TY - JOUR A1 - Leussu, R. A1 - Usoskin, IIlya G. A1 - Valliappan, Senthamizh Pavai A1 - Diercke, Andrea A1 - Arlt, Rainer A1 - Denker, Carsten A1 - Mursula, K. T1 - Wings of the butterfly BT - sunspot groups for 1826-2015 JF - Astronomy and astrophysics : an international weekly journal N2 - The spatio-temporal evolution of sunspot activity, the so-called Maunder butterfly diagram, has been continously available since 1874 using data from the Royal Greenwich Observatory, extended by SOON network data after 1976. Here we present a new extended butterfly diagram of sunspot group occurrence since 1826, using the recently digitized data from Schwabe (1826-1867) and Sporer (1866-1880). The wings of the diagram are separated using a recently developed method based on an analysis of long gaps in sunspot group occurrence in different latitude bands. We define characteristic latitudes, corresponding to the start, end, and the largest extent of the wings (the F, L, and H latitudes). The H latitudes (30 degrees-45 degrees) are highly significantly correlated with the strength of the wings (quantified by the total sum of the monthly numbers of sunspot groups). The F latitudes (20 degrees-30 degrees) depict a weak tendency, especially in the southern hemisphere, to follow the wing strength. The L latitudes (2 degrees-10 degrees) show no clear relation to the wing strength. Overall, stronger cycle wings tend to start at higher latitudes and have a greater wing extent. A strong (5-6)-cycle periodic oscillation is found in the start and end times of the wings and in the overlap and gaps between successive wings of one hemisphere. While the average wing overlap is zero in the southern hemisphere, it is two to three months in the north. A marginally significant oscillation of about ten solar cycles is found in the asymmetry of the L latitudes. The new long database of butterfly wings provides new observational constraints to solar dynamo models that discuss the spatio-temporal distribution of sunspot occurrence over the solar cycle and longer. KW - Sun: activity KW - sunspots KW - history and philosophy of astronomy Y1 - 2017 U6 - https://doi.org/10.1051/0004-6361/201629533 SN - 1432-0746 VL - 599 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Gonzalez Manrique, Sergio Javier A1 - Kuckein, Christoph A1 - Collados, M. A1 - Denker, Carsten A1 - Solanki, S. K. A1 - Gomory, P. A1 - Verma, Meetu A1 - Balthasar, H. A1 - Lagg, A. A1 - Diercke, Andrea T1 - Temporal evolution of arch filaments as seen in He I 10 830 angstrom JF - Astronomy and astrophysics : an international weekly journal N2 - Aims. We study the evolution of an arch filament system (AFS) and of its individual arch filaments to learn about the processes occurring in them. Methods. We observed the AFS at the GREGOR solar telescope on Tenerife at high cadence with the very fast spectroscopic mode of the GREGOR Infrared Spectrograph (GRIS) in the He I 10 830 angstrom spectral range. The He I triplet profiles were fitted with analytic functions to infer line-of-sight (LOS) velocities to follow plasma motions within the AFS. Results. We tracked the temporal evolution of an individual arch filament over its entire lifetime, as seen in the He I 10 830 angstrom triplet. The arch filament expanded in height and extended in length from 13 ' to 21 '. The lifetime of this arch filament is about 30 min. About 11 min after the arch filament is seen in He I, the loop top starts to rise with an average Doppler velocity of 6 km s(-1). Only two minutes later, plasma drains down with supersonic velocities towards the footpoints reaching a peak velocity of up to 40 km s(-1) in the chromosphere. The temporal evolution of He I 10 830 angstrom profiles near the leading pore showed almost ubiquitous dual red components of the He I triplet, indicating strong downflows, along with material nearly at rest within the same resolution element during the whole observing time. KW - Sun: chromosphere KW - Sun: activity KW - methods: observational KW - methods: data analysis KW - techniques: high angular resolution Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201832684 SN - 1432-0746 VL - 617 PB - EDP Sciences CY - Les Ulis ER - 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 - Valliappan, Senthamizh Pavai A1 - Arlt, Rainer A1 - Diercke, Andrea A1 - Denker, Carsten A1 - Vaquero, J. M. T1 - Sunspot group tilt angle measurements from historical observations JF - Advances in space research N2 - Sunspot positions from various historical sets of solar drawings are analyzed with respect to the tilt angles of bipolar sunspot groups. Data by Scheiner, Hevelius, Staudacher, Zucconi, Schwabe, and Sporer