker, C.; Berkefeld, T.; Hofmann, A.; Kiess, C.; Nicklas, H.;
Pastor Yabar, A.; Rezaei, R.; Schmidt, D.; Schmidt, W.; Sobotka, M.;
Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe,
O.; Waldmann, T.
2016A&A...596A...8J Altcode: 2016arXiv160801988J
 Aims: The fine-structure of the magnetic field in a sunspot
penumbra in the upper chromosphere is to be explored and compared
to that in the photosphere. Methods: Spectropolarimetric
observations with high spatial resolution were recorded with the 1.5-m
GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The
observed spectral domain includes the upper chromospheric Hei triplet
at 10 830 Å and the photospheric Sii 10 827.1 Å and Cai 10 833.4 Å
spectral lines. The upper chromospheric magnetic field is obtained
by inverting the Hei triplet assuming a Milne-Eddington-type model
atmosphere. A height-dependent inversion was applied to the Sii 10
827.1 Å and Cai 10 833.4 Å lines to obtain the photospheric magnetic
field. Results: We find that the inclination of the magnetic
field varies in the azimuthal direction in the photosphere and in the
upper chromosphere. The chromospheric variations coincide remarkably
well with the variations in the inclination of the photospheric field
and resemble the well-known spine and interspine structure in the
photospheric layers of penumbrae. The typical peak-to-peak variations
in the inclination of the magnetic field in the upper chromosphere
are found to be 10°-15°, which is roughly half the variation in
the photosphere. In contrast, the magnetic field strength of the
observed penumbra does not vary on small spatial scales in the upper
chromosphere. Conclusions: Thanks to the high spatial resolution
of the observations that is possible with the GREGOR telescope at 1.08
microns, we find that the prominent small-scale fluctuations in the
magnetic field inclination, which are a salient part of the property
of sunspot penumbral photospheres, also persist in the chromosphere,
although at somewhat reduced amplitudes. Such a complex magnetic
configuration may facilitate penumbral chromospheric dynamic phenomena,
such as penumbral micro-jets or transient bright dots.

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Title: Probing deep photospheric layers of the quiet Sun with high
magnetic sensitivity
Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González,
M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.;
Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.;
Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.;
Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier,
K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.;
Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.;
Schmidt, D.; Kiess, C.; Nicklas, H.
2016A&A...596A...6L Altcode: 2016arXiv160506324L
Context. Investigations of the magnetism of the quiet Sun are hindered
by extremely weak polarization signals in Fraunhofer spectral
lines. Photon noise, straylight, and the systematically different
sensitivity of the Zeeman effect to longitudinal and transversal
magnetic fields result in controversial results in terms of the strength
and angular distribution of the magnetic field vector. Aims:
The information content of Stokes measurements close to the diffraction
limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of
spatial straylight and photon noise into account. Methods: Highly
sensitive full Stokes measurements of a quiet-Sun region at disk center
in the deep photospheric Fe I lines in the 1.56 μm region were obtained
with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise
statistics and Stokes V asymmetries were analyzed and compared to a
similar data set of the Hinode spectropolarimeter (SOT/SP). Simple
diagnostics based directly on the shape and strength of the profiles
were applied to the GRIS data. We made use of the magnetic line ratio
technique, which was tested against realistic magneto-hydrodynamic
simulations (MURaM). Results: About 80% of the GRIS spectra
of a very quiet solar region show polarimetric signals above a 3σ
level. Area and amplitude asymmetries agree well with small-scale
surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio
analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss
range with some concentrations of kilo-Gauss fields. Conclusions:
The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4
are so far unique in the combination of high spatial resolution scans
and high magnetic field sensitivity. Nevertheless, the unavoidable
effect of spatial straylight and the resulting dilution of the weak
Stokes profiles means that inversion techniques still bear a high risk
of misinterpretating the data.

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Title: Flow and magnetic field properties in the trailing sunspots
of active region NOAA 12396
Authors: Verma, M.; Denker, C.; Böhm, F.; Balthasar, H.; Fischer,
C. E.; Kuckein, C.; Bello González, N.; Berkefeld, T.; Collados,
M.; Diercke, A.; Feller, A.; González Manrique, S. J.; Hofmann, A.;
Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pator Yabar, A.; Rezaei,
R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.;
Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2016AN....337.1090V Altcode:
Improved measurements of the photospheric and chromospheric
three-dimensional magnetic and flow fields are crucial for a precise
determination of the origin and evolution of active regions. We present
an illustrative sample of multi-instrument data acquired during a
two-week coordinated observing campaign in August 2015 involving,
among others, the GREGOR solar telescope (imaging and near-infrared
spectroscopy) and the space missions Solar Dynamics Observatory (SDO)
and Interface Region Imaging Spectrograph (IRIS). The observations
focused on the trailing part of active region NOAA 12396 with complex
polarity inversion lines and strong intrusions of opposite polarity
flux. The GREGOR Infrared Spectrograph (GRIS) provided Stokes IQUV
spectral profiles in the photospheric Si I λ1082.7 nm line, the
chromospheric He I λ1083.0 nm triplet, and the photospheric Ca I
λ1083.9 nm line. Carefully calibrated GRIS scans of the active region
provided maps of Doppler velocity and magnetic field at different
atmospheric heights. We compare quick-look maps with those obtained
with the “Stokes Inversions based on Response functions” (SIR)
code, which furnishes deeper insight into the magnetic properties
of the region. We find supporting evidence that newly emerging flux
and intruding opposite polarity flux are hampering the formation
of penumbrae, i.e., a penumbra fully surrounding a sunspot is only
expected after cessation of flux emergence in proximity to the sunspots.

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Title: Three-dimensional structure of a sunspot light bridge
Authors: Felipe, T.; Collados, M.; Khomenko, E.; Kuckein, C.; Asensio
Ramos, A.; Balthasar, H.; Berkefeld, T.; Denker, C.; Feller, A.;
Franz, M.; Hofmann, A.; Joshi, J.; Kiess, C.; Lagg, A.; Nicklas, H.;
Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier,
R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki,
S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.;
von der Lühe, O.; Waldmann, T.
2016A&A...596A..59F Altcode: 2016arXiv161104803F
Context. Active regions are the most prominent manifestations of solar
magnetic fields; their generation and dissipation are fundamental
problems in solar physics. Light bridges are commonly present during
sunspot decay, but a comprehensive picture of their role in the
removal of the photospheric magnetic field is still lacking. 
Aims: We study the three-dimensional configuration of a sunspot,
and in particular, its light bridge, during one of the last stages of
its decay. Methods: We present the magnetic and thermodynamical
stratification inferred from full Stokes inversions of the photospheric
Si I 10 827 Å and Ca I 10 839 Å lines obtained with the GREGOR
Infrared Spectrograph of the GREGOR telescope at the Observatorio del
Teide, Tenerife, Spain. The analysis is complemented by a study of
continuum images covering the disk passage of the active region, which
are provided by the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory. Results: The sunspot shows a light bridge
with penumbral continuum intensity that separates the central umbra from
a smaller umbra. We find that in this region the magnetic field lines
form a canopy with lower magnetic field strength in the inner part. The
photospheric light bridge is dominated by gas pressure (high-β),
as opposed to the surrounding umbra, where the magnetic pressure
is higher. A convective flow is observed in the light bridge. This
flow is able to bend the magnetic field lines and to produce field
reversals. The field lines merge above the light bridge and become
as vertical and strong as in the surrounding umbra. We conclude that
this occurs because two highly magnetized regions approach each other
during the sunspot evolution. Movies associated to Figs. 2 and 13
are available at <A href="http://www.aanda.org">http://www.aanda.org</A>

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Title: Inference of magnetic fields in the very quiet Sun
Authors: Martínez González, M. J.; Pastor Yabar, A.; Lagg, A.;
Asensio Ramos, A.; Collados, M.; Solanki, S. K.; Balthasar, H.;
Berkefeld, T.; Denker, C.; Doerr, H. P.; Feller, A.; Franz, M.;
González Manrique, S. J.; Hofmann, A.; Kneer, F.; Kuckein, C.;
Louis, R.; von der Lühe, O.; Nicklas, H.; Orozco, D.; Rezaei, R.;
Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Waldman,
T.; Volkmer, R.
2016A&A...596A...5M Altcode: 2018arXiv180410089M
Context. Over the past 20 yr, the quietest areas of the solar surface
have revealed a weak but extremely dynamic magnetism occurring
at small scales (&lt;500 km), which may provide an important
contribution to the dynamics and energetics of the outer layers of
the atmosphere. Understanding this magnetism requires the inference
of physical quantities from high-sensitivity spectro-polarimetric
data with high spatio-temporal resolution. Aims: We present
high-precision spectro-polarimetric data with high spatial resolution
(0.4”) of the very quiet Sun at 1.56 μm obtained with the GREGOR
telescope to shed some light on this complex magnetism. Methods:
We used inversion techniques in two main approaches. First, we assumed
that the observed profiles can be reproduced with a constant magnetic
field atmosphere embedded in a field-free medium. Second, we assumed
that the resolution element has a substructure with either two constant
magnetic atmospheres or a single magnetic atmosphere with gradients of
the physical quantities along the optical depth, both coexisting with
a global stray-light component. Results: Half of our observed
quiet-Sun region is better explained by magnetic substructure within
the resolution element. However, we cannot distinguish whether this
substructure comes from gradients of the physical parameters along the
line of sight or from horizontal gradients (across the surface). In
these pixels, a model with two magnetic components is preferred, and
we find two distinct magnetic field populations. The population with
the larger filling factor has very weak ( 150 G) horizontal fields
similar to those obtained in previous works. We demonstrate that the
field vector of this population is not constrained by the observations,
given the spatial resolution and polarimetric accuracy of our data. The
topology of the other component with the smaller filling factor is
constrained by the observations for field strengths above 250 G:
we infer hG fields with inclinations and azimuth values compatible
with an isotropic distribution. The filling factors are typically
below 30%. We also find that the flux of the two polarities is not
balanced. From the other half of the observed quiet-Sun area 50% are
two-lobed Stokes V profiles, meaning that 23% of the field of view
can be adequately explained with a single constant magnetic field
embedded in a non-magnetic atmosphere. The magnetic field vector and
filling factor are reliable inferred in only 50% based on the regular
profiles. Therefore, 12% of the field of view harbour hG fields with
filling factors typically below 30%. At our present spatial resolution,
70% of the pixels apparently are non-magnetised.

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Title: Spectroscopy at the Solar Limb: II. Are Spicules Heated to
Coronal Temperatures?
Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.; Fabbian, D.
2016SoPh..291.2281B Altcode: 2016arXiv160606132B; 2016SoPh..tmp..132B
Spicules of the so-called type II were suggested to be relevant for
coronal heating because of their ubiquity on the solar surface and
their eventual extension into the corona. We investigate whether solar
spicules are heated to transition-region or coronal temperatures and
reach coronal heights (≫6 Mm) using multiwavelength observations
of limb spicules in different chromospheric spectral lines (Ca II H,
Hε , Hα , Ca II IR at 854.2 nm, He I at 1083 nm) taken with slit
spectrographs and imaging spectrometers. We determine the line width
of spectrally resolved line profiles in individual spicules and
throughout the field of view, and estimate the maximal height that
different types of off-limb features reach. We derive estimates of
the kinetic temperature and the non-thermal velocity from the line
width of spectral lines from different chemical elements. We find that
most regular, i.e. thin and elongated, spicules reach a height of at
most about 6 Mm above the solar limb. The majority of features found
at larger heights are irregularly shaped with a significantly larger
lateral extension, of up to a few Mm, than spicules. Both individual and
average line profiles in all spectral lines show a decrease in their
line width with height above the limb with very few exceptions. The
kinetic temperature and the non-thermal velocity decrease with height
above the limb. We find no indications that the spicules in our data
reach coronal heights or transition-region or coronal temperatures.

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Title: Quiet Sun Magnetic Field Evolution Observed with Hinode SOT
and IRIS
Authors: Fischer, C. E.; Bello González, N.; Rezaei, R.
2016ASPC..504...19F Altcode:
We study two physical processes that can be commonly observed in
the quiet sun and involve temporal evolution of the magnetic field:
convective collapse and flux cancellation. The aim is to investigate
the response of the chromosphere to the magnetic events in the
photosphere below. We have calibrated and aligned a co-spatial and
co-temporal 3 hour quiet sun time series observed with the Hinode
SOT (Solar Optical Telescope) and the IRIS (Interface Region Imaging
Spectrograph) satellites. Convective collapse events are identified in
the photosphere by inverting spectropolarimetric data and searching for
magnetic field intensification, preceded by a downflow and accompanied
by the development of a bright point in Ca II H images. We find a
corresponding downflow in the low chromosphere as deduced from IRIS
Mg II k and h spectra and an ensuing oscillatory velocity pattern. We
use magnetograms in the high photosphere to study pairs of magnetic
elements involved in flux cancellation and find an increase in the
entire quasi-continuum of the IRIS Mg II k and h spectrum following
the flux cancellation process and indicating a substantial energy
deposit into the lower atmosphere.

