beck

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Author name code: beck
ADS astronomy entries on 2022-09-14
author:"Beck, Christian" AND (aff:"Freiburg" OR aff:"Solar")

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Title: The magnetic topology of the inverse Evershed flow
Authors: Prasad, A.; Ranganathan, M.; Beck, C.; Choudhary, D. P.;
Hu, Q.
2022A&A...662A..25P Altcode: 2022arXiv220302702P
Context. The inverse Evershed flow (IEF) is a mass motion towards
sunspots at chromospheric heights. Aims: We combined
high-resolution observations of NOAA 12418 from the Dunn Solar
Telescope and vector magnetic field measurements from the Helioseismic
and Magnetic Imager (HMI) to determine the driver of the IEF. 
Methods: We derived chromospheric line-of-sight (LOS) velocities from
spectra of Hα and Ca II IR. The HMI data were used in a non-force-free
magnetic field extrapolation to track closed field lines near the
sunspot in the active region. We determined their length and height,
located their inner and outer foot points, and derived flow velocities
along them. Results: The magnetic field lines related to the
IEF reach on average a height of 3 megameter (Mm) over a length
of 13 Mm. The inner (outer) foot points are located at 1.2 (1.9)
sunspot radii. The average field strength difference ΔB between inner
and outer foot points is +400 G. The temperature difference ΔT is
anti-correlated with ΔB with an average value of −100 K. The pressure
difference Δp is dominated by ΔB and is primarily positive with a
driving force towards the inner foot points of 1.7 kPa on average. The
velocities predicted from Δp reproduce the LOS velocities of 2-10 km
s−1 with a square-root dependence. Conclusions:
We find that the IEF is driven along magnetic field lines connecting
network elements with the outer penumbra by a gas pressure difference
that results from a difference in field strength as predicted by the
classical siphon flow scenario.

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Title: Velocities of an Erupting Filament
Authors: Wang, Shuo; Jenkins, Jack M.; Muglach, Karin; Martinez Pillet,
Valentin; Beck, Christian; Long, David M.; Choudhary, Debi Prasad;
McAteer, James
2022ApJ...926...18W Altcode: 2021arXiv211107830W
Solar filaments exist as stable structures for extended periods of
time before many of them form the core of a coronal mass ejection
(CME). We examine the properties of an erupting filament on 2017 May
29-30 with high-resolution He I 10830 Å and Hα spectra from the
Dunn Solar Telescope, full-disk Dopplergrams of He I 10830 Å from
the Chromospheric Telescope, and EUV and coronograph data from SDO
and STEREO. Pre-eruption line-of-sight velocities from an inversion
of He I with the HAZEL code exhibit coherent patches of 5 Mm extent
that indicate counter-streaming and/or buoyant behavior. During the
eruption, individual, aligned threads appear in the He I velocity
maps. The distribution of velocities evolves from Gaussian to strongly
asymmetric. The maximal optical depth of He I 10830 Å decreased from
τ = 1.75 to 0.25, the temperature increased by 13 kK, and the average
speed and width of the filament increased from 0 to 25 km s-1
and 10 to 20 Mm, respectively. All data sources agree that the filament
rose with an exponential acceleration reaching 7.4 m s-2
that increased to a final velocity of 430 km s-1 at 22:24
UT; a CME was associated with this filament eruption. The properties
during the eruption favor a kink/torus instability, which requires
the existence of a flux rope. We conclude that full-disk chromospheric
Dopplergrams can be used to trace the initial phase of on-disk filament
eruptions in real time, which might potentially be useful for modeling
the source of any subsequent CMEs.

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Title: The Visible Spectro-Polarimeter of the Daniel K. Inouye
Solar Telescope
Authors: de Wijn, A. G.; Casini, R.; Carlile, A.; Lecinski, A. R.;
Sewell, S.; Zmarzly, P.; Eigenbrot, A. D.; Beck, C.; Wöger, F.;
Knölker, M.
2022SoPh..297...22D Altcode: 2022arXiv220300117D
The Daniel K. Inouye Solar Telescope (DKIST) Visible Spectro-Polarimeter
(ViSP) is a traditional slit-scanning spectrograph with the ability
to observe solar regions up to a 120 ×78 arcsec2 area. The
design implements dual-beam polarimetry, a polychromatic polarization
modulator, a high-dispersion echelle grating, and three spectral
channels that can be automatically positioned. A defining feature of
the instrument is its capability to tune anywhere within the 380 - 900
nm range of the solar spectrum, allowing for a virtually infinite number
of combinations of three wavelengths to be observed simultaneously. This
enables the ViSP user to pursue well-established spectro-polarimetric
studies of the magnetic structure and plasma dynamics of the solar
atmosphere, as well as completely novel investigations of the solar
spectrum. Within the suite of first-generation instruments at the DKIST,
ViSP is the only wavelength-versatile spectro-polarimeter available to
the scientific community. It was specifically designed as a discovery
instrument to explore new spectroscopic and polarimetric diagnostics
and test improved models of polarized line formation through high
spatial-, spectral-, and temporal-resolution observations of the Sun's
polarized spectrum. In this instrument article, we describe the science
requirements and design drivers of ViSP and present preliminary science
data collected during the commissioning of the instrument.

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Title: Heating of the solar atmosphere by electric currents
Authors: Choudhary, Debi Prasad; Louis, Rohan; Prasad, Avijeet; Beck,
Christian; Yalim, Mehmet
2021AGUFMSH12B..07C Altcode:
We present direct evidence of Ohmic dissipation of electric currents
that leads to heating of the solar chromosphere above a light bridge
in a sunspot by combining high-resolution spectroscopic Ca II IR data
from the Dunn Solar Telescope with vector magnetic field observations
from HMI. An extrapolation of the photospheric magnetic field from
HMI to the corona using a non-force-free field technique provided the
three-dimensional distribution of electric currents at locations of
magnetic discontinuities, while the inversion of the chromospheric
Ca II IR line spectra with the CAISAR code delivered the temperature
stratifications from the photosphere to the chromosphere. The comparison
of these results clearly shows that the light bridge is a site of
strong electric currents of about 0.3 A m2 at the bottom boundary, which
extend to about 0.7 Mm while decreasing monotonically with height. The
dissipation of these currents produces a chromospheric temperature
excess of about 600800 K relative to the umbra. Only the light bridge,
where relatively weak and highly inclined magnetic fields emerge
over a duration of 13 hrs, shows a spatial coincidence of thermal
enhancements and electric currents. The temperature enhancements
and the Cowling heating are primarily confined to a height range
of 0.40.7 Mm above the light bridge. The corresponding increase in
internal energy of 200 J m3 can be supplied by the heating in about
10 min. This heating process by Ohmic dissipation can happen at any
place in the solar atmosphere where large electric currents co-exist
with non-zero resistivity. Large-scale corona loops embedded in a more
vertical background magnetic field could experience the same effect
leading to a localized heating of coronal plasma.

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Title: Heating of the solar chromosphere in a sunspot light bridge
by electric currents
Authors: Louis, Rohan E.; Prasad, Avijeet; Beck, Christian; Choudhary,
Debi P.; Yalim, Mehmet S.
2021A&A...652L...4L Altcode: 2021arXiv210712066L
Context. Resistive Ohmic dissipation has been suggested as a mechanism
for heating the solar chromosphere, but few studies have established
this association. Aims: We aim to determine how Ohmic dissipation
by electric currents can heat the solar chromosphere. Methods:
We combine high-resolution spectroscopic Ca II data from the Dunn Solar
Telescope and vector magnetic field observations from the Helioseismic
and Magnetic Imager (HMI) to investigate thermal enhancements in a
sunspot light bridge. The photospheric magnetic field from HMI was
extrapolated to the corona using a non-force-free field technique that
provided the three-dimensional distribution of electric currents, while
an inversion of the chromospheric Ca II line with a local thermodynamic
equilibrium and a nonlocal thermodynamic equilibrium spectral archive
delivered the temperature stratifications from the photosphere to the
chromosphere. Results: We find that the light bridge is a site of
strong electric currents, of about 0.3 A m−2 at the bottom
boundary, which extend to about 0.7 Mm while decreasing monotonically
with height. These currents produce a chromospheric temperature excess
of about 600−800 K relative to the umbra. Only the light bridge,
where relatively weak and highly inclined magnetic fields emerge over a
duration of 13 h, shows a spatial coincidence of thermal enhancements
and electric currents. The temperature enhancements and the Cowling
heating are primarily confined to a height range of 0.4−0.7 Mm above
the light bridge. The corresponding increase in internal energy of 200 J
m−3 can be supplied by the heating in about 10 min. 
Conclusions: Our results provide direct evidence for currents heating
the lower solar chromosphere through Ohmic dissipation.

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Title: The National Science Foundation's Daniel K. Inouye Solar
Telescope — Status Update
Authors: Rimmele, T.; Woeger, F.; Tritschler, A.; Casini, R.; de Wijn,
A.; Fehlmann, A.; Harrington, D.; Jaeggli, S.; Anan, T.; Beck, C.;
Cauzzi, G.; Schad, T.; Criscuoli, S.; Davey, A.; Lin, H.; Kuhn, J.;
Rast, M.; Goode, P.; Knoelker, M.; Rosner, R.; von der Luehe, O.;
Mathioudakis, M.; Dkist Team
2021AAS...23810601R Altcode:
The National Science Foundation's 4m Daniel K. Inouye Solar Telescope
(DKIST) on Haleakala, Maui is now the largest solar telescope in the
world. DKIST's superb resolution and polarimetric sensitivity will
enable astronomers to unravel many of the mysteries the Sun presents,
including the origin of solar magnetism, the mechanisms of coronal
heating and drivers of flares and coronal mass ejections. Five
instruments, four of which provide highly sensitive measurements
of solar magnetic fields, including the illusive magnetic field of
the faint solar corona. The DKIST instruments will produce large and
complex data sets, which will be distributed through the NSO/DKIST Data
Center. DKIST has achieved first engineering solar light in December
of 2019. Due to COVID the start of the operations commissioning phase
is delayed and is now expected for fall of 2021. We present a status
update for the construction effort and progress with the operations
commissioning phase.

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Title: Spectropolarimetric Measurements Of The Inverse Evershed Flow
Authors: Choudhary, D. P.; Beck, C.; Prasad, A.; Monankrishna, R.;
Dhara, S.
2021AAS...23831319C Altcode:
We use high spatial resolution observations with spectropolarimeters
at the Dunn Solar Telescope in multiple spectral lines originating
in the chromosphere to study inverse Evershed flow channels that
connect the outer penumbra and the moat region around sunspots at
various heliocentric positions. The measurements were combined with
extrapolated magnetic field lines to determine the three-dimensional
topology of the flow channels. The magnetic field lines guiding the
flows reach on average a height of 2-5 Mm over a length of 10-20 Mm,
with cold inner (hot outer) foot points located at 1.2 (1.9) sunspot
radii. The average difference in field strength between the inner and
outer foot point is about +400 G, while the temperature difference is
about -100 K. This configuration can drive and sustain a siphon flow
from the moat boundary towards the penumbra over the lifetime of the
flow channels of about one hour. This study clearly demonstrates the
inverse Evershed flow to be a siphon flow along arched chromospheric
loops.

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Title: Magnetoacoustic wave energy dissipation in the atmosphere of
solar pores
Authors: Gilchrist-Millar, Caitlin A.; Jess, David B.; Grant, Samuel
D. T.; Keys, Peter H.; Beck, Christian; Jafarzadeh, Shahin; Riedl,
Julia M.; Van Doorsselaere, Tom; Ruiz Cobo, Basilio
2021RSPTA.37900172G Altcode: 2020arXiv200711594G
The suitability of solar pores as magnetic wave guides has been a key
topic of discussion in recent years. Here, we present observational
evidence of propagating magnetohydrodynamic wave activity in a group
of five photospheric solar pores. Employing data obtained by the
Facility Infrared Spectropolarimeter at the Dunn Solar Telescope,
oscillations with periods of the order of 5 min were detected at
varying atmospheric heights by examining Si ɪ 10827 Å line bisector
velocities. Spectropolarimetric inversions, coupled with the spatially
resolved root mean square bisector velocities, allowed the wave energy
fluxes to be estimated as a function of atmospheric height for each
pore. We find propagating magnetoacoustic sausage mode waves with
energy fluxes on the order of 30 kW m-2 at an atmospheric
height of 100 km, dropping to approximately 2 kW m-2
at an atmospheric height of around 500 km. The cross-sectional
structuring of the energy fluxes reveals the presence of both body-
and surface-mode sausage waves. Examination of the energy flux decay
with atmospheric height provides an estimate of the damping length,
found to have an average value across all five pores of Ld
≈ 268 km, similar to the photospheric density scale height. We find
the damping lengths are longer for body mode waves, suggesting that
surface mode sausage oscillations are able to more readily dissipate
their embedded wave energies. This work verifies the suitability of
solar pores to act as efficient conduits when guiding magnetoacoustic
wave energy upwards into the outer solar atmosphere. This article
is part of the Theo Murphy meeting issue `High-resolution wave dynamics
in the lower solar atmosphere'.