deliver a series of average tilt angles spanning a period of 270 years, additional to previously found values for 20th-century data obtained by other authors. We find that the average tilt angles before the Maunder minimum were not significantly different from the modem values. However, the average tilt angles of a period 50 years after the Maunder minimum, namely for cycles 0 and 1, were much lower and near zero. The normal tilt angles before the Maunder minimum suggest that it was not abnormally low tilt angles which drove the solar cycle into a grand minimum. (C) 2016 COSPAR. Published by Elsevier Ltd. All rights reserved. KW - Sun: sunspots KW - Tilt angles KW - Cycle-averaged tilt angle Y1 - 2016 U6 - https://doi.org/10.1016/j.asr.2016.03.002 SN - 0273-1177 SN - 1879-1948 VL - 58 SP - 1468 EP - 1474 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Balthasar, H. A1 - Gömöry, P. A1 - González Manrique, Sergio Javier A1 - Kuckein, Christoph A1 - Kavka, J. A1 - Kucera, A. A1 - Schwartz, P. A1 - Vaskova, R. A1 - Berkefeld, T. A1 - Collados Vera, M. A1 - Denker, Carsten A1 - Feller, A. A1 - Hofmann, A. A1 - Lagg, A. A1 - Nicklas, H. A1 - Suarez, D. A1 - Pastor Yabar, A. A1 - Rezaei, R. A1 - Schlichenmaier, R. A1 - Schmidt, D. A1 - Schmidt, W. A1 - Sigwarth, M. A1 - Sobotka, M. A1 - Solanki, S. K. A1 - Soltau, D. A1 - Staude, J. A1 - Strassmeier, Klaus G. A1 - Volkmer, R. A1 - von der Lühe, O. A1 - Waldmann, T. T1 - Spectropolarimetric observations of an arch filament system with the GREGOR solar telescope JF - Astronomische Nachrichten = Astronomical notes N2 - Arch filament systems occur in active sunspot groups, where a fibril structure connects areas of opposite magnetic polarity, in contrast to active region filaments that follow the polarity inversion line. We used the GREGOR Infrared Spectrograph (GRIS) to obtain the full Stokes vector in the spectral lines SiI lambda 1082.7 nm, He I lambda 1083.0 nm, and Ca I lambda 1083.9 nm. We focus on the near-infrared calcium line to investigate the photospheric magnetic field and velocities, and use the line core intensities and velocities of the helium line to study the chromospheric plasma. The individual fibrils of the arch filament system connect the sunspot with patches of magnetic polarity opposite to that of the spot. These patches do not necessarily coincide with pores, where the magnetic field is strongest. Instead, areas are preferred not far from the polarity inversion line. These areas exhibit photospheric downflows of moderate velocity, but significantly higher downflows of up to 30 km s(-1) in the chromospheric helium line. Our findings can be explained with new emerging flux where the matter flows downward along the field lines of rising flux tubes, in agreement with earlier results. (C) 2016 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim KW - Sun: filaments KW - Sun: photosphere KW - techniques: polarimetric KW - techniques: spectroscopic Y1 - 2016 U6 - https://doi.org/10.1002/asna.201612432 SN - 0004-6337 SN - 1521-3994 VL - 337 SP - 1050 EP - 1056 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Denker, Carsten A1 - Heibel, C. A1 - Rendtel, J. A1 - Arlt, K. A1 - Balthasar, H. A1 - Diercke, Andrea A1 - Gonzalez Manrique, Sergio Javier A1 - Hofmann, A. A1 - Kuckein, Christoph A1 - Önel, H. A1 - Valliappan, Senthamizh Pavai A1 - Staude, J. A1 - Verma, Meetu T1 - Solar physics at the Einstein Tower JF - Astronomische Nachrichten = Astronomical notes KW - history and philosophy of astronomy KW - Sun: photosphere KW - Sun: magnetic fields KW - techniques: spectroscopic KW - telescopes Y1 - 2016 U6 - https://doi.org/10.1002/asna.201612442 SN - 0004-6337 SN - 1521-3994 VL - 337 SP - 1105 EP - 1113 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Diercke, Andrea A1 - Kuckein, Christoph A1 - Cauley, Paul Wilson A1 - Poppenhäger, Katja A1 - Alvarado-Gómez, Julián David A1 - Dineva, Ekaterina Ivanova A1 - Denker, Carsten T1 - Solar H alpha excess during Solar Cycle 24 from full-disk filtergrams of the Chromospheric Telescope JF - Astronomy and astrophysics : an international weekly journal N2 - Context The chromospheric H alpha spectral line is a strong line in the spectrum of the Sun and other stars. In the stellar regime, this spectral line is already used as a powerful tracer of stellar activity. For the Sun, other tracers, such as Ca II K, are typically used to monitor solar activity. Nonetheless, the Sun is observed constantly in H alpha with globally distributed ground-based full-disk imagers. Aims The aim of this study is to introduce the imaging H alpha excess and deficit as tracers of solar activity and compare them to other established