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Title: Multiwavelength spectropolarimetric observations of an
Ellerman bomb
Authors: Rezaei, R.; Beck, C.
2015A&A...582A.104R Altcode:
Context. Ellerman bombs (EBs) are enhanced emission in the wings of the
Hα line in the solar spectrum. Aims: We study the structure of
an EB in the photosphere and chromosphere. Methods: We analyze
simultaneous observations of four chromospheric lines (Hα, Ca ii H,
Ca ii IR 854 nm, and He i 1083 nm) as well as two photospheric lines
(Fe i 630 and Si i 1082.7 nm) along with high-cadence 160 and 170
nm ultraviolet (UV) continuum filtergrams. Full Stokes data from the
Helioseismic and Magnetic Imager (HMI) are used to trace the temporal
evolution of the magnetic structure. Results: We identify the
EB by excess emission in the wings of the Hα line, a brightening in
the UV continuum, and large emission peaks in the core of the two Ca
ii lines. The EB shows a blueshift in all chromospheric lines, while
no shifts are observed in the photospheric lines. The blueshift in
the chromospheric layer causes very asymmetric emission peaks in the
Ca ii H line. The photospheric Si i spectral line shows a shallower
line depth at the location of the EB. The UV continuum maps show that
the EB was substantially brighter than its surroundings for about 30
min. The continuum contrast of the EB from 170 nm to 1080 nm shows a
power-law dependency on the wavelength. The temperature enhancement
amounts to 130 K in the low photosphere and 400 K at the temperature
minimum level. This temperature excess is also seen in an LTE inversion
of the Ca ii spectra. The total thermal and radiative energy content
of the EB is about 1020 J and 1018 J in the
photosphere and chromosphere, respectively. The HMI data hints at a
photospheric magnetic flux cancellation as the driver of the EB. Conclusions: Ellerman bombs release the energy in a height range
of several pressure scale heights around the temperature minimum such
that they affect both the photosphere and the lower chromosphere.

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Title: A distinct magnetic property of the inner penumbral
boundary. Formation of a stable umbra-penumbra boundary in a sunspot
Authors: Jurčák, J.; Bello González, N.; Schlichenmaier, R.;
Rezaei, R.
2015A&A...580L...1J Altcode:
Context. A sunspot emanates from a growing pore or protospot. In order
to trigger the formation of a penumbra, large inclinations at the
outskirts of the protospot are necessary. The penumbra develops and
establishes by colonising both umbral areas and granulation. Evidence
for a unique stable boundary value for the vertical component of the
magnetic field strength, Bstablever, was found
along the umbra-penumbra boundary of developed sunspots. Aims: We
study the changing value of Bver as the penumbra forms and as
it reaches a stable state. We compare this with the corresponding value
in fully developed penumbrae. Methods: We use broadband G-band
images and spectropolarimetric GFPI/VTT data to study the evolution
of and the vertical component of the magnetic field on a forming
umbra-penumbra boundary. For comparison with stable sunspots, we also
analyse the two maps observed by Hinode/SP on the same spot after the
penumbra formed. Results: The vertical component of the magnetic
field, Bver, at the umbra-penumbra boundary increases during
penumbra formation owing to the incursion of the penumbra into umbral
areas. After 2.5 h, the penumbra reaches a stable state as shown
by the GFPI data. At this stable stage, the simultaneous Hinode/SP
observations show a Bver value comparable to that of
umbra-penumbra boundaries of fully fledged sunspots. Conclusions:
We confirm that the umbra-penumbra boundary, traditionally defined by
an intensity threshold, is also characterised by a distinct canonical
magnetic property, namely by Bverstable. During
the penumbra formation process, the inner penumbra extends
into regions where the umbra previously prevailed. Hence, in
areas where Bver&lt;Bstablever,
the magneto-convection mode operating in the umbra turns into a
penumbral mode. Eventually, the inner penumbra boundary settles at
Bverstable, which hints toward the role of
Bverstable as inhibitor of the penumbral mode
of magneto-convection.

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Title: Variation in sunspot properties between 1999 and 2014
Authors: Rezaei, R.; Beck, C.; Lagg, A.; Borrero, J. M.; Schmidt,
W.; Collados, M.
2015A&A...578A..43R Altcode:
 Aims: We study the variation in the magnetic field strength,
area, and continuum intensity of umbrae in solar cycles 23 and 24. Methods: We analyzed a sample of 374 sunspots observed from 1999
until 2014 with the Tenerife Infrared Polarimeter at the German
Vacuum Tower Telescope and the Facility InfRared Spectropolarimeter
at the Dunn Solar Telescope. The sample of field strength, area,
and intensities was used to trace any long-term or cyclic trend of
umbral properties in the last 15 years. Results: Sunspots
are systematically weaker, that is, have a weaker field strength and
stronger continuum intensity, toward the end of cycle 23 than they
had at the maximum of cycle 23. The linear trend reverses with the
onset of cycle 24. We find that the field strength decreases in the
declining phase of cycle 23 by about 112 (± 16) G yr-1,
while it increases in the rising phase of cycle 24 by about 138 (±
72) G yr-1. The umbral intensity shows the opposite trend:
the intensity increases with a rate of 0.7 (± 0.3)% of Ic
yr-1 toward the end of cycle 23 and decreases with a rate
of 3.8 (± 1.5)% of Ic yr-1 toward the maximum of
cycle 24. The distribution of the umbral maximum field strength in cycle
24 is similar to that of cycle 23, but is slightly shifted toward lower
values by about 80 G, corresponding to a possible long-term gradient in
umbral field strength of about 7 ± 4 G yr-1. If instead of
the maximum umbral field we consider the average value over the entire
umbra, the distribution shifts by about 44 Gauss. Conclusions:
The umbral brightness decreases in the rising stage of a solar cycle,
but increases from maximum toward the end of the cycle. Our results
do not indicate a drastic change of the solar cycle toward a grand
minimum in the near future.

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Title: A distinct magnetic property of the inner penumbral boundary
Authors: Jurčák, Jan; Bello Gonzalez, Nazaret; Schlichenmaier,
Rolf; Rezaei, Reza
2015arXiv150608574J Altcode:
A sunspot emanates from a growing pore or protospot. In order to
trigger the formation of a penumbra, large inclinations at the
outskirts of the protospot are necessary. The penumbra develops and
establishes by colonising both umbral areas and granulation. Evidence
for a unique stable boundary value for the vertical component of the
magnetic field strength, $B^{\rm stable}_{\rm ver}$, was found along
the umbra-penumbra boundary of developed sunspots. We use broadband
G-band images and spectropolarimetric GFPI/VTT data to study the
evolution of and the vertical component of the magnetic field on a
forming umbra-penumbra boundary. For comparison with stable sunspots,
we also analyse the two maps observed by Hinode/SP on the same spot
after the penumbra formed. The vertical component of the magnetic
field, $B_{\rm ver}$, at the umbra-penumbra boundary increases
during penumbra formation owing to the incursion of the penumbra into
umbral areas. After 2.5 hours, the penumbra reaches a stable state
as shown by the GFPI data. At this stable stage, the simultaneous
Hinode/SP observations show a $B_{\rm ver}$ value comparable to that of
umbra-penumbra boundaries of fully fledged sunspots. We confirm that
the umbra-penumbra boundary, traditionally defined by an intensity
threshold, is also characterised by a distinct canonical magnetic
property, namely by $B^{\rm stable}_{\rm ver}$. During the penumbra
formation process, the inner penumbra extends into regions where the
umbra previously prevailed. Hence, in areas where $B_{\rm ver} &lt;
B^{\rm stable}_{\rm ver}$, the magneto-convection mode operating in
the umbra turns into a penumbral mode. Eventually, the inner penumbra
boundary settles at $B^{\rm stable}_{\rm ver}$, which hints toward the
role of $B_{\rm ver}^{\rm stable}$ as inhibitor of the penumbral mode
of magneto-convection.

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Title: Three Dimensional Chromospheric Thermal Structure of Sunspot
Authors: Prasad Choudhary, Debi; Beck, Christian; Rezaei, R.
2015TESS....131201P Altcode:
We have observed sunspots using the Spectropolarimeter for infrared
and optical wavelength ranges at the Dunn Solar Telescope during
29 July to 4 August 2013. The data consists of full Stokes profiles
in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of
these Stokes spectra provides the magnetic, thermal and velocity
structure at photospheric and chromospheric heights of sunspots. In
this contribution, we present the results on the 3D thermal structure
in the super-penumbral canopy of a well rounded sunspot, derived
by a novel approach for the inversion of Ca II IR spectra. Tracing
individual fibrils in the super-penumbral canopy, we find that about
half of them form only short loops of a a few Mm length that return
to the photosphere in the close surroundings of the sunspot instead
of connecting to more remote magnetic network at the outer end of the
moat flow.

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Title: Fast Inversion of Solar Ca II Spectra
Authors: Beck, C.; Choudhary, D. P.; Rezaei, R.; Louis, R. E.
2015ApJ...798..100B Altcode: 2014arXiv1410.8451B
We present a fast (Lt1 s per profile) inversion code for solar Ca
II lines. The code uses an archive of spectra that are synthesized
prior to the inversion under the assumption of local thermodynamic
equilibrium (LTE). We show that it can be successfully applied to
spectrograph data or more sparsely sampled spectra from two-dimensional
spectrometers. From a comparison to a non-LTE inversion of the same
set of spectra, we derive a first-order non-LTE correction to the
temperature stratifications derived in the LTE approach. The correction
factor is close to unity up to log τ ~ -3 and increases to values of
2.5 and 4 at log τ = -6 in the quiet Sun and the umbra, respectively.

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Title: Evolution of magnetic field inclination in a forming penumbra
Authors: Jurčák, Jan; Bello González, Nazaret; Schlichenmaier,
Rolf; Rezaei, Reza
2014PASJ...66S...3J Altcode: 2014PASJ..tmp...93J
As a sunspot penumbra forms, the magnetic field vector at the outer
boundary of the protospot undergoes a transformation. We study the
changes of the magnetic field vector at this boundary as a penumbral
segment forms. We analyze a set of spectropolarimetric maps covering
2 hr during the formation of a sunspot in NOAA 11024. The data were
recorded with the GFPI instrument attached to the German VTT. We
observe a stationary umbra/quiet Sun boundary, where the magnetic
field becomes more horizontal with time. The magnetic field inclination
increases by 5°, reaching a maximum value of about 59°. The maximum
inclination coincides with the onset of filament formation. In time,
the penumbra filaments become longer and the penumbral bright grains
protrude into the umbra, where the magnetic field is stronger and
more vertical. Consequently, we observe a decrease in the magnetic
field inclination at the boundary as the penumbra grows. In summary,
in order to initiate the formation of the penumbra, the magnetic
field at the umbral (protospot) boundary becomes more inclined. As
the penumbra grows, the umbra/penumbra boundary migrates inwards,
and at this boundary the magnetic field turns more vertical again,
while it remains inclined in the outer penumbra.

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Title: Comparison of inversion codes for polarized line formation
in MHD simulations. I. Milne-Eddington codes
Authors: Borrero, J. M.; Lites, B. W.; Lagg, A.; Rezaei, R.; Rempel, M.
2014A&A...572A..54B Altcode: 2014arXiv1409.3376B
Milne-Eddington (M-E) inversion codes for the radiative transfer
equation are the most widely used tools to infer the magnetic field
from observations of the polarization signals in photospheric and
chromospheric spectral lines. Unfortunately, a comprehensive comparison
between the different M-E codes available to the solar physics
community is still missing, and so is a physical interpretation of their
inferences. In this contribution we offer a comparison between three
of those codes (VFISV, ASP/HAO, and HeLIx+). These codes are
used to invert synthetic Stokes profiles that were previously obtained
from realistic non-grey three-dimensional magnetohydrodynamical (3D MHD)
simulations. The results of the inversion are compared with each other
and with those from the MHD simulations. In the first case, the M-E
codes retrieve values for the magnetic field strength, inclination and
line-of-sight velocity that agree with each other within σB
≤ 35 (Gauss), σγ ≤ 1.2°, and σv ≤
10 m s-1, respectively. Additionally, M-E inversion codes
agree with the numerical simulations, when compared at a fixed optical
depth, within σB ≤ 130 (Gauss), σγ ≤ 5°,
and σv ≤ 320 m s-1. Finally, we show that
employing generalized response functions to determine the height at
which M-E codes measure physical parameters is more meaningful than
comparing at a fixed geometrical height or optical depth. In this case
the differences between M-E inferences and the 3D MHD simulations
decrease to σB ≤ 90 (Gauss), σγ ≤ 3°,
and σv ≤ 90 m s-1.