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Title: The Formation of an Atypical Sunspot Light Bridge as a Result
of Large-scale Flux Emergence
Authors: Louis, Rohan E.; Beck, Christian; Choudhary, Debi P.
2020ApJ...905..153L Altcode: 2020arXiv201014085L
We use a combination of full-disk data from the Solar Dynamics
Observatory and high-resolution data from the Dunn Solar Telescope (DST)
to study the formation, structure, and evolution of an atypical light
bridge (LB) in a regular sunspot. The LB results from the emergence of
magnetic flux with one footpoint rooted in a pore outside the parent
sunspot that appears about 17 hr before the LB. The pore has a polarity
opposite to that of the sunspot and recedes from it at a speed of
about 0.4 km s-1. This is accompanied by the development of
an elongated magnetic channel in the outer penumbra that triggers the
formation of the LB when it reaches the inner penumbral boundary. The
LB is a nearly horizontal structure with a field strength of about
1.2 kG that exhibits long-lived photospheric blueshifts of about 0.85
km s-1 along its entire length. The emergence of the LB
leads to dynamic surges in the chromosphere and transition region
about 13 minutes later. We derive the photospheric and chromospheric
structure of the LB in the DST data from spectral line parameters and
inversions of He I at 1083 nm, Si I at 1082.7 nm, Ca II IR at 854
nm, and Hα at 656 nm and speckle-reconstructed imaging
at 700 and 430 nm. The LB shows an elongated filamentary shape in
the photosphere without lateral extrusions. The thermal inversion
of Ca II IR reveals the LB to be about 600-800 K hotter than the
umbra. Different sections of the LB are elevated to heights between 400
and 700 km. Our results indicate that LB formation is part of a flux
emergence event with the LB envelope reaching a height of about 29 Mm
before dissolving after about 13 hr. We conclude that the existence
of persistent, large-scale photospheric blueshifts in LBs is the most
likely criterion for distinguishing between flux emergence events and
overturning convection in field-free umbral intrusions.

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Title: The Daniel K. Inouye Solar Telescope - Observatory Overview
Authors: Rimmele, Thomas R.; Warner, Mark; Keil, Stephen L.; Goode,
Philip R.; Knölker, Michael; Kuhn, Jeffrey R.; Rosner, Robert R.;
McMullin, Joseph P.; Casini, Roberto; Lin, Haosheng; Wöger, Friedrich;
von der Lühe, Oskar; Tritschler, Alexandra; Davey, Alisdair; de Wijn,
Alfred; Elmore, David F.; Fehlmann, André; Harrington, David M.;
Jaeggli, Sarah A.; Rast, Mark P.; Schad, Thomas A.; Schmidt, Wolfgang;
Mathioudakis, Mihalis; Mickey, Donald L.; Anan, Tetsu; Beck, Christian;
Marshall, Heather K.; Jeffers, Paul F.; Oschmann, Jacobus M.; Beard,
Andrew; Berst, David C.; Cowan, Bruce A.; Craig, Simon C.; Cross,
Eric; Cummings, Bryan K.; Donnelly, Colleen; de Vanssay, Jean-Benoit;
Eigenbrot, Arthur D.; Ferayorni, Andrew; Foster, Christopher; Galapon,
Chriselle Ann; Gedrites, Christopher; Gonzales, Kerry; Goodrich, Bret
D.; Gregory, Brian S.; Guzman, Stephanie S.; Guzzo, Stephen; Hegwer,
Steve; Hubbard, Robert P.; Hubbard, John R.; Johansson, Erik M.;
Johnson, Luke C.; Liang, Chen; Liang, Mary; McQuillen, Isaac; Mayer,
Christopher; Newman, Karl; Onodera, Brialyn; Phelps, LeEllen; Puentes,
Myles M.; Richards, Christopher; Rimmele, Lukas M.; Sekulic, Predrag;
Shimko, Stephan R.; Simison, Brett E.; Smith, Brett; Starman, Erik;
Sueoka, Stacey R.; Summers, Richard T.; Szabo, Aimee; Szabo, Louis;
Wampler, Stephen B.; Williams, Timothy R.; White, Charles
2020SoPh..295..172R Altcode:
We present an overview of the National Science Foundation's Daniel
K. Inouye Solar Telescope (DKIST), its instruments, and support
facilities. The 4 m aperture DKIST provides the highest-resolution
observations of the Sun ever achieved. The large aperture of
DKIST combined with state-of-the-art instrumentation provide the
sensitivity to measure the vector magnetic field in the chromosphere
and in the faint corona, i.e. for the first time with DKIST we will
be able to measure and study the most important free-energy source
in the outer solar atmosphere - the coronal magnetic field. Over its
operational lifetime DKIST will advance our knowledge of fundamental
astronomical processes, including highly dynamic solar eruptions
that are at the source of space-weather events that impact our
technological society. Design and construction of DKIST took over two
decades. DKIST implements a fast (f/2), off-axis Gregorian optical
design. The maximum available field-of-view is 5 arcmin. A complex
thermal-control system was implemented in order to remove at prime
focus the majority of the 13 kW collected by the primary mirror and
to keep optical surfaces and structures at ambient temperature, thus
avoiding self-induced local seeing. A high-order adaptive-optics
system with 1600 actuators corrects atmospheric seeing enabling
diffraction limited imaging and spectroscopy. Five instruments, four
of which are polarimeters, provide powerful diagnostic capability
over a broad wavelength range covering the visible, near-infrared,
and mid-infrared spectrum. New polarization-calibration strategies
were developed to achieve the stringent polarization accuracy
requirement of 5×10−4. Instruments can be combined and
operated simultaneously in order to obtain a maximum of observational
information. Observing time on DKIST is allocated through an open,
merit-based proposal process. DKIST will be operated primarily in
"service mode" and is expected to on average produce 3 PB of raw
data per year. A newly developed data center located at the NSO
Headquarters in Boulder will initially serve fully calibrated data to
the international users community. Higher-level data products, such as
physical parameters obtained from inversions of spectro-polarimetric
data will be added as resources allow.

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Title: Center-to-limb Variation of the Inverse Evershed Flow
Authors: Beck, C.; Choudhary, D. P.; Ranganathan, M.
2020ApJ...902...30B Altcode: 2020arXiv200812748B
We present the properties of the inverse Evershed flow (IEF)
based on the center-to-limb variation of the plasma speed and loop
geometry of chromospheric superpenumbral fibrils in eleven sunspots
that were located at a wide range of heliocentric angles from 12° to
79°. The observations were acquired at the Dunn Solar Telescope in the
spectral line of Hα at 656 nm to determine chromospheric flows and the
photospheric Si I line at 1082.7 nm to estimate the photospheric umbral
magnetic field strength. All sunspots display opposite line-of-sight
(LOS) velocities on the limb and center side with a distinct shock
signature near the outer penumbral edge. We developed a simplified
flexible sunspot model assuming axisymmetry and prescribing the radial
flow speed profile at a known loop geometry to replicate the observed
two-dimensional IEF patterns under different viewing angles. The
simulated flow maps match the observations for chromospheric loops with
10-20 Mm length starting at 0.8-1.1 sunspot radii, an apex height of
1-3 Mm, and a flow speed of 2-9 km s-1. We find on average
a good agreement of the simulated velocities and the observations
on elliptical annuli around the sunspot. Individual IEF channels
show a significant range of variation in their properties and reach
maximal LOS speeds of up to 12 km s-1. Upwards or downwards
directed flows do not show a change of sign in the LOS velocities for
heliocentric angles above 30°. Our results are consistent with the IEF
being caused by a siphon flow mechanism driving a flow at about sonic
speed along elevated loops with a flattened top in the chromosphere.

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Title: Magnetic Structure of an Erupting Filament
Authors: Wang, Shuo; Jenkins, Jack M.; Martinez Pillet, Valentin;
Beck, Christian; Long, David M.; Prasad Choudhary, Debi; Muglach,
Karin; McAteer, James
2020ApJ...892...75W Altcode: 2020arXiv200202104W
The full 3D vector magnetic field of a solar filament prior to
eruption is presented. The filament was observed with the Facility
Infrared Spectropolarimeter at the Dunn Solar Telescope in the
chromospheric He I line at 10830 Å on 2017 May 29 and 30. We inverted
the spectropolarimetric observations with the Hanle and Zeeman Light
code to obtain the chromospheric magnetic field. A bimodal distribution
of field strength was found in or near the filament. The average field
strength was 24 Gauss, but prior to the eruption we find the 90th
percentile of field strength was 435 Gauss for the observations on May
29. The field inclination was about 67° from the solar vertical. The
field azimuth made an angle of about 47°-65° to the spine axis. The
results suggest an inverse configuration indicative of a flux rope
topology. He I intensity threads were found to be coaligned with the
magnetic field direction. The filament had a sinistral configuration
as expected for the southern hemisphere. The filament was stable on
2017 May 29 and started to rise during two observations on May 30,
before erupting and causing a minor coronal mass ejection. There
was no obvious change of the magnetic topology during the eruption
process. Such information on the magnetic topology of erupting filaments
could improve the prediction of the geoeffectiveness of solar storms.

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Title: Temporal Evolution of the Inverse Evershed Flow
Authors: Beck, C.; Choudhary, D. P.
2020ApJ...891..119B Altcode: 2020arXiv200204560B
The inverse Evershed flow (IEF) is an inflow of material into the
penumbra of sunspots in the solar chromosphere that occurs along dark,
elongated super-penumbral fibrils extending from about the outer edge
of the moat cell to the sunspot. The IEF channels exhibit brightenings
in the penumbra, where the supersonic IEF descends to the photosphere
causing shock fronts with localized heating. We used an 1 hr time series
of spectroscopic observations of the chromospheric spectral lines of
Ca II IR at 854 nm and Hα at 656 nm taken with the Interferometric
Bidimensional Spectrometer at the Dunn Solar Telescope to investigate
the temporal evolution of IEF channels. Complementary information on
the photospheric magnetic field was obtained from observations with the
Facility Infrared Spectropolarimeter at 1083 nm and the Helioseismic
and Magnetic Imager. We find that individual IEF channels are long-lived
(10-60 minutes) and only show minor changes in position and flow speed
during their lifetime. Initiation and termination of IEF channels
takes several minutes. The IEF channels with line-of-sight velocities
of about 10 km s-1 show no lasting impact from transient or
oscillatory phenomena with maximal velocity amplitudes of only about 1
km s-1 that run along them. We could not detect any clear
correlation of the location and evolution of IEF channels to local
magnetic field properties in the photosphere in the penumbra or moving
magnetic features in the sunspot moat. Our results support a picture
of the IEF as a field-aligned siphon flow along arched loops. From our
data we cannot determine if their evolution is controlled by events
at the outer end in the moat or at the inner end in the penumbra.

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Title: Stokes Line Parameters as Possible Indicators of Flaring
Activity: A Comparison of Flaring and Non-Flaring Active Regions
Authors: Romich, K.; Cadavid, A. C.; Choudhary, D. P.; Beck, C.
2019AGUFMSH31D3338R Altcode:
While the association between solar active regions (ARs) and solar
flares is well-established, there is currently no reliable means
of determining when (or if) a given AR will flare. Much of flare
forecasting is based on the application of machine-learning statistical
techniques that use parameters derived from the local magnetic field
as predictors; these are commonly obtained through spectropolarimetric
inversions using Stokes profiles from the observed radiation. The
standard inversion codes, such as those based on the Milne-Eddington
approximation, yield the average magnetic field values at or near flare
locations. However, this fails to utilize the rich information contained
in the shape of Stokes profiles, such as the existence of line-of-sight
(LOS) magnetic and velocity gradients and multiple magnetic components
along the LOS or within the resolving element. The resulting loss
of information can potentially lead to inaccurate forecasts. We
propose a novel approach, in which line parameters derived from
spectrally-resolved Stokes profiles are considered as possible
precursors to flare events. Using data from the spectropolarimeter
onboard the Hinode satellite, we examine the amplitude asymmetry, net
circular polarization, and degree of complexity of Stokes V profiles
from several flaring and non-flaring ARs. Particular attention is given
to regions near the polarity inversion line (PIL) due to its documented
role in flare initiation. We define the PIL using the magnetic field
vector relative to the solar surface, which we calculate from the Stokes
Q, U, and V profiles; this corrects for projection effects arising
from LOS observations of magnetic flux from ARs off disc center. In
light of the shortcomings of existing methods, we hope to expand the
set of viable indicators of flaring activity with the long-term goal
of improving flare forecasting models.

