Author name code: beck ADS astronomy entries on 2022-09-14 author:"Beck, Christian" AND (aff:"Freiburg" OR aff:"Solar") ------------------------------------------------------------------------ Title: The magnetic topology of the inverse Evershed flow Authors: Prasad, A.; Ranganathan, M.; Beck, C.; Choudhary, D. P.; Hu, Q. Bibcode: 2022A&A...662A..25P Altcode: 2022arXiv220302702P Context. The inverse Evershed flow (IEF) is a mass motion towards sunspots at chromospheric heights.
Aims: We combined high-resolution observations of NOAA 12418 from the Dunn Solar Telescope and vector magnetic field measurements from the Helioseismic and Magnetic Imager (HMI) to determine the driver of the IEF.
Methods: We derived chromospheric line-of-sight (LOS) velocities from spectra of Hα and Ca II IR. The HMI data were used in a non-force-free magnetic field extrapolation to track closed field lines near the sunspot in the active region. We determined their length and height, located their inner and outer foot points, and derived flow velocities along them.
Results: The magnetic field lines related to the IEF reach on average a height of 3 megameter (Mm) over a length of 13 Mm. The inner (outer) foot points are located at 1.2 (1.9) sunspot radii. The average field strength difference ΔB between inner and outer foot points is +400 G. The temperature difference ΔT is anti-correlated with ΔB with an average value of −100 K. The pressure difference Δp is dominated by ΔB and is primarily positive with a driving force towards the inner foot points of 1.7 kPa on average. The velocities predicted from Δp reproduce the LOS velocities of 2-10 km s−1 with a square-root dependence.
Conclusions: We find that the IEF is driven along magnetic field lines connecting network elements with the outer penumbra by a gas pressure difference that results from a difference in field strength as predicted by the classical siphon flow scenario. Title: Velocities of an Erupting Filament Authors: Wang, Shuo; Jenkins, Jack M.; Muglach, Karin; Martinez Pillet, Valentin; Beck, Christian; Long, David M.; Choudhary, Debi Prasad; McAteer, James Bibcode: 2022ApJ...926...18W Altcode: 2021arXiv211107830W Solar filaments exist as stable structures for extended periods of time before many of them form the core of a coronal mass ejection (CME). We examine the properties of an erupting filament on 2017 May 29-30 with high-resolution He I 10830 Å and Hα spectra from the Dunn Solar Telescope, full-disk Dopplergrams of He I 10830 Å from the Chromospheric Telescope, and EUV and coronograph data from SDO and STEREO. Pre-eruption line-of-sight velocities from an inversion of He I with the HAZEL code exhibit coherent patches of 5 Mm extent that indicate counter-streaming and/or buoyant behavior. During the eruption, individual, aligned threads appear in the He I velocity maps. The distribution of velocities evolves from Gaussian to strongly asymmetric. The maximal optical depth of He I 10830 Å decreased from τ = 1.75 to 0.25, the temperature increased by 13 kK, and the average speed and width of the filament increased from 0 to 25 km s-1 and 10 to 20 Mm, respectively. All data sources agree that the filament rose with an exponential acceleration reaching 7.4 m s-2 that increased to a final velocity of 430 km s-1 at 22:24 UT; a CME was associated with this filament eruption. The properties during the eruption favor a kink/torus instability, which requires the existence of a flux rope. We conclude that full-disk chromospheric Dopplergrams can be used to trace the initial phase of on-disk filament eruptions in real time, which might potentially be useful for modeling the source of any subsequent CMEs. Title: The Visible Spectro-Polarimeter of the Daniel K. Inouye Solar Telescope Authors: de Wijn, A. G.; Casini, R.; Carlile, A.; Lecinski, A. R.; Sewell, S.; Zmarzly, P.; Eigenbrot, A. D.; Beck, C.; Wöger, F.; Knölker, M. Bibcode: 2022SoPh..297...22D Altcode: 2022arXiv220300117D The Daniel K. Inouye Solar Telescope (DKIST) Visible Spectro-Polarimeter (ViSP) is a traditional slit-scanning spectrograph with the ability to observe solar regions up to a 120 ×78 arcsec2 area. The design implements dual-beam polarimetry, a polychromatic polarization modulator, a high-dispersion echelle grating, and three spectral channels that can be automatically positioned. A defining feature of the instrument is its capability to tune anywhere within the 380 - 900 nm range of the solar spectrum, allowing for a virtually infinite number of combinations of three wavelengths to be observed simultaneously. This enables the ViSP user to pursue well-established spectro-polarimetric studies of the magnetic structure and plasma dynamics of the solar atmosphere, as well as completely novel investigations of the solar spectrum. Within the suite of first-generation instruments at the DKIST, ViSP is the only wavelength-versatile spectro-polarimeter available to the scientific community. It was specifically designed as a discovery instrument to explore new spectroscopic and polarimetric diagnostics and test improved models of polarized line formation through high spatial-, spectral-, and temporal-resolution observations of the Sun's polarized spectrum. In this instrument article, we describe the science requirements and design drivers of ViSP and present preliminary science data collected during the commissioning of the instrument. Title: Heating of the solar atmosphere by electric currents Authors: Choudhary, Debi Prasad; Louis, Rohan; Prasad, Avijeet; Beck, Christian; Yalim, Mehmet Bibcode: 2021AGUFMSH12B..07C Altcode: We present direct evidence of Ohmic dissipation of electric currents that leads to heating of the solar chromosphere above a light bridge in a sunspot by combining high-resolution spectroscopic Ca II IR data from the Dunn Solar Telescope with vector magnetic field observations from HMI. An extrapolation of the photospheric magnetic field from HMI to the corona using a non-force-free field technique provided the three-dimensional distribution of electric currents at locations of magnetic discontinuities, while the inversion of the chromospheric Ca II IR line spectra with the CAISAR code delivered the temperature stratifications from the photosphere to the chromosphere. The comparison of these results clearly shows that the light bridge is a site of strong electric currents of about 0.3 A m2 at the bottom boundary, which extend to about 0.7 Mm while decreasing monotonically with height. The dissipation of these currents produces a chromospheric temperature excess of about 600800 K relative to the umbra. Only the light bridge, where relatively weak and highly inclined magnetic fields emerge over a duration of 13 hrs, shows a spatial coincidence of thermal enhancements and electric currents. The temperature enhancements and the Cowling heating are primarily confined to a height range of 0.40.7 Mm above the light bridge. The corresponding increase in internal energy of 200 J m3 can be supplied by the heating in about 10 min. This heating process by Ohmic dissipation can happen at any place in the solar atmosphere where large electric currents co-exist with non-zero resistivity. Large-scale corona loops embedded in a more vertical background magnetic field could experience the same effect leading to a localized heating of coronal plasma. Title: Heating of the solar chromosphere in a sunspot light bridge by electric currents Authors: Louis, Rohan E.; Prasad, Avijeet; Beck, Christian; Choudhary, Debi P.; Yalim, Mehmet S. Bibcode: 2021A&A...652L...4L Altcode: 2021arXiv210712066L Context. Resistive Ohmic dissipation has been suggested as a mechanism for heating the solar chromosphere, but few studies have established this association.
Aims: We aim to determine how Ohmic dissipation by electric currents can heat the solar chromosphere.
Methods: We combine high-resolution spectroscopic Ca II data from the Dunn Solar Telescope and vector magnetic field observations from the Helioseismic and Magnetic Imager (HMI) to investigate thermal enhancements in a sunspot light bridge. The photospheric magnetic field from HMI was extrapolated to the corona using a non-force-free field technique that provided the three-dimensional distribution of electric currents, while an inversion of the chromospheric Ca II line with a local thermodynamic equilibrium and a nonlocal thermodynamic equilibrium spectral archive delivered the temperature stratifications from the photosphere to the chromosphere.
Results: We find that the light bridge is a site of strong electric currents, of about 0.3 A m−2 at the bottom boundary, which extend to about 0.7 Mm while decreasing monotonically with height. These currents produce a chromospheric temperature excess of about 600−800 K relative to the umbra. Only the light bridge, where relatively weak and highly inclined magnetic fields emerge over a duration of 13 h, shows a spatial coincidence of thermal enhancements and electric currents. The temperature enhancements and the Cowling heating are primarily confined to a height range of 0.4−0.7 Mm above the light bridge. The corresponding increase in internal energy of 200 J m−3 can be supplied by the heating in about 10 min.
Conclusions: Our results provide direct evidence for currents heating the lower solar chromosphere through Ohmic dissipation. Title: The National Science Foundation's Daniel K. Inouye Solar Telescope — Status Update Authors: Rimmele, T.; Woeger, F.; Tritschler, A.; Casini, R.; de Wijn, A.; Fehlmann, A.; Harrington, D.; Jaeggli, S.; Anan, T.; Beck, C.; Cauzzi, G.; Schad, T.; Criscuoli, S.; Davey, A.; Lin, H.; Kuhn, J.; Rast, M.; Goode, P.; Knoelker, M.; Rosner, R.; von der Luehe, O.; Mathioudakis, M.; Dkist Team Bibcode: 2021AAS...23810601R Altcode: The National Science Foundation's 4m Daniel K. Inouye Solar Telescope (DKIST) on Haleakala, Maui is now the largest solar telescope in the world. DKIST's superb resolution and polarimetric sensitivity will enable astronomers to unravel many of the mysteries the Sun presents, including the origin of solar magnetism, the mechanisms of coronal heating and drivers of flares and coronal mass ejections. Five instruments, four of which provide highly sensitive measurements of solar magnetic fields, including the illusive magnetic field of the faint solar corona. The DKIST instruments will produce large and complex data sets, which will be distributed through the NSO/DKIST Data Center. DKIST has achieved first engineering solar light in December of 2019. Due to COVID the start of the operations commissioning phase is delayed and is now expected for fall of 2021. We present a status update for the construction effort and progress with the operations commissioning phase. Title: Spectropolarimetric Measurements Of The Inverse Evershed Flow Authors: Choudhary, D. P.; Beck, C.; Prasad, A.; Monankrishna, R.; Dhara, S. Bibcode: 2021AAS...23831319C Altcode: We use high spatial resolution observations with spectropolarimeters at the Dunn Solar Telescope in multiple spectral lines originating in the chromosphere to study inverse Evershed flow channels that connect the outer penumbra and the moat region around sunspots at various heliocentric positions. The measurements were combined with extrapolated magnetic field lines to determine the three-dimensional topology of the flow channels. The magnetic field lines guiding the flows reach on average a height of 2-5 Mm over a length of 10-20 Mm, with cold inner (hot outer) foot points located at 1.2 (1.9) sunspot radii. The average difference in field strength between the inner and outer foot point is about +400 G, while the temperature difference is about -100 K. This configuration can drive and sustain a siphon flow from the moat boundary towards the penumbra over the lifetime of the flow channels of about one hour. This study clearly demonstrates the inverse Evershed flow to be a siphon flow along arched chromospheric loops. Title: Magnetoacoustic wave energy dissipation in the atmosphere of solar pores Authors: Gilchrist-Millar, Caitlin A.; Jess, David B.; Grant, Samuel D. T.; Keys, Peter H.; Beck, Christian; Jafarzadeh, Shahin; Riedl, Julia M.; Van Doorsselaere, Tom; Ruiz Cobo, Basilio Bibcode: 2021RSPTA.37900172G Altcode: 2020arXiv200711594G The suitability of solar pores as magnetic wave guides has been a key topic of discussion in recent years. Here, we present observational evidence of propagating magnetohydrodynamic wave activity in a group of five photospheric solar pores. Employing data obtained by the Facility Infrared Spectropolarimeter at the Dunn Solar Telescope, oscillations with periods of the order of 5 min were detected at varying atmospheric heights by examining Si ɪ 10827 Å line bisector velocities. Spectropolarimetric inversions, coupled with the spatially resolved root mean square bisector velocities, allowed the wave energy fluxes to be estimated as a function of atmospheric height for each pore. We find propagating magnetoacoustic sausage mode waves with energy fluxes on the order of 30 kW m-2 at an atmospheric height of 100 km, dropping to approximately 2 kW m-2 at an atmospheric height of around 500 km. The cross-sectional structuring of the energy fluxes reveals the presence of both body- and surface-mode sausage waves. Examination of the energy flux decay with atmospheric height provides an estimate of the damping length, found to have an average value across all five pores of Ld ≈ 268 km, similar to the photospheric density scale height. We find the damping lengths are longer for body mode waves, suggesting that surface mode sausage oscillations are able to more readily dissipate their embedded wave energies. This work verifies the suitability of solar pores to act as efficient conduits when guiding magnetoacoustic wave energy upwards into the outer solar atmosphere.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: The Formation of an Atypical Sunspot Light Bridge as a Result of Large-scale Flux Emergence Authors: Louis, Rohan E.; Beck, Christian; Choudhary, Debi P. Bibcode: 2020ApJ...905..153L Altcode: 2020arXiv201014085L We use a combination of full-disk data from the Solar Dynamics Observatory and high-resolution data from the Dunn Solar Telescope (DST) to study the formation, structure, and evolution of an atypical light bridge (LB) in a regular sunspot. The LB results from the emergence of magnetic flux with one footpoint rooted in a pore outside the parent sunspot that appears about 17 hr before the LB. The pore has a polarity opposite to that of the sunspot and recedes from it at a speed of about 0.4 km s-1. This is accompanied by the development of an elongated magnetic channel in the outer penumbra that triggers the formation of the LB when it reaches the inner penumbral boundary. The LB is a nearly horizontal structure with a field strength of about 1.2 kG that exhibits long-lived photospheric blueshifts of about 0.85 km s-1 along its entire length. The emergence of the LB leads to dynamic surges in the chromosphere and transition region about 13 minutes later. We derive the photospheric and chromospheric structure of the LB in the DST data from spectral line parameters and inversions of He I at 1083 nm, Si I at 1082.7 nm, Ca II IR at 854 nm, and Hα at 656 nm and speckle-reconstructed imaging at 700 and 430 nm. The LB shows an elongated filamentary shape in the photosphere without lateral extrusions. The thermal inversion of Ca II IR reveals the LB to be about 600-800 K hotter than the umbra. Different sections of the LB are elevated to heights between 400 and 700 km. Our results indicate that LB formation is part of a flux emergence event with the LB envelope reaching a height of about 29 Mm before dissolving after about 13 hr. We conclude that the existence of persistent, large-scale photospheric blueshifts in LBs is the most likely criterion for distinguishing between flux emergence events and overturning convection in field-free umbral intrusions. 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 Bibcode: 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. Title: Center-to-limb Variation of the Inverse Evershed Flow Authors: Beck, C.; Choudhary, D. P.; Ranganathan, M. Bibcode: 2020ApJ...902...30B Altcode: 2020arXiv200812748B We present the properties of the inverse Evershed flow (IEF) based on the center-to-limb variation of the plasma speed and loop geometry of chromospheric superpenumbral fibrils in eleven sunspots that were located at a wide range of heliocentric angles from 12° to 79°. The observations were acquired at the Dunn Solar Telescope in the spectral line of Hα at 656 nm to determine chromospheric flows and the photospheric Si I line at 1082.7 nm to estimate the photospheric umbral magnetic field strength. All sunspots display opposite line-of-sight (LOS) velocities on the limb and center side with a distinct shock signature near the outer penumbral edge. We developed a simplified flexible sunspot model assuming axisymmetry and prescribing the radial flow speed profile at a known loop geometry to replicate the observed two-dimensional IEF patterns under different viewing angles. The simulated flow maps match the observations for chromospheric loops with 10-20 Mm length starting at 0.8-1.1 sunspot radii, an apex height of 1-3 Mm, and a flow speed of 2-9 km s-1. We find on average a good agreement of the simulated velocities and the observations on elliptical annuli around the sunspot. Individual IEF channels show a significant range of variation in their properties and reach maximal LOS speeds of up to 12 km s-1. Upwards or downwards directed flows do not show a change of sign in the LOS velocities for heliocentric angles above 30°. Our results are consistent with the IEF being caused by a siphon flow mechanism driving a flow at about sonic speed along elevated loops with a flattened top in the chromosphere. Title: Magnetic Structure of an Erupting Filament Authors: Wang, Shuo; Jenkins, Jack M.; Martinez Pillet, Valentin; Beck, Christian; Long, David M.; Prasad Choudhary, Debi; Muglach, Karin; McAteer, James Bibcode: 2020ApJ...892...75W Altcode: 2020arXiv200202104W The full 3D vector magnetic field of a solar filament prior to eruption is presented. The filament was observed with the Facility Infrared Spectropolarimeter at the Dunn Solar Telescope in the chromospheric He I line at 10830 Å on 2017 May 29 and 30. We inverted the spectropolarimetric observations with the Hanle and Zeeman Light code to obtain the chromospheric magnetic field. A bimodal distribution of field strength was found in or near the filament. The average field strength was 24 Gauss, but prior to the eruption we find the 90th percentile of field strength was 435 Gauss for the observations on May 29. The field inclination was about 67° from the solar vertical. The field azimuth made an angle of about 47°-65° to the spine axis. The results suggest an inverse configuration indicative of a flux rope topology. He I intensity threads were found to be coaligned with the magnetic field direction. The filament had a sinistral configuration as expected for the southern hemisphere. The filament was stable on 2017 May 29 and started to rise during two observations on May 30, before erupting and causing a minor coronal mass ejection. There was no obvious change of the magnetic topology during the eruption process. Such information on the magnetic topology of erupting filaments could improve the prediction of the geoeffectiveness of solar storms. Title: Temporal Evolution of the Inverse Evershed Flow Authors: Beck, C.; Choudhary, D. P. Bibcode: 2020ApJ...891..119B Altcode: 2020arXiv200204560B The inverse Evershed flow (IEF) is an inflow of material into the penumbra of sunspots in the solar chromosphere that occurs along dark, elongated super-penumbral fibrils extending from about the outer edge of the moat cell to the sunspot. The IEF channels exhibit brightenings in the penumbra, where the supersonic IEF descends to the photosphere causing shock fronts with localized heating. We used an 1 hr time series of spectroscopic observations of the chromospheric spectral lines of Ca II IR at 854 nm and Hα at 656 nm taken with the Interferometric Bidimensional Spectrometer at the Dunn Solar Telescope to investigate the temporal evolution of IEF channels. Complementary information on the photospheric magnetic field was obtained from observations with the Facility Infrared Spectropolarimeter at 1083 nm and the Helioseismic and Magnetic Imager. We find that individual IEF channels are long-lived (10-60 minutes) and only show minor changes in position and flow speed during their lifetime. Initiation and termination of IEF channels takes several minutes. The IEF channels with line-of-sight velocities of about 10 km s-1 show no lasting impact from transient or oscillatory phenomena with maximal velocity amplitudes of only about 1 km s-1 that run along them. We could not detect any clear correlation of the location and evolution of IEF channels to local magnetic field properties in the photosphere in the penumbra or moving magnetic features in the sunspot moat. Our results support a picture of the IEF as a field-aligned siphon flow along arched loops. From our data we cannot determine if their evolution is controlled by events at the outer end in the moat or at the inner end in the penumbra. Title: Stokes Line Parameters as Possible Indicators of Flaring Activity: A Comparison of Flaring and Non-Flaring Active Regions Authors: Romich, K.; Cadavid, A. C.; Choudhary, D. P.; Beck, C. Bibcode: 2019AGUFMSH31D3338R Altcode: While the association between solar active regions (ARs) and solar flares is well-established, there is currently no reliable means of determining when (or if) a given AR will flare. Much of flare forecasting is based on the application of machine-learning statistical techniques that use parameters derived from the local magnetic field as predictors; these are commonly obtained through spectropolarimetric inversions using Stokes profiles from the observed radiation. The standard inversion codes, such as those based on the Milne-Eddington approximation, yield the average magnetic field values at or near flare locations. However, this fails to utilize the rich information contained in the shape of Stokes profiles, such as the existence of line-of-sight (LOS) magnetic and velocity gradients and multiple magnetic components along the LOS or within the resolving element. The resulting loss of information can potentially lead to inaccurate forecasts. We propose a novel approach, in which line parameters derived from spectrally-resolved Stokes profiles are considered as possible precursors to flare events. Using data from the spectropolarimeter onboard the Hinode satellite, we examine the amplitude asymmetry, net circular polarization, and degree of complexity of Stokes V profiles from several flaring and non-flaring ARs. Particular attention is given to regions near the polarity inversion line (PIL) due to its documented role in flare