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Title: Three Dimensional Chromospheric Temperature Structure of
Sunspot
Authors: Choudhary, D. P.; Beck, C.; Rezaei, R.
2014AGUFMSH41B4132C Altcode:
We have observed sunspots using the Spectropolarimeter for infrared
and optical wavelength ranges at the Dunn Solar Telescope during
29 July to 4 August 2013. The data consists of full Stokes profiles
in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of
these Stokes spectra provides the magnetic, thermal and velocity
structure at photospheric and chromospheric heights of sunspots. In
this contribution, we present the results on the 3D thermal structure
in the super-penumbral canopy of a well rounded sunspot, derived
by a novel approach for the inversion of Ca II IR spectra. Tracing
individual fibrils in the super-penumbral canopy, we find that about
half of them form only short loops of a a few Mm length that return
to the photosphere in the close surroundings of the sunspot instead
of connecting to more remote magnetic network at the outer end of the
moat flow.

---------------------------------------------------------
Title: A Three-dimensional View of the Thermal Structure in a
Super-penumbral Canopy
Authors: Beck, C.; Choudhary, D. P.; Rezaei, R.
2014ApJ...788..183B Altcode: 2014arXiv1405.1473B
We investigate the three-dimensional (3D) thermal topology in a
super-penumbral canopy of an active region (AR). We derive temperature
stratifications in the AR by an inversion of the Ca II IR line at 854.2
nm, assuming local thermal equilibrium. We find that about half of the
radially oriented fibrils in the super-penumbral canopy form short,
low-lying (h &lt; 1 Mm) loops in the 3D temperature cube. These closed
loops connect from bright grains in or close to the penumbra to the
photosphere a few Mms away from the sunspot. The other half of the
fibrils monotonically rise with distance from the sunspot. Many of
the fibrils show a central dark core and two lateral brightenings in
line-core intensity images. The corresponding velocity image shows
fibrils that are as wide as the fibrils seen in intensity without a
lateral substructure. Additionally, we study a feature from a different
class of structures without prominent mass flows. Its 3D topology is
formed by two parallel, closed loops that connect patches of opposite
polarity. We present evidence that the inverse Evershed flow into the
sunspot in the lower chromosphere is the consequence of siphon flows
along short loops that connect photospheric foot points. The dark-cored
structure of the chromospheric fibrils cannot have a convective origin
because of their location above regular granulation. The dark core
most likely results from an opacity difference between the central
axis and the lateral edges caused by the significant flow speed along
the fibrils.

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Title: Properties of sunspot umbrae observed in cycle 24
Authors: Kiess, Christoph; Rezaei, Reza; Schmidt, Wolfgang
2014A&A...565A..52K Altcode: 2014arXiv1402.2881K
 Aims: There is an ongoing debate whether the solar activity cycle
is overlaid with a long-term decline that may lead to another grand
minimum in the near future. We used the size, intensity, and magnetic
field strength of sunspot umbrae to compare the present cycle 24 with
the previous one. Methods: We used data of the Helioseismic and
Magnetic Imager on board the Solar Dynamics Observatory and selected
all sunspots between May 2010 and October 2012, using one image per
day. We created two subsets of this dataset with a manual tracking
algorithm, both without duplication. One contains each sunspot (910
umbrae within 488 spots) and was used to analyze the distribution of
umbral areas, selected with an automated thresholding method. The other
subset contains 205 fully evolved sunspots. We estimated their magnetic
field and the total magnetic flux and discuss the relations between
umbral size, minimum continuum intensity, maximum field strength,
and total magnetic flux. Results: We find non-linear relations
between umbral minimum intensity and size and between maximum magnetic
field strength and size. The field strength scales linearly with
the intensity and the umbral size scales roughly linearly with the
total magnetic flux, while the size and field strength level off with
stronger flux. When separated into hemispheres and averaged temporally,
the southern umbrae show a temporal increase in size and the northern
umbrae remain constant. We detected no temporal variation in the umbral
mean intensity. The probability density function of the umbral area
in the ascending phase of the current solar cycle is similar to that
of the last solar cycle. Conclusions: From our investigation
of umbral area, magnetic field, magnetic flux, and umbral intensity
of the sunspots of the rising phase of cycle 24, we do not find a
significant difference to the previous cycle, and hence no indication
for a long-term decline of solar activity.

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Title: Comparison of sunspot properties in cycles 23 and 24
Authors: Rezaei, Reza; Schmidt, Wolfgang; Beck, Christian
2014cosp...40E2740R Altcode:
Sunspots form by coalescence of small-scale magnetic elements and
pores in magnetic flux emergence areas. By observing recently formed
sunspots just after their initial growth and before substantial decay,
one samples a magnetic signal which has been least disturbed by
granulation. Properties of the emergence events have a direct impact
on the results. Failed active regions, e.g. the ones which cannot form
a sunspot, are a clear example: in several cases, they would harbor
enough magnetic flux to form a small sunspot but fail to do so. Another
way to evaluate secular variations of flux emergence events is to
quantify long-term trends of sunspot properties. The 11-year solar
magnetic activity cycle has been known for centuries. During this
time the activity level changed dramatically from the Maunder minimum
(1650-1700) to the Modern maximum in mid 20-th century. The extended
minimum of the last solar cycle alerted solar physicist about possible
long-term variation in the solar magnetic activity. While some argue
that the Sun was unusually active in mid 20-th century, others find
it unusually inactive now. This caused speculations whether the solar
activity cycle is overlaid with a long-term decline that may lead to
another grand minimum in the near future. Some extrapolations predicted
that there will be no sunspots in the next cycle. Detailed observations
of sunspot properties were performed only in the last few cycles. Such
spectropolarimetric observations enable us to accurately derive the
magnetic field strengths of spots and their physical properties. We
present measurements of sunspot intensity, area, and magnetic field
strength and compare the present cycle 24 with the previous one. We
analyze a sample of about 400 sunspots observed from 1999 until 2014
with the Tenerife Infrared Polarimeter at the German Vacuum Tower
Telescope as well as with the Facility Infrared Spectropolarimeter
of the Dunn Solar Telescope of the NSO. The magnetic field strength
is derived from the Zeeman splitting of the Stokes-V signal in a
near-infrared spectral line. We take into account the center-to-limb
variation of umbral intensities and apply a correction to compensate the
variation of magnetic sensitivity of different spectral lines. There is
a systematic trend for sunspots in the late stage of the solar cycle
to have a smaller maximum magnetic field strength than those at the
start of the cycle. At the same time, the continuum intensity of umbrae
gradually increases with phase of the solar cycle, while the umbral
area does not show any trend above the statistical variance. Sunspots
in cycle 24 show higher field strengths and lower continuum intensities
than those at the end of cycle 23. There is a slight decrease in field
strength and an increase in intensity as a long-term trend across the
cycles. We find that the cyclic variations are dominating over any
long-term trend that continues across cycles. From our investigation of
umbral area, magnetic field, and umbral intensity of the sunspots of
cycle 23 and the first half of cycle 24, we do not find a significant
indication of variation in either sunspot physical properties. This
indicates that if a long-term trend exists, its amplitude is smaller
than the cyclic variation of umbral properties.

---------------------------------------------------------
Title: Three Dimensional Chromospheric Structure of Sunspot
Authors: Choudhary, Debi Prasad; Rezaei, Reza; Beck, Christian
2014cosp...40E.544C Altcode:
We have observed sunspots using the Spectropolarimeter for infrared
and optical wavelength ranges at the Dunn Solar Telescope during 29
July to 4 August 2013. The data consists of full Stokes profiles
in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion
of these Stokes spectra provides the magnetic, thermal and velocity
structure at photospheric and chromospheric heights of sunspots. In this
contribution, we present the first results on the 3D thermal structure
in the super-penumbral canopy of a well rounded sunspot, derived
by a novel approach for the inversion of Ca II IR spectra. Tracing
individual fibrils in the super-penumbral canopy, we find that about
half of them form only short loops of a a few Mm length that return
to the photosphere in the close surroundings of the sunspot instead
of connecting to more remote magnetic network at the outer end of the
moat flow.

---------------------------------------------------------
Title: Thermodynamic fluctuations in solar photospheric
three-dimensional convection simulations and observations
Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.;
Rezaei, R.
2013A&A...557A.109B Altcode: 2013arXiv1306.6093B
Context. Numerical three-dimensional (3D) radiative
(magneto-)hydrodynamical [(M)HD] simulations of solar convection
are nowadays used to understand the physical properties of the solar
photosphere and convective envelope, and, in particular, to determine
the Sun's photospheric chemical abundances. To validate this approach,
it is important to check that no excessive thermodynamic fluctuations
arise as a consequence of the partially incomplete treatment of
radiative transfer causing radiative damping that is too modest. Aims: We investigate the realism of the thermodynamics in recent
state-of-the-art 3D convection simulations of the solar atmosphere
carried out with the Stagger code. Methods: We compared the
characteristic properties of several Fe i lines (557.6 nm, 630 nm, 1565
nm) and one Si i line at 1082.7 nm in solar disc-centre observations
of different spatial resolution with spectra synthesized from 3D
convection simulations. The observations were taken with ground-based
(Echelle spectrograph, Göttingen Fabry-Pérot Interferometer (GFPI),
POlarimetric LIttrow Spectrograph, Tenerife Infrared Polarimeter, all
at the Vacuum Tower Telescope on Tenerife) and space-based instruments
(Hinode/Spectropolarimeter). We degraded the synthetic spectra to
the spatial resolution of the observations, based on the distribution
of the continuum intensity Ic. We estimated the spectral
degradation to be applied to the simulation results by comparing atlas
spectra with averaged observed spectra. In addition to deriving a set
of line parameters directly from the intensity profiles, we used the
SIR (Stokes Inversion based on Response functions) code to invert
the spectra. Results: The spatial degradation kernels yield
a similar generic spatial stray-light contamination of about 30%
for all instruments. The spectral stray light inside the different
spectrometers is found to be between 2% and 20%. Most of the line
parameters from the observational data are matched by the degraded
HD simulation spectra. The inversions predict a macroturbulent
velocity vmac below 10 m s-1 for the HD
simulation spectra at full spatial resolution, whereas they yield
vmac ≲ 1000 m s-1 at a spatial resolution of
0.″3. The temperature fluctuations in the inversion of the degraded
HD simulation spectra do not exceed those from the observational data
(of the order of 100-200 K rms for -2 ⪉ log τ500 nm
⪉ -0.5). The comparison of line parameters in spatially averaged
profiles with the averaged values of line parameters in spatially
resolved profiles indicates a significant change in (average) line
properties on a spatial scale between 0.″13 and 0.″3. 
Conclusions: Up to a spatial resolution of 0.″3 (GFPI spectra),
we find no indications of excessive thermodynamic fluctuations
in the 3D HD simulation. To definitely confirm that simulations
without spatial degradation contain fully realistic thermodynamic
fluctuations requires observations at even higher spatial resolution
(i.e. &lt;0.″13). Appendices A and B are available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Can spicules be detected at disc centre in broad-band Ca ii
H filter imaging data?
Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.
2013A&A...556A.127B Altcode: 2013arXiv1306.5199B
Context. Recently, a possible identification of type II spicules
in broad-band (full-width at half-maximum (FWHM) of ~0.3 nm) filter
imaging data in Ca ii H on the solar disc was reported. Aims:
We estimate the formation height range contributing to broad-band and
narrow-band filter imaging data in Ca ii H to investigate whether
spicules can be detected in such observations at the centre of the
solar disc. Methods: We applied spectral filters of FWHMs
from 0.03 nm to 1 nm to observed Ca ii H line profiles to simulate
Ca imaging data. We used observations across the limb to estimate the
relative intensity contributions of off-limb and on-disc structures. We
compared the synthetic Ca filter imaging data with intensity maps of
Ca spectra at different wavelengths and temperature maps at different
optical depths obtained by an inversion of these spectra. In addition,
we determined the intensity response function for the wavelengths
covered by the filters of different FWHM. Results: In broad-band
(FWHM = 0.3 nm) Ca imaging data, the intensity emitted off the solar
limb is about 5% of the intensity at disc centre. For a 0.3-nm-wide
filter centred at the Ca ii H line core, up to about one third of
the off-limb intensity comes from emission in Hɛ. On the disc, only
about 10 to 15% of the intensity transmitted through a broad-band
filter comes from the line-core region between the H1
minima (396.824 to 396.874 nm). No traces of elongated fibrillar
structures are visible in the synthetic Ca broad-band imaging data at
disc centre, in contrast to the line-core images of the Ca spectra. The
intensity-weighted response function for a 0.3-nm-wide filter centred at
the Ca ii H line core peaks at about log τ ~ -2 (z ~ 200 km). Relative
contributions from atmospheric layers above 800 km are about 10%. The
inversion results suggest that the slightly enhanced emission around
the photospheric magnetic network in broad-band Ca imaging data
is caused by a thermal canopy at a height of about 600 km. 
Conclusions: Broad-band (~0.3 nm) Ca ii H imaging data do not trace
upper chromospheric structures such as spicules in observations at the
solar disc because of the too small relative contribution of the line
core to the total wavelength-integrated filter intensity. The faint haze
around network elements in broad-band Ca imaging observations at disc
centre presumably traces thermal canopies in the vicinity of magnetic
flux concentrations instead. Appendix A is available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>

