schmidt

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Author name code: schmidt
ADS astronomy entries on 2022-09-14
author:"Schmidt, Wolfgang" AND aff:"Freiburg"

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Title: The magnetic drivers of campfires seen by the Polarimetric
and Helioseismic Imager (PHI) on Solar Orbiter
Authors: Kahil, F.; Hirzberger, J.; Solanki, S. K.; Chitta, L. P.;
Peter, H.; Auchère, F.; Sinjan, J.; Orozco Suárez, D.; Albert,
K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero, A.; Blanco
Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero, L.; Gutiérrez
Márquez, P.; Kolleck, M.; del Toro Iniesta, J. C.; Volkmer, R.;
Woch, J.; Fiethe, B.; Gómez Cama, J. M.; Pérez-Grande, I.; Sanchis
Kilders, E.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Calchetti,
D.; Carmona, M.; Deutsch, W.; Fernández-Rico, G.; Fernández-Medina,
A.; García Parejo, P.; Gasent-Blesa, J. L.; Gizon, L.; Grauf, B.;
Heerlein, K.; Lagg, A.; Lange, T.; López Jiménez, A.; Maue, T.;
Meller, R.; Michalik, H.; Moreno Vacas, A.; Müller, R.; Nakai,
E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub, J.; Strecker, H.;
Torralbo, I.; Valori, G.; Aznar Cuadrado, R.; Teriaca, L.; Berghmans,
D.; Verbeeck, C.; Kraaikamp, E.; Gissot, S.
2022A&A...660A.143K Altcode: 2022arXiv220213859K
Context. The Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter
(SO) spacecraft observed small extreme ultraviolet (EUV) bursts,
termed campfires, that have been proposed to be brightenings near the
apexes of low-lying loops in the quiet-Sun atmosphere. The underlying
magnetic processes driving these campfires are not understood. Aims: During the cruise phase of SO and at a distance of 0.523
AU from the Sun, the Polarimetric and Helioseismic Imager on Solar
Orbiter (SO/PHI) observed a quiet-Sun region jointly with SO/EUI,
offering the possibility to investigate the surface magnetic field
dynamics underlying campfires at a spatial resolution of about 380
km. Methods: We used co-spatial and co-temporal data of the
quiet-Sun network at disc centre acquired with the High Resolution
Imager of SO/EUI at 17.4 nm (HRIEUV, cadence 2 s) and the
High Resolution Telescope of SO/PHI at 617.3 nm (HRT, cadence 2.5
min). Campfires that are within the SO/PHI−SO/EUI common field
of view were isolated and categorised according to the underlying
magnetic activity. Results: In 71% of the 38 isolated events,
campfires are confined between bipolar magnetic features, which seem to
exhibit signatures of magnetic flux cancellation. The flux cancellation
occurs either between the two main footpoints, or between one of the
footpoints of the loop housing the campfire and a nearby opposite
polarity patch. In one particularly clear-cut case, we detected the
emergence of a small-scale magnetic loop in the internetwork followed
soon afterwards by a campfire brightening adjacent to the location
of the linear polarisation signal in the photosphere, that is to
say near where the apex of the emerging loop lays. The rest of the
events were observed over small scattered magnetic features, which
could not be identified as magnetic footpoints of the campfire hosting
loops. Conclusions: The majority of campfires could be driven
by magnetic reconnection triggered at the footpoints, similar to the
physical processes occurring in the burst-like EUV events discussed
in the literature. About a quarter of all analysed campfires, however,
are not associated to such magnetic activity in the photosphere, which
implies that other heating mechanisms are energising these small-scale
EUV brightenings.

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Title: Newly formed downflow lanes in exploding granules in the
solar photosphere
Authors: Ellwarth, M.; Fischer, C. E.; Vitas, N.; Schmiz, S.;
Schmidt, W.
2021A&A...653A..96E Altcode: 2021arXiv210700582E
Context. Exploding granules have drawn renewed interest because of
their interaction with the magnetic field (either emerging or already
present). Especially the newly forming downflow lanes developing in
their centre seem to be eligible candidates for the intensification
of magnetic fields. We analyse spectroscopic data from two different
instruments in order to study the intricate velocity pattern within
the newly forming downflow lanes in detail. Aims: We aim to
examine general properties of a number of exploding granules, such
as their lifetime and extend. To gain a better understanding of the
formation process of the developing intergranular lane in exploding
granules, we study the temporal evolution and height dependence of the
line-of-sight velocities at their formation location. Additionally, we
search for evidence that exploding granules act as acoustic sources. Methods: We investigated the evolution of several exploding granules
using data taken with the Interferometric Bidimensional Spectrometer and
the Imaging Magnetograph eXperiment. Velocities for different heights
of the solar atmosphere were determined by computing bisectors of the
Fe I 6173.0 Å and the Fe I 5250.2 Å lines. We performed a wavelet
analysis to study the intensity and velocity oscillations within
and around exploding granules. We also compared our observational
findings with predictions of numerical simulations. Results:
Exploding granules have significantly longer lifetimes (10 to 15 min)
than regular granules. Exploding granules larger than 3.8″ form an
independent intergranular lane during their decay phase, while smaller
granules usually fade away or disappear into the intergranular area
(we find only one exception of a smaller exploding granule that also
forms an intergranular lane). For all exploding granules that form a new
intergranular downflow lane, we find a temporal height-dependent shift
with respect to the maximum of the downflow velocity. Our suggestion
that this results from a complex atmospheric structure within the newly
forming downflow lane is supported by the comparison with synthesised
profiles inferred from the simulations. We found an enhanced wavelet
power with periods between 120 s to 190 s seen in the intensity and
velocity oscillations of high photospheric or chromospheric spectral
lines in the region of the dark core of an exploding granule.

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Title: On the (in)stability of sunspots
Authors: Strecker, H.; Schmidt, W.; Schlichenmaier, R.; Rempel, M.
2021A&A...649A.123S Altcode: 2021arXiv210311487S
Context. The stability of sunspots is one of the long-standing unsolved
puzzles in the field of solar magnetism and the solar cycle. The thermal
and magnetic structure of the sunspot beneath the solar surface is
not accessible through observations, thus processes in these regions
that contribute to the decay of sunspots can only be studied through
theoretical and numerical studies. Aims: We study the effects
that destabilise and stabilise the flux tube of a simulated sunspot
in the upper convection zone. The depth-varying effects of fluting
instability, buoyancy forces, and timescales on the flux tube are
analysed. Methods: We analysed a numerical simulation of a
sunspot calculated with the MURaM code. The simulation domain has a
lateral extension of more than 98 Mm × 98 Mm and extends almost 18
Mm below the solar surface. The analysed data set of 30 hours shows a
stable sunspot at the solar surface. We studied the evolution of the
flux tube at defined horizontal layers (1) by means of the relative
change in perimeter and area, that is, its compactness; and (2) with
a linear stability analysis. Results: The simulation shows a
corrugation along the perimeter of the flux tube (sunspot) that proceeds
fastest at a depth of about 8 Mm below the solar surface. Towards
the surface and towards deeper layers, the decrease in compactness is
damped. From the stability analysis, we find that above a depth of 2
Mm, the sunspot is stabilised by buoyancy forces. The spot is least
stable at a depth of about 3 Mm because of the fluting instability. In
deeper layers, the flux tube is marginally unstable. The stability
of the sunspot at the surface affects the behaviour of the field
lines in deeper layers by magnetic tension. Therefore the fluting
instability is damped at depths of about 3 Mm, and the decrease in
compactness is strongest at a depth of about 8 Mm. The more vertical
orientation of the magnetic field and the longer convective timescale
lead to slower evolution of the corrugation process in layers deeper
than 10 Mm. Conclusions: The formation of large intrusions of
field-free plasma below the surface destabilises the flux tube of
the sunspot. This process is not visible at the surface, where the
sunspot is stabilised by buoyancy forces. The onset of sunspot decay
occurs in deeper layers, while the sunspot still appears stable in
the photosphere. The intrusions eventually lead to the disruption
and decay of the sunspot. The animation is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202040199/olm">https://www.aanda.org</A>
This paper is mainly based on Part I of
the Ph.D. thesis "On the decay of sunspots", <A
href="https://freidok.uni-freiburg.de/data/165760">https://freidok.uni-freiburg.de/data/165760</A>

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Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
2021SoPh..296...70R Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.

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Title: Power spectrum of turbulent convection in the solar photosphere
Authors: Yelles Chaouche, L.; Cameron, R. H.; Solanki, S. K.;
Riethmüller, T. L.; Anusha, L. S.; Witzke, V.; Shapiro, A. I.;
Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; van Noort,
M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez,
D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2020A&A...644A..44Y Altcode: 2020arXiv201009037Y
The solar photosphere provides us with a laboratory for understanding
turbulence in a layer where the fundamental processes of transport
vary rapidly and a strongly superadiabatic region lies very closely
to a subadiabatic layer. Our tools for probing the turbulence are
high-resolution spectropolarimetric observations such as have recently
been obtained with the two balloon-borne SUNRISE missions, and numerical
simulations. Our aim is to study photospheric turbulence with the
help of Fourier power spectra that we compute from observations
and simulations. We also attempt to explain some properties of the
photospheric overshooting flow with the help of its governing equations
and simulations. We find that quiet-Sun observations and smeared
simulations are consistent with each other and exhibit a power-law
behavior in the subgranular range of their Doppler velocity power
spectra with a power-law index of ≈ - 2. The unsmeared simulations
exhibit a power law that extends over the full range between the
integral and Taylor scales with a power-law index of ≈ - 2.25. The
smearing, reminiscent of observational conditions, considerably reduces
the extent of the power-law-like portion of the power spectra. This
suggests that the limited spatial resolution in some observations
might eventually result in larger uncertainties in the estimation of
the power-law indices. The simulated vertical velocity power spectra
as a function of height show a rapid change in the power-law index
(at the subgranular range) from roughly the optical depth unity layer,
that is, the solar surface, to 300 km above it. We propose that the
cause of the steepening of the power-law index is the transition from
a super- to a subadiabatic region, in which the dominant source of
motions is overshooting convection. A scale-dependent transport of
the vertical momentum occurs. At smaller scales, the vertical momentum
is more efficiently transported sideways than at larger scales. This
results in less vertical velocity power transported upward at small
scales than at larger scales and produces a progressively steeper
vertical velocity power law below 180 km. Above this height, the
gravity work progressively gains importance at all relevant scales,
making the atmosphere progressively more hydrostatic and resulting
in a gradually less steep power law. Radiative heating and cooling of
the plasma is shown to play a dominant role in the plasma energetics
in this region, which is important in terms of nonadiabatic damping
of the convective motions.

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Title: 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: The Polarimetric and Helioseismic Imager on Solar Orbiter
Authors: Solanki, S. K.; del Toro Iniesta, J. C.; Woch, J.; Gandorfer,
A.; Hirzberger, J.; Alvarez-Herrero, A.; Appourchaux, T.; Martínez
Pillet, V.; Pérez-Grande, I.; Sanchis Kilders, E.; Schmidt, W.;
Gómez Cama, J. M.; Michalik, H.; Deutsch, W.; Fernandez-Rico, G.;
Grauf, B.; Gizon, L.; Heerlein, K.; Kolleck, M.; Lagg, A.; Meller, R.;
Müller, R.; Schühle, U.; Staub, J.; Albert, K.; Alvarez Copano, M.;
Beckmann, U.; Bischoff, J.; Busse, D.; Enge, R.; Frahm, S.; Germerott,
D.; Guerrero, L.; Löptien, B.; Meierdierks, T.; Oberdorfer, D.;
Papagiannaki, I.; Ramanath, S.; Schou, J.; Werner, S.; Yang, D.;
Zerr, A.; Bergmann, M.; Bochmann, J.; Heinrichs, J.; Meyer, S.;
Monecke, M.; Müller, M. -F.; Sperling, M.; Álvarez García, D.;
Aparicio, B.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Cobos
Carracosa, J. P.; Girela, F.; Hernández Expósito, D.; Herranz, M.;
Labrousse, P.; López Jiménez, A.; Orozco Suárez, D.; Ramos, J. L.;
Barandiarán, J.; Bastide, L.; Campuzano, C.; Cebollero, M.; Dávila,
B.; Fernández-Medina, A.; García Parejo, P.; Garranzo-García, D.;
Laguna, H.; Martín, J. A.; Navarro, R.; Núñez Peral, A.; Royo, M.;
Sánchez, A.; Silva-López, M.; Vera, I.; Villanueva, J.; Fourmond,
J. -J.; de Galarreta, C. Ruiz; Bouzit, M.; Hervier, V.; Le Clec'h,
J. C.; Szwec, N.; Chaigneau, M.; Buttice, V.; Dominguez-Tagle, C.;
Philippon, A.; Boumier, P.; Le Cocguen, R.; Baranjuk, G.; Bell,
A.; Berkefeld, Th.; Baumgartner, J.; Heidecke, F.; Maue, T.; Nakai,
E.; Scheiffelen, T.; Sigwarth, M.; Soltau, D.; Volkmer, R.; Blanco
Rodríguez, J.; Domingo, V.; Ferreres Sabater, A.; Gasent Blesa,
J. L.; Rodríguez Martínez, P.; Osorno Caudel, D.; Bosch, J.; Casas,
A.; Carmona, M.; Herms, A.; Roma, D.; Alonso, G.; Gómez-Sanjuan, A.;
Piqueras, J.; Torralbo, I.; Fiethe, B.; Guan, Y.; Lange, T.; Michel,
H.; Bonet, J. A.; Fahmy, S.; Müller, D.; Zouganelis, I.
2020A&A...642A..11S Altcode: 2019arXiv190311061S
 Aims: This paper describes the Polarimetric and Helioseismic
Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and
helioseismology instrument to observe the Sun from outside the Sun-Earth
line. It is the key instrument meant to address the top-level science
question: How does the solar dynamo work and drive connections between
the Sun and the heliosphere? SO/PHI will also play an important role
in answering the other top-level science questions of Solar Orbiter,
while hosting the potential of a rich return in further science. Methods: SO/PHI measures the Zeeman effect and the Doppler shift
in the Fe I 617.3 nm spectral line. To this end, the instrument
carries out narrow-band imaging spectro-polarimetry using a tunable
LiNbO3 Fabry-Perot etalon, while the polarisation modulation
is done with liquid crystal variable retarders. The line and the nearby
continuum are sampled at six wavelength points and the data are recorded
by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data
are reduced on board, including being inverted under the assumption of
a Milne-Eddington atmosphere, although simpler reduction methods are
also available on board. SO/PHI is composed of two telescopes; one,
the Full Disc Telescope, covers the full solar disc at all phases of
the orbit, while the other, the High Resolution Telescope, can resolve
structures as small as 200 km on the Sun at closest perihelion. The high
heat load generated through proximity to the Sun is greatly reduced by
the multilayer-coated entrance windows to the two telescopes that allow
less than 4% of the total sunlight to enter the instrument, most of
it in a narrow wavelength band around the chosen spectral line. 
Results: SO/PHI was designed and built by a consortium having partners
in Germany, Spain, and France. The flight model was delivered to
Airbus Defence and Space, Stevenage, and successfully integrated into
the Solar Orbiter spacecraft. A number of innovations were introduced
compared with earlier space-based spectropolarimeters, thus allowing
SO/PHI to fit into the tight mass, volume, power and telemetry budgets
provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal)
challenges posed by the mission's highly elliptical orbit.

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Title: Subpixel real-time jitter detection algorithm and
implementation for polarimetric and helioseismic imager
Authors: Roma, David; Carmona, Manuel; Bosch, Jose; Casas, Albert;
Herms, Atila; Lopez, Manel; Ruiz, Oscar; Sabater, Josep; Berkefeld,
Thomas; Maue, Thorsten; Nakai, Eiji; Schmidt, Wolfgang; Soltau, Dirk;
Volkmer, Reiner; Gomez, Jose M.
2019JATIS...5c9003R Altcode:
The polarimetric and helioseismic imager instrument for the Solar
Orbiter mission from the European Space Agency requires a high
stability while capturing images, specially for the polarimetric
ones. For this reason, an image stabilization system has been included
in the instrument. It uses global motion estimation techniques to
estimate the jitter in real time with subpixel resolution. Due to
instrument requirements, the algorithm has to be implemented in a Xilinx
Virtex-4QV field programmable gate array. The algorithm includes a 2-D
paraboloid interpolation algorithm based on 2-D bisection. We describe
the algorithm implementation and the tests that have been made to
verify its performance. The jitter estimation has a mean error of 125
pixel of the correlation tracking camera. The paraboloid interpolation
algorithm provides also better results in terms of resources and time
required for the calculation (at least a 20% improvement in both cases)
than those based on direct calculation.

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Title: Spectropolarimetric Observations of an Arch Filament System
with GREGOR
Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.;
Kuckein, C.; Kučera, A.; Schwartz, P.; Berkefeld, T.; Collados, M.;
Denker, C.; Feller, A.; Hofmann, A.; Schlichenmaier, R.; Schmidt,
D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau,
D.; Staude, J.; Strassmeier, K. G.; von der Lühe, O.
2019ASPC..526..217B Altcode: 2018arXiv180401789B
We observed an arch filament system (AFS) in a sunspot group with the
GREGOR Infrared Spectrograph attached to the GREGOR solar telescope. The
AFS was located between the leading sunspot of negative polarity and
several pores of positive polarity forming the following part of the
sunspot group. We recorded five spectro-polarimetric scans of this
region. The spectral range included the spectral lines Si I 1082.7
nm, He I 1083.0 nm, and Ca I 1083.9 nm. In this work we concentrate
on the silicon line which is formed in the upper photosphere. The
line profiles are inverted with the code 'Stokes Inversion based
on Response functions' to obtain the magnetic field vector. The
line-of-sight velocities are determined independently with a Fourier
phase method. Maximum velocities are found close to the ends of AFS
fibrils. These maximum values amount to 2.4 km s-1 next
to the pores and to 4 km s-1 at the sunspot side. Between
the following pores, we encounter an area of negative polarity that
is decreasing during the five scans. We interpret this by new emerging
positive flux in this area canceling out the negative flux. In summary,
our findings confirm the scenario that rising magnetic flux tubes
cause the AFS.

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Title: Convective blueshifts in the solar
atmosphere. III. High-accuracy observations of spectral lines in
the visible
Authors: Löhner-Böttcher, J.; Schmidt, W.; Schlichenmaier, R.;
Steinmetz, T.; Holzwarth, R.
2019A&A...624A..57L Altcode: 2019arXiv190107606L
Context. Convective motions in the solar atmosphere cause spectral lines
to become asymmetric and shifted in wavelength. For photospheric lines,
this differential Doppler shift varies from the solar disk center
to the limb. Aims: Precise and comprehensive observations of
the convective blueshift and its center-to-limb variation improve
our understanding of the atmospheric hydrodynamics and ensuing line
formation, and provide the basis to refine 3D models of the solar
atmosphere. Methods: We performed systematical spectroscopic
measurements of the convective blueshift of the quiet Sun with the
Laser Absolute Reference Spectrograph (LARS) at the German Vacuum
Tower Telescope. The spatial scanning of the solar disk covered
11 heliocentric positions each along four radial (meridional and
equatorial) axes. The high-resolution spectra of 26 photospheric
to chromospheric lines in the visible range were calibrated with a
laser frequency comb to absolute wavelengths at the 1 m s-1
accuracy. Applying ephemeris and reference corrections, the bisector
analysis provided line asymmetries and Doppler shifts with an
uncertainty of only few m s-1. To allow for a comparison
with other observations, we convolved the results to lower spectral
resolutions. Results: All spectral line bisectors exhibit
a systematic center-to-limb variation. Typically, a blueshifted
"C"-shaped curve at disk center transforms into a less blueshifted
"\"-shape toward the solar limb. The comparison of all lines reveals
the systematic dependence of the convective blueshift on the line
depth. The blueshift of the line minima describe a linear decrease
with increasing line depths. The slope of the center-to-limb variation
develops a reversal point at heliocentric positions between μ = 0.7
and 0.85, seen as the effect of horizontal granular flows in the mid
photosphere. Line minima formed in the upper photosphere to chromosphere
exhibit hardly any blueshift or even a slight redshift. Synthetic
models yield considerable deviations from the observed center-to-limb
variation. Conclusions: The obtained Doppler shifts of the
quiet Sun can serve as an absolute reference for other observations,
the relative calibration of Dopplergrams, and the necessary refinement
of atmospheric models. Based on this, the development of high-precision
models of stellar surface convection will advance the detection of
(potentially habitable) exoplanets by radial velocity measurements. The reduced spectra are only available at the CDS via anonymous ftp to
<A href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A57">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A57</A>

---------------------------------------------------------
Title: Convective blueshifts in the solar
atmosphere. II. High-accuracy observations of the Fe I 6173.3 Å
line and deviations of full-disk Dopplergrams
Authors: Stief, F.; Löhner-Böttcher, J.; Schmidt, W.; Steinmetz,
T.; Holzwarth, R.
2019A&A...622A..34S Altcode: 2018arXiv181108685S
Context. Granular convective motions reach into the lower solar
atmosphere, typically causing photospheric spectral lines to exhibit
a differential line shift. This Doppler shift to shorter wavelengths
is commonly known as convective blueshift. Aims: Spectroscopic
high-accuracy measurements provide us with a refined determination of
the absolute convective blueshift and its atmospheric distribution from
disk center to the solar limb. Methods: We performed systematic
observations of the quiet Sun with the Laser Absolute Reference
Spectrograph (LARS) at the German Vacuum Tower Telescope. The solar disk
was scanned along the meridian and the equator, from the disk center
toward the limb. The solar spectrum around 6173 Å was calibrated with a
laser frequency comb on an absolute wavelength scale with an accuracy
of a few meters per second. We applied a bisector analysis on the
spectral lines to reveal the changes of convective blueshift and line
asymmetry at different heliocentric positions. Results: Being a
signature for convective motions, the bisector curve of Fe I 6173.3 Å
describes a "C"-shape at disk center. When approaching the solar limb,
the bisector transforms into a "\"-shape. The analysis of the time- and
bisector-averaged line shifts yields three distinct results. Firstly,
the center-to-limb variation of Doppler velocities measured with
LARS reveals a significant discrepancy (up to 200 m s-1)
to the full-disk Dopplergrams of the Helioseismic and Magnetic Imager
(HMI). Secondly, we obtained a significant decrease of convective
blueshift toward the solar limb. Thirdly, the line-of-sight effect of
solar activity, including p-mode oscillations and supergranular flows,
leads to a scatter of up to ±100 m s-1 at intermediate
heliocentric positions. Conclusions: The accurate observation of
the absolute convective blueshift with LARS allows the identification
of systematic discrepancy with Doppler velocities measured by HMI. The
center-to-limb variation of HMI suffers from an additional blueshift for
μ &lt; 0.9 that is incompatible with our results. LARS measurements
can be taken as a reference for the correction of systematic errors
in the synoptic HMI Dopplergrams.