initiation. We define the PIL using the magnetic field vector relative to the solar surface, which we calculate from the Stokes Q, U, and V profiles; this corrects for projection effects arising from LOS observations of magnetic flux from ARs off disc center. In light of the shortcomings of existing methods, we hope to expand the set of viable indicators of flaring activity with the long-term goal of improving flare forecasting models. Title: Time dependent properties of Inverse Evershed Flow and Perspectives with Daniel K. Inouye Solar Telescope (DKIST) Authors: Choudhary, D. P.; Beck, C. Bibcode: 2019AGUFMSH41F3334C Altcode: We observed isolated leading sunspot, located at a heliocentric angle of 43 degrees, of a decaying active region (AR) NOAA 12418 on 16 September 2015 with the Interferometric Bidimensional, the Facility InfraRed Spectropolarimeter at the Dunn Solar Telescope to study the time dependent chromospheric flow properties along the fibril structure, which is widely known as Inverse Evershed Flow. The observations were carried out in spectral lines of H_alpha at 656 nm and Ca ii IR at 854.2 nm, photospheric Zeeman-sensitive Si i line at 1082.7 nm and the chromospheric He i line at 1083 nm. Our data is complemented with the Milne-Eddington inversion results for the photospheric magnetic field obtained with the derived from observations by the Helioseismic and Magnetic Imager on-board the Solar Dynamics Observatory. We find that all three chromospheric lines show a very similar behavior in their line-core intensity and the LOS velocity with matching spatial and temporal properties. We find that individual IEF channels persist for a few ten minutes to more than one hour. IEF channels that disappear are often rapidly replaced by a new channel at the about the same location after a short time. The IEF channels show little radial or lateral motion and usually end in the mid to outer penumbra. Initiation of the flow takes about 10min, while the termination is faster and takes only about 5min. The IEF channels seem to appear at preferred azimuth angles that are spaced at about 10◦ distance. The transient events were found to have almost no effect on the overall Inverse Evershed Flow pattern. In this paper, we shall present the results from our current study and perspectives for further observational study with upcoming 4 meter Daniel K. Inouye Solar Telescope. Title: An Erupting Solar Filament Observed at the DST Authors: Wang, S.; Jenkins, J. M.; Pillet, V. M.; Beck, C.; Long, D.; Choudhary, D. P.; McAteer, J. Bibcode: 2019AGUFMSH33B3376W Altcode: An erupting filament that lead to a coronal mass ejection (CME) was observed at the Dunn Solar Telescope (DST). We present HAZEL inversions of spectropolarimetric observations of the quiescent filament acquired with the Facility Infrared Spectropolarimeter at the DST. This study includes three observations of the He I triplet at 10830 Å on May 29 and 30, 2017. The filament was stable on May 29, and was observed in the process of rising at speeds of 20-30 km/s during the two spatial maps taken on May 30. Vector magnetic fields along the filament were obtained that show an inverse configuration indicative of a flux rope topology, including co-aligned threads. To take advantage of the better spatial and temporal resolution of the Daniel K. Inouye Solar Telescope (DKIST), future collaborations of the DKIST and the DST to study solar filaments are discussed. Title: Filament Magnetic Fields at the DST and DKIST Authors: Wang, Shuo; Jenkins, Jack; Pillet, Valentin; Beck, Christian; Long, David; Choudhary, Debi Prasad; McAteer, James Bibcode: 2019AAS...23422603W Altcode: Observations from the 0.8-m Dunn Solar Telescope (DST) are qualitatively similar to data that will be produced by the 4-m Daniel K. Inouye Solar Telescope (DKIST), albeit at a lower spatial resolution and polarimetric sensitivity. We present HAZEL inversions of spectropolarimetric observations of a quiescent filament acquired with the Facility Infrared Spectropolarimeter at the DST. This study includes three observations of the He I triplet at 10830 Å on May 29 and 30, 2017. The filament was stable on May 29, and was observed in the process of rising at speeds of 20-30 km/s during the two spatial maps taken on May 30. Vector magnetic fields along the filament were obtained that show an inverse configuration indicative of a flux rope topology, including co-aligned threads. To take advantage of the better spatial and temporal resolution of the DKIST, future collaborations of the DKIST and the DST to study solar filaments are discussed. We propose to further study the evolution of solar filaments that erupt and lead to Coronal Mass Ejections using interspaced observations from the DKIST and DST spectropolarimeters. While the DST observations will give information about the global evolution of physical properties leading to the destabilization, the DKIST observations will provide the information about the physical conditions in the small-scale structures that support the filament material. Title: Fast Inversion of Solar Ca II Spectra in Non-local Thermodynamic Equilibrium Authors: Beck, C.; Gosain, S.; Kiessner, C. Bibcode: 2019ApJ...878...60B Altcode: 2019arXiv190411843B Present-day solar imaging spectrometers typically yield a few hundred million spectra in one hour of observing time. This number will increase by an order of magnitude for future instruments with larger 4k × 4k sensors, such as those planned to be used for the upcoming Daniel K. Inouye Solar Telescope. A fast quantitative analysis of such huge data volumes can be done by comparing the observations to an archive of pre-calculated synthetic spectra to infer the thermodynamic properties of the atmosphere. To analyze intensity spectra of the Ca II IR line at 854 nm in the solar atmosphere, we generated an archive with 2,000,000 spectra under the assumption of non-local thermodynamic equilibrium (NLTE) with the NICOLE code. We tested its performance by inverting 60 spectral scans of Ca II IR at 854 nm in the magnetically quiet Sun with 700,000 profiles each. Based on the inversion results obtained using the full archive, we constructed a smaller archive by keeping only the 70,000 archive profiles that were actually used. We can reproduce the observed intensity spectra to within a few percent using either the full or the small archive. For spectra with 30 wavelength points, this NLTE inversion approach takes 0.02 (0.35) s per profile to obtain a temperature stratification when using the small (full) archive, i.e., it can invert a single spectral scan in about 4 (68) hr. The code is able to simultaneously deal with an arbitrary number of spectral lines. This makes it a promising tool for deriving thermodynamic properties of the solar atmosphere from current or future solar high-resolution observations of photospheric and chromospheric lines. Title: NLTE inversions of Ca II 854.2 nm spectra from SOLIS/VSM: Temperature diagnostics Authors: Gusain, Sanjay; Beck, Christian Bibcode: 2019AAS...23430203G Altcode: We present non-local thermodynamic equilibrium (NLTE) inversions of Ca II 854.2 nm spectra from SOLIS/VSM observations. We show that precomputed spectra from a database of model atmospheres can be applied for a reliable inference of temperature stratifications in the solar atmosphere. We show examples of such inferences in different solar structures such as the umbra, penumbra, circum-facular regions and the quiet Sun. We also fit full Stokes profiles assuming a simple magnetic model with an exponential dependence with optical depth to derive vector magnetic field information and compare with the values derived from the weak-field approximation. Title: Magnetic Flux Density in 3D MHD Simulations and Observations Authors: Beck, C.; Fabbian, D.; Rezaei, R.; Puschmann, K. G. Bibcode: 2019ASPC..526..191B Altcode: We compare the polarization signals induced in three-dimensional (3D) magneto-hydrodynamical (MHD) simulations by the Zeeman effect in the presence of photospheric magnetic fields to those in observations at disc centre. We consider quantities determined from Stokes vector profiles of observations of photospheric spectral lines in the visible and near-infrared, and in corresponding synthetic spectra obtained from numerical 3D MHD simulations with an average magnetic flux density of 20-200 G. We match the spatial resolution of observations by degrading the spectra of the simulations. We find that the total unsigned vertical magnetic flux density in the simulation should be less than 50 G to reproduce the observed polarization signals in the quiet Sun internetwork. A value of ∼30 G best agrees with all observations we employed. Title: Magnetic Structures of a Quiecent Filament Authors: Wang, Shuo; Jenkins, Jack; Pillet, Valentin; Beck, Christian; Long, David; Choudhary, Debi; McAteer, James Bibcode: 2019shin.confE..52W Altcode: A quiecent filament is observed from the Dunn Solar Telescope (DST) on May 29 and 30, 2017. We present HAZEL inversions of spectropolarimetric observations acquired with the Facility Infrared Spectropolarimeter at the DST. This study includes three observations of the He I triplet at 1083.0 nm. Title: Magnetic Properties and Flow Angle of the Inverse Evershed Flow at Its Downflow Points Authors: Beck, C.; Choudhary, D. P. Bibcode: 2019ApJ...874....6B Altcode: 2019arXiv190204660B We determined the direction and strength of the photospheric and lower chromospheric magnetic field in the umbra and penumbra of a sunspot from inversions of spectropolarimetric observations of photospheric lines at 617 nm and 1565 nm and the chromospheric Ca II IR line at 854 nm, respectively. We compare the magnetic field vector with the direction of 75 flow channels that harbor the chromospheric inverse Evershed effect (IEF) near their downflow points (DFPs) in the sunspot’s penumbra. The azimuth and inclination of the IEF channels to the line of sight (LOS) were derived from spatial maps of the LOS velocity and line-core intensity of the Ca II IR line and a thermal inversion of the Ca II IR spectra to obtain temperature cubes. We find that the flow direction of the IEF near the DFPs is aligned with the photospheric magnetic field to within about ±15°. The IEF flow fibrils make an angle of 30°-90° to the local vertical with an average value of about 65°. The average field strength at the DFPs is about 1.3 kG. Our findings suggest that the IEF in the lower chromosphere is a field-aligned siphon flow, where the larger field strength at the inner footpoints together with the lower temperature in the penumbra causes the necessary gas pressure difference relative to the outer footpoints in the hotter quiet Sun with lower magnetic field strength. The IEF connects to magnetic field lines that are not, like in the case of the regular Evershed flow, but which continue upward into the chromosphere, indicating an “uncombed” penumbral structure. Title: The First Solar Seeing Profile Measurement with Two Apertures and Multiple Guide Regions Authors: Ren, Deqing; Zhao, Gang; Wang, Xin; Beck, Christian; Broadfoot, Robert Bibcode: 2019SoPh..294....1R Altcode: 2018SoPh..294....1R Ground-based observations suffer from atmospheric turbulence perturbations, which seriously degrade the image quality. The seeing profile associated with the turbulence is critical to characterize an astronomical site. The optimal design and performance estimation of future solar ground-layer adaptive optics (GLAO) and multi-conjugate adaptive optics (MCAO) heavily rely on our knowledge of the seeing profile at a specific site. Many current optical seeing profile measurement techniques require one to use a large solar telescope for that purpose. The development of a portable instrument to measure and characterize the seeing profile is thus highly needed, in particular for testing potential new sites or for the regularly monitoring of the seeing condition at existing sites. Recently, we proposed the Advanced Multiple Aperture Seeing Profiler (A-MASP), which uses multiple small telescopes and multiple regions of interest (ROIs) on the solar surface to measure the seeing profile up to an altitude of 30 km. Here, we report our recent proof-of-concept observation run of the A-MASP technique with the Dunn Solar Telescope (DST) of the National Solar Observatory (NSO). We found that the Fried parameter, r0, was about 12 cm at the observed wavelength of 630 nm in the early morning and that there were three main turbulence layers. The strongest one was the mix layer near the ground. We observed the evolution of the top of the mix layer and found that it can rise to about 1.5 km in about 18 min, which is consistent with the theory of daytime boundary layer evolution. Another turbulence layer was observed from 8 to 15 km, which is at the top of the convective layer. Comparing an instrument with two sub-apertures with a real A-MASP instrument, we found that they should lead to similar results except for the altitude h =0. Title: Inferring telescope polarization properties through spectral lines without linear polarization Authors: Derks, A.; Beck, C.; Martínez Pillet, V. Bibcode: 2018A&A...615A..22D Altcode: 2018arXiv180401153D Context. Polarimetric observations taken with ground- or space-based telescopes usually need to be corrected for changes of the polarization state in the optical path.
Aims: We present a technique to determine the polarization properties of a telescope through observations of spectral lines that have no or negligible intrinsic linear polarization signals. For such spectral lines, any observed linear polarization must be induced by the telescope optics. We apply the technique to observations taken with the Spectropolarimeter for Infrared and Optical Regions (SPINOR) at the Dunn Solar Telescope (DST) and demonstrate that we can retrieve the characteristic polarization properties of the DST at three wavelengths of 459, 526, and 615 nm.
Methods: We determine the amount of crosstalk between the intensity Stokes I and the linear and circular polarization states Stokes Q, U, and V, and between Stokes V and Stokes Q and U in spectropolarimetric observations of active regions. We fit a set of parameters that describe the polarization properties of the DST to the observed crosstalk values. We compare our results to parameters that were derived using a conventional telescope calibration unit (TCU).
Results: The values for the ratio of reflectivities X = rs/rp and the retardance τ of the DST turret mirrors from the analysis of the crosstalk match those derived with the TCU within the error bars. We find a negligible contribution of retardance from the entrance and exit windows of the evacuated part of the DST. Residual crosstalk after applying a correction for the telescope polarization stays at a level of 3-10% regardless of which parameter set is used, but with an rms fluctuation in the input data of already a few percent. The accuracy in the determination of the telescope properties is thus more limited by the quality of the input data than the method itself.
Conclusions: It is possible to derive the parameters that describe the polarization properties of a telescope from observations of spectral lines without intrinsic linear polarization signal. Such spectral lines have a dense coverage (about 50 nm separation) in the visible part of the spectrum (400-615 nm), but none were found at longer wavelengths. Using spectral lines without intrinsic linear polarization is a promising tool for the polarimetric calibration of current or future solar telescopes such as the Daniel K. Inouye Solar Telescope (DKIST). Title: Status of the Daniel K. Inouye Solar Telescope: unraveling the mysteries the Sun. Authors: Rimmele, Thomas R.; Martinez Pillet, Valentin; Goode, Philip R.; Knoelker, Michael; Kuhn, Jeffrey Richard; Rosner, Robert; Casini, Roberto; Lin, Haosheng; von der Luehe, Oskar; Woeger, Friedrich; Tritschler, Alexandra; Fehlmann, Andre; Jaeggli, Sarah A.; Schmidt, Wolfgang; De Wijn, Alfred; Rast, Mark; Harrington, David M.; Sueoka, Stacey R.; Beck, Christian; Schad, Thomas A.; Warner, Mark; McMullin, Joseph P.; Berukoff, Steven J.; Mathioudakis, Mihalis; DKIST Team Bibcode: 2018AAS...23231601R Altcode: The 4m Daniel K. Inouye Solar Telescope (DKIST) currently under construction on Haleakala, Maui will be the world’s largest solar telescope. Designed to meet the needs of critical high resolution and high sensitivity spectral and polarimetric observations of the sun, this facility will perform key observations of our nearest star that matters most to humankind. DKIST’s superb resolution and sensitivity will enable astronomers to address many of the fundamental problems in solar and stellar astrophysics, including the origin of stellar magnetism, the mechanisms of coronal heating and drivers of the solar wind, flares, coronal mass ejections and variability in solar and stellar output. DKIST will also address basic research aspects of Space Weather and help improve predictive capabilities. In combination with synoptic observations and theoretical modeling DKIST will unravel the many remaining mysteries of the Sun.The construction of DKIST is progressing on schedule with 80% of the facility complete. Operations are scheduled to begin early 2020. DKIST will replace the NSO facilities on Kitt Peak and Sac Peak with a national facility with worldwide unique capabilities. The design allows DKIST to operate as a coronagraph. Taking advantage of its large aperture and infrared polarimeters DKIST will be capable to routinely measure the currently illusive coronal magnetic fields. The state-of-the-art adaptive optics system provides diffraction limited imaging and the ability to resolve features approximately 20 km on the Sun. Achieving this resolution is critical for the ability to observe magnetic structures at their intrinsic, fundamental scales. Five instruments will be available at the start of operations, four of which will provide highly sensitive measurements of solar magnetic fields throughout the solar atmosphere - from the photosphere to the corona. The data from these instruments will be distributed to the world wide community via the NSO/DKIST data center located in Boulder. We present examples of science objectives and provide an overview of the facility and project status, including the ongoing efforts of the community to develop the critical science plan for the first 2-3 years of operations. Title: The Magnetic Response of the Solar Atmosphere to Umbral Flashes Authors: Houston, S. J.; Jess, D. B.; Asensio Ramos, A.; Grant, S. D. T.; Beck, C.; Norton, A. A.; Krishna Prasad, S. Bibcode: 2018ApJ...860...28H Altcode: 2018arXiv180300018H Chromospheric observations of sunspot umbrae offer an exceptional view of magnetoacoustic shock phenomena and the impact they have on the surrounding magnetically dominated plasma. We employ simultaneous slit-based spectro-polarimetry and spectral imaging observations of the chromospheric He I 10830 Å and Ca II 8542 Å lines to examine fluctuations in the umbral magnetic field caused by the steepening of magnetoacoustic waves into umbral flashes. Following the application of modern inversion routines, we find evidence to support the scenario that umbral shock events cause expansion of the embedded magnetic field lines due to the increased adiabatic pressure. The large number statistics employed allow us to calculate the adiabatic index, γ = 1.12 ± 0.01, for chromospheric umbral locations. Examination of the vector magnetic field fluctuations perpendicular to the solar normal revealed changes up to ∼200 G at the locations of umbral flashes. Such transversal magnetic field fluctuations have not been described before. Through comparisons with nonlinear force-free field extrapolations, we find that the perturbations of the transverse field components are oriented in the same direction as the quiescent field geometries. This implies that magnetic field enhancements produced by umbral flashes are directed along the motion path of the developing shock, hence producing relatively small changes, up to a maximum of ∼8°, in the inclination and/or azimuthal directions of the magnetic field. Importantly, this work highlights that umbral flashes are able to modify the full vector magnetic field, with the detection of the weaker transverse magnetic field components made possible by high-resolution data combined with modern inversion routines. Title: Thermodynamic Properties of the Inverse Evershed Flow at Its Downflow Points Authors: Choudhary, D. P.; Beck, C. Bibcode: 2018ApJ...859..139C Altcode: We used spectropolarimetric observations of a sunspot in the active region NOAA 11809 in the Ca II line at 854.2 nm taken with the SpectroPolarimeter for Optical and Infrared Regions at the Dunn Solar Telescope to infer thermodynamic parameters along 100 super-penumbral fibrils that harbor the inverse Evershed flow. The fibrils were identified in line-of-sight (LOS) velocity and line-core intensity maps. The chromospheric LOS velocity abruptly decreases from 3 to 15 km s-1 to zero at the inner footpoints of the fibrils that are located from the mid penumbra to about 1.4 spot radii. The spectra often show multiple absorption components, indicating spatially or vertically unresolved structures. Synthetic spectra with a 100% fill factor of a flow channel in the upper atmosphere yield strongly asymmetric profiles but no multiple separate components. The line-core intensity always peaks slightly closer to the umbra than the LOS velocity. Using the CAlcium Inversion using a Spectral ARchive code, we find that the fibrils make an angle of 30°-60° to the local vertical away from the umbra. The temperature near the downflow points is enhanced by 200 K at log τ ∼ -2 and up to 2000 K at log τ ∼ (-6) compared to the quiet Sun, without any signature in the low photosphere. Our results are consistent with a critical, i.e., sonic, or supersonic siphon flow along super-penumbral flux tubes in which accelerating plasma abruptly attains subcritical velocity through a standing shock in or near the penumbra. Title: Inferring Telescope Polarization Properties Through Spectral Lines Without Linear Polarization Authors: Derks, Alysa; Beck, Christian; Martinez Pillet, Valentin Bibcode: 2018tess.conf21060D Altcode: We present a technique to determine the polarization properties of a telescope through observations of spectral lines that have no or negligible intrinsic linear polarization signals. For such spectral lines, any observed linear polarization must be induced by the telescope optics. We apply the technique to observations taken with SPINOR at the Dunn Solar Telescope (DST) and demonstrate that we can retrieve the characteristic polarization properties of the DST at three wavelengths of 459, 526, and 615 nm. The values for the ratio of reflectivities X = rs/rp and the retardance τ of the DST turret mirrors from the analysis of the crosstalk match those derived with the TCU within the error bars.