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Title: The energy of waves in the photosphere and lower
chromosphere. IV. Inversion results of Ca II H spectra
Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.
2013A&A...553A..73B Altcode: 2013arXiv1302.6936B
Context. Most semi-empirical static one-dimensional (1D) models of
the solar atmosphere in the magnetically quiet Sun (QS) predict an
increase in temperature at chromospheric layers. Numerical simulations
of the solar chromosphere with a variable degree of sophistication,
i.e. from 1D to three-dimensional (3D) simulations; assuming local
thermal equilibrium (LTE) or non-LTE (NLTE), on the other hand,
only yielded an increase in the brightness temperature without any
stationary increase in the gas temperature. Aims: We investigate
the thermal structure in the solar chromosphere as derived from an
LTE inversion of observed Ca ii H spectra in QS and active regions
(ARs). Methods: We applied an inversion strategy based on the
SIR (Stokes inversion by response functions) code to Ca ii H spectra to
obtain 1D temperature stratifications. We investigated the temperature
stratifications on differences between magnetic and field-free regions
in the QS, and on differences between QS and ARs. We determined the
energy content of individual calcium bright grains (BGs) as specific
candidates of chromospheric heating events. We compared observed with
synthetic NLTE spectra to estimate the significance of the LTE inversion
results. Results: The fluctuations of observed intensities yield
a variable temperature structure with spatio-temporal rms fluctuations
below 100 K in the photosphere and between 200 and 300 K in the QS
chromosphere. The average temperature stratification in the QS does not
exhibit a clear chromospheric temperature rise, unlike the AR case. We
find a characteristic energy content of about 7 × 1018 J
for BGs that repeat with a cadence of about 160 s. The precursors of BGs
have a vertical extent of about 200 km and a horizontal extent of about
1 Mm. The comparison of observed with synthetic NLTE profiles partly
confirms the results of the LTE inversion that the solar chromosphere
in the QS oscillates between an atmosphere in radiative equilibrium and
one with a moderate chromospheric temperature rise. Two-dimensional x -
z temperature maps exhibit nearly horizontal canopy-like structures with
an extent of a few Mm around photospheric magnetic field concentrations
at a height of about 600 km. Conclusions: The large difference
between QS regions and ARs and the better match of AR and NLTE reference
spectra suggest that magnetic heating processes are more important
than commonly assumed. The temperature fluctuations in QS derived by
the LTE inversion do not suffice on average to maintain a stationary
chromospheric temperature rise. The spatially and vertically resolved
information on the temperature structure allows one to investigate
in detail the topology and evolution of the thermal structure in the
lower solar atmosphere. Appendix A is available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>

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Title: On the Effects of the SDO Orbital Motion on the HMI Vector
Magnetic Field Measurements
Authors: Fleck, B.; Centeno, R.; Cheung, M.; Couvidat, S.; Hayashi,
K.; Rezaei, R.; Steiner, O.; Straus, T.
2013enss.confE.145F Altcode:
In a previous study we have investigated the magnetic field diagnostics
potential of SDO/HMI. We have used the output of high-resolution
3D, time-dependent, radiative magneto-hydrodynamics simulations to
calculate Stokes profiles for the Fe I 6173 Å line. From these we
constructed Stokes filtergrams using a representative set of HMI filter
response functions. The magnetic field vector (x,y) and line-of-sight
Doppler velocities V(x,y) were determined from these filtergrams using
a simplified version of the HMI magnetic field processing pipeline,
and the reconstructed magnetic field (x,y) and line-of-sight velocity
V(x,y) were compared to the actual magnetic field (x,y,z) and vertical
velocity V0(x,y,z) in the simulations. The present investigation expands
this analysis to include the effects of the significant orbital motions
of SDO, which, given the limited wavelength range of the HMI filter
profiles, affects the outer wing measurements and therefore might impact
the magnetic field measurements. We find that the effects of the orbital
movement of SDO are noticeable, in particular for the strongest fields
(B &gt; 3 kG) and the maximum wavelength shift of 5.5 km/s (3.5 km/s
orbital movement + 2 km/s solar rotation). Saturation effects for strong
fields (B &gt; 3 kG) are already visible for wavelength shifts of 3.2
km/s (orbital movement, disk center). The measurements of inclination
and vertical velocity are more robust. Compared to other factors of
uncertainty in the inversion of HMI Stokes measurements the orbital
movement is not a major concern or source of error.

---------------------------------------------------------
Title: The energy of waves in the photosphere and lower
chromosphere. III. Inversion setup for Ca II H spectra in local
thermal equilibrium
Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.
2013A&A...549A..24B Altcode: 2012arXiv1209.6194B
Context. The Ca II H line is one of the strongest lines in the solar
spectrum, and it provides continuous information on the solar atmosphere
from the photosphere to the lower chromosphere. Aims: We describe
an inversion approach that reproduces observed Ca II H spectra by
assuming local thermal equilibrium (LTE). Methods: We developed
an inversion strategy based on the SIR code that reproduces Ca II H
spectra in the LTE approximation. The approach uses a two-step procedure
with an archive of pre-calculated spectra to fit the line core and a
subsequent iterative modification to improve the fit mainly in the line
wing. Simultaneous spectra in the 630 nm range can optionally be used
to fix the continuum temperature. The method retrieves one-dimensional
(1D) temperature stratifications while neglecting lateral radiative
transport. Line-of-sight velocities are included post facto with
an empirical approach. Results: An archive of about 300 000
pre-calculated spectra is more than sufficient to reproduce the line
core of observed Ca II H spectra both in the quiet Sun and in active
regions. The subsequent iterative adjustment of the thermodynamical
stratification matches observed and best-fit spectra to a level of about
0.5% of Ic in the line wing and about 1% of Ic
in the line core. Conclusions: The successful application of
the LTE inversion strategy suggests that inversion schemes based on
pre-calculated spectra allow a reliable and relatively fast retrieval of
solar properties from observed chromospheric spectra. The approach can
be easily extended to a 1D non-LTE (NLTE) case by a simple exchange
of the pre-calculated archive spectra. Using synthetic NLTE spectra
from numerical three-dimensional (3D) simulations instead will
finally allow one to extend the approach from the static 1D-case
to dynamical atmosphere models, including the complete 3D radiative
transport. The animation is available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

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Title: Chromospheric Multi-Wavelength Observations near the Solar
Limb: Techniques and Prospects
Authors: Beck, C.; Rezaei, R.
2012ASPC..463..257B Altcode: 2012arXiv1203.2114B
Observations of chromospheric spectral lines near and beyond the
solar limb provide information on the solar chromosphere without any
photospheric contamination. For ground-based observations near and
off the limb with real-time image correction by adaptive optics (AO),
some technical requirements have to be met, such as an AO lock point
that is independent of the location of the field of view observed by
the science instruments, both for 1D and 2D instruments. We show how
to obtain simultaneous AO-corrected spectra in Ca II H, Hα, Ca II IR
at 854 nm, and He I at 1083 nm with the instrumentation at the German
Vacuum Tower Telescope in Izaña, Tenerife. We determined the spectral
properties of an active-region macrospicule inside the field of view in
the four chromospheric lines, including its signature in polarization
in He I at 1083 nm. Compared to the line-core intensities, the Doppler
shifts of the lines change on a smaller spatial scale in the direction
parallel to the limb, suggesting the presence of coherent rotating
structures or the passage of upwards propagating helical waves on the
surfaces of expanding flux tubes.

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Title: Comparing Simultaneous Measurements of two High-Resolution
Imaging Spectropolarimeters: The `Göttingen' FPI@VTT and CRISP@SST
Authors: Bello González, N.; Bellot Rubio, L. R.; Ortiz, A.; Rezaei,
R.; Rouppe van der Voort, L.; Schlichenmaier, R.
2012ASPC..463..251B Altcode: 2012arXiv1204.1023B
In July 2009, the leading spot of the active region NOAA11024 was
observed simultaneously and independently with the ‘Göttingen’
FPI at VTT and CRISP at SST, i.e., at two different sites,
telescopes, instruments and using different spectral lines. The data
processing and data analysis have been carried out independently
with different techniques. Maps of physical parameters retrieved
from 2D spectro-polarimetric data observed with ‘Göttingen’
FPI and CRISP show an impressive agreement. In addition, the
‘Göttingen’ FPI maps also exhibit a notable resemblance with
simultaneous TIP (spectrographic) observations. The consistency in the
results demonstrates the excellent capabilities of these observing
facilities. Besides, it confirms the solar origin of the detected
signals and the reliability of FPI-based spectro-polarimeters.

---------------------------------------------------------
Title: The energy of waves in the photosphere and lower
chromosphere. II. Intensity statistics
Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.
2012A&A...544A..46B Altcode: 2012arXiv1206.1759B
Context. The energy source powering the solar chromosphere is still
undetermined, but leaves its traces in observed intensities. 
Aims: We investigate the statistics of the intensity distributions
as a function of the wavelength for Ca ii H and the Ca ii IR line at
854.2 nm to estimate the energy content in the observed intensity
fluctuations. Methods: We derived the intensity variations
at different heights of the solar atmosphere, as traced by the line
wings and line cores of the two spectral lines. We converted the
observed intensities to absolute energy units employing reference
profiles calculated in non-local thermal equilibrium (NLTE). We also
converted the intensity fluctuations to corresponding brightness
temperatures assuming LTE. Results: The root-mean-square
(rms) fluctuations of the emitted intensity are about 0.6 (1.2)
W m-2 ster-1 pm-1 near the core of
the Ca ii IR line at 854.2 nm (Ca ii H), corresponding to relative
intensity fluctuations of about 20% (30%). For the line wing,
we find rms values of about 0.3 W m-2 ster-1
pm-1 for both lines, corresponding to relative fluctuations
below 5%. The relative rms values show a local minimum for wavelengths
forming at a height of about 130 km, but otherwise increase smoothly
from the wing to the core, i.e., from photosphere to chromosphere. The
corresponding rms brightness temperature fluctuations are below 100 K
for the photosphere and up to 500 K in the chromosphere. The skewness
of the intensity distributions is close to zero in the outer line
wing and positive throughout the rest of the line spectrum, owing to
the frequent occurrence of high-intensity events. The skewness shows a
pronounced local maximum at locations with photospheric magnetic fields
for wavelengths in-between those of the line wing and the line core
(z ≈ 150-300 km), and a global maximum at the very core (z ≈ 1000
km) for both magnetic and field-free locations. Conclusions:
The energy content of the intensity fluctuations is insufficient to
create a chromospheric temperature rise that would be similar to the
one in most reference models of the solar atmosphere. The increase in
the rms fluctuations with height indicates the presence of upwardly
propagating acoustic waves of increasing oscillation amplitude. The
intensity and temperature variations indicate that there is a
clear increase in dynamical activity from photosphere towards the
chromosphere, but the variations fall short of the magnitude predicted
by fully dynamical chromospheric models by a factor of about five. The
enhanced skewness between the photosphere and lower solar chromosphere
at magnetic locations is indicative of a mechanism that acts solely
on magnetized plasma. Appendices are available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>

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Title: Recent Advances in the Exploration of the Small-Scale Structure
of the Quiet Solar Atmosphere: Vortex Flows, the Horizontal Magnetic
Field, and the Stokes- V Line-Ratio Method
Authors: Steiner, O.; Rezaei, R.
2012ASPC..456....3S Altcode: 2012arXiv1202.4040S
We review (i) observations and numerical simulations of vortical flows
in the solar atmosphere and (ii) measurements of the horizontal magnetic
field in quiet Sun regions. First, we discuss various manifestations of
vortical flows and emphasize the role of magnetic fields in mediating
swirling motion created near the solar surface to the higher layers
of the photosphere and to the chromosphere. We reexamine existing
simulation runs of solar surface magnetoconvection with regard to
vortical flows and compare to previously obtained results. Second,
we reviews contradictory results and problems associated with
measuring the angular distribution of the magnetic field in quiet Sun
regions. Furthermore, we review the Stokes-V-amplitude ratio method
for the lines Fe i λλ 630.15 and 630.25 nm. We come to the conclusion
that the recently discovered two distinct populations in scatter plots
of this ratio must not bee interpreted in terms of “uncollapsed”
and “collapsed” fields but stem from weak granular magnetic fields
and weak canopy fields located at the boundaries between granules and
the intergranular space. Based on new simulation runs, we reaffirm
earlier findings of a predominance of the horizontal field components
over the vertical one, particularly in the upper photosphere and at
the base of the chromosphere.