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Title: Time dependent properties of Inverse Evershed Flow and
Perspectives with Daniel K. Inouye Solar Telescope (DKIST)
Authors: Choudhary, D. P.; Beck, C.
2019AGUFMSH41F3334C Altcode:
We observed isolated leading sunspot, located at a heliocentric angle
of 43 degrees, of a decaying active region (AR) NOAA 12418 on 16
September 2015 with the Interferometric Bidimensional, the Facility
InfraRed Spectropolarimeter at the Dunn Solar Telescope to study the
time dependent chromospheric flow properties along the fibril structure,
which is widely known as Inverse Evershed Flow. The observations were
carried out in spectral lines of H_alpha at 656 nm and Ca ii IR at
854.2 nm, photospheric Zeeman-sensitive Si i line at 1082.7 nm and the
chromospheric He i line at 1083 nm. Our data is complemented with the
Milne-Eddington inversion results for the photospheric magnetic field
obtained with the derived from observations by the Helioseismic and
Magnetic Imager on-board the Solar Dynamics Observatory. We find that
all three chromospheric lines show a very similar behavior in their
line-core intensity and the LOS velocity with matching spatial and
temporal properties. We find that individual IEF channels persist for a
few ten minutes to more than one hour. IEF channels that disappear are
often rapidly replaced by a new channel at the about the same location
after a short time. The IEF channels show little radial or lateral
motion and usually end in the mid to outer penumbra. Initiation of
the flow takes about 10min, while the termination is faster and takes
only about 5min. The IEF channels seem to appear at preferred azimuth
angles that are spaced at about 10◦ distance. The transient events
were found to have almost no effect on the overall Inverse Evershed
Flow pattern. In this paper, we shall present the results from our
current study and perspectives for further observational study with
upcoming 4 meter Daniel K. Inouye Solar Telescope.

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Title: An Erupting Solar Filament Observed at the DST
Authors: Wang, S.; Jenkins, J. M.; Pillet, V. M.; Beck, C.; Long,
D.; Choudhary, D. P.; McAteer, J.
2019AGUFMSH33B3376W Altcode:
An erupting filament that lead to a coronal mass ejection (CME) was
observed at the Dunn Solar Telescope (DST). We present HAZEL inversions
of spectropolarimetric observations of the quiescent filament acquired
with the Facility Infrared Spectropolarimeter at the DST. This study
includes three observations of the He I triplet at 10830 Å on May 29
and 30, 2017. The filament was stable on May 29, and was observed in
the process of rising at speeds of 20-30 km/s during the two spatial
maps taken on May 30. Vector magnetic fields along the filament were
obtained that show an inverse configuration indicative of a flux rope
topology, including co-aligned threads. To take advantage of the better
spatial and temporal resolution of the Daniel K. Inouye Solar Telescope
(DKIST), future collaborations of the DKIST and the DST to study solar
filaments are discussed.

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Title: Filament Magnetic Fields at the DST and DKIST
Authors: Wang, Shuo; Jenkins, Jack; Pillet, Valentin; Beck, Christian;
Long, David; Choudhary, Debi Prasad; McAteer, James
2019AAS...23422603W Altcode:
Observations from the 0.8-m Dunn Solar Telescope (DST) are
qualitatively similar to data that will be produced by the 4-m
Daniel K. Inouye Solar Telescope (DKIST), albeit at a lower spatial
resolution and polarimetric sensitivity. We present HAZEL inversions
of spectropolarimetric observations of a quiescent filament acquired
with the Facility Infrared Spectropolarimeter at the DST. This study
includes three observations of the He I triplet at 10830 Å on May 29
and 30, 2017. The filament was stable on May 29, and was observed in
the process of rising at speeds of 20-30 km/s during the two spatial
maps taken on May 30. Vector magnetic fields along the filament were
obtained that show an inverse configuration indicative of a flux rope
topology, including co-aligned threads. To take advantage of the better
spatial and temporal resolution of the DKIST, future collaborations
of the DKIST and the DST to study solar filaments are discussed. We
propose to further study the evolution of solar filaments that erupt
and lead to Coronal Mass Ejections using interspaced observations from
the DKIST and DST spectropolarimeters. While the DST observations will
give information about the global evolution of physical properties
leading to the destabilization, the DKIST observations will provide the
information about the physical conditions in the small-scale structures
that support the filament material.

---------------------------------------------------------
Title: Fast Inversion of Solar Ca II Spectra in Non-local
Thermodynamic Equilibrium
Authors: Beck, C.; Gosain, S.; Kiessner, C.
2019ApJ...878...60B Altcode: 2019arXiv190411843B
Present-day solar imaging spectrometers typically yield a few hundred
million spectra in one hour of observing time. This number will increase
by an order of magnitude for future instruments with larger 4k ×
4k sensors, such as those planned to be used for the upcoming Daniel
K. Inouye Solar Telescope. A fast quantitative analysis of such huge
data volumes can be done by comparing the observations to an archive of
pre-calculated synthetic spectra to infer the thermodynamic properties
of the atmosphere. To analyze intensity spectra of the Ca II IR line at
854 nm in the solar atmosphere, we generated an archive with 2,000,000
spectra under the assumption of non-local thermodynamic equilibrium
(NLTE) with the NICOLE code. We tested its performance by inverting 60
spectral scans of Ca II IR at 854 nm in the magnetically quiet Sun with
700,000 profiles each. Based on the inversion results obtained using
the full archive, we constructed a smaller archive by keeping only
the 70,000 archive profiles that were actually used. We can reproduce
the observed intensity spectra to within a few percent using either
the full or the small archive. For spectra with 30 wavelength points,
this NLTE inversion approach takes 0.02 (0.35) s per profile to obtain a
temperature stratification when using the small (full) archive, i.e.,
it can invert a single spectral scan in about 4 (68) hr. The code
is able to simultaneously deal with an arbitrary number of spectral
lines. This makes it a promising tool for deriving thermodynamic
properties of the solar atmosphere from current or future solar
high-resolution observations of photospheric and chromospheric lines.

---------------------------------------------------------
Title: NLTE inversions of Ca II 854.2 nm spectra from SOLIS/VSM:
Temperature diagnostics
Authors: Gusain, Sanjay; Beck, Christian
2019AAS...23430203G Altcode:
We present non-local thermodynamic equilibrium (NLTE) inversions
of Ca II 854.2 nm spectra from SOLIS/VSM observations. We show that
precomputed spectra from a database of model atmospheres can be applied
for a reliable inference of temperature stratifications in the solar
atmosphere. We show examples of such inferences in different solar
structures such as the umbra, penumbra, circum-facular regions and
the quiet Sun. We also fit full Stokes profiles assuming a simple
magnetic model with an exponential dependence with optical depth to
derive vector magnetic field information and compare with the values
derived from the weak-field approximation.

---------------------------------------------------------
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.

---------------------------------------------------------
Title: Magnetic Structures of a Quiecent Filament
Authors: Wang, Shuo; Jenkins, Jack; Pillet, Valentin; Beck, Christian;
Long, David; Choudhary, Debi; McAteer, James
2019shin.confE..52W Altcode:
A quiecent filament is observed from the Dunn Solar Telescope (DST) on
May 29 and 30, 2017. We present HAZEL inversions of spectropolarimetric
observations acquired with the Facility Infrared Spectropolarimeter
at the DST. This study includes three observations of the He I triplet
at 1083.0 nm.

---------------------------------------------------------
Title: Magnetic Properties and Flow Angle of the Inverse Evershed
Flow at Its Downflow Points
Authors: Beck, C.; Choudhary, D. P.
2019ApJ...874....6B Altcode: 2019arXiv190204660B
We determined the direction and strength of the photospheric and lower
chromospheric magnetic field in the umbra and penumbra of a sunspot from
inversions of spectropolarimetric observations of photospheric lines
at 617 nm and 1565 nm and the chromospheric Ca II IR line at 854 nm,
respectively. We compare the magnetic field vector with the direction
of 75 flow channels that harbor the chromospheric inverse Evershed
effect (IEF) near their downflow points (DFPs) in the sunspot’s
penumbra. The azimuth and inclination of the IEF channels to the line
of sight (LOS) were derived from spatial maps of the LOS velocity and
line-core intensity of the Ca II IR line and a thermal inversion of the
Ca II IR spectra to obtain temperature cubes. We find that the flow
direction of the IEF near the DFPs is aligned with the photospheric
magnetic field to within about ±15°. The IEF flow fibrils make
an angle of 30°-90° to the local vertical with an average value
of about 65°. The average field strength at the DFPs is about 1.3
kG. Our findings suggest that the IEF in the lower chromosphere is a
field-aligned siphon flow, where the larger field strength at the inner
footpoints together with the lower temperature in the penumbra causes
the necessary gas pressure difference relative to the outer footpoints
in the hotter quiet Sun with lower magnetic field strength. The IEF
connects to magnetic field lines that are not, like in the case of the
regular Evershed flow, but which continue upward into the chromosphere,
indicating an “uncombed” penumbral structure.

---------------------------------------------------------
Title: The First Solar Seeing Profile Measurement with Two Apertures
and Multiple Guide Regions
Authors: Ren, Deqing; Zhao, Gang; Wang, Xin; Beck, Christian;
Broadfoot, Robert
2019SoPh..294....1R Altcode: 2018SoPh..294....1R
Ground-based observations suffer from atmospheric turbulence
perturbations, which seriously degrade the image quality. The seeing
profile associated with the turbulence is critical to characterize an
astronomical site. The optimal design and performance estimation of
future solar ground-layer adaptive optics (GLAO) and multi-conjugate
adaptive optics (MCAO) heavily rely on our knowledge of the seeing
profile at a specific site. Many current optical seeing profile
measurement techniques require one to use a large solar telescope for
that purpose. The development of a portable instrument to measure and
characterize the seeing profile is thus highly needed, in particular
for testing potential new sites or for the regularly monitoring of
the seeing condition at existing sites. Recently, we proposed the
Advanced Multiple Aperture Seeing Profiler (A-MASP), which uses multiple
small telescopes and multiple regions of interest (ROIs) on the solar
surface to measure the seeing profile up to an altitude of 30 km. Here,
we report our recent proof-of-concept observation run of the A-MASP
technique with the Dunn Solar Telescope (DST) of the National Solar
Observatory (NSO). We found that the Fried parameter, r0, was
about 12 cm at the observed wavelength of 630 nm in the early morning
and that there were three main turbulence layers. The strongest one
was the mix layer near the ground. We observed the evolution of the
top of the mix layer and found that it can rise to about 1.5 km in
about 18 min, which is consistent with the theory of daytime boundary
layer evolution. Another turbulence layer was observed from 8 to 15 km,
which is at the top of the convective layer. Comparing an instrument
with two sub-apertures with a real A-MASP instrument, we found that
they should lead to similar results except for the altitude h =0.

---------------------------------------------------------
Title: Inferring telescope polarization properties through spectral
lines without linear polarization
Authors: Derks, A.; Beck, C.; Martínez Pillet, V.
2018A&A...615A..22D Altcode: 2018arXiv180401153D
Context. Polarimetric observations taken with ground- or space-based
telescopes usually need to be corrected for changes of the polarization
state in the optical path. Aims: We present a technique
to determine the polarization properties of a telescope through
observations of spectral lines that have no or negligible intrinsic
linear polarization signals. For such spectral lines, any observed
linear polarization must be induced by the telescope optics. We apply
the technique to observations taken with the Spectropolarimeter for
Infrared and Optical Regions (SPINOR) at the Dunn Solar Telescope
(DST) and demonstrate that we can retrieve the characteristic
polarization properties of the DST at three wavelengths of 459,
526, and 615 nm. Methods: We determine the amount of crosstalk
between the intensity Stokes I and the linear and circular polarization
states Stokes Q, U, and V, and between Stokes V and Stokes Q and U in
spectropolarimetric observations of active regions. We fit a set of
parameters that describe the polarization properties of the DST to
the observed crosstalk values. We compare our results to parameters
that were derived using a conventional telescope calibration unit
(TCU). Results: The values for the ratio of reflectivities X =
rs/rp and the retardance τ of the DST turret
mirrors from the analysis of the crosstalk match those derived with
the TCU within the error bars. We find a negligible contribution of
retardance from the entrance and exit windows of the evacuated part
of the DST. Residual crosstalk after applying a correction for the
telescope polarization stays at a level of 3-10% regardless of which
parameter set is used, but with an rms fluctuation in the input data
of already a few percent. The accuracy in the determination of the
telescope properties is thus more limited by the quality of the input
data than the method itself. Conclusions: It is possible to
derive the parameters that describe the polarization properties of a
telescope from observations of spectral lines without intrinsic linear
polarization signal. Such spectral lines have a dense coverage (about
50 nm separation) in the visible part of the spectrum (400-615 nm),
but none were found at longer wavelengths. Using spectral lines without
intrinsic linear polarization is a promising tool for the polarimetric
calibration of current or future solar telescopes such as the Daniel
K. Inouye Solar Telescope (DKIST).