---------------------------------------------------------
Title: Absolute velocity measurements in sunspot umbrae
Authors: Löhner-Böttcher, J.; Schmidt, W.; Schlichenmaier, R.;
Doerr, H. -P.; Steinmetz, T.; Holzwarth, R.
2018A&A...617A..19L Altcode: 2018arXiv180408304L
Context. In sunspot umbrae, convection is largely suppressed by the
strong magnetic field. Previous measurements reported on negligible
convective flows in umbral cores. Based on this, numerous studies have
taken the umbra as zero reference to calculate Doppler velocities of
the ambient active region. Aims: We aim to clarify the amount of
convective motion in the darkest part of umbrae, by directly measuring
Doppler velocities with an unprecedented accuracy and precision. 
Methods: We performed spectroscopic observations of sunspot umbrae with
the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum
Tower Telescope. A laser frequency comb enabled the calibration of
the high-resolution spectrograph and absolute wavelength positions for
13 observation sequences. A thorough spectral calibration, including
the measurement of the reference wavelength, yielded Doppler shifts
of the spectral line Ti I 5713.9 Å with an uncertainty of around 5
m s-1. A bisector analysis gave the depth-dependent line
asymmetry. Results: The measured Doppler shifts are a composition
of umbral convection and magneto-acoustic waves. For the analysis of
convective shifts, we temporally averaged each sequence to reduce the
superimposed wave signal. Compared to convective blueshifts of up to
-350 m s-1 in the quiet Sun, sunspot umbrae yield strongly
reduced convective blueshifts around -30 m s-1. We find
that the velocity in a sunspot umbra correlates significantly with the
magnetic field strength, but also with the umbral temperature defining
the depth of the Ti I 5713.9 Å line. The vertical upward motion
decreases with increasing field strength. Extrapolating the linear
approximation to zero magnetic field reproduces the measured quiet Sun
blueshift. In the same manner, we find that the convective blueshift
decreases as a function of increasing line depth. Conclusions:
Simply taking the sunspot umbra as a zero velocity reference for the
calculation of photospheric Dopplergrams can imply a systematic velocity
error reaching 100 m s-1, or more. Setting up a relationship
between vertical velocities and magnetic field strength provides a
remedy for solar spectropolarimetry. We propose a novel approach of
substantially increasing the accuracy of the Doppler velocities of a
sunspot region by including the magnetic field information to define
the umbral reference velocity.

---------------------------------------------------------
Title: Convective blueshifts in the solar atmosphere. I. Absolute
measurements with LARS of the spectral lines at 6302 Å
Authors: Löhner-Böttcher, J.; Schmidt, W.; Stief, F.; Steinmetz,
T.; Holzwarth, R.
2018A&A...611A...4L Altcode: 2017arXiv171207059L
Context. The solar convection manifests as granulation and
intergranulation at the solar surface. In the photosphere, convective
motions induce differential Doppler shifts to spectral lines. The
observed convective blueshift varies across the solar disk. Aim. We
focus on the impact of solar convection on the atmosphere and aim
to resolve its velocity stratification in the photosphere. 
Methods: We performed high-resolution spectroscopic observations of the
solar spectrum in the 6302 Å range with the Laser Absolute Reference
Spectrograph at the Vacuum Tower Telescope. A laser frequency comb
enabled the calibration of the spectra to an absolute wavelength scale
with an accuracy of 1 m s-1. We systematically scanned the
quiet Sun from the disk center to the limb at ten selected heliocentric
positions. The analysis included 99 time sequences of up to 20 min in
length. By means of ephemeris and reference corrections, we translated
wavelength shifts into absolute line-of-sight velocities. A bisector
analysis on the line profiles yielded the shapes and convective shifts
of seven photospheric lines. Results: At the disk center,
the bisector profiles of the iron lines feature a pronounced C-shape
with maximum convective blueshifts of up to -450 m s-1 in
the spectral line wings. Toward the solar limb, the bisectors change
into a "\"-shape with a saturation in the line core at a redshift of
+100 m s-1. The center-to-limb variation of the line core
velocities shows a slight increase in blueshift when departing the
disk center for larger heliocentric angles. This increase in blueshift
is more pronounced for the magnetically less active meridian than
for the equator. Toward the solar limb, the blueshift decreases and
can turn into a redshift. In general, weaker lines exhibit stronger
blueshifts. Conclusions: Best spectroscopic measurements enabled
the accurate determination of absolute convective shifts in the solar
photosphere. We convolved the results to lower spectral resolution to
permit a comparison with observations from other instruments.

---------------------------------------------------------
Title: The Maximum Entropy Limit of Small-scale Magnetic Field
Fluctuations in the Quiet Sun
Authors: Gorobets, A. Y.; Berdyugina, S. V.; Riethmüller, T. L.;
Blanco Rodríguez, J.; Solanki, S. K.; Barthol, P.; Gandorfer, A.;
Gizon, L.; Hirzberger, J.; van Noort, M.; Del Toro Iniesta, J. C.;
Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..233....5G Altcode: 2017arXiv171008361G
The observed magnetic field on the solar surface is characterized by a
very complex spatial and temporal behavior. Although feature-tracking
algorithms have allowed us to deepen our understanding of this behavior,
subjectivity plays an important role in the identification and tracking
of such features. In this paper, we continue studies of the temporal
stochasticity of the magnetic field on the solar surface without relying
either on the concept of magnetic features or on subjective assumptions
about their identification and interaction. We propose a data analysis
method to quantify fluctuations of the line-of-sight magnetic field by
means of reducing the temporal field’s evolution to the regular Markov
process. We build a representative model of fluctuations converging to
the unique stationary (equilibrium) distribution in the long time limit
with maximum entropy. We obtained different rates of convergence to the
equilibrium at fixed noise cutoff for two sets of data. This indicates
a strong influence of the data spatial resolution and mixing-polarity
fluctuations on the relaxation process. The analysis is applied to
observations of magnetic fields of the relatively quiet areas around an
active region carried out during the second flight of the Sunrise/IMaX
and quiet Sun areas at the disk center from the Helioseismic and
Magnetic Imager on board the Solar Dynamics Observatory satellite.

---------------------------------------------------------
Title: Flows along arch filaments observed in the GRIS `very fast
spectroscopic mode'
Authors: González Manrique, S. J.; Denker, C.; Kuckein, C.; Pastor
Yabar, A.; Collados, M.; Verma, M.; Balthasar, H.; Diercke, A.;
Fischer, C. E.; Gömöry, P.; Bello González, N.; Schlichenmaier,
R.; Cubas Armas, M.; Berkefeld, T.; Feller, A.; Hoch, S.; Hofmann,
A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Schmidt, D.; Schmidt,
W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude,
J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2017IAUS..327...28G Altcode: 2017arXiv170102206G
A new generation of solar instruments provides improved spectral,
spatial, and temporal resolution, thus facilitating a better
understanding of dynamic processes on the Sun. High-resolution
observations often reveal multiple-component spectral line profiles,
e.g., in the near-infrared He i 10830 Å triplet, which provides
information about the chromospheric velocity and magnetic fine
structure. We observed an emerging flux region, including two small
pores and an arch filament system, on 2015 April 17 with the `very
fast spectroscopic mode' of the GREGOR Infrared Spectrograph (GRIS)
situated at the 1.5-meter GREGOR solar telescope at Observatorio del
Teide, Tenerife, Spain. We discuss this method of obtaining fast (one
per minute) spectral scans of the solar surface and its potential to
follow dynamic processes on the Sun. We demonstrate the performance
of the `very fast spectroscopic mode' by tracking chromospheric
high-velocity features in the arch filament system.

---------------------------------------------------------
Title: LARS: An Absolute Reference Spectrograph for solar
observations. Upgrade from a prototype to a turn-key system
Authors: Löhner-Böttcher, J.; Schmidt, W.; Doerr, H. -P.; Kentischer,
T.; Steinmetz, T.; Probst, R. A.; Holzwarth, R.
2017A&A...607A..12L Altcode: 2017arXiv170701573L
Context. We designed a Laser-based Absolute Reference Spectrograph
(LARS) for ultra-precise solar observations. The high-resolution
echelle spectrograph of the Vacuum Tower Telescope is supported by
a laser frequency comb to calibrate the solar spectrum on an absolute
wavelength scale. In this article, we describe the scientific instrument
and focus on the upgrades carried out in the last two years to turn the
prototype into a turn-key system. Aims: The goal was to improve
the short-term and long-term stability of the systems, and to enable
a user-friendly and more versatile operation of the instrument. 
Methods: The first upgrade involved the modernization of the frequency
comb. The laser system generating the comb spectrum was renewed. The
Fabry-Pérot cavities were adjusted to filter to a repetition
frequency of 8 GHz. A technologically matured photonic crystal fiber was
implemented for spectral broadening which simplified and stabilized the
setup. The new control software facilitates an automated operation of
the frequency comb. The second, quite recent upgrade was performed on
the optics which feed the sunlight into a single-mode fiber connected
to the spectrograph. A motorized translation stage was deployed to allow
the automated selection of three different fields of view with diameters
of 1”, 3”, and 10” for the analysis of the solar spectrum. 
Results: The successful upgrades allow for long-term observations
of up to several hours per day with a stable spectral accuracy of 1
m s-1 limited by the spectrograph. The instrument covers
a wavelength range between 480 nm and 700 nm in the visible. Stable,
user-friendly operation of the instrument is supported. The selection
of the pre-aligned fiber to change the field of view can now be done
within seconds. Conclusions: LARS offers the possibility to
observe absolute wavelength positions of spectral lines and Doppler
velocities in the solar atmosphere. First results demonstrate the
capabilities of the instrument for solar science. The accurate
measurement of the solar convection, p-modes, and atmospheric waves
will enhance our knowledge of the solar atmosphere and its physical
conditions to improve current atmospheric models.

---------------------------------------------------------
Title: Erratum: Morphological Properties of
Slender CaII H Fibrils Observed by sunrise II (<A
href="http://doi.org/10.3847/1538-4365/229/1/6">ApJS 229, 1, 6</A>)
Authors: Gafeira, R.; Lagg, A.; Solanki, S. K.; Jafarzadeh, S.;
van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta,
J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco
Suárez, D.; Riethmüller, T. L.; Schmidt, W.
2017ApJS..230...11G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Slender Ca II H Fibrils Mapping Magnetic Fields in the Low
Solar Chromosphere
Authors: Jafarzadeh, S.; Rutten, R. J.; Solanki, S. K.; Wiegelmann, T.;
Riethmüller, T. L.; van Noort, M.; Szydlarski, M.; Blanco Rodríguez,
J.; Barthol, P.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.;
Hirzberger, J.; Knölker, M.; Martínez Pillet, V.; Orozco Suárez,
D.; Schmidt, W.
2017ApJS..229...11J Altcode: 2016arXiv161003104J
A dense forest of slender bright fibrils near a small solar active
region is seen in high-quality narrowband Ca II H images from the SuFI
instrument onboard the Sunrise balloon-borne solar observatory. The
orientation of these slender Ca II H fibrils (SCF) overlaps with the
magnetic field configuration in the low solar chromosphere derived
by magnetostatic extrapolation of the photospheric field observed
with Sunrise/IMaX and SDO/HMI. In addition, many observed SCFs are
qualitatively aligned with small-scale loops computed from a novel
inversion approach based on best-fit numerical MHD simulation. Such
loops are organized in canopy-like arches over quiet areas that differ
in height depending on the field strength near their roots.

---------------------------------------------------------
Title: Magneto-static Modeling from Sunrise/IMaX: Application to an
Active Region Observed with Sunrise II
Authors: Wiegelmann, T.; Neukirch, T.; Nickeler, D. H.; Solanki, S. K.;
Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller,
T. L.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.;
Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229...18W Altcode: 2017arXiv170101458N; 2017arXiv170101458W
Magneto-static models may overcome some of the issues facing force-free
magnetic field extrapolations. So far they have seen limited use
and have faced problems when applied to quiet-Sun data. Here we
present a first application to an active region. We use solar vector
magnetic field measurements gathered by the IMaX polarimeter during
the flight of the Sunrise balloon-borne solar observatory in 2013
June as boundary conditions for a magneto-static model of the higher
solar atmosphere above an active region. The IMaX data are embedded
in active region vector magnetograms observed with SDO/HMI. This work
continues our magneto-static extrapolation approach, which was applied
earlier to a quiet-Sun region observed with Sunrise I. In an active
region the signal-to-noise-ratio in the measured Stokes parameters
is considerably higher than in the quiet-Sun and consequently the
IMaX measurements of the horizontal photospheric magnetic field allow
us to specify the free parameters of the model in a special class of
linear magneto-static equilibria. The high spatial resolution of IMaX
(110-130 km, pixel size 40 km) enables us to model the non-force-free
layer between the photosphere and the mid-chromosphere vertically
by about 50 grid points. In our approach we can incorporate some
aspects of the mixed beta layer of photosphere and chromosphere, e.g.,
taking a finite Lorentz force into account, which was not possible with
lower-resolution photospheric measurements in the past. The linear model
does not, however, permit us to model intrinsic nonlinear structures
like strongly localized electric currents.

---------------------------------------------------------
Title: The Second Flight of the Sunrise Balloon-borne Solar
Observatory: Overview of Instrument Updates, the Flight, the Data,
and First Results
Authors: Solanki, S. K.; Riethmüller, T. L.; Barthol, P.; Danilovic,
S.; Deutsch, W.; Doerr, H. -P.; Feller, A.; Gandorfer, A.; Germerott,
D.; Gizon, L.; Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.;
Lagg, A.; Meller, R.; Tomasch, G.; van Noort, M.; Blanco Rodríguez,
J.; Gasent Blesa, J. L.; Balaguer Jiménez, M.; Del Toro Iniesta,
J. C.; López Jiménez, A. C.; Orozco Suarez, D.; Berkefeld, T.;
Halbgewachs, C.; Schmidt, W.; Álvarez-Herrero, A.; Sabau-Graziati,
L.; Pérez Grande, I.; Martínez Pillet, V.; Card, G.; Centeno, R.;
Knölker, M.; Lecinski, A.
2017ApJS..229....2S Altcode: 2017arXiv170101555S
The Sunrise balloon-borne solar observatory, consisting of a 1 m
aperture telescope that provides a stabilized image to a UV filter
imager and an imaging vector polarimeter, carried out its second science
flight in 2013 June. It provided observations of parts of active regions
at high spatial resolution, including the first high-resolution images
in the Mg II k line. The obtained data are of very high quality, with
the best UV images reaching the diffraction limit of the telescope
at 3000 Å after Multi-Frame Blind Deconvolution reconstruction
accounting for phase-diversity information. Here a brief update is
given of the instruments and the data reduction techniques, which
includes an inversion of the polarimetric data. Mainly those aspects
that evolved compared with the first flight are described. A tabular
overview of the observations is given. In addition, an example time
series of a part of the emerging active region NOAA AR 11768 observed
relatively close to disk center is described and discussed in some
detail. The observations cover the pores in the trailing polarity of
the active region, as well as the polarity inversion line where flux
emergence was ongoing and a small flare-like brightening occurred in
the course of the time series. The pores are found to contain magnetic
field strengths ranging up to 2500 G, and while large pores are clearly
darker and cooler than the quiet Sun in all layers of the photosphere,
the temperature and brightness of small pores approach or even exceed
those of the quiet Sun in the upper photosphere.

---------------------------------------------------------
Title: A Tale of Two Emergences: Sunrise II Observations of Emergence
Sites in a Solar Active Region
Authors: Centeno, R.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.;
Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger,
J.; Riethmüller, T. L.; van Noort, M.; Orozco Suárez, D.; Berkefeld,
T.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229....3C Altcode: 2016arXiv161003531C
In 2013 June, the two scientific instruments on board the second Sunrise
mission witnessed, in detail, a small-scale magnetic flux emergence
event as part of the birth of an active region. The Imaging Magnetograph
Experiment (IMaX) recorded two small (∼ 5\prime\prime )
emerging flux patches in the polarized filtergrams of a photospheric Fe
I spectral line. Meanwhile, the Sunrise Filter Imager (SuFI) captured
the highly dynamic chromospheric response to the magnetic fields pushing
their way through the lower solar atmosphere. The serendipitous capture
of this event offers a closer look at the inner workings of active
region emergence sites. In particular, it reveals in meticulous detail
how the rising magnetic fields interact with the granulation as they
push through the Sun’s surface, dragging photospheric plasma in
their upward travel. The plasma that is burdening the rising field
slides along the field lines, creating fast downflowing channels at
the footpoints. The weight of this material anchors this field to the
surface at semi-regular spatial intervals, shaping it in an undulatory
fashion. Finally, magnetic reconnection enables the field to release
itself from its photospheric anchors, allowing it to continue its
voyage up to higher layers. This process releases energy that lights
up the arch-filament systems and heats the surrounding chromosphere.

---------------------------------------------------------
Title: Photospheric Response to an Ellerman Bomb-like Event—An
Analogy of Sunrise/IMaX Observations and MHD Simulations
Authors: Danilovic, S.; Solanki, S. K.; Barthol, P.; Gandorfer,
A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.;
Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.;
Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229....5D Altcode: 2016arXiv160903817D
Ellerman Bombs are signatures of magnetic reconnection, which is an
important physical process in the solar atmosphere. How and where they
occur is a subject of debate. In this paper, we analyze Sunrise/IMaX
data, along with 3D MHD simulations that aim to reproduce the exact
scenario proposed for the formation of these features. Although
the observed event seems to be more dynamic and violent than the
simulated one, simulations clearly confirm the basic scenario for the
production of EBs. The simulations also reveal the full complexity of
the underlying process. The simulated observations show that the Fe I
525.02 nm line gives no information on the height where reconnection
takes place. It can only give clues about the heating in the aftermath
of the reconnection. However, the information on the magnetic field
vector and velocity at this spatial resolution is extremely valuable
because it shows what numerical models miss and how they can be
improved.

---------------------------------------------------------
Title: Transverse Oscillations in Slender Ca II H Fibrils Observed
with Sunrise/SuFI
Authors: Jafarzadeh, S.; Solanki, S. K.; Gafeira, R.; van Noort, M.;
Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer,
A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco Suárez, D.;
Riethmüller, T. L.; Schmidt, W.
2017ApJS..229....9J Altcode: 2016arXiv161007449J
We present observations of transverse oscillations in slender Ca II
H fibrils (SCFs) in the lower solar chromosphere. We use a 1 hr long
time series of high- (spatial and temporal-) resolution seeing-free
observations in a 1.1 Å wide passband covering the line core of Ca
II H 3969 Å from the second flight of the Sunrise balloon-borne solar
observatory. The entire field of view, spanning the polarity inversion
line of an active region close to the solar disk center, is covered with
bright, thin, and very dynamic fine structures. Our analysis reveals
the prevalence of transverse waves in SCFs with median amplitudes and
periods on the order of 2.4 ± 0.8 km s-1 and 83 ± 29 s,
respectively (with standard deviations given as uncertainties). We
find that the transverse waves often propagate along (parts of) the
SCFs with median phase speeds of 9 ± 14 km s-1. While the
propagation is only in one direction along the axis in some of the
SCFs, propagating waves in both directions, as well as standing waves
are also observed. The transverse oscillations are likely Alfvénic
and are thought to be representative of magnetohydrodynamic kink
waves. The wave propagation suggests that the rapid high-frequency
transverse waves, often produced in the lower photosphere, can
penetrate into the chromosphere with an estimated energy flux of ≈15
kW m-2. Characteristics of these waves differ from those
reported for other fibrillar structures, which, however, were observed
mainly in the upper solar chromosphere.

---------------------------------------------------------
Title: Kinematics of Magnetic Bright Features in the Solar Photosphere
Authors: Jafarzadeh, S.; Solanki, S. K.; Cameron, R. H.; Barthol, P.;
Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon,
L.; Hirzberger, J.; Knölker, M.; Martínez Pillet, V.; Orozco Suárez,
D.; Riethmüller, T. L.; Schmidt, W.; van Noort, M.
2017ApJS..229....8J Altcode: 2016arXiv161007634J
Convective flows are known as the prime means of transporting magnetic
fields on the solar surface. Thus, small magnetic structures are good
tracers of turbulent flows. We study the migration and dispersal
of magnetic bright features (MBFs) in intergranular areas observed
at high spatial resolution with Sunrise/IMaX. We describe the flux
dispersal of individual MBFs as a diffusion process whose parameters are
computed for various areas in the quiet-Sun and the vicinity of active
regions from seeing-free data. We find that magnetic concentrations
are best described as random walkers close to network areas (diffusion
index, γ =1.0), travelers with constant speeds over a supergranule
(γ =1.9{--}2.0), and decelerating movers in the vicinity of flux
emergence and/or within active regions (γ =1.4{--}1.5). The three
types of regions host MBFs with mean diffusion coefficients of 130
km2 s-1, 80-90 km2 s-1,
and 25-70 km2 s-1, respectively. The MBFs in
these three types of regions are found to display a distinct kinematic
behavior at a confidence level in excess of 95%.

---------------------------------------------------------
Title: Spectropolarimetric Evidence for a Siphon Flow along an
Emerging Magnetic Flux Tube
Authors: Requerey, Iker S.; Ruiz Cobo, B.; Del Toro Iniesta, J. C.;
Orozco Suárez, D.; Blanco Rodríguez, J.; Solanki, S. K.; Barthol,
P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.;
van Noort, M.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229...15R Altcode: 2016arXiv161106732R
We study the dynamics and topology of an emerging magnetic flux
concentration using high spatial resolution spectropolarimetric data
acquired with the Imaging Magnetograph eXperiment on board the sunrise
balloon-borne solar observatory. We obtain the full vector magnetic
field and the line of sight (LOS) velocity through inversions of
the Fe I line at 525.02 nm with the SPINOR code. The derived vector
magnetic field is used to trace magnetic field lines. Two magnetic flux
concentrations with different polarities and LOS velocities are found
to be connected by a group of arch-shaped magnetic field lines. The
positive polarity footpoint is weaker (1100 G) and displays an upflow,
while the negative polarity footpoint is stronger (2200 G) and shows
a downflow. This configuration is naturally interpreted as a siphon
flow along an arched magnetic flux tube.

---------------------------------------------------------
Title: Morphological Properties of Slender Ca II H Fibrils Observed
by SUNRISE II
Authors: Gafeira, R.; Lagg, A.; Solanki, S. K.; Jafarzadeh, S.;
van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta,
J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco
Suárez, D.; Riethmüller, T. L.; Schmidt, W.
2017ApJS..229....6G Altcode: 2016arXiv161200319G
We use seeing-free high spatial resolution Ca II H data obtained by
the SUNRISE observatory to determine properties of slender fibrils
in the lower solar chromosphere. In this work we use intensity images
taken with the SuFI instrument in the Ca II H line during the second
scientific flight of the SUNRISE observatory to identify and track
elongated bright structures. After identification, we analyze theses
structures to extract their morphological properties. We identify
598 slender Ca II H fibrils (SCFs) with an average width of around
180 km, length between 500 and 4000 km, average lifetime of ≈400
s, and average curvature of 0.002 arcsec-1. The maximum
lifetime of the SCFs within our time series of 57 minutes is ≈2000
s. We discuss similarities and differences of the SCFs with other
small-scale, chromospheric structures such as spicules of type I and
II, or Ca II K fibrils.

---------------------------------------------------------
Title: A New MHD-assisted Stokes Inversion Technique
Authors: Riethmüller, T. L.; Solanki, S. K.; Barthol, P.; Gandorfer,
A.; Gizon, L.; Hirzberger, J.; van Noort, M.; Blanco Rodríguez, J.;
Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez
Pillet, V.; Knölker, M.
2017ApJS..229...16R Altcode: 2016arXiv161105175R
We present a new method of Stokes inversion of spectropolarimetric
data and evaluate it by taking the example of a Sunrise/IMaX
observation. An archive of synthetic Stokes profiles is obtained
by the spectral synthesis of state-of-the-art magnetohydrodynamics
(MHD) simulations and a realistic degradation to the level of the
observed data. The definition of a merit function allows the archive
to be searched for the synthetic Stokes profiles that best match the
observed profiles. In contrast to traditional Stokes inversion codes,
which solve the Unno-Rachkovsky equations for the polarized radiative
transfer numerically and fit the Stokes profiles iteratively, the new
technique provides the full set of atmospheric parameters. This gives
us the ability to start an MHD simulation that takes the inversion
result as an initial condition. After a relaxation process of half an
hour solar time we obtain physically consistent MHD data sets with
a target similar to the observation. The new MHD simulation is used
to repeat the method in a second iteration, which further improves
the match between observation and simulation, resulting in a factor
of 2.2 lower mean {χ }2 value. One advantage of the new
technique is that it provides the physical parameters on a geometrical
height scale. It constitutes a first step toward inversions that give
results consistent with the MHD equations.