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Title: On The Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, Bernard; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno,
R.; Cheung, M.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.;
Viticchie, B.
2012AAS...22020701F Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk
in the Fe I absorption line at 6173 Å. We use the output of three
high-resolution 3D, time-dependent, radiative magneto-hydrodynamics
simulations (two based on the MURaM code, one on the CO5BOLD
code) to calculate Stokes profiles for the Fe I 6173 Å line
for snapshots of a sunspot, a plage area and an enhanced network
region. Stokes filtergrams are constructed for the 6 nominal HMI
wavelengths by multiplying the Stokes profiles with a representative
set of HMI filter response functions. The magnetic field vector B(x,y)
and line-of-sight Doppler velocities V(x,y) are determined from these
filtergrams using a simplified version of the HMI magnetic field
processing pipeline. Finally, the reconstructed magnetic field B(x,y)
and line-of-sight velocity V(x,y) are compared to the actual magnetic
field B0(x,y,z) and vertical velocity V0(x,y,z)
in the simulations.

---------------------------------------------------------
Title: Detection of Vortex Tubes in Solar Granulation from
Observations SUNRISE
Authors: Steiner, O.; Franz, M.; González, N. B.; Nutto, C.; Rezaei,
R.; Pillet, V. M.; Bonet, J. A.; Iniesta, J. C. d. T.; Domingo, V.;
Solanki, S. K.; Knölker, M.; Schmidt, W.; Barthol, P.; Gandorfer, A.
2012ASPC..455...35S Altcode:
We investigated a time series of continuum intensity maps and
Dopplergrams of granulation in a very quiet solar region at the disk
center, recorded with the Imaging Magnetograph eXperiment (IMaX)
on board the balloon-borne solar observatory SUNRISE. We find that
granules frequently show substructure in the form of lanes composed of
a leading bright rim and a trailing dark edge, which move together
from the boundary of a granule into the granule itself. We find
strikingly similar events in synthesized intensity maps from an ab
initio numerical simulation of solar surface convection. We conclude
that these granular lanes are the visible signature of (horizontally
oriented) vortex tubes. The characteristic optical appearance of vortex
tubes at the solar surface is explained. This paper is a summary and
update of the results previously presented in Steiner et al. (2010).

---------------------------------------------------------
Title: Variation Of Sunspot Properties Between 1999 And 2011
Authors: Schmidt, Wolfgang; Rezaei, R.; Beck, C.
2012AAS...22020608S Altcode:
We study the magnetic field and the umbral intensity of sunspots for
the period 1999 to 2011. We analyze full Stokes spectra of 183 spots
observed with the Tenerife Infrared Polarimeter at the German Vacuum
Tower Telescope on Tenerife. We derive the magnetic field strength
in the umbra from the Zeeman splitting of a near-infrared spectral
line. This procedure eliminates the influence of non-magnetic stray
light from the spot surroundings. The systematic decrease of umbral
magnetic field strength observed during the declining phase of cycle
23 does not continue into cycle 24, instead, we observe a significant
increase of magnetic field strength in spots of the new cycle. This
indicates that the observed variations of the magnetic field strength
are dominated by a cyclic effect rather than by a long-term trend.

---------------------------------------------------------
Title: On the Formation of Penumbrae as Observed with the German
VTT SOHO/MDI, and SDO/HMI
Authors: Schlichenmaier, R.; Rezaei, R.; González, N. B.
2012ASPC..455...61S Altcode: 2011arXiv1102.0965S
Solar magnetic fields are generated in the solar interior and pop up
at the solar surface to form active regions. As the magnetic field
appears on the surface, it forms various structures like small magnetic
elements, pores, and sunspots. The nature of these formation processes
is largely unknown. In this contribution we elaborate on the formation
of sunspots and particularly on the formation of penumbrae. We report
on observations that we obtained at the German Vacuum Tower Telescope
(VTT) on July 4, 2009 on the formation of the spot in AR 11024. This
data set is complemented with data from the Michelson Doppler Imager
(MDI) aboard SOHO, which offers an entire time coverage. Moreover, the
evolution of AR 11024 is compared with a particular event of penumbra
formation in AR 11124 around November 13, 2010, using intensity images
from the Helioseismic and Magnetic Imager (HMI) onboard SDO. We conclude
that two processes contribute to the increase of the magnetic flux of a
sunspot: (1) merging pores, and (2) emerging bipoles of which the spot
polarity migrates towards and merges with the spot. As the penumbra
forms, the area, magnetic flux, and maximum field strength in the
umbra stay constant or increase slightly, i.e., the formation of the
penumbra is associated with flux emergence and flux increase of the
proto-spot. If two pores merge or if a pore merges with a proto-spot
a light bridge is created. This initial light bridge dissolves in the
further evolution.

---------------------------------------------------------
Title: Variation in sunspot properties between 1999 and 2011 as
observed with the Tenerife Infrared Polarimeter
Authors: Rezaei, R.; Beck, C.; Schmidt, W.
2012A&A...541A..60R Altcode: 2012arXiv1203.1444R
 Aims: We study the variation in the magnetic field strength
and the umbral intensity of sunspots during the declining phase of
the solar cycle No. 23 and in the beginning of cycle No. 24. 
Methods: We analyze a sample of 183 sunspots observed from 1999 until
2011 with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum
Tower Telescope (VTT). The magnetic field strength is derived from the
Zeeman splitting of the Stokes-V signal in one near-infrared spectral
line, either Fe i 1564.8 nm, Fe i 1089.6 nm, or Si i 1082.7 nm. This
avoids the effects of the unpolarized stray light from the field-free
quiet Sun surroundings that can affect the splitting seen in Stokes-I
in the umbra. The minimum umbral continuum intensity and umbral area are
also measured. Results: We find that there is a systematic trend
for sunspots in the late stage of the solar cycle No. 23 to be weaker,
i.e., to have a smaller maximum magnetic field strength than those at
the start of the cycle. The decrease in the field strength with time of
about 94 Gyr-1 is well beyond the statistical fluctuations
that would be expected because of the larger number of sunspots close
to cycle maximum (14 Gyr-1). In the same time interval, the
continuum intensity of the umbra increases with a rate of 1.3 (±0.4)%
of Ic yr-1, while the umbral area does not show
any trend above the statistical variance. Sunspots in the new cycle
No. 24 show higher field strengths and lower continuum intensities
than those at the end of cycle No. 23, interrupting the trend. 
Conclusions: Sunspots have an intrinsically weaker field strength and
brighter umbrae at the late stages of solar cycles compared to their
initial stages, without any significant change in their area. The abrupt
increase in field strength in sunspots of the new cycle suggests that
the cyclic variations are dominating over any long-term trend that
continues across cycles. We find a slight decrease in field strength
and an increase in intensity as a long-term trend across the cycles.

---------------------------------------------------------
Title: On the Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, B.; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno,
R.; Cheung, M.; Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.;
Viticchie, B.
2012decs.confE.104F Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk
in the Fe I absorption line at 6173 Å. We use the output of three
high-resolution 3D, time-dependent, radiative magneto-hydrodynamics
simulations (two based on the MURaM code, one on the CO5BOLD code)
to calculate Stokes profiles Fi(λ,x,y; i=I, V, Q, U) for the Fe I
6173 Å line for snapshots of a sunspot, a plage area and an enhanced
network region. Stokes filtergrams are constructed for the 6 nominal
HMI wavelengths by multiplying the Stokes profiles with a representative
set of HMI filter response functions. The magnetic field vector B(x,y)
and line-of-sight Doppler velocities V(x,y) are determined from these
filtergrams using a simplified version of the HMI magnetic field
processing pipeline. Finally, the reconstructed magnetic field B(x,y)
and line-of-sight velocity V(x,y) are compared to the actual magnetic
field B0(x,y,z) and vertical velocity V0(x,y,z) in the simulations.

---------------------------------------------------------
Title: The formation of sunspot penumbra. Magnetic field properties
Authors: Rezaei, R.; Bello González, N.; Schlichenmaier, R.
2012A&A...537A..19R Altcode: 2011arXiv1111.3189R
 Aims: We study the magnetic flux emergence and formation of
a sunspot penumbra in the active region NOAA 11024. Methods:
We simultaneously observed the Stokes parameters of the photospheric
iron lines at 1089.6 nm with the TIP and 617.3 nm with the GFPI
spectropolarimeters along with broad-band images using G-band and
Ca ii K filters at the German VTT. The photospheric magnetic field
vector was reconstructed from an inversion of the measured Stokes
profiles. Using the AZAM code, we converted the inclination from
line-of-sight (LOS) to the local reference frame (LRF). Results:
Individual filaments are resolved in maps of magnetic parameters. The
formation of the penumbra is intimately related to the inclined
magnetic field. No penumbra forms in areas with strong magnetic field
strength and small inclination. Within 4.5 h observing time, the LRF
magnetic flux of the penumbra increases from 9.7 × 1020
to 18.2 × 1020 Mx, while the magnetic flux of the umbra
remains constant at ~3.8 × 1020 Mx. Magnetic flux in the
immediate surroundings is incorporated into the spot, and new flux is
supplied via small flux patches (SFPs), which on average have a flux
of 2-3 × 1018 Mx. The spot's flux increase rate of 4.2 ×
1016 Mx s-1 corresponds to the merging of one
SFP per minute. We also find that, during the formation of the spot
penumbra, a) the maximum magnetic field strength of the umbra does not
change; b) the magnetic neutral line keeps the same position relative
to the umbra; c) the new flux arrives on the emergence side of the
spot while the penumbra forms on the opposite side; d) the average
LRF inclination of the light bridges decreases from 50° to 37°;
and e) as the penumbra develops, the mean magnetic field strength
at the spot border decreases from 1.0 to 0.8 kG. Conclusions:
The SFPs associated with elongated granules are the building blocks of
structure formation in active regions. During the sunspot formation,
their contribution is comparable to the coalescence of pores. Besides a
set of critical parameters for the magnetic field, a quiet environment
in the surroundings is important for penumbral formation. As remnants
of trapped granulation between merging pores, the light bridges are
found to play a crucial role in the formation process. They seem to
channel the magnetic flux through the spot during its formation. Light
bridges are also the locations where the first penumbral filaments form.

---------------------------------------------------------
Title: Stray-light contamination and spatial deconvolution of
slit-spectrograph observations
Authors: Beck, C.; Rezaei, R.; Fabbian, D.
2011A&A...535A.129B Altcode: 2011arXiv1109.2421B
Context. Stray light caused by scattering on optical surfaces
and in the Earth's atmosphere degrades the spatial resolution
of observations. Whereas post-facto reconstruction techniques
are common for 2D imaging and spectroscopy, similar options for
slit-spectrograph data are rarely applied. Aims: We study the
contribution of stray light to the two channels of the POlarimetric
LIttrow Spectrograph (POLIS) at 396 nm and 630 nm as an example of
a slit-spectrograph instrument. We test the performance of different
methods of stray-light correction and spatial deconvolution to improve
the spatial resolution post-facto. Methods: We model the stray
light as having two components: a spectrally dispersed component and
a "parasitic" component of spectrally undispersed light caused by
scattering inside the spectrograph. We used several measurements to
estimate the two contributions: a) observations with a (partly) blocked
field of view (FOV); b) a convolution of the FTS spectral atlas; c)
imaging of the spider mounting in the pupil plane; d) umbral profiles;
and e) spurious polarization signal in telluric spectral lines. The
measurements with a partly blocked FOV in the focal plane allowed us
to estimate the spatial point spread function (PSF) of POLIS and the
main spectrograph of the German Vacuum Tower Telescope (VTT). We then
used the obtained PSF for a deconvolution of both spectroscopic and
spectropolarimetric data and investigated the effect on the spectra. Results: The parasitic contribution can be directly and accurately
determined for POLIS, amounting to about 5% (0.3%) of the (continuum)
intensity at 396 nm (630 nm). The spectrally dispersed stray light is
less accessible because of its many contributing sources. We estimate
a lower limit of about 10% across the full FOV for the dispersed stray
light from umbral profiles. In quiet Sun regions, the stray-light level
from the close surroundings (d &lt; 2”) of a given spatial point is
about 20%. The stray light reduces to below 2% at a distance of 20”
from a lit area for both POLIS and the main spectrograph. The spatial
deconvolution using the PSF obtained improves the spatial resolution
and increases the contrast, with a minor amplification of noise. Conclusions: A two-component model of the stray-light contributions
seems to be sufficient for a basic correction of observed spectra. The
instrumental PSF obtained can be used to model the off-limb stray light,
to determine the stray-light contamination accurately for observation
targets with large spatial intensity gradients such as sunspots,
and also to improve the spatial resolution of observations post-facto.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Spectroscopy at the solar
limb. I. Average off-limb profiles and Doppler shifts of Ca II H.
Authors: Beck, C.; Rezaei, R.
2011yCat..35310173B Altcode: 2011yCat..35319173B
The data used in the publication consist of a set of CaII H
spectra taken near and beyond the solar limb on 25/08/2009 at UT
08:43-09:15. The data were obtained with the POLIS instrument at the
German VTT. The wavelength range covers the core and the blue wing of
the CaII H line from 396.332nm to 396.969nm in 326 steps of 1.96pm. The
observations were done by moving the solar image across the slit of
the spectrograph with a step width of 0.3-arcsec, yielding in total
134 slit spectra of 326 wavelength points on 244 CCD rows along the
slit. The spatial sampling along the slit was 0.3-arcsec. The center
of the field-of-view was located at (x,y)=(+37",+920") relative to the
center of the solar disk. The data have been corrected for stray-light
with the methods described in the article and are normalized to
the continuum intensity on disc center. The corresponding data file
data.fit is organized as floating array (x,y,wavelength)=(134x244x326)
pixels. (1 data file).

