Huairou Solar Observing Station Research Articles

Calibration of Polarization Data for Vector Magnetographs at the Huairou Solar Observing Station over the Past Four Decades

Calibration of Polarization Data for Vector Magnetographs at the Huairou Solar Observing Station over the Past Four Decades

Solar Observation with the Fourier Transform Spectrometer. II. Preliminary Results of Solar Spectrum near the CO 4.66 μm and Mg i 12.32 μm

An infrared solar spectrum observed by ground-based telescopes is seriously affected by the background radiation both from the telescope and sky, relative to the visible wavelengths. Its accuracy is also influenced by the spectral resolution of the Fourier transform spectrometer. In the paper, we developed a CO2 gas cell and installed it in the sample compartment to calibrate the spectral resolution of the Bruker IFS-125HR at infrared wavelengths. The measured spectral resolution is 0.00342 ± 0.00086 cm−1 and 0.0059 ± 0.00024 cm−1 at the wavenumbers of 798 cm−1 and 2136 cm−1, respectively. We also updated a fully reflective sunlight feeding system to observe the solar spectrum near CO 4.66 μm and Mg i 12.32 μm. By quickly pointing the sunlight feeding system about 1 degree away from the solar disk center, we are able to measure the background radiation from the telescope and the sky at Huairou Solar Observing Station. After removing the background radiation, our observed solar spectrum at CO 4.66 μm is consistent with that from the National Solar Observatory. The Mg i 12.32 μm working line selected by the Accurate Infrared Magnetic Field Measurements of the Sun (AIMS) project is also identified. Our method is helpful not only for the spectral resolution calibration and background radiation correction of AIMS but also for other infrared astronomical telescopes.

A Comparison of Co-temporal Vector Magnetograms Obtained with HMI/SDO and SP/Hinode

Accurate measurement of magnetic fields is very important for understanding the formation and evolution of solar magnetic fields. Currently, there are two types of solar magnetic field measurement instruments: filter-based magnetographs and Stokes polarimeters. The former gives high temporal resolution magnetograms and the latter provides more accurate measurements of magnetic fields. Calibrating the magnetograms obtained by filter-based magnetographs with those obtained by Stokes polarimeters is a good way to combine the advantages of the two types. Our previous studies have shown that, compared to the magnetograms obtained by the Spectro-Polarimeter (SP) on board Hinode, those magnetograms obtained by both the filter-based Solar Magnetic Field Telescope (SMFT) of the Huairou Solar Observing Station and by the filter-based Michelson Doppler Imager (MDI) aboard SOHO have underestimated the flux densities in their magnetograms and systematic center-to-limb variations present in the magnetograms of both instruments. Here, using a sample of 75 vector magnetograms of stable alpha sunspots, we compare the vector magnetograms obtained by the Helioseismic and Magnetic Imager (HMI) aboard Solar Dynamics Observatory (SDO) with co-temporal vector magnetograms acquired by SP/Hinode. Our analysis shows that both the longitudinal and transverse flux densities in the HMI/SDO magnetograms are very close to those in the SP/Hinode magnetograms and the systematic center-to-limb variations in the HMI/SDO magnetograms are very minor. Our study suggests that using a filter-based magnetograph to construct a low spectral resolution Stokes profile, as done by HMI/SDO, can largely remove the disadvantages of the filter-type measurements and yet still possess the advantage of high temporal resolution.

Nonlinearity Correction of Fourier Transform Spectrometer in Solar Observation Using Simulated Annealing

A Fourier transform spectrometer (FTS) has been used to observe solar activities due to its ultra-high spectral resolution. However, the FTS in-band spectra are usually distorted and some artifacts appear in out-of-band regions due to nonlinear effects. Therefore, the FTS nonlinear problem must be corrected. In this study, we proposed a novel method to correct the nonlinear effects using simulated annealing. We simulated several nonlinear spectra to evaluate the performance of our method. The calculated quadratic coefficients are extremely close to the given values, demonstrating that the method is effective and accurate. The proposed method is further used to correct the blackbody and solar spectra with nonlinearity obtained by Bruker IFS-125HR installed at the Huairou Solar Observing Station, which is a pathfinder for the accurate infrared magnetic field measurements of the Sun project. To the blackbody spectra, the nonlinearity in low- and high-frequency regions are corrected by 89.09% and 60.84%. The nonlinear correction of the solar spectra in the low- and high-frequency regions have reached 65.34% and 81.04%, respectively. These results prove that our method can correct the nonlinear problem to improve the data accuracy.