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Title: Oscillations on Width and Intensity of Slender Ca II H Fibrils
from Sunrise/SuFI
Authors: Gafeira, R.; Jafarzadeh, S.; Solanki, S. K.; Lagg, A.;
van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta,
J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco
Suárez, D.; Riethmüller, T. L.; Schmidt, W.
2017ApJS..229....7G Altcode: 2017arXiv170102801G
We report the detection of oscillations in slender Ca II H fibrils
(SCFs) from high-resolution observations acquired with the Sunrise
balloon-borne solar observatory. The SCFs show obvious oscillations in
their intensity, but also their width. The oscillatory behaviors are
investigated at several positions along the axes of the SCFs. A large
majority of fibrils show signs of oscillations in intensity. Their
periods and phase speeds are analyzed using a wavelet analysis. The
width and intensity perturbations have overlapping distributions
of the wave period. The obtained distributions have median values
of the period of 32 ± 17 s and 36 ± 25 s, respectively. We
find that the fluctuations of both parameters propagate in
the SCFs with speeds of {11}-11+49 km
s-1 and {15}-15+34 km s-1,
respectively. Furthermore, the width and intensity oscillations have a
strong tendency to be either in anti-phase or, to a smaller extent, in
phase. This suggests that the oscillations of both parameters are caused
by the same wave mode and that the waves are likely propagating. Taking
all the evidence together, the most likely wave mode to explain all
measurements and criteria is the fast sausage mode.

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Title: Solar Coronal Loops Associated with Small-scale Mixed Polarity
Surface Magnetic Fields
Authors: Chitta, L. P.; Peter, H.; Solanki, S. K.; Barthol, P.;
Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van
Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco
Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229....4C Altcode: 2016arXiv161007484C
How and where are coronal loops rooted in the solar lower
atmosphere? The details of the magnetic environment and its evolution
at the footpoints of coronal loops are crucial to understanding the
processes of mass and energy supply to the solar corona. To address
the above question, we use high-resolution line-of-sight magnetic
field data from the Imaging Magnetograph eXperiment instrument on the
Sunrise balloon-borne observatory and coronal observations from the
Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory
of an emerging active region. We find that the coronal loops are
often rooted at the locations with minor small-scale but persistent
opposite-polarity magnetic elements very close to the larger dominant
polarity. These opposite-polarity small-scale elements continually
interact with the dominant polarity underlying the coronal loop through
flux cancellation. At these locations we detect small inverse Y-shaped
jets in chromospheric Ca II H images obtained from the Sunrise Filter
Imager during the flux cancellation. Our results indicate that magnetic
flux cancellation and reconnection at the base of coronal loops due
to mixed polarity fields might be a crucial feature for the supply of
mass and energy into the corona.

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Title: Moving Magnetic Features around a Pore
Authors: Kaithakkal, A. J.; Riethmüller, T. L.; Solanki, S. K.; Lagg,
A.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; vanNoort,
M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez,
D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
2017ApJS..229...13K Altcode: 2016arXiv160905664K
Spectropolarimetric observations from Sunrise/IMaX, obtained in 2013
June, are used for a statistical analysis to determine the physical
properties of moving magnetic features (MMFs) observed near a pore. MMFs
of the same and opposite polarity, with respect to the pore, are found
to stream from its border at an average speed of 1.3 km s-1
and 1.2 km s-1, respectively, with mainly same-polarity MMFs
found further away from the pore. MMFs of both polarities are found to
harbor rather weak, inclined magnetic fields. Opposite-polarity MMFs
are blueshifted, whereas same-polarity MMFs do not show any preference
for up- or downflows. Most of the MMFs are found to be of sub-arcsecond
size and carry a mean flux of ∼1.2 × 1017 Mx.

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Title: Convectively Driven Sinks and Magnetic Fields in the Quiet-Sun
Authors: Requerey, Iker S.; Del Toro Iniesta, Jose Carlos; Bellot
Rubio, Luis R.; Martínez Pillet, Valentín; Solanki, Sami K.;
Schmidt, Wolfgang
2017ApJS..229...14R Altcode: 2016arXiv161007622R
We study the relation between mesogranular flows, convectively
driven sinks and magnetic fields using high spatial resolution
spectropolarimetric data acquired with the Imaging Magnetograph
eXperiment on board Sunrise. We obtain the horizontal velocity
flow fields of two quiet-Sun regions (31.2 × 31.2 Mm2)
via local correlation tracking. Mesogranular lanes and the central
position of sinks are identified using Lagrange tracers. We find
6.7× {10}-2 sinks per Mm2 in the two observed
regions. The sinks are located at the mesogranular vertices and turn
out to be associated with (1) horizontal velocity flows converging to a
central point and (2) long-lived downdrafts. The spatial distribution
of magnetic fields in the quiet-Sun is also examined. The strongest
magnetic fields are preferentially located at sinks. We find that 40% of
the pixels with longitudinal components of the magnetic field stronger
than 500 G are located in the close neighborhood of sinks. In contrast,
the small-scale magnetic loops detected by Martínez González et al. in
the same two observed areas do not show any preferential distribution
at mesogranular scales. The study of individual examples reveals
that sinks can play an important role in the evolution of quiet-Sun
magnetic features.

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Title: Far side Helioseismology with Solar Orbiter
Authors: Appourchaux, T.; Birch, A.; Gizon, L. C.; Löptien, B.;
Schou, J.; Solanki, S. K.; del Toro Iniesta, J. C.; Gandorfer, A.;
Hirzberger, J.; Alvarez-Herrero, A.; Woch, J. G.; Schmidt, W.
2016AGUFMSH43A2554A Altcode:
The Solar Orbiter mission, to be launched in October 2018, will
carry a suite of remote sensing and in-situ instruments, including
the Polarimetric and Helioseismic Imager (PHI). PHI will deliver
high-cadence images of the Sun in intensity and Doppler velocity
suitable for carrying out novel helioseismic studies. The orbit
of the Solar Orbiter spacecraft will reach a solar latitude up to
34 degrees by the end of the extended mission and thus will enable
the first local helioseismology studies of the polar regions. The
full range of Earth-Sun-spacecraft angles provided by the orbit will
enable helioseismology from two vantage points by combining PHI with
another instrument: stereoscopic helioseismology will allow the study
of the deep solar interior and a better understanding of the physics
of solar oscillations in both quiet Sun and sunspots. In this paper
we will review the helioseismic objectives achievable with PHI, and
will also give a short status report of the development of the Flight
Model of PHI.

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Title: Slipping reconnection in a solar flare observed in high
resolution with the GREGOR solar telescope
Authors: Sobotka, M.; Dudík, J.; Denker, C.; Balthasar, H.; Jurčák,
J.; Liu, W.; Berkefeld, T.; Collados Vera, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Kuckein, C.; Lagg, A.; Louis, R. E.; von der Lühe, O.;
Nicklas, H.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.;
Volkmer, R.; Waldmann, T.
2016A&A...596A...1S Altcode: 2016arXiv160500464S
A small flare ribbon above a sunspot umbra in active region 12205 was
observed on November 7, 2014, at 12:00 UT in the blue imaging channel
of the 1.5 m GREGOR telescope, using a 1 Å Ca II H interference
filter. Context observations from the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO), the Solar Optical
Telescope (SOT) onboard Hinode, and the Interface Region Imaging
Spectrograph (IRIS) show that this ribbon is part of a larger one
that extends through the neighboring positive polarities and also
participates in several other flares within the active region. We
reconstructed a time series of 140 s of Ca II H images by means of the
multiframe blind deconvolution method, which resulted in spatial and
temporal resolutions of 0.1″ and 1 s. Light curves and horizontal
velocities of small-scale bright knots in the observed flare ribbon
were measured. Some knots are stationary, but three move along the
ribbon with speeds of 7-11 km s-1. Two of them move in the
opposite direction and exhibit highly correlated intensity changes,
which provides evidence of a slipping reconnection at small spatial
scales. Movies associated to Figs. 1 and 2 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201527966/olm">http://www.aanda.org</A>

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Title: Magnetic field reconstruction based on sunspot oscillations
Authors: Löhner-Böttcher, J.; Bello González, N.; Schmidt, W.
2016AN....337.1040L Altcode: 2016arXiv160105925L
The magnetic field of a sunspot guides magnetohydrodynamic waves
toward higher atmospheric layers. In the upper photosphere and lower
chromosphere, wave modes with periods longer than the acoustic
cut-off period become evanescent. The cut-off period essentially
changes due to the atmospheric properties, e.g., increases for
larger zenith inclinations of the magnetic field. In this work, we
aim at introducing a novel technique of reconstructing the magnetic
field inclination on the basis of the dominating wave periods in
the sunspot chromosphere and upper photosphere. On 2013 August 21,
we observed an isolated, circular sunspot (NOAA11823) for 58 min in
a purely spectroscopic multi-wavelength mode with the Interferometric
Bidimensional Spectro-polarimeter (IBIS) at the Dunn Solar Telescope. By
means of a wavelet power analysis, we retrieved the dominating wave
periods and reconstructed the zenith inclinations in the chromosphere
and upper photosphere. The results are in good agreement with the
lower photospheric HMI magnetograms. The sunspot's magnetic field
in the chromosphere inclines from almost vertical (0°)
in the umbra to around 60° in the outer penumbra. With
increasing altitude in the sunspot atmosphere, the magnetic field of
the penumbra becomes less inclined. We conclude that the reconstruction
of the magnetic field topology on the basis of sunspot oscillations
yields consistent and conclusive results. The technique opens up a
new possibility to infer the magnetic field inclination in the solar
chromosphere.

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Title: Daniel K. Inouye Solar Telescope: High-resolution observing
of the dynamic Sun
Authors: Tritschler, A.; Rimmele, T. R.; Berukoff, S.; Casini, R.;
Kuhn, J. R.; Lin, H.; Rast, M. P.; McMullin, J. P.; Schmidt, W.;
Wöger, F.; DKIST Team
2016AN....337.1064T Altcode:
The 4-m aperture Daniel K. Inouye Solar Telescope (DKIST) formerly
known as the Advanced Technology Solar Telescope (ATST) is currently
under construction on Haleakalā (Maui, Hawai'i) projected to
start operations in 2019. At the time of completion, DKIST will be
the largest ground-based solar telescope providing unprecedented
resolution and photon collecting power. The DKIST will be equipped
with a set of first-light facility-class instruments offering unique
imaging, spectroscopic and spectropolarimetric observing opportunities
covering the visible to infrared wavelength range. This first-light
instrumentation suite will include: a Visible Broadband Imager (VBI) for
high-spatial and -temporal resolution imaging of the solar atmosphere; a
Visible Spectro-Polarimeter (ViSP) for sensitive and accurate multi-line
spectropolarimetry; a Fabry-Pérot based Visible Tunable Filter
(VTF) for high-spatial resolution spectropolarimetry; a fiber-fed
Diffraction-Limited Near Infra-Red Spectro-Polarimeter (DL-NIRSP)
for two-dimensional high-spatial resolution spectropolarimetry
(simultaneous spatial and spectral information); and a Cryogenic Near
Infra-Red Spectro-Polarimeter (Cryo-NIRSP) for coronal magnetic field
measurements and on-disk observations of, e.g., the CO lines at 4.7
μm. We will provide an overview of the DKIST's unique capabilities
with strong focus on the first-light instrumentation suite, highlight
some of the additional properties supporting observations of transient
and dynamic solar phenomena, and touch on some operational strategies
and the DKIST critical science plan.

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

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

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

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Title: Horizontal flow fields in and around a small active region. The
transition period between flux emergence and decay
Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González
Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke,
A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor
Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert,
M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2016A&A...596A...3V Altcode: 2016arXiv160507462V
Context. The solar magnetic field is responsible for all aspects
of solar activity. Thus, emergence of magnetic flux at the surface
is the first manifestation of the ensuing solar activity. 
Aims: Combining high-resolution and synoptic observations aims to
provide a comprehensive description of flux emergence at photospheric
level and of the growth process that eventually leads to a mature
active region. Methods: The small active region NOAA 12118
emerged on 2014 July 17 and was observed one day later with the 1.5-m
GREGOR solar telescope on 2014 July 18. High-resolution time-series
of blue continuum and G-band images acquired in the blue imaging
channel (BIC) of the GREGOR Fabry-Pérot Interferometer (GFPI) were
complemented by synoptic line-of-sight magnetograms and continuum
images obtained with the Helioseismic and Magnetic Imager (HMI) onboard
the Solar Dynamics Observatory (SDO). Horizontal proper motions and
horizontal plasma velocities were computed with local correlation
tracking (LCT) and the differential affine velocity estimator (DAVE),
respectively. Morphological image processing was employed to measure
the photometric and magnetic area, magnetic flux, and the separation
profile of the emerging flux region during its evolution. 
Results: The computed growth rates for photometric area, magnetic
area, and magnetic flux are about twice as high as the respective
decay rates. The space-time diagram using HMI magnetograms of five days
provides a comprehensive view of growth and decay. It traces a leaf-like
structure, which is determined by the initial separation of the two
polarities, a rapid expansion phase, a time when the spread stalls,
and a period when the region slowly shrinks again. The separation
rate of 0.26 km s-1 is highest in the initial stage, and
it decreases when the separation comes to a halt. Horizontal plasma
velocities computed at four evolutionary stages indicate a changing
pattern of inflows. In LCT maps we find persistent flow patterns such
as outward motions in the outer part of the two major pores, a diverging
feature near the trailing pore marking the site of upwelling plasma and
flux emergence, and low velocities in the interior of dark pores. We
detected many elongated rapidly expanding granules between the two
major polarities, with dimensions twice as large as the normal granules.

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

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Title: Active region fine structure observed at 0.08 arcsec resolution
Authors: Schlichenmaier, R.; von der Lühe, O.; Hoch, S.; Soltau, D.;
Berkefeld, T.; Schmidt, D.; Schmidt, W.; Denker, C.; Balthasar, H.;
Hofmann, A.; Strassmeier, K. G.; Staude, J.; Feller, A.; Lagg, A.;
Solanki, S. K.; Collados, M.; Sigwarth, M.; Volkmer, R.; Waldmann,
T.; Kneer, F.; Nicklas, H.; Sobotka, M.
2016A&A...596A...7S Altcode: 2016arXiv160707094S
Context. The various mechanisms of magneto-convective energy transport
determine the structure of sunspots and active regions. 
Aims: We characterise the appearance of light bridges and other
fine-structure details and elaborate on their magneto-convective
nature. Methods: We present speckle-reconstructed images taken
with the broad-band imager (BBI) at the 1.5 m GREGOR telescope in the
486 nm and 589 nm bands. We estimate the spatial resolution from the
noise characteristics of the image bursts and obtain 0.08″ at 589
nm. We describe structure details in individual best images as well
as the temporal evolution of selected features. Results: We
find branched dark lanes extending along thin (≈1″) light bridges
in sunspots at various heliocentric angles. In thick (≳ 2″) light
bridges the branches are disconnected from the central lane and have a Y
shape with a bright grain toward the umbra. The images reveal that light
bridges exist on varying intensity levels and that their small-scale
features evolve on timescales of minutes. Faint light bridges show
dark lanes outlined by the surrounding bright features. Dark lanes are
very common and are also found in the boundary of pores. They have a
characteristic width of 0.1″ or smaller. Intergranular dark lanes of
that width are seen in active region granulation. Conclusions: We
interpret our images in the context of magneto-convective simulations
and findings: while central dark lanes in thin light bridges are
elevated and associated with a density increase above upflows, the dark
lane branches correspond to locations of downflows and are depressed
relative to the adjacent bright plasma. Thick light bridges with central
dark lanes show no projection effect. They have a flat elevated plateau
that falls off steeply at the umbral boundary. There, Y-shaped filaments
form as they do in the inner penumbra. This indicates the presence of
inclined magnetic fields, meaning that the umbral magnetic field is
wrapped around the convective light bridge.

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

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

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

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

---------------------------------------------------------
Title: Fitting peculiar spectral profiles in He I 10830Å absorption
features
Authors: González Manrique, S. J.; Kuckein, C.; Pastor Yabar, A.;
Collados, M.; Denker, C.; Fischer, C. E.; Gömöry, P.; Diercke, A.;
Bello González, N.; Schlichenmaier, R.; Balthasar, H.; Berkefeld, T.;
Feller, A.; Hoch, S.; Hofmann, A.; Kneer, F.; Lagg, A.; Nicklas, H.;
Orozco Suárez, D.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma,
M.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2016AN....337.1057G Altcode: 2016arXiv160300679G
The new generation of solar instruments provides better
spectral, spatial, and temporal resolution for a better
understanding of the physical processes that take place on the
Sun. Multiple-component profiles are more commonly observed with these
instruments. Particularly, the He I 10830 Å triplet presents such
peculiar spectral profiles, which give information on the velocity
and magnetic fine structure of the upper chromosphere. The purpose
of this investigation is to describe a technique to efficiently fit
the two blended components of the He I 10830 Å triplet, which are
commonly observed when two atmospheric components are located within
the same resolution element. The observations used in this study were
taken on 2015 April 17 with the very fast spectroscopic mode of the
GREGOR Infrared Spectrograph (GRIS) attached to the 1.5-m GREGOR solar
telescope, located at the Observatorio del Teide, Tenerife, Spain. We
apply a double-Lorentzian fitting technique using Levenberg-Marquardt
least-squares minimization. This technique is very simple and much
faster than inversion codes. Line-of-sight Doppler velocities can
be inferred for a whole map of pixels within just a few minutes. Our
results show sub- and supersonic downflow velocities of up to 32 km
s-1 for the fast component in the vicinity of footpoints of
filamentary structures. The slow component presents velocities close
to rest.

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Title: Flows in and around Active Region NOAA12118 Observed with
the GREGOR Solar Telescope and SDO/HMI
Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González
Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke,
A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor
Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert,
M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2016ASPC..504...29V Altcode: 2016arXiv160301109V
Accurate measurements of magnetic and velocity fields in and around
solar active regions are key to unlocking the mysteries of the
formation and the decay of sunspots. High spatial resolution images
and spectral sequences with a high cadence obtained with the GREGOR
solar telescope give us an opportunity to scrutinize 3-D flow fields
with local correlation tracking and imaging spectroscopy. We present
GREGOR early science data acquired in 2014 July - August with the GREGOR
Fabry-Pérot Interferometer and the Blue Imaging Channel. Time-series
of blue continuum (λ 450.6 nm) images of the small active region
NOAA 12118 were restored with the speckle masking technique to derive
horizontal proper motions and to track the evolution of morphological
changes. In addition, high-resolution observations are discussed in
the context of synoptic data from the Solar Dynamics Observatory.

---------------------------------------------------------
Title: The Polarimetric and Helioseismic Imager for Solar Orbiter:
SO/PHI
Authors: Solanki, Sami K.; del Toro Iniesta, Jose Carlos; Woch,
Joachim; Gandorfer, Achim; Hirzberger, Johann; Schmidt, Wolfgang;
Appourchaux, Thierry; Alvarez-Herrero, Alberto
2015IAUS..305..108S Altcode: 2015arXiv150203368S
The Solar Orbiter is the next solar physics mission of the European
Space Agency, ESA, in collaboration with NASA, with a launch planned in
2018. The spacecraft is designed to approach the Sun to within 0.28 AU
at perihelion of a highly eccentric orbit. The proximity with the Sun
will also allow its observation at uniformly high resolution at EUV and
visible wavelengths. Such observations are central for learning more
about the magnetic coupling of the solar atmosphere. At a later phase
in the mission the spacecraft will leave the ecliptic and study the
enigmatic poles of the Sun from a heliographic latitude of up to 33°.

---------------------------------------------------------
Title: Where does the Thermospheric Ionospheric GEospheric Research
(TIGER) Program go?
Authors: Schmidtke, G.; Avakyan, S. V.; Berdermann, J.; Bothmer,
V.; Cessateur, G.; Ciraolo, L.; Didkovsky, L.; Dudok de Wit, T.;
Eparvier, F. G.; Gottwald, A.; Haberreiter, M.; Hammer, R.; Jacobi,
Ch.; Jakowski, N.; Kretzschmar, M.; Lilensten, J.; Pfeifer, M.;
Radicella, S. M.; Schäfer, R.; Schmidt, W.; Solomon, S. C.; Thuillier,
G.; Tobiska, W. K.; Wieman, S.; Woods, T. N.
2015AdSpR..56.1547S Altcode:
At the 10th Thermospheric Ionospheric GEospheric Research (TIGER/COSPAR)
symposium held in Moscow in 2014 the achievements from the start of
TIGER in 1998 were summarized. During that period, great progress was
made in measuring, understanding, and modeling the highly variable
UV-Soft X-ray (XUV) solar spectral irradiance (SSI), and its effects
on the upper atmosphere. However, after more than 50 years of work the
radiometric accuracy of SSI observation is still an issue and requires
further improvement. Based on the extreme ultraviolet (EUV) data from
the SOLAR/SolACES, and SDO/EVE instruments, we present a combined data
set for the spectral range from 16.5 to 105.5 nm covering a period
of 3.5 years from 2011 through mid of 2014. This data set is used
in ionospheric modeling of the global Total Electron Content (TEC),
and in validating EUV SSI modeling. For further investigations the
period of 3.5 years is being extended to about 12 years by including
data from SOHO/SEM and TIMED/SEE instruments. Similarly, UV data are
used in modeling activities. After summarizing the results, concepts
are proposed for future real-time SSI measurements with in-flight
calibration as experienced with the ISS SOLAR payload, for the
development of a space weather camera for observing and investigating
space weather phenomena in real-time, and for providing data sets for
SSI and climate modeling. Other planned topics are the investigation
of the relationship between solar EUV/UV and visible/near-infrared
emissions, the impact of X-rays on the upper atmosphere, the development
of solar EUV/UV indices for different applications, and establishing a
shared TIGER data system for EUV/UV SSI data distribution and real-time
streaming, also taking into account the achievements of the FP7 SOLID
(First European SOLar Irradiance Data Exploitation) project. For further
progress it is imperative that coordinating activities in this special
field of solar-terrestrial relations and solar physics is emphasized.

---------------------------------------------------------
Title: Dynamics of Multi-cored Magnetic Structures in the Quiet Sun
Authors: Requerey, Iker S.; Del Toro Iniesta, Jose Carlos; Bellot
Rubio, Luis R.; Martínez Pillet, Valentín; Solanki, Sami K.;
Schmidt, Wolfgang
2015ApJ...810...79R Altcode: 2015arXiv150806998R
We report on the dynamical interaction of quiet-Sun magnetic fields and
granular convection in the solar photosphere as seen by Sunrise. We
use high spatial resolution (0.″15-0.″18) and temporal cadence
(33 s) spectropolarimetric Imaging Magnetograph eXperiment data,
together with simultaneous CN and Ca ii H filtergrams from Sunrise
Filter Imager. We apply the SIR inversion code to the polarimetric
data in order to infer the line of sight velocity and vector magnetic
field in the photosphere. The analysis reveals bundles of individual
flux tubes evolving as a single entity during the entire 23 minute data
set. The group shares a common canopy in the upper photospheric layers,
while the individual tubes continually intensify, fragment and merge in
the same way that chains of bright points in photometric observations
have been reported to do. The evolution of the tube cores are driven
by the local granular convection flows. They intensify when they
are “compressed” by surrounding granules and split when they are
“squeezed” between two moving granules. The resulting fragments
are usually later regrouped in intergranular lanes by the granular
flows. The continual intensification, fragmentation and coalescence of
flux results in magnetic field oscillations of the global entity. From
the observations we conclude that the magnetic field oscillations first
reported by Martínez González et al. correspond to the forcing by
granular motions and not to characteristic oscillatory modes of thin
flux tubes.

