Fuxian Solar Observatory Research Articles

Correction performance uniformity on ground-layer adaptive optics

ABSTRACTGround-layer adaptive optics (GLAO) is generally used to improve the seeing for large-aperture ground-based optical telescopes across a wide field of view with uniform imaging quality. In this paper, a new criterion, which is defined as the weighted sum of the mean and mean-square-error values of the full width at half-maximum of the field-dependent point spread function of the image over the whole field of view, is proposed to evaluate GLAO performance uniformity. An analytic model of the GLAO-corrected point spread function based on spatial filtering methods is used to investigate the uniformity and image quality over the whole field of view for different guide-star layouts, telescope apertures, numbers of actuators for the deformable mirror, and turbulence profiles. Simulations are performed on the GLAO configurations for the 1-m New Vacuum Solar Telescope at Fuxian Solar Observatory. The results show that, compared with that of the classical evaluation, the correction uniformity improves by 63 per cent, with the resolution sacrificing only 5 per cent for a measured turbulence profile at Fuxian Solar Observatory. Furthermore, this criterion is applied to various requirements, and some critical system parameters are suggested for the wide field-of-view GLAO.

The Formation and Eruption of a Sigmoidal Filament Driven by Rotating Network Magnetic Fields

Abstract We present the formation and eruption of a sigmoidal filament driven by rotating network magnetic fields (RNFs) near the center of the solar disk, which was observed by the 1 m aperture New Vacuum Solar Telescope at the Fuxian Solar Observatory on 2018 July 12. Counterclockwise RNFs twist two small-scale filaments at their northeastern foot-point region, giving a rotation of nearly 200° within about 140 minutes. The motion of the RNF has a tendency to accelerate at first and then decelerate obviously, as the average rotation speed increased from 10 to 150° hr−1, and then slowed down to 50° hr−1. Coalescence then occurs between filaments F1 and F2. Meanwhile the fine structures in the southwestern region of the filament was involved in another interaction of coalescence. The subsequent EUV brightening due to plasma heating is observed in the two interaction regions. These interacting structures, including F1, F2 and the fine structures in the southwestern region, eventually evolve into a larger-scale sigmoidal filament twisted in the same direction as the RNFs gave. The twist of the sigmoidal filament has exceeded 4π and the filament finally erupted. The motion of the sigmoidal filament remains uniform until a nearby jet collides, causing the filament to erupt faster. These results provide evidence that RNF plays an important role in the formation and eruption of the sigmoidal filament. The phenomena also suggests that the kink instability is the trigger mechanism for the filament eruption.

Automatic disturbance identification for linear quadratic Gaussian control in adaptive optics

ABSTRACT Linear quadratic Gaussian (LQG) control is an appealing control strategy to mitigate disturbances in adaptive optics (AO) systems. The key of this method is to quickly and consecutively build an accurate dynamical model to track time-varying disturbances such as turbulence, wind load and vibrations. In order to address this problem, we propose an automatic identification method consisting mainly of an improved spectrum separation procedure and a parameter optimization process based on the particle swarm optimization (PSO) algorithm. The improved spectrum separation can pick out perturbation peaks more accurately, especially when some peaks are very close together. Moreover, compared with the Levenberg–Marquardt method and the maximum-likelihood technique based on grids, the PSO algorithm has a faster convergence speed and lower computational burden, and thus is easier to implement. The entire identification process can run automatically online without human intervention. This identification method is verified with a synthetic disturbance profile in a simulation. Furthermore, the performance of the method is evaluated with consecutive measurement data recorded by the 1-m New Vacuum Solar Telescope at the Fuxian Solar Observatory.

Energy Origination and Triggering Mechanism of a Series of Homologous Confined Flares

Abstract Using the Hα data from the New Vacuum Solar Telescope at the Fuxian Solar Observatory together with multiwavelength images and magnetograms obtained by the Solar Dynamics Observatory, we study the detailed process of three homologous confined flares in active region NOAA 11861 on 2013 October 12. All of the three flares occurred at the same location, with similar morphologies and comparable classes. Through analyzing the evolution of magnetic field and flow field, we found an emergence of magnetic flux and a strong shearing motion between two opposite polarities near the following sunspot. The magnetic flux and the average transverse field strength exhibited a decrease before each eruption and reached the lowest point at the onset of each eruption. By calculating the shearing and the emergence energy in the photosphere, we found that the integral of energy injected from the photosphere, for a few hours, could provide enough energy for the flares. The reconnection between different loops was observed in Hα images during the occurrence of each flare. These results suggest that the emerging magnetic flux and the shearing motion in the photosphere can inject the energy to the sheared magnetic loops and the energy was finally released via magnetic reconnection to power the solar flares.