indicators. Furthermore, we investigate whether the active region coverage fraction or the changing H alpha excess in the active regions dominates temporal variability in solar H alpha observations. Methods We used observations of full-disk H alpha filtergrams of the Chromospheric Telescope and morphological image processing techniques to extract the imaging H alpha excess and deficit, which were derived from the intensities above or below 10% of the median intensity in the filtergrams, respectively. These thresholds allowed us to filter for bright features (plage regions) and dark absorption features (filaments and sunspots). In addition, the thresholds were used to calculate the mean intensity I-mean(E/D) for H alpha excess and deficit regions. We describe the evolution of the H alpha excess and deficit during Solar Cycle 24 and compare it to the mean intensity and other well established tracers: the relative sunspot number, the F10.7 cm radio flux, and the Mg II index. In particular, we tried to determine how constant the H alpha excess and number density of H alpha excess regions are between solar maximum and minimum. The number of pixels above or below the intensity thresholds were used to calculate the area coverage fraction of H alpha excess and deficit regions on the Sun, which was compared to the imaging H alpha excess and deficit and the respective mean intensities averaged for the length of one Carrington rotation. In addition, we present the H alpha excess and mean intensity variation of selected active regions during their disk passage in comparison to the number of pixels of H alpha excess regions. Results. The H alpha excess and deficit follow the behavior of the solar activity over the course of the cycle. They both peak around solar maximum, whereby the peak of the H alpha deficit is shortly after the solar maximum. Nonetheless, the correlation of the monthly averages of the H alpha excess and deficit is high with a Spearman correlation of rho =  0.91. The H alpha excess is closely correlated to the chromospheric Mg II index with a correlation of 0.95. The highest correlation of the H alpha deficit is found with the F10.7 cm radio flux, with a correlation of 0.89, due to their peaks after the solar activity maximum. Furthermore, the H alpha deficit reflects the cyclic behavior of polar crown filaments and their disappearance shortly before the solar maximum. We investigated the mean intensity distribution for H alpha excess regions for solar minimum and maximum. The shape of the distributions for solar minimum and maximum is very similar, but with different amplitudes. Furthermore, we found that the area coverage fraction of H alpha excess regions and the H alpha excess are strongly correlated with an overall Spearman correlation of 0.92. The correlation between the H alpha excess and the mean intensity of H alpha excess regions is 0.75. The correlation of the area coverage fraction and the mean intensity of H alpha excess regions is in general relatively low (rho = 0.45) and only for few active regions is this correlation above 0.7. The weak correlation between the area coverage fraction and mean intensity leaves us pessimistic that the degeneracy between these two quantities can be broken for the modeling of unresolved stellar surfaces. KW - methods: observational KW - Sun: chromosphere KW - Sun: activity KW - Sun: faculae, plages KW - Sun: filaments KW - stars: atmospheres KW - prominences Y1 - 2022 U6 - https://doi.org/10.1051/0004-6361/202040091 SN - 1432-0746 SN - 0004-6361 VL - 661 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Verma, Meetu A1 - Kummerow, P. A1 - Denker, Carsten T1 - On the extent of the moat flow in axisymmetric sunspots JF - Astronomische Nachrichten = Astronomical notes N2 - Unipolar, axisymmetric sunspots are figuratively called “theoretician's sunspots” because their simplicity supposedly makes them more suitable for theoretical descriptions or numerical models. On November 18, 2013, a very large specimen (active region NOAA 11899) crossed the central meridian of the sun. The moat flow associated with this very large spot is quantitatively compared to that of a medium and a small sunspot to determine the extent of the moat flow in different environments. We employ continuum images and magnetograms of the Helioseismic and Magnetic Imager (HMI) as well as extreme ultraviolet (EUV) images at λ160 nm of the Atmospheric Imaging Assembly (AIA), both on board the Solar Dynamics Observatory (SDO), to measure horizontal proper motions with Local Correlation Tracking (LCT) and flux transport velocities with the Differential Affine Velocity Estimator (DAVE). We compute time-averaged flow