---------------------------------------------------------
Title: On the Magnetic-Field Diagnostics Potential of SDO/HMI
Authors: Fleck, B.; Hayashi, K.; Rezaei, R.; Vitas, N.; Centeno, R.;
Couvidat, S.; Fischer, C.; Steiner, O.; Straus, T.; Viticchie, B.
2011sdmi.confE..74F Altcode:
The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO) is designed to study oscillations and the magnetic
field in the solar photosphere. It observes the full solar disk in the
Fe I 6173 absorption line. We use the output of two high-resolution 3D,
time-dependent, radiative magneto-hydrodynamics simulations (one based
on the MURAM code, the other one on the COBOLD code) to calculate
Stokes profiles for the Fe I 6173 line for a snapshot of a plage
region and a snapshot of an enhanced network region. After spatially
degrading the Stokes profiles to HMI resolution, they are multiplied
by a representative set of HMI filter response functions and Stokes
filtergrams are constructed for the 6 nominal HMI wavelengths. The
magnetic field vector and line-of-sight Doppler velocities are
determined from these filtergrams using a simplified version of the HMI
magnetic field processing pipeline. Finally, the reconstructed magnetic
field is compared to the actual magnetic field in the simulation.

---------------------------------------------------------
Title: The formation of a penumbra as observed with the German VTT
and SoHO/MDI
Authors: Schlichenmaier, Rolf; González, Nazaret Bello; Rezaei, Reza
2011IAUS..273..134S Altcode: 2010arXiv1009.4457S
The generation of magnetic flux in the solar interior and its transport
to the outer solar atmosphere will be in the focus of solar physics
research for the next decades. One key-ingredient is the process
of magnetic flux emergence into the solar photosphere, and the
reorganization to form the magnetic phenomena of active regions
like sunspots and pores. On July 4, 2009, we observed a region
of emerging magnetic flux, in which a proto-spot without penumbra
forms a penumbra within some 4.5 hours. This process is documented
by multi-wavelength observations at the German VTT: (a) imaging, (b)
data with high resolution and temporal cadence acquired in Fe I 617.3
nm with the 2D imaging spectropolarimter GFPI, and (c) scans with the
slit based spectropolarimeter TIP in Fe I 1089.6 nm. MDI contiuum maps
and magnetograms are used to follow the formation of the proto-spot, and
the subsequent evolution of the entire active region. During the
formation of the penumbra, the area and the magnetic flux of the spot
increases. The additional magnetic flux is supplied by the adjacent
region of emerging magnetic flux: As emerging bipole separate, the
poles of the spot polarity migrate towards the spot, and finally merge
with it. As more and more flux is accumulated, a penumbra forms. From
inversions we infer maps for the magnetic field and the Doppler velocity
(being constant along the line-of-sight). We calculate the magnetic flux
of the forming spot and of the bipole footpoints that merge with the
proto-spot. We witness the onset of the Evershed flow and the associated
enhance of the field inclination as individual penumbral filaments
form. Prior to the formation of individual penumbral sectors we detect
the existence of `counter' Evershed flows. These in-flows turn into
the classical radial Evershed outflows as stable penumbra segments form.

---------------------------------------------------------
Title: Spectroscopy at the solar limb. I. Average off-limb profiles
and Doppler shifts of Ca II H
Authors: Beck, C. A. R.; Rezaei, R.
2011A&A...531A.173B Altcode: 2011arXiv1106.0646B
 Aims: We present constraints on the thermodynamical structure
of the chromosphere from ground-based observations of the Ca
ii H line profile near and off the solar limb. Methods:
We obtained a slit-spectrograph data set of the Ca ii H line with
a high signal-to-noise ratio in a field of view extending 20”
across the limb. We analyzed the spectra for the characteristic
properties of average and individual off-limb spectra. We used
various tracers of the Doppler shifts, such as the location of the
absorption core, the ratio of the two emission peaks H2V
and H2R, and intensity images at a fixed wavelength. Results: The average off-limb profiles show a smooth variation
with increasing limb distance. The line width increases up to a
height of about 2 Mm above the limb. The profile shape is fairly
symmetric with nearly identical H2V and H2R
intensities; at a height of 5 Mm, it changes into a single Gaussian
without emission peaks. We find that all off-limb spectra show large
Doppler shifts that fluctuate on the smallest resolved spatial
scales. The variation is more prominent in cuts parallel to the
solar limb than on those perpendicular to it. As far as individual
structures can be unequivocally identified at our spatial resolution,
we find a specific relation between intensity enhancements and Doppler
shifts: elongated brightenings are often flanked all along their
extension by velocities in opposite directions. Conclusions:
The average off-limb spectra of Ca ii H present a good opportunity
to test static chromospheric atmosphere models because they lack the
photospheric contribution that is present in disk-center spectra. We
suggest that the observed relation between intensity enhancements
and Doppler shifts could be caused by waves propagating along the
surfaces of flux tubes: an intrinsic twist of the flux tubes or a wave
propagation inclined to the tube axis would cause a helical shape of
the Doppler excursions, visible as opposite velocity at the sides of
the flux tube. Spectroscopic data allow one to distinguish this from
a sausage-mode oscillation where the maximum Doppler shift and the
tube axis would coincide. Appendices are available in electronic
form at <A href="http://www.andaa.org">http://www.andaa.org</A>The
Data set is available in electronic form at the CDS via
anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A173">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A173</A>

---------------------------------------------------------
Title: Detection of Vortex Tubes in Solar Granulation from
Observations with SUNRISE
Authors: Steiner, O.; Franz, M.; Bello González, N.; Nutto, Ch.;
Rezaei, R.; Martínez Pillet, V.; Bonet Navarro, J. A.; del Toro
Iniesta, J. C.; Domingo, V.; Solanki, S. K.; Knölker, M.; Schmidt,
W.; Barthol, P.; Gandorfer, A.
2010ApJ...723L.180S Altcode: 2010arXiv1009.4723S
We have investigated a time series of continuum intensity maps and
corresponding Dopplergrams of granulation in a very quiet solar region
at the disk center, recorded with the Imaging Magnetograph eXperiment
(IMaX) on board the balloon-borne solar observatory SUNRISE. We
find that granules frequently show substructure in the form of lanes
composed of a leading bright rim and a trailing dark edge, which move
together from the boundary of a granule into the granule itself. We
find strikingly similar events in synthesized intensity maps from an
ab initio numerical simulation of solar surface convection. From cross
sections through the computational domain of the simulation, we conclude
that these granular lanes are the visible signature of (horizontally
oriented) vortex tubes. The characteristic optical appearance of vortex
tubes at the solar surface is explained. We propose that the observed
vortex tubes may represent only the large-scale end of a hierarchy of
vortex tubes existing near the solar surface.

---------------------------------------------------------
Title: The role of emerging bipoles in the formation of a sunspot
penumbra
Authors: Schlichenmaier, R.; Bello González, N.; Rezaei, R.; Waldmann,
T. A.
2010AN....331..563S Altcode: 2010arXiv1003.1313S
The generation of magnetic flux in the solar interior and its transport
from the convection zone into the photosphere, the chromosphere,
and the corona will be in the focus of solar physics research for
the next decades. With 4 m class telescopes, one plans to measure
essential processes of radiative magneto-hydrodynamics that are needed
to understand the nature of solar magnetic fields. One key-ingredient
to understand the behavior of solar magnetic field is the process
of flux emergence into the solar photosphere, and how the magnetic
flux reorganizes to form the magnetic phenomena of active regions
like sunspots and pores. Here, we present a spectropolarimetric and
imaging data set from a region of emerging magnetic flux, in which a
proto-spot without penumbra forms a penumbra. During the formation of
the penumbra the area and the magnetic flux of the spot increases. First
results of our data analysis demonstrate that the additional magnetic
flux, which contributes to the increasing area of the penumbra, is
supplied by the region of emerging magnetic flux. We observe emerging
bipoles that are aligned such that the spot polarity is closer to the
spot. As an emerging bipole separates, the pole of the spot polarity
migrates towards the spot, and finally merges with it. We speculate
that this is a fundamental process, which makes the sunspot accumulate
magnetic flux. As more and more flux is accumulated a penumbra forms
and transforms the proto-spot into a full-fledged sunspot.

---------------------------------------------------------
Title: The formation of a sunspot penumbra
Authors: Schlichenmaier, R.; Rezaei, R.; Bello González, N.; Waldmann,
T. A.
2010A&A...512L...1S Altcode:
Context. The formation of a penumbra is crucial for our understanding
of solar magnetism, but it has not been observed in detail. 
Aims: We aim to enhance our knowledge of how a sunspot penumbra forms
and how sunspots grow in size. Methods: We present a data
set of the active region NOAA 11024 acquired at the German VTT with
speckle-reconstructed images in the G-band and Ca ii K. The data set
includes spectropolarimetric profiles from GFPI in Fe i 617.3 nm and
TIP in Fe i 1089.6 nm. Results: On 2009 July 4, at 08:30 UT,
a leading spot without penumbra and pores of opposite polarity were
present in the active region. For the next 4:40 h, we observed the
formation of a penumbra in the leading spot at a cadence of 5 images per
second. We produced speckle reconstructed images of 0.3 arcsec spatial
resolution or better, interrupted by one large gap of 35 min and a
few more small gaps of about 10 min. The leading spot initially has a
size of 230 arcsec2 with only a few penumbral filaments and
then grows to a size of 360 arcsec2. The penumbra forms in
segments, and it takes about 4 h until it encircles half of the umbra,
on the side opposite the following polarity. On the side towards the
following polarity, elongated granules mark a region of magnetic flux
emergence. Conclusions: This ongoing emergence appears to prevent
a steady penumbra from forming on this side. While the penumbra forms,
the umbral area is constant; i.e., the increase in the total spot
area is caused exclusively by the growth of the penumbra. From this
we conclude that the umbra has reached an upper size limit and that
any new magnetic flux that joins the spot is linked to the process of
penumbral formation. Movies are only available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: The Horizontal Magnetic Field of the Quiet Sun: Numerical
Simulations in Comparison to Observations with Hinode
Authors: Steiner, O.; Rezaei, R.; Schlichenmaier, R.; Schaffenberger,
W.; Wedemeyer-Böhm, S.
2009ASPC..415...67S Altcode: 2009arXiv0904.2030S
Three-dimensional magnetohydrodynamic simulations of the surface layers
of the Sun intrinsically produce a predominantly horizontal magnetic
field in the photosphere. This is a robust result in the sense that it
arises from simulations with largely different initial and boundary
conditions for the magnetic field. While the disk-center synthetic
circular and linear polarization signals agree with measurements from
Hinode, their center-to-limb variation sensitively depends on the
height variation of the horizontal and the vertical field component
and they seem to be at variance with the observed behavior.

---------------------------------------------------------
Title: The energy of waves in the photosphere and lower
chromosphere. I. Velocity statistics
Authors: Beck, C.; Khomenko, E.; Rezaei, R.; Collados, M.
2009A&A...507..453B Altcode: 2009arXiv0905.1011B
Context: Acoustic waves are one of the primary suspects besides magnetic
fields for the chromospheric heating process to temperatures above
radiative equilibrium (RE). Aims: We derived the mechanical
wave energy as seen in line-core velocities on disc centre to obtain
a measure of mechanical energy flux with height for a comparison
with the energy requirements in a semi-empirical atmosphere model,
the Harvard-Smithsonian reference atmosphere (HSRA). Methods: We
analyzed a 1-hour time series and a large-area map of Ca II H spectra
on the traces of propagating waves. We analyzed the velocity statistics
of several spectral lines in the wing of Ca II H, and the line-core
velocity of Ca II H. We converted the velocity amplitudes into volume
(∝ ρ v^2) and mass energy densities (∝ v^2). For comparison, we
used the increase of internal energy (∝ R ρ Δ T) necessary to lift
a RE atmosphere to the HSRA temperature stratification. Results:
We find that the velocity amplitude grows in agreement with linear
wave theory and thus slower with height than predicted from energy
conservation. The mechanical energy of the waves above around z ~ 500 km
is insufficient to maintain on a long-term average the chromospheric
temperature rise in the semi-empirical HSRA model. The intensity
variations of the Ca line core (z ~ 1000 km) can, however, be traced
back to the velocity variations of the lowermost forming spectral line
considered (z ~ 250 km). Conclusions: The chromospheric intensity,
and hence, (radiation) temperature variations are seen to be induced by
passing waves originating in the photosphere. The wave energy is found
to be insufficient to maintain the temperature stratification of the
semi-empirical HSRA model above 500 km. We will in a following paper of
this series investigate the energy contained in the intensity variations
to see if the semi-empirical model is appropriate for the spectra.

