TY - JOUR A1 - Louis, Rohan E. A1 - Puschmann, Klaus G. A1 - Kliem, Bernhard A1 - Balthasar, Horst A1 - Denker, Carsten T1 - Sunspot splitting triggering an eruptive flare JF - Astronomy and astrophysics : an international weekly journal N2 - Aims. We investigate how the splitting of the leading sunspot and associated flux emergence and cancellation in active region NOAA 11515 caused an eruptive M5.6 flare on 2012 July 2. Methods. Continuum intensity, line-of-sight magnetogram, and dopplergram data of the Helioseismic and Magnetic Imager were employed to analyse the photospheric evolution. Filtergrams in H alpha and He I 10830 angstrom of the Chromospheric Telescope at the Observatorio del Teide, Tenerife, track the evolution of the flare. The corresponding coronal conditions were derived from 171 angstrom and 304 angstrom images of the Atmospheric Imaging Assembly. Local correlation tracking was utilized to determine shear flows. Results. Emerging flux formed a neutral line ahead of the leading sunspot and new satellite spots. The sunspot splitting caused a long-lasting flow towards this neutral line, where a filament formed. Further flux emergence, partly of mixed polarity, as well as episodes of flux cancellation occurred repeatedly at the neutral line. Following a nearby C-class precursor flare with signs of interaction with the filament, the filament erupted nearly simultaneously with the onset of the M5.6 flare and evolved into a coronal mass ejection. The sunspot stretched without forming a light bridge, splitting unusually fast (within about a day, complete approximate to 6 h after the eruption) in two nearly equal parts. The front part separated strongly from the active region to approach the neighbouring active region where all its coronal magnetic connections were rooted. It also rotated rapidly (by 4.9 degrees h(-1)) and caused significant shear flows at its edge. Conclusions. The eruption resulted from a complex sequence of processes in the (sub-)photosphere and corona. The persistent flows towards the neutral line likely caused the formation of a flux rope that held the filament. These flows, their associated flux cancellation, the emerging flux, and the precursor flare all contributed to the destabilization of the flux rope. We interpret the sunspot splitting as the separation of two flux bundles differently rooted in the convection zone and only temporarily joined in the spot. This explains the rotation as the continued rise of the separating flux, and it implies that at least this part of the sunspot was still connected to its roots deep in the convection zone. KW - Sun: flares KW - sunspots KW - Sun: photosphere KW - Sun: chromosphere KW - techniques: photometric Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201321106 SN - 0004-6361 SN - 1432-0746 VL - 562 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Kuckein, Christoph A1 - Diercke, Andrea A1 - González Manrique, Sergio Javier A1 - Verma, Meetu A1 - Loehner-Boettcher, Johannes A1 - Socas-Navarro, H. A1 - Balthasar, Horst A1 - Sobotka, M. A1 - Denker, Carsten T1 - Ca II 8542 angstrom brightenings induced by a solar microflare JF - Astronomy and astrophysics : an international weekly journal N2 - Aims. We study small-scale brightenings in Ca II 8542 angstrom line-core images to determine their nature and effect on localized heating and mass transfer in active regions. Methods. High-resolution two-dimensional spectroscopic observations of a solar active region in the near-infrared Ca II 8542 angstrom line were acquired with the GREGOR Fabry-Perot Interferometer attached to the 1.5-m GREGOR telescope. Inversions of the spectra were carried out using the NICOLE code to infer temperatures and line-of-sight (LOS) velocities. Response functions of the Ca II line were computed for temperature and LOS velocity variations. Filtergrams of the Atmospheric Imaging Assembly (AIA) and magnetograms of the Helioseismic and Magnetic Imager (HMI) were coaligned to match the ground-based observations and to follow the Ca II brightenings along all available layers of the atmosphere. Results. We identified three brightenings of sizes up to 2 ' x 2 ' that appeared in the Ca II 8542 angstrom line-core images. Their lifetimes were at least 1.5 min. We found evidence that the brightenings belonged to the footpoints of a microflare (MF). The properties of the observed brightenings disqualified the scenarios of Ellerman bombs or Interface Region Imaging Spectrograph (IRIS) bombs. However, this MF shared some common properties with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs and MFs are both apparent in chromospheric and coronal layers according to the AIA channels; and (2) both show flaring arches with lifetimes of about 3.0-3.5 min and lengths of similar to 20 ' next to the brightenings. The inversions revealed heating by 600 K at the footpoint location in the ambient chromosphere during the impulsive phase. Connecting the footpoints, a dark filamentary structure appeared in the Ca II line-core images. Before the start of the MF, the spectra of this structure already indicated average blueshifts, meaning upward motions of the plasma along the LOS. During the impulsive phase, these velocities increased up to -2.2 km s(-1). The structure did not disappear during the observations. Downflows dominated at the footpoints. However, in the upper photosphere, slight upflows occurred during the impulsive phase. Hence, bidirectional flows are present in the footpoints of the MF. KW - Sun: photosphere KW - Sun: chromosphere KW - Sun: corona KW - Sun: activity KW - techniques: imaging spectroscopy Y1 - 2017 U6 - https://doi.org/10.1051/0004-6361/201731319 SN - 1432-0746 VL - 608 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Gömöry, Peter A1 - Balthasar, Horst A1 - Kuckein, Christoph A1 - Koza, Julis A1 - Veronig, Astrid M. A1 - González Manrique, Sergio Javier A1 - Kucera, Ales A1 - Schwartz, Pavol A1 - Hanslmeier, Arnold T1 - Flare-induced changes of the photospheric magnetic field in a delta-spot deduced from ground-based observations JF - Astronomy and astrophysics : an international weekly journal N2 - Aims. Changes of the magnetic field and the line-of-sight velocities in the photosphere are being reported for an