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In a joint observing campaign at GREGOR in June 2017, scientists from three different institutes (IAC, KIS, and AIP) observed the oscillations in the umbra of a sunspot. Two instruments, GRIS and GFPI, were used to acquire spectropolarimetric and spectroscopic data, respectively, at different wavelengths. With this setup, it was possible to infer the velocities across several heights of the solar atmosphere.

T. Felipe (IAC), C. Kuckein (AIP), and I. Thaler (KIS, Racah Institute of Physics) found evidences of the variation of the cutoff frequency with height in the umbra of the sunspot. The acoustic cutoff frequency is a local quantity which depends on the height of the atmosphere. It separates the high-frequency propagating waves from the low-frequency transient waves. The height dependence of the cutoff frequency has implications for helioseismology as well as for a better understanding of the wave propagation on the Sun.

The results will appear soon in the refereed journal Astronomy & Astrophysics and are already available online on arXiv.

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We observed an arch filament system (AFS) in a sunspot group with the GREGOR Infrared Spectrograph attached to the GREGOR solar telescope. The AFS was located between the leading sunspot of negative polarity and several pores of positive polarity forming the following part of the sunspot group. We recorded five spectro-polarimetric scans of this region. The spectral range included the spectral lines Si I 1082.7 nm, He I 1083.0 nm, and Ca I 1083.9 nm. In this work we concentrate on the silicon line which is formed in the upper photosphere. The line profiles are inverted with the code »Stokes Inversion based on Response functions« to obtain the magnetic field vector. The line-of-sight velocities are determined independently with a Fourier phase method. Maximum velocities are found close to the ends of AFS fibrils. These maximum values amount to 2.4 km/s next to the pores and to 4 km/s at the sunspot side. Between the following pores, we encounter an area of negative polarity that is decreasing during the five scans. We interpret this by new emerging positive flux in this area canceling out the negative flux. In summary, our findings confirm the scenario that rising magnetic flux tubes cause the AFS.

This article is accessible as a preprint in arXiv.

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The solar magnetic field is responsible for all aspects of solar activity. Sunspots are the main manifestation of the ensuing solar activity. Combining high-resolution and synoptic observations has the ambition to provide a comprehensive description of the sunspot growth and decay processes. Active region NOAA 12396 emerged on 2015 August 3 and was observed three days later with the 1.5-meter GREGOR solar telescope on 2015 August 6. High-resolution spectro-polarimetric data from the GREGOR Infrared Spectrograph (GRIS) are obtained in the photospheric Si I λ1082.7 nm and Ca I λ1083.9 nm lines, together with the chromospheric He I λ1083.0 nm triplet. These near-infrared spectro-polarimetric observations were complemented by synoptic line-of-sight magnetograms and continuum images of the Helioseismic and Magnetic Imager (HMI) and EUV images of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO).

This article is accessible as a preprint in arXiv.

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The GREGOR Bibliography was used to create a presentation with a bibliometric analysis of the refereed GREGOR article published since 2001. The list includes currently only the 52 refereed publications. In the next edition, we will also add the 60+ conference publications. Note that all other publications such as abstracts or poster presentations were not included. In the future, we can also include a more elaborate bibliometric analysis. Please let me know if you have ideas or suggestions.

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In high-resolution solar physics, the volume and complexity of photometric, spectroscopic, and polarimetric ground-based data significantly increased in the last decade, reaching data acquisition rates of terabytes per hour. This is driven by the desire to capture fast processes on the Sun and the necessity for short exposure times “freezing” the atmospheric seeing, thus enabling ex post facto image restoration. Consequently, large-format and high-cadence detectors are nowadays used in solar observations to facilitate image restoration. Based on our experience during the “early science” phase with the 1.5 m GREGOR solar telescope (2014–2015) and the subsequent transition to routine observations in 2016, we describe data collection and data management tailored toward image restoration and imaging spectroscopy. We outline our approaches regarding data processing, analysis, and archiving for two of GREGOR’s post-focus instruments (see http://gregor.aip.de), i.e., the GREGOR Fabry–Pérot Interferometer (GFPI) and the newly installed High-Resolution Fast Imager (HiFI). The heterogeneous and complex nature of multidimensional data arising from high-resolution solar observations provides an intriguing but also a challenging example for »big data« in astronomy. The big data challenge has two aspects: (1) establishing a workflow for publishing the data for the whole community and beyond and (2) creating a collaborative research environment (CRE), where computationally intense data and postprocessing tools are colocated and collaborative work is enabled for scientists of multiple institutes. This requires either collaboration with a data center or frameworks and databases capable of dealing with huge data sets based on virtual observatory (VO) and other community standards and procedures.