maps (±6 hr around meridian passage) and radial averages of photometric, magnetic, and flow properties. Flow fields of a small- and a medium-sized axisymmetric sunspot provide the context for interpreting the results. All sunspots show outward moat flow and the advection of moving magnetic features (MMFs). However, the extent of the moat flow varies from spot to spot, and a correlation of flow properties with size is tenuous, if at all present. The moat flow is asymmetric and predominantly in the east–west direction, whereby deviations are related to the tilt angle of the sunspot group as well as to the topology and activity level of the trailing plage. KW - activity KW - data analysis KW - image processing KW - photosphere KW - sunspots Y1 - 2018 U6 - https://doi.org/10.1002/asna.201813482 SN - 0004-6337 SN - 1521-3994 VL - 339 IS - 4 SP - 268 EP - 276 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Martinez Gonzalez, M. J. A1 - Pastor Yabar, A. A1 - Lagg, A. A1 - Asensio Ramos, A. A1 - Collados Vera, M. A1 - Solanki, S. K. A1 - Balthasar, H. A1 - Berkefeld, T. A1 - Denker, Carsten A1 - Doerr, H. P. A1 - Feller, A. A1 - Franz, M. A1 - González Manrique, Sergio Javier A1 - Hofmann, A. A1 - Kneer, F. A1 - Kuckein, Christoph A1 - Louis, R. A1 - von der Lühe, O. A1 - Nicklas, H. A1 - Orozco, D. A1 - Rezaei, R. A1 - Schlichenmaier, R. A1 - Schmidt, D. A1 - Schmidt, W. A1 - Sigwarth, M. A1 - Sobotka, M. A1 - Soltau, D. A1 - Staude, J. A1 - Strassmeier, Klaus G. A1 - Verma, Meetu A1 - Waldman, T. A1 - Volkmer, R. T1 - Inference of magnetic fields in the very quiet Sun JF - Journal of geophysical research : Earth surface N2 - Context. Over the past 20 yr, the quietest areas of the solar surface have revealed a weak but extremely dynamic magnetism occurring at small scales (<500 km), which may provide an important contribution to the dynamics and energetics of the outer layers of the atmosphere. Understanding this magnetism requires the inference of physical quantities from high-sensitivity spectro-polarimetric data with high spatio-temporal resolution. Aims. We present high-precision spectro-polarimetric data with high spatial resolution (0.4") of the very quiet Sun at 1.56 mu m obtained with the GREGOR telescope to shed some light on this complex magnetism. Methods. We used inversion techniques in two main approaches. First, we assumed that the observed profiles can be reproduced with a constant magnetic field atmosphere embedded in a field-free medium. Second, we assumed that the resolution element has a substructure with either two constant magnetic atmospheres or a single magnetic atmosphere with gradients of the physical quantities along the optical depth, both coexisting with a global stray-light component. Results. Half of our observed quiet-Sun region is better explained by magnetic substructure within the resolution element. However, we cannot distinguish whether this substructure comes from gradients of the physical parameters along the line of sight or from horizontal gradients (across the surface). In these pixels, a model with two magnetic components is preferred, and we find two distinct magnetic field populations. The population with the larger filling factor has very weak (similar to 150 G) horizontal fields similar to those obtained in previous works. We demonstrate that the field vector of this population is not constrained by the observations, given the spatial resolution and polarimetric accuracy of our data. The topology of the other component with the smaller filling factor is constrained by the observations for field strengths above 250 G: we infer hG fields with inclinations and azimuth values compatible with an isotropic distribution. The filling factors are typically below 30%. We also find that the flux of the two polarities is not balanced. From the other half of the observed quiet-Sun area similar to 50% are two-lobed Stokes V profiles, meaning that 23% of the field of view can be adequately explained with a single constant magnetic field embedded in a non-magnetic atmosphere. The magnetic field vector and filling factor are reliable inferred in only 50% based on the regular profiles. Therefore, 12% of the field of view harbour hG fields with filling factors typically below 30%. At our present spatial resolution, 70% of the pixels apparently are non-magnetised. KW - Sun: atmosphere KW - Sun: magnetic fields KW - techniques: polarimetric KW - methods: observational Y1 - 2016 U6 - https://doi.org/10.1051/0004-6361/201628449 SN - 1432-0746 VL - 596 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Denker, Carsten A1 - Dineva, Ekaterina Ivanova 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 -