We conclude that it is possible to derive the parameters that describe the polarization properties of a telescope from observations of spectral lines without intrinsic linear polarization signal. Such spectral lines have a dense coverage (about 50 nm separation) in the visible part of the spectrum (400-615 nm), but none were found at longer wavelengths. Using spectral lines without intrinsic linear polarization is a promising tool for the polarimetric calibration of current or future solar telescopes such as the Daniel K. Inouye Solar Telescope (DKIST). Title: Alfvén wave dissipation in the solar chromosphere Authors: Grant, Samuel D. T.; Jess, David B.; Zaqarashvili, Teimuraz V.; Beck, Christian; Socas-Navarro, Hector; Aschwanden, Markus J.; Keys, Peter H.; Christian, Damian J.; Houston, Scott J.; Hewitt, Rebecca L. Bibcode: 2018NatPh..14..480G Altcode: 2018arXiv181007712G Magnetohydrodynamic Alfvén waves1 have been a focus of laboratory plasma physics2 and astrophysics3 for over half a century. Their unique nature makes them ideal energy transporters, and while the solar atmosphere provides preferential conditions for their existence4, direct detection has proved difficult as a result of their evolving and dynamic observational signatures. The viability of Alfvén waves as a heating mechanism relies upon the efficient dissipation and thermalization of the wave energy, with direct evidence remaining elusive until now. Here we provide the first observational evidence of Alfvén waves heating chromospheric plasma in a sunspot umbra through the formation of shock fronts. The magnetic field configuration of the shock environment, alongside the tangential velocity signatures, distinguish them from conventional umbral flashes5. Observed local temperature enhancements of 5% are consistent with the dissipation of mode-converted Alfvén waves driven by upwardly propagating magneto-acoustic oscillations, providing an unprecedented insight into the behaviour of Alfvén waves in the solar atmosphere and beyond. Title: High-resolution Observations of Hα Spectra with a Subtractive Double Pass Authors: Beck, C.; Rezaei, R.; Choudhary, D. P.; Gosain, S.; Tritschler, A.; Louis, R. E. Bibcode: 2018SoPh..293...36B Altcode: 2017arXiv171207077B High-resolution imaging spectroscopy in solar physics has relied on Fabry-Pérot interferometers (FPIs) in recent years. FPI systems, however, become technically challenging and expensive for telescopes larger than the 1 m class. A conventional slit spectrograph with a diffraction-limited performance over a large field of view (FOV) can be built at much lower cost and effort. It can be converted into an imaging spectro(polari)meter using the concept of a subtractive double pass (SDP). We demonstrate that an SDP system can reach a similar performance as FPI-based systems with a high spatial and moderate spectral resolution across a FOV of 100″×100″ with a spectral coverage of 1 nm. We use Hα spectra taken with an SDP system at the Dunn Solar Telescope and complementary full-disc data to infer the properties of small-scale superpenumbral filaments. We find that the majority of all filaments end in patches of opposite-polarity fields. The internal fine-structure in the line-core intensity of Hα at spatial scales of about 0.″5 exceeds that in other parameters such as the line width, indicating small-scale opacity effects in a larger-scale structure with common properties. We conclude that SDP systems in combination with (multi-conjugate) adaptive optics are a valid alternative to FPI systems when high spatial resolution and a large FOV are required. They can also reach a cadence that is comparable to that of FPI systems, while providing a much larger spectral range and a simultaneous multi-line capability. Title: The Polarization Signature of Photospheric Magnetic Fields in 3D MHD Simulations and Observations at Disk Center Authors: Beck, C.; Fabbian, D.; Rezaei, R.; Puschmann, K. G. Bibcode: 2017ApJ...842...37B Altcode: 2017arXiv170506812B Before using three-dimensional (3D) magnetohydrodynamical (MHD) simulations of the solar photosphere in the determination of elemental abundances, one has to ensure that the correct amount of magnetic flux is present in the simulations. The presence of magnetic flux modifies the thermal structure of the solar photosphere, which affects abundance determinations and the solar spectral irradiance. The amount of magnetic flux in the solar photosphere also constrains any possible heating in the outer solar atmosphere through magnetic reconnection. We compare the polarization signals in disk-center observations of the solar photosphere in quiet-Sun regions with those in Stokes spectra computed on the basis of 3D MHD simulations having average magnetic flux densities of about 20, 56, 112, and 224 G. This approach allows us to find the simulation run that best matches the observations. The observations were taken with the Hinode SpectroPolarimeter (SP), the Tenerife Infrared Polarimeter (TIP), the Polarimetric Littrow Spectrograph (POLIS), and the GREGOR Fabry-Pèrot Interferometer (GFPI), respectively. We determine characteristic quantities of full Stokes profiles in a few photospheric spectral lines in the visible (630 nm) and near-infrared (1083 and 1565 nm). We find that the appearance of abnormal granulation in intensity maps of degraded simulations can be traced back to an initially regular granulation pattern with numerous bright points in the intergranular lanes before the spatial degradation. The linear polarization signals in the simulations are almost exclusively related to canopies of strong magnetic flux concentrations and not to transient events of magnetic flux emergence. We find that the average vertical magnetic flux density in the simulation should be less than 50 G to reproduce the observed polarization signals in the quiet-Sun internetwork. A value of about 35 G gives the best match across the SP, TIP, POLIS, and GFPI observations. Title: Design and calibration of a high-sensitivity and high-accuracy polarimeter based on liquid crystal variable retarders Authors: Guo, Jing; Ren, De-Qing; Liu, Cheng-Chao; Zhu, Yong-Tian; Dou, Jiang-Pei; Zhang, Xi; Beck, Christian Bibcode: 2017RAA....17....8G Altcode: Polarimetry plays an important role in the measurement of solar magnetic fields. We developed a high-sensitivity and high-accuracy polarimeter (HHP) based on nematic liquid crystal variable retarders (LCVRs), which has a compact setup and no mechanical moving parts. The system design and calibration methods are discussed in detail. The azimuth error of the transmission axis of the polarizer as well as the fast axes of the two LCVRs and the quarter-wave plate were determined using dedicated procedures. Linearly and circularly polarized light were employed to evaluate the performance of the HHP. The experimental results indicate that a polarimetric sensitivity of better than \[5.7 × {10 - 3}\] can be achieved by using a single short-exposure image, while an accuracy on the order of 10-5 can be reached by using a large number of short-exposure images. This makes the HHP a high-performance system that can be used with a ground-based solar telescope for high-precision solar magnetic field investigations. Title: Absorption by Mercury's Exosphere During the May 9th, 2016 Solar Transit. Authors: Schmidt, C.; Reardon, K.; Killen, R. M.; Gary, D. E.; Ahn, K.; Leblanc, F.; Baumgardner, J. L.; Mendillo, M.; Beck, C.; Mangano, V. Bibcode: 2016AGUFM.P53B2198S Altcode: Observations of Mercury during a solar transit have the unique property that line absorption may be used to retrieve the exosphere's column density at all points above the terminator simultaneously. We report on measurements during the 9 May 2016 transit with the Dunn Solar Telescope (Interferometric BIdimensional Spectropolarimeter: IBIS & Horizontal Spectrograph: HSG) and the Big Bear Solar Observatory (Fast Imaging Solar Spectrograph: FISS). The sodium exosphere was observed via Fabry-Perot imaging with IBIS in 9 mA increments, and with FISS at a dispersion of 17 mA/pixel by scanning the spectrograph slit over the planet's disk. A search for potassium D line absorption was performed using slit spectroscopy with HSG at a resolution of R 270,000. In each instrument, exposures of 20-40 ms and adaptive optics enable spatial structure to be resolved on sub-arcsecond scales. The line profiles at every spatial bin are divided by a shifted and scaled reference spectrum in order to isolate the exosphere's absorption from line absorption in the solar atmosphere and structures inherent to granulation. Analysis of these data sets is ongoing, but preliminary findings clearly show the densest column of sodium near the poles and the content at dawn enhanced several times with respect to dusk. Such is consistent with 2003 transit results taken at the same Mercury season (Schleicher et al., 2004), however the data volumes herein permit a more in-depth study in which time-dependence of the exosphere may be considered. Title: Spectroscopy at the Solar Limb: II. Are Spicules Heated to Coronal Temperatures? Authors: Beck, C.; Rezaei, R.; Puschmann, K. G.; Fabbian, D. Bibcode: 2016SoPh..291.2281B Altcode: 2016arXiv160606132B; 2016SoPh..tmp..132B Spicules of the so-called type II were suggested to be relevant for coronal heating because of their ubiquity on the solar surface and their eventual extension into the corona. We investigate whether solar spicules are heated to transition-region or coronal temperatures and reach coronal heights (≫6 Mm) using multiwavelength observations of limb spicules in different chromospheric spectral lines (Ca II H, Hε , Hα , Ca II IR at 854.2 nm, He I at 1083 nm) taken with slit spectrographs and imaging spectrometers. We determine the line width of spectrally resolved line profiles in individual spicules and throughout the field of view, and estimate the maximal height that different types of off-limb features reach. We derive estimates of the kinetic temperature and the non-thermal velocity from the line width of spectral lines from different chemical elements. We find that most regular, i.e. thin and elongated, spicules reach a height of at most about 6 Mm above the solar limb. The majority of features found at larger heights are irregularly shaped with a significantly larger lateral extension, of up to a few Mm, than spicules. Both individual and average line profiles in all spectral lines show a decrease in their line width with height above the limb with very few exceptions. The kinetic temperature and the non-thermal velocity decrease with height above the limb. We find no indications that the spicules in our data reach coronal heights or transition-region or coronal temperatures. Title: On the Magnetism and Dynamics of Prominence Legs Hosting Tornadoes Authors: Martínez González, M. J.; Asensio Ramos, A.; Arregui, I.; Collados, M.; Beck, C.; de la Cruz Rodríguez, J. Bibcode: 2016ApJ...825..119M Altcode: 2016arXiv160501183M Solar tornadoes are dark vertical filamentary structures observed in the extreme ultraviolet associated with prominence legs and filament barbs. Their true nature and relationship to prominences requires an understanding of their magnetic structure and dynamic properties. Recently, a controversy has arisen: is the magnetic field organized forming vertical, helical structures or is it dominantly horizontal? And concerning their dynamics, are tornadoes really rotating or is it just a visual illusion? Here we analyze four consecutive spectro-polarimetric scans of a prominence hosting tornadoes on its legs, which helps us shed some light on their magnetic and dynamical properties. We show that the magnetic field is very smooth in all the prominence, which is probably an intrinsic property of the coronal field. The prominence legs have vertical helical fields that show slow temporal variation that is probably related to the motion of the fibrils. Concerning the dynamics, we argue that (1) if rotation exists, it is intermittent, lasting no more than one hour, and (2) the observed velocity pattern is also consistent with an oscillatory velocity pattern (waves). Title: Three-Dimensional Chromospheric Thermal Structure of Sunspots Authors: Prasad Choudhary, Debi; Beck, Christian Bibcode: 2016SPD....47.0401P Altcode: We have observed several sunspots using the Spectropolarimeter for Infrared and Optical wavelength Ranges at the Dunn Solar Telescope during 29 July to 4 August 2013. The data consists of full Stokes profiles in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of these Stokes spectra provides the magnetic, thermal and velocity structure at photospheric and chromospheric heights of sunspots. In this contribution, we present the results on the 3D thermal structure in the super-penumbral canopy of a round sunspot, derived by a novel approach for the inversion of Ca II IR spectra. Tracing individual fibrils in the super-penumbral canopy, we find that about half of them form only short loops of a a few Mm length that return to the photosphere in the close surroundings of the sunspot instead of connecting to more remote magnetic network at the outer end of the moat flow. We also find indications for standing shocks at the inner foot points of the flow channels that are compatible with a supersonic siphon flow scenario. Title: Cross-Calibrating Sunspot Magnetic Field Strength Measurements from the McMath-Pierce Solar Telescope and the Dunn Solar Telescope Authors: Watson, Fraser T.; Beck, Christian; Penn, Matthew J.; Tritschler, Alexandra; Pillet, Valentín Martinez; Livingston, William C. Bibcode: 2015SoPh..290.3267W Altcode: 2015arXiv151107315W; 2015SoPh..tmp..158W In this article we describe a recent effort to cross-calibrate data from an infrared detector at the McMath-Pierce Solar Telescope and the Facility InfraRed Spectropolarimeter (FIRS) at the Dunn Solar Telescope. A synoptic observation program at the McMath-Pierce has measured umbral magnetic field strengths since 1998, and this data set has recently been compared with umbral magnetic field observations from SOHO/MDI and SDO/HMI. To further improve on the data from McMath-Pierce, we compared the data with measurements taken at the Dunn Solar Telescope with far greater spectral resolution than has been possible with space instrumentation. To minimise potential disruption to the study, concurrent umbral measurements were made so that the relationship between the two datasets can be most accurately characterised. We find that there is a strong agreement between the umbral magnetic field strengths recorded by each instrument, and we reduced the FIRS data in two different ways to successfully test this correlation further. Title: Multiwavelength spectropolarimetric observations of an Ellerman bomb Authors: Rezaei, R.; Beck, C. Bibcode: 2015A&A...582A.104R Altcode: Context. Ellerman bombs (EBs) are enhanced emission in the wings of the Hα line in the solar spectrum.
Aims: We study the structure of an EB in the photosphere and chromosphere.
Methods: We analyze simultaneous observations of four chromospheric lines (Hα, Ca ii H, Ca ii IR 854 nm, and He i 1083 nm) as well as two photospheric lines (Fe i 630 and Si i 1082.7 nm) along with high-cadence 160 and 170 nm ultraviolet (UV) continuum filtergrams. Full Stokes data from the Helioseismic and Magnetic Imager (HMI) are used to trace the temporal evolution of the magnetic structure.
Results: We identify the EB by excess emission in the wings of the Hα line, a brightening in the UV continuum, and large emission peaks in the core of the two Ca ii lines. The EB shows a blueshift in all chromospheric lines, while no shifts are observed in the photospheric lines. The blueshift in the chromospheric layer causes very asymmetric emission peaks in the Ca ii H line. The photospheric Si i spectral line shows a shallower line depth at the location of the EB. The UV continuum maps show that the EB was substantially brighter than its surroundings for about 30 min. The continuum contrast of the EB from 170 nm to 1080 nm shows a power-law dependency on the wavelength. The temperature enhancement amounts to 130 K in the low photosphere and 400 K at the temperature minimum level. This temperature excess is also seen in an LTE inversion of the Ca ii spectra. The total thermal and radiative energy content of the EB is about 1020 J and 1018 J in the photosphere and chromosphere, respectively. The HMI data hints at a photospheric magnetic flux cancellation as the driver of the EB.
Conclusions: Ellerman bombs release the energy in a height range of several pressure scale heights around the temperature minimum such that they affect both the photosphere and the lower chromosphere. Title: Variation in sunspot properties between 1999 and 2014 Authors: Rezaei, R.; Beck, C.; Lagg, A.; Borrero, J. M.; Schmidt, W.; Collados, M. Bibcode: 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: Three Dimensional Chromospheric Thermal Structure of Sunspot Authors: Prasad Choudhary, Debi; Beck, Christian; Rezaei, R. Bibcode: 2015TESS....131201P Altcode: We have observed sunspots using the Spectropolarimeter for infrared and optical wavelength ranges at the Dunn Solar Telescope during 29 July to 4 August 2013. The data consists of full Stokes profiles in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of these Stokes spectra provides the magnetic, thermal and velocity structure at photospheric and chromospheric heights of sunspots. In this contribution, we present the results on the 3D thermal structure in the super-penumbral canopy of a well rounded sunspot, derived by a novel approach for the inversion of Ca II IR spectra. Tracing individual fibrils in the super-penumbral canopy, we find that about half of them form only short loops of a a few Mm length that return to the photosphere in the close surroundings of the sunspot instead of connecting to more remote magnetic network at the outer end of the moat flow. Title: Spectro-Polarimetric Imaging Reveals Helical Magnetic Fields in Solar Prominence Feet Authors: Martínez González, M. J.; Manso Sainz, R.; Asensio Ramos, A.; Beck, C.; de la Cruz Rodríguez, J.; Díaz, A. J. Bibcode: 2015ApJ...802....3M Altcode: 2015arXiv150103295M Solar prominences are clouds of cool plasma levitating above the solar surface and insulated from the million-degree corona by magnetic fields. They form in regions of complex magnetic topology, characterized by non-potential fields, which can evolve abruptly, disintegrating the prominence and ejecting magnetized material into the heliosphere. However, their physics is not yet fully understood because mapping such complex magnetic configurations and their evolution is extremely challenging, and must often be guessed by proxy from photometric observations. Using state-of-the-art spectro-polarimetric data, we reconstruct the structure of the magnetic field in a prominence. We find that prominence feet harbor helical magnetic fields connecting the prominence to the solar surface below. Title: Fast Inversion of Solar Ca II Spectra Authors: Beck, C.; Choudhary, D. P.; Rezaei, R.; Louis, R. E. Bibcode: 2015ApJ...798..100B Altcode: 2014arXiv1410.8451B We present a fast (Lt1 s per profile) inversion code for solar Ca II lines. The code uses an archive of spectra that are synthesized prior to the inversion under the assumption of local thermodynamic equilibrium (LTE). We show that it can be successfully applied to spectrograph data or more sparsely sampled spectra from two-dimensional spectrometers. From a comparison to a non-LTE inversion of the same set of spectra, we derive a first-order non-LTE correction to the temperature stratifications derived in the LTE approach. The correction factor is close to unity up to log τ ~ -3 and increases to values of 2.5 and 4 at log τ = -6 in the quiet Sun and the umbra, respectively. Title: The relation between umbral magnetic field strength and area density of umbral dots Authors: Ferguson, Sierra; Beck, Christian Bibcode: 2015AAS...22513704F Altcode: Interiors of sunspots are modeled either as a monolithic block of magnetic flux or groups of flux bundles. Umbral dots (UDs) - small bright grains inside the dark umbra - are harder to explain in the monolithic model, but they fit into the group of flux bundle model as field-free intrusions that push magnetic field lines aside due to their convective nature. The goal of this project was to determine if there is a relationship between the UD density and the area of the umbra as well as the magnetic field strength of the umbra. Routines were developed in IDL that located the UDs and then would find the respective areas and magnetic field strength of the umbrae. These routines can be applied to both numerical simulations and observational data. Our findings show that there is an indication of correlation between the UD density and the umbral area, but there is no clear indication of a correlation with the UD density and the magnetic field strength. The simulations show a significantly larger UD density than the observations This work is carried out through the National Solar Observatory Summer Research Assistantship (SRA) Program. The National Solar Observatory is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation. Title: Three Dimensional Chromospheric Temperature Structure of Sunspot Authors: Choudhary, D. P.; Beck, C.; Rezaei, R. Bibcode: 2014AGUFMSH41B4132C Altcode: We have observed sunspots using the Spectropolarimeter for infrared and optical wavelength ranges at the Dunn Solar Telescope during 29 July to 4 August 2013. The data consists of full Stokes profiles in the Ca II 854.2 nm and Fe I 1.56 micron lines. The inversion of these Stokes spectra provides the magnetic, thermal and velocity structure at photospheric and chromospheric heights of sunspots. In this contribution, we present the results on the 3D thermal structure in the super-penumbral canopy of a well rounded sunspot, derived by a novel approach for the inversion of Ca II IR spectra. Tracing individual fibrils in the super-penumbral canopy, we find that about half of them form only short loops of a a few Mm length that return to the photosphere in the close surroundings of the sunspot instead of connecting to more remote magnetic network at the outer end of the moat flow. Title: Anomalous flows in a sunspot penumbra Authors: Louis, Rohan E.; Beck, Christian; Mathew, Shibu K.; Venkatakrishnan, P. Bibcode: 2014A&A...570A..92L Altcode: 2014arXiv1408.6690L Context. The photospheric Evershed flow is a distinct characteristic of penumbrae that is closely associated with the photometric and magnetic structure of sunspots.
Aims: We analyse the properties of an anomalous flow in the photosphere in a sunspot penumbra and compare it with those of the regular Evershed flow.
Methods: High-resolution spectropolarimetric observations of active region NOAA 11271 were obtained with the spectro-polarimeter (SP) on board Hinode. We used the magnetic field properties derived by a Milne-Eddington inversion. In addition, we used Ca II H images obtained by the broad-band filter instrument to study the low chromospheric response to the photospheric structure and Dopplergrams from the Helioseismic and Magnetic Imager to follow the evolution of the photospheric flows.
Results: We detect a blueshifted feature that appeared on the limb-side penumbra of a sunspot and that was present intermittently during the next two hours. It exhibited a maximum blueshift of 1.6 km s-1, an area of 5.2 arcsec2, and a maximum uninterrupted lifetime of 1 h. The blueshifted feature, when present, lies parallel to redshifts. Both blue- and redshifts flank a highly inclined or horizontal magnetic structure that is radially oriented in the penumbra. The low-cadence SP maps reveal changes in size, radial position in the penumbra, and line-of-sight (LOS) velocity of the blueshifted feature, from one scan to the next. There was an increase of nearly 500 G in the field strength with the onset of the blueshifts, particularly when the LOS velocity in the feature exceeded 1.5 km s-1. There was only a marginal reduction in the field inclination of about 10° with the increase in blueshifts. In the chromosphere, intense, arc-shaped brightenings were observed close to the location of the photospheric blueshifts, which extend from the edge of the umbral core to the penumbra-quiet Sun boundary. The intensity of these brightenings exceeds the background intensity by 30% to 65% with the strongest and largest brightenings observed about 30 min after the strongest blueshifts were detected at the photosphere. The close spatial proximity of the two phenomenon strongly suggests a causal relationship.
Conclusions: The blueshifted feature represents plasma motion that could be related to a magnetic structure that rises in the solar atmosphere and subsequently reconnects with the ambient chromospheric magnetic field of the sunspot or an inverse Evershed flow, which would be unique in the photosphere. This transient phenomena is presumably related to the dynamic stability of the sunspot because the corresponding umbral core separated two days later at the location of the blueshifts and fragmented subsequently. Title: Small-scale chromospheric jets above a sunspot light bridge Authors: Louis, Rohan E.; Beck, Christian; Ichimoto, Kiyoshi Bibcode: 2014A&A...567A..96L Altcode: 2014arXiv1406.0103L Context. The chromosphere above sunspot umbrae and penumbrae shows several different types of fast dynamic events such as running penumbral waves, umbral flashes, and penumbral microjets.
Aims: The aim of this paper is to identify the physical driver responsible for the dynamic and small-scale chromospheric jets above a sunspot light bridge.
Methods: High-resolution broadband filtergrams of active region NOAA 11271 in Ca ii H and G band were obtained with the Solar Optical Telescope on board Hinode. We identified the jets in the Ca ii H images using a semi-automatic routine and determined their length and orientation. We applied local correlation tracking (LCT) to the G-band images to obtain the photospheric horizontal velocity field. The magnetic field topology was derived from a Milne-Eddington inversion of a simultaneous scan with the Spectropolarimeter.
Results: The chromospheric jets consist of a bright, triangular-shaped blob that lies on the light bridge, while the apex of this blob extends into a spike-like structure that is bright against the dark umbral background. Most of the jets have apparent lengths of less than 1000 km and about 30% of the jets have lengths between 1000-1600 km. The jets are oriented within ±35° to the normal of the spine of the light bridge. Most of them are clustered near the central part of the light bridge within a 2'' area. The jets are seen to move rapidly along the light bridge and many of them cannot be identified in successive images taken with a 2 min cadence. The jets are primarily located on one side of the light bridge and are directed into the umbral core. The Stokes profiles at or close to the location of the blobs on the LB exhibit both a significant net circular polarization and multiple components, including opposite-polarity lobes. The magnetic field diverges from the light bridge towards the umbral cores that it separates. The LCT reveals that in the photosphere there is a predominantly uni-directional flow with speeds of 100-150 m s-1 along the light bridge. This unidirectional flow is interrupted by a patch of weak or very small motions on the light bridge which also moves along the light bridge.
Conclusions: The dynamic short-lived chromospheric jets above the LB seem to be guided by the magnetic field lines. Reconnection events are a likely trigger for such phenomenon since they occur at locations where the magnetic field changes orientation sharply and where we also observe isolated patches of opposite-polarity magnetic components. We find no clear relation between the jets and the photospheric flow pattern. Title: A Three-dimensional View of the Thermal Structure in a Super-penumbral Canopy Authors: Beck, C.; Choudhary, D. P.; Rezaei, R. Bibcode: 2014ApJ...788..183B Altcode: 2014arXiv1405.1473B We investigate the three-dimensional (3D) thermal topology in a super-penumbral canopy of an active region (AR). We derive temperature stratifications in the AR by an inversion of the Ca II IR line at 854.2 nm, assuming local thermal equilibrium. We find that about half of the radially oriented fibrils in the super-penumbral canopy form short, low-lying (h < 1 Mm) loops in the 3D temperature cube. These closed loops connect from bright grains in or close to the penumbra to the photosphere a few Mms away from the sunspot. The other half of the fibrils monotonically rise with distance from the sunspot. Many of the fibrils show a central dark core and two lateral brightenings in line-core intensity images. The corresponding velocity image shows fibrils that are as wide as the fibrils seen in intensity without a lateral substructure. Additionally, we study a feature from a different class of structures without prominent mass flows. Its 3D topology is formed by two parallel, closed loops that connect patches of opposite polarity. We present evidence that the inverse Evershed flow into the sunspot in the lower chromosphere is the consequence of siphon flows along short loops that connect photospheric foot points. The dark-cored structure of the chromospheric fibrils cannot have a convective origin because of their location above regular granulation. The dark core most likely results from an opacity difference between the central axis and the lateral edges caused by the significant flow speed along the fibrils. Title: A Spectro-polarimetric Analysis of Sunspot Umbrae Authors: Watson, Fraser; Tritschler, Alexandra; Penn, Matthew J.; Beck, Christian; Livingston, William; Martinez Pillet, Valentin Bibcode: 2014AAS...22411202W Altcode: The recent quiet solar cycle has invited new questions as to the nature of the solar magnetic field and how it changes over time. To investigate this, we use the National Solar Observatory’s McMath-Pierce Solar Telescope Facility (McMP) and Dunn Solar Telescope (DST) to compare measurements of sunspots from five active regions observed in 2013. Both BABO at the McMP and FIRS at the DST were used to provide spectra of the Fe 1564.8nm line, which is affected by the presence of magnetic fields. The magnetic field is derived from Zeeman splitting in Stokes-I by BABO, and by inversion of the Stokes parameters from FIRS data allowing for comparisons of sunspot properties between the two instruments.. We present the first results from this study including the magnetic fields in sunspot umbrae from five active regions measured simultaneously by BABO and FIRS. Title: Near-infrared spectropolarimetry of a δ-spot Authors: Balthasar, H.; Beck, C.; Louis, R. E.; Verma, M.; Denker, C. Bibcode: 2014A&A...562L...6B Altcode: 2014arXiv1401.4386B Sunspots harboring umbrae of both magnetic polarities within a common penumbra (δ-spots) are often but not always related to flares. We present first near-infrared observations (Fe i λ1078.3 nm and Si i λ1078.6 nm spectra) obtained with the Tenerife Infrared Polarimeter at the Vacuum Tower Telescope in Tenerife on 2012 June 17, which afford accurate and sensitive diagnostics to scrutinize the complex fields along the magnetic neutral line of a δ-spot within active region NOAA 11504. We examined the vector magnetic field, line-of-sight (LOS) velocities, and horizontal proper motions of this rather inactive δ-spot. We find a smooth transition of the magnetic vector field from the main umbra to that of opposite polarity (δ-umbra), but a discontinuity of the horizontal magnetic field at some distance from the δ-umbra on the polarity inversion line. The magnetic field decreases faster with height by a factor of two above the δ-umbra. The latter is surrounded by its own Evershed flow. The Evershed flow coming from the main umbra ends at a line dividing the spot into two parts. This line is marked by the occurrence of central emission in the Ca iiλ854.2 nm line. Along this line, high chromospheric LOS-velocities of both signs appear. We detect a shear flow within the horizontal flux transport velocities parallel to the dividing line. Title: Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations (Corrigendum) Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.; Rezaei, R. Bibcode: 2013A&A...559C...1B Altcode: No abstract at ADS Title: Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations Authors: Beck, C.; Fabbian, D.; Moreno-Insertis, F.; Puschmann, K. G.; Rezaei, R. Bibcode: 2013A&A...557A.109B Altcode: 2013arXiv1306.6093B Context. Numerical three-dimensional (3D) radiative (magneto-)hydrodynamical [(M)HD] simulations of solar convection are nowadays used to understand the physical properties of the solar photosphere and convective envelope, and, in particular, to determine the Sun's photospheric chemical abundances. To validate this approach, it is important to check that no excessive thermodynamic fluctuations arise as a consequence of the partially incomplete treatment of radiative transfer causing radiative damping that is too modest.
Aims: We investigate the realism of the thermodynamics in recent state-of-the-art 3D convection simulations of the solar atmosphere carried out with the Stagger code.
Methods: We compared the characteristic properties of several Fe i lines (557.6 nm, 630 nm, 1565 nm) and one Si i line at 1082.7 nm in solar disc-centre observations of different spatial resolution with spectra synthesized from 3D convection simulations. The observations were taken with ground-based (Echelle spectrograph, Göttingen Fabry-Pérot Interferometer (GFPI), POlarimetric LIttrow Spectrograph, Tenerife Infrared Polarimeter, all at the Vacuum Tower Telescope on Tenerife) and space-based instruments (Hinode/Spectropolarimeter). We degraded the synthetic spectra to the spatial resolution of the observations, based on the distribution of the continuum intensity Ic. We estimated the spectral degradation to be applied to the simulation results by comparing atlas spectra with averaged observed spectra. In addition to deriving a set of line parameters directly from the intensity profiles, we used the SIR (Stokes Inversion based on Response functions) code to invert the spectra.
Results: The spatial degradation kernels yield a similar generic spatial stray-light contamination of about 30% for all instruments. The spectral stray light inside the different spectrometers is found to be between 2% and 20%. Most of the line parameters from the observational data are matched by the degraded HD simulation spectra. The inversions predict a macroturbulent velocity vmac below 10 m s-1 for the HD simulation spectra at full spatial resolution, whereas they yield vmac ≲ 1000 m s-1 at a spatial resolution of 0.″3. The temperature fluctuations in the inversion of the degraded HD simulation spectra do not exceed those from the observational data (of the order of 100-200 K rms for -2 ⪉ log τ500 nm ⪉ -0.5). The comparison of line parameters in spatially averaged profiles with the averaged values of line parameters in spatially resolved profiles indicates a significant change in (average) line properties on a spatial scale between 0.″13 and 0.″3.
Conclusions: Up to a spatial resolution of 0.″3 (GFPI spectra), we find no indications of excessive thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that simulations without spatial degradation contain fully realistic thermodynamic fluctuations requires observations at even higher spatial resolution (i.e. <0.″13).

