TY - JOUR A1 - Amaya, Jorge A1 - Musset, Sophie A1 - Andersson, Viktor A1 - Diercke, Andrea A1 - Hoeller, Christian A1 - Iliev, Sergiu A1 - Juhasz, Lilla A1 - Kiefer, Rene A1 - Lasagni, Riccardo A1 - Lejosne, Solene A1 - Madi, Mohammad A1 - Rummelhagen, Mirko A1 - Scheucher, Markus A1 - Sorba, Arianna A1 - Thonhofer, Stefan T1 - The PAC2MAN mission BT - a new tool to understand and predict solar energetic events JF - Journal of space weather and space climate N2 - An accurate forecast of flare and coronal mass ejection (CME) initiation requires precise measurements of the magnetic energy buildup and release in the active regions of the solar atmosphere. We designed a new space weather mission that performs such measurements using new optical instruments based on the Hanle and Zeeman effects. The mission consists of two satellites, one orbiting the L1 Lagrangian point (Spacecraft Earth, SCE) and the second in heliocentric orbit at 1AU trailing the Earth by 80 degrees (Spacecraft 80, SC80). Optical instruments measure the vector magnetic field in multiple layers of the solar atmosphere. The orbits of the spacecraft allow for a continuous imaging of nearly 73% of the total solar surface. In-situ plasma instruments detect solar wind conditions at 1AU and ahead of our planet. Earth-directed CMEs can be tracked using the stereoscopic view of the spacecraft and the strategic placement of the SC80 satellite. Forecasting of geoeffective space weather events is possible thanks to an accurate surveillance of the magnetic energy buildup in the Sun, an optical tracking through the interplanetary space, and in-situ measurements of the near-Earth environment. KW - Space weather KW - Spacecraft KW - Missions KW - Coronal mass ejection (CME) KW - Flare Y1 - 2015 U6 - https://doi.org/10.1051/swsc/2015005 SN - 2115-7251 VL - 5 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 - 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 - 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 Ivanova 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 - THES A1 - Diercke, Andrea T1 - Physical environment of large-scale high-latitude and polar crown filaments T1 - Die Physik von polaren Filamenten N2 - Filaments are omnipresent features in the solar chromosphere, one of the atmospheric layers of the Sun, which is located above the photosphere, the visible surface of the Sun. They are clouds of plasma reaching from the photosphere to the chromosphere, and even to the outer-most atmospheric layer, the corona. They are stabalized by the magnetic field. If the magnetic field is disturbed, filaments can erupt as coronal mass ejections (CME), releasing plasma into space, which can also hit the Earth. A special type of filaments are polar crown filaments, which form at the interface of the unipolar field of the poles and flux of opposite magnetic polarity, which was transported towards the poles. This flux transport is related to the global dynamo of the Sun and can therefore be analyzed indirectly with polar crown filaments. The main objective of this thesis is to better understand the physical properties and environment of high-latitude and polar crown filaments, which can be approached from two perspectives: (1) analyzing the large-scale properties of high-latitude and polar crown filaments with full-disk Hα observations from the Chromospheric Telescope (ChroTel) and (2) determining the relation of polar crown and high-latitude filaments from the chromosphere to the lower-lying photosphere with high-spatial resolution observations of the Vacuum Tower Telescope (VTT), which reveal the smallest details. The Chromospheric Telescope (ChroTel) is a small 10-cm robotic telescope at Observatorio del Teide on Tenerife (Spain), which observes the entire Sun in Hα, Ca IIK, and He I 10830 Å. We present a new calibration method that includes limb-darkening correction, removal of non-uniform filter transmission, and determination of He I Doppler velocities. Chromospheric full-disk filtergrams are often obtained with Lyot filters, which may display non-uniform transmission causing large-scale intensity variations across the solar disk. Removal of a 2D symmetric limb-darkening function from full-disk images results in a flat background. However, transmission artifacts remain and are even more distinct in these contrast-enhanced images. Zernike polynomials are uniquely appropriate to fit these large-scale intensity variations of the background. The Zernike coefficients show a distinct temporal evolution for ChroTel data, which is likely related to the telescope’s alt-azimuth mount that introduces image rotation. In addition, applying this calibration to sets of