---------------------------------------------------------
Title: The photospheric solar oxygen project. III. Investigation of
the centre-to-limb variation of the 630 nm [O I]-Ni I blend
Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Livingston, W.;
Bonifacio, P.; Malherbe, J. -M.; Doerr, H. -P.; Schmidt, W.
2015A&A...579A..88C Altcode: 2015arXiv150600931C
Context. The solar photospheric abundance of oxygen is still a matter
of debate. For about ten years some determinations have favoured a
low oxygen abundance which is at variance with the value inferred by
helioseismology. Among the oxygen abundance indicators, the forbidden
line at 630 nm has often been considered the most reliable even
though it is blended with a Ni i line. In Papers I and II of this
series we reported a discrepancy in the oxygen abundance derived
from the 630 nm and the subordinate [O I] line at 636 nm in dwarf
stars, including the Sun. Aims: Here we analyse several,
in part new, solar observations of the centre-to-limb variation
of the spectral region including the blend at 630 nm in order to
separate the individual contributions of oxygen and nickel. 
Methods: We analyse intensity spectra observed at different limb
angles in comparison with line formation computations performed on a
CO5BOLD 3D hydrodynamical simulation of the solar atmosphere. 
Results: The oxygen abundances obtained from the forbidden line at
different limb angles are inconsistent if the commonly adopted nickel
abundance of 6.25 is assumed in our local thermodynamic equilibrium
computations. With a slightly lower nickel abundance, A(Ni) ≈ 6.1,
we obtain consistent fits indicating an oxygen abundance of A(O) = 8.73
± 0.05. At this value the discrepancy with the subordinate oxygen
line remains. Conclusions: The derived value of the oxygen
abundance supports the notion of a rather low oxygen abundance in the
solar photosphere. However, it is disconcerting that the forbidden
oxygen lines at 630 and 636 nm give noticeably different results,
and that the nickel abundance derived here from the 630 nm blend is
lower than expected from other nickel lines.

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

---------------------------------------------------------
Title: Comb-calibrated solar spectroscopy through a multiplexed
single-mode fiber channel
Authors: Probst, R. A.; Wang, L.; Doerr, H. -P.; Steinmetz, T.;
Kentischer, T. J.; Zhao, G.; Hänsch, T. W.; Udem, Th; Holzwarth,
R.; Schmidt, W.
2015NJPh...17b3048P Altcode: 2015arXiv150204942P
We investigate a new scheme for astronomical spectrograph calibration
using the laser frequency comb at the Solar Vacuum Tower Telescope on
Tenerife. Our concept is based upon a single-mode fiber channel, that
simultaneously feeds the spectrograph with comb light and sunlight. This
yields nearly perfect spatial mode matching between the two sources. In
combination with the absolute calibration provided by the frequency
comb, this method enables extremely robust and accurate spectroscopic
measurements. The performance of this scheme is compared to a sequence
of alternating comb and sunlight, and to absorption lines from Earth’s
atmosphere. We also show how the method can be used for radial-velocity
detection by measuring the well-explored 5 min oscillations averaged
over the full solar disk. Our method is currently restricted to solar
spectroscopy, but with further evolving fiber-injection techniques it
could become an option even for faint astronomical targets.

---------------------------------------------------------
Title: DKIST: Observing the Sun at High Resolution
Authors: Tritschler, A.; Rimmele, T. R.; Berukoff, S.; Casini, R.;
Craig, S. C.; Elmore, D. F.; Hubbard, R. P.; Kuhn, J. R.; Lin, H.;
McMullin, J. P.; Reardon, K. P.; Schmidt, W.; Warner, M.; Woger, F.
2015csss...18..933T Altcode:
The 4-m aperture Daniel K. Inouye Solar Telescope (DKIST) formerly
known as the Advanced Technology Solar Telescope (ATST) and currently
under construction on Haleakalā (Maui, Hawai'i) will be the largest
solar ground-based telescope and leading resource for studying the
dynamic Sun and its phenomena at high spatial, spectral and temporal
resolution. Accurate and sensitive polarimetric observations at
high-spatial resolution throughout the solar atmosphere including the
corona is a high priority and a major science driver. As such the DKIST
will offer a combination of state-of-the-art instruments with imaging
and/or spectropolarimetric capabilities covering a broad wavelength
range. This first-light instrumentation suite will include: a Visible
Broadband Imager (VBI) for high-spatial and -temporal resolution
imaging of the solar atmosphere; a Visible Spectro-Polarimeter (ViSP)
for sensitive and accurate multi-line spectropolarimetry; a double
Fabry-Pérot based Visible Tunable Filter (VTF) for high-spatial
resolution spectropolarimetry; a fiber-fed 2D Diffraction-Limited Near
Infra-Red Spectro-Polarimeter (DL-NIRSP); and a Cryogenic Near Infra-Red
Spectro-Polarimeter (Cryo-NIRSP) for coronal magnetic field measurements
and on-disk observations of e.g. the CO lines at 4.7 microns. We
will provide a brief overview of the DKIST's unique capabilities to
perform spectroscopic and spectropolarimetric measurements of the solar
atmosphere using its first-light instrumentation suite, the status of
the construction project, and how facility and data access is provided
to the US and international community.

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Title: The Formation and Disintegration of Magnetic Bright Points
Observed by Sunrise/IMaX
Authors: Utz, D.; del Toro Iniesta, J. C.; Bellot Rubio, L. R.;
Jurčák, J.; Martínez Pillet, V.; Solanki, S. K.; Schmidt, W.
2014ApJ...796...79U Altcode: 2014arXiv1411.3240U
The evolution of the physical parameters of magnetic bright points
(MBPs) located in the quiet Sun (mainly in the interwork) during
their lifetime is studied. First, we concentrate on the detailed
description of the magnetic field evolution of three MBPs. This
reveals that individual features follow different, generally complex,
and rather dynamic scenarios of evolution. Next, we apply statistical
methods on roughly 200 observed MBP evolutionary tracks. MBPs are found
to be formed by the strengthening of an equipartition field patch,
which initially exhibits a moderate downflow. During the evolution,
strong downdrafts with an average velocity of 2.4 km s-1
set in. These flows, taken together with the concurrent strengthening of
the field, suggest that we are witnessing the occurrence of convective
collapses in these features, although only 30% of them reach kG field
strengths. This fraction might turn out to be larger when the new 4
m class solar telescopes are operational as observations of MBPs with
current state of the art instrumentation could still be suffering from
resolution limitations. Finally, when the bright point disappears
(although the magnetic field often continues to exist) the magnetic
field strength has dropped to the equipartition level and is generally
somewhat weaker than at the beginning of the MBP's evolution. Also,
only relatively weak downflows are found on average at this stage of
the evolution. Only 16% of the features display upflows at the time
that the field weakens, or the MBP disappears. This speaks either for
a very fast evolving dynamic process at the end of the lifetime, which
could not be temporally resolved, or against strong upflows as the cause
of the weakening of the field of these magnetic elements, as has been
proposed based on simulation results. It is noteworthy that in about 10%
of the cases, we observe in the vicinity of the downflows small-scale
strong (exceeding 2 km s-1) intergranular upflows related
spatially and temporally to these downflows. The paper is complemented
by a detailed discussion of aspects regarding the applied methods,
the complementary literature, and in depth analysis of parameters
like magnetic field strength and velocity distributions. An important
difference to magnetic elements and associated bright structures in
active region plage is that most of the quiet Sun bright points display
significant downflows over a large fraction of their lifetime (i.e.,
in more than 46% of time instances/measurements they show downflows
exceeding 1 km s-1).

---------------------------------------------------------
Title: The History of a Quiet-Sun Magnetic Element Revealed by
IMaX/SUNRISE
Authors: Requerey, Iker S.; Del Toro Iniesta, Jose Carlos; Bellot
Rubio, Luis R.; Bonet, José A.; Martínez Pillet, Valentín; Solanki,
Sami K.; Schmidt, Wolfgang
2014ApJ...789....6R Altcode: 2014arXiv1405.2837R
Isolated flux tubes are considered to be fundamental magnetic building
blocks of the solar photosphere. Their formation is usually attributed
to the concentration of magnetic field to kG strengths by the convective
collapse mechanism. However, the small size of the magnetic elements in
quiet-Sun areas has prevented this scenario from being studied in fully
resolved structures. Here, we report on the formation and subsequent
evolution of one such photospheric magnetic flux tube, observed in
the quiet Sun with unprecedented spatial resolution (0.”15-0.”18)
and high temporal cadence (33 s). The observations were acquired by
the Imaging Magnetograph eXperiment on board the SUNRISE balloon-borne
solar observatory. The equipartition field strength magnetic element
is the result of the merging of several same polarity magnetic flux
patches, including a footpoint of a previously emerged loop. The
magnetic structure is then further intensified to kG field strengths
by convective collapse. The fine structure found within the flux
concentration reveals that the scenario is more complex than can be
described by a thin flux tube model with bright points and downflow
plumes being established near the edges of the kG magnetic feature. We
also observe a daisy-like alignment of surrounding granules and a
long-lived inflow toward the magnetic feature. After a subsequent
weakening process, the field is again intensified to kG strengths. The
area of the magnetic feature is seen to change in anti-phase with the
field strength, while the brightness of the bright points and the speed
of the downflows varies in phase. We also find a relation between the
brightness of the bright point and the presence of upflows within it.

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

---------------------------------------------------------
Title: Comparison between Mg II k and Ca II H Images Recorded by
SUNRISE/SuFI
Authors: Danilovic, S.; Hirzberger, J.; Riethmüller, T. L.; Solanki,
S. K.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Gizon, L.; Knölker,
M.; Schmidt, W.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.
2014ApJ...784...20D Altcode:
We present a comparison of high-resolution images of the solar surface
taken in the Mg II k and Ca II H channels of the Filter Imager on the
balloon-borne solar observatory SUNRISE. The Mg and Ca lines are sampled
with 0.48 nm and 0.11 nm wide filters, respectively. The two channels
show remarkable qualitative and quantitative similarities in the quiet
Sun, in an active region plage and during a small flare. However, the Mg
filtergrams display 1.4-1.7 times higher intensity contrast and appear
more smeared and smoothed in the quiet Sun. In addition, the fibrils
in a plage are wider. Although the exposure time is 100 times longer
for Mg images, the evidence suggests that these differences cannot be
explained only with instrumental effects or the evolution of the solar
scene. The differences at least partially arise because of different
line-formation heights, the stronger response of Mg k emission peaks
to the higher temperatures, and the larger height range sampled by
the broad Mg filter used here. This is evidently manifested during
the flare when a surge in Mg evolves differently than in Ca.

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Title: Prominence Science with ATST Instrumentation
Authors: Rimmele, Thomas; Berger, Thomas; Casini, Roberto; Elmore,
David; Kuhn, Jeff; Lin, Haosheng; Schmidt, Wolfgang; Wöger, Friedrich
2014IAUS..300..362R Altcode:
The 4m Advance Technology Solar Telescope (ATST) is under construction
on Maui, HI. With its unprecedented resolution and photon collecting
power ATST will be an ideal tool for studying prominences and filaments
and their role in producing Coronal Mass Ejections that drive Space
Weather. The ATST facility will provide a set of first light instruments
that enable imaging and spectroscopy of the dynamic filament and
prominence structure at 8 times the resolution of Hinode. Polarimeters
allow high precision chromospheric and coronal magnetometry at visible
and infrared (IR) wavelengths. This paper summarizes the capabilities
of the ATST first-light instrumentation with focus on prominence and
filament science.

---------------------------------------------------------
Title: Dynamics of the Fe II 396.94 nm emission line observed at
solar disk center
Authors: Schmidt, W.; Fisher, J.
2013A&A...560A..50S Altcode:
 Aims: We have studied the structure and dynamics of the upper
photosphere and lower chromosphere of the quiet Sun. Methods:
We analyzed a sequence of scans of slit spectra of the Ca II H line
at 396.8 nm, with a spectral resolution of 850 000, and a spatial
resolution of about one arcsec, taken in the quiet Sun at disk center,
with a duration of about 55 min and an area coverage of 185″ ×
3.5″. We used statistical methods to characterize size, lifetime,
and dynamics of emission features of the Fe II line at 396.94 nm. 
Results: We have identified about 780 distinct features where the Fe II
line shows emission stronger than 3% above the local continuum. Their
lifetime is on the order of one minute, and the spatial extent is
about or less than 2″. On average, Fe II emission features show a
significant redshift of 1.3 km s-1, with respect to the
position of the absorption line. The redshift of the Fe II emission
is absent close to the solar limb. The Fe II emission is coupled with
a strongly enhanced intensity level of the Ca II wings. A time line
analysis shows that other photospheric lines show significant redshift
one minute before, and a blueshift after, the occurrence of an emission
event. A redshift of the Ca II H line core precedes the redshift of
the Fe II line by about 30 s. The occurrence of the H2v emissions is
strongly reduced before and significantly increased after the Fe II
emission events. Conclusions: The temporal behavior of the Fe II
emission line and both the Ca II H line core and H2v emission suggest
a connection between Fe II emission and chromospheric activity. The
presence of a significant redshift during the Fe II emissions and
the absence of this redshift near the limb suggest that the observed
redshift is indeed caused by a downflow in the line-forming region. From
our result concerning the intensity fluctuations in the core of the Fe
II line, and from the magnitude of the downflow velocities we conclude
that the Fe II emissions at 396.94 nm occur in the photosphere.

---------------------------------------------------------
Title: First High-resolution Images of the Sun in the 2796 Å Mg II
k Line
Authors: Riethmüller, T. L.; Solanki, S. K.; Hirzberger, J.;
Danilovic, S.; Barthol, P.; Berkefeld, T.; Gandorfer, A.; Gizon, L.;
Knölker, M.; Schmidt, W.; Del Toro Iniesta, J. C.
2013ApJ...776L..13R Altcode: 2013arXiv1309.5213R
We present the first high-resolution solar images in the Mg II k 2796
Å line. The images, taken through a 4.8 Å broad interference filter,
were obtained during the second science flight of Sunrise in 2013 June
by the Sunrise Filter Imager (SuFI) instrument. The Mg II k images
display structures that look qualitatively very similar to images taken
in the core of Ca II H. The Mg II images exhibit reversed granulation
(or shock waves) in the internetwork regions of the quiet Sun, at
intensity contrasts that are similar to those found in Ca II H. Very
prominent in Mg II are bright points, both in the quiet Sun and in plage
regions, particularly near the disk center. These are much brighter than
at other wavelengths sampled at similar resolution. Furthermore, Mg II k
images also show fibril structures associated with plage regions. Again,
the fibrils are similar to those seen in Ca II H images, but tend to
be more pronounced, particularly in weak plage.

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Title: Is Magnetic Reconnection the Cause of Supersonic Upflows in
Granular Cells?
Authors: Borrero, J. M.; Martínez Pillet, V.; Schmidt, W.; Quintero
Noda, C.; Bonet, J. A.; del Toro Iniesta, J. C.; Bellot Rubio, L. R.
2013ApJ...768...69B Altcode: 2013arXiv1303.2557B
In a previous work, we reported on the discovery of supersonic magnetic
upflows on granular cells in data from the SUNRISE/IMaX instrument. In
the present work, we investigate the physical origin of these events
employing data from the same instrument but with higher spectral
sampling. By means of the inversion of Stokes profiles we are able
to recover the physical parameters (temperature, magnetic field,
line-of-sight velocity, etc.) present in the solar photosphere at the
time of these events. The inversion is performed in a Monte-Carlo-like
fashion, that is, repeating it many times with different initializations
and retaining only the best result. We find that many of the events are
characterized by a reversal in the polarity of the magnetic field along
the vertical direction in the photosphere, accompanied by an enhancement
in the temperature and by supersonic line-of-sight velocities. In
about half of the studied events, large blueshifted and redshifted
line-of-sight velocities coexist above/below each other. These features
can be explained in terms of magnetic reconnection, where the energy
stored in the magnetic field is released in the form of kinetic
and thermal energy when magnetic field lines of opposite polarities
coalesce. However, the agreement with magnetic reconnection is not
perfect and, therefore, other possible physical mechanisms might also
play a role.

---------------------------------------------------------
Title: Evolution of the Fine Structure of Magnetic Fields in the
Quiet Sun: Observations from Sunrise/IMaX and Extrapolations
Authors: Wiegelmann, T.; Solanki, S. K.; Borrero, J. M.; Peter,
H.; Barthol, P.; Gandorfer, A.; Martínez Pillet, V.; Schmidt, W.;
Knölker, M.
2013SoPh..283..253W Altcode:
Observations with the balloon-borne Sunrise/Imaging Magnetograph
eXperiment (IMaX) provide high spatial resolution (roughly 100 km at
disk center) measurements of the magnetic field in the photosphere of
the quiet Sun. To investigate the magnetic structure of the chromosphere
and corona, we extrapolate these photospheric measurements into
the upper solar atmosphere and analyze a 22-minute long time series
with a cadence of 33 seconds. Using the extrapolated magnetic-field
lines as tracer, we investigate temporal evolution of the magnetic
connectivity in the quiet Sun's atmosphere. The majority of magnetic
loops are asymmetric in the sense that the photospheric field strength
at the loop foot points is very different. We find that the magnetic
connectivity of the loops changes rapidly with a typical connection
recycling time of about 3±1 minutes in the upper solar atmosphere and
12±4 minutes in the photosphere. This is considerably shorter than
previously found. Nonetheless, our estimate of the energy released by
the associated magnetic-reconnection processes is not likely to be the
sole source for heating the chromosphere and corona in the quiet Sun.

---------------------------------------------------------
Title: Gregor@night: The future high-resolution stellar spectrograph
for the GREGOR solar telescope
Authors: Strassmeier, K. G.; Ilyin, I. V.; Woche, M.; Granzer,
T.; Weber, M.; Weingrill, J.; Bauer, S. -M.; Popow, E.; Denker, C.;
Schmidt, W.; von der Lühe, O.; Berdyugina, S.; Collados, M.; Koubsky,
P.; Hackman, T.; Mantere, M. J.
2012AN....333..901S Altcode:
We describe the future night-time spectrograph for the GREGOR solar
telescope and present its science core projects. The spectrograph
provides a 3-pixel resolution of up to R=87 000 in 45 échelle orders
covering the wavelength range 390-900 nm with three grating settings. An
iodine cell can be used for high-precision radial velocity work in the
500-630 nm range. The operation of the spectrograph and the telescope
will be fully automated without the presence of humans during night-time
and will be based on the successful STELLA control system. Future
upgrades include a second optical camera for even higher spectral
resolution, a Stokes-V polarimeter and a link to the laser-frequency
comb at the Vacuum Tower Telescope. The night-time core projects are a
study of the angular-momentum evolution of “The Sun in Time” and a
continuation of our long-term Doppler imaging of active stars.

---------------------------------------------------------
Title: A retrospective of the GREGOR solar telescope in scientific
literature
Authors: Denker, C.; von der Lühe, O.; Feller, A.; Arlt, K.;
Balthasar, H.; Bauer, S. -M.; Bello González, N.; Berkefeld, Th.;
Caligari, P.; Collados, M.; Fischer, A.; Granzer, T.; Hahn, T.;
Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Kentischer, T.; Klva{ňa,
M.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.;
Rendtel, J.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Solanki, S. K.;
Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; Waldmann,
T.; Wiehr, E.; Wittmann, A. D.; Woche, M.
2012AN....333..810D Altcode: 2012arXiv1210.3167D
In this review, we look back upon the literature, which had the
GREGOR solar telescope project as its subject including science cases,
telescope subsystems, and post-focus instruments. The articles date
back to the year 2000, when the initial concepts for a new solar
telescope on Tenerife were first presented at scientific meetings. This
comprehensive bibliography contains literature until the year 2012,
i.e., the final stages of commissioning and science verification. Taking
stock of the various publications in peer-reviewed journals and
conference proceedings also provides the “historical” context
for the reference articles in this special issue of Astronomische
Nachrichten/Astronomical Notes.

---------------------------------------------------------
Title: Open-foldable domes with high-tension textile membranes:
The GREGOR dome
Authors: Hammerschlag, R. H.; Kommers, J. N.; Visser, S.; Bettonvil,
F. C. M.; van Schie, A. G. M.; van Leverink, S. J.; Sliepen, G.;
Jägers, A. P. L.; Schmidt, W.; Volkmer, R.
2012AN....333..830H Altcode:
Double layers of high-tensioned textile membranes were applied to the
completely open-foldable dome for the GREGOR telescope for the first
time. Simultaneous climate measurements inside and outside the dome
have proven the thermal-insulating capability of this double-layer
construction. The GREGOR dome is the result of the continuation of the
ESO research on open-foldable domes with textile structures, followed by
the research for the DOT dome with high-tensioned textile membranes. It
cleared the way to extreme stability required for astronomical practice
on high mountain sites with heavy storms and ice formation. The storm
Delta with 245 km/h 1-minute mean maximum at the location of the GREGOR
caused no problems, nor did other storms afterwards. Opening and closing
experiences up to wind speeds of 90 km/h were without problems. New
technical developments were implemented and tested at the GREGOR dome,
opening the way for application to much larger domes up to the 30 m
diameter-class range.

---------------------------------------------------------
Title: The 1.5 meter solar telescope GREGOR
Authors: Schmidt, W.; von der Lühe, O.; Volkmer, R.; Denker, C.;
Solanki, S. K.; Balthasar, H.; Bello Gonzalez, N.; Berkefeld, Th.;
Collados, M.; Fischer, A.; Halbgewachs, C.; Heidecke, F.; Hofmann,
A.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.;
Schmidt, D.; Sigwarth, M.; Sobotka, M.; Soltau, D.; Staude, J.;
Strassmeier, K. G.; Waldmann , T. A.
2012AN....333..796S Altcode:
The 1.5 m telescope GREGOR opens a new window to the understanding
of solar small-scale magnetism. The first light instrumentation
includes the Gregor Fabry Pérot Interferometer (GFPI), a filter
spectro-polarimeter for the visible wavelength range, the GRating
Infrared Spectro-polarimeter (GRIS) and the Broad-Band Imager (BBI). The
excellent performance of the first two instruments has already been
demonstrated at the Vacuum Tower Telescope. GREGOR is Europe's largest
solar telescope and number 3 in the world. Its all-reflective Gregory
design provides a large wavelength coverage from the near UV up to at
least 5 microns. The field of view has a diameter of 150 arcsec. GREGOR
is equipped with a high-order adaptive optics system, with a subaperture
size of 10 cm, and a deformable mirror with 256 actuators. The science
goals are focused on, but not limited to, solar magnetism. GREGOR
allows us to measure the emergence and disappearance of magnetic flux
at the solar surface at spatial scales well below 100 km. Thanks to its
spectro-polarimetric capabilities, GREGOR will measure the interaction
between the plasma flows, different kinds of waves, and the magnetic
field. This will foster our understanding of the processes that heat the
chromosphere and the outer layers of the solar atmosphere. Observations
of the surface magnetic field at very small spatial scales will shed
light on the variability of the solar brightness.

---------------------------------------------------------
Title: Mechanical design of the solar telescope GREGOR
Authors: Volkmer, R.; Eisenträger, P.; Emde, P.; Fischer, A.; von
der Lühe, O.; Nicklas, H.; Soltau, D.; Schmidt, W.; Weis, U.
2012AN....333..816V Altcode:
The mechanical structure of the GREGOR telescope was installed at the
Observatorio del Teide, Tenerife, in 2004. New concepts for mounting
and cooling of the 1.5-meter primary mirror were introduced. GREGOR
is an open telescope, therefore the dome is completely open during
observations to allow for air flushing through the open, but stiff
telescope structure. Backside cooling system of the primary mirror
keeps the mirror surface close to ambient temperature to prevent mirror
seeing. The large collecting area of the primary mirror results in high
energy density at the field stop at the prime focus of the primary
which needs to be removed. The optical elements are supported by
precision alignment systems and should provide a stable solar image
at the optical lab. The coudé train can be evacuated and serves
as a natural barrier between the outer environmental conditions and
the air-conditioned optical laboratory with its sensitive scientific
instrumentation. The telescope was successfully commissioned and will
start its nominal operation during 2013.

---------------------------------------------------------
Title: GRIS: The GREGOR Infrared Spectrograph
Authors: Collados, M.; López, R.; Páez, E.; Hernández, E.; Reyes,
M.; Calcines, A.; Ballesteros, E.; Díaz, J. J.; Denker, C.; Lagg,
A.; Schlichenmaier, R.; Schmidt, W.; Solanki, S. K.; Strassmeier,
K. G.; von der Lühe, O.; Volkmer, R.
2012AN....333..872C Altcode:
This paper describes the main characteristics of GRIS (GREGOR Infrared
Spectrograph), the grating spectrograph installed in the recently
inaugurated (May 2012) 1.5-meter GREGOR telescope located at the
Observatorio del Teide in Tenerife. The spectrograph has a standard
Czerny-Turner configuration with parabolic collimator and camera mirrors
that belong to the same conic surface. Although nothing prevents its
use at visible wavelengths, the spectrograph will be initially used
in combination with the infrared detector of the Tenerife Infrared
Polarimeter (TIP-II) in standard spectroscopic mode as well as for
spectropolarimetric measurements.

