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
α
λ
6563 Å, Ca
ii
K
λ
3933 Å, and He
i
λ
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 the declining phase of Solar Cycle 24. This demonstrates that ChroTel provides a promising data set to study the solar cycle.
Aim. The giant solar filament was visible on the solar surface from 2011 November 8–23. Multiwavelength data from the Solar Dynamics Observatory (SDO) were used to examine counter-streaming flows ...within the spine of the filament. Methods. We use data from two SDO instruments, the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI), covering the whole filament, which stretched over more than half a solar diameter. Hα images from the Kanzelhöhe Solar Observatory (KSO) provide context information of where the spine of the filament is defined and the barbs are located. We apply local correlation tracking (LCT) to a two-hour time series on 2011 November 16 of the AIA images to derive horizontal flow velocities of the filament. To enhance the contrast of the AIA images, noise adaptive fuzzy equalization (NAFE) is employed, which allows us to identify and quantify counter-streaming flows in the filament. We observe the same cool filament plasma in absorption in both Hα and EUV images. Hence, the counter-streaming flows are directly related to this filament material in the spine. In addition, we use directional flow maps to highlight the counter-streaming flows. Results. We detect counter-streaming flows in the filament, which are visible in the time-lapse movies in all four examined AIA wavelength bands (λ171 Å, λ193 Å, λ304 Å, and λ211 Å). In the time-lapse movies we see that these persistent flows lasted for at least two hours, although they became less prominent towards the end of the time series. Furthermore, by applying LCT to the images we clearly determine counter-streaming flows in time series of λ171 Å and λ193 Å images. In the λ304 Å wavelength band, we only see minor indications for counter-streaming flows with LCT, while in the λ211 Å wavelength band the counter-streaming flows are not detectable with this method. The diverse morphology of the filament in Hα and EUV images is caused by different absorption processes, i.e., spectral line absorption and absorption by hydrogen and helium continua, respectively. The horizontal flows reach mean flow speeds of about 0.5 km s−1 for all wavelength bands. The highest horizontal flow speeds are identified in the λ171 Å band with flow speeds of up to 2.5 km s−1. The results are averaged over a time series of 90 minutes. Because the LCT sampling window has finite width, a spatial degradation cannot be avoided leading to lower estimates of the flow velocities as compared to feature tracking or Doppler measurements. The counter-streaming flows cover about 15–20% of the whole area of the EUV filament channel and are located in the central part of the spine. Conclusions. Compared to the ground-based observations, the absence of seeing effects in AIA observations reveal counter-streaming flows in the filament even with a moderate image scale of 0. ′′6 pixel−1. Using a contrast enhancement technique, these flows can be detected and quantified with LCT in different wavelengths. We confirm the omnipresence of counter-streaming flows also in giant quiet-Sun filaments.
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 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 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 . 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.
Ground-based solar observations are severely affected by Earth’s turbulent atmosphere. As a consequence, observed image quality and prevailing seeing conditions are closely related. Partial ...correction of image degradation is nowadays provided in real time by adaptive optics (AO) systems. In this study, different metrics of image quality are compared with parameters characterizing the prevailing seeing conditions, i.e. Median Filter Gradient Similarity (MFGS), Median Filter Laplacian Similarity (MFLS), Helmli–Scherer mean, granular rms-contrast, differential image motion, and Fried-parameter
r
0
. The quiet-Sun observations at disk center were carried out at the Vacuum Tower Telescope (VTT), Observatorio del Teide (OT), Izaña, Tenerife, Spain. In July and August 2016, time series of short-exposure images were recorded with the High-resolution Fast Imager (HiFI) at various wavelengths in the visible and near-infrared parts of the spectrum. Correlation analysis yields the wavelength dependence of the image quality metrics and seeing parameters, and Uniform Manifold Approximation and Projection (UMAP) is employed to characterize the seeing on a particular observing day. In addition, the image quality metrics and seeing parameters are used to determine the field dependence of the correction provided by the AO system. Management of high-resolution imaging data from large-aperture, ground-based telescopes demands reliable image quality metrics and meaningful characterization of prevailing seeing conditions and AO performance. The present study offers guidance on how retrieving such information ex post facto.
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 Hα data and spectropolarimetric data that was obtained in the near-infrared (NIR) Ca IIλ8542 Å line. We derived the corresponding line-of-sight (LOS) velocities and used He IIλ304 Å 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 Ca II 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 quiet-Sun network, converging footpoint motions, and longer lifetimes of individual arch filaments of about one hour, while the extended AFS is still very dynamic.
Aims.
In this work, we explore the spectral window containing Fraunhofer lines formed in the solar photosphere, around the magnetically sensitive Cr
I
lines at 5780.9, 5781.1, 5781.7, 5783.0, and ...5783.8 Å, with Landé
g
-factors between 1.6 and 2.5. The goal is to simultaneously analyze 15 spectral lines, comprising Cr
I
, Cu
I
, Fe
I
, Mn
I
, and Si
I
lines, without the use of polarimetry, to infer the thermodynamic and magnetic properties in strongly magnetized plasmas using an inversion code.