Algorithm Development of Cloud Removal from Solar Images Based on Pix2Pix Network

Sky clouds affect solar observations significantly. Their shadows obscure the details of solar features in observed images. Cloud-covered solar images are difficult to be used for further research without pre-processing. In this paper, the solar image cloud removing problem is converted to an image-to-image translation problem, with a used algorithm of the Pixel to Pixel Network (Pix2Pix), which generates a cloudless solar image without relying on the physical scattering model. Pix2Pix is consists of a generator and a discriminator. The generator is a well-designed U-Net. The discriminator uses PatchGAN structure to improve the details of the generated solar image, which guides the generator to create a pseudo realistic solar image. The image generation model and the training process are optimized, and the generator is jointly trained with the discriminator. So the generation model which can stably generate cloudless solar image is obtained. Extensive experiment results on Huairou Solar Observing Station, National Astronomical Observatories, and Chinese Academy of Sciences (HSOS, NAOC and CAS) datasets show that Pix2Pix is superior to the traditional methods based on physical prior knowledge in peak signal-to-noise ratio, structural similarity, perceptual index, and subjective visual effect. The result of the PSNR, SSIM and PI are 27.2121 dB, 0.8601 and 3.3341.

Solar Image Cloud Removal based on Improved Pix2Pix Network

In ground-based observations of the Sun, solar images are often affected by appearance of thin clouds, which contaminate the images and affect the scientific results from data analysis. In this paper, the improved Pixel to Pixel Network (Pix2Pix) network is used to convert polluted images to clear images to remove the cloud shadow in the solar images. By adding attention module to the model, the hidden layer of Pix2Pix model can infer the attention map of the input feature vector according to the input feature vector. And then, the attention map is multiplied by the input feature map to give different weights to the hidden features in the feature map, adaptively refine the input feature map to make the model pay attention to important feature information and achieve better recovery effect. In order to further enhance the model’s ability to recover detailed features, perceptual loss is added to the loss function. The model was tested on the full disk H-alpha images datasets provided by Huairou Solar Observing Station, National Astronomical Observatories. The experimental results show that the model can effectively remove the influence of thin clouds on the picture and restore the details of solar activity. The peak signal-to-noise ratio (PSNR) reaches 27.3012 and the learned perceptual image patch similarity (LPIPS) reaches 0.330, which is superior to the existed dehaze algorithms.

Improvements of the Longitudinal Magnetic Field Measurement from the Solar Magnetic Field Telescope at the Huairou Solar Observing Station

The weak-field approximation, implying a linear relationship between Stokes- $V/I$ and the longitudinal magnetic field [ $B_{\Vert }$ ], often suffers from saturation observed in strong magnetic-field regions such as sunspot umbrae. In this work, we seek to improve the magnetic-field observations carried out by the Solar Magnetic Field Telescope (SMFT) at the Huairou Solar Observing Station, China. We propose using a non-linear relationship between Stokes- $V/I$ and $B_{\Vert }$ to derive the magnetic field. To determine the form of the relationship, we perform a cross-calibration of the observed SMFT data and magnetograms provided by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory. The algorithm for the magnetic-field derivation is described in detail. We show that using a non-linear relationship between Stokes- $V/I$ and $B_{\Vert }$ allows us to eliminate magnetic-field saturation inside the sunspot umbra. The proposed technique enables one to enhance the reliability of the SMFT magnetic-field data obtained even long before the space-based instrumentation era, since 1987.