---------------------------------------------------------
Title: Status of the Daniel K. Inouye Solar Telescope: unraveling
the mysteries the Sun.
Authors: Rimmele, Thomas R.; Martinez Pillet, Valentin; Goode, Philip
R.; Knoelker, Michael; Kuhn, Jeffrey Richard; Rosner, Robert; Casini,
Roberto; Lin, Haosheng; von der Luehe, Oskar; Woeger, Friedrich;
Tritschler, Alexandra; Fehlmann, Andre; Jaeggli, Sarah A.; Schmidt,
Wolfgang; De Wijn, Alfred; Rast, Mark; Harrington, David M.; Sueoka,
Stacey R.; Beck, Christian; Schad, Thomas A.; Warner, Mark; McMullin,
Joseph P.; Berukoff, Steven J.; Mathioudakis, Mihalis; DKIST Team
2018AAS...23231601R Altcode:
The 4m Daniel K. Inouye Solar Telescope (DKIST) currently under
construction on Haleakala, Maui will be the world’s largest solar
telescope. Designed to meet the needs of critical high resolution and
high sensitivity spectral and polarimetric observations of the sun,
this facility will perform key observations of our nearest star that
matters most to humankind. DKIST’s superb resolution and sensitivity
will enable astronomers to address many of the fundamental problems
in solar and stellar astrophysics, including the origin of stellar
magnetism, the mechanisms of coronal heating and drivers of the
solar wind, flares, coronal mass ejections and variability in solar
and stellar output. DKIST will also address basic research aspects of
Space Weather and help improve predictive capabilities. In combination
with synoptic observations and theoretical modeling DKIST will unravel
the many remaining mysteries of the Sun.The construction of DKIST is
progressing on schedule with 80% of the facility complete. Operations
are scheduled to begin early 2020. DKIST will replace the NSO
facilities on Kitt Peak and Sac Peak with a national facility with
worldwide unique capabilities. The design allows DKIST to operate as
a coronagraph. Taking advantage of its large aperture and infrared
polarimeters DKIST will be capable to routinely measure the currently
illusive coronal magnetic fields. The state-of-the-art adaptive optics
system provides diffraction limited imaging and the ability to resolve
features approximately 20 km on the Sun. Achieving this resolution
is critical for the ability to observe magnetic structures at their
intrinsic, fundamental scales. Five instruments will be available at
the start of operations, four of which will provide highly sensitive
measurements of solar magnetic fields throughout the solar atmosphere
- from the photosphere to the corona. The data from these instruments
will be distributed to the world wide community via the NSO/DKIST data
center located in Boulder. We present examples of science objectives
and provide an overview of the facility and project status, including
the ongoing efforts of the community to develop the critical science
plan for the first 2-3 years of operations.

---------------------------------------------------------
Title: The Magnetic Response of the Solar Atmosphere to Umbral Flashes
Authors: Houston, S. J.; Jess, D. B.; Asensio Ramos, A.; Grant,
S. D. T.; Beck, C.; Norton, A. A.; Krishna Prasad, S.
2018ApJ...860...28H Altcode: 2018arXiv180300018H
Chromospheric observations of sunspot umbrae offer an exceptional
view of magnetoacoustic shock phenomena and the impact they have on
the surrounding magnetically dominated plasma. We employ simultaneous
slit-based spectro-polarimetry and spectral imaging observations of
the chromospheric He I 10830 Å and Ca II 8542 Å lines to examine
fluctuations in the umbral magnetic field caused by the steepening of
magnetoacoustic waves into umbral flashes. Following the application
of modern inversion routines, we find evidence to support the scenario
that umbral shock events cause expansion of the embedded magnetic
field lines due to the increased adiabatic pressure. The large number
statistics employed allow us to calculate the adiabatic index, γ =
1.12 ± 0.01, for chromospheric umbral locations. Examination of
the vector magnetic field fluctuations perpendicular to the solar
normal revealed changes up to ∼200 G at the locations of umbral
flashes. Such transversal magnetic field fluctuations have not been
described before. Through comparisons with nonlinear force-free field
extrapolations, we find that the perturbations of the transverse field
components are oriented in the same direction as the quiescent field
geometries. This implies that magnetic field enhancements produced by
umbral flashes are directed along the motion path of the developing
shock, hence producing relatively small changes, up to a maximum
of ∼8°, in the inclination and/or azimuthal directions of the
magnetic field. Importantly, this work highlights that umbral flashes
are able to modify the full vector magnetic field, with the detection
of the weaker transverse magnetic field components made possible by
high-resolution data combined with modern inversion routines.

---------------------------------------------------------
Title: Thermodynamic Properties of the Inverse Evershed Flow at Its
Downflow Points
Authors: Choudhary, D. P.; Beck, C.
2018ApJ...859..139C Altcode:
We used spectropolarimetric observations of a sunspot in the active
region NOAA 11809 in the Ca II line at 854.2 nm taken with the
SpectroPolarimeter for Optical and Infrared Regions at the Dunn Solar
Telescope to infer thermodynamic parameters along 100 super-penumbral
fibrils that harbor the inverse Evershed flow. The fibrils were
identified in line-of-sight (LOS) velocity and line-core intensity
maps. The chromospheric LOS velocity abruptly decreases from 3 to 15 km
s-1 to zero at the inner footpoints of the fibrils that are
located from the mid penumbra to about 1.4 spot radii. The spectra often
show multiple absorption components, indicating spatially or vertically
unresolved structures. Synthetic spectra with a 100% fill factor
of a flow channel in the upper atmosphere yield strongly asymmetric
profiles but no multiple separate components. The line-core intensity
always peaks slightly closer to the umbra than the LOS velocity. Using
the CAlcium Inversion using a Spectral ARchive code, we find that the
fibrils make an angle of 30°-60° to the local vertical away from the
umbra. The temperature near the downflow points is enhanced by 200 K
at log τ ∼ -2 and up to 2000 K at log τ ∼ (-6) compared to the
quiet Sun, without any signature in the low photosphere. Our results are
consistent with a critical, i.e., sonic, or supersonic siphon flow along
super-penumbral flux tubes in which accelerating plasma abruptly attains
subcritical velocity through a standing shock in or near the penumbra.

---------------------------------------------------------
Title: Inferring Telescope Polarization Properties Through Spectral
Lines Without Linear Polarization
Authors: Derks, Alysa; Beck, Christian; Martinez Pillet, Valentin
2018tess.conf21060D Altcode:
We present a technique to determine the polarization properties
of a telescope through observations of spectral lines that have
no or negligible intrinsic linear polarization signals. For such
spectral lines, any observed linear polarization must be induced by
the telescope optics. We apply the technique to observations taken
with SPINOR at the Dunn Solar Telescope (DST) and demonstrate that we
can retrieve the characteristic polarization properties of the DST at
three wavelengths of 459, 526, and 615 nm. The values for the ratio
of reflectivities X = rs/rp and the retardance τ of the DST turret
mirrors from the analysis of the crosstalk match those derived with
the TCU within the error bars. We conclude that it is possible
to derive the parameters that describe the polarization properties
of a telescope from observations of spectral lines without intrinsic
linear polarization signal. Such spectral lines have a dense coverage
(about 50 nm separation) in the visible part of the spectrum (400-615
nm), but none were found at longer wavelengths. Using spectral lines
without intrinsic linear polarization is a promising tool for the
polarimetric calibration of current or future solar telescopes such
as the Daniel K. Inouye Solar Telescope (DKIST).

---------------------------------------------------------
Title: Alfvén wave dissipation in the solar chromosphere
Authors: Grant, Samuel D. T.; Jess, David B.; Zaqarashvili, Teimuraz
V.; Beck, Christian; Socas-Navarro, Hector; Aschwanden, Markus J.;
Keys, Peter H.; Christian, Damian J.; Houston, Scott J.; Hewitt,
Rebecca L.
2018NatPh..14..480G Altcode: 2018arXiv181007712G
Magnetohydrodynamic Alfvén waves1 have been a focus of
laboratory plasma physics2 and astrophysics3
for over half a century. Their unique nature makes them ideal energy
transporters, and while the solar atmosphere provides preferential
conditions for their existence4, direct detection has proved
difficult as a result of their evolving and dynamic observational
signatures. The viability of Alfvén waves as a heating mechanism relies
upon the efficient dissipation and thermalization of the wave energy,
with direct evidence remaining elusive until now. Here we provide the
first observational evidence of Alfvén waves heating chromospheric
plasma in a sunspot umbra through the formation of shock fronts. The
magnetic field configuration of the shock environment, alongside the
tangential velocity signatures, distinguish them from conventional
umbral flashes5. Observed local temperature enhancements
of 5% are consistent with the dissipation of mode-converted Alfvén
waves driven by upwardly propagating magneto-acoustic oscillations,
providing an unprecedented insight into the behaviour of Alfvén waves
in the solar atmosphere and beyond.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
Title: Design and calibration of a high-sensitivity and high-accuracy
polarimeter based on liquid crystal variable retarders
Authors: Guo, Jing; Ren, De-Qing; Liu, Cheng-Chao; Zhu, Yong-Tian;
Dou, Jiang-Pei; Zhang, Xi; Beck, Christian
2017RAA....17....8G Altcode:
Polarimetry plays an important role in the measurement of solar magnetic
fields. We developed a high-sensitivity and high-accuracy polarimeter
(HHP) based on nematic liquid crystal variable retarders (LCVRs), which
has a compact setup and no mechanical moving parts. The system design
and calibration methods are discussed in detail. The azimuth error of
the transmission axis of the polarizer as well as the fast axes of the
two LCVRs and the quarter-wave plate were determined using dedicated
procedures. Linearly and circularly polarized light were employed to
evaluate the performance of the HHP. The experimental results indicate
that a polarimetric sensitivity of better than \[5.7 × {10 -
3}\] can be achieved by using a single short-exposure image,
while an accuracy on the order of 10-5 can be reached by
using a large number of short-exposure images. This makes the HHP a
high-performance system that can be used with a ground-based solar
telescope for high-precision solar magnetic field investigations.

---------------------------------------------------------
Title: Absorption by Mercury's Exosphere During the May 9th, 2016
Solar Transit.
Authors: Schmidt, C.; Reardon, K.; Killen, R. M.; Gary, D. E.; Ahn, K.;
Leblanc, F.; Baumgardner, J. L.; Mendillo, M.; Beck, C.; Mangano, V.
2016AGUFM.P53B2198S Altcode:
Observations of Mercury during a solar transit have the unique property
that line absorption may be used to retrieve the exosphere's column
density at all points above the terminator simultaneously. We report
on measurements during the 9 May 2016 transit with the Dunn Solar
Telescope (Interferometric BIdimensional Spectropolarimeter: IBIS &amp;
Horizontal Spectrograph: HSG) and the Big Bear Solar Observatory (Fast
Imaging Solar Spectrograph: FISS). The sodium exosphere was observed
via Fabry-Perot imaging with IBIS in 9 mA increments, and with FISS at
a dispersion of 17 mA/pixel by scanning the spectrograph slit over the
planet's disk. A search for potassium D line absorption was performed
using slit spectroscopy with HSG at a resolution of R 270,000. In each
instrument, exposures of 20-40 ms and adaptive optics enable spatial
structure to be resolved on sub-arcsecond scales. The line profiles at
every spatial bin are divided by a shifted and scaled reference spectrum
in order to isolate the exosphere's absorption from line absorption in
the solar atmosphere and structures inherent to granulation. Analysis
of these data sets is ongoing, but preliminary findings clearly show
the densest column of sodium near the poles and the content at dawn
enhanced several times with respect to dusk. Such is consistent with
2003 transit results taken at the same Mercury season (Schleicher et
al., 2004), however the data volumes herein permit a more in-depth
study in which time-dependence of the exosphere may be considered.

---------------------------------------------------------
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.