---------------------------------------------------------
Title: A Laser Frequency Comb System for Absolute Calibration of
the VTT Echelle Spectrograph
Authors: Doerr, H. -P.; Steinmetz, T.; Holzwarth, R.; Kentischer,
T.; Schmidt, W.
2012SoPh..280..663D Altcode: 2012arXiv1204.0948D; 2012SoPh..tmp...59D
A wavelength calibration system based on a laser frequency comb (LFC)
was developed in a co-operation between the Kiepenheuer-Institut für
Sonnenphysik, Freiburg, Germany and the Max-Planck-Institut für
Quantenoptik, Garching, Germany for permanent installation at the
German Vacuum Tower Telescope (VTT) on Tenerife, Canary Islands. The
system was installed successfully in October 2011. By simultaneously
recording the spectra from the Sun and the LFC, for each exposure a
calibration curve can be derived from the known frequencies of the
comb modes that is suitable for absolute calibration at the meters
per second level. We briefly summarize some topics in solar physics
that benefit from absolute spectroscopy and point out the advantages
of LFC compared to traditional calibration techniques. We also sketch
the basic setup of the VTT calibration system and its integration with
the existing echelle spectrograph.

---------------------------------------------------------
Title: Resolving the Internal Magnetic Structure of the Solar Network
Authors: Martínez González, M. J.; Bellot Rubio, L. R.; Solanki,
S. K.; Martínez Pillet, V.; Del Toro Iniesta, J. C.; Barthol, P.;
Schmidt, W.
2012ApJ...758L..40M Altcode: 2012arXiv1209.2584M
We analyze the spectral asymmetry of Stokes V (circularly polarized)
profiles of an individual network patch in the quiet Sun observed
by Sunrise/IMaX. At a spatial resolution of 0farcs15-0farcs18, the
network elements contain substructure which is revealed by the spatial
distribution of Stokes V asymmetries. The area asymmetry between the
red and blue lobes of Stokes V increases from nearly zero at the core
of the structure to values close to unity at its edges (single-lobed
profiles). Such a distribution of the area asymmetry is consistent
with magnetic fields expanding with height, i.e., an expanding
magnetic canopy (which is required to fulfill pressure balance and
flux conservation in the solar atmosphere). Inversion of the Stokes
I and V profiles of the patch confirms this picture, revealing a
decreasing field strength and increasing height of the canopy base
from the core to the periphery of the network patch. However, the
non-roundish shape of the structure and the presence of negative area
and amplitude asymmetries reveal that the scenario is more complex than
a canonical flux tube expanding with height surrounded by downflows.

---------------------------------------------------------
Title: 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.

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Title: Diffusivity of Isolated Internetwork Ca II H Bright Points
Observed by SuFI/SUNRISE
Authors: Jafarzadeh, S.; Solanki, S. K.; Cameron, R. H.; Feller, A.;
Pietarila, A.; Lagg, A.; Barthol, P.; Berkefeld, T.; Gandorfer, A.;
Knoelker, M.; Martinez Pillet, V.; Schmidt, W.; Title, A.
2012decs.confE..99J Altcode:
We analyze trajectories of the proper motion of intrinsically magnetic,
isolated internetwork Ca II H BPs (with mean lifetime of 461 sec) to
obtain their diffusivity behaviors. We use high spatial and temporal
resolution image sequences of quiet-Sun, disc-centre observations
obtained in the Ca II H 397 nm passband of the Sunrise Filter Imager
(SuFI) on board the SUNRISE balloon-borne solar observatory. In
order to avoid misidentification, the BPs are semi-manually selected
and then automatically tracked. The trajectory of each BP is then
calculated and its diffusion index is described by a power law
exponent, using which we classify the BPs' trajectories into sub-,
normal and super- diffusive. In addition, the corresponding diffusion
coefficients (D) based on the observed displacements are consequently
computed. We find a strong super-diffusivity at a height sampled by the
SuFI/SUNRISE Ca II H passband (i.e. a height corresponding roughly to
the temperature minimum). We find that 74% of the identified tiny BPs
are super-diffusive, 18% move randomly (i.e. their motion corresponds
to normal diffusion) and only 8% belong to the sub-diffusion regime. In
addition, we find that 53% of the super-diffusion regime (i.e. 39% of
all BPs) have the diffusivity index of 2 which are termed as "Ballistic
BPs". Finally, we explore the distribution of diffusion index with the
help of a simple simulation. The results suggest that the BPs are random
walkers superposed by a systematic (background) velocity in which the
magnitude of each component (and hence their ratio) depends on the time
and spatial scales. We further discuss a simple sketch to explain the
diffusivity of observed BPs while they migrate within a supergranule
(i.e. internetwork areas) or close to the network regions.

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Title: The Frontier between Small-scale Bipoles and Ephemeral Regions
in the Solar Photosphere: Emergence and Decay of an Intermediate-scale
Bipole Observed with SUNRISE/IMaX
Authors: Guglielmino, S. L.; Martínez Pillet, V.; Bonet, J. A.;
del Toro Iniesta, J. Carlos; Bellot Rubio, L. R.; Solanki, S. K.;
Schmidt, W.; Gandorfer, A.; Barthol, P.; Knölker, M.
2012ApJ...745..160G Altcode: 2011arXiv1110.1405G
We report on the photospheric evolution of an intermediate-scale (≈4
Mm footpoint separation) magnetic bipole, from emergence to decay,
observed in the quiet Sun at high spatial (0farcs3) and temporal (33 s)
resolution. The observations were acquired by the Imaging Magnetograph
Experiment imaging magnetograph during the first science flight of the
SUNRISE balloon-borne solar observatory. The bipole flux content is 6 ×
1017 Mx, representing a structure bridging the gap between
granular scale bipoles and the smaller ephemeral regions. Footpoints
separate at a speed of 3.5 km s-1 and reach a maximum
distance of 4.5 Mm before the field dissolves. The evolution of the
bipole is revealed to be very dynamic: we found a proper motion of
the bipole axis and detected a change of the azimuth angle of 90° in
300 s, which may indicate the presence of some writhe in the emerging
structure. The overall morphology and behavior are in agreement with
previous analyses of bipolar structures emerging at the granular scale,
but we also found several similarities with emerging flux structures
at larger scales. The flux growth rate is 2.6 × 1015 Mx
s-1, while the mean decay rate is one order of magnitude
smaller. We describe in some detail the decay phase of the bipole
footpoints that includes break up into smaller structures, and
interaction with preexisting fields leading to cancellation, but it
appears to be dominated by an as-yet unidentified diffusive process
that removes most of the flux with an exponential flux decay curve. The
diffusion constant (8 × 102 km2 s-1)
associated with this decay is similar to the values used to describe
the large-scale diffusion in flux transport models.

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Title: Magnetic field emergence in mesogranular-sized exploding
granules observed with sunrise/IMaX data
Authors: Palacios, J.; Blanco Rodríguez, J.; Vargas Domínguez, S.;
Domingo, V.; Martínez Pillet, V.; Bonet, J. A.; Bellot Rubio, L. R.;
Del Toro Iniesta, J. C.; Solanki, S. K.; Barthol, P.; Gandorfer, A.;
Berkefeld, T.; Schmidt, W.; Knölker, M.
2012A&A...537A..21P Altcode: 2011arXiv1110.4555P
We report on magnetic field emergences covering significant
areas of exploding granules. The balloon-borne mission Sunrise
provided high spatial and temporal resolution images of the solar
photosphere. Continuum images, longitudinal and transverse magnetic
field maps and Dopplergrams obtained by IMaX onboard Sunrise are
analyzed by local correlation traking (LCT), divergence calculation
and time slices, Stokes inversions and numerical simulations are also
employed. We characterize two mesogranular-scale exploding granules
where ~1018 Mx of magnetic flux emerges. The emergence
of weak unipolar longitudinal fields (~100 G) start with a single
visible magnetic polarity, occupying their respective granules' top
and following the granular splitting. After a while, mixed polarities
start appearing, concentrated in downflow lanes. The events last around
20 min. LCT analyses confirm mesogranular scale expansion, displaying
a similar pattern for all the physical properties, and divergence
centers match between all of them. We found a similar behaviour
with the emergence events in a numerical MHD simulation. Granule
expansion velocities are around 1 kms-1 while magnetic
patches expand at 0.65 kms-1. One of the analyzed events
evidences the emergence of a loop-like structure. Advection of
the emerging magnetic flux features is dominated by convective
motion resulting from the exploding granule due to the magnetic
field frozen in the granular plasma. Intensification of the
magnetic field occurs in the intergranular lanes, probably
because of being directed by the downflowing plasma. Movies
associated to Figs. 2-4 are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

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Title: Venus transit 2004: Illustrating the capability of exoplanet
transmission spectroscopy
Authors: Hedelt, P.; Alonso, R.; Brown, T.; Collados Vera, M.; Rauer,
H.; Schleicher, H.; Schmidt, W.; Schreier, F.; Titz, R.
2011A&A...533A.136H Altcode: 2011arXiv1107.3700H
The transit of Venus in 2004 offered the rare possibility to remotely
sense a well-known planetary atmosphere using ground-based absorption
spectroscopy. Transmission spectra of Venus' atmosphere were obtained
in the near infrared using the Vacuum Tower Telescope (VTT) in
Tenerife. Since the instrument was designed to measure the very bright
photosphere of the Sun, extracting Venus' atmosphere was challenging. We
were able to identify CO2 absorption lines in the upper
Venus atmosphere. Moreover, the relative abundance of the three
most abundant CO2 isotopologues could be determined. The
observations resolved Venus' limb, showing Doppler-shifted absorption
lines that are probably caused by high-altitude winds. We demonstrate
the utility of ground-based measurements in analyzing the atmospheric
constituents of a terrestrial planet atmosphere using methods that
might be applied in future to terrestrial extrasolar planets.

---------------------------------------------------------
Title: The Sun at high resolution: first results from the Sunrise
mission
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller,
A.; Gandorfer, A.; Hirzberger, J.; Lagg, A.; Riethmüller, T. L.;
Schüssler, M.; Wiegelmann, T.; Bonet, J. A.; Pillet, V. Martínez;
Khomenko, E.; del Toro Iniesta, J. C.; Domingo, V.; Palacios, J.;
Knölker, M.; González, N. Bello; Borrero, J. M.; Berkefeld, T.;
Franz, M.; Roth, M.; Schmidt, W.; Steiner, O.; Title, A. M.
2011IAUS..273..226S Altcode:
The Sunrise balloon-borne solar observatory consists of a 1m aperture
Gregory telescope, a UV filter imager, an imaging vector polarimeter,
an image stabilization system and further infrastructure. The first
science flight of Sunrise yielded high-quality data that reveal the
structure, dynamics and evolution of solar convection, oscillations
and magnetic fields at a resolution of around 100 km in the quiet
Sun. Here we describe very briefly the mission and the first results
obtained from the Sunrise data, which include a number of discoveries.

---------------------------------------------------------
Title: Unnoticed Magnetic Field Oscillations in the Very Quiet Sun
Revealed by SUNRISE/IMaX
Authors: Martínez González, M. J.; Asensio Ramos, A.; Manso Sainz,
R.; Khomenko, E.; Martínez Pillet, V.; Solanki, S. K.; López Ariste,
A.; Schmidt, W.; Barthol, P.; Gandorfer, A.
2011ApJ...730L..37M Altcode: 2011arXiv1103.0145M
We present observational evidence for oscillations of magnetic flux
density in the quiet areas of the Sun. The majority of magnetic
fields on the solar surface have strengths of the order of or lower
than the equipartition field (300-500 G). This results in a myriad of
magnetic fields whose evolution is largely determined by the turbulent
plasma motions. When granules evolve they squash the magnetic field
lines together or pull them apart. Here, we report on the periodic
deformation of the shapes of features in circular polarization observed
at high resolution with SUNRISE. In particular, we note that the
area of patches with a constant magnetic flux oscillates with time,
which implies that the apparent magnetic field intensity oscillates
in antiphase. The periods associated with this oscillatory pattern
are compatible with the granular lifetime and change abruptly, which
suggests that these oscillations might not correspond to characteristic
oscillatory modes of magnetic structures, but to the forcing by granular
motions. In one particular case, we find three patches around the same
granule oscillating in phase, which means that the spatial coherence
of these oscillations can reach 1600 km. Interestingly, the same kind
of oscillatory phenomenon is also found in the upper photosphere.

---------------------------------------------------------
Title: Mesogranulation and the Solar Surface Magnetic Field
Distribution
Authors: Yelles Chaouche, L.; Moreno-Insertis, F.; Martínez Pillet,
V.; Wiegelmann, T.; Bonet, J. A.; Knölker, M.; Bellot Rubio, L. R.;
del Toro Iniesta, J. C.; Barthol, P.; Gandorfer, A.; Schmidt, W.;
Solanki, S. K.
2011ApJ...727L..30Y Altcode: 2010arXiv1012.4481Y
The relation of the solar surface magnetic field with mesogranular
cells is studied using high spatial (≈100 km) and temporal (≈30
s) resolution data obtained with the IMaX instrument on board
SUNRISE. First, mesogranular cells are identified using Lagrange
tracers (corks) based on horizontal velocity fields obtained through
local correlation tracking. After ≈20 minutes of integration, the
tracers delineate a sharp mesogranular network with lanes of width
below about 280 km. The preferential location of magnetic elements in
mesogranular cells is tested quantitatively. Roughly 85% of pixels with
magnetic field higher than 100 G are located in the near neighborhood
of mesogranular lanes. Magnetic flux is therefore concentrated in
mesogranular lanes rather than intergranular ones. Second, magnetic
field extrapolations are performed to obtain field lines anchored in
the observed flux elements. This analysis, therefore, is independent
of the horizontal flows determined in the first part. A probability
density function (PDF) is calculated for the distribution of distances
between the footpoints of individual magnetic field lines. The PDF has
an exponential shape at scales between 1 and 10 Mm, with a constant
characteristic decay distance, indicating the absence of preferred
convection scales in the mesogranular range. Our results support
the view that mesogranulation is not an intrinsic convective scale
(in the sense that it is not a primary energy-injection scale of solar
convection), but also give quantitative confirmation that, nevertheless,
the magnetic elements are preferentially found along mesogranular lanes.

---------------------------------------------------------
Title: The Imaging Magnetograph eXperiment (IMaX) for the Sunrise
Balloon-Borne Solar Observatory
Authors: Martínez Pillet, V.; del Toro Iniesta, J. C.;
Álvarez-Herrero, A.; Domingo, V.; Bonet, J. A.; González Fernández,
L.; López Jiménez, A.; Pastor, C.; Gasent Blesa, J. L.; Mellado, P.;
Piqueras, J.; Aparicio, B.; Balaguer, M.; Ballesteros, E.; Belenguer,
T.; Bellot Rubio, L. R.; Berkefeld, T.; Collados, M.; Deutsch, W.;
Feller, A.; Girela, F.; Grauf, B.; Heredero, R. L.; Herranz, M.;
Jerónimo, J. M.; Laguna, H.; Meller, R.; Menéndez, M.; Morales, R.;
Orozco Suárez, D.; Ramos, G.; Reina, M.; Ramos, J. L.; Rodríguez,
P.; Sánchez, A.; Uribe-Patarroyo, N.; Barthol, P.; Gandorfer, A.;
Knoelker, M.; Schmidt, W.; Solanki, S. K.; Vargas Domínguez, S.
2011SoPh..268...57M Altcode: 2010SoPh..tmp..181M; 2010arXiv1009.1095M
The Imaging Magnetograph eXperiment (IMaX) is a spectropolarimeter
built by four institutions in Spain that flew on board the Sunrise
balloon-borne solar observatory in June 2009 for almost six days over
the Arctic Circle. As a polarimeter, IMaX uses fast polarization
modulation (based on the use of two liquid crystal retarders),
real-time image accumulation, and dual-beam polarimetry to reach
polarization sensitivities of 0.1%. As a spectrograph, the instrument
uses a LiNbO3 etalon in double pass and a narrow band
pre-filter to achieve a spectral resolution of 85 mÅ. IMaX uses the
high-Zeeman-sensitive line of Fe I at 5250.2 Å and observes all four
Stokes parameters at various points inside the spectral line. This
allows vector magnetograms, Dopplergrams, and intensity frames to be
produced that, after reconstruction, reach spatial resolutions in the
0.15 - 0.18 arcsec range over a 50×50 arcsec field of view. Time
cadences vary between 10 and 33 s, although the shortest one only
includes longitudinal polarimetry. The spectral line is sampled in
various ways depending on the applied observing mode, from just two
points inside the line to 11 of them. All observing modes include
one extra wavelength point in the nearby continuum. Gauss equivalent
sensitivities are 4 G for longitudinal fields and 80 G for transverse
fields per wavelength sample. The line-of-sight velocities are estimated
with statistical errors of the order of 5 - 40 m s−1. The
design, calibration, and integration phases of the instrument,
together with the implemented data reduction scheme, are described in
some detail.

---------------------------------------------------------
Title: The Wave-Front Correction System for the Sunrise Balloon-Borne
Solar Observatory
Authors: Berkefeld, T.; Schmidt, W.; Soltau, D.; Bell, A.;
Doerr, H. P.; Feger, B.; Friedlein, R.; Gerber, K.; Heidecke, F.;
Kentischer, T.; v. d. Lühe, O.; Sigwarth, M.; Wälde, E.; Barthol,
P.; Deutsch, W.; Gandorfer, A.; Germerott, D.; Grauf, B.; Meller, R.;
Álvarez-Herrero, A.; Knölker, M.; Martínez Pillet, V.; Solanki,
S. K.; Title, A. M.
2011SoPh..268..103B Altcode: 2010SoPh..tmp..236B; 2010arXiv1009.3196B
This paper describes the wave-front correction system developed for
the Sunrise balloon telescope, and it provides information about its
in-flight performance. For the correction of low-order aberrations,
a Correlating Wave-Front Sensor (CWS) was used. It consisted of a
six-element Shack - Hartmann wave-front sensor (WFS), a fast tip-tilt
mirror for the compensation of image motion, and an active telescope
secondary mirror for focus correction. The CWS delivered a stabilized
image with a precision of 0.04 arcsec (rms), whenever the coarse
pointing was better than ± 45 arcsec peak-to-peak. The automatic
focus adjustment maintained a focus stability of 0.01 waves in the
focal plane of the CWS. During the 5.5 day flight, good image quality
and stability were achieved during 33 hours, containing 45 sequences,
which lasted between 10 and 45 min.

---------------------------------------------------------
Title: The Sunrise Mission
Authors: Barthol, P.; Gandorfer, A.; Solanki, S. K.; Schüssler,
M.; Chares, B.; Curdt, W.; Deutsch, W.; Feller, A.; Germerott, D.;
Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.; Meller, R.;
Müller, R.; Riethmüller, T. L.; Tomasch, G.; Knölker, M.; Lites,
B. W.; Card, G.; Elmore, D.; Fox, J.; Lecinski, A.; Nelson, P.;
Summers, R.; Watt, A.; Martínez Pillet, V.; Bonet, J. A.; Schmidt,
W.; Berkefeld, T.; Title, A. M.; Domingo, V.; Gasent Blesa, J. L.;
del Toro Iniesta, J. C.; López Jiménez, A.; Álvarez-Herrero, A.;
Sabau-Graziati, L.; Widani, C.; Haberler, P.; Härtel, K.; Kampf,
D.; Levin, T.; Pérez Grande, I.; Sanz-Andrés, A.; Schmidt, E.
2011SoPh..268....1B Altcode: 2010arXiv1009.2689B; 2010SoPh..tmp..224B
The first science flight of the balloon-borne Sunrise telescope took
place in June 2009 from ESRANGE (near Kiruna/Sweden) to Somerset
Island in northern Canada. We describe the scientific aims and
mission concept of the project and give an overview and a description
of the various hardware components: the 1-m main telescope with its
postfocus science instruments (the UV filter imager SuFI and the imaging
vector magnetograph IMaX) and support instruments (image stabilizing
and light distribution system ISLiD and correlating wavefront sensor
CWS), the optomechanical support structure and the instrument mounting
concept, the gondola structure and the power, pointing, and telemetry
systems, and the general electronics architecture. We also explain
the optimization of the structural and thermal design of the complete
payload. The preparations for the science flight are described,
including AIV and ground calibration of the instruments. The course
of events during the science flight is outlined, up to the recovery
activities. Finally, the in-flight performance of the instrumentation
is discussed.

---------------------------------------------------------
Title: The Filter Imager SuFI and the Image Stabilization and Light
Distribution System ISLiD of the Sunrise Balloon-Borne Observatory:
Instrument Description
Authors: Gandorfer, A.; Grauf, B.; Barthol, P.; Riethmüller, T. L.;
Solanki, S. K.; Chares, B.; Deutsch, W.; Ebert, S.; Feller, A.;
Germerott, D.; Heerlein, K.; Heinrichs, J.; Hirche, D.; Hirzberger,
J.; Kolleck, M.; Meller, R.; Müller, R.; Schäfer, R.; Tomasch,
G.; Knölker, M.; Martínez Pillet, V.; Bonet, J. A.; Schmidt, W.;
Berkefeld, T.; Feger, B.; Heidecke, F.; Soltau, D.; Tischenberg, A.;
Fischer, A.; Title, A.; Anwand, H.; Schmidt, E.
2011SoPh..268...35G Altcode: 2010SoPh..tmp..176G; 2010arXiv1009.1037G
We describe the design of the Sunrise Filter Imager (SuFI) and the
Image Stabilization and Light Distribution (ISLiD) unit onboard the
Sunrise balloon borne solar observatory. This contribution provides the
necessary information which is relevant to understand the instruments'
working principles, the relevant technical data, and the necessary
information about calibration issues directly related to the science
data.

---------------------------------------------------------
Title: SUNRISE: Instrument, Mission, Data, and First Results
Authors: Solanki, S. K.; Barthol, P.; Danilovic, S.; Feller, A.;
Gandorfer, A.; Hirzberger, J.; Riethmüller, T. L.; Schüssler, M.;
Bonet, J. A.; Martínez Pillet, V.; del Toro Iniesta, J. C.; Domingo,
V.; Palacios, J.; Knölker, M.; Bello González, N.; Berkefeld, T.;
Franz, M.; Schmidt, W.; Title, A. M.
2010ApJ...723L.127S Altcode: 2010arXiv1008.3460S
The SUNRISE balloon-borne solar observatory consists of a 1 m aperture
Gregory telescope, a UV filter imager, an imaging vector polarimeter,
an image stabilization system, and further infrastructure. The first
science flight of SUNRISE yielded high-quality data that revealed the
structure, dynamics, and evolution of solar convection, oscillations,
and magnetic fields at a resolution of around 100 km in the quiet
Sun. After a brief description of instruments and data, the first
qualitative results are presented. In contrast to earlier observations,
we clearly see granulation at 214 nm. Images in Ca II H display narrow,
short-lived dark intergranular lanes between the bright edges of
granules. The very small-scale, mixed-polarity internetwork fields
are found to be highly dynamic. A significant increase in detectable
magnetic flux is found after phase-diversity-related reconstruction
of polarization maps, indicating that the polarities are mixed right
down to the spatial resolution limit and probably beyond.