Appendices A and B are available in electronic form at http://www.aanda.org Title: GREGOR Fabry-Pérot interferometer and its companion the blue imaging solar spectrometer Authors: Puschmann, Klaus G.; Denker, Carsten; Balthasar, Horst; Louis, Rohan E.; Popow, Emil; Woche, Manfred; Beck, Christian; Seelemann, Thomas; Volkmer, Reiner Bibcode: 2013OptEn..52h1606P Altcode: 2013arXiv1302.7157P The GREGOR Fabry-Pérot Interferometer (GFPI) is one of three first-light instruments of the German 1.5-m GREGOR solar telescope at the Observatorio del Teide, Tenerife, Spain. The GFPI allows fast narrow-band imaging and postfactum image restoration. The retrieved physical parameters will be a fundamental building block for understanding the dynamic sun and its magnetic field at spatial scales down to ∼50 km on the solar surface. The GFPI is a tunable dual-etalon system in a collimated mounting. It is designed for spectrometric and spectropolarimetric observations between 530-860 nm and 580-660 nm, respectively, and possesses a theoretical spectral resolution of R≈250,000. Large-format, high-cadence charged coupled device detectors with sophisticated computer hard- and software enable the scanning of spectral lines in time-spans equivalent to the evolution time of solar features. The field-of-view (FOV) of 50″×38″ covers a significant fraction of the typical area of active regions in the spectroscopic mode. In case of Stokes-vector spectropolarimetry, the FOV reduces to 25″×38″. The main characteristics of the GFPI including advanced and automated calibration and observing procedures are presented. Improvements in the optical design of the instrument are discussed and first observational results are shown. Finally, the first concrete ideas for the integration of a second FPI, the blue imaging solar spectrometer, are laid out, which will explore the blue spectral region below 530 nm. Title: Can spicules be detected at disc centre in broad-band Ca ii H filter imaging data? Authors: Beck, C.; Rezaei, R.; Puschmann, K. G. Bibcode: 2013A&A...556A.127B Altcode: 2013arXiv1306.5199B Context. Recently, a possible identification of type II spicules in broad-band (full-width at half-maximum (FWHM) of ~0.3 nm) filter imaging data in Ca ii H on the solar disc was reported.
Aims: We estimate the formation height range contributing to broad-band and narrow-band filter imaging data in Ca ii H to investigate whether spicules can be detected in such observations at the centre of the solar disc.
Methods: We applied spectral filters of FWHMs from 0.03 nm to 1 nm to observed Ca ii H line profiles to simulate Ca imaging data. We used observations across the limb to estimate the relative intensity contributions of off-limb and on-disc structures. We compared the synthetic Ca filter imaging data with intensity maps of Ca spectra at different wavelengths and temperature maps at different optical depths obtained by an inversion of these spectra. In addition, we determined the intensity response function for the wavelengths covered by the filters of different FWHM.
Results: In broad-band (FWHM = 0.3 nm) Ca imaging data, the intensity emitted off the solar limb is about 5% of the intensity at disc centre. For a 0.3-nm-wide filter centred at the Ca ii H line core, up to about one third of the off-limb intensity comes from emission in Hɛ. On the disc, only about 10 to 15% of the intensity transmitted through a broad-band filter comes from the line-core region between the H1 minima (396.824 to 396.874 nm). No traces of elongated fibrillar structures are visible in the synthetic Ca broad-band imaging data at disc centre, in contrast to the line-core images of the Ca spectra. The intensity-weighted response function for a 0.3-nm-wide filter centred at the Ca ii H line core peaks at about log τ ~ -2 (z ~ 200 km). Relative contributions from atmospheric layers above 800 km are about 10%. The inversion results suggest that the slightly enhanced emission around the photospheric magnetic network in broad-band Ca imaging data is caused by a thermal canopy at a height of about 600 km.
Conclusions: Broad-band (~0.3 nm) Ca ii H imaging data do not trace upper chromospheric structures such as spicules in observations at the solar disc because of the too small relative contribution of the line core to the total wavelength-integrated filter intensity. The faint haze around network elements in broad-band Ca imaging observations at disc centre presumably traces thermal canopies in the vicinity of magnetic flux concentrations instead.

