Carsten Denker

In 2026 September, we tested and evaluated an ORCA-Quest 2 qCMOS camera at the 0.7 m Vacuum Tower Telescope (VTT) at the Observatorio del Teide, Tenerife, Spain. The 9.4 megapixel detector is characterized by an extremely low readout noise (0.43 e ⁻ ) and a fast readout speed (120 Hz) in the standard operating mode. These properties make the camera ideal for solar image restoration, which requires bursts of short exposure images that are needed to “freeze” the seeing. Here, observation of active region NOAA 14523 are presented, taken in the Fraunhofer G -band at 430.7 nm. Speckle masking imaging successfully restored the G -band images, even during moderately good seeing conditions. The performance increase delivered by the detector promises to expand the observing intervals for science quality data.

Accurate modeling of stellar active regions (ARs) remains a major bottleneck for radial-velocity and transmission-spectroscopy studies aimed at finding Earth-like planets. While much effort has been devoted to AR modeling, their center-to-limb variations (CLVs) have been largely overlooked. We took a step toward remedying this by measuring the CLV of the 6173 Å continuum intensity for sunspots (the whole spot, separate umbrae, and penumbrae), faculae, network, and the quiet Sun using the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). This study is based on four simple round α -sunspots and their surroundings, as well as one strongly evolving region. After correcting for stray light, we find that relative to the quiet Sun, all components except for the umbra display reduced darkening toward the limb. Additionally, strongly evolving ARs do not appear to display significantly altered CLV profiles compared to stable ARs. Faculae and network show contrast enhancements that peak near µ ≈ 0.3 before declining toward the limb, reaching maxima of approximately 4% and 2%, respectively, in contrast excess relative to the quiet Sun, while the spot-to-quiet-Sun contrast rises to approximately 15% near the limb. For both types of AR, this change in CLV behavior near the limb is likely related to the 3D structure of the ARs and the rapidly changing viewing geometry. This behavior is not captured by synthetic CLVs based on PHOENIX and ATLAS model atmospheres with solar values and a different effective temperature, underscoring the need for more realistic treatments of stellar activity.

Context. The Fraunhofer G -band around 4304 Å is widely used as a photospheric diagnostic and is generally not expected to show signatures of chromospheric or coronal structures. However, recent amateur observations have suggested the presence of off-limb prominence emission in this spectral region. Aims. We investigated the origin of the prominence emission in the G -band to determine whether this is caused by methylene (CH) or other lines in this band. We also tested these lines for the presence of ionization memory effects in neutral lines. Methods. We present a case study of two prominences, one obtained with a Solar Explorer (Sol’Ex) spectroheliograph, and the other with the high-resolution Fast Multi-Line Universal Spectrograph (FaMuLUS) camera system at the echelle spectrograph of the German Vacuum Tower Telescope (VTT). The line widths were measured for simultaneously observed neutral and ionized metal lines, allowing a comparison of thermal and nonthermal broadening components to determine whether these lines exhibit any ionization memory effects. Results. We report clear prominence emission in several metal lines within the G -band, primarily from Ti II and Ca I lines, while contributions from CH molecular lines are not observed. A comparison of the simultaneously observed ionized and neutral lines revealed no clear evidence of an ionization memory effect. Conclusions. Since the prominence emission does not originate from CH lines, we do not call them G -band prominences, but prominences in the G -band, as they are independent of the primary diagnostic in this spectral window. In addition, the absence of a clear ionization memory effect suggests that these effects may be less pronounced for weak neutral lines.

We performed high-spatial-resolution spectropolarimetric observations of the active region NOAA 13363 during a C-class flare with the Gregor Infrared Spectrograph (GRIS) on 16 July 2023. We examined the coupling between the photosphere and the chromosphere, studying the polarimetric signals during a period that encompasses the decaying phase of a C-class flare and the appearance of a new C-class flare at the same location. We focused on the analysis of various spectral lines. In particular, we studied the Si I 10827 Å, Ca I 10833.4 Å, Na I 10834.9 Å, and Ca I 10838.9 Å photospheric lines, as well as the He I 10830 Å triplet. GRIS data revealed the presence of flare-related red- and blueshifted spectral line components, reaching Doppler velocities of up to ∼90 km s ⁻¹ , and complex Si I profiles in which the He I spectral line contribution is blueshifted. In contrast, the photospheric Ca I and Na I transitions remain unchanged, indicating that the flare did not modify the physical conditions of the lower photosphere. We combined that information with simultaneous imaging in the Ca II H line and TiO band with the improved High-resolution Fast Imager (HiFI+), finding that the flare emission did not affect the inverse granulation or nearby plage, in agreement with the results from GRIS. We also complemented the previous studies with a forward modelling computation, concluding that the He I spectral line emission reflects a complex response of the flaring chromosphere. Radiative excitation from coronal EUV irradiation, energy deposition by flare-accelerated electrons, and dynamic field-aligned plasma flows likely act together to produce the observed supersonic downflows and upflows. We plan to expand these findings through inversions of the He I 10830 Å triplet signals in the future.

We present high-resolution spectroscopic observations of the quiet-Sun center-to-limb variations (CLV) of the He I triplet at 10 830 Å and the nearby Si I 10 827 Å line, observed with GREGOR Infrared Spectrograph (GRIS) and the improved High-resolution Fast Imager (HiFI+). The observations cover the interval μ = [0.1, 1.0], where μ is the cosine of the heliocentric angle. At each μ position, the spectra were spatially averaged over 0.02 μ and the resulting CLVs were given both as these averaged data points and as smooth polynomial curves fitted across each wavelength point. The He I spectra were inverted using the HAnle and ZEeman Light (HAZEL) code, showing an increase in optical depth towards the limb and a reversed convective blueshift for the red component, while the blue component was entirely absent. In addition, we found a strong increase in the steepness of the He I CLV compared to that of the nearby continuum. The Si I showed a behavior more typical of photospheric lines, namely shallower CLV, a reduction in width and depth, and a more typical convective blueshift.