---------------------------------------------------------
Title: Supersonic Magnetic Upflows in Granular Cells Observed with
SUNRISE/IMAX
Authors: Borrero, J. M.; Martínez-Pillet, V.; Schlichenmaier, R.;
Solanki, S. K.; Bonet, J. A.; del Toro Iniesta, J. C.; Schmidt, W.;
Barthol, P.; Gandorfer, A.; Domingo, V.; Knölker, M.
2010ApJ...723L.144B Altcode: 2010arXiv1009.1227B
Using the IMaX instrument on board the SUNRISE stratospheric balloon
telescope, we have detected extremely shifted polarization signals
around the Fe I 5250.217 Å spectral line within granules in the solar
photosphere. We interpret the velocities associated with these events
as corresponding to supersonic and magnetic upflows. In addition, they
are also related to the appearance of opposite polarities and highly
inclined magnetic fields. This suggests that they are produced by the
reconnection of emerging magnetic loops through granular upflows. The
events occupy an average area of 0.046 arcsec2 and last for
about 80 s, with larger events having longer lifetimes. These supersonic
events occur at a rate of 1.3 × 10-5 occurrences per second
per arcsec2.

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

---------------------------------------------------------
Title: Where the Granular Flows Bend
Authors: Khomenko, E.; Martínez Pillet, V.; Solanki, S. K.; del Toro
Iniesta, J. C.; Gandorfer, A.; Bonet, J. A.; Domingo, V.; Schmidt,
W.; Barthol, P.; Knölker, M.
2010ApJ...723L.159K Altcode: 2010arXiv1008.0517K
Based on IMaX/SUNRISE data, we report on a previously undetected
phenomenon in solar granulation. We show that in a very narrow region
separating granules and intergranular lanes, the spectral line width
of the Fe I 5250.2 Å line becomes extremely small. We offer an
explanation of this observation with the help of magneto-convection
simulations. These regions with extremely small line widths correspond
to the places where the granular flows bend from upflow in granules
to downflow in intergranular lanes. We show that the resolution and
image stability achieved by IMaX/SUNRISE are important requisites to
detect this interesting phenomenon.

---------------------------------------------------------
Title: Bright Points in the Quiet Sun as Observed in the Visible
and Near-UV by the Balloon-borne Observatory SUNRISE
Authors: Riethmüller, T. L.; Solanki, S. K.; Martínez Pillet, V.;
Hirzberger, J.; Feller, A.; Bonet, J. A.; Bello González, N.; Franz,
M.; Schüssler, M.; Barthol, P.; Berkefeld, T.; del Toro Iniesta,
J. C.; Domingo, V.; Gandorfer, A.; Knölker, M.; Schmidt, W.
2010ApJ...723L.169R Altcode: 2010arXiv1009.1693R
Bright points (BPs) are manifestations of small magnetic elements
in the solar photosphere. Their brightness contrast not only gives
insight into the thermal state of the photosphere (and chromosphere) in
magnetic elements, but also plays an important role in modulating the
solar total and spectral irradiance. Here, we report on simultaneous
high-resolution imaging and spectropolarimetric observations of
BPs using SUNRISE balloon-borne observatory data of the quiet Sun
at the disk center. BP contrasts have been measured between 214 nm
and 525 nm, including the first measurements at wavelengths below
388 nm. The histograms of the BP peak brightness show a clear trend
toward broader contrast distributions and higher mean contrasts at
shorter wavelengths. At 214 nm, we observe a peak brightness of up to
five times the mean quiet-Sun value, the highest BP contrast so far
observed. All BPs are associated with a magnetic signal, although in
a number of cases it is surprisingly weak. Most of the BPs show only
weak downflows, the mean value being 240 m s-1, but some
display strong down- or upflows reaching a few km s-1.

---------------------------------------------------------
Title: Transverse Component of the Magnetic Field in the Solar
Photosphere Observed by SUNRISE
Authors: Danilovic, S.; Beeck, B.; Pietarila, A.; Schüssler, M.;
Solanki, S. K.; Martínez Pillet, V.; Bonet, J. A.; del Toro Iniesta,
J. C.; Domingo, V.; Barthol, P.; Berkefeld, T.; Gandorfer, A.;
Knölker, M.; Schmidt, W.; Title, A. M.
2010ApJ...723L.149D Altcode: 2010arXiv1008.1535D
We present the first observations of the transverse component of
a photospheric magnetic field acquired by the imaging magnetograph
SUNRISE/IMaX. Using an automated detection method, we obtain statistical
properties of 4536 features with significant linear polarization
signal. We obtain a rate of occurrence of 7 × 10-4
s-1 arcsec-2, which is 1-2 orders of magnitude
larger than the values reported by previous studies. We show that
these features have no characteristic size or lifetime. They appear
preferentially at granule boundaries with most of them being caught
in downflow lanes at some point. Only a small percentage are entirely
and constantly embedded in upflows (16%) or downflows (8%).

---------------------------------------------------------
Title: Detection of Large Acoustic Energy Flux in the Solar Atmosphere
Authors: Bello González, N.; Franz, M.; Martínez Pillet, V.; Bonet,
J. A.; Solanki, S. K.; del Toro Iniesta, J. C.; Schmidt, W.; Gandorfer,
A.; Domingo, V.; Barthol, P.; Berkefeld, T.; Knölker, M.
2010ApJ...723L.134B Altcode: 2010arXiv1009.4795B
We study the energy flux carried by acoustic waves excited by convective
motions at sub-photospheric levels. The analysis of high-resolution
spectropolarimetric data taken with IMaX/SUNRISE provides a total
energy flux of ~6400-7700 W m-2 at a height of ~250 km
in the 5.2-10 mHz range, i.e., at least twice the largest energy
flux found in previous works. Our estimate lies within a factor of
two of the energy flux needed to balance radiative losses from the
chromosphere according to the estimates of Anderson &amp; Athay and
revives interest in acoustic waves for transporting energy to the
chromosphere. The acoustic flux is mainly found in the intergranular
lanes but also in small rapidly evolving granules and at the bright
borders, forming dark dots and lanes of splitting granules.

---------------------------------------------------------
Title: Magnetic Loops in the Quiet Sun
Authors: Wiegelmann, T.; Solanki, S. K.; Borrero, J. M.; Martínez
Pillet, V.; del Toro Iniesta, J. C.; Domingo, V.; Bonet, J. A.;
Barthol, P.; Gandorfer, A.; Knölker, M.; Schmidt, W.; Title, A. M.
2010ApJ...723L.185W Altcode: 2010arXiv1009.4715W
We investigate the fine structure of magnetic fields in the atmosphere
of the quiet Sun. We use photospheric magnetic field measurements from
SUNRISE/IMaX with unprecedented spatial resolution to extrapolate
the photospheric magnetic field into higher layers of the solar
atmosphere with the help of potential and force-free extrapolation
techniques. We find that most magnetic loops that reach into the
chromosphere or higher have one footpoint in relatively strong magnetic
field regions in the photosphere. Ninety-one percent of the magnetic
energy in the mid-chromosphere (at a height of 1 Mm) is in field
lines, whose stronger footpoint has a strength of more than 300 G,
i.e., above the equipartition field strength with convection. The
loops reaching into the chromosphere and corona are also found to be
asymmetric in the sense that the weaker footpoint has a strength B &lt;
300 G and is located in the internetwork (IN). Such loops are expected
to be strongly dynamic and have short lifetimes, as dictated by the
properties of the IN fields.

---------------------------------------------------------
Title: SUNRISE/IMaX Observations of Convectively Driven Vortex Flows
in the Sun
Authors: Bonet, J. A.; Márquez, I.; Sánchez Almeida, J.; Palacios,
J.; Martínez Pillet, V.; Solanki, S. K.; del Toro Iniesta, J. C.;
Domingo, V.; Berkefeld, T.; Schmidt, W.; Gandorfer, A.; Barthol, P.;
Knölker, M.
2010ApJ...723L.139B Altcode: 2010arXiv1009.1992B
We characterize the observational properties of the convectively driven
vortex flows recently discovered on the quiet Sun, using magnetograms,
Dopplergrams, and images obtained with the 1 m balloon-borne SUNRISE
telescope. By visual inspection of time series, we find some 3.1
× 10-3 vortices Mm-2 minute-1,
which is a factor of ~1.7 larger than previous estimates. The mean
duration of the individual events turns out to be 7.9 minutes, with
a standard deviation of 3.2 minutes. In addition, we find several
events appearing at the same locations along the duration of the time
series (31.6 minutes). Such recurrent vortices show up in the proper
motion flow field map averaged over the time series. The typical
vertical vorticities are lsim6 × 10-3 s-1,
which corresponds to a period of rotation of some 35 minutes. The
vortices show a preferred counterclockwise sense of rotation, which
we conjecture may have to do with the preferred vorticity impinged by
the solar differential rotation.

---------------------------------------------------------
Title: Surface Waves in Solar Granulation Observed with SUNRISE
Authors: Roth, M.; Franz, M.; Bello González, N.; Martínez Pillet,
V.; Bonet, J. A.; Gandorfer, A.; Barthol, P.; Solanki, S. K.;
Berkefeld, T.; Schmidt, W.; del Toro Iniesta, J. C.; Domingo, V.;
Knölker, M.
2010ApJ...723L.175R Altcode: 2010arXiv1009.4790R
Solar oscillations are expected to be excited by turbulent flows in
the intergranular lanes near the solar surface. Time series recorded
by the IMaX instrument on board the SUNRISE observatory reveal solar
oscillations at high spatial resolution, which allow the study of
the properties of oscillations with short wavelengths. We analyze
two time series with synchronous recordings of Doppler velocity and
continuum intensity images with durations of 32 minutes and 23 minutes,
respectively, recorded close to the disk center of the Sun to study
the propagation and excitation of solar acoustic oscillations. In
the Doppler velocity data, both the standing acoustic waves and the
short-lived, high-degree running waves are visible. The standing
waves are visible as temporary enhancements of the amplitudes of the
large-scale velocity field due to the stochastic superposition of
the acoustic waves. We focus on the high-degree small-scale waves by
suitable filtering in the Fourier domain. Investigating the propagation
and excitation of f- and p 1-modes with wavenumbers k&gt;1.4
Mm-1, we also find that exploding granules contribute to
the excitation of solar p-modes in addition to the contribution of
intergranular lanes.

---------------------------------------------------------
Title: Fully Resolved Quiet-Sun Magnetic flux Tube Observed with
the SUNRISE/IMAX Instrument
Authors: Lagg, A.; Solanki, S. K.; Riethmüller, T. L.; Martínez
Pillet, V.; Schüssler, M.; Hirzberger, J.; Feller, A.; Borrero,
J. M.; Schmidt, W.; del Toro Iniesta, J. C.; Bonet, J. A.; Barthol, P.;
Berkefeld, T.; Domingo, V.; Gandorfer, A.; Knölker, M.; Title, A. M.
2010ApJ...723L.164L Altcode: 2010arXiv1009.0996L
Until today, the small size of magnetic elements in quiet-Sun areas has
required the application of indirect methods, such as the line-ratio
technique or multi-component inversions, to infer their physical
properties. A consistent match to the observed Stokes profiles could
only be obtained by introducing a magnetic filling factor that specifies
the fraction of the observed pixel filled with magnetic field. Here,
we investigate the properties of a small magnetic patch in the quiet
Sun observed with the IMaX magnetograph on board the balloon-borne
telescope SUNRISE with unprecedented spatial resolution and low
instrumental stray light. We apply an inversion technique based on
the numerical solution of the radiative transfer equation to retrieve
the temperature stratification and the field strength in the magnetic
patch. The observations can be well reproduced with a one-component,
fully magnetized atmosphere with a field strength exceeding 1 kG and
a significantly enhanced temperature in the mid to upper photosphere
with respect to its surroundings, consistent with semi-empirical flux
tube models for plage regions. We therefore conclude that, within the
framework of a simple atmospheric model, the IMaX measurements resolve
the observed quiet-Sun flux tube.

---------------------------------------------------------
Title: Quiet-sun Intensity Contrasts in the Near-ultraviolet as
Measured from SUNRISE
Authors: Hirzberger, J.; Feller, A.; Riethmüller, T. L.; Schüssler,
M.; Borrero, J. M.; Afram, N.; Unruh, Y. C.; Berdyugina, S. V.;
Gandorfer, A.; Solanki, S. K.; Barthol, P.; Bonet, J. A.; Martínez
Pillet, V.; Berkefeld, T.; Knölker, M.; Schmidt, W.; Title, A. M.
2010ApJ...723L.154H Altcode:
We present high-resolution images of the Sun in the near-ultraviolet
spectral range between 214 nm and 397 nm as obtained from the first
science flight of the 1 m SUNRISE balloon-borne solar telescope. The
quiet-Sun rms intensity contrasts found in this wavelength range are
among the highest values ever obtained for quiet-Sun solar surface
structures—up to 32.8% at a wavelength of 214 nm. We compare the
rms contrasts obtained from the observational data with theoretical
intensity contrasts obtained from numerical magnetohydrodynamic
simulations. For 388 nm and 312 nm the observations agree well with
the numerical simulations whereas at shorter wavelengths discrepancies
between observed and simulated contrasts remain.

---------------------------------------------------------
Title: A Chromospheric Conundrum?
Authors: Judge, Philip; Knölker, Michael; Schmidt, Wolfgang;
Steiner, Oskar
2010ApJ...720..776J Altcode: 2010arXiv1007.1203J
We examine spectra of the Ca II H line, obtained under good seeing
conditions with the VTT Echelle Spectrograph in 2007 June, and
higher resolution data of the Ca II λ8542 line from Fabry-Pérot
instruments. The VTT targets were areas near disk center which included
quiet Sun and some dispersed plage. The infrared data included quiet
Sun and plage associated with small pores. Bright chromospheric network
emission patches expand little with wavelength from line wing to line
center, i.e., with increasing line opacity and height. We argue that
this simple observation has implications for the force and energy
balance of the chromosphere, since bright chromospheric network
emission is traditionally associated with enhanced local mechanical
heating which increases temperatures and pressures. Simple physical
considerations then suggest that the network chromosphere may not be
able to reach horizontal force balance with its surroundings, yet the
network is a long-lived structure. We speculate on possible reasons for
the observed behavior. By drawing attention to a potential conundrum,
we hope to contribute to a better understanding of a long-standing
unsolved problem: the heating of the chromospheric network.

---------------------------------------------------------
Title: SUNRISE Impressions from a successful science flight
Authors: Schmidt, W.; Solanki, S. K.; Barthol, P.; Berkefeld, T.;
Gandorfer, A.; Knölker, M.; Martínez Pillet, V.; Schüssler, M.;
Title, A.
2010AN....331..601S Altcode:
SUNRISE is a balloon-borne telescope with an aperture of one meter. It
is equipped with a filter imager for the UV wavelength range between
214 nm and 400 nm (SUFI), and with a spectro-polarimeter that measures
the magnetic field of the photosphere using the Fe I line at 525.02
nm that has a Landé factor of 3. SUNRISE performed its first science
flight from 8 to 14 June 2009. It was launched at the Swedish ESRANGE
Space Center and cruised at an altitude of about 36 km and geographic
latitudes between 70 and 74 degrees to Somerset Island in northern
Canada. There, all data, the telescope and the gondola were successfully
recovered. During its flight, Sunrise achieved high pointing stability
during 33 hours, and recorded about 1.8 TB of science data. Already at
this early stage of data processing it is clear that SUNRISE recorded
UV images of the solar photosphere, and spectropolarimetric measurements
of the quiet Sun's magnetic field of unprecedented quality.

---------------------------------------------------------
Title: GREGOR solar telescope: Design and status
Authors: Volkmer, R.; von der Lühe, O.; Denker, C.; Solanki, S. K.;
Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Fischer, A.;
Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Klvaňa, M.; Kneer, F.;
Lagg, A.; Popow, E.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau,
D.; Strassmeier, K. G.
2010AN....331..624V Altcode:
The integration and verification phase of the GREGOR telescope
reached an important milestone with the installation of the
interim 1 m SolarLite primary mirror. This was the first time
that the entire light path had seen sunlight. Since then extensive
testing of the telescope and its subsystems has been carried out. The
integration and verification phase will culminate with the delivery and
installation of the final 1.5 m Zerodur primary mirror in the summer of
2010. Observatory level tests and science verification will commence in
the second half of 2010 and in 2011. This phase includes testing of the
main optics, adaptive optics, cooling and pointing systems. In addition,
assuming the viewpoint of a typical user, various observational modes
of the GREGOR Fabry-Pérot Interferometer (GFPI), the Grating Infrared
Spectrograph (GRIS), and high-speed camera systems will be tested to
evaluate if they match the expectations and science requirements. This
ensures that GREGOR will provide high-quality observations with its
combination of (multi-conjugate) adaptive optics and advanced post-focus
instruments. Routine observations are expected for 2012.

---------------------------------------------------------
Title: Laser Frequency Combs for Astronomical Observations
Authors: Steinmetz, Tilo; Wilken, Tobias; Araujo-Hauck, Constanza;
Holzwarth, Ronald; Hänsch, Theodor W.; Pasquini, Luca; Manescau,
Antonio; D'Odorico, Sandro; Murphy, Michael T.; Kentischer, Thomas;
Schmidt, Wolfgang; Udem, Thomas
2008Sci...321.1335S Altcode: 2008arXiv0809.1663S
A direct measurement of the universe’s expansion history could be made
by observing in real time the evolution of the cosmological redshift
of distant objects. However, this would require measurements of Doppler
velocity drifts of ~1 centimeter per second per year, and astronomical
spectrographs have not yet been calibrated to this tolerance. We
demonstrated the first use of a laser frequency comb for wavelength
calibration of an astronomical telescope. Even with a simple analysis,
absolute calibration is achieved with an equivalent Doppler precision
of ~9 meters per second at ~1.5 micrometers—beyond state-of-the-art
accuracy. We show that tracking complex, time-varying systematic
effects in the spectrograph and detector system is a particular
advantage of laser frequency comb calibration. This technique promises
an effective means for modeling and removal of such systematic effects
to the accuracy required by future experiments to see direct evidence
of the universe’s putative acceleration.

---------------------------------------------------------
Title: SUNRISE: High resolution UV/VIS observations of the sun from
the stratosphere
Authors: Sunrise Team; Barthol, P.; Gandorfer, A. M.; Solanki,
S. K.; Knölker, M.; Martinez Pillet, V.; Schmidt, W.; Title, A. M.;
SUNRISE Team
2008AdSpR..42...70S Altcode:
SUNRISE is an international project for the development, construction
and operation of a balloon-borne solar telescope with an aperture
of 1 m, working in the UV/VIS spectral domain. The main scientific
goal of SUNRISE is to understand the structure and dynamics of the
magnetic field in the atmosphere of the Sun. SUNRISE will provide
near diffraction-limited images of the photosphere and chromosphere
with an unprecedented resolution down to 35 km on the solar surface
at wavelengths around 220 nm. Active in-flight alignment and image
stabilization techniques are used. The focal-plane instrumentation
consists of a polarization sensitive spectrograph, a Fabry Perot
filter magnetograph and a phase-diverse filter imager working in
the near UV. The first stratospheric long-duration balloon flight
of SUNRISE is planned in summer 2009 from the Swedish ESRANGE
station. SUNRISE is a joint project of the German Max-Planck-Institut
für Sonnensystemforschung (MPS), Katlenburg-Lindau, with the
Kiepenheuer-Institut für Sonnenphysik (KIS), Freiburg, Germany, the
High-Altitude Observatory (HAO), Boulder, USA, the Lockheed-Martin
Solar and Astrophysics Laboratory (LMSAL), Palo Alto, USA, and the
Spanish IMaX consortium. This paper will give an overview about the
mission and a description of its scientific and technological aspects.

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

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

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

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

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

---------------------------------------------------------
Title: On the fine structure of the quiet solar Ca II K atmosphere
Authors: Tritschler, A.; Schmidt, W.; Uitenbroek, H.; Wedemeyer-Böhm,
S.
2007A&A...462..303T Altcode: 2006astro.ph.11402T
Aims:We investigate the morphological, dynamical, and evolutionary
properties of the internetwork and network fine structure of the quiet
sun at disk centre. Methods: The analysis is based on a ~6 h
time sequence of narrow-band filtergrams centred on the inner-wing
Ca II K2v reversal at 393.3 nm. To examine the temporal evolution of
network and internetwork areas separately we employ a double-Gaussian
decomposition of the mean intensity distribution. An autocorrelation
analysis is performed to determine the respective characteristic
time scales. In order to analyse statistical properties of the fine
structure we apply image segmentation techniques. Results: The
results for the internetwork are related to predictions derived from
numerical simulations of the quiet sun. The average evolutionary time
scale of the internetwork in our observations is 52 s. Internetwork
grains show a tendency to appear on a mesh-like pattern with a
mean cell size of ~4-5 arcsec. Based on this size and the spatial
organisation of the mesh we speculate that this pattern is related to
the existence of photospheric downdrafts as predicted by convection
simulations. The image segmentation shows that typical sizes of both
network and internetwork grains are in the order of 1.6 arcsec.

---------------------------------------------------------
Title: Observation of a short-lived pattern in the solar chromosphere
Authors: Wöger, F.; Wedemeyer-Böhm, S.; Schmidt, W.; von der
Lühe, O.
2006A&A...459L...9W Altcode: 2006astro.ph..9382W
Aims.In this work we investigate the dynamic behavior of inter-network
regions of the solar chromosphere. Methods: .We observed the
chromosphere of the quiet Sun using a narrow-band Lyot filter centered
at the Ca II K2v emission peak with a bandpass of 0.3 Å. We achieved
a spatial resolution of on average 0.7 arcsec at a cadence of 10 s. Results: .In the inter-network we find a mesh-like pattern that
features bright grains at the vertices. The pattern has a typical
spatial scale of 1.95 arcsec and a mean evolution time scale of 53 s
with a standard deviation of 10 s. A comparison of our results with a
recent three-dimensional radiation hydrodynamical model implies that
the observed pattern is of chromospheric origin. The measured time
scales are not compatible with those of reversed granulation in the
photosphere although the appearance is similar. A direct comparison
between network and inter-network structure shows that their typical
time scales differ by at least a factor of two. Conclusions:
.The existence of a rapidly evolving small-scale pattern in the
inter-network regions supports the picture of the lower chromosphere
as a highly dynamical and intermittent phenomenon.

---------------------------------------------------------
Title: The influence of image reconstruction on two-dimensional
spectrograms of the solar photosphere
Authors: Mikurda, K.; Tritschler, A.; Schmidt, W.
2006A&A...454..359M Altcode:
Aims.We present a spectral analysis of small-scale structures in
the solar photosphere and investigate the influence of the speckle
deconvolution technique on the line profiles. Methods: . A
short sequence of two-dimensional spectra is used, taken with the
Telecentric Etalon Solar Spectrometer (TESOS) at the German Vacuum
Tower Telescope on Tenerife. We observed two small pores surrounded by
disturbed and by regular granulation in the non-magnetic neutral Iron
line at 557.6 nm. In a first step, a speckle reconstruction is computed
by applying an extended Knox-Thompson algorithm to the broad-band
data. In a second step, the individual narrow-band filtergrams are
deconvolved utilizing the information gained in the first step. We
then perform a spectral analysis of the 2D spectra and compare the
results obtained with the raw and the restored data. Results:
.Important spectral quantities, e.g. line position, line depression
and line asymmetry are largely unchanged by the image reconstruction
process. We derive the line asymmetry and the line-of-sight flow for
granules and intergranular lanes and also for an isolated G-band bright
point and find important differences between quiet and magnetically
disturbed granulation: the granule centers in the quiet region show a
strong asymmetry with significant blue shift (300 m/s) toward deeper
layers, while the velocity in the disturbed area show virtually
no height dependence. For the intergranular lanes the situation is
reversed: no height dependence in the quiet area, significant red-shift
toward deeper layers in the disturbed part. An isolated G-band bright
point does not show any line-of-sight motion relative to its immediate
surroundings. The map of LOS velocities derived from line-wing shifts
shows a significant downflow around one of the pores measured in deep
layers of the photosphere. Conclusions: .In most cases we do
not find any artefacts in the reconstructed line profiles that would
compromise their usage for quantitative spectroscopy.