Chromospheric Cannonballs on the Sun

Abstract In the highly dynamic chromosphere, there exist many kinds of small-scale activities, such as spicules, surges, and Ellerman bombs. Here, we report the discovery of a new phenomenon in the chromosphere observed with the New Vacuum Solar Telescope at the Fuxian Solar Observatory. In the high tempo-spatial-resolution Hα images, some dark or bright structures are found to fly along the curved trajectory, looking like cannonballs. Their average size, mass, and velocity are about 1.5 × 109 km3, 1.5 × 108 kg, and 56 km s−1, respectively. In the simultaneous (extreme-)ultraviolet images obtained by the Solar Dynamics Observatory, these cannonballs appear as brighter features compared to the surrounding area, implying that there exists some kind of heating during this process. The photospheric magnetograms show the magnetic flux emergence and interaction with the pre-existing fields. These observations reveal that the cannonballs are chromospheric material blobs launched due to the magnetic reconnection between emerging magnetic flux and the pre-existing loops.

Coordinated observations between China and Europe to follow active region 12709

AbstractWe present the first images of a coordinated campaign to follow active region NOAA 12709 on 2018 May 13 as part of a joint effort between three observatories (China-Europe). The active region was close to disk center and enclosed a small pore, a tight polarity inversion line and a filament in the chromosphere. The active region was observed with the 1.5-meter GREGOR solar telescope on Tenerife (Spain) with spectropolarimetry using GRIS in the He i 10830 Å spectral range and with HiFI using two broad-band filter channels. In addition, the Lomnicky Stit Observatory (LSO, Slovakia) recorded the same active region with the new Solar Chromospheric Detector (SCD) in spectroscopic mode at Hα 6562 Å. The third ground-based telescope was located at the Fuxian Solar Observatory (China), where the active region was observed with the 1-meter New Vacuum Solar Telescope (NVST), using the Multi-Channel High Resolution Imaging System at Hα 6562 Å. Overlapping images of the active region from all three telescopes will be shown as well as preliminary Doppler line-of-sight (LOS) velocities. The potential of such observations are discussed.

Climatological analysis of the seeing at Fuxian Solar Observatory

There is a significant seasonal variation in the seeing of Fuxian Solar Observatory (FSO). The seeing in summer and autumn is better than that in winter and spring. The overall seeing is divided into the boundary layer seeing and free atmosphere seeing to investigate the climatic phenomena or meteorological events that might lead to seasonal variation in the seeing. The overall seeing was measured by the solar difference image motion monitor (SDIMM). The boundary layer seeing is calculated from the temperature difference between air and water. The analysis results show that the seasonal variation in seeing is caused by the alternation of subtropical high and westerly jet. The decrease of seeing in winter and spring at FSO is probably related to the westerly jet. A complete analysis of the seeing at FSO is given in this paper. It is also the first time to describe FSO’s boundary layer seeing and its measurement method.

The relationship between the 5-min oscillation and 3-min oscillations at the umbral/penumbral sunspot boundary

Observations of the main sunspot of AR 11692 were carried out with the 1 m New Vacuum Solar Telescope (NVST) located on the Fuxian Solar Observatory (FSO) in \(H\alpha\) on March 13, 2013. The high cadence (up to 12 s) \(H\alpha\) intensity images help us to investigate the relationship between the 5-min oscillation and 3-min oscillation. It is found that running waves, periodically formed at the wave sources within umbra, propagate outward with the shape of partial arcs. The running waves run across the umbra–penumbra boundary and eventually disappear at the edge of penumbra. But there are obvious differences when we measure the period of running waves in different regions of a sunspot. The period is about 150 s when the running waves are located in umbra, which is a typical 3-min oscillation, and the period is about 300 s when the running waves are located in the penumbra, which is a typical 5-min oscillation. On the basis of time-slice images, we conclude that the waves form in the umbral region with the 5-min oscillation period, which can cause the brightness periodicity change in the umbra region with the 3-min period (in fact, is half of 5-min oscillation) and 5-min in the penumbra.