This article was published in a special issue on »Data: Insights and Challenges in a Time of Abundance« of the Astrophysical Journal Supplement Series and is accessible as a preprint in arXiv. This article about the »GREGOR Project and Archive @ AIP« was also featured in AIP’s News Section.

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Broad-band imaging and even imaging with a moderate bandpass (about 1 nm) provides a »photon-rich« environment, where frame selection (»lucky imaging«) becomes a helpful tool in image restoration allowing us to perform a cost-benefit analysis on how to design observing sequences for high-spatial resolution imaging in combination with real-time correction provided by an adaptive optics (AO) system. This study presents high-cadence (160 Hz) G-band and blue continuum image sequences obtained with the High-resolution Fast Imager (HiFI) at the 1.5-meter GREGOR solar telescope, where the speckle masking technique is used to restore images with nearly diffraction-limited resolution. HiFI employs two synchronized large-format and high-cadence sCMOS detectors. The Median Filter Gradient Similarity (MFGS) image quality metric is applied, among others, to AO-corrected image sequences of a pore and a small sunspot observed on 2017 June 4 and 5. A small region-of-interest, which was selected for fast imaging performance, covered these contrast-rich features and their neighborhood, which were part of active region NOAA 12661. Modifications of the MFGS algorithm uncover the field- and structure-dependency of this image quality metric. However, MFGS still remains a good choice for determining image quality without a priori knowledge, which is an important characteristic when classifying the huge number of high-resolution images contained in data archives. In addition, this investigation demonstrates that a fast cadence and millisecond exposure times are still insufficient to reach the coherence time of daytime seeing. Nonetheless, the analysis shows that data acquisition rates exceeding 50 Hz are required to capture a substantial fraction of the best seeing moments, significantly boosting the performance of post-facto image restoration.

This article was published in Solar Physics and is accessible as a preprint in arXiv.

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Combining high-resolution spectropolarimetric and imaging data is key to understanding the decay process of sunspots as it allows us scrutinizing the velocity and magnetic fields of sunspots and their surroundings. Active region NOAA 12597 was observed on 24/09/2016 with the 1.5-m GREGOR solar telescope using high-spatial resolution imaging (HiFI) as well as imaging spectroscopy (GFPI) and near-infrared (NIR) spectropolarimetry (GRIS). Horizontal proper motions were estimated with LCT, whereas LOS velocities were computed with spectral line fitting methods. The magnetic field properties were inferred with the SIR code for the Si I and Ca I NIR lines. At the time of the GREGOR observations, the leading sunspot had two light-bridges indicating the onset of its decay. One of the light-bridges disappeared, and an elongated, dark umbral core at its edge appeared in a decaying penumbral sector facing the newly emerging flux. The flow and magnetic field properties of this penumbral sector exhibited weak Evershed flow, moat flow, and horizontal magnetic field. The penumbral gap adjacent to the elongated umbral core and the penumbra in that penumbral sector displayed LOS velocities similar to granulation. The separating polarities of a new flux system interacted with the leading and central part of the already established active region. As a consequence, the leading spot rotated 55-degree in clockwise direction over 12 hours. In the high-resolution observations of a decaying sunspot, the penumbral filaments facing flux emergence site contained a darkened area resembling an umbral core filled with umbral dots. This umbral core had velocity and magnetic field properties similar to the sunspot umbra. This implies that the horizontal magnetic fields in the decaying penumbra became vertical as observed in flare-induced rapid penumbral decay, but on a very different time-scale.

The article was published in arXiv and in Astronomy & Astrophysics as a forthcoming article.

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The German-Czech research collaboration »Quiet-Sun Magnetic Fields and Newly Emerging Flux – Dynamics, Energetics, and Upper Atmospheric Response« started on 2018 January 1 (PIs: Carsten Denker and Michal Sobotka, co-Is: Horst Balthasar, Petr Heinzel, Jan Jurčák, Christoph Kuckein, Michal Švanda, and Meetu Verma). This research project is jointly funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) and the Czech Science Foundation (GACR).