A new comprehensive dataset of solar filaments of one-hundred-year interval (II) the poleward migration of polar crown filaments

Solar filaments, hypothermia and dense structures suspended in the solar corona are formed above the magnetic polarity inversion line. Polar crown filaments (PCFs) at high-latitude regions of the Sun are of profound significance to the periodic variation of solar activities. In this paper, we statistically analyze PCFs by using full disk Hα data from 1912 to 2018, which were obtained by Kodaikanal Solar Observatory (KODA, India), National Solar Observatory (NSO, USA), Kanzelhöhe Solar Observatory (KSO, Austria), Big Bear Solar Observatory (BBSO, USA), and Huairou Solar Observing Station (HSOS, China). We first manually identify PCFs from every solar image based on the centennial data, and record the latitude and other features corresponding to the PCFs. Then we plot the PCF latitude distribution as a function of time, which clearly shows that PCFs rush to the poles at the ascending phase of each solar cycle. Our results show that the filaments drift toward mid-latitude covering solar cycle 15 to 24 after the PCFs reach the highest latitudes. The poleward migration rates of PCFs are calculated in ten solar cycles, and the range is about 0.12 degree to 0.50 degree per Carrington Rotation (CR). We also investigate the north-south (N-S) asymmetry of migration rates and the normalized N-S asymmetry index.

The passband integration properties of Birefringent filter

In this article, we discuss an observation phenomenon where the total amount of photons in the full passband of the Birefringent filter is a constant number that is considered by removing the spectrum of the light source irrespective of the instrument transmittance. This conclusion is only noticed and considered to be correct in Huairou Solar Observing Station since 1980’s. This article will give a further discussion to the question that had been proposed by the previous researchers. The article structure is organized as history (Sec. 1), experiment (Sec. 2), math (Sec. 3), and discussion (Sec. 4). This issue should be the Paseval-Theorem manifesting itself in astronomical measurement, even though we rigorously demonstrate that this photons conservation has its mathematical generality in Sec. 3.

A New Approach for the Regression of the Center Coordinates and Radius of the Solar Disk Using a Deep Convolutional Neural Network

Abstract This paper presents a new approach for the regression of the center coordinates and radius of the solar disk in Hα solar full-disk images by using a Deep Convolutional Neural Network. We use ∼100,000 original Hα solar full-disk images obtained from Huairou Solar Observing Station as the experimental data set. The data set includes two parts: the original image and three numeric values (center coordinates and radius). In order to deal with the uneven distribution of the solar disk position in the original image, we randomly shift the solar disk during image preprocessing. Furthermore, data augmentation is also used to increase the robustness of the model. By evaluating the model with R-square and relative error, the center coordinates and the radius of the solar disk are proved to be effectively regressed. The data sets we constructed and source code are available as open source on GitHub.

A full-disk image standardization of the chromosphere observation at Huairou Solar Observing Station

Observations of local features in the solar chromosphere began in 1992 at Huairou Solar Observing Station, while the full-disk chromosphere observations were carried out since 2000. In order to facilitate researchers to use full-disk chromosphere observation, algorithms have been developed to standardize the full-disk images. The algorithms include the determination of the center of the image and size standardization, geometric correction and intensity normalization. The solar limb of each image is determined from a histogram analysis of its intensity distribution. The center and radius are then calculated and the image is corrected for geometric distortions. Images are re-scaled to have a fixed radius of 500 pixels and centered within the 1024 × 1024 frame. Finally, large-scale variations in intensity, such as limb-darkening, are removed using a median filter. This paper provides a detailed description of these algorithms, and a summary of the properties of these chromospheric full-disk observations to be used for further scientific investigations.