seven filtergrams that cover the He I triplet facilitates determining chromospheric Doppler velocities. To validate the method, we use three datasets with varying levels of solar activity. The Doppler velocities are benchmarked with respect to co-temporal high-resolution spectroscopic data of the GREGOR Infrared Spectrograph (GRIS). Furthermore, this technique can be applied to ChroTel Hα and Ca IIK data. The calibration method for ChroTel filtergrams can be easily adapted to other full-disk data exhibiting unwanted large-scale variations. The spectral region of the He I triplet is a primary choice for high-resolution near-infrared spectropolarimetry. Here, the improved calibration of ChroTel data will provide valuable context data. Polar crown filaments form above the polarity inversion line between the old magnetic flux of the previous cycle and the new magnetic flux of the current cycle. Studying their appearance and their properties can lead to a better understanding of the solar cycle. We use full-disk data of the ChroTel at Observatorio del Teide, Tenerife, Spain, which were taken in three different chromospheric absorption lines (Hα 6563 Å, Ca IIK 3933 Å, and He I 10830 Å), and we create synoptic maps. In addition, the spectroscopic He I data allow us to compute Doppler velocities and to create synoptic Doppler maps. ChroTel data cover the rising and decaying phase of Solar Cycle 24 on about 1000 days between 2012 and 2018. Based on these data, we automatically extract polar crown filaments with image-processing tools and study their properties. We compare contrast maps of polar crown filaments with those of quiet-Sun filaments. Furthermore, we present a super-synoptic map summarizing the entire ChroTel database. In summary, we provide statistical properties, i.e. number and location of filaments, area, and tilt angle for both the maximum and declining phase of Solar Cycle 24. This demonstrates that ChroTel provides a promising dataset to study the solar cycle. The cyclic behavior of polar crown filaments can be monitored by regular full-disk Hα observations. ChroTel provides such regular observations of the Sun in three chromospheric wavelengths. To analyze the cyclic behavior and the statistical properties of polar crown filaments, we have to extract the filaments from the images. Manual extraction is tedious, and extraction with morphological image processing tools produces a large number of false positive detections and the manual extraction of these takes too much time. Automatic object detection and extraction in a reliable manner allows us to process more data in a shorter time. We will present an overview of the ChroTel database and a proof of concept of a machine learning application, which allows us a unified extraction of, for example, filaments from ChroTel data. The chromospheric Hα spectral line dominates the spectrum of the Sun and other stars. In the stellar regime, this spectral line is already used as a powerful tracer of magnetic activity. For the Sun, other tracers are typically used to monitor solar activity. Nonetheless, the Sun is observed constantly in Hα with globally distributed ground-based full-disk imagers. The aim of this study is to introduce Hα as a tracer of solar activity and compare it to other established indicators. We discuss the newly created imaging Hα excess in the perspective of possible application for modelling of stellar atmospheres. In particular, we try to determine how constant is the mean intensity of the Hα excess and number density of low-activity regions between solar maximum and minimum. Furthermore, we investigate whether the active region coverage fraction or the changing emission strength in the active regions dominates time variability in solar Hα observations. We use ChroTel observations of full-disk Hα filtergrams and morphological image processing techniques to extract the positive and negative imaging Hα excess, for bright features (plage regions) and dark absorption features (filaments and sunspots), respectively. We describe the evolution of the Hα excess during Solar Cycle 24 and compare it to other well established tracers: the relative sunspot number, the F10.7 cm radio flux, and the Mg II index. Moreover, we discuss possible applications of the Hα excess for stellar activity diagnostics and the contamination of exoplanet transmission spectra. The positive and negative Hα excess follow the behavior of the solar activity over the course of the cycle. Thereby, positive Hα excess is closely correlated to the chromospheric Mg II index. On the other hand, the negative Hα excess, created from dark features like filaments and sunspots, is introduced as a tracer