Appendix A is available in electronic form at http://www.aanda.org Title: First Results from the EUNIS 2013 Sounding Rocket Campaign Authors: Daw, Adrian N.; Rabin, D. M.; Brosius, J. W.; Haas, J. P.; Plummer, T.; Cauzzi, G.; Reardon, K. P.; Beck, C. Bibcode: 2013SPD....4410501D Altcode: The Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) sounding rocket launched 23 April 2013 at 17:30 UT, as part of a campaign including co-ordinated observations with the Dunn Solar Telescope/IBIS, Hinode/EIS, SoHO/CDS, RHESSI and SDO. EUNIS obtained the highest-resolution observations of the solar spectrum from 52-63 nm observed to date, as well as observations with the previously-flown waveband from 30-37 nm. The broad spectral coverage of the EUV observations includes emission lines of ionization stages from He I to Fe XIX, and thus a wide temperature range of 0.025 to 10 MK. Absolute radiometric calibration of EUNIS provides underflight calibration of CDS, EIS and AIA. Spectra were obtained with a 1.3 s cadence as the 660-arcsec long slit was rastered across two different regions. The observations captured a B-class flare in active region NOAA 11726 as well as active regions 11723, 11724, off-limb, quiet sun and a coronal hole. We discuss first results from anaysis of this rich and extensive data set. Title: The energy of waves in the photosphere and lower chromosphere. IV. Inversion results of Ca II H spectra Authors: Beck, C.; Rezaei, R.; Puschmann, K. G. Bibcode: 2013A&A...553A..73B Altcode: 2013arXiv1302.6936B Context. Most semi-empirical static one-dimensional (1D) models of the solar atmosphere in the magnetically quiet Sun (QS) predict an increase in temperature at chromospheric layers. Numerical simulations of the solar chromosphere with a variable degree of sophistication, i.e. from 1D to three-dimensional (3D) simulations; assuming local thermal equilibrium (LTE) or non-LTE (NLTE), on the other hand, only yielded an increase in the brightness temperature without any stationary increase in the gas temperature.
Aims: We investigate the thermal structure in the solar chromosphere as derived from an LTE inversion of observed Ca ii H spectra in QS and active regions (ARs).
Methods: We applied an inversion strategy based on the SIR (Stokes inversion by response functions) code to Ca ii H spectra to obtain 1D temperature stratifications. We investigated the temperature stratifications on differences between magnetic and field-free regions in the QS, and on differences between QS and ARs. We determined the energy content of individual calcium bright grains (BGs) as specific candidates of chromospheric heating events. We compared observed with synthetic NLTE spectra to estimate the significance of the LTE inversion results.
Results: The fluctuations of observed intensities yield a variable temperature structure with spatio-temporal rms fluctuations below 100 K in the photosphere and between 200 and 300 K in the QS chromosphere. The average temperature stratification in the QS does not exhibit a clear chromospheric temperature rise, unlike the AR case. We find a characteristic energy content of about 7 × 1018 J for BGs that repeat with a cadence of about 160 s. The precursors of BGs have a vertical extent of about 200 km and a horizontal extent of about 1 Mm. The comparison of observed with synthetic NLTE profiles partly confirms the results of the LTE inversion that the solar chromosphere in the QS oscillates between an atmosphere in radiative equilibrium and one with a moderate chromospheric temperature rise. Two-dimensional x - z temperature maps exhibit nearly horizontal canopy-like structures with an extent of a few Mm around photospheric magnetic field concentrations at a height of about 600 km.
Conclusions: The large difference between QS regions and ARs and the better match of AR and NLTE reference spectra suggest that magnetic heating processes are more important than commonly assumed. The temperature fluctuations in QS derived by the LTE inversion do not suffice on average to maintain a stationary chromospheric temperature rise. The spatially and vertically resolved information on the temperature structure allows one to investigate in detail the topology and evolution of the thermal structure in the lower solar atmosphere.