Context. The photospheric Evershed flow is normally oriented radially outward, yet sometimes opposite velocities are observed not only in the chromosphere but also in the photospheric layers of the penumbra. Aims. We studied the velocity field in a special case of an active region with two mature sunspots, the lesser of which formed several days after the main one. Flux emergence between the two spots is still ongoing, influencing the velocity pattern. Methods. We observed the active region NOAA 12146 on August 24, 2014, with the GREGOR Fabry-Pérot Interferometer and the Blue Imaging Channel of the GREGOR solar telescope at Observatorio del Teide on Tenerife. Context data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory complement the high-resolution data. Results. In the penumbra of a newly formed spot, we observe opposite Doppler velocity streams of up to ±2 km s ⁻¹ very close to each other. These velocities extend beyond the outer penumbral boundary and cross the polarity-inversion line. The properties of the magnetic field do not change significantly between these two streams. Although the magnetic field is almost horizontal, we do not detect high transversal velocities in horizontal flow maps obtained via the local correlation technique. Conclusions. The ongoing emergence of magnetic flux in an active region causes flows of opposite directions that penetrate the penumbra of a preexisting sunspot.

Active region NOAA 14274 produced some of the strongest flares of Solar Cycle 25, including the X1.2 and X5.1 flares on 10 and 11 November 2025, respectively. We present the first large mosaic of speckle-restored images obtained with the improved High-resolution Fast Imager (HiFI+) at the 1.5-meter GREGOR solar telescope at the Observatorio del Teide in Izaña, Tenerife, Spain. The observations were obtained approximately 30 minutes before the onset of the X1.2 flare. The active region exhibited strongly curved penumbral filaments, sunspot rotation, and shear motions along the polarity inversion line (PIL), which led to a highly stressed magnetic field configuration that stored sufficient energy to release multiple M- and X-class flares. The first flare signatures appeared as small-scale brightenings, each with a width of a few tenths of an arcsecond, that trace penumbral filaments in the trailing sunspot.

Active region NOAA 14274 produced some of the strongest flares of Solar Cycle 25, including the X1.2 and X5.1 flares on 2025 November 10 and 11, respectively. We present the first large mosaic of speckle-restored images obtained with the improved High-resolution Fast Imager at the 1.5 m GREGOR solar telescope at the Observatorio del Teide in Izaña, Tenerife, Spain. The observations were obtained approximately 30 minutes before the onset of the X1.2 flare. The active region exhibited strongly curved penumbral filaments, sunspot rotation, and shear motions along the polarity inversion line, which led to a highly stressed magnetic field configuration that stored sufficient energy to release multiple M- and X-class flares. The first flare signatures appeared as small-scale brightenings, each with a width of a few tenths of an arcsecond, that trace penumbral filaments in the trailing sunspot.

The photospheric Evershed flow is normally oriented radially outward, yet sometimes opposite velocities are observed not only in the chromosphere but also in the photospheric layers of the penumbra. We study the velocity field in a special case of an active region with two mature sunspots, where one of them formed several days later than the main one. Between the two spots, flux emergence is still ongoing influencing the velocity pattern. We observed the active region NOAA 12146 on August 24, 2014, with the GREGOR Fabry-Pérot Interferometer (GFPI) and the Blue Imaging Channel (BIC) of the GREGOR solar telescope at Observatorio del Teide on Tenerife. Context data from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) complement the high-resolution data. In the penumbra of a newly formed spot, we observe opposite Doppler velocity streams of up to +-2 km/s very close to each other. These velocities extend beyond the outer penumbral boundary and cross also the polarity-inversion line. The properties of the magnetic field do not change significantly between these two streams. Although the magnetic field is almost horizontal, we do not detect large transversal velocities in horizontal flow maps obtained with the local correlation technique. The ongoing emergence of magnetic flux in an active region causes flows of opposite directions intruding the penumbra of a pre-existing sunspot.

Context . Differential rotation is a key driver of magnetic activity and dynamo processes in the Sun and other stars, especially as the rate differs across the solar layers, and also in active regions. Aims . We aim to accurately quantify the velocity at which round α spots traverse the solar disk as a function of their latitude, and compare these rates to those of the quiet Sun and other sunspot types. We then extend this work to other stars and investigate how differential rotation affects the modulation of stellar light curves by introducing a generalized stellar differential rotation law. Methods . We manually identified and tracked 105 α sunspots in the 6173 Å continuum using the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO). We measured the angular velocities of each spot through center-of-mass and geometric ellipse-fitting methods to derive a differential rotation law for round α sunspots. Results . Using over a decade of HMI data, we derived a differential rotation law for α sunspots. Compared to previous measurements, we find that α sunspots rotate 1.56% faster than the surrounding quiet Sun, but 1.35% slower than the average sunspot population. This supports the hypothesis that the depth at which flux tubes are anchored influences sunspot motions across the solar disk. We extend this analysis to other stars by introducing a scaling law based on the rotation rates of these stars. This scaling law is implemented with the Stellar Activity Grid for Exoplanets (SAGE) code to illustrate how differential rotation alters the photometric modulation of active stars. Conclusions . Our findings emphasize the necessity of considering differential rotation effects when modeling stellar activity and exoplanet transit signatures.