of solar activity for the first time. We investigated the mean intensity distribution for active regions for solar minimum and maximum and found that the shape of both distributions is very similar but with different amplitudes. This might be related with the relatively stable coronal temperature component during the solar cycle. Furthermore, we found that the coverage fraction of Hα excess and the Hα excess of bright features are strongly correlated, which will influence modelling of stellar and exoplanet atmospheres. High-resolution observations of polar crown and high-latitude filaments are scarce. We present a unique sample of such filaments observed in high-resolution Hα narrow-band filtergrams and broad-band images, which were obtained with a new fast camera system at the VTT. ChroTel provided full-disk context observations in Hα, Ca IIK, and He I 10830 Å. The Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) provided line-of-sight magnetograms and ultraviolet (UV) 1700 Å filtergrams, respectively. We study filigree in the vicinity of polar crown and high-latitude filaments and relate their locations to magnetic concentrations at the filaments’ footpoints. Bright points are a well studied phenomenon in the photosphere at low latitudes, but they were not yet studied in the quiet network close to the poles. We examine size, area, and eccentricity of bright points and find that their morphology is very similar to their counterparts at lower latitudes, but their sizes and areas are larger. Bright points at the footpoints of polar crown filaments are preferentially located at stronger magnetic flux concentrations, which are related to bright regions at the border of supergranules as observed in UV filtergrams. Examining the evolution of bright points on three consecutive days reveals that their amount increases while the filament decays, which indicates they impact the equilibrium of the cool plasma contained in filaments. N2 - Filamente sind omnipräsente Strukturen in der Chromosphäre der Sonne. Diese Schicht befindet sich über der Photosphäre, welche die sichtbare Oberfläche der Sonne darstellt. Filamente sind Plasmagebilde, welche in der Photosphäre verankert sind und von der Chromosphäre in die Korona reichen, der äußersten Atmosphärenschicht der Sonne. Diese Strukturen werden durch das Magnetfeld der Sonne stabilisiert. Durch Störungen des Magnetfelds, destabilisiert sich das Filament und das dort enthaltene Plasma kann als Sonneneruption, ein sogenannter koronaler Massenauswurf, ins Weltall geschleudert werden, welcher auch die Erde treffen könnte. Das Verständnis von Filamenten, deren Stabilität und Verbindung zum Magnetfeld sind ungemein wichtig, um Sonneneruptionen besser verstehen und vorhersagen zu können. Ein spezieller Typ von Filamenten, sind polare Filamente, (engl. polar crown filaments). Diese bilden sich an der Grenzfläche des unipolaren Magnetfelds an den Polen und dem Magnetfeld von gemischten Polaritäten in den Aktivitätsgürteln der Sonne. In letzteren werden Reste von bipolaren und zerfallenen Sonnenfleckengruppen zum Pol transportiert. Dieser Transport wird durch den Sonnendynamo initialisiert, so dass die Untersuchung polarer Filamente indirekt Rückschlüsse auf den Sonnendynamo zulässt. Die vorliegende Arbeit untersucht die polaren Filamente aus zwei Perspektiven. Zum einen aus der globalen Perspektive, bei der wir synoptische Beobachtungen der gesamten Sonnenscheibe nutzen, um das zyklische Verhalten der Filamente zu untersuchen. Zum anderen aus einer detailorientierten Perspektive, wobei wir hochaufgelöste Beobachtungen der Filamente auswerten, um mehr über die Verbindung von kühlem chromosphärischem Plasma zum Magnetfeld zu erfahren. Für die Untersuchung des zyklischen Verhaltens von polaren Filamenten nutzen wir Daten des Chromospheric Telescope (ChroTel), welches alle drei Minuten Aufnahmen der Chromosphäre in drei verschiedenen Wellenlängen macht. Die Wasserstofflinie der Balmerserie Hα ist dabei die beste Möglichkeit Filamente in der Chromosphäre abzubilden. Eine während dieser Arbeit entwickelte Methode, zum Korrigieren von Intensitätsungleichmäßigkeiten in Sonnenbildern, legt den Grundstein für alle weiteren Studien mit diesen Daten. Die Filamente können somit aus den Bildern heraus extrahiert werden und damit kann der aktuelle Sonnenzyklus zwischen Maximum und Minimum untersucht werden. Wir konnten die Wanderung der polaren Filamente für den Sonnenzyklus 24 in den Daten lokalisieren und die polwärtsgerichtete Geschwindigkeit bestimmen, welche wir mit vorherigen Ergebnissen verglichen haben. Da die morphologischen Bildbearbeitungsmethode, welche wir zur Extraktion der Filamente benutzt