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

---------------------------------------------------------
Title: Diagnostic spectroscopy of G-band brightenings in the
photosphere of the sun
Authors: Langhans, K.; Schmidt, W.; Rimmele, T.
2004A&A...423.1147L Altcode:
A detailed analysis of high-resolution spectra obtained in three
different wavelength regions (at 430 nm, 526 nm and 569 nm) of G-band
bright points in the solar photosphere is presented. They show an
average intensity contrast of 11% with respect to the “quiet” sun
reference. The CH lines are weakened in the bright point interior. The
atomic lines, too, e.g. the Fe I line at 569.15 nm, weaken in the bright
point interior. In contrast thereto, the absorption line of single
ionized iron at 526.48 nm remains almost constant between bright point
interior and the immediate surroundings. Line-of-sight velocities show a
stronger downflow within bright points than in the close environment. A
net downflow relative to the intergranular surroundings of around 80
m/s is measured. Filling factors are calculated from a comparison with
synthesized spectra for different flux tube models and are used to
estimate the “true” velocity in bright points with respect to their
immediate surroundings. We obtain up- and downflows in the order of one
km s-1, in agreement with the magneto-convective picture of
the formation and dispersal of magnetic flux tubes. From the different
behavior of the metallic lines and the CH lines we conclude that the
line-weakening process that leads to the G-band bright points is mainly
due to hot-wall radiation. This confirms that these bright points are
indeed magnetic flux elements.

---------------------------------------------------------
Title: On the geometry of sunspot penumbral filaments
Authors: Schmidt, W.; Fritz, G.
2004A&A...421..735S Altcode:
We investigate the center-to-limb variation of the brightness of
the penumbrae of sunspots. The analysis includes narrow-band and
broad-band continuum images of about 80 sunspots observed with the
Vacuum Tower Telescope on Tenerife and with the Transition Region And
Coronal Explorer (TRACE). We find that the azimuthal intensity variation
depends on the viewing angle, i.e. on the position of the spot on the
solar disk. With increasing viewing angle, the average intensity remains
almost constant on the center side and on the limb side of the spot,
but increases in the lateral parts. This is explained by the existence
of elevated bright filaments that, when observed from the side and
at increasing viewing angle, more and more hide the dark and deeper
lying background. In addition to this azimuthal intensity modulation,
we observe an enhanced brightness on the limb-side penumbra for large
viewing angles.

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

---------------------------------------------------------
Title: GREGOR - optical design considerations
Authors: Soltau, D.; Berkefeld, Th.; von der Lühe, O.; Hofmann, A.;
Schmidt, W.; Volkmer, R.; Wiehr, E.
2003AN....324..292S Altcode:
After many years of successful work the Gregory Coudé Telescope
(GCT) is going to be replaced by GREGOR. This new 1.5 m telescope is
the result of design considerations which to a wide extent are based
on new technologies. Special aspects of the design are presented,
including measures to compensate for optical aberrations caused by
instrumental and atmospheric effects (Adaptive Optics). First light
is expected for the end of 2004.

---------------------------------------------------------
Title: Material flow in sunspots
Authors: Schmidt, W.
2003AN....324..374S Altcode:
Observations nowadays reveal more and more details about the small-scale
structure of the penumbra and umbra. Recent measurements of the motions
in the penumbra indicate that (i) the Evershed flow is confined to a
thin photospheric layer and (ii) the material rises from and sinks to
subphotospheric layers within the penumbra. This flow pattern solves
the question of the mass budget of the Evershed flow in a natural
way. The nature of umbral dots and their role for the spot's energy
budget remains unclear, since they are obviously unresolved even in
the best existing measurements.

---------------------------------------------------------
Title: Observations of G-band bright structures with TESOS
Authors: Langhans, K.; Schmidt, W.; Tritschler, A.
2003AN....324..354L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: High-resolution solar spectroscopy with TESOS - Upgrade from
a double to a triple system
Authors: Tritschler, A.; Schmidt, W.; Langhans, K.; Kentischer, T.
2002SoPh..211...17T Altcode:
We present the characteristics and demonstrate the performance of the
Triple Etalon SOlar Spectrometer (TESOS) operated at the German Vacuum
Tower Telescope (VTT) on Tenerife. The Fabry-Pérot interferometer
TESOS is ideally suited for precise measurements of photospheric and
chromospheric motion. Installed in 1997 and equipped with two etalons,
TESOS has recently been completed with a third etalon and upgraded
with two high-speed, backside-illuminated CCD cameras. The image scale
of 0.089 arc sec pixel−1 is adapted to the resolution of
the telescope. The improved system enables frame rates up to 5 frames
per second. The spectral resolution of 300 000 allows for spectral
diagnostics of weak photospheric lines, including individual CH-lines
within the G-band at 430.6 nm.

---------------------------------------------------------
Title: 2D-spectroscopic observations of vec G-band bright structures
in the solar photosphere
Authors: Langhans, K.; Schmidt, W.; Tritschler, A.
2002A&A...394.1069L Altcode:
We took two-dimensional spectra with the filter spectrometer TESOS
at the German Vacuum Tower Telescope, Tenerife, of an absorption
line of the CH molecule and a Fe II-line in the G-band at 430.3
nm. We observed a region, close to disk center of the Sun, that
showed a lot of structures with enhanced G-band intensity (up to 1.3
times the mean intensity of normal granulation). Our spectroscopic
investigation of these structures suggests two classes which differ
in their spectroscopic signature: (a) Bright structures caused by
significant (up to 40\%) weakening of absorption lines of the CH
molecule; (b) bright structures only caused by an enhanced continuum
intensity. In order to distinguish between those two classes we
introduce a Bright Point Index (BPI) defined by the ratio of the
normalized line depressions of the Fe~II and the CH-line. The bright
structures caused by weakening of the CH-lines have high BPI values and
are accompanied by downflows. The remaining G-band bright structures
have low BPI and are related to granules.

---------------------------------------------------------
Title: Sunspot photometry with phase diversity. II. Fine-structure
characteristics
Authors: Tritschler, A.; Schmidt, W.
2002A&A...388.1048T Altcode:
We investigate the thermal and morphological fine structure of a small
sunspot, which includes the determination of brightness temperatures
and characteristic spatial scales as well as their distribution
inside the sunspot. The identification and isolation of sunspot fine
structure is accomplished by means of a feature-finding algorithm
applied to a high-resolution time sequence taken simultaneously in
three continuum bands of the solar spectrum. In order to compensate for
seeing and instrumental effects, we apply the phase-diversity technique
combined with a deconvolution method. The findings can be summarized
as follows: (1) umbral dots are found to be on average 760 K cooler
than the immediate surrounding photosphere outside the spot. (2) Some
exceptional hot penumbral grains exceed the average temperature of the
brightest granules of the spots surroundings by typically 150 K. (3)
The size distribution of umbral dots and penumbral grains support the
idea that the smallest structures are still spatially unresolved. (4)
The distribution function of umbral dot peak intensities points to
the existence of two umbral dot “populations” indicating different
efficiency of energy transport. (5) The classification of penumbral
filaments into “dark” and “bright” depends on the immediate
surroundings.

---------------------------------------------------------
Title: Sunspot photometry with phase diversity. I. Methods and global
sunspot parameters
Authors: Tritschler, A.; Schmidt, W.
2002A&A...382.1093T Altcode:
The global brightness structure of a small sunspot is
investigated. Seeing and instrumental effects are compensated by
application of the phase-diversity technique and a conventional
deconvolution method. We calculate brightness temperatures for the
reconstructed data in three simultaneously observed continuum bands of
the solar spectrum (402.1 nm, 569.5 nm, 709.1 nm). The darkest umbral
regions are on average 0.17Isun, 0.27Isun and
0.32Isun bright. The corresponding temperatures lie in the
range around 4790 K, 4600 K and 4460 K. The spatially averaged penumbral
brightness amounts to 0.72Isun, 0.81Isun and
0.85Isun, which corresponds to 5910 K, 5750 K and 5640 K,
respectively. Although the spectral distribution of the umbral and
penumbral intensities is consistent with former measurements, the
derived values support the idea that there exist a real difference in
the thermal properties between the umbrae of small and large sunspots.

---------------------------------------------------------
Title: High resolution spectro-polarimetry with a large balloon-borne
solar telescope
Authors: Schmidt, Wolfgang
2002AdSpR..29.2055S Altcode:
The balloon mission "Sunrise" consists of a 1m lightweight solar
telescope equipped with a powerful spectro-graph-polarimeter and a
multi-channel filtergraph for the visible and the UV. The science goals
of Sunrise are focussed on, but not limited to, the investigation of
the solar surface magnetism. The key questions include the generation
and maintenance of the variable magnetic field, the structure and
dynamics of the chromosphere, and the interaction of material flow
and magnetic field in sunspots.

---------------------------------------------------------
Title: Center-to-limb-variation of the G-band lines at 430.5 nm
Authors: Langhans, K.; Schmidt, W.
2002A&A...382..312L Altcode:
We measure the center-to-limb-variation of the CH lines in the
wavelength range from 430.40 to 430.61 nm (G-band) for both quiet
sun and for the cooler atmophere of sunspot umbrae at different
heliographic positions. The limb effect of the CH lines is about 650
ms-1 at mu =0.1 with a slope similar to weak Fe lines. We
investigate the behaviour of the CH abundance in the range of optical
depth (tau =-0.05 to tau =-0.25) that is accessible by center-to-limb
observations. The CH abundance decreases with height in accordance
with recent numerical models.

---------------------------------------------------------
Title: Height and dynamics of the quiet solar chromosphere at the limb
Authors: Muglach, K.; Schmidt, W.
2001A&A...379..592M Altcode:
In this paper we present observations of He I 1083.0 nm, He D_3
587.6 nm and Hβ 486.1 nm taken at various positions at the solar
limb. We determine and compare the emission of the lines in terms
of line-parameters. The height of the chromosphere as seen in the
helium lines varies in space and time and reaches values between
1100 and 1800 km above the continuum and is the same for both helium
lines within the errors of the measurement. From a time-sequence of
slit-spectra of about 23 min we study the oscillation signature of
the chromosphere near the solar limb. We find velocity oscillations
in He I 1083.0 nm that do not drop to zero near the limb as would be
expected of vertically oriented oscillations, we even get horizontal
oscillations in the off-limb emission data of both helium lines.

---------------------------------------------------------
Title: GREGOR, a 1.5 m Gregory-type Telescope for Solar Observation
Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Schmidt, W.;
Soltau, D.; Staude, J.; Wiehr, E.; Wittmann, A. D.
2001AGM....18.P223K Altcode:
GREGOR is a high-resolution solar telescope with an aperture of 1.5
m. It will be equipped with an Adaptive Optics system and is designed
for high-precision measurements of magnetic fields and plasma motions
in the solar atmosphere and chromosphere with a resolution of 70 km
on the Sun. GREGOR will replace the Gregory Coudé Teleskope at the
Observatorio del Teide on Tenerife. We describe the optical design
and the focal plane instrumentation. In concert with the other solar
telescopes at Teide Observatory it will be useful for studying the
dynamics of the solar atmosphere and the underlying processes. GREGOR
will also serve as a test bed for next generation solar telescopes.

---------------------------------------------------------
Title: High-resolution Solar Polarimetry with Sunrise
Authors: Schmidt, W.; Solanki, S. K.; Schüssler, M.; Curdt, W.;
Lites, B. W.; Title, A. M.; Martinez Pillet, V.
2001AGM....18S1001S Altcode:
Sunrise is a 1m balloon-borne solar telescope. It is equipped with
a spectrograph polarimeter which combines vector-polarimetry in the
visible with diagnostic spectroscopy in the visible and the UV, down
to 200 nm. The instrumentation includes a filter-magnetograph and a
medium-band filtergraph. The wavelength bands of the latter include
the CH-band (430.6 nm) and a UV continuum at 205 nm. Diffraction
limited resolution in the UV will be achieved by employing a phase
diversity technique. The main telescope is based on a lightweight
silicon-carbide mirror, developed within the Solar Lite program. During
the long-duration flight at Antarctica, foreseen for late 2005, Sunrise
will continuously observe the sun for a period of about ten days,
with constant image quality across the full field of view. In-flight
alignment of the telescope optics will be controlled by a wavefront
sensor. The main goal of Sunrise is to understand the structure and
dynamics of the magnetic field in the atmosphere of the sun. To this
end, Sunrise will observe small magnetic flux concentrations with
dimensions of less than 70 km with high polarimetric accuracy. At the
same time, Sunrise will provide diffraction-limited filtergrams of
the photosphere and chromosphere with a resolution down to 35 km at
a wavelength of 200 nm.

---------------------------------------------------------
Title: Small-scale flow field in a sunspot penumbra
Authors: Schmidt, W.; Schlichenmaier, R.
2000A&A...364..829S Altcode:
We have measured velocity maps of a round sunspot near the center of
the solar disk. We derive the small-scale velocity field of the penumbra
in the deep photosphere. Superimposed on a radial outflow, we find up-
and downflows, which are associated with bright and dark features: The
flow in the dark component of the penumbra has a larger inclination
angle with respect to the surface normal than the bright component,
everywhere in the penumbra. The maximum inclination difference is
8°. In the inner and mid penumbra, bright features (penumbral grains)
are associated with an upflow of about 0.6 km s-1. At the
outer spot boundary, the flow is inclined downwards predominantly in
the dark component (-7degr ).

---------------------------------------------------------
Title: Free-fall Downflow Observed in HE I 1083.0 Nanometers and Hβ
Authors: Schmidt, W.; Muglach, K.; Knölker, M.
2000ApJ...544..567S Altcode:
In a short time sequence of simultaneously observed slit spectra
of He I 1083.0 nm and Hβ we find the signature of material flowing
toward the solar surface with up to 42 km s-1, in addition
to material which is almost at rest. The constant acceleration of the
moving material is about 200 m s-2. These multiple velocities
occur in a small region of about 5” in a plage region. We
observe a highly dynamical phenomenon which lasts a few minutes. The
duration and constant acceleration suggest free fall of matter
unobstructed by magnetic structures or along vertical field lines.

---------------------------------------------------------
Title: Flow geometry in a sunspot penumbra
Authors: Schlichenmaier, R.; Schmidt, W.
2000A&A...358.1122S Altcode:
We have measured the material flow in the penumbra of a large symmetric
sunspot during the passage of the spot across the solar meridian. The
line-of-sight velocity field has been obtained from Doppler measurements
in a Fe II line using a filter spectrometer with a large field of
view. From data sets taken on different days, i.e. at different view
angles, we have reconstructed the magnitude and orientation of the
penumbral flow field in the deep photosphere. We find upflows near
the inner and downflows at the outer boundary of the penumbra with
nearly horizontal outflow in between. From our measurements we derive
the following flow geometry: narrow upflow channels rise at different
penumbral radii, they bend outwards with a nearly horizontal outflow,
and are finally tilted a few degrees downwards at the outer penumbral
boundary, but still inside the spot. The flow reaches its maximum speed
of about 3.5 km s-1 in the outer part of the penumbra. Our
findings are a significant step towards an understanding of the mass
balance of the Evershed flow. The proposed geometry is consistent with
recent numerical models of penumbral filaments.

---------------------------------------------------------
Title: Solar equatorial plasma rotation: a comparison of different
spectroscopic measurements
Authors: Wöhl, H.; Schmidt, W.
2000A&A...357..763W Altcode:
We present solar equatorial rotation velocities measured with two
different spectral lines (Fe I 557.6 nm and Ni I 676.8 nm) and two
different spectrometers at the German Vacuum Tower Telescope (VTT) on
Tenerife. The `classical' sidereal solar equatorial rotation velocity
of about 2000 m/s has been confirmed. The results are compared with
those from velocity data of the Michelson Doppler Imager (MDI) onboard
the Solar Heliospheric Observatory (SOHO) obtained on 10 June 1996 and
27 May 1999. From both data sets of MDI, a rotation velocity about 100
to 200 m/s below the `classical' value cited above was found. Possible
explanations of this discrepancy are discussed.

---------------------------------------------------------
Title: Dynamics of solar magnetic elements
Authors: Sigwarth, M.; Balasubramaniam, K. S.; Knölker, M.;
Schmidt, W.
1999A&A...349..941S Altcode:
We present observational results that demonstrate a strong increase in
the dynamic behavior of magnetic elements at the solar photosphere,
when observed at high spatial and temporal resolution. The HAO/NSO
Advanced Stokes Polarimeter was used to obtain an extensive set of
high resolution Stokes-V spectra from network, intranetwork and active
region magnetic fields at a low noise level. We performed a statistical
analysis of Doppler shifts and asymmetries of the V spectra of FeI
630.15 and 630.25 nm to obtain information on the dynamics of magnetic
elements of different sizes. The spatial resolution of 0.8-1 arcsec in
combination with high polarimetric precision allowed us to investigate
Stokes-V spectra of magnetic elements down to a size of ~ 150 km. The
Doppler velocity within magnetic elements as well as the amplitude and
area asymmetries of the Stokes-V profiles show a strong dependence on
the size of the magnetic elements as well as on the granular velocity
in their vicinity. Applying an absolute velocity calibration we find
that the smallest magnetic features have velocities of up to 5 km s(-1)
in both up- and downflows whereas for larger elements or clusters of
several flux tubes the velocities become smaller and more uniform. The
V-profile asymmetries are larger (both positive and negative) for
small fill fraction than for higher fill fraction within the resolution
element. Averaged over all individual profiles, there remains a positive
amplitude and area asymmetry and a downflow exceeding 0.5 km s(-1)
. The properties of spatially and temporally integrated V profiles are
consistent with results from FTS-observations. Our observations are
in qualitative agreement with results from numerical MHD simulations.

---------------------------------------------------------
Title: Vertical mass flux in a sunspot penumbra
Authors: Schlichenmaier, R.; Schmidt, W.
1999A&A...349L..37S Altcode:
We present the first direct measurement of vertical motion in the
deepest atmospheric layers of a penumbra, obtained at hitherto
unreached spatial resolution. Isolated hot upflows in the inner
penumbra feed the horizontal Evershed flow that is observed in the
mid and outer penumbra. We discover cool downflows which surround the
hot upflows in the inner penumbra. At the outer edge of the penumbra,
the Evershed flow terminates in a ring of downflow channels. The
upflows transport a sufficient amount of energy to account for the
observed brightness of the penumbra. These measurements have therefore
significant implications for understanding the mass balance and the
energy transport in a sunspot penumbra.

---------------------------------------------------------
Title: Center-to-limb variation of the solar oscillation. New results
from MDI data
Authors: Schmidt, W.; Stix, M.; Wöhl, H.
1999A&A...346..633S Altcode:
{Using 17 hours of full-disk MDI Doppler data we have investigated the
center-to-limb variation of the oscillatory wave spectrum of the solar
photosphere. Power distributions in the k_hnu -plane are calculated
for fields of 20(deg) x 20(deg) on the Sun, centered at every 10(deg)
heliographic latitude. From the center-to-limb variation of the power
in the f mode and the lowest 7 p modes we obtain information about the
mean inclination of the oscillatory velocity vector with respect to
the vertical direction. We find qualitative agreement with a model of
adiabatic waves in an isothermal atmosphere, but generally the solar
oscillations appear to be less inclined than the model oscillations. We
find no indication for the existence of horizontal sound waves on
the Sun.}

---------------------------------------------------------
Title: GREGOR, a 1.5 M Solar Telescope
Authors: von der Lühe, O.; Schmidt, W.; Soltau, D.; Kneer, F.;
Staude, J.
1999AGAb...15....5V Altcode: 1999AGM....15..A01V
We present the design of a new large solar telescope which is going to
be installed at the Observatorio del Teide, in the tower that presently
houses the 45cm Gregory-Coudé-Telescope. The new telescope has an
aperture of 1.5 meters and its optical design is basically a Gregorian
configuration. It will be an open telescope in an azimuthal mount. An
adaptive optics system is incorporated in the optical design as well as
a polarimetry package. The feasibility of lightweight optics for the
primary mirror has been investigated in an industrial pre-study. The
focal plane instrumentation will include a high resolution filter
spectrometer similar to the existing TESOS instrument at the VTT and
a new spectro-polarimeter for the visible and the near UV. The latter
instrument is presently being developed jointly by the KIS and the
High Altitude Observatory in Boulder, USA. Budget permitting, detailed
(Phase-B) planning will start in 2000, and the telescope will be
developed and built in 2002 and 2003 with first light in spring of 2004.

---------------------------------------------------------
Title: Chromospheric Oscillations Observed by SOHO/SUMER and VTT/GCT
on Tenerife
Authors: Curdt, W.; Heinzel, P.; Schmidt, W.; von Uexküll, M.;
Wilken, V.
1999AGAb...15R..11C Altcode: 1999AGM....15..A12C
Previous studies have revealed various oscillations in higher Lyman
lines of hydrogen, as observed with SUMER on SOHO (Curdt &amp; Heinzel
1998). The assumption that these periodic phenomena might be related
to similar effects observed in optical lines like Ca II K led to the
idea of simultaneous observations with the SOHO/SUMER instrument and
the VTT/GCT telescopes on Tenerife. First results are presented here
for the Lyman series and Lyman continuum. Power spectra in network and
internetwork regions have been derived and a preliminary interpretation
based on the wave propagation concept is given. In SUMER spectra
we found a well-pronounced asymmetry of all Lyman lines, which is
related to atmospheric dynamics. The time variation of co-spatial UV
and optical data is presented and basic characteristics are discussed.

---------------------------------------------------------
Title: Vertical and horizontal mass flux in a sunspot penumbra.
Authors: Schlichenmaier, R.; Schmidt, W.
1999AGAb...15....8S Altcode: 1999AGM....15..A06S
We present direct measurements of vertical and horizontal motions in
the deepest atmospheric layers of a sunspot penumbra with a spatial
resolution of about 0.5 arcsec. The observations have been made with
the Vacuum Tower Telescope of the Kiepenheuer-Institut, operated
at the Observatorio del Teide on Tenerife, using the Telecentric
Solar Spectrometer (TESOS), which is based on two Fabry-Perot
interferometers. &gt;From two-dimensional spectra we infer the
line-of-sight velocities for the weak C I line at 538.0 nm, which forms
in the deepest photospheric layers, and for the Fe II 542.5 nm line,
a rather temperature sensitive line which forms in the mid-photosphere
at a height of about 100 km above the continuum level. We have observed
a fairly round sunspot (NOAA 8578) with a diameter of 32 000 km on its
passage through the central meridian on four consecutive days. We find
that isolated hot upflows in the inner penumbra feed the horizontal
Evershed flow that is observed in the mid and outer penumbra. At the
outer edge of the penumbra, the Evershed flow terminates in a ring of
downflow channels. Measurements at various position angles allow us
to derive the inclination angles of these flows. Based on our measured
upflow velocities, we estimate that the heat flux that emerges from our
brightest feature compares with the observed brightness of penumbral
grains. These measurements have therefore profound implications for
understanding the mass balance and the energy transport in a sunspot
penumbra.

---------------------------------------------------------
Title: Chromospheric Oscillations
Authors: Schmidt, W.; von Uexküll, M.
1999AGM....15..A14S Altcode:
Sequences of high-resolution filtergrams taken at Ca K2v show regions
in the interior of the Ca network with oscillations at 11 mHz. These
regions do also have strong 3min oscillations. Periodic signals around
2.2 mHz are concentrated in small scale structures located at the cell
boundaries. The observations have been carried at at the Vacuum-Tower
Telescope on Tenerife in May 1999, along with a coordinated SOHO-SUMER
campaign.

---------------------------------------------------------
Title: TESOS, a double Fabry-Perot instrument for solar spectroscopy
Authors: Kentischer, T. J.; Schmidt, W.; Sigwarth, M.; Uexkuell, M. V.
1998A&A...340..569K Altcode:
A double Fabry-Perot spectrometer in a telecentric configuration
(TESOS: Telecentric Etalon SOlar Spectrometer) has been developed and
installed in the Vacuum Tower Telescope at the Observatorio del Teide,
Tenerife. The control system allows to switch from one wavelength band
to another within 2 seconds. The telecentric configuration provides a
constant wavelength passband across the field of view. Servo-stabilized
etalons and a stable thermal environment provide a wavelength stability
of &lt;0.3 picometer per hour. We present initial results obtained
with this instrument.