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

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

The animation is available in electronic form at http://www.aanda.org Title: The GREGOR Fabry-Pérot Interferometer Authors: Puschmann, K. G.; Denker, C.; Kneer, F.; Al Erdogan, N.; Balthasar, H.; Bauer, S. M.; Beck, C.; Bello González, N.; Collados, M.; Hahn, T.; Hirzberger, J.; Hofmann, A.; Louis, R. E.; Nicklas, H.; Okunev, O.; Martínez Pillet, V.; Popow, E.; Seelemann, T.; Volkmer, R.; Wittmann, A. D.; Woche, M. Bibcode: 2012AN....333..880P Altcode: 2012arXiv1210.2921P The GREGOR Fabry-Pérot Interferometer (GFPI) is one of three first-light instruments of the German 1.5-meter GREGOR solar telescope at the Observatorio del Teide, Tenerife, Spain. The GFPI uses two tunable etalons in collimated mounting. Thanks to its large-format, high-cadence CCD detectors with sophisticated computer hard- and software it is capable of scanning spectral lines with a cadence that is sufficient to capture the dynamic evolution of the solar atmosphere. The field-of-view (FOV) of 50 arcsec × 38 arcsec is well suited for quiet Sun and sunspot observations. However, in the vector spectropolarimetric mode the FOV reduces to 25 arcsec × 38 arcsec. The spectral coverage in the spectroscopic mode extends from 530-860 nm with a theoretical spectral resolution of R ≈ 250,000, whereas in the vector spectropolarimetric mode the wavelength range is at present limited to 580-660 nm. The combination of fast narrow-band imaging and post-factum image restoration has the potential for discovery science concerning the dynamic Sun and its magnetic field at spatial scales down to ∼50 km on the solar surface. Title: The GREGOR Fabry-Perot interferometer: status report and prospects Authors: Puschmann, Klaus G.; Balthasar, Horst; Beck, Christian; Louis, Rohan E.; Popow, Emil; Seelemann, Thomas; Volkmer, Reiner; Woche, Manfred; Denker, Carsten Bibcode: 2012SPIE.8446E..79P Altcode: 2012arXiv1207.2084P The GREGOR Fabry-Ṕerot Interferometer (GFPI) is one of three first-light instruments of the German 1.5-meter GREGOR solar telescope at the Observatorio del Teide, Tenerife, Spain. The GFPI allows fast narrow-band imaging and post-factum image restoration. The retrieved physical parameters will be a fundamental building block for understanding the dynamic Sun and its magnetic field at spatial scales down to 50 km on the solar surface. The GFPI is a tunable dual-etalon system in a collimated mounting. It is designed for spectropolarimetric observations over the wavelength range from 530-860 nm with a theoretical spectral resolution of R ≍ 250,000. The GFPI is equipped with a full-Stokes polarimeter. Large-format, high-cadence CCD detectors with powerful computer hard- and software enable the scanning of spectral lines in time spans equivalent to the evolution time of solar features. The field-of-view of 50''×38'' covers a significant fraction of the typical area of active regions. We present the main characteristics of the GFPI including advanced and automated calibration and observing procedures. We discuss improvements in the optical design of the instrument and show first observational results. Finally, we lay out first concrete ideas for the integration of a second FPI, the Blue Imaging Solar Spectrometer, which will explore the blue spectral region below 530 nm. Title: The energy of waves in the photosphere and lower chromosphere. II. Intensity statistics Authors: Beck, C.; Rezaei, R.; Puschmann, K. G. Bibcode: 2012A&A...544A..46B Altcode: 2012arXiv1206.1759B Context. The energy source powering the solar chromosphere is still undetermined, but leaves its traces in observed intensities.
Aims: We investigate the statistics of the intensity distributions as a function of the wavelength for Ca ii H and the Ca ii IR line at 854.2 nm to estimate the energy content in the observed intensity fluctuations.
Methods: We derived the intensity variations at different heights of the solar atmosphere, as traced by the line wings and line cores of the two spectral lines. We converted the observed intensities to absolute energy units employing reference profiles calculated in non-local thermal equilibrium (NLTE). We also converted the intensity fluctuations to corresponding brightness temperatures assuming LTE.
Results: The root-mean-square (rms) fluctuations of the emitted intensity are about 0.6 (1.2) W m-2 ster-1 pm-1 near the core of the Ca ii IR line at 854.2 nm (Ca ii H), corresponding to relative intensity fluctuations of about 20% (30%). For the line wing, we find rms values of about 0.3 W m-2 ster-1 pm-1 for both lines, corresponding to relative fluctuations below 5%. The relative rms values show a local minimum for wavelengths forming at a height of about 130 km, but otherwise increase smoothly from the wing to the core, i.e., from photosphere to chromosphere. The corresponding rms brightness temperature fluctuations are below 100 K for the photosphere and up to 500 K in the chromosphere. The skewness of the intensity distributions is close to zero in the outer line wing and positive throughout the rest of the line spectrum, owing to the frequent occurrence of high-intensity events. The skewness shows a pronounced local maximum at locations with photospheric magnetic fields for wavelengths in-between those of the line wing and the line core (z ≈ 150-300 km), and a global maximum at the very core (z ≈ 1000 km) for both magnetic and field-free locations.
Conclusions: The energy content of the intensity fluctuations is insufficient to create a chromospheric temperature rise that would be similar to the one in most reference models of the solar atmosphere. The increase in the rms fluctuations with height indicates the presence of upwardly propagating acoustic waves of increasing oscillation amplitude. The intensity and temperature variations indicate that there is a clear increase in dynamical activity from photosphere towards the chromosphere, but the variations fall short of the magnitude predicted by fully dynamical chromospheric models by a factor of about five. The enhanced skewness between the photosphere and lower solar chromosphere at magnetic locations is indicative of a mechanism that acts solely on magnetized plasma.

Appendices are available in electronic form at http://www.aanda.org Title: Variation in sunspot properties between 1999 and 2011 as observed with the Tenerife Infrared Polarimeter Authors: Rezaei, R.; Beck, C.; Schmidt, W. Bibcode: 2012A&A...541A..60R Altcode: 2012arXiv1203.1444R
Aims: We study the variation in the magnetic field strength and the umbral intensity of sunspots during the declining phase of the solar cycle No. 23 and in the beginning of cycle No. 24.
Methods: We analyze a sample of 183 sunspots observed from 1999 until 2011 with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT). The magnetic field strength is derived from the Zeeman splitting of the Stokes-V signal in one near-infrared spectral line, either Fe i 1564.8 nm, Fe i 1089.6 nm, or Si i 1082.7 nm. This avoids the effects of the unpolarized stray light from the field-free quiet Sun surroundings that can affect the splitting seen in Stokes-I in the umbra. The minimum umbral continuum intensity and umbral area are also measured.
Results: We find that there is a systematic trend for sunspots in the late stage of the solar cycle No. 23 to be weaker, i.e., to have a smaller maximum magnetic field strength than those at the start of the cycle. The decrease in the field strength with time of about 94 Gyr-1 is well beyond the statistical fluctuations that would be expected because of the larger number of sunspots close to cycle maximum (14 Gyr-1). In the same time interval, the continuum intensity of the umbra increases with a rate of 1.3 (±0.4)% of Ic yr-1, while the umbral area does not show any trend above the statistical variance. Sunspots in the new cycle No. 24 show higher field strengths and lower continuum intensities than those at the end of cycle No. 23, interrupting the trend.
Conclusions: Sunspots have an intrinsically weaker field strength and brighter umbrae at the late stages of solar cycles compared to their initial stages, without any significant change in their area. The abrupt increase in field strength in sunspots of the new cycle suggests that the cyclic variations are dominating over any long-term trend that continues across cycles. We find a slight decrease in field strength and an increase in intensity as a long-term trend across the cycles. Title: Siphon flow in a cool magnetic loop Authors: Bethge, C.; Beck, C.; Peter, H.; Lagg, A. Bibcode: 2012A&A...537A.130B Altcode: 2011arXiv1111.5564B Context. Siphon flows that are driven by a gas pressure difference between two photospheric footpoints of different magnetic field strength connected by magnetic field lines are a well-studied phenomenon in theory, but observational evidence is scarce. Aims. We investigate the properties of a structure in the solar chromosphere in an active region to find out whether the feature is consistent with a siphon flow in a magnetic loop filled with chromospheric material.

Methods. We derived the line-of-sight (LOS) velocity of several photospheric spectral lines and two chromospheric spectral lines, Ca II H 3968.5 *Aring; and He I 10830 Å, in spectropolarimetric observations of NOAA 10978 done with the Tenerife Infrared Polarimeter (TIP-II) and the POlarimetric LIttrow Spectrograph (POLIS). The structure can be clearly traced in the LOS velocity maps and the absorption depth of He I. The magnetic field configuration in the photosphere is inferred directly from the observed Stokes parameters and from inversions with the HELIX+ code. Data from the full-disk Chromospheric Telescope (ChroTel) in He I in intensity and LOS velocity are used for tracking the temporal evolution of the flow, along with TRACE Fe IX/X 171 Å data for additional information about coronal regions related to the structure under investigation.

Results. The inner end of the structure is located in the penumbra of a sunspot. It shows downflows whose strength decreases with decreasing height in the atmosphere. The flow velocity in He I falls abruptly from above 40 km s-1 to about zero further into the penumbra. A slight increase of emission is seen in the Ca II H spectra at the endpoint. At the outer end of the structure, the photospheric lines that form higher up in the atmosphere show upflows that accelerate with height. The polarization signal near the outer end shows a polarity opposite to that of the sunspot, the magnetic field strength of 580 G is roughly half as large as at the inner end. The structure exists for about 90 min. Its appearance is preceeded by a brightening in its middle in the coronal TRACE data.

Conclusions. The observed flows match theoretical predictions of chromospheric and coronal siphon flows, with accelerating upflowing plasma at one footpoint with low field strength and decelerating downflowing plasma at the other end. A tube shock at the inner end is probable, but the evidence is not conclusive. The TRACE data suggest that the structure forms because of a reorganization of field lines after a reconnection event. Title: Stray-light contamination and spatial deconvolution of slit-spectrograph observations Authors: Beck, C.; Rezaei, R.; Fabbian, D. Bibcode: 2011A&A...535A.129B Altcode: 2011arXiv1109.2421B Context. Stray light caused by scattering on optical surfaces and in the Earth's atmosphere degrades the spatial resolution of observations. Whereas post-facto reconstruction techniques are common for 2D imaging and spectroscopy, similar options for slit-spectrograph data are rarely applied.
Aims: We study the contribution of stray light to the two channels of the POlarimetric LIttrow Spectrograph (POLIS) at 396 nm and 630 nm as an example of a slit-spectrograph instrument. We test the performance of different methods of stray-light correction and spatial deconvolution to improve the spatial resolution post-facto.
Methods: We model the stray light as having two components: a spectrally dispersed component and a "parasitic" component of spectrally undispersed light caused by scattering inside the spectrograph. We used several measurements to estimate the two contributions: a) observations with a (partly) blocked field of view (FOV); b) a convolution of the FTS spectral atlas; c) imaging of the spider mounting in the pupil plane; d) umbral profiles; and e) spurious polarization signal in telluric spectral lines. The measurements with a partly blocked FOV in the focal plane allowed us to estimate the spatial point spread function (PSF) of POLIS and the main spectrograph of the German Vacuum Tower Telescope (VTT). We then used the obtained PSF for a deconvolution of both spectroscopic and spectropolarimetric data and investigated the effect on the spectra.
Results: The parasitic contribution can be directly and accurately determined for POLIS, amounting to about 5% (0.3%) of the (continuum) intensity at 396 nm (630 nm). The spectrally dispersed stray light is less accessible because of its many contributing sources. We estimate a lower limit of about 10% across the full FOV for the dispersed stray light from umbral profiles. In quiet Sun regions, the stray-light level from the close surroundings (d < 2'') of a given spatial point is about 20%. The stray light reduces to below 2% at a distance of 20'' from a lit area for both POLIS and the main spectrograph. The spatial deconvolution using the PSF obtained improves the spatial resolution and increases the contrast, with a minor amplification of noise.
Conclusions: A two-component model of the stray-light contributions seems to be sufficient for a basic correction of observed spectra. The instrumental PSF obtained can be used to model the off-limb stray light, to determine the stray-light contamination accurately for observation targets with large spatial intensity gradients such as sunspots, and also to improve the spatial resolution of observations post-facto. Title: The Chromospheric Telescope Authors: Bethge, C.; Peter, H.; Kentischer, T. J.; Halbgewachs, C.; Elmore, D. F.; Beck, C. Bibcode: 2011A&A...534A.105B Altcode: 2011arXiv1108.4880B
Aims: We introduce the Chromospheric Telescope (ChroTel) at the Observatorio del Teide in Izaña on Tenerife as a new multi-wavelength imaging telescope for full-disk synoptic observations of the solar chromosphere. We describe the design of the instrument and summarize its performance during the first one and a half years of operation. We present a method to derive line-of-sight velocity maps of the full solar disk from filtergrams taken in and near the He i infrared line at 10 830 Å.
Methods: ChroTel observations are conducted using Lyot-type filters for the chromospheric lines of Ca ii K, Hα, and He i 10 830 Å. The instrument operates autonomically and gathers imaging data in all three channels with a cadence of down to one minute. The use of a tunable filter for the He i line allows us to determine line-shifts by calibrating the line-of-sight velocity maps derived from the filtergram intensities with spectrographic data from the Tenerife Infrared Polarimeter at high spatial and spectral resolution.
Results: The robotic operation and automated data reduction have proven to operate reliably in the first one and and half years. The achieved spatial resolution of the data is close to the theoretical limit of 2 arcsec in Hα and Ca ii K and 3 arcsec in He i. Line-of-sight velocities in He i can be determined with a precision of better than 3-4 km s-1 when co-temporal spectrographic maps are available for calibration.
Conclusions: ChroTel offers a unique combination of imaging in the most important chromospheric lines, along with the possibility to determine line-of-sight velocities in one of the lines. This is of interest for scientific investigations of large-scale structures in the solar chromosphere, as well as for context imaging of high-resolution solar observations. Title: Spectroscopy at the solar limb. I. Average off-limb profiles and Doppler shifts of Ca II H Authors: Beck, C. A. R.; Rezaei, R. Bibcode: 2011A&A...531A.173B Altcode: 2011arXiv1106.0646B
Aims: We present constraints on the thermodynamical structure of the chromosphere from ground-based observations of the Ca ii H line profile near and off the solar limb.
Methods: We obtained a slit-spectrograph data set of the Ca ii H line with a high signal-to-noise ratio in a field of view extending 20'' across the limb. We analyzed the spectra for the characteristic properties of average and individual off-limb spectra. We used various tracers of the Doppler shifts, such as the location of the absorption core, the ratio of the two emission peaks H2V and H2R, and intensity images at a fixed wavelength.
Results: The average off-limb profiles show a smooth variation with increasing limb distance. The line width increases up to a height of about 2 Mm above the limb. The profile shape is fairly symmetric with nearly identical H2V and H2R intensities; at a height of 5 Mm, it changes into a single Gaussian without emission peaks. We find that all off-limb spectra show large Doppler shifts that fluctuate on the smallest resolved spatial scales. The variation is more prominent in cuts parallel to the solar limb than on those perpendicular to it. As far as individual structures can be unequivocally identified at our spatial resolution, we find a specific relation between intensity enhancements and Doppler shifts: elongated brightenings are often flanked all along their extension by velocities in opposite directions.
Conclusions: The average off-limb spectra of Ca ii H present a good opportunity to test static chromospheric atmosphere models because they lack the photospheric contribution that is present in disk-center spectra. We suggest that the observed relation between intensity enhancements and Doppler shifts could be caused by waves propagating along the surfaces of flux tubes: an intrinsic twist of the flux tubes or a wave propagation inclined to the tube axis would cause a helical shape of the Doppler excursions, visible as opposite velocity at the sides of the flux tube. Spectroscopic data allow one to distinguish this from a sausage-mode oscillation where the maximum Doppler shift and the tube axis would coincide.