Flat-fielding of the solar spectrograph with the engineered diffuser

A flat-fielding method with an engineered diffuser is proposed, especially being suitable for the solar spectrograph with relative larger aperture and smaller field-of-view. According to the field-of-view of solar observation and the configuration of the solar spectrograph, we finally chose the engineered diffuser with a scattering angle of 1° and the top-hat intensity distribution. We simulated the uniformity of the scattered sunlight after the engineer diffuser at the focal plane within the field-of-view of 189″×189″ near the solar disk center. The simulation result shows that the uniformity is 99.23%, which is very close to the ideal uniform light source. The engineered diffuser is employed for the flat-field measurement of the solar spectrograph installed at Huairou Solar Observing Station, belonging to National Astronomical Observatories, Chinese Academy of Sciences. The size of the engineered diffuser can be reduced significantly by installing it near the focus plane. During the flat-field measurement time, we also rotate the grating at several fixed positon with a tiny rotation angle. The result indicates that the non-uniformity of the solar spectrograph could be well corrected, including both the non-uniformity from the slit and that from the spectral direction. The large scale distribution of the flat-fielding spectral map in the slit direction agrees well with that from the Helioseismic and Magnetic Imager (HMI). The dirty dots can also be removed after the flat-fielding process. The validity of flat-field measurement with the engineered diffuser is conformed with the self-calibration and the cross calibration.

From polarimetry to helicity: Studies of solar magnetic fields at the Huairou Solar Observing Station

In this paper, we briefly introduce the basic questions in the measurements of solar magnetic fields and the possible error sources due to the approximation of the theory of radiation transfer of spectral lines in the solar atmosphere. We introduce some basic research progress in magnetic field measurement at Huairou Solar Observing Station of National Astronomical Observatories of the Chinese Academy of Sciences, especially concerning the non-potentiality in solar active regions, such as the magnetic shear, current and helicity. We also discuss some basic questions for the measurements of the magnetic fields and corresponding challenges for the future studies.

On the Measurements of Full-Disk Longitudinal Magnetograms at Huairou Solar Observing Station

Reliable information on the distribution of magnetic fields across the whole surface of the Sun is urgently needed to predict conditions in the solar corona, in the interplanetary medium, and in the near-Earth space (space weather). Several space- and ground-based solar instruments currently provide full-disk magnetograms. However, these measurements sometimes differ very significantly, which makes a cross-calibration of different datasets and searching for the reasons for such differences a very crucial task. Here, we analyze the Huairou Solar Observing Station (HSOS) Solar Magnetism and Activity Telescope (SMAT) full-disk line-of-sight magnetograms in comparison with magnetograms taken at the Solar Dynamic Observatory/Helioseismic and Magnetic Imager (SDO/HMI) and Solar Telescope for Operative Predictions (STOP) instruments. We show systematic differences between original SMAT magnetograms and those of other telescopes. The differences are caused by some SMAT instrumental problems, which we investigate. We suggest methods for compensating for these effects that have improved the quality of SMAT magnetograms. These methods will enable us to use SMAT measurements to solve many solar physics problems that are related to studying global solar magnetism and space weather.

Intermittency spectra of current helicity in solar active regions

We analyse the spatial distribution of current helicity in solar active regions. A comparison of current helicity maps derived from three different instruments (Helioseismic and Magnetc Imager on board the Solar Dynamics Observatory, SDO/HMI, Spectro-Polarimeter on board the Hinode, and Solar Magnetic Field Telescope at the Huairou Solar Observing Station, China, HSOS/SMFT) is carried out. The comparison showed an excellent correlation between the maps derived from the spaceborne instruments and moderate correlation between the maps derived from SDO/HMI and HSOS/SMFT vector magnetograms. The results suggest that the obtained maps characterize real spatial distribution of current helicity over an active region. To analyse intermittency of current helicity, we traditionally use the high-order structure functions and flatness function approach. The slope of a flatness function within some range of scales - the flatness exponent - is a measure of the degree of intermittency. SDO/HMI vector magnetograms for 3 ARs (NOAA 11158, 12494, and 12673) were used to calculate the flatness exponent time variations. All three ARs exhibited emergence of a new magnetic flux during the observational interval. The flatness exponent indicated the increase of intermittency 12-20 hours before the emergence of a new flux. We suppose that this behaviour can indicate subphotospheric fragmentation or distortion of the pre-existed electric current system by emerging magnetic flux.

Standard Magnetic Field Production at Huairou Solar