---------------------------------------------------------
Title: Upwelling in a young sunspot
Authors: Sigwarth, M.; Schmidt, W.; Schuessler, M.
1998A&A...339L..53S Altcode:
An upflow of with a velocity exceeding 0.5 km*s(-1) is found in the
umbra of the preceding spot in a young active region with ongoing
flux emergence. A weak downflow is indicated in the corresponding
spot of follower polarity. Such a flow pattern is consistent with the
counter-rotation flow along a rising magnetic flux loop driven by the
Coriolis force as predicted by numerical simulations.

---------------------------------------------------------
Title: The lithium abundance in sunspots
Authors: Ritzenhoff, S.; Schroter, E. H.; Schmidt, W.
1997A&A...328..695R Altcode:
The solar lithium abundance was determined using an empirical model
for an observed sunspot umbra. The atmospheric model is deduced from a
number of spectral lines. In addition to the lithium (I) line doublets
from Li(6) and Li(7) at 670.8 nm eight different line profiles were
observed with high spectrographic resolution. A magnetically sensitive
line was included in order to determine the magnetic field strength. An
estimation for the influence of molecular blends caused by CN and
TiO was taken into account. Under the assumption of LTE a lithium
abundance of epsilon_Li =1.02 +/-0.12 was derived. Some evidence for the
existence of a small but notable amount of Li(6) is found, however only
an upper limit of Li(6/Li^7&lt;=) 0.03 can be derived. A more precise
determination of the isotopic ratio is hampered by the uncertainties
induced by the presence of TiO blends. On the logarithmic scale,
where the abundance of hydrogen is epsilon_H =12.

---------------------------------------------------------
Title: Multiple velocities observed in HeI 1083 nm
Authors: Muglach, K.; Schmidt, W.; Knölker, M.
1997SoPh..172..103M Altcode: 1997SoPh..172..103S; 1997ESPM....8..103M
We present a time sequence of slit spectra of Hei 1083.03 nm (and Hβ)
taken with the German Vacuum Tower Telescope at Tenerife. We find the
signature of an onset of a downflow accelerating up to 42 km/s . The
Hei 1083.03 nm line splits into two well separated components during
the event, one of which shows the usual absorption profile of material
which is almost at rest. We calculate the velocity, line depth and
area of both line profiles. Possible explanations for these observed
multiple velocities are discussed.

---------------------------------------------------------
Title: Phase Diversity Applied to Sunspot Observations
Authors: Tritschler, A.; Schmidt, W.; Knolker, M.
1997ASPC..118..170T Altcode: 1997fasp.conf..170T
We present preliminary results of a multi-colour phase diversity
experiment carried out with the Multichannel Filter System of the
Vacuum Tower Telescope at the Observatorio del Teide on Tenerife. We
apply phase-diversity imaging to a time sequence of sunspot filtergrams
taken in three continuum bands and correct the seeing influence for
each image. A newly developed phase diversity device allowing for
the projection of both the focused and the defocused image onto a
single CCD chip was used in one of the wavelength channels. With the
information about the wavefront obtained by the image reconstruction
algorithm the restoration of the other two bands can be performed as
well. The processed and restored data set will then be used to derive
the temperature and proper motion of the umbral dots. Data analysis
is still under way, and final results will be given in a forthcoming
article.

---------------------------------------------------------
Title: Adaptive Optics at the German VTT on Tenerife
Authors: Soltau, D.; Acton, D. S.; Kentischer, Th.; Roser, M.; Schmidt,
W.; Stix, M.; von der Luhe, O.
1997ASPC..118..351S Altcode: 1997fasp.conf..351S
Experiences and considerations are presented which play an important
role for an Adaptive Optics system at our telescope.

---------------------------------------------------------
Title: Solar Magnetic Fields
Authors: Schüssler, Manfred; Schmidt, Wolfgang
1994smf..conf.....S Altcode: 1994QB539.M23S65...
The study of solar magnetic fields includes such issues as how
large-scale magnetic fields are generated in the Sun and how magnetic
structures are spontaneously formed and then interact with convective
flows. These are just a sample of the fundamental questions answered in
this timely review of our understanding of solar magnetic fields. This
volume collects together articles and research papers ranging from
such topics as large-scale patterns and global dynamo action to tiny
flux tubes, from the overshoot layer below the convection zone up to
the corona, and from instrumental problems and theoretical methods
to the latest ground-based and satellite observations. This volume
provides an essential review of our knowledge to date for graduate
students and researchers.

---------------------------------------------------------
Title: Limb observations of the HeI 1083.0 NM line.
Authors: Schmidt, W.; Knoelker, M.; Westendorp Plaza, C.
1994A&A...287..229S Altcode:
The He 1083.0 nm line has been observed at the solar limb and the
strength of the line has been measured as a function of height above
it. The maximum of the emission is found to be at 2400 km.

---------------------------------------------------------
Title: Polarimetry and spectroscopy of a simple sunspot. 3: Velocity
and magnetic field of sunspot umbral dots
Authors: Schmidt, W.; Balthasar, H.
1994A&A...283..241S Altcode:
Velocities and magnetic fields in sunspot umbrae and in umbral dots
have been determined from high resolution Stokes-I spectra obtained at
the Vacuum Tower Telescope (VTT) on Tenerife. Except for oscillatory
motions the umbra is at rest and flow velocities in umbral dots,
measured at an optical depth of about 0.01, are smaller than 25
m/s. The magnetic field strength is weakened in the dots by 10-20%
for a central umbral dot and by 5-10% for a peripheral umbral dot. The
line of sight decrease of magnetic field in a peripheral dot shows no
difference to the surrounding umbra and amounts to 2.5-3 G/km.

---------------------------------------------------------
Title: The Impact of Fragment “L” of Comet SL-9 on Jupiter
Authors: Schleicher, H.; Balthasar, H.; Knolker, M.; Schmidt, W.;
Jockers, K.
1994EM&P...66...13S Altcode:
Filtergrams of high spatial and temporal resolution were obtained in
the methane band centred at 892 nm during the impact of fragment L of
comet Shoemaker-Levy 9 on Jupiter. The light curve shows two maxima of
an emission ball observed above the limb shortly after the impact. The
second maximum was the brightest and had a short life time of about 90
seconds. During it's life, the apparent height of the emission ball
declined towards the surface of Jupiter; the amount of displacement
is larger than the expected effect caused by Jupiter's rotation. About
half an hour after the impact, a domelike feature became visible when
the location of the impact rotated into the illuminated hemisphere
of Jupiter.

---------------------------------------------------------
Title: Polarimetry and spectroscopy of a simple sunspot. 2: On the
height and temperature dependence of the magnetic field
Authors: Balthasar, H.; Schmidt, W.
1993A&A...279..243B Altcode:
We have observed a fairly regular sunspot located near disk center
and have taken series of spectrograms in three different photospheric
lines. The magnetic field strength across the spot has been measured
in different heights of the sunspot photosphere and shows a change in
slope at the umbra-penumbra boundary. The line depression contribution
functions have been computed in order to obtain the height of formation
of the spectral lines used in this analysis. Along the line-of-sight,
we find an upward decrease of the field strength of 2.5 to 3 G/km in
the penumbra and the adjacent outer part of the umbra. A relation has
been deduced between the magnetic field and the brightness temperature
measured across the sunspot which is strongly nonlinear and shows
pronounced differences between the umbra and the penumbra.

---------------------------------------------------------
Title: On the Magnetic and Velocity Field Geometry of Simple Sunspots
Authors: Title, Alan M.; Frank, Zoe A.; Shine, Richard A.; Tarbell,
Theodore D.; Topka, Kenneth P.; Scharmer, Goran; Schmidt, Wolfgang
1993ApJ...403..780T Altcode:
It is presently shown that a simple sunspot model with azimuthal
variations in inclination, but lacking azimuthal field-strength
variations, is free from azimuthal Lorentz forces. The meridional
currents arising from the inclination variations are parallel to the
field lines, suggesting that a cylindrically symmetric magnetostatic
sunspot model can be perturbed into one with azimuthal variations in
inclination with adjustment of the meridional force balance.

---------------------------------------------------------
Title: Polarimetry and spectroscopy of a simple sunspot. I - On the
magnetic field of a sunspot penumbra
Authors: Schmidt, W.; Hofmann, A.; Balthasar, H.; Tarbell, T. D.;
Frank, Z. A.
1992A&A...264L..27S Altcode:
We investigate the magnetic field structure of a medium sized sunspot
using high resolution magnetograms and spectrograms and derive a
relationship between the brightness of penumbral structures and the
inclination of the magnetic field. The field inclination to the spot
normal is larger in the dark structures than in the bright ones. We
show that the field strength does not vary between dark and bright
structures. At the inner penumbral boundary the field strength is 2000
Gauss and about 1000 Gauss at the outer penumbral edge. The line-of
sight component of the material flow decreases rapidly within one
arcsecond at the photospheric boundary of the spot.

---------------------------------------------------------
Title: Chromospheric fine structure revisited
Authors: Grossmann-Doerth, U.; Schmidt, W.
1992A&A...264..236G Altcode:
We obtained H-alpha spectra from both spicules at the limb and mottles
at disk center. We compare our results with data obtained by other
groups and derive two distributions of axial spicule velocity, one
based on limb, the other on disk observations. The two distributions
disagree. We conclude that our knowledge on the basic properties of
the chromospheric fine structure has not much improved in the last
two decades; it is still rather scanty.

---------------------------------------------------------
Title: On the Differences between Plage and Quiet Sun in the Solar
Photosphere
Authors: Title, Alan M.; Topka, Kenneth P.; Tarbell, Theodore D.;
Schmidt, Wolfgang; Balke, Christiaan; Scharmer, Goran
1992ApJ...393..782T Altcode:
Time sequences of interleaved observations of the continuum intensity,
longitudinal magnetic field, vertical velocity in the midphotosphere,
and the line-center intensity in Ni I 6768 A were obtained in an
active-region plage and the surrounding relatively field-free area near
disk center. Spacetime Fourier filtering techniques are used to separate
the convective and oscillatory components of the solar atmosphere. The
properties of the photosphere are found to differ qualitatively and
quantitatively between the plage, where the field is 150 G or more,
and its quiet surroundings. The scale of granulation is smaller, the
contrast lower, and the temporal evolution slower in the plage than the
quiet sun. In the plage, the vertical velocity is reduced in amplitude
compared to the quiet sun, and there is little evidence of a granulation
pattern, while in the quiet sun the vertical flow pattern is similar in
size and shape to the underlying granulation pattern in the continuum.

---------------------------------------------------------
Title: Beobachtungen mit dem Vakuum-Turm-Teleskop auf Teneriffa.
Authors: Schmidt, Wolfgang
1992S&W....31..167S Altcode:
The Tenerife Vacuum Tower Telescope, which is designed to study solar
magnetic fields with high spatial resolution, are discussed. The
spectrograph and the structure of the associated universal filter
are described. Observations of the magnetic field structure inside a
sunspot umbra are presented and discussed.

---------------------------------------------------------
Title: High Resolution Observations of the Magnetic and Velocity
Field of Simple Sunspots
Authors: Title, Alan M.; Frank, Zoe A.; Shine, Richard A.; Tarbell,
T. D.; Topka, K. P.; Scharmer, Goran; Schmidt, Wolfgang
1992ASIC..375..195T Altcode: 1992sto..work..195T
We have observed the disk passage of relatively simple round sunspots
using a narrowband filter and a large format CCD detector and have
created magnetograms, Dopplergrams, and continuum images nearly
simultaneously. In addition the spectral resolution of the filter
allows the construction of 'spectra' for all points in the field of
view. The mean inclination of the magnetic field increases from 45-50
deg to 70-75 deg across the penumbra and there is a fluctuation of the
inclination angle about the mean of about 4 +/- 18 deg. The variation in
inclination is large enough that substantial amounts of magnetic field
are parallel to the solar surface from the mid to outer penumbra. The
Evershed flow tends to occur in the regions where the magnetic field
is horizontal. This suggests that the Evershed flow is confined to the
regions of horizontal fields. We show that a simple sunspot model with
azimuthal variations in inclination but no azimuthal variations of
field strength is free from azimuthal Lorentz forces. The meridional
component of the currents which arise from the azimuthal variation
in inclination are parallel to the field lines. This suggests that a
cylindrically symmetric magnetostatic sunspot model can be perturbed
into one with azimuthal variations in inclination with some adjustment
in the meridional force balance.

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Title: On the Umbra / Penumbra Area Ratio of Sunspots
Authors: Brandt, P. N.; Schmidt, W.; Steinegger, M.
1990SoPh..129..191B Altcode:
The area ratio of umbra to penumbra (Au/Ap) was
investigated for a series of 126 sunspots observed around the maximum
of solar activity in 1980. Plotting log (Au) as function of
log (Ap) a linear relation is obtained with a slope of 1.10,
yielding an average ratio Au/Ap of 0.24 for small
spots and of 0.32 for large spots. This deviates noticeably from the
commonly adopted constant value of Au/Ap = 0.21
(Allen, 1973) used e.g. for modelling the sunspot irradiance deficit
(cf. Willson et al., 1981) from the Solar Geophysical Data.

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Title: Sunspot Photometry and the Total Solar Irradiance Deficit
Measured in 1980 BY ACRIM
Authors: Steinegger, M.; Brandt, P. N.; Pap, J.; Schmidt, W.
1990Ap&SS.170..127S Altcode:
Until now a simple Photometric Sunspot Index (PSI) model was used
(e.g. Willsonet al., 1981) to describe the contribution of sunspots
to the solar irradiance deficit measurement by ACRIM. In this work we
replace this model by a photometry of sunspot pictures for the period
of 19 August to 4 September, 1980 taking into account the individual
features, like lightbridges or umbral dots, of each spot. The main
results of this preliminary analysis are: (1) theA u/A
p ratios and alsos the α values vary in a wide range and
are by no means constant as in the PSI model; (2) the general trend of
the irradiance deficit from our analysis agrees well with the ACRIM
measurements; (3) on some days there are differences of more than
50% between the deficits derived from our measurements and from the
PSI model.

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Title: The solar granulation in the vicinity of sunspots
Authors: Schmidt, W.; Grossmann-Doerth, U.; Schroeter, E. H.
1988A&A...197..306S Altcode:
The mean size of the granulation close to sunspots is about 10 percent
smaller than the corresponding value in the normal photosphere. The
percentage of small granules increases steadily over a distance of
about 30 arcsedc, as one approaches the sunspots. The observed rms
intensity-contrast of the granulation at distances of less than 10
arcse to the penumbra is lower than in the quiet photosphere. This is
the result of a power spectrum analysis of 11 white light pictures of
different sunspots taken in 1980 with the 40 cm Newtonian Telescope
of the Kiepenheuer-Institut operated at the Observatorio del Teide,
Tenerife.

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Title: Size and temperature of umbral dots
Authors: Grossmann-Doerth, U.; Schmidt, W.; Schroeter, E. H.
1986A&A...156..347G Altcode:
In 12 white light broad band pictures of 6 different sunspots, taken
in 1980 and 1981 with the 40 cm Newtonian Vacuum Telescope operated at
the Observatorio del Teide (Iazaña), the authors identified nearly 100
umbral dots and attempted to derive their temperature and diameter. They
found strong evidence that the diameter of the dots is in the range
0arcsec.4 to 0arcsec.9 and the temperature is several 100K up to more
than 1000K cooler than the photosphere.

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Title: Oscillations of the sun's chromosphere. III - Simultaneous
H-alpha observations from two sites
Authors: von Uexkuell, M.; Kneer, F.; Mattig, W.; Nesis, A.;
Schmidt, W.
1985A&A...146..192V Altcode:
The authors analyze time sequences of Hα filtergrams taken
simultaneously from two distant observatories, Capri and Izaña. By
means of a coherence analysis the authors discriminate between
instrumental effects including seeing and truly solar intensity
fluctuations. Waves with periods as short as 60 s are present in the
solar chromosphere; the lower limit is set by the time resolution of
the observations.

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Title: New information about solar rotation
Authors: Woehl, H.; Balthasar, H.; Koch, A.; Kueveler, G.; Roca-Cortes,
T.; Schmidt, W.; Vazquez, M.
1984S&W....23...73W Altcode:
It is pointed out that the phenomenon of solar rotation is known
since the introduction of the telescope into astronomy by Galilei in
1610. Regular measurements concerning the positions of sunspots were
conducted with the aid of photographic plates at the Royal Greenwich
Observatory during the time from 1874 to 1976. The obtained data
provide an excellent basis for statistical analysis. Information
obtained as a result of such analyses is discussed. Attention is given
to the determination of the Wilson depression, details regarding
solar differential rotation, plans for an evaluation of data with
the aid of a computer, the rotation of the solar plasma, a comparison
of sunspots and plasma, the theory of differential rotation, and new
information concerning solar rotation partly obtained with the aid of
solar telescopes in a Spanish observatory on the island of Tenerife.

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Title: Balloon-borne imagery of the solar granulation. IV - The
centre-to-limb variation of the intensity fluctuations
Authors: Durrant, C. J.; Mattig, W.; Nesis, A.; Schmidt, W.
1983A&A...123..319D Altcode:
A reanalysis of the white-light photographs of the granulation obtained
during the flight of the balloon-borne Spektrostratoskop experiment
is presented. A detailed examination of the power spectra of the
intensity fluctuations at various times during the flight reveals a
steady increase in a straylight and a substantial degree of astigmatism
that changed with the focus setting. Allowing for the latter and
normalizing all measurements to that with the least straylight, the
rms relative intensity fluctuation at the center of the disk at 556 nm
is increased from the previously reported value of 8.6 percent to 11.3
percent. The overall change of rms fluctuation from the center of the
disk to 0.3 micron is remarkably small, dropping some 10 percent. The
results agree quite well with those of the Soviet Solar Stratospheric
Observatory, when corrected roughly for instrumental degradation, and
with a granulation model in which the granular temperature perturbation
cuts off at a height of 35 km in the atmosphere.

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Title: Two comments of the sun's differential rotation
Authors: Schmidt, W.; Stix, M.
1983A&A...118....1S Altcode:
The authors argue that the effect of rotation should be neglected
when the convection velocity is estimated through a comparison of the
kinetic energy with the work done by the superadiabatic temperature
gradient. A recent model of the Sun's differential rotation by Durney
(1981) would thus not be restricted to small rates of rotation. The
authors present a simple model where convection cells elongated
in north-south direction are simulated by means of an anisotropic
turbulent viscosity tensor. The resulting mean flow has equatorial
acceleration, as observed on the Sun, with negligible concomitant
meridional circulation and pole-equator temperature difference.

---------------------------------------------------------
Title: Models of solar differential rotation
Authors: Schmidt, W.
1982GApFD..21...27S Altcode:
Models of a differentially rotating compressible convection zone
are calculated, considering the inertial forces in the poloidal
components of the equations of motion. Two driving mechanisms have
been considered: latitude dependent heat transport and anisotropic
viscosity. In the former case a meridional circulation is induced
initially which in turn generates differential rotation, whereas
in the latter case differential rotation is directly driven by the
anisotropic viscosity, and the meridional circulation is a secondary
effect. In the case of anisotropic viscosity the choice of boundary
conditions has a big influence on the results: depending on whether or
not the conditions of vanishing pressure perturbation are imposed at
the bottom of the convection zone, one obtains differential rotation
with a fast ( 10 ms-1) or a slow ( 1 ms-1) circulation. In the latter
case the rotation law is mainly a function of radius and the rotation
rate increases inwards if the viscosity is larger in radial direction
than in the horizontal directions. The models with latitude dependent
heat transport exhibit a strong dependence on the Prandtl number. For
values of the Prandtl number less than 0.2 the pole-equator temperature
difference and the surface velocity of the meridional circulation are
compatible with observations. For sufficiently small values of the
Prandtl number the convection zone becomes globally unstable like a
layer of fluid for which the critical Rayleigh number is exceeded.

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Title: Rms-value and power spectrum of the photospheric
intensity-fluctuations
Authors: Schmidt, W.; Knoelker, M.; Schroeter, E. H.
1981SoPh...73..217S Altcode:
The power spectrum and the rms-value of the granular intensity
fluctuations were studied using granulation photographs of
excellent quality obtained during the JOSO site testing campaign
1979 at Izaña. The observed power spectrum was corrected using
various effective modulation transfer functions of the system:
telescope+aberrations+atmospheric seeing, assuming different
contributions of the atmospheric seeing. With this procedure a
lower and upper limit for the `true' power spectrum of the granular
intensity fluctuations and thus for the rms-value could be derived:
7.2% &lt;Irms &lt;12% at λ = 550 nm, with a most probable
value of Irms = 10.5%. We checked the validity of the
upper limit by applying to our data a MTF (Deubner and Mattig, 1975),
which certainly must lead to an overcorrection. This procedure lead
to Irms = 13.4%. Thus we can state that the true rms-value
of the granular intensity fluctuations does certainly not exceed 13%
at λ = 550 nm.

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Title: The brightness distribution in sunspot penumbrae
Authors: Grossmann-Doerth, U.; Schmidt, W.
1981A&A....95..366G Altcode:
From a set of high quality sunspot photographs the intensity
distribution in the penumbrae as well as mean intensities and rms
contrast were derived. For image restoration a Fourier technique was
employed. The intensity distribution was found to be almost symmetrical
and singly peaked.

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Title: On the center to limb variation of the granular brightness
fluctuations.
Authors: Schmidt, W.; Deubner, F. -L.; Mattig, W.; Mehltretter, J. P.
1979A&A....75..223S Altcode:
The center to limb variation of the granular brightness contrast
was studied on the basis of slit-jaw photographs obtained with the
balloon-borne Spektro-Stratoskop telescope, flown at an altitude of
about 28 km on May 17, 1975 and during the partial solar eclipse of
April 26, 1976 on Tenerife. Positions concentrate at cosine theta
values of about 1, 0.6 and 0.2. The granular contrast was found to
decrease monotonically towards the solar limb, with the magnitude of
contrast about three times smaller at cosine theta 2 than at the disk
center. The relative center to limb variation was found to be 10.4%
(eclipse observations by a 40 cm evacuated Newtonian reflector at 422
nm) and 8.6% (Spektro-Stratoskop observations at 556 nm). Discrepancies
between these results and those reported by Deubner and Mattig (1975)
are thought to be due to corrections for photographic noise and other
factors.

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Title: Studies of granular velocities. VIII. The height dependence
of the vertical granular velocity component.
Authors: Durrant, C. J.; Mattig, W.; Nesis, A.; Reiss, G.; Schmidt, W.
1979SoPh...61..251D Altcode:
Spectral observations of solar velocity fields made during a partial
solar eclipse are described. Continuum intensity measurements at
the lunar limb allow the modulation transfer function to be derived
and the true spatial power spectrum of the velocity field to be
reconstructed. The oscillatory and granular components are separated
by applying spatial filters cutting off at 3″.7. The oscillatory
component values are in good agreement with those of Canfield (1976) but
the granular component has substantially more power and a smaller height
gradient. The discrepancy can be resolved by noting the uncertainties
of the seeing corrections and of the separation into components in
the work of Canfield.

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Title: Study of the 19F(3He,
6Li)16O reaction at 11 MeV
Authors: Strohbusch, U.; Schmidt, W.; Huber, G.
1971NuPhA.163..453S Altcode:
Angular distributions of the 19F
(3He, 6Li)16O
reaction have been measured at 11 MeV for the transitions
6Lig.s. (16Og.s),
6Li3.56 (16Og.s.)
and 6Lig.s. (16O6.06,
6.14). Zero-range DWBA analysis on the basis of the triton
pickup mechanism revealed good fits for the two transitions to
16Og.s. A ratio of 1.6 +/- 0.2 is obtained for the
spectroscopic factors S (6Lig.s¦3He,
t) and S (6Li3.56¦3He, t). Published
angular distributions of the reaction 19F(p,α)
16O for Ep = 22.8 and 30.5 MeV have been
analysed on the basis of the same mechanism using the same triton
well geometry as for the (3He, 6Li) calculations
and yield a spectroscopic factor S (19F16O, t)
= 0.06. A rather strong transition 19F (3He,
6Lig.s166.06. 6.14 is
observed which is not compatible with one-step triton pickup. The
corresponding angular distribution shows no pronounced structure.