---------------------------------------------------------
Title: The magnetic flux of the quiet Sun internetwork as observed
with the Tenerife infrared polarimeter
Authors: Beck, C.; Rezaei, R.
2009A&A...502..969B Altcode: 2009arXiv0903.3158B
Context: Observations made with the spectropolarimeter onboard the
HINODE satellite have detected abundant horizontal magnetic fields in
the internetwork quiet Sun. Aims: We compare the results for the
horizontal fields obtained at 630 nm with ground-based observations at
1.56 μm, where the sensitivity to magnetic fields is higher than in the
visible. Methods: We obtained 30-s integrated spectropolarimetric
data of the quiet Sun on disc centre during a period of extremely
stable and good seeing. The data have a rms noise in polarization of
around 2 × 10-4 of the continuum intensity. The low noise
level allows the spectra to be inverted with the SIR code. We compare
the inversion results with proxies to determine the magnetic flux. Results: We confirm the presence of the horizontal fields in the
quiet Sun internetwork as reported for the satellite data, including
voids without linear polarization signal that extend over an area of
a few granules. Voids in the circular polarization signal are only
of granular scale. More than 60% of the surface show polarization
signals of above four times the rms noise level. We find that the
total magnetic flux contained in the more inclined to horizontal fields
(γ &gt; 45°) is lower by a factor of around 2 than that of the less
inclined fields. The proxies for flux determination are strongly
affected by the thermodynamic state of the atmosphere, and hence,
seem to be unreliable. Conclusions: During spells of good seeing
conditions, adaptive optics can render ground-based slit-spectrograph
observations at a 70-cm telescope equivalent to the seeing-free
space-based data of half-meter class telescopes. We suggest that the
difference in the ratio of horizontal to transversal flux between the
ground-based infrared data and the satellite-based visible data is
due to the different formation heights of the respective spectral
lines. We emphasize that the true amount of magnetic flux cannot
be derived directly from the spectra. For purely horizontal flux,
one would need its vertical extension that has to be estimated by
explicit modeling, using the observed spectra as boundary conditions,
or be taken from MHD simulations. Time-series of the evolution of the
magnetic flux and chromospheric diagnostics are needed to address its
possible contribution to chromospheric heating. Appendices A and
B are only available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: Freeware Solutions for Spectropolarimetric Data Reduction
Authors: Paletou, F.; Rezaei, R.; Léger, L.
2009ASPC..405...51P Altcode:
Most of the solar physicists use very expensive software for data
reduction and visualization. We present hereafter a reliable freeware
solution based on the Python language. This is made possible by the
association of the latter with a small set of additional libraries
developed in the scientific community. It provides then a very powerful
and economical alternative to other interactive data languages. Although
it can also be used for any kind of post-processing of data, we
demonstrate the capabilities of such a set of freeware tools using
THéMIS observations of the second solar spectrum.

---------------------------------------------------------
Title: Temporal Evolution of Magnetic Elements
Authors: Rezaei, R.; Schlichenmaier, R.; Schmidt, W.; Beck, C.
2009ASPC..405..195R Altcode: 2007arXiv0712.0234R
We study the structure and evolution of the magnetic field of the quiet
Sun by investigating weak spectro-polarimetric signals. To this end,
we observed a quiet region close to the disk center with the German VTT
in Tenerife, July 07, 2006. We recorded 38 scans of the same area. Each
scan was eight arcsec wide and observed within about 100 seconds. We
used POLIS to simultaneously observe Stokes profiles of the neutral
iron lines at 630.15 and 630.25 nm, the Stokes-I profile of the Ca
II H line at 396.8 nm, and a continuum speckle channel at 500 nm. We
witness two examples of magnetic flux cancellation of small-scale
opposite-polarity patches, followed by an enhanced chromospheric
emission. In each case, the two opposite-polarity patches gradually
became smaller and, within a few minutes, the smaller one completely
disappeared. The larger patch also diminished significantly. We provide
evidence for a cancellation scenario in the photosphere which leaves
minor traces at the chromospheric level.

---------------------------------------------------------
Title: Magnetic coupling of the solar photosphere and chromosphere
Authors: Rezaei, R.
2008PhDT........16R Altcode:
The solar surface outside sunspots and active regions, i.e., the quiet
Sun, shows the ubiquitous pattern of granulation in the photosphere. The
quiet solar photosphere harbors small-scale magnetic structures inside
and between granules. This thesis presents thermodynamic properties
of the small-scale magnetic flux concentrations in the quiet Sun using
high spatial and temporal resolution observations along with numerical
simulations. Spectral line profiles of the Fe I 630 nm pair and Ca
II H were used to trace the photospheric and chromospheric layers of
the magnetic elements. In the presence of magnetic field spectral
lines split and are polarized via the Zeeman effect. The difference of
a spectral line profile, measured in left and right circular polarized
light, is a Stokes-V profile with two lobes. In the absence of any
gradients of velocity or magnetic field along the line of sight,
Stokes-V profiles are anti-symmetric. The different area of the
two lobes, the Stokes-V area asymmetry, provides information about
the gradients of the magnetic and velocity fields along the line of
sight. Comparing high resolution spectropolarimetric data with
synthetic maps of a 3D MHD simulation, we found several magnetic
elements in the photosphere showing a central region of negative
Stokes-V area asymmetry surrounded by a peripheral region with
larger positive asymmetry. This finding was the first observational
confirmation of the existence of a sharp boundary layer between magnetic
elements and their immediate surroundings. Such boundary layers had
been theoretically predicted ten years go. Furthermore, we found for
the first time two Stokes-V profiles of the Fe I 630 nm line pair in a
single spectrum showing opposite magnetic polarities. These two lines
form in slightly different layers, so they trace the magnetic field in
different geometrical heights. The temporal evolution of these
profiles showed a magnetic flux cancellation, suggesting a magnetic
reconnection in the photosphere. A 1D numerical model that reproduced
the observed profiles was interpreted as an indication for the magnetic
reconnection. To verify the existence of vigorous gradients in the
magnetic and thermal properties of the atmosphere as suggested by the
cancellation event, extreme cases of asymmetry in Stokes-V profiles,
i.e., profiles with only one lobe instead of two, were found in a large
sample of data. We find strong evidence for concentrated magnetic
flux structures with sharp boundaries sustaining a strong gradient
or a discontinuity in thermodynamic parameters. In other words,
we find current sheets in boundary layers which separate magnetic
from non-magnetic plasma. This supports the existence of structured
magnetic entities like flux tubes. The second part of this thesis
was devoted to study the thermal structure of the chromosphere and its
relation to underlying photospheric magnetic flux concentrations. The
Ca II H line is one of the strongest absorption lines in the visible
solar spectrum. Generally, there are two emission peaks (H2v and H2r)
on either side of the line core which form in the chromosphere. The
existence of the emission peaks implies a temperature rise in the
chromosphere. Parts of the line outside these two emission peaks mainly
form in the photosphere. The integrated intensity in a 0.1 nm band
around the core which contains the two emission peaks is a measure for
the chromospheric emission. The minimum chromospheric emission is as
large as half of the average emission. We found that the chromospheric
emission in excess to the minimum emission scales with the magnetic flux
density. To establish a relation between the chromospheric emission
and temperature, we looked for the Ca II H line profiles without any
emission peak at H2v and H2r wavelengths. A quarter of the observed
calcium profiles did not show any emission peak. A comparison of
these profiles with synthesized calcium profiles of one-dimensional
time-independent models provides strong indication for chromospheric
temperatures cooler than that of the spatially and temporally averaged
quiet Sun. Our results suggest that a large fraction of the solar
chromosphere consists of hot plasma, T &gt; 10000 K. Beside this, there
is a smaller fraction which is cooler than the underlying photosphere,
T &lt; 5000 K, but cannot be much cooler than, e.g., 2000 K.

---------------------------------------------------------
Title: On the Possible Sources of Chromospheric Heating
Authors: Beck, C.; Collados, M. Vera; Khomenko, E.; Rezaei, R.
2008ESPM...12.2.14B Altcode:
The chromospheric temperature rise to values above the photospheric
temperature cannot be due to radiative energy transport alone. We will
outline different possibilities for the additional energy transport in
the solar atmosphere by processes that require (or exclude) the presence
of magnetic fields. We will discuss which of them could be identified
and studied in detail using current data. To find the signature of
the different heating processes and derive quantitative estimates
of their efficiency, we analyzed simultaneous spectropolarimetric
observations of photospheric magnetic fields (@630 nm) and intensity
spectra of the chromospheric Ca II H line (396 nm). The mechanical
energy flux at several height layers was derived from the velocity
amplitudes of propagating acoustic waves seen in different spectral
lines. The enhancement of chromospheric (radiation) temperature above
the radiative equilibrium values was taken from an inversion of the
Ca II H spectra with the SIR code assuming local thermal equilibrium
(LTE) and complete redistribution (CRD). We compare the obtained energy
values with each other and with the energy requirements demanded by
theoretical/semi-empirical atmospheric models. We find that
the most important agent of chromospheric heating are propagating
(magneto-)acoustic waves, which suffice to explain the brightenings in
Ca II H spectra and their corresponding temperature enhancements. The
energy contained in these intensity variations of the Ca II H line,
however, is found to be insufficient to maintain a full-time and
full-volume "hot" chromosphere. Additional energy transport mechanisms
without a signature in the Ca II H spectra are thus necessary. Finally,
we will outline which improvements are to be expected with future
observations of higher quality (spatial resolution, enhanced
polarimetric sensitivity, temporal cadence, other spectral lines)
to be achieved with new ground-based telescopes like GREGOR or EST.

---------------------------------------------------------
Title: Reversal-free Ca II H Profiles: a Challenge for Solar
Chromosphere Modeling in Quiet Inter-Network
Authors: Rezaei, R.; Bruls, J.; Beck, C.; Schmidt, W.; Kalkofen, W.;
Schlichenmaier, R.
2008ESPM...12.2.13R Altcode:
There is no agreement on the thermal structure of the solar
chromosphere. While results of the CO observations and 3D MHD
simulations suggest very cool structures in the upper atmosphere,
SUMER observations of UV spectral lines is interpreted as signature
of a full-time hot chromosphere. We tried to look for cool structures
in the solar chromosphere. We observed the intensity profile of the
Ca II H line in a quiet Sun region close to the disk center at the
German Vacuum Tower Telescope. We analyze over 10^5 line profiles from
inter-network regions. For comparison with the observed profiles, we
synthesize spectra for a variety of model atmospheres with a non local
thermodynamic equilibrium(NLTE) radiative transfer code. A fraction of
about 25% of the observed Ca II H line profiles do not show a measurable
emission peak in H2v and H2r wavelength bands (reversal-free). All of
the chosen model atmospheres with a temperature rise fail to reproduce
such profiles. On the other hand, the synthetic calcium profile of a
model atmosphere that has a monotonic decline of the temperature with
height shows a reversal-free profile that has much lower intensities
than any observed line profile. The observed reversal-free profiles,
at a spatial resolution of 1 arcs and a temporal resolution of 5 s,
indicate the existence of cool patches in the interior of chromospheric
network cells, at least for short time intervals. Our finding is not
only in conflict with a full-time hot chromosphere (e.g., FALC), but
also with a very cool chromosphere as found in some dynamic simulations.

---------------------------------------------------------
Title: The Horizontal Internetwork Magnetic Field: Numerical
Simulations in Comparison to Observations with Hinode
Authors: Steiner, O.; Rezaei, R.; Schaffenberger, W.; Wedemeyer-Böhm,
S.
2008ESPM...12.3.22S Altcode:
Observations with the Hinode space observatory led to the discovery
of predominantly horizontal magnetic fields in the photosphere of the
quiet internetwork region. Here we investigate realistic numerical
simulations of the surface layers of the Sun with respect to horizontal
magnetic fields and compute the corresponding polarimetric response
in the Fe I 630 nm line pair. We find a local maximum in the mean
strength of the horizontal field component at a height of around 500
km in the photosphere, where, depending on the initial state or the
boundary condition, it surpasses the vertical component by a factor
of 2.0 or 5.6. From the synthesized Stokes profiles, we derive a mean
horizontal field component that is 1.6 or 4.3 times stronger than
the vertical component, depending on the initial state or the boundary
condition. This is a consequence of both the intrinsically stronger flux
density of and the larger area occupied by the horizontal fields. We
find that convective overshooting expels horizontal fields to the upper
photosphere, making the Poynting flux positive in the photosphere,
whereas it is negative in the convectively unstable layer below it.