M-class flare that originated at a delta-spot belonging to active region NOAA 11865. Methods. High-resolution ground-based near-infrared spectropolarimetric observations were acquired simultaneously in two photospheric spectral lines, Fe I 10783 angstrom and Si I 10786 angstrom, with the Tenerife Infrared Polarimeter at the Vacuum Tower Telescope (VTT) in Tenerife on 2013 October 15. The observations covered several stages of the M-class flare. Inversions of the full-Stokes vector of both lines were carried out and the results were put into context using (extreme)-ultraviolet filtergrams from the Solar Dynamics Observatory (SDO). Results. The active region showed high flaring activity during the whole observing period. After the M-class flare, the longitudinal magnetic field did not show significant changes along the polarity inversion line (PIL). However, an enhancement of the transverse magnetic field of approximately 550G was found that bridges the PIL and connects umbrae of opposite polarities in the delta-spot. At the same time, a newly formed system of loops appeared co-spatially in the corona as seen in 171 angstrom filtergrams of the Atmospheric Imaging Assembly (AIA) on board SDO. However, we cannot exclude that the magnetic connection between the umbrae already existed in the upper atmosphere before the M-class flare and became visible only later when it was filled with hot plasma. The photospheric Doppler velocities show a persistent upflow pattern along the PIL without significant changes due to the flare. Conclusions. The increase of the transverse component of the magnetic field after the flare together with the newly formed loop system in the corona support recent predictions of flare models and flare observations. KW - Sun: magnetic fields KW - sunspots KW - Sun: photosphere KW - Sun: flares KW - techniques: polarimetric Y1 - 2017 U6 - https://doi.org/10.1051/0004-6361/201730644 SN - 1432-0746 VL - 602 SP - 14 EP - 27 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Denker, Carsten A1 - Kuckein, Christoph A1 - Verma, Meetu A1 - Manrique Gonzalez, Sergio Javier Gonzalez A1 - Diercke, Andrea A1 - Enke, Harry A1 - Klar, Jochen A1 - Balthasar, Horst A1 - Louis, Rohan E. A1 - Dineva, Ekaterina T1 - High-cadence Imaging and Imaging Spectroscopy at the GREGOR Solar Telescope-A Collaborative Research Environment for High-resolution Solar Physics JF - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Supplement series N2 - 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. KW - astronomical databases KW - methods: data analysis KW - Sun: chromosphere KW - Sun: photosphere KW - techniques: image processing KW - techniques: spectroscopic Y1 - 2018 U6 - https://doi.org/10.3847/1538-4365/aab773 SN - 0067-0049 SN - 1538-4365 VL - 236 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Denker, Carsten A1 - Dineva, Ekaterina A1 - Balthasar, Horst A1 - Verma, Meetu A1 - Kuckein, Christoph A1 - Diercke, Andrea A1 - Manrique Gonzalez, Sergio Javier Gonzalez T1 - Image Quality in High-resolution and High-cadence Solar Imaging JF - Solar physics : a journal for solar and solar-stellar research and the study of solar terrestrial physics N2 - 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 imaging with high spatial resolution 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. The 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 contrastrich features and their neighborhood, which were part of Active Region NOAA 12661. Modifications of theMFGS algorithm uncover the field-and structure-dependency of this imagequality 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. KW - Granulation KW - Sunspots KW - Instrumental effects KW - Instrumentation and data management Y1 - 2018 U6 - https://doi.org/10.1007/s11207-018-1261-1 SN - 0038-0938 SN - 1573-093X VL - 293 IS - 3 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Kontogiannis, Ioannis A1 - Dineva, Ekaterina Ivanova A1 - Diercke, Andrea A1 - Verma, Meetu A1 - Kuckein, Christoph A1 - Balthasar, Horst A1 - Denker, Carsten T1 - High-resolution spectroscopy of an erupting minifilament and its impact on the nearby chromosphere JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We study the evolution of a minifilament eruption in a quiet region at the center of the solar disk and its impact on the ambient atmosphere. We used high spectral resolution imaging spectroscopy in H alpha acquired by the echelle spectrograph of the Vacuum Tower Telescope, Tenerife, Spain; photospheric magnetic field observations from the Helioseismic Magnetic Imager; and UV/EUV imaging from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory. The H alpha line profiles were noise-stripped using principal component analysis and then inverted to produce physical and cloud model parameter maps. The minifilament formed between small-scale, opposite-polarity magnetic features through a series of small reconnection events, and it erupted within an hour after its appearance in H alpha. Its development and eruption exhibited similarities to large-scale erupting filaments, indicating the action of common mechanisms. Its eruption took place in two phases, namely, a slow rise and a fast expansion, and it produced a coronal dimming, before the minifilament disappeared. During its eruption, we detected a complicated velocity pattern, indicative of a twisted, thread-like structure. Part of its material returned to the chromosphere, producing observable effects on nearby low-lying magnetic structures. Cloud model analysis showed that the minifilament was initially similar to other chromospheric fine structures, in terms of optical depth, source function, and Doppler width, but it resembled a large-scale filament on its course to eruption. High spectral resolution observations of the chromosphere can provide a wealth of information regarding the dynamics and properties of minifilaments and their interactions with the surrounding atmosphere. KW - the sun KW - solar chromosphere KW - active solar chromosphere KW - active solar KW - corona KW - solar filament eruptions KW - high resolution spectroscopy Y1 - 2020 U6 - https://doi.org/10.3847/1538-4357/aba117 SN - 0004-637X SN - 1538-4357 VL - 898 IS - 2 PB - Institute of Physics Publ. CY - London ER -