INSTRUMENT DESCRIPTION AND PERFORMANCE EVALUATION OF A HIGH-ORDER ADAPTIVE OPTICS SYSTEM FOR THE 1 m NEW VACUUM SOLAR TELESCOPE AT FUXIAN SOLAR OBSERVATORY

ABSTRACT A high-order solar adaptive optics (AO) system including a fine tracking loop and a high-order wavefront correction loop has been installed at the 1 m New Vacuum Solar Telescope of the Fuxian Solar Observatory, in routine operation since 2016. The high-order wavefront correction loop consists of a deformable mirror with 151 actuators, a correlating Shack–Hartmann wavefront sensor with 102 subapertures of which the Absolute Difference Square Algorithm is used to extract the gradients, and a custom-built real-time controller based on a Field-Programmable Gate Array (FPGA) and multi-core Digital Signal Processor (DSP). The frame rate of the wavefront sensor is up to 3500 Hz and this is, to our knowledge, the fastest solar AO system. This AO system can work with a Fried parameter r 0, at the 500 nm wavelength, of larger than 3 cm. The first 65 modes of the Zernike aberrations can be efficiently corrected and the Strehl ratio of the corrected TiO image for the solar pore is superior to 0.75 with the Fried parameter r 0 larger than 10 cm. In this paper, the design of the system is described, and high-resolution solar observational images are presented. Furthermore, the performances of the AO system are evaluated according to the data recorded by the real-time controller.

Statistical research of the umbral and penumbral oscillations

Observation of the main sunspot of AR11692 were carried out with the 1m New Vacuum Solar Telescope (NVST) located on the Fuxian Solar Observatory (FSO) with the Hα on 2013 march 13. The high cadence Hα intensity images show that running waves formed periodically within umbra, and propagate outwards with the shape of partial arcs. The running waves run across the umbra-penumbra boundary and disappear at the outer edge of the penumbra eventually. Confusingly, the period of the running waves within umbra is half of that within penumbra, i.e., the former is a typical 3-min oscillation with a period of 156s and the latter is typical 5-min oscillation with a period of 312s. To investigate the nature of the umbral and penumbral oscillations, we selected 300 sample points from the umbra and 150 sample points from the penumbra, respectively, of the sunspot to analyze the distributions of the extreme values related to the intensity profiles of the sample points. It is found that there are significant difference between the distributions of the extreme values related to umbral and penumbral oscillation intensity profiles. When a possible steady solar radiation, which was replaced by the average of troughs of the umbral oscillation intensity profile, was filtered from the umbral oscillation intensity, the distribution of the trough values is a normal distribution symmetrical about zero, the distribution of the crest values is a skewed distribution. When another possible steady solar radiation, which was replaced by the average of the penumbral oscillation intensity profile, was filtered from the penumbral oscillation intensity, while the distribution of the crest values and trough values were a pair of skewed distribution symmetrical about zero. According to the statistical characteristic of umbral and penumbral oscillations, an interpretation seemingly no having physical meaning was used to explain the statistical results. The possible interpretation for the phenomenon is that the pair of oscillations with different period in the intensity are produced by a same wave with a basis oscillation period of 312s. The basis wave can be expressed as Z(x,y,t)=A(x,y)cos(ωt+θ), where ω=(2π)/312 is the angular frequency of the wave. In the umbra, the intensity due to the wave is proportional to the square of the displacement of the wave, IU=a2Z2=a22A2(x,y)[1+cos(2ωt+2θ)], while the intensity in the penumbra due to the wave is proportional to the basis wave, IP=a1Z=A(x,y)cos(ωt+θ). Correspondingly, the period of the intensity within umbra is TU=156s, while the period of the intensity within penumbra is TP=312s.

First generation solar adaptive optics system for 1-m New Vacuum Solar Telescope at Fuxian Solar Observatory

The first generation solar adaptive optics (AO) system, which consists of a fine tracking loop with a tip-tilt mirror (TTM) and a correlation tracker, and a high-order correction loop with a 37-element deformable mirror (DM), a correlating Shack-Hartmann (SH) wavefront sensor (WFS) based on the absolute difference algorithm and a real time controller (RTC), has been developed and installed at the 1-m New Vacuum Solar Telescope (NVST) that is part of Fuxian Solar Observatory (FSO). Compared with the 37-element solar AO system developed for the 26-cm Solar Fine Structure Telescope, administered by Yunnan Astronomical Observatories, this AO system has two updates: one is the subaperture arrangement of the WFS changed from square to hexagon; the other is the high speed camera of the WFS and the corresponding real time controller. The WFS can be operated at a frame rate of 2100 Hz and the error correction bandwidth can exceed 100 Hz. After AO correction, the averaged residual image motion and the averaged RMS wavefront error are reduced to 0.06″ and 45 nm, respectively. The results of on-sky testing observations demonstrate better contrast and finer structures of the images taken with AO than those without AO.