---------------------------------------------------------
Title: Reversal-free Ca II H profiles: a challenge for solar
chromosphere modeling in quiet inter-network
Authors: Rezaei, R.; Bruls, J. H. M. J.; Schmidt, W.; Beck, C.;
Kalkofen, W.; Schlichenmaier, R.
2008A&A...484..503R Altcode: 2008arXiv0804.2325R
Aims: We study chromospheric emission to understand the temperature
stratification in the solar chromosphere. Methods: We observed
the intensity profile of the Ca II H line in a quiet Sun region close
to the disk center at the German Vacuum Tower Telescope. We analyze
over 105 line profiles from inter-network regions. For
comparison with the observed profiles, we synthesize spectra for a
variety of model atmospheres with a non local thermodynamic equilibrium
(NLTE) radiative transfer code. Results: A fraction of about
25% of the observed Ca II H line profiles do not show a measurable
emission peak in H2v and H2r wavelength bands
(reversal-free). All of the chosen model atmospheres with a temperature
rise fail to reproduce such profiles. On the other hand, the synthetic
calcium profile of a model atmosphere that has a monotonic decline of
the temperature with height shows a reversal-free profile that has much
lower intensities than any observed line profile. Conclusions:
The observed reversal-free profiles indicate the existence of cool
patches in the interior of chromospheric network cells, at least
for short time intervals. Our finding is not only in conflict with a
full-time hot chromosphere, but also with a very cool chromosphere as
found in some dynamic simulations.

---------------------------------------------------------
Title: The Horizontal Internetwork Magnetic Field: Numerical
Simulations in Comparison to Observations with Hinode
Authors: Steiner, O.; Rezaei, R.; Schaffenberger, W.; Wedemeyer-Böhm,
S.
2008ApJ...680L..85S Altcode: 2008arXiv0801.4915S
Observations with the Hinode space observatory led to the discovery
of predominantly horizontal magnetic fields in the photosphere of the
quiet internetwork region. Here we investigate realistic numerical
simulations of the surface layers of the Sun with respect to horizontal
magnetic fields and compute the corresponding polarimetric response
in the Fe I 630 nm line pair. We find a local maximum in the mean
strength of the horizontal field component at a height of around 500
km in the photosphere, where, depending on the initial state or the
boundary condition, it surpasses the vertical component by a factor
of 2.0 or 5.6. From the synthesized Stokes profiles, we derive a mean
horizontal field component that is 1.6 or 4.3 times stronger than
the vertical component, depending on the initial state or the boundary
condition. This is a consequence of both the intrinsically stronger flux
density of and the larger area occupied by the horizontal fields. We
find that convective overshooting expels horizontal fields to the upper
photosphere, making the Poynting flux positive in the photosphere,
whereas the Poynting flux is negative in the convectively unstable
layer below it.

---------------------------------------------------------
Title: The signature of chromospheric heating in Ca II H spectra
Authors: Beck, C.; Schmidt, W.; Rezaei, R.; Rammacher, W.
2008A&A...479..213B Altcode: 2007arXiv0712.2538B
Context: The heating process that balances the solar chromospheric
energy losses has not yet been determined. Conflicting views exist on
the source of the energy and the influence of photospheric magnetic
fields on chromospheric heating. Aims: We analyze a 1-h time
series of cospatial Ca II H intensity spectra and photospheric
polarimetric spectra around 630 nm to derive the signature of the
chromospheric heating process in the spectra and to investigate its
relation to photospheric magnetic fields. The data were taken in a
quiet Sun area on disc center without strong magnetic activity. Methods: We have derived several characteristic quantities of Ca
II H to define the chromospheric atmosphere properties. We study the
power of the Fourier transform at different wavelengths and the phase
relations between them. We perform local thermodynamic equilibrium (LTE)
inversions of the spectropolarimetric data to obtain the photospheric
magnetic field, once including the Ca intensity spectra. Results:
We find that the emission in the Ca II H line core at locations
without detectable photospheric polarization signal is due to waves
that propagate in around 100 s from low forming continuum layers in
the line wing up to the line core. The phase differences of intensity
oscillations at different wavelengths indicate standing waves for ν
&lt; 2 mHz and propagating waves for higher frequencies. The waves
steepen into shocks in the chromosphere. On average, shocks are both
preceded and followed by intensity reductions. In field-free regions,
the profiles show emission about half of the time. The correlation
between wavelengths and the decorrelation time is significantly higher
in the presence of magnetic fields than for field-free areas. The
average Ca II H profile in the presence of magnetic fields contains
emission features symmetric to the line core and an asymmetric
contribution, where mainly the blue H2V emission peak is increased
(shock signature). Conclusions: We find that acoustic waves
steepening into shocks are responsible for the emission in the Ca II H
line core for locations without photospheric magnetic fields. We suggest
using wavelengths in the line wing of Ca II H, where LTE still applies,
to compare theoretical heating models with observations. Appendices
are only available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: Freeware solutions for spectropolarimetric data reduction
Authors: Paletou, F.; Rezaei, R.; Leger, L.
2007arXiv0712.1522P Altcode:
Most of the solar physicists use very expensive software for
data reduction and visualization. We present hereafter a reliable
freeware solution based on the Python language. This is made possible
by the association of the latter with a small set of additional
libraries developed in the scientific community. It provides then
a very powerful and economical alternative to other interactive data
languages. Although it can also be used for any kind of post-processing
of data, we demonstrate the capabities of such a set of freeware tools
using THeMIS observations of the second solar spectrum.

---------------------------------------------------------
Title: Hinode observations reveal boundary layers of magnetic elements
in the solar photosphere
Authors: Rezaei, R.; Steiner, O.; Wedemeyer-Böhm, S.; Schlichenmaier,
R.; Schmidt, W.; Lites, B. W.
2007A&A...476L..33R Altcode: 2007arXiv0711.0408R
Aims:We study the structure of the magnetic elements in network-cell
interiors. Methods: A quiet Sun area close to the disc centre was
observed with the spectro-polarimeter of the Solar Optical Telescope
on board the Hinode space mission, which yielded the best spatial
resolution ever achieved in polarimetric data of the Fe I 630 nm line
pair. For comparison and interpretation, we synthesize a similar data
set from a three-dimensional magneto-hydrodynamic simulation. Results: We find several examples of magnetic elements, either
roundish (tube) or elongated (sheet), which show a central area of
negative Stokes-V area asymmetry framed or surrounded by a peripheral
area with larger positive asymmetry. This pattern was predicted
some eight years ago on the basis of numerical simulations. Here,
we observationally confirm its existence for the first time. Conclusions: We gather convincing evidence that this pattern of
Stokes-V area asymmetry is caused by the funnel-shaped boundary of
magnetic elements that separates the flux concentration from the
weak-field environment. On this basis, we conclude that electric
current sheets induced by such magnetic boundary layers are common in
the photosphere.

---------------------------------------------------------
Title: Variation of the Stokes-V area asymmetry across magnetic
elements
Authors: Rezaei, R.; Steiner, O.; Wedemeyer-Böhm, S.; Schlichenmaier,
R.; Lites, B. W.
2007AN....328..706R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Opposite magnetic polarity of two photospheric lines in single
spectrum of the quiet Sun
Authors: Rezaei, R.; Schlichenmaier, R.; Schmidt, W.; Steiner, O.
2007A&A...469L...9R Altcode: 2007arXiv0704.3135R
Aims:We study the structure of the photospheric magnetic field of the
quiet Sun by investigating weak spectro-polarimetric signals. Methods: We took a sequence of Stokes spectra of the Fe I 630.15
nm and 630.25 nm lines in a region of quiet Sun near the disk
center, using the POLIS spectro-polarimeter at the German VTT on
Tenerife. The line cores of these two lines form at different heights
in the atmosphere. The 3σ noise level of the data is about 1.8
× 10-3 I_c. Results: We present co-temporal and
co-spatial Stokes-V profiles of the Fe I 630 nm line pair, where
the two lines show opposite polarities in a single spectrum. We
compute synthetic line profiles and reproduce these spectra with a
two-component model atmosphere: a non-magnetic component and a magnetic
component. The magnetic component consists of two magnetic layers with
opposite polarity: the upper one moves upwards while the lower one moves
downward. In-between, there is a region of enhanced temperature. Conclusions: The Stokes-V line pair of opposite polarity in a single
spectrum can be understood as a magnetic reconnection event in the
solar photosphere. We demonstrate that such a scenario is realistic,
but the solution may not be unique.

---------------------------------------------------------
Title: Relation between photospheric magnetic field and chromospheric
emission
Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C. A. R.; Bruls,
J. H. M. J.; Schmidt, W.
2007A&A...466.1131R Altcode: 2007astro.ph..1896R
Aims: We investigate the relationship between the photospheric
magnetic field and the emission of the mid chromosphere of the
Sun. Methods: We simultaneously observed the Stokes parameters
of the photospheric iron line pair at 630.2 nm and the intensity
profile of the chromospheric Ca II H line at 396.8 nm in a quiet
Sun region at a heliocentric angle of 53°. Various line parameters
have been deduced from the Ca II H line profile. The photospheric
magnetic field vector has been reconstructed from an inversion of
the measured Stokes profiles. After alignment of the Ca and Fe maps,
a common mask has been created to define network and inter-network
regions. We perform a statistical analysis of network and inter-network
properties. The H-index is the integrated emission in a 0.1 nm band
around the Ca core. We separate a non-magnetically, Hnon,
and a magnetically, Hmag, heated component from a
non-heated component, Hco in the H-index. Results:
The average network and inter-network H-indices are equal to 12 and
10 pm, respectively. The emission in the network is correlated with
the magnetic flux density, approaching a value of H ≈ 10 pm for
vanishing flux. The inter-network magnetic field is dominated by weak
field strengths with values down to 200 G and has a mean absolute
flux density of about 11 Mx cm-2. Conclusions:
We find that a dominant fraction of the calcium emission caused by
the heated atmosphere in the magnetic network has non-magnetic origin
(Hmag≈2 pm, Hnon≈3 pm). Considering the effect
of straylight, the contribution from an atmosphere with no temperature
rise to the H-index (Hco≈6 pm) is about half of the
observed H-index in the inter-network. The H-index in the inter-network
is not correlated to any property of the photospheric magnetic field,
suggesting that magnetic flux concentrations have a negligible role
in the chromospheric heating in this region. The height range of the
thermal coupling between the photosphere and low/mid chromosphere
increases in presence of magnetic field. In addition, we demonstrate
that a poor signal-to-noise level in the Stokes profiles leads to a
significant over-estimation of the magnetic field strength.

---------------------------------------------------------
Title: Photospheric magnetic field and chromospheric emission
Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C.; Schmidt, W.
2007msfa.conf..169R Altcode: 2007astro.ph..1681R
We present a statistical analysis of network and internetwork
properties in the photosphere and the chromosphere. For the first
time we simultaneously observed (a) the four Stokes parameters of the
photospheric iron line pair at 630.2 nm and (b) the intensity profile
of the Ca H line at 396.8 nm. The vector magnetic field was inferred
from the inversion of the iron lines. We aim at an understanding of the
coupling between photospheric magnetic field and chromospheric emission.

---------------------------------------------------------
Title: The flow field in the sunspot canopy
Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C.; Bellot Rubio, L. R.
2006A&A...454..975R Altcode: 2006astro.ph..4301R
Aims.We investigate the flow field in the sunspot canopy using
simultaneous Stokes vector spectropolarimetry of three sunspots
(θ=27°, 50°, 75°) and their surroundings in visible (630.15 and
630.25 nm) and near infrared (1564.8 and 1565.2 nm) neutral iron
lines. Methods: .To calibrate the Doppler shifts, we compare
an absolute velocity calibration using the telluric O_2-line at
630.20 nm and a relative velocity calibration using the Doppler
shift of Stokes V profiles in the umbra under the assumption that
the umbra is at rest. Both methods yield the same result within the
calibration uncertainties (~150 m s-1). We study the radial
dependence of Stokes V profiles in the directions of disk center and
limb side. Results: .Maps of Stokes V profile shifts, polarity,
amplitude asymmetry, field strength and magnetic field azimuth provide
strong evidence for the presence of a magnetic canopy and for the
existence of a radial outflow in the canopy. Conclusions: .Our
findings indicate that the Evershed flow does not cease abruptly at the
white-light spot boundary, but that at least a part of the penumbral
Evershed flow continues into the magnetic canopy.