Methods.
Our study is based on a new setup at the Vacuum Tower Telescope (VTT, Tenerife), which includes fast spectroscopic scans in the wavelength range around the Cr
I
5781.75 Å line. The oscillator strengths log(
gf
) of all spectral lines, as well as their response functions to temperature, magnetic field, and Doppler velocity, were determined using the Stokes Inversion based on Response functions (SIR) code. Snapshot 385 of the enhanced network simulation from the Bifrost code serves to synthesize all the lines, which are, in turn, inverted simultaneously with SIR to establish the best inversion strategy. We applied this strategy to VTT observations of a sunspot belonging to NOAA 12723 on 2018 September 30 and compared the results to full-disk vector field data obtained with the Helioseismic and Magnetic Imager (HMI).
Results.
The 15 simultaneously inverted intensity profiles (Stokes
I
) delivered accurate temperatures and Doppler velocities when compared with the simulations. The derived magnetic fields and inclinations achieve the best level of accuracy when the fields are oriented along the line-of-sight (LOS) and less accurate when the fields are transverse to the LOS. In general, the results appear similar to what is reported in the HMI vector-field data, although some discrepancies exist.
Conclusions.
The analyzed spectral range has the potential to deliver thermal, dynamic, and magnetic information for strongly magnetized features on the Sun, such as pores and sunspots, even without the use of polarimetry. The highest sensitivity of the lines is found in the lower photosphere, on average, around log
τ
= −1. The multiple-line inversions provide smooth results across the whole field of view (FOV). The presented spectral range and inversion strategy will be used for future VTT observing campaigns.
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 ii K, and He i 10 830 Å. We present a ...new calibration method that includes limb‐darkening correction, removal of nonuniform filter transmission, and determination of He i Doppler velocities. Chromospheric full‐disk filtergrams are often obtained with Lyot filters, which may display nonuniform 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 the determination of 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 cotemporal high‐resolution spectroscopic data of the GREGOR Infrared Spectrograph (GRIS). Furthermore, this technique can be applied to ChroTel Hα and Ca ii K 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.
Aims.
The regular pattern of quiet-Sun magnetic fields was disturbed by newly emerging magnetic flux, which led a day later to two homologous surges after renewed flux emergence, affecting all ...atmospheric layers. Hence, simultaneous observations in different atmospheric heights are needed to understand the interaction of rising flux tubes with the surrounding plasma, in particular by exploiting the important diagnostic capabilities provided by the strong chromospheric H
α
line regarding morphology and energetic processes in active regions.
Methods.
A newly emerged active region NOAA 12722 was observed with the Vacuum Tower Telescope (VTT) at Observatorio del Teide, Tenerife, Spain, on 11 September 2018. High spectral resolution observations using the echelle spectrograph in the chromospheric H
α
λ
6562.8 Å line were obtained in the early growth phase. Noise-stripped H
α
line profiles yield maps of line-core and bisector velocities, which were contrasted with velocities inferred from Cloud Model inversions. A high-resolution imaging system recorded simultaneously broad- and narrowband H
α
context images. The Solar Dynamics Observatory provided additional continuum images, line-of-sight (LOS) magnetograms, and UV and extreme UV (EUV) images, which link the different solar atmospheric layers.
Results.
The active region started as a bipolar region with continuous flux emergence when a new flux system emerged in the leading part during the VTT observations, resulting in two homologous surges. While flux cancellation at the base of the surges provided the energy for ejecting the cool plasma, strong proper motions of the leading pores changed the magnetic field topology making the region susceptible to surging. Despite the surge activity in the leading part, an arch filament system in the trailing part of the old flux remained stable. Thus, stable and violently expelled mass-loaded ascending magnetic structures can coexist in close proximity. Investigating the height dependence of LOS velocities revealed the existence of neighboring strong up- and downflows. However, downflows occur with a time lag. The opacity of the ejected cool plasma decreases with distance from the base of the surge, while the speed of the ejecta increases. The location at which the surge becomes invisible in H
α
corresponds to the interface where the surge brightens in He
II
λ
304 Å. Broad-shouldered and dual-lobed H
α
profiles suggests accelerated or decelerated and highly structured LOS plasma flows. Significantly broadened H
α
profiles imply significant heating at the base of the surges, which is also supported by bright kernels in UV and EUV images uncovered by swaying motions of dark fibrils at the base of the surges.
Conclusions.
The interaction of newly emerging flux with pre-existing flux concentrations of a young, diffuse active region provided suitable conditions for two homologous surges. High-resolution spectroscopy revealed broadened and dual-lobed H
α
profiles tracing accelerated or decelerated flows of cool plasma along the multi-threaded structure of the surge.
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 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.
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