---------------------------------------------------------
Title: On the Magnetism and Dynamics of Prominence Legs Hosting
Tornadoes
Authors: Martínez González, M. J.; Asensio Ramos, A.; Arregui, I.;
Collados, M.; Beck, C.; de la Cruz Rodríguez, J.
2016ApJ...825..119M Altcode: 2016arXiv160501183M
Solar tornadoes are dark vertical filamentary structures observed
in the extreme ultraviolet associated with prominence legs and
filament barbs. Their true nature and relationship to prominences
requires an understanding of their magnetic structure and dynamic
properties. Recently, a controversy has arisen: is the magnetic field
organized forming vertical, helical structures or is it dominantly
horizontal? And concerning their dynamics, are tornadoes really rotating
or is it just a visual illusion? Here we analyze four consecutive
spectro-polarimetric scans of a prominence hosting tornadoes on its
legs, which helps us shed some light on their magnetic and dynamical
properties. We show that the magnetic field is very smooth in all the
prominence, which is probably an intrinsic property of the coronal
field. The prominence legs have vertical helical fields that show
slow temporal variation that is probably related to the motion of
the fibrils. Concerning the dynamics, we argue that (1) if rotation
exists, it is intermittent, lasting no more than one hour, and (2)
the observed velocity pattern is also consistent with an oscillatory
velocity pattern (waves).

---------------------------------------------------------
Title: Three-Dimensional Chromospheric Thermal Structure of Sunspots
Authors: Prasad Choudhary, Debi; Beck, Christian
2016SPD....47.0401P Altcode:
We have observed several 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 round 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. We also find
indications for standing shocks at the inner foot points of the flow
channels that are compatible with a supersonic siphon flow scenario.

---------------------------------------------------------
Title: Cross-Calibrating Sunspot Magnetic Field Strength Measurements
from the McMath-Pierce Solar Telescope and the Dunn Solar Telescope
Authors: Watson, Fraser T.; Beck, Christian; Penn, Matthew J.;
Tritschler, Alexandra; Pillet, Valentín Martinez; Livingston,
William C.
2015SoPh..290.3267W Altcode: 2015arXiv151107315W; 2015SoPh..tmp..158W
In this article we describe a recent effort to cross-calibrate data
from an infrared detector at the McMath-Pierce Solar Telescope and
the Facility InfraRed Spectropolarimeter (FIRS) at the Dunn Solar
Telescope. A synoptic observation program at the McMath-Pierce has
measured umbral magnetic field strengths since 1998, and this data set
has recently been compared with umbral magnetic field observations from
SOHO/MDI and SDO/HMI. To further improve on the data from McMath-Pierce,
we compared the data with measurements taken at the Dunn Solar Telescope
with far greater spectral resolution than has been possible with
space instrumentation. To minimise potential disruption to the study,
concurrent umbral measurements were made so that the relationship
between the two datasets can be most accurately characterised. We find
that there is a strong agreement between the umbral magnetic field
strengths recorded by each instrument, and we reduced the FIRS data
in two different ways to successfully test this correlation further.

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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: 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: 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: Spectro-Polarimetric Imaging Reveals Helical Magnetic Fields
in Solar Prominence Feet
Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos,
A.; Beck, C.; de la Cruz Rodríguez, J.; Díaz, A. J.
2015ApJ...802....3M Altcode: 2015arXiv150103295M
Solar prominences are clouds of cool plasma levitating above
the solar surface and insulated from the million-degree corona by
magnetic fields. They form in regions of complex magnetic topology,
characterized by non-potential fields, which can evolve abruptly,
disintegrating the prominence and ejecting magnetized material
into the heliosphere. However, their physics is not yet fully
understood because mapping such complex magnetic configurations
and their evolution is extremely challenging, and must often be
guessed by proxy from photometric observations. Using state-of-the-art
spectro-polarimetric data, we reconstruct the structure of the magnetic
field in a prominence. We find that prominence feet harbor helical
magnetic fields connecting the prominence to the solar surface below.

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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: The relation between umbral magnetic field strength and area
density of umbral dots
Authors: Ferguson, Sierra; Beck, Christian
2015AAS...22513704F Altcode:
Interiors of sunspots are modeled either as a monolithic block of
magnetic flux or groups of flux bundles. Umbral dots (UDs) - small
bright grains inside the dark umbra - are harder to explain in the
monolithic model, but they fit into the group of flux bundle model
as field-free intrusions that push magnetic field lines aside due to
their convective nature. The goal of this project was to determine if
there is a relationship between the UD density and the area of the
umbra as well as the magnetic field strength of the umbra. Routines
were developed in IDL that located the UDs and then would find the
respective areas and magnetic field strength of the umbrae. These
routines can be applied to both numerical simulations and observational
data. Our findings show that there is an indication of correlation
between the UD density and the umbral area, but there is no clear
indication of a correlation with the UD density and the magnetic field
strength. The simulations show a significantly larger UD density than
the observations This work is carried out through the National Solar
Observatory Summer Research Assistantship (SRA) Program. The National
Solar Observatory is operated by the Association of Universities for
Research in Astronomy, Inc. (AURA) under cooperative agreement with
the National Science Foundation.

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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.

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Title: Anomalous flows in a sunspot penumbra
Authors: Louis, Rohan E.; Beck, Christian; Mathew, Shibu K.;
Venkatakrishnan, P.
2014A&A...570A..92L Altcode: 2014arXiv1408.6690L
Context. The photospheric Evershed flow is a distinct characteristic of
penumbrae that is closely associated with the photometric and magnetic
structure of sunspots. Aims: We analyse the properties of an
anomalous flow in the photosphere in a sunspot penumbra and compare it
with those of the regular Evershed flow. Methods: High-resolution
spectropolarimetric observations of active region NOAA 11271 were
obtained with the spectro-polarimeter (SP) on board Hinode. We used the
magnetic field properties derived by a Milne-Eddington inversion. In
addition, we used Ca II H images obtained by the broad-band filter
instrument to study the low chromospheric response to the photospheric
structure and Dopplergrams from the Helioseismic and Magnetic Imager
to follow the evolution of the photospheric flows. Results:
We detect a blueshifted feature that appeared on the limb-side
penumbra of a sunspot and that was present intermittently during
the next two hours. It exhibited a maximum blueshift of 1.6 km
s-1, an area of 5.2 arcsec2, and a maximum
uninterrupted lifetime of 1 h. The blueshifted feature, when present,
lies parallel to redshifts. Both blue- and redshifts flank a highly
inclined or horizontal magnetic structure that is radially oriented
in the penumbra. The low-cadence SP maps reveal changes in size,
radial position in the penumbra, and line-of-sight (LOS) velocity
of the blueshifted feature, from one scan to the next. There was an
increase of nearly 500 G in the field strength with the onset of the
blueshifts, particularly when the LOS velocity in the feature exceeded
1.5 km s-1. There was only a marginal reduction in the
field inclination of about 10° with the increase in blueshifts. In the
chromosphere, intense, arc-shaped brightenings were observed close to
the location of the photospheric blueshifts, which extend from the edge
of the umbral core to the penumbra-quiet Sun boundary. The intensity
of these brightenings exceeds the background intensity by 30% to 65%
with the strongest and largest brightenings observed about 30 min after
the strongest blueshifts were detected at the photosphere. The close
spatial proximity of the two phenomenon strongly suggests a causal
relationship. Conclusions: The blueshifted feature represents
plasma motion that could be related to a magnetic structure that rises
in the solar atmosphere and subsequently reconnects with the ambient
chromospheric magnetic field of the sunspot or an inverse Evershed flow,
which would be unique in the photosphere. This transient phenomena is
presumably related to the dynamic stability of the sunspot because the
corresponding umbral core separated two days later at the location of
the blueshifts and fragmented subsequently.

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Title: Small-scale chromospheric jets above a sunspot light bridge
Authors: Louis, Rohan E.; Beck, Christian; Ichimoto, Kiyoshi
2014A&A...567A..96L Altcode: 2014arXiv1406.0103L
Context. The chromosphere above sunspot umbrae and penumbrae shows
several different types of fast dynamic events such as running
penumbral waves, umbral flashes, and penumbral microjets. Aims:
The aim of this paper is to identify the physical driver responsible
for the dynamic and small-scale chromospheric jets above a sunspot
light bridge. Methods: High-resolution broadband filtergrams
of active region NOAA 11271 in Ca ii H and G band were obtained with
the Solar Optical Telescope on board Hinode. We identified the jets in
the Ca ii H images using a semi-automatic routine and determined their
length and orientation. We applied local correlation tracking (LCT)
to the G-band images to obtain the photospheric horizontal velocity
field. The magnetic field topology was derived from a Milne-Eddington
inversion of a simultaneous scan with the Spectropolarimeter. 
Results: The chromospheric jets consist of a bright, triangular-shaped
blob that lies on the light bridge, while the apex of this blob extends
into a spike-like structure that is bright against the dark umbral
background. Most of the jets have apparent lengths of less than 1000
km and about 30% of the jets have lengths between 1000-1600 km. The
jets are oriented within ±35° to the normal of the spine of the light
bridge. Most of them are clustered near the central part of the light
bridge within a 2” area. The jets are seen to move rapidly along the
light bridge and many of them cannot be identified in successive images
taken with a 2 min cadence. The jets are primarily located on one side
of the light bridge and are directed into the umbral core. The Stokes
profiles at or close to the location of the blobs on the LB exhibit
both a significant net circular polarization and multiple components,
including opposite-polarity lobes. The magnetic field diverges from the
light bridge towards the umbral cores that it separates. The LCT reveals
that in the photosphere there is a predominantly uni-directional flow
with speeds of 100-150 m s-1 along the light bridge. This
unidirectional flow is interrupted by a patch of weak or very small
motions on the light bridge which also moves along the light bridge. Conclusions: The dynamic short-lived chromospheric jets above the LB
seem to be guided by the magnetic field lines. Reconnection events are
a likely trigger for such phenomenon since they occur at locations where
the magnetic field changes orientation sharply and where we also observe
isolated patches of opposite-polarity magnetic components. We find no
clear relation between the jets and the photospheric flow pattern.

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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: A Spectro-polarimetric Analysis of Sunspot Umbrae
Authors: Watson, Fraser; Tritschler, Alexandra; Penn, Matthew J.;
Beck, Christian; Livingston, William; Martinez Pillet, Valentin
2014AAS...22411202W Altcode:
The recent quiet solar cycle has invited new questions as to
the nature of the solar magnetic field and how it changes over
time. To investigate this, we use the National Solar Observatory’s
McMath-Pierce Solar Telescope Facility (McMP) and Dunn Solar Telescope
(DST) to compare measurements of sunspots from five active regions
observed in 2013. Both BABO at the McMP and FIRS at the DST were
used to provide spectra of the Fe 1564.8nm line, which is affected
by the presence of magnetic fields. The magnetic field is derived
from Zeeman splitting in Stokes-I by BABO, and by inversion of the
Stokes parameters from FIRS data allowing for comparisons of sunspot
properties between the two instruments.. We present the first results
from this study including the magnetic fields in sunspot umbrae from
five active regions measured simultaneously by BABO and FIRS.

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Title: Near-infrared spectropolarimetry of a δ-spot
Authors: Balthasar, H.; Beck, C.; Louis, R. E.; Verma, M.; Denker, C.
2014A&A...562L...6B Altcode: 2014arXiv1401.4386B
Sunspots harboring umbrae of both magnetic polarities within a common
penumbra (δ-spots) are often but not always related to flares. We
present first near-infrared observations (Fe i λ1078.3 nm and Si i
λ1078.6 nm spectra) obtained with the Tenerife Infrared Polarimeter at
the Vacuum Tower Telescope in Tenerife on 2012 June 17, which afford
accurate and sensitive diagnostics to scrutinize the complex fields
along the magnetic neutral line of a δ-spot within active region NOAA
11504. We examined the vector magnetic field, line-of-sight (LOS)
velocities, and horizontal proper motions of this rather inactive
δ-spot. We find a smooth transition of the magnetic vector field
from the main umbra to that of opposite polarity (δ-umbra), but a
discontinuity of the horizontal magnetic field at some distance from the
δ-umbra on the polarity inversion line. The magnetic field decreases
faster with height by a factor of two above the δ-umbra. The latter is
surrounded by its own Evershed flow. The Evershed flow coming from the
main umbra ends at a line dividing the spot into two parts. This line
is marked by the occurrence of central emission in the Ca iiλ854.2 nm
line. Along this line, high chromospheric LOS-velocities of both signs
appear. We detect a shear flow within the horizontal flux transport
velocities parallel to the dividing line.