Appendices are available in electronic form at http://www.andaa.orgThe Data set is available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A173 Title: Two-dimensional solar spectropolarimetry with the KIS/IAA Visible Imaging Polarimeter Authors: Beck, C.; Bellot Rubio, L. R.; Kentischer, T. J.; Tritschler, A.; Del Toro Iniesta, J. C. Bibcode: 2010A&A...520A.115B Altcode: 2010arXiv1007.1153B Context. Spectropolarimetry at high spatial and spectral resolution is a basic tool to characterize the magnetic properties of the solar atmosphere.
Aims: We introduce the KIS/IAA Visible Imaging Polarimeter (VIP), a new post-focus instrument that upgrades the TESOS spectrometer at the German Vacuum Tower Telescope (VTT) into a full vector polarimeter. VIP is a collaboration between the Kiepenheuer Institut für Sonnenphysik (KIS) and the Instituto de Astrofísica de Andalucía (IAA-CSIC).
Methods: We describe the optical setup of VIP, the data acquisition procedure, and the calibration of the spectropolarimetric measurements. We show examples of data taken between 2005 and 2008 to illustrate the potential of the instrument.
Results: VIP is capable of measuring the four Stokes profiles of spectral lines in the range from 420 to 700 nm with a spatial resolution better than 0farcs5. Lines can be sampled at 40 wavelength positions in 60 s, achieving a noise level of about 2 × 10-3 with exposure times of 300 ms and pixel sizes of 0farcs17 × 0farcs17 (2 × 2 binning). The polarization modulation is stable over periods of a few days, ensuring high polarimetric accuracy. The excellent spectral resolution of TESOS allows the use of sophisticated data analysis techniques such as Stokes inversions. One of the first scientific results of VIP presented here is that the ribbon-like magnetic structures of the network are associated with a distinct pattern of net circular polarization away from disk center.
Conclusions: VIP performs spectropolarimetric measurements of solar magnetic fields at a spatial resolution that is only slightly worse than that of the Hinode spectropolarimeter, while providing a 2D field field of view and the possibility to observe up to four spectral regions sequentially with high cadence. VIP can be used as a stand-alone instrument or in combination with other spectropolarimeters and imaging systems of the VTT for extended wavelength coverage. Title: A chromospheric dark-cored fibril in Ca II IR spectra Authors: Beck, C.; Tritschler, A.; Wöger, F. Bibcode: 2010AN....331..574B Altcode: We investigate the thermodynamical and magnetic properties of a ``dark-cored" fibril seen in the chromospheric Ca II IR line at 854.2 nm to determine the physical process behind its appearance. We analyse a time series of spectropolarimetric observations obtained in the Ca II IR line at 854.2 nm and the photospheric Fe I line at 630.25 nm. We simultaneously invert the spectra in both wavelength ranges with the SIR code to obtain the temperature and velocity stratification with height in the solar atmosphere and the magnetic field properties in the photosphere. The structure can be clearly traced in the line-of-sight (LOS) velocity and the temperature maps. It connects from a small pore with kG fields to a region with lower field strength. The flow velocity and the temperature indicate that the height of the structure increases with increasing distance from the inner footpoint. The Stokes V signal of 854.2 nm shows a Doppler-shifted polarization signal with the same displacement as in the intensity profile, indicating that the supersonic flow seen in the LOS velocity is located within magnetized plasma. We conclude that the chromospheric dark-cored fibril traces a siphon flow along magnetic field lines, driven by the gas pressure difference caused by the higher magnetic field strength at the inner footpoint. We suggest that fast flows guided by the magnetic field lead to the appearance of ``dark-cored" fibrils in intensity images. Although the observations included the determination of the polarization signal in the chromospheric Ca II IR line, the signal could not be analysed quantitatively due to the low S/N. Chromospheric polarimetry will thus require telescopes of larger aperture able to collect a sufficient number of photons for a reliable determination of polarization in deep and only weakly polarized spectral lines. Title: Linear wavelength correlation matrices of photospheric and chromospheric spectral lines. I. Observations vs. modeling Authors: Beck, C. A. R.; Rammacher, W. Bibcode: 2010A&A...510A..66B Altcode: 2009arXiv0909.2224B Context. The process that heats the solar chromosphere is a difficult target for observational studies because the assumption of local thermal equilibrium (LTE) is not valid in the upper solar atmosphere, which complicates the analysis of spectra.
Aims: We investigate the linear correlation coefficient between the intensities at different wavelengths in photospheric and chromospheric spectral lines because the correlation can be determined directly for any spectra from observations or modeling. Waves which propagate vertically through the stratified solar atmosphere affect different wavelengths at different times when the contribution functions for each wavelength peak in different layers. This leads to a characteristic pattern of (non-)coherence of the intensity at various wavelengths with respect to each other which carries information on the physical processes.
Methods: We derived the correlation matrices for several photospheric and chromospheric spectral lines from observations. We separated locations with a significant photospheric polarization signal and thus magnetic fields from those without a polarization signal. For comparison with the observations, we calculated correlation matrices for spectra from simplified LTE modeling approaches, 1-D NLTE simulations, and a 3-D MHD simulation run. We applied the correlation method also to temperature maps at different optical depth layers derived from a LTE inversion of Ca II H spectra.
Results: We find that all photospheric spectral lines show a similar pattern: a pronounced asymmetry of the correlation between line core and red or blue wing. The pattern cannot be reproduced with a simulation of the granulation pattern, but with waves that travel upwards through the formation heights of the lines. The correct asymmetry between red and blue wing only appears when a temperature enhancement occurs simultaneously with a downflow velocity in the wave simulation. All chromospheric spectral lines show a more complex pattern. The 1-D NLTE simulations of monochromatic waves produce a correlation matrix that qualitatively matches the observations near the very core of the Ca II H line. The photospheric signature is well reproduced in the correlation matrix derived from the 3-D MHD simulation.
Conclusions: The correlation matrices of observed photospheric and chromospheric spectral lines are highly structured with characteristic and different patterns in every spectral line. The comparison with matrices derived from simulations and simple modeling suggests that the main driver of the detected patterns are upwards propagating waves. Application of the correlation method to 3-D temperature cubes seems to be a promising tool for a detailed comparison of simulation results and observations in future studies. Title: Magnetic loop emergence within a granule Authors: Gömöry, P.; Beck, C.; Balthasar, H.; Rybák, J.; Kučera, A.; Koza, J.; Wöhl, H. Bibcode: 2010A&A...511A..14G Altcode: 2009arXiv0910.4449G
Aims: We investigate the temporal evolution of magnetic flux emerging within a granule in the quiet-Sun internetwork at disk center.
Methods: We combined IR spectropolarimetry of high angular resolution performed in two Fe i lines at 1565 nm with speckle-reconstructed G-band imaging. We determined the magnetic field parameters by a LTE inversion of the full Stokes vector using the SIR code, and followed their evolution in time. To interpret the observations, we created a geometrical model of a rising loop in 3D. The relevant parameters of the loop were matched to the observations where possible. We then synthesized spectra from the 3D model for a comparison to the observations.
Results: We found signatures of magnetic flux emergence within a growing granule. In the early phases, a horizontal magnetic field with a distinct linear polarization signal dominated the emerging flux. Later on, two patches of opposite circular polarization signal appeared symmetrically on either side of the linear polarization patch, indicating a small loop-like structure. The mean magnetic flux density of this loop was roughly 450 G, with a total magnetic flux of around 3 × 1017 Mx. During the ~12 min episode of loop occurrence, the spatial extent of the loop increased from about 1 to 2 arcsec. The middle part of the appearing feature was blueshifted during its occurrence, supporting the scenario of an emerging loop. There is also clear evidence for the interaction of one loop footpoint with a preexisting magnetic structure of opposite polarity. The temporal evolution of the observed spectra is reproduced to first order by the spectra derived from the geometrical model. During the phase of clearest visibility of the loop in the observations, the observed and synthetic spectra match quantitatively.
Conclusions: The observed event can be explained as a case of flux emergence in the shape of a small-scale loop. The fast disappearance of the loop at the end could possibly be due to magnetic reconnection. Title: The energy of waves in the photosphere and lower chromosphere. I. Velocity statistics Authors: Beck, C.; Khomenko, E.; Rezaei, R.; Collados, M. Bibcode: 2009A&A...507..453B Altcode: 2009arXiv0905.1011B Context: Acoustic waves are one of the primary suspects besides magnetic fields for the chromospheric heating process to temperatures above radiative equilibrium (RE).
Aims: We derived the mechanical wave energy as seen in line-core velocities on disc centre to obtain a measure of mechanical energy flux with height for a comparison with the energy requirements in a semi-empirical atmosphere model, the Harvard-Smithsonian reference atmosphere (HSRA).
Methods: We analyzed a 1-hour time series and a large-area map of Ca II H spectra on the traces of propagating waves. We analyzed the velocity statistics of several spectral lines in the wing of Ca II H, and the line-core velocity of Ca II H. We converted the velocity amplitudes into volume (∝ ρ v^2) and mass energy densities (∝ v^2). For comparison, we used the increase of internal energy (∝ R ρ Δ T) necessary to lift a RE atmosphere to the HSRA temperature stratification.
Results: We find that the velocity amplitude grows in agreement with linear wave theory and thus slower with height than predicted from energy conservation. The mechanical energy of the waves above around z ~ 500 km is insufficient to maintain on a long-term average the chromospheric temperature rise in the semi-empirical HSRA model. The intensity variations of the Ca line core (z ~ 1000 km) can, however, be traced back to the velocity variations of the lowermost forming spectral line considered (z ~ 250 km).
Conclusions: The chromospheric intensity, and hence, (radiation) temperature variations are seen to be induced by passing waves originating in the photosphere. The wave energy is found to be insufficient to maintain the temperature stratification of the semi-empirical HSRA model above 500 km. We will in a following paper of this series investigate the energy contained in the intensity variations to see if the semi-empirical model is appropriate for the spectra. Title: The magnetic flux of the quiet Sun internetwork as observed with the Tenerife infrared polarimeter Authors: Beck, C.; Rezaei, R. Bibcode: 2009A&A...502..969B Altcode: 2009arXiv0903.3158B Context: Observations made with the spectropolarimeter onboard the HINODE satellite have detected abundant horizontal magnetic fields in the internetwork quiet Sun.
Aims: We compare the results for the horizontal fields obtained at 630 nm with ground-based observations at 1.56 μm, where the sensitivity to magnetic fields is higher than in the visible.
Methods: We obtained 30-s integrated spectropolarimetric data of the quiet Sun on disc centre during a period of extremely stable and good seeing. The data have a rms noise in polarization of around 2 × 10-4 of the continuum intensity. The low noise level allows the spectra to be inverted with the SIR code. We compare the inversion results with proxies to determine the magnetic flux.
Results: We confirm the presence of the horizontal fields in the quiet Sun internetwork as reported for the satellite data, including voids without linear polarization signal that extend over an area of a few granules. Voids in the circular polarization signal are only of granular scale. More than 60% of the surface show polarization signals of above four times the rms noise level. We find that the total magnetic flux contained in the more inclined to horizontal fields (γ > 45°) is lower by a factor of around 2 than that of the less inclined fields. The proxies for flux determination are strongly affected by the thermodynamic state of the atmosphere, and hence, seem to be unreliable.
Conclusions: During spells of good seeing conditions, adaptive optics can render ground-based slit-spectrograph observations at a 70-cm telescope equivalent to the seeing-free space-based data of half-meter class telescopes. We suggest that the difference in the ratio of horizontal to transversal flux between the ground-based infrared data and the satellite-based visible data is due to the different formation heights of the respective spectral lines. We emphasize that the true amount of magnetic flux cannot be derived directly from the spectra. For purely horizontal flux, one would need its vertical extension that has to be estimated by explicit modeling, using the observed spectra as boundary conditions, or be taken from MHD simulations. Time-series of the evolution of the magnetic flux and chromospheric diagnostics are needed to address its possible contribution to chromospheric heating.