Identification and morphology analysis of granules using the New Vacuum Solar Telephone Observations

Solar granulation is a plasma convective phenomenon from the solar convection zone to the photosphere, which appears as a cellular pattern on the solar surface. Morphological characteristics and evolution process of granules can help us to better understand the mechanism of the convective motion and the relationship to solar magnetic field. Therefore, an accurate identification method is crucial to study the mechanism and relationship. However, the traditional identification methods based on intensity- and gradient-threshold are very difficult to identify granules because the intensity distribution of granules is non-uniform and their boundaries appear to be blurred. In this paper, we proposed a new method based on phase congruency to identify granules and analyzed their morphological characteristics. The identification procedure includes three steps: (1) obtaining phase congruency feature from an original image; (2) obtaining binary image of phase congruency feature; (3) morphological filter to extract granule shapes. We selected two high-resolution image sequences taken from the New Vacuum Solar Telescope (NVST) at Fuxian Solar Observatory (FSO) to illustrate the identification procedure. For evaluating the accuracy of the proposed method, two traditional methods based on intensity threshold and marked watershed were compare to our method. Furthermore, we also selected three different thresholds to inspect the influence. The experiment result demonstrates that our proposed method is effective and accurate, and insensitive to those selected threshold. This demonstrates that our proposed method can be used to study granules evolution and their physical mechanism.

Prototype of solar ground layer adaptive optics at the 1 m New Vacuum Solar Telescope

A prototype of a solar ground-layer adaptive optics (GLAO) system, which consists of a multi-direction correlating Shack–Hartmann wavefront sensor with 30 effective subapertures and about a 1 arcmin field of view (FoV) in each subaperture, a deformable mirror with 151 actuators conjugated to the telescope entrance pupil, and a custom-built real-time controller based on field-programmable gate array and multi-core digital signal processor (DSP), is implemented at the 1 m New Vacuum Solar Telescope at Fuxian Solar Observatory and saw its first light on January 12th, 2016. The on-sky observational results show that the solar image is apparently improved in the whole FoV over 1 arcmin with the GLAO correction.

Investigation of Umbral Dots with the New Vacuum Solar Telescope

Umbral dots (UDs) are small isolated brightenings observed in sunspot umbrae. They are convective phenomena existing inside umbrae. UDs are usually divided into central UDs (CUDs) and peripheral UDs (PUDs) according to their positions inside an umbra. Our purpose is to investigate UD properties and analyze their relationships, and further to find whether or not the properties depend on umbral magnetic field strengths. Thus, we selected high-resolution TiO images of four active regions (ARs) taken under the best seeing conditions with the \textit{New Vacuum Solar Telescope} in the Fuxian Solar Observatory of the Yunnan Astronomical Observatory, China. The four ARs (NOAA 11598, 11801, 12158, and 12178) include six sunspots. A total of 1220 CUDs and 603 PUDs were identified. Meanwhile, the radial component of the vector magnetic field of the sunspots taken with the \textit{Helioseismic and Magnetic Imager} on-board the \textit{Solar Dynamics Observatory} was used to analyze relationships between UD properties and umbral magnetic field strengths. We find that diameters and lifetimes of UDs exhibit an increasing trend with the brightness, but velocities do not. Moreover, diameters, intensities, lifetimes and velocities depend on the surrounding magnetic field. A CUD diameter was found larger, the CUD brighter, its lifetime longer, and its motion slower in a weak umbral magnetic field environment than in a strong one.

Objective Image-Quality Assessment for High-Resolution Photospheric Images by Median Filter-Gradient Similarity

All next-generation ground-based and space-based solar telescopes require a good quality-assessment metric to evaluate their imaging performance. In this paper, a new image quality metric, the median filter-gradient similarity (MFGS) is proposed for photospheric images. MFGS is a no-reference/blind objective image-quality metric (IQM) by a measurement result between 0 and 1 and has been performed on short-exposure photospheric images captured by the New Vacuum Solar Telescope (NVST) of the Fuxian Solar Observatory and by the Solar Optical Telescope (SOT) onboard the Hinode satellite, respectively. The results show that (1) the measured value of the MFGS changes monotonically from 1 to 0 with degradation of image quality; (2) there exists a linear correlation between the measured values of the MFGS and the root-mean-square contrast (RMS-contrast) of the granulation; (3) the MFGS is less affected by the image contents than the granular RMS-contrast. Overall, the MFGS is a good alternative for the quality assessment of photospheric images.