haben, auch andere Strukturen, wie Sonnenflecken nicht ausschließen konnte, haben wir neue Methoden entwickelt, die auf Maschinellem Lernen mit tiefen neuronalen Netzwerken beruhen. Die vorläufigen Ergebnisse sind sehr vielversprechend und auch auf Hα Bildern von anderen Teleskopen leicht übertragbar. Für die Untersuchung der polaren Filamente mit hochaufgelösten Bildern verwenden wir Beobachtungen vom Vakuumturmteleskop (VTT) auf Teneriffa, Spanien. Die Bilder wurden mit Hα Schmal- und Breitbandfiltern aufgenommen und zeigen sowohl die Chromosphäre als auch die Photosphäre. Wir untersuchen dabei die kleinsten auflösbaren Aufhellungen, (engl. bright points), welche in Verbindung mit dem Magnetfeld stehen. Diese kleinskaligen Aufhellungen finden wir vor allem an den Fußpunktregionen der Filamente, die mit starken Konzentrationen vom Magnetfeld korrelieren. Solche hellen Punkte in der Nähe von polaren Filamenten wurden bisher nie mit hochaufgelösten Beobachtungen untersucht. Die statistische Auswertung dieser Strukturen zeigt, dass sie sich kaum von ihren äquatornahen Gegenstücken unterscheiden, mit Ausnahme einer tendenziell größeren Fläche. KW - Solar Physics KW - Sonnenphysik KW - Filaments KW - Filamente KW - Chromosphere KW - Chromosphäre Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-511301 ER - TY - JOUR A1 - Diercke, Andrea A1 - Arlt, Rainer A1 - Denker, Carsten T1 - Digitization of sunspot drawings by Sporer made in 1861-1894 JF - Astronomische Nachrichten = Astronomical notes N2 - Most of our knowledge about the Sun's activity cycle arises from sunspot observations over the last centuries since telescopes have been used for astronomy. The German astronomer Gustav Sporer observed almost daily the Sun from 1861 until the beginning of 1894 and assembled a 33-year collection of sunspot data covering a total of 445 solar rotation periods. These sunspot drawings were carefully placed on an equidistant grid of heliographic longitude and latitude for each rotation period, which were then copied to copper plates for a lithographic reproduction of the drawings in astronomical journals. In this article, we describe in detail the process of capturing these data as digital images, correcting for various effects of the aging print materials, and preparing the data for contemporary scientific analysis based on advanced image processing techniques. With the processed data we create a butterfly diagram aggregating sunspot areas, and we present methods to measure the size of sunspots (umbra and penumbra) and to determine tilt angles of active regions. A probability density function of the sunspot area is computed, which conforms to contemporary data after rescaling. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KW - astronomical databases: miscellaneous KW - history and philosophy of astronomy KW - Sun: activity KW - Sun: photosphere KW - Sun: sunspots KW - techniques: image processing Y1 - 2015 U6 - https://doi.org/10.1002/asna.201412138 SN - 0004-6337 SN - 1521-3994 VL - 336 IS - 1 SP - 53 EP - 62 PB - Wiley-VCH CY - Weinheim ER - TY - GEN A1 - Diercke, Andrea A1 - Arlt, Rainer A1 - Denker, Carsten T1 - Digitization of Spörer’s sunspot drawings T2 - Postprints der Universität Potsdam : Mathematisch Naturwissenschaftliche Reihe N2 - Much of our knowledge about the solar dynamo is based on sunspot observations. It is thus desirable to extend the set of positional and morphological data of sunspots into the past. Gustav Spörer observed in Germany from Anklam (1861–1873) and Potsdam (1874–1894). He left detailed prints of sunspot groups, which we digitized and processed to mitigate artifacts left in the print by the passage of time. After careful geometrical correction, the sunspot data are now available as synoptic charts for almost 450 solar rotation periods. Individual sunspot positions can thus be precisely determined and spot areas can be accurately measured using morphological image processing techniques. These methods also allow us to determine tilt angles of active regions (Joy’s law) and to assess the complexity of an active region. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 577 KW - sun: sunspots KW - sun: photosphere KW - sun: activity KW - techniques: image processing KW - astronomical databases: miscellaneous KW - history and philosophy of astronomy Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-413922 SN - 1866-8372 IS - 577 SP - 63 EP - 64 ER - TY - JOUR A1 - Diercke, Andrea A1 - Denker, Carsten T1 - Chromospheric Synoptic Maps of Polar Crown Filaments JF - Solar physics : a journal for solar and solar-stellar research and the study of solar terrestrial physics N2 - Polar crown filaments form above the polarity inversion line between the old magnetic flux of the previous cycle and the new magnetic flux of the current cycle. Studying their appearance and their properties can lead to a better understanding of the solar cycle. We use full-disk data of the Chromospheric Telescope (ChroTel) at the Observatorio del Teide, Tenerife, Spain, which were taken in three different chromospheric absorption lines (H alpha lambda 6563 angstrom, Ca II K lambda 3933 angstrom, and He I lambda 10830 angstrom), and we create synoptic maps. In addition, the spectroscopic He I data allow us to compute Doppler velocities and to create synoptic Doppler maps. ChroTel data cover the rising and decaying phase of Solar Cycle 24 on about 1000 days between 2012 and 2018. Based on these data, we automatically extract polar crown filaments with image-processing tools and study their properties. We compare contrast maps of polar crown filaments with those of quiet-Sun filaments. Furthermore, we present a super-synoptic map summarizing the entire ChroTel database. In summary, we provide statistical properties, i.e. number and location of filaments, area, and tilt angle for both the maximum and the declining phase of Solar Cycle 24. This demonstrates that ChroTel provides a promising data set to study the solar cycle. KW - Chromosphere, quiet KW - Prominences, quiescent KW - Prominences, magnetic field KW - Solar Cycle, observations KW - Instrumentation and data management Y1 - 0201 U6 - https://doi.org/10.1007/s11207-019-1538-z SN - 0038-0938 SN - 1573-093X VL - 294 IS - 11 PB - Springer CY - Dordrecht 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 VL - 661 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Diercke, Andrea A1 - Kuckein, Christoph A1 - Denker, Carsten T1 - Dynamics and connectivity of an extended arch filament system JF - Astronomy and astrophysics : an international weekly journal N2 - Aims. In this study, we analyzed a filament system, which expanded between moving magnetic features (MMFs) of a decaying sunspot and opposite flux outside of the active region from the nearby quiet-Sun network. This configuration deviated from a classical arch filament system (AFS), which typically connects two pores in an emerging flux region. Thus, we called this system an extended AFS. We contrasted classical and extended AFSs with an emphasis on the complex magnetic structure of the latter. Furthermore, we examined the physical properties of the extended AFS and described its dynamics and connectivity. Methods. The extended AFS was observed with two instruments at the Dunn Solar Telescope (DST). The Rapid Oscillations in the Solar Atmosphere (ROSA) imager provided images in three different wavelength regions, which covered the dynamics of the extended AFS at different atmospheric heights. The Interferometric Bidimensional Spectropolarimeter (IBIS) provided spectroscopic Ha data and spectropolarimetric data that was obtained in the near-infrared (NIR) Call lambda 8542 angstrom line. We derived the corresponding line-of-sight (LOS) velocities and used He II lambda 304 angstrom extreme ultraviolet (EUV) images of the Atmospheric Imaging Assembly (AIA) and LOS magnetograms of the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) as context data. Results. The NIR Call Stokes-V maps are not suitable to definitively define a clear polarity inversion line and to classify this chromospheric structure. Nevertheless, this unusual AFS connects the MMFs of a decaying sunspot with the network field. At the southern footpoint, we measured that the flux decreases over time. We find strong downflow velocities at the footpoints of the extended AFS, which increase in a time period of 30 min. The velocities are asymmetric at both footpoints with higher velocities at the southern footpoint. An EUV brigthening appears in one of the arch filaments, which migrates from the northern footpoint toward the southern one. This activation likely influences the increasing redshift at the southern footpoint. Conclusions. The extended AFS exhibits a similar morphology as classical AFSs, for example, threaded filaments of comparable length and width. Major differences concern the connection from MMFs around the sunspot with the flux of the neighboring quietSun network, converging footpoint motions, and longer lifetimes of individual arch filaments of about one hour, while the extended AFS is still very dynamic. KW - methods: observational KW - Sun: filaments, prominences KW - Sun: activity KW - techniques: image processing KW - Sun: chromosphere Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201935583 SN - 1432-0746 VL - 629 PB - EDP Sciences CY - Les Ulis ER -