Appendices A and B are only available in electronic form at http://www.aanda.org Title: 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. Bibcode: 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: A 3D sunspot model derived from an inversion of spectropolarimetric observations and its implications for the penumbral heating Authors: Beck, C. Bibcode: 2008A&A...480..825B Altcode: 2007arXiv0712.3168B Aims:I deduced a 3D sunspot model that is in full agreement with spectropolarimetric observations, in order to address the question of a possible penumbral heating process by the repetitive rise of hot flow channels.
Methods: I performed inversions of spectropolarimetric data taken simultaneously in infrared (1.5 μm) and visible (630 nm) spectral lines. I used two independent magnetic components inside each pixel to reproduce the irregular Stokes profiles in the penumbra and studied the averaged and individual properties of the two components. By integrating the field inclination to the surface, I developed a 3D model of the spot from inversion results without intrinsic height information.
Results: I find that the Evershed flow is harbored by the weaker of the two field components. This component forms flow channels that show upstreams in the inner and mid penumbra, continue almost horizontally as slightly elevated loops throughout the penumbra, and finally bend down in the outer penumbra. I find several examples where two or more flow channels are found along a radial cut from the umbra to the outer boundary of the spot.
Conclusions: I find that a model of horizontal flow channels in a static background field is in good agreement with the observed spectra. The properties of the flow channels correspond very well to the moving tube simulations of Schlichenmaier et al. (1998, A&A, 337, 897). From the temporal evolution in intensity images and the properties of the flow channels in the inversion, I conclude that interchange convection of rising hot flux tubes in a thick penumbra still seems a possible mechanism for maintaining the penumbral energy balance. Title: The signature of chromospheric heating in Ca II H spectra Authors: Beck, C.; Schmidt, W.; Rezaei, R.; Rammacher, W. Bibcode: 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 < 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: Magnetic properties of G-band bright points in a sunspot moat Authors: Beck, C.; Bellot Rubio, L. R.; Schlichenmaier, R.; Sütterlin, P. Bibcode: 2007A&A...472..607B Altcode: 2007arXiv0707.1232B We present simultaneous spectropolarimetric observations of four visible (630 nm) and three infrared (1565 nm) spectral lines from the German Vacuum Tower Telescope, together with speckle-reconstructed filtergrams in the G-band and the Ca II H line core from the Dutch Open Telescope. After alignment of the data sets, we used the G-band intensity to locate bright points (BPs) in the moat of a regular sunspot. With the cospatial and cotemporal information provided by the polarimetric data, we characterize the magnetic, kinematic, and thermal properties of the BPs. We find that (a) 94% of the BPs are associated with magnetic fields; (b) their field strengths range between 500 and 1400 G, with a rather flat distribution; (c) the contrast of BPs in the G-band depends on the angle between the vector magnetic field and the line of sight; (d) the BPs harbor downflows of magnetized plasma and exhibit Stokes V profiles with large area and amplitude asymmetries; (e) the magnetic interior of BPs is hotter than the immediate field-free surroundings by about 1000 K at equal optical depth; and (f) the mean effective diameter of BPs in our data set is 150 km, with very few BPs larger than 300 km. Most of these properties can be explained by the classical magnetic flux tube model. However, the wide range of BP parameters found in this study indicates that not all G-band BPs are identical to stable long-lived flux tubes or sheets of kG strength.

Appendices A-C are only available in electronic form at http://www.aanda.org 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. Bibcode: 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: The multi-component field topology of sunspot penumbrae. A diagnostic tool for spectropolarimetric measurements Authors: Müller, D. A. N.; Schlichenmaier, R.; Fritz, G.; Beck, C. Bibcode: 2006A&A...460..925M Altcode: 2006astro.ph..9632M Context: .Sunspot penumbrae harbor highly structured magnetic fields and flows. The moving flux tube model offers an explanation for several observed phenomena, e.g. the Evershed effect and bright penumbral grains.
Aims: .A wealth of information can be extracted from spectropolarimetric observations. In order to deduce the structure of the magnetic field in sunspot penumbrae, detailed forward modeling is necessary. On the one hand, it gives insight into the sensitivity of various spectral lines to different physical scenarios. On the other hand, it is a very useful tool to guide inversion techniques. In this work, we present a generalized 3D geometrical model that embeds an arbitrarily shaped flux tube in a stratified magnetized atmosphere.
Methods: .The new semi-analytical geometric model serves as a frontend for a polarized radiative transfer code. The advantage of this model is that it preserves the discontinuities of the physical parameters across the flux tube boundaries. This is important for the detailed shape of the emerging Stokes Profiles and the resulting net circular polarization (NCP).
Results: .(a) The inclination of downflows in the outer penumbra must be shallower than approximately 15° (b) observing the limb-side NCP of sunspots in the Fe I 1564.8 nm line offers a promising way to identify a reduced magnetic field strength in flow channels; (c) the choice of the background atmosphere can significantly influence the shape of the Stokes profiles, but does not change the global characteristics of the resulting NCP curves for the tested atmospheric models.
Title: Evershed Clouds as Precursors of Moving Magnetic Features around Sunspots Authors: Cabrera Solana, D.; Bellot Rubio, L. R.; Beck, C.; del Toro Iniesta, J. C. Bibcode: 2006ApJ...649L..41C Altcode: 2006astro.ph..9108C The relation between the Evershed flow and moving magnetic features (MMFs) is studied using high-cadence, simultaneous spectropolarimetric measurements of a sunspot in visible (630.2 nm) and near-infrared (1565 nm) lines. Doppler velocities, magnetograms, and total linear polarization maps are calculated from the observed Stokes profiles. We follow the temporal evolution of two Evershed clouds that move radially outward along the same penumbral filament. Eventually, the clouds cross the visible border of the spot and enter the moat region, where they become MMFs. The flux patch farther from the sunspot has the same polarity of the spot, while the MMF closer to it has opposite polarity and exhibits abnormal circular polarization profiles. Our results provide strong evidence that at least some MMFs are the continuation of the penumbral Evershed flow into the moat. This, in turn, suggests that MMFs are magnetically connected to sunspots. Title: The flow field in the sunspot canopy Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C.; Bellot Rubio, L. R. Bibcode: 2006A&A...454..975R Altcode: 2006astro.ph..4301R Aims.We investigate the flow field in the sunspot canopy using simultaneous Stokes vector spectropolarimetry of three sunspots (θ=27°, 50°, 75°) and their surroundings in visible (630.15 and 630.25 nm) and near infrared (1564.8 and 1565.2 nm) neutral iron lines.
Methods: .To calibrate the Doppler shifts, we compare an absolute velocity calibration using the telluric O_2-line at 630.20 nm and a relative velocity calibration using the Doppler shift of Stokes V profiles in the umbra under the assumption that the umbra is at rest. Both methods yield the same result within the calibration uncertainties (~150 m s-1). We study the radial dependence of Stokes V profiles in the directions of disk center and limb side.
Results: .Maps of Stokes V profile shifts, polarity, amplitude asymmetry, field strength and magnetic field azimuth provide strong evidence for the presence of a magnetic canopy and for the existence of a radial outflow in the canopy.
Conclusions: .Our findings indicate that the Evershed flow does not cease abruptly at the white-light spot boundary, but that at least a part of the penumbral Evershed flow continues into the magnetic canopy. Title: VIP - 2D Vector Spectropolarimetry of the Solar Atmosphere near the Diffraction Limit Authors: Bellot Rubio, L. R.; Tritschler, A.; Kentischer, T.; Beck, C.; Del Toro Iniesta, J. C. Bibcode: 2006IAUJD...3E..58B Altcode: The KIS/IAA Vector Imaging Polarimeter (VIP) is a new instrument for two-dimensional spectropolarimetry of the solar atmosphere. It is used with TESOS, the triple etalon Fabry-Perot interferometer installed at the German Vacuum Tower Telescope of Observatorio del Teide (Tenerife, Spain). The polarimeter is based on a pair of nematic liquid crystal retarders and a Wollaston prism. VIP and TESOS are able to observe any spectral line in the range from 450 nm to 750 nm with a spectral resolving power of about 250000 and spatial resolutions better than 0.5" (thanks to the Kiepenheuer Adaptive Optics System). Typically, the four Stokes parameters of a line can be measured at 40 wavelength points in less than 60 s, with a noise level of 1-2 x 10^-3 and a pixel size of 0.18" x 0.18" (2x2 binning). The modulation matrix of VIP is derived using the polarimetric calibration unit installed at the telescope. Here we present first-light observations of VIP carried out in November 2005. We recorded the full Stokes profiles of the 630.1 and 630.2 nm Fe I lines emerging from a solar pore and its surroundings at a spatial resolution of ~0.4". Based on these data, we discuss the performance and capabilities of VIP as a powerful instrument for high spatial and temporal resolution measurements of vector magnetic fields in the solar atmosphere. Title: Observational Evidence for the "Hot Wall" Effect in Small Magnetic Flux Concentrations Authors: Mikurda, K.; Beck, C. Bibcode: 2006IAUJD...3E..91M Altcode: Introduction: When lacking polarimetric observations, the Bright Points (BPs) visible in the G-band at 430 nm are commonly used as tracers for magnetic fields. Methods: Observations presented in this paper were taken on October 11, 2005 at the German Vacuum Tower Telescope and involved the Tenerife Infrared Polarimeter (TIP) at 1.5 micron, the Telecentric Etalon Solar Spectrometer (TESOS) in the Fe I spectral line at 557.6 nm, and a speckle setup in G-band. The area scanned by TIP was 75''x33'' and covered a pore surrounded by network. The TIP spectra were inverted with the SIR (Stokes Inversion based on Response functions) code to retrieve the magnetic field vector. Results: We find that G-band BPs are not cospatial with the central part of the flux concentrations. Even at the small heliocentric angle of 12 degree, the BPs appear projected on the limb side walls of the granules (Fig. 1), whereas the fields are concentrated in the intergranular lanes. Discussion: Our findings indicate that the G-band Bright Points are a result of the "hot wall effect". The downward shift of the optical depth scale in the presence of magnetic fields allows to see deeper and hotter layers, where CH dissociates, in the granules next to the field concentrations. Thus, information drawn from the observations of BPs cannot be used to conclude on the actual variation of the magnetic field structure, as only the outer parts of the flux concentrations are seen in the BPs. Figure 1. Non-cospatiality of BPs and field concentrations. Top row, left to right: line-core intensity, LOS velocity (range =+/- 1 km/s), magnetic flux , polarity, LOS velocity from the inversion (range =+/- 1.5 km/s), field azimuth. Bottom row, left to right: polarization degree, G-band intensity, continuum intensity at 1.5 micron, BP mask, field strength, field inclination. The white arrow in the G-band image denotes the limb direction, the cross marks the center of the 6.7 x 6.7 Mm field-of-view shown. Title: The 3-D topology of magnetic fields in and around sunspots Authors: Beck, Christian Bibcode: 2006PhDT........30B Altcode: The most prominent features on the surface of the solar disc are the sunspots, which have been studied since their detection in the 17th century. Despite this years and centuries of scientific investigations, surprisingly many facets of sunspots are not well understood. In this thesis, the properties of a sunspots' penumbra are derived from the observed spectra by means of an inversion, a forward modeling technique. A method for the construction of a 3-dimensional model of the field topology from the inversion results is presented. It offers the possibility to build a toy-model of the spot. This allows to investigate the relationships between the geometry and other properties of the sunspot like flow velocities, intensity, temperature, or field strength. The application of the procedure to time series of observations will allow to study the temporal evolution of the geometry and the other properties, to determine the physical processes happening and their respective drivers.

A second part of the thesis is devoted to the analysis of isolated field concentrations in the surroundings of the sunspot. These isolated magnetic elements are assumed to be the elementary building blocks of the solar surface magnetism. In the turbulent surroundings of the solar atmosphere, only relatively strong concentrations with a field strength above 1 kG are assumed to be stable at all. Such field concentrations produce brightenings in the G band, where a lot of spectral lines of the CH molecule are present. Inside magnetic fields, the density is reduced, as the magnetic pressure contributes to the pressure balance equation. Thus, the reduced density allows to see to deeper layers of the atmosphere with higher temperature. If the CH-molecules dissociate due to the higher temperature, the spectral lines disappear and the intensity increases strongly. Due to the small size of the field concentration, the brightenings are strongly localized (''G-band Bright Points''). In this study, we find a much larger variability of the Bright Point properties than expected, which puts their elementary nature in some doubt.

In the conclusions of this work, the resulting 3-D topology of the sunspot is used in an attempt to develop a consistent picture of the development and the fine structure of sunspots. Title: A polarization model for the German Vacuum Tower Telescope from in situ and laboratory measurements Authors: Beck, C.; Schlichenmaier, R.; Collados, M.; Bellot Rubio, L.; Kentischer, T. Bibcode: 2005A&A...443.1047B Altcode: It is essential to properly calibrate the polarimetric properties of telescopes, if one wants to take advantage of the capabilities of high precision spectro-polarimeters. We have constructed a model for the German Vacuum Tower Telescope (VTT) that describes its time-dependent polarization properties. Since the coelostat of the telescope changes the polarization state of the light by introducing cross talk among different polarization states, such a model is necessary to correct the measurements, in order to retrieve the true polarization as emitted from the Sun. The telescope model is quantified by a time-dependent Mueller matrix that depends on the geometry of the light beam through the telescope, and on material properties: the refractive indices of the coelostat mirrors, and the birefringence of the entrance window to the vacuum tube. These material properties were determined experimentally in-situ by feeding the telescope with known states of polarization (including unpolarized light) and by measuring its response, and from measurements of an aluminum-coated sample in the laboratory. Accuracy can in our case be determined only for the combination of telescope and spectro-polarimeter used; for the instrument POLIS at the VTT, we estimate an accuracy of ±4-5× 10-3 for the cross talk correction coefficients. Title: Polarimetric Littrow Spectrograph - instrument calibration and first measurements Authors: Beck, C.; Schmidt, W.; Kentischer, T.; Elmore, D. Bibcode: 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: Magnetic flux in the internetwork quiet Sun Authors: Khomenko, E. V.; Martínez González, M. J.; Collados, M.; Vögler, A.; Solanki, S. K.; Ruiz Cobo, B.; Beck, C. Bibcode: 2005A&A...436L..27K Altcode: We report a direct comparison of the amplitudes of Stokes spectra of the Fe i 630 nm and 1.56 μm lines produced by realistic MHD simulations with simultaneous observations in the same spectral regions. The Stokes spectra were synthesized in snapshots with a mixed polarity magnetic field having a spatially averaged strength, < B >, between 10 and 30 G. The distribution of Stokes V amplitudes depends sensitively on < B >. A quiet inter-network region was observed at the German VTT simultaneously with TIP (1.56 μm) and POLIS (630 nm). We find that the Stokes V amplitudes of both infrared and visible observations are best reproduced by the simulation snapshot with < B > = 20 G. In observations with 1 resolution, up to 2/3 of the magnetic flux can remain undetected. Title: Magnetic Flux Cancellation in the Moat of Sunspots: Results from Simultaneous Vector Spectropolarimetry in the Visible and the Infrared Authors: Bellot Rubio, Luis R.; Beck, Christian Bibcode: 2005ApJ...626L.125B Altcode: We analyze multiwavelength observations of the cancellation of a moving magnetic feature and a plage element at the outer edge of the moat of an isolated, round sunspot. The event lasted for some 35 minutes until the smaller flux concentration disappeared completely from the photosphere. The data set consists of high-resolution, full vector spectropolarimetric measurements of four visible lines and two near-infrared lines, along with speckle-reconstructed G-band and Ca II H filtergrams. The observations reveal strong chromospheric emission at the neutral line separating the two magnetic poles; it becomes visible 18 minutes after the cancellation has started and persists for 25 minutes. We have carried out an inversion of the observed Stokes profiles to determine the variation of the vector magnetic field, temperature, and line-of-sight velocity during the cancellation. No significant changes in field strength, field inclination, or temperature are observed in either of the two opposite-polarity patches. The decrease in magnetic flux is primarily due to a decrease in magnetic filling factor, which is accompanied by strong upflows (of at least 1.1 km s-1) in the smaller flux concentration. These results suggest that the cancellation is due to magnetic reconnection in the photosphere. Title: POLIS: A spectropolarimeter for the VTT and for GREGOR Authors: Schmidt, W.; Beck, C.; Kentischer, T.; Elmore, D.; Lites, B. Bibcode: 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.