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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>

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Title: GREGOR Fabry-Pérot interferometer and its companion the blue
imaging solar spectrometer
Authors: Puschmann, Klaus G.; Denker, Carsten; Balthasar, Horst; Louis,
Rohan E.; Popow, Emil; Woche, Manfred; Beck, Christian; Seelemann,
Thomas; Volkmer, Reiner
2013OptEn..52h1606P Altcode: 2013arXiv1302.7157P
The GREGOR Fabry-Pérot Interferometer (GFPI) is one of three
first-light instruments of the German 1.5-m GREGOR solar telescope
at the Observatorio del Teide, Tenerife, Spain. The GFPI allows
fast narrow-band imaging and postfactum image restoration. The
retrieved physical parameters will be a fundamental building block
for understanding the dynamic sun and its magnetic field at spatial
scales down to ∼50 km on the solar surface. The GFPI is a tunable
dual-etalon system in a collimated mounting. It is designed for
spectrometric and spectropolarimetric observations between 530-860
nm and 580-660 nm, respectively, and possesses a theoretical spectral
resolution of R≈250,000. Large-format, high-cadence charged coupled
device detectors with sophisticated computer hard- and software enable
the scanning of spectral lines in time-spans equivalent to the evolution
time of solar features. The field-of-view (FOV) of 50″×38″ covers
a significant fraction of the typical area of active regions in the
spectroscopic mode. In case of Stokes-vector spectropolarimetry,
the FOV reduces to 25″×38″. The main characteristics of the GFPI
including advanced and automated calibration and observing procedures
are presented. Improvements in the optical design of the instrument
are discussed and first observational results are shown. Finally,
the first concrete ideas for the integration of a second FPI, the blue
imaging solar spectrometer, are laid out, which will explore the blue
spectral region below 530 nm.

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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: First Results from the EUNIS 2013 Sounding Rocket Campaign
Authors: Daw, Adrian N.; Rabin, D. M.; Brosius, J. W.; Haas, J. P.;
Plummer, T.; Cauzzi, G.; Reardon, K. P.; Beck, C.
2013SPD....4410501D Altcode:
The Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS)
sounding rocket launched 23 April 2013 at 17:30 UT, as part of a
campaign including co-ordinated observations with the Dunn Solar
Telescope/IBIS, Hinode/EIS, SoHO/CDS, RHESSI and SDO. EUNIS obtained
the highest-resolution observations of the solar spectrum from 52-63 nm
observed to date, as well as observations with the previously-flown
waveband from 30-37 nm. The broad spectral coverage of the EUV
observations includes emission lines of ionization stages from He I to
Fe XIX, and thus a wide temperature range of 0.025 to 10 MK. Absolute
radiometric calibration of EUNIS provides underflight calibration of
CDS, EIS and AIA. Spectra were obtained with a 1.3 s cadence as the
660-arcsec long slit was rastered across two different regions. The
observations captured a B-class flare in active region NOAA 11726 as
well as active regions 11723, 11724, off-limb, quiet sun and a coronal
hole. We discuss first results from anaysis of this rich and extensive
data set.

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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>

---------------------------------------------------------
Title: Formation of a penumbra in a decaying sunspot
Authors: Louis, R. E.; Mathew, S. K.; Puschmann, K. G.; Beck, C.;
Balthasar, H.
2013A&A...552L...7L Altcode: 2013arXiv1303.3599L
Context. Penumbrae are an important characteristic of sunspots, whose
formation is intricately related to the nature of sub-photospheric
magnetic fields. Aims: We study the formation of a penumbra in
a decaying sunspot and compare its properties with those seen during
the development of a proto-spot. Methods: High-resolution
spectropolarimetric observations of active region NOAA 11283 were
obtained from the spectro-polarimeter on board Hinode. These were
complemented with full-disk filtergrams of continuum intensity,
line-of-sight magnetograms, and dopplergrams from the Helioseismic and
Magnetic Imager at high cadence. Results: The formation of a
penumbra in the decaying sunspot occurs after the coalescence of the
sunspot with a magnetic fragment/pore, which initially formed in the
quiet Sun close to an emerging flux region. At first, a smaller set of
penumbral filaments develop near the location of the merger with very
bright penumbral grains with intensities of 1.2 IQS, upflows
of 4 km s-1, and a lifetime of 10 h. During the decay of
these filaments, a larger segment of a penumbra forms at the location
of the coalescence. These new filaments are characterized by nearly
supersonic downflows of 6.5 km s-1 that change to a regular
Evershed flow nearly 3 h later. Conclusions: The coalescence of
the pore with the decaying sunspot provided sufficient magnetic flux for
the penumbra to form in the sunspot. The emerging flux region could have
played a decisive role in this process because the formation occurred
at the location of the merger and not on the opposite side of the
sunspot. An animation of the HMI data is available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
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>

---------------------------------------------------------
Title: The GREGOR Fabry-Pérot Interferometer
Authors: Puschmann, K. G.; Denker, C.; Kneer, F.; Al Erdogan, N.;
Balthasar, H.; Bauer, S. M.; Beck, C.; Bello González, N.; Collados,
M.; Hahn, T.; Hirzberger, J.; Hofmann, A.; Louis, R. E.; Nicklas, H.;
Okunev, O.; Martínez Pillet, V.; Popow, E.; Seelemann, T.; Volkmer,
R.; Wittmann, A. D.; Woche, M.
2012AN....333..880P Altcode: 2012arXiv1210.2921P
The GREGOR Fabry-Pérot Interferometer (GFPI) is one of three
first-light instruments of the German 1.5-meter GREGOR solar telescope
at the Observatorio del Teide, Tenerife, Spain. The GFPI uses two
tunable etalons in collimated mounting. Thanks to its large-format,
high-cadence CCD detectors with sophisticated computer hard- and
software it is capable of scanning spectral lines with a cadence
that is sufficient to capture the dynamic evolution of the solar
atmosphere. The field-of-view (FOV) of 50 arcsec × 38 arcsec is well
suited for quiet Sun and sunspot observations. However, in the vector
spectropolarimetric mode the FOV reduces to 25 arcsec × 38 arcsec. The
spectral coverage in the spectroscopic mode extends from 530-860 nm
with a theoretical spectral resolution of R ≈ 250,000, whereas in
the vector spectropolarimetric mode the wavelength range is at present
limited to 580-660 nm. The combination of fast narrow-band imaging and
post-factum image restoration has the potential for discovery science
concerning the dynamic Sun and its magnetic field at spatial scales
down to ∼50 km on the solar surface.

---------------------------------------------------------
Title: The GREGOR Fabry-Perot interferometer: status report and
prospects
Authors: Puschmann, Klaus G.; Balthasar, Horst; Beck, Christian;
Louis, Rohan E.; Popow, Emil; Seelemann, Thomas; Volkmer, Reiner;
Woche, Manfred; Denker, Carsten
2012SPIE.8446E..79P Altcode: 2012arXiv1207.2084P
The GREGOR Fabry-Ṕerot Interferometer (GFPI) is one of three
first-light instruments of the German 1.5-meter GREGOR solar telescope
at the Observatorio del Teide, Tenerife, Spain. The GFPI allows
fast narrow-band imaging and post-factum image restoration. The
retrieved physical parameters will be a fundamental building block
for understanding the dynamic Sun and its magnetic field at spatial
scales down to 50 km on the solar surface. The GFPI is a tunable
dual-etalon system in a collimated mounting. It is designed for
spectropolarimetric observations over the wavelength range from 530-860
nm with a theoretical spectral resolution of R ≍ 250,000. The GFPI is
equipped with a full-Stokes polarimeter. Large-format, high-cadence CCD
detectors with powerful computer hard- and software enable the scanning
of spectral lines in time spans equivalent to the evolution time of
solar features. The field-of-view of 50”×38” covers a significant
fraction of the typical area of active regions. We present the main
characteristics of the GFPI including advanced and automated calibration
and observing procedures. We discuss improvements in the optical design
of the instrument and show first observational results. Finally, we
lay out first concrete ideas for the integration of a second FPI, the
Blue Imaging Solar Spectrometer, which will explore the blue spectral
region below 530 nm.

---------------------------------------------------------
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>

---------------------------------------------------------
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: Siphon flow in a cool magnetic loop
Authors: Bethge, C.; Beck, C.; Peter, H.; Lagg, A.
2012A&A...537A.130B Altcode: 2011arXiv1111.5564B
Context. Siphon flows that are driven by a gas pressure difference
between two photospheric footpoints of different magnetic field strength
connected by magnetic field lines are a well-studied phenomenon in
theory, but observational evidence is scarce. Aims. We investigate the
properties of a structure in the solar chromosphere in an active region
to find out whether the feature is consistent with a siphon flow in
a magnetic loop filled with chromospheric material. Methods. We
derived the line-of-sight (LOS) velocity of several photospheric
spectral lines and two chromospheric spectral lines, Ca II H 3968.5
*Aring; and He I 10830 Å, in spectropolarimetric observations of
NOAA 10978 done with the Tenerife Infrared Polarimeter (TIP-II) and
the POlarimetric LIttrow Spectrograph (POLIS). The structure can be
clearly traced in the LOS velocity maps and the absorption depth of
He I. The magnetic field configuration in the photosphere is inferred
directly from the observed Stokes parameters and from inversions with
the HELIX+ code. Data from the full-disk Chromospheric
Telescope (ChroTel) in He I in intensity and LOS velocity are used for
tracking the temporal evolution of the flow, along with TRACE Fe IX/X
171 Å data for additional information about coronal regions related to
the structure under investigation. Results. The inner end of the
structure is located in the penumbra of a sunspot. It shows downflows
whose strength decreases with decreasing height in the atmosphere. The
flow velocity in He I falls abruptly from above 40 km s-1
to about zero further into the penumbra. A slight increase of emission
is seen in the Ca II H spectra at the endpoint. At the outer end of the
structure, the photospheric lines that form higher up in the atmosphere
show upflows that accelerate with height. The polarization signal near
the outer end shows a polarity opposite to that of the sunspot, the
magnetic field strength of 580 G is roughly half as large as at the
inner end. The structure exists for about 90 min. Its appearance is
preceeded by a brightening in its middle in the coronal TRACE data. Conclusions. The observed flows match theoretical predictions of
chromospheric and coronal siphon flows, with accelerating upflowing
plasma at one footpoint with low field strength and decelerating
downflowing plasma at the other end. A tube shock at the inner end is
probable, but the evidence is not conclusive. The TRACE data suggest
that the structure forms because of a reorganization of field lines
after a reconnection event.

---------------------------------------------------------
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: The Chromospheric Telescope
Authors: Bethge, C.; Peter, H.; Kentischer, T. J.; Halbgewachs, C.;
Elmore, D. F.; Beck, C.
2011A&A...534A.105B Altcode: 2011arXiv1108.4880B
 Aims: We introduce the Chromospheric Telescope (ChroTel) at the
Observatorio del Teide in Izaña on Tenerife as a new multi-wavelength
imaging telescope for full-disk synoptic observations of the solar
chromosphere. We describe the design of the instrument and summarize
its performance during the first one and a half years of operation. We
present a method to derive line-of-sight velocity maps of the full
solar disk from filtergrams taken in and near the He i infrared line
at 10 830 Å. Methods: ChroTel observations are conducted
using Lyot-type filters for the chromospheric lines of Ca ii K,
Hα, and He i 10 830 Å. The instrument operates autonomically and
gathers imaging data in all three channels with a cadence of down to
one minute. The use of a tunable filter for the He i line allows us
to determine line-shifts by calibrating the line-of-sight velocity
maps derived from the filtergram intensities with spectrographic data
from the Tenerife Infrared Polarimeter at high spatial and spectral
resolution. Results: The robotic operation and automated data
reduction have proven to operate reliably in the first one and and
half years. The achieved spatial resolution of the data is close to
the theoretical limit of 2 arcsec in Hα and Ca ii K and 3 arcsec in He
i. Line-of-sight velocities in He i can be determined with a precision
of better than 3-4 km s-1 when co-temporal spectrographic
maps are available for calibration. Conclusions: ChroTel offers a
unique combination of imaging in the most important chromospheric lines,
along with the possibility to determine line-of-sight velocities in
one of the lines. This is of interest for scientific investigations
of large-scale structures in the solar chromosphere, as well as for
context imaging of high-resolution solar observations.