Second generation solar adaptive optics for 1-m New Vacuum Solar Telescope at the Fuxian Solar Observatory

A second generation solar adaptive optics (AO) system is built and installed at the 1-m New Vacuum Solar Telescope (NVST) of the Fuxian Solar Observatory (FSO) in 2015. The AO high-order correction system consists of a 151-element deformable mirror (DM), a correlating Shack–Hartmann (SH) wavefront sensor (WFS) with a 3500 Hz frame rate, and a real-time controller. The system saw first light on Mar. 16, 2015. The simultaneous high-resolution photosphere and chromosphere images with AO are obtained. The on-sky observational results show that the contrast and resolution of the images are apparently improved after the wavefront correction by AO.

FINE STRUCTURES AND OVERLYING LOOPS OF CONFINED SOLAR FLARES

Using the H$\alpha$ observations from the New Vacuum Solar Telescope at Fuxian Solar Observatory, we focus on the fine structures of three confined flares and the issue why all the three flares are confined instead of eruptive. All the three confined flares take place successively at the same location and have similar morphologies, so can be termed homologous confined flares. In the simultaneous images obtained by the Solar Dynamics Observatory, many large-scale coronal loops above the confined flares are clearly observed in multi-wavelengths. At the pre-flare stage, two dipoles emerge near the negative sunspot, and the dipolar patches are connected by small loops appearing as arch-shaped H$\alpha$ fibrils. There exists a reconnection between the small loops, and thus the H$\alpha$ fibrils change their configuration. The reconnection also occurs between a set of emerging H$\alpha$ fibrils and a set of pre-existing large loops, which are rooted in the negative sunspot, a nearby positive patch, and some remote positive faculae, forming a typical three-legged structure. During the flare processes, the overlying loops, some of which are tracked by activated dark materials, do not break out. These direct observations may illustrate the physical mechanism of confined flares, i.e., magnetic reconnection between the emerging loops and the pre-existing loops triggers flares and the overlying loops prevent the flares from being eruptive.

New vacuum solar telescope and observations with high resolution

The New Vacuum Solar Telescope (NVST) is a one meter vacuum solar telescope that aims to observe fine structures on the Sun. The main goals of NVST are high resolution imaging and spectral observations, including measurements of the solar magnetic field. NVST is the primary ground-based facility used by the Chinese solar research community in this solar cycle. It is located by Fuxian Lake in southwest China, where the seeing is good enough to perform high resolution observations. We first introduce the general conditions at the Fuxian Solar Observatory and the primary science cases of NVST. Then, the basic structures of this telescope and instruments are described in detail. Finally, some typical high resolution data of the solar photosphere and chromosphere are also shown.

Decimetric and metric digital solar radio spectrometers of the Yunnan Astronomical Observatories and the first-light results

Based on an old decimetric solar radio spectrometer working in the frequency range of 625–1500MHz of the Yunnan Astronomical Observatories (YNAO) during the last solar cycle, we designed a fully digital Fast Fourier Transform (FFT) spectrometer to upgrade the old one. The new digital spectrometer has the spectral resolution of 200kHz, much higher than the old one (about 1.3MHz). In addition, we also established a new metric solar radio telescope working in the frequency range of 70–700MHz located at the Fuxian Solar Observatory of YNAO, deploying the same type of the digital FFT spectrometer. The two instruments have begun to operate in a daily survey mode since September 2009 and March 2012, respectively, and many solar radio bursts have been observed. In these events, various types of decimetric and metric fine structures with fairly meticulous spectral features were recognized. These features were never resolved in previous observation and studies. We have introduced these two instruments with their detailed technological components, as well as a set of observational data obtained during the first-light of the instruments. The information revealed by these data can improve our knowledge and understanding of the physics of the energy conversion, particle acceleration and transportation during the solar eruption.

The first observation and data reduction of the Multi-wavelength Spectrometer on the New Vacuum Solar Telescope

The Multi-wavelength Spectrometer is a medium-dispersion (R ∼ 130000) grating spectrometer installed on the New Vacuum Solar Telescope at the Fuxian Solar Observatory, Yunnan Astronomical Observatory, Chinese Academy of Sciences. It is designed to accurately observe the velocity and magnetic fields of the Sun. The present configuration of this spectrometer allows us to simultaneously observe three different solar spectral lines. This work is dedicated to showing the first observations carried out in both the Hα and Ca II 8542 Å lines. We give a detailed description of the data reduction process, focusing on the retrieval of a flat field from the high-resolution spectral data. Two different methods are also performed and compared to eliminate the residual fringe in the reduced data. The real spectral resolution and instrumental profile are analyzed based on the final results, which indicate that this spectrometer presently satisfies the expected performance and it is ready for further scientific observations.