Project Summary. Exploring the complex tension and intricate relationship between the ubiquitous quiet-Sun magnetic fields and newly emerging flux motivates this German-Czech research cooperation. The Leibniz Institute for Astrophysics Potsdam (AIP) and the Astronomical Institute – Academy of Sciences of the Czech Republic (ASU) propose to jointly investigate small-scale photospheric and chromospheric magnetic fields. Newly emerging flux regions (EFRs) exhibit photospheric signatures affecting the regular granulation pattern, creating micro-pores and pores, and eventually leading to larger, more complex structures like active regions including groups of pores and sunspots. The chromospheric response to emerging flux are field-aligned dark structures containing cool plasma. These arch filament systems (AFSs) are seen prominently in the strong chromospheric absorption line Hα and in the near-infrared (NIR) He I triplet at 1083.0 nm. A rising system of Ω-loops best describes the observed properties of EFRs and AFSs but only provides an illustration of the flux emergence process rather than a full description of the underlying physics. Therefore, we will trace newly emerging flux from the photosphere, over chromosphere and transition region, to the corona, derive its three-dimensional magnetic field topology, evaluate the stability of the magnetic field configuration, and examine energy input and balance in various atmospheric layers. We will investigate how (magneto)acoustic chromospheric waves and small-scale magnetic reconnection contribute to the energy input of the corona. The general theme of the proposed project is the fundamental solar process of the interaction between plasma motions and magnetic fields. Our investigation is founded in imaging and NIR spectropolarimetry with the GREGOR solar telescope, combining high-resolution ground-based observations with magnetic field and extreme ultraviolet data from space missions. The 1.5-meter GREGOR telescope is Europe’s largest solar telescope and a premier facility for high-resolution solar physics. The proposed research project will significantly contribute to definition of science cases, design of observing campaign, evaluation of data reduction and analysis methods, and preparation of researchers for the next generation of large-aperture solar telescopes like the U.S. American Daniel K. Inouye Solar Telescope (DKIST) and the European Solar Telescope (EST).

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The GREGOR archive at AIP and it data analysis and management plan were presented as an invited talk at the EST Meeting in Bairisch-Kölldorf, Austria on 2017 October 10.

In high-resolution solar physics, the volume and complexity of photometric, spectroscopic, and polarimetric ground-based data significantly increased in the last decade reaching data acquisition rates of terabytes per hour. This is driven on the one hand by the desire to capture fast processes on the Sun and on the other hand by the necessity for short exposure times »freezing« the atmospheric seeing, thus enabling post-facto image restoration. Solar features move with velocities of several kilometers per second in the photosphere and several tens of kilometers per second in the chromosphere, often exceeding the speed of sound. Eruptive phenomena in the chromosphere reach even higher velocities in excess of 100 kilometers per second. The coherence time of wavefront distortions is of the order of milliseconds under daytime seeing conditions. Consequently, large-format and high-cadence detectors are nowadays used in solar observations to facilitate image restoration. Based on our experience during the “early science” phase with the 1.5-meter GREGOR solar telescope (2014–2015) and the subsequent transition to routine observations in 2016, we describe data analysis and data management tailored towards image restoration and imaging spectroscopy. We outline our approaches regarding data processing, analysis, and archiving for two of GREGOR’s post-focus instruments, i.e., the GREGOR Fabry-Pérot Interferometer (GFPI) and the newly installed High-Resolution Fast Imager (HiFI). The heterogeneous and complex nature of multi-dimensional data arising from highresolution solar observations provides an intriguing but also a challenging example for »big data« in astronomy – in particular when considering the next generation of 4-meter aperture solar telescopes. The big data challenge has two aspects: (1) creating a Collaborative Research Environment, where computationally intense data and post-processing tools are co-located and collaborative work is enabled for scientists of multiple institutes and (2) establishing a workflow for publishing the data for the whole community and beyond. This requires either collaboration with a data center or frameworks and databases capable of dealing with huge data sets based on Virtual Observatory and other community standards and procedures. We present working approaches for both.

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In this work we investigated small-scale brightenings in Ca II 854.2 nm line-core images of the Sun to determine their nature and effect on localized heating and mass transfer in active regions. To this end, we used 2D spectroscopic observations of an active region in the Ca II 854.2 nm line acquired with the GREGOR Fabry-Pérot Interferometer (GFPI). Spectral-line inversions were carried out using the inversion code NICOLE. We identified three brightenings of sizes up to 2″ × 2″ . We found evidence that the brightenings belonged to the footpoints of a microflare. However, this microflare shared some common properties with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs and microflares are both apparent in chromospheric and coronal layers according to the AIA channels on board of SDO, and (2) both show flaring arches with lifetimes of about 3.0-3.5 min and lengths of about 20″. The inversions revealed heating by 600 K at the footpoint location in the ambient chromosphere during the impulsive phase. The detected Ca II brightenings coincided with the footpoint location of the microflare. The microflare event led to a rise of plasma in the upper photosphere, both before and during the impulsive phase. Excess mass, previously raised to at most chromospheric layers, slowly drained downward along arches toward the footpoints of the microflare.

The article was published in arXiv and in Astronomy & Astrophysics as a forthcoming article.

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