---------------------------------------------------------
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: Two-dimensional solar spectropolarimetry with the KIS/IAA
Visible Imaging Polarimeter
Authors: Beck, C.; Bellot Rubio, L. R.; Kentischer, T. J.; Tritschler,
A.; Del Toro Iniesta, J. C.
2010A&A...520A.115B Altcode: 2010arXiv1007.1153B
Context. Spectropolarimetry at high spatial and spectral resolution
is a basic tool to characterize the magnetic properties of the solar
atmosphere. Aims: We introduce the KIS/IAA Visible Imaging
Polarimeter (VIP), a new post-focus instrument that upgrades the TESOS
spectrometer at the German Vacuum Tower Telescope (VTT) into a full
vector polarimeter. VIP is a collaboration between the Kiepenheuer
Institut für Sonnenphysik (KIS) and the Instituto de Astrofísica
de Andalucía (IAA-CSIC). Methods: We describe the optical
setup of VIP, the data acquisition procedure, and the calibration of
the spectropolarimetric measurements. We show examples of data taken
between 2005 and 2008 to illustrate the potential of the instrument. Results: VIP is capable of measuring the four Stokes profiles of
spectral lines in the range from 420 to 700 nm with a spatial resolution
better than 0farcs5. Lines can be sampled at 40 wavelength positions
in 60 s, achieving a noise level of about 2 × 10-3 with
exposure times of 300 ms and pixel sizes of 0farcs17 × 0farcs17 (2
× 2 binning). The polarization modulation is stable over periods of a
few days, ensuring high polarimetric accuracy. The excellent spectral
resolution of TESOS allows the use of sophisticated data analysis
techniques such as Stokes inversions. One of the first scientific
results of VIP presented here is that the ribbon-like magnetic
structures of the network are associated with a distinct pattern of
net circular polarization away from disk center. Conclusions:
VIP performs spectropolarimetric measurements of solar magnetic fields
at a spatial resolution that is only slightly worse than that of the
Hinode spectropolarimeter, while providing a 2D field field of view and
the possibility to observe up to four spectral regions sequentially
with high cadence. VIP can be used as a stand-alone instrument or in
combination with other spectropolarimeters and imaging systems of the
VTT for extended wavelength coverage.

---------------------------------------------------------
Title: A chromospheric dark-cored fibril in Ca II IR spectra
Authors: Beck, C.; Tritschler, A.; Wöger, F.
2010AN....331..574B Altcode:
We investigate the thermodynamical and magnetic properties of a
“dark-cored" fibril seen in the chromospheric Ca II IR line at 854.2
nm to determine the physical process behind its appearance. We analyse
a time series of spectropolarimetric observations obtained in the Ca
II IR line at 854.2 nm and the photospheric Fe I line at 630.25 nm. We
simultaneously invert the spectra in both wavelength ranges with the
SIR code to obtain the temperature and velocity stratification with
height in the solar atmosphere and the magnetic field properties in the
photosphere. The structure can be clearly traced in the line-of-sight
(LOS) velocity and the temperature maps. It connects from a small
pore with kG fields to a region with lower field strength. The flow
velocity and the temperature indicate that the height of the structure
increases with increasing distance from the inner footpoint. The Stokes
V signal of 854.2 nm shows a Doppler-shifted polarization signal with
the same displacement as in the intensity profile, indicating that the
supersonic flow seen in the LOS velocity is located within magnetized
plasma. We conclude that the chromospheric dark-cored fibril traces
a siphon flow along magnetic field lines, driven by the gas pressure
difference caused by the higher magnetic field strength at the inner
footpoint. We suggest that fast flows guided by the magnetic field lead
to the appearance of “dark-cored" fibrils in intensity images. Although
the observations included the determination of the polarization signal
in the chromospheric Ca II IR line, the signal could not be analysed
quantitatively due to the low S/N. Chromospheric polarimetry will thus
require telescopes of larger aperture able to collect a sufficient
number of photons for a reliable determination of polarization in deep
and only weakly polarized spectral lines.

---------------------------------------------------------
Title: Linear wavelength correlation matrices of photospheric and
chromospheric spectral lines. I. Observations vs. modeling
Authors: Beck, C. A. R.; Rammacher, W.
2010A&A...510A..66B Altcode: 2009arXiv0909.2224B
Context. The process that heats the solar chromosphere is a difficult
target for observational studies because the assumption of local
thermal equilibrium (LTE) is not valid in the upper solar atmosphere,
which complicates the analysis of spectra. Aims: We investigate
the linear correlation coefficient between the intensities at different
wavelengths in photospheric and chromospheric spectral lines because the
correlation can be determined directly for any spectra from observations
or modeling. Waves which propagate vertically through the stratified
solar atmosphere affect different wavelengths at different times when
the contribution functions for each wavelength peak in different
layers. This leads to a characteristic pattern of (non-)coherence
of the intensity at various wavelengths with respect to each other
which carries information on the physical processes. Methods:
We derived the correlation matrices for several photospheric and
chromospheric spectral lines from observations. We separated locations
with a significant photospheric polarization signal and thus magnetic
fields from those without a polarization signal. For comparison with
the observations, we calculated correlation matrices for spectra from
simplified LTE modeling approaches, 1-D NLTE simulations, and a 3-D MHD
simulation run. We applied the correlation method also to temperature
maps at different optical depth layers derived from a LTE inversion
of Ca II H spectra. Results: We find that all photospheric
spectral lines show a similar pattern: a pronounced asymmetry of
the correlation between line core and red or blue wing. The pattern
cannot be reproduced with a simulation of the granulation pattern, but
with waves that travel upwards through the formation heights of the
lines. The correct asymmetry between red and blue wing only appears
when a temperature enhancement occurs simultaneously with a downflow
velocity in the wave simulation. All chromospheric spectral lines show
a more complex pattern. The 1-D NLTE simulations of monochromatic
waves produce a correlation matrix that qualitatively matches the
observations near the very core of the Ca II H line. The photospheric
signature is well reproduced in the correlation matrix derived from
the 3-D MHD simulation. Conclusions: The correlation matrices
of observed photospheric and chromospheric spectral lines are highly
structured with characteristic and different patterns in every spectral
line. The comparison with matrices derived from simulations and simple
modeling suggests that the main driver of the detected patterns are
upwards propagating waves. Application of the correlation method to
3-D temperature cubes seems to be a promising tool for a detailed
comparison of simulation results and observations in future studies.

---------------------------------------------------------
Title: Magnetic loop emergence within a granule
Authors: Gömöry, P.; Beck, C.; Balthasar, H.; Rybák, J.; Kučera,
A.; Koza, J.; Wöhl, H.
2010A&A...511A..14G Altcode: 2009arXiv0910.4449G
 Aims: We investigate the temporal evolution of magnetic
flux emerging within a granule in the quiet-Sun internetwork at
disk center. Methods: We combined IR spectropolarimetry of
high angular resolution performed in two Fe i lines at 1565 nm with
speckle-reconstructed G-band imaging. We determined the magnetic
field parameters by a LTE inversion of the full Stokes vector using
the SIR code, and followed their evolution in time. To interpret
the observations, we created a geometrical model of a rising loop in
3D. The relevant parameters of the loop were matched to the observations
where possible. We then synthesized spectra from the 3D model for a
comparison to the observations. Results: We found signatures of
magnetic flux emergence within a growing granule. In the early phases,
a horizontal magnetic field with a distinct linear polarization signal
dominated the emerging flux. Later on, two patches of opposite circular
polarization signal appeared symmetrically on either side of the linear
polarization patch, indicating a small loop-like structure. The mean
magnetic flux density of this loop was roughly 450 G, with a total
magnetic flux of around 3 × 1017 Mx. During the ~12 min
episode of loop occurrence, the spatial extent of the loop increased
from about 1 to 2 arcsec. The middle part of the appearing feature
was blueshifted during its occurrence, supporting the scenario of an
emerging loop. There is also clear evidence for the interaction of
one loop footpoint with a preexisting magnetic structure of opposite
polarity. The temporal evolution of the observed spectra is reproduced
to first order by the spectra derived from the geometrical model. During
the phase of clearest visibility of the loop in the observations,
the observed and synthetic spectra match quantitatively. 
Conclusions: The observed event can be explained as a case of flux
emergence in the shape of a small-scale loop. The fast disappearance
of the loop at the end could possibly be due to magnetic reconnection.

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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.

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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

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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.

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Title: A 3D sunspot model derived from an inversion of
spectropolarimetric observations and its implications for the
penumbral heating
Authors: Beck, C.
2008A&A...480..825B Altcode: 2007arXiv0712.3168B
Aims:I deduced a 3D sunspot model that is in full agreement with
spectropolarimetric observations, in order to address the question of
a possible penumbral heating process by the repetitive rise of hot flow
channels. Methods: I performed inversions of spectropolarimetric
data taken simultaneously in infrared (1.5 μm) and visible (630 nm)
spectral lines. I used two independent magnetic components inside each
pixel to reproduce the irregular Stokes profiles in the penumbra and
studied the averaged and individual properties of the two components. By
integrating the field inclination to the surface, I developed a 3D
model of the spot from inversion results without intrinsic height
information. Results: I find that the Evershed flow is harbored
by the weaker of the two field components. This component forms flow
channels that show upstreams in the inner and mid penumbra, continue
almost horizontally as slightly elevated loops throughout the penumbra,
and finally bend down in the outer penumbra. I find several examples
where two or more flow channels are found along a radial cut from the
umbra to the outer boundary of the spot. Conclusions: I find
that a model of horizontal flow channels in a static background field
is in good agreement with the observed spectra. The properties of the
flow channels correspond very well to the moving tube simulations of
Schlichenmaier et al. (1998, A&amp;A, 337, 897). From the temporal
evolution in intensity images and the properties of the flow channels
in the inversion, I conclude that interchange convection of rising
hot flux tubes in a thick penumbra still seems a possible mechanism
for maintaining the penumbral energy balance.

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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

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Title: Magnetic properties of G-band bright points in a sunspot moat
Authors: Beck, C.; Bellot Rubio, L. R.; Schlichenmaier, R.;
Sütterlin, P.
2007A&A...472..607B Altcode: 2007arXiv0707.1232B
We present simultaneous spectropolarimetric observations of four
visible (630 nm) and three infrared (1565 nm) spectral lines from the
German Vacuum Tower Telescope, together with speckle-reconstructed
filtergrams in the G-band and the Ca II H line core from the Dutch
Open Telescope. After alignment of the data sets, we used the G-band
intensity to locate bright points (BPs) in the moat of a regular
sunspot. With the cospatial and cotemporal information provided by the
polarimetric data, we characterize the magnetic, kinematic, and thermal
properties of the BPs. We find that (a) 94% of the BPs are associated
with magnetic fields; (b) their field strengths range between 500 and
1400 G, with a rather flat distribution; (c) the contrast of BPs in the
G-band depends on the angle between the vector magnetic field and the
line of sight; (d) the BPs harbor downflows of magnetized plasma and
exhibit Stokes V profiles with large area and amplitude asymmetries;
(e) the magnetic interior of BPs is hotter than the immediate field-free
surroundings by about 1000 K at equal optical depth; and (f) the mean
effective diameter of BPs in our data set is 150 km, with very few
BPs larger than 300 km. Most of these properties can be explained
by the classical magnetic flux tube model. However, the wide range
of BP parameters found in this study indicates that not all G-band
BPs are identical to stable long-lived flux tubes or sheets of kG
strength. Appendices A-C are only available in electronic form
at http://www.aanda.org

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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.

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Title: The multi-component field topology of sunspot penumbrae. A
diagnostic tool for spectropolarimetric measurements
Authors: Müller, D. A. N.; Schlichenmaier, R.; Fritz, G.; Beck, C.
2006A&A...460..925M Altcode: 2006astro.ph..9632M
Context: .Sunspot penumbrae harbor highly structured magnetic fields
and flows. The moving flux tube model offers an explanation for several
observed phenomena, e.g. the Evershed effect and bright penumbral
grains. Aims: .A wealth of information can be extracted from
spectropolarimetric observations. In order to deduce the structure of
the magnetic field in sunspot penumbrae, detailed forward modeling is
necessary. On the one hand, it gives insight into the sensitivity of
various spectral lines to different physical scenarios. On the other
hand, it is a very useful tool to guide inversion techniques. In this
work, we present a generalized 3D geometrical model that embeds an
arbitrarily shaped flux tube in a stratified magnetized atmosphere. Methods: .The new semi-analytical geometric model serves as a
frontend for a polarized radiative transfer code. The advantage of
this model is that it preserves the discontinuities of the physical
parameters across the flux tube boundaries. This is important for the
detailed shape of the emerging Stokes Profiles and the resulting net
circular polarization (NCP). Results: .(a) The inclination of
downflows in the outer penumbra must be shallower than approximately
15° (b) observing the limb-side NCP of sunspots in the Fe I 1564.8
nm line offers a promising way to identify a reduced magnetic field
strength in flow channels; (c) the choice of the background atmosphere
can significantly influence the shape of the Stokes profiles, but does
not change the global characteristics of the resulting NCP curves for
the tested atmospheric models.

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Title: Evershed Clouds as Precursors of Moving Magnetic Features
around Sunspots
Authors: Cabrera Solana, D.; Bellot Rubio, L. R.; Beck, C.; del Toro
Iniesta, J. C.
2006ApJ...649L..41C Altcode: 2006astro.ph..9108C
The relation between the Evershed flow and moving magnetic features
(MMFs) is studied using high-cadence, simultaneous spectropolarimetric
measurements of a sunspot in visible (630.2 nm) and near-infrared
(1565 nm) lines. Doppler velocities, magnetograms, and total linear
polarization maps are calculated from the observed Stokes profiles. We
follow the temporal evolution of two Evershed clouds that move radially
outward along the same penumbral filament. Eventually, the clouds
cross the visible border of the spot and enter the moat region, where
they become MMFs. The flux patch farther from the sunspot has the same
polarity of the spot, while the MMF closer to it has opposite polarity
and exhibits abnormal circular polarization profiles. Our results
provide strong evidence that at least some MMFs are the continuation
of the penumbral Evershed flow into the moat. This, in turn, suggests
that MMFs are magnetically connected to sunspots.

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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.

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Title: VIP - 2D Vector Spectropolarimetry of the Solar Atmosphere
near the Diffraction Limit
Authors: Bellot Rubio, L. R.; Tritschler, A.; Kentischer, T.; Beck,
C.; Del Toro Iniesta, J. C.
2006IAUJD...3E..58B Altcode:
The KIS/IAA Vector Imaging Polarimeter (VIP) is a new instrument for
two-dimensional spectropolarimetry of the solar atmosphere. It is used
with TESOS, the triple etalon Fabry-Perot interferometer installed at
the German Vacuum Tower Telescope of Observatorio del Teide (Tenerife,
Spain). The polarimeter is based on a pair of nematic liquid crystal
retarders and a Wollaston prism. VIP and TESOS are able to observe
any spectral line in the range from 450 nm to 750 nm with a spectral
resolving power of about 250000 and spatial resolutions better than
0.5" (thanks to the Kiepenheuer Adaptive Optics System). Typically,
the four Stokes parameters of a line can be measured at 40 wavelength
points in less than 60 s, with a noise level of 1-2 x 10^-3 and a
pixel size of 0.18" x 0.18" (2x2 binning). The modulation matrix of
VIP is derived using the polarimetric calibration unit installed at
the telescope. Here we present first-light observations of VIP carried
out in November 2005. We recorded the full Stokes profiles of the 630.1
and 630.2 nm Fe I lines emerging from a solar pore and its surroundings
at a spatial resolution of ~0.4". Based on these data, we discuss the
performance and capabilities of VIP as a powerful instrument for high
spatial and temporal resolution measurements of vector magnetic fields
in the solar atmosphere.

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Title: Observational Evidence for the "Hot Wall" Effect in Small
Magnetic Flux Concentrations
Authors: Mikurda, K.; Beck, C.
2006IAUJD...3E..91M Altcode:
Introduction: When lacking polarimetric observations, the Bright Points
(BPs) visible in the G-band at 430 nm are commonly used as tracers
for magnetic fields. Methods: Observations presented in this paper
were taken on October 11, 2005 at the German Vacuum Tower Telescope
and involved the Tenerife Infrared Polarimeter (TIP) at 1.5 micron,
the Telecentric Etalon Solar Spectrometer (TESOS) in the Fe I spectral
line at 557.6 nm, and a speckle setup in G-band. The area scanned by
TIP was 75”x33” and covered a pore surrounded by network. The TIP
spectra were inverted with the SIR (Stokes Inversion based on Response
functions) code to retrieve the magnetic field vector. Results: We find
that G-band BPs are not cospatial with the central part of the flux
concentrations. Even at the small heliocentric angle of 12 degree,
the BPs appear projected on the limb side walls of the granules
(Fig. 1), whereas the fields are concentrated in the intergranular
lanes. Discussion: Our findings indicate that the G-band Bright Points
are a result of the "hot wall effect". The downward shift of the optical
depth scale in the presence of magnetic fields allows to see deeper
and hotter layers, where CH dissociates, in the granules next to the
field concentrations. Thus, information drawn from the observations of
BPs cannot be used to conclude on the actual variation of the magnetic
field structure, as only the outer parts of the flux concentrations
are seen in the BPs. Figure 1. Non-cospatiality of BPs and field
concentrations. Top row, left to right: line-core intensity, LOS
velocity (range =+/- 1 km/s), magnetic flux , polarity, LOS velocity
from the inversion (range =+/- 1.5 km/s), field azimuth. Bottom
row, left to right: polarization degree, G-band intensity, continuum
intensity at 1.5 micron, BP mask, field strength, field inclination. The
white arrow in the G-band image denotes the limb direction, the cross
marks the center of the 6.7 x 6.7 Mm field-of-view shown.

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Title: The 3-D topology of magnetic fields in and around sunspots
Authors: Beck, Christian
2006PhDT........30B Altcode:
The most prominent features on the surface of the solar disc are
the sunspots, which have been studied since their detection in
the 17th century. Despite this years and centuries of scientific
investigations, surprisingly many facets of sunspots are not well
understood. In this thesis, the properties of a sunspots' penumbra
are derived from the observed spectra by means of an inversion,
a forward modeling technique. A method for the construction of a
3-dimensional model of the field topology from the inversion results
is presented. It offers the possibility to build a toy-model of the
spot. This allows to investigate the relationships between the geometry
and other properties of the sunspot like flow velocities, intensity,
temperature, or field strength. The application of the procedure to
time series of observations will allow to study the temporal evolution
of the geometry and the other properties, to determine the physical
processes happening and their respective drivers. A second
part of the thesis is devoted to the analysis of isolated field
concentrations in the surroundings of the sunspot. These isolated
magnetic elements are assumed to be the elementary building blocks
of the solar surface magnetism. In the turbulent surroundings of
the solar atmosphere, only relatively strong concentrations with a
field strength above 1 kG are assumed to be stable at all. Such field
concentrations produce brightenings in the G band, where a lot of
spectral lines of the CH molecule are present. Inside magnetic fields,
the density is reduced, as the magnetic pressure contributes to the
pressure balance equation. Thus, the reduced density allows to see
to deeper layers of the atmosphere with higher temperature. If the
CH-molecules dissociate due to the higher temperature, the spectral
lines disappear and the intensity increases strongly. Due to the
small size of the field concentration, the brightenings are strongly
localized (”G-band Bright Points”). In this study, we find a much
larger variability of the Bright Point properties than expected, which
puts their elementary nature in some doubt. In the conclusions
of this work, the resulting 3-D topology of the sunspot is used in
an attempt to develop a consistent picture of the development and the
fine structure of sunspots.

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Title: A polarization model for the German Vacuum Tower Telescope
from in situ and laboratory measurements
Authors: Beck, C.; Schlichenmaier, R.; Collados, M.; Bellot Rubio,
L.; Kentischer, T.
2005A&A...443.1047B Altcode:
It is essential to properly calibrate the polarimetric properties of
telescopes, if one wants to take advantage of the capabilities of high
precision spectro-polarimeters. We have constructed a model for the
German Vacuum Tower Telescope (VTT) that describes its time-dependent
polarization properties. Since the coelostat of the telescope changes
the polarization state of the light by introducing cross talk among
different polarization states, such a model is necessary to correct the
measurements, in order to retrieve the true polarization as emitted
from the Sun. The telescope model is quantified by a time-dependent
Mueller matrix that depends on the geometry of the light beam through
the telescope, and on material properties: the refractive indices of the
coelostat mirrors, and the birefringence of the entrance window to the
vacuum tube. These material properties were determined experimentally
in-situ by feeding the telescope with known states of polarization
(including unpolarized light) and by measuring its response, and from
measurements of an aluminum-coated sample in the laboratory. Accuracy
can in our case be determined only for the combination of telescope
and spectro-polarimeter used; for the instrument POLIS at the VTT,
we estimate an accuracy of ±4-5× 10-3 for the cross talk
correction coefficients.

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Title: Polarimetric Littrow Spectrograph - instrument calibration
and first measurements
Authors: Beck, C.; Schmidt, W.; Kentischer, T.; Elmore, D.
2005A&A...437.1159B Altcode:
We present first measurements and the calibration procedure for the
Polarimetric Littrow Spectrograph (POLIS) operated at the Vacuum Tower
Telescope on Tenerife, together with a brief summary of the technical
characteristics of the instrument. In its present configuration,
we achieve a polarimetric accuracy of about 3 × 10-3
in the visible channel (630 nm) of the instrument. The accuracy is
limited by cross talk among the different polarization states. The
detection limit for polarized light is about 2 × 10-3
for a 7 s exposure. Polarimetric measurements in the blue channel (Ca
II H line, 396.7 nm) are strongly limited by the low photon flux. At
this wavelength we present Stokes-V maps with a spatial resolution of
about 0.5 arcs. The polarimetric quality of any spectropolarimeter is
limited by the precision of the instrument calibration. We present
a new method for self-calibration that reduces cross talk among the
polarization components to 0.1%. This improvement results from a
measurement of the retardance of the calibration waveplate with an
accuracy of 0.1°. We demonstrate the capability of the simultaneous
use of POLIS and the Tenerife Infrared Polarimeter which is integrated
in the main spectrograph of the Vacuum Tower Telescope.

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Title: Magnetic flux in the internetwork quiet Sun
Authors: Khomenko, E. V.; Martínez González, M. J.; Collados, M.;
Vögler, A.; Solanki, S. K.; Ruiz Cobo, B.; Beck, C.
2005A&A...436L..27K Altcode:
We report a direct comparison of the amplitudes of Stokes spectra of the
Fe i 630 nm and 1.56 μm lines produced by realistic MHD simulations
with simultaneous observations in the same spectral regions. The
Stokes spectra were synthesized in snapshots with a mixed polarity
magnetic field having a spatially averaged strength, &lt; B &gt;,
between 10 and 30 G. The distribution of Stokes V amplitudes depends
sensitively on &lt; B &gt;. A quiet inter-network region was observed
at the German VTT simultaneously with TIP (1.56 μm) and POLIS (630
nm). We find that the Stokes V amplitudes of both infrared and visible
observations are best reproduced by the simulation snapshot with &lt;
B &gt; = 20 G. In observations with 1 resolution, up to 2/3 of the
magnetic flux can remain undetected.

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Title: Magnetic Flux Cancellation in the Moat of Sunspots: Results
from Simultaneous Vector Spectropolarimetry in the Visible and
the Infrared
Authors: Bellot Rubio, Luis R.; Beck, Christian
2005ApJ...626L.125B Altcode:
We analyze multiwavelength observations of the cancellation of a
moving magnetic feature and a plage element at the outer edge of
the moat of an isolated, round sunspot. The event lasted for some 35
minutes until the smaller flux concentration disappeared completely
from the photosphere. The data set consists of high-resolution, full
vector spectropolarimetric measurements of four visible lines and
two near-infrared lines, along with speckle-reconstructed G-band and
Ca II H filtergrams. The observations reveal strong chromospheric
emission at the neutral line separating the two magnetic poles;
it becomes visible 18 minutes after the cancellation has started
and persists for 25 minutes. We have carried out an inversion of
the observed Stokes profiles to determine the variation of the
vector magnetic field, temperature, and line-of-sight velocity
during the cancellation. No significant changes in field strength,
field inclination, or temperature are observed in either of the two
opposite-polarity patches. The decrease in magnetic flux is primarily
due to a decrease in magnetic filling factor, which is accompanied by
strong upflows (of at least 1.1 km s-1) in the smaller flux
concentration. These results suggest that the cancellation is due to
magnetic reconnection in the photosphere.

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Title: POLIS: A spectropolarimeter for the VTT and for GREGOR
Authors: Schmidt, W.; Beck, C.; Kentischer, T.; Elmore, D.; Lites, B.
2003AN....324..300S Altcode:
The polarimetric Littrow Spectrograph POLIS is designed for vector
polarimetry at high angular and spectral resolution. It measures the
magnetic field simultaneously in the photosphere and the chromosphere
of the sun. Both branches of the polarimetry unit are dual beam systems
with a single rotating modulator for both wavelengths and polarizing
beam splitters in front of each CCD camera. POLIS has been installed
at the VTT on Tenerife and has seen First Light on 17 May 2002. A
modified version of POLIS will be developed for the balloon mission
Sunrise. That version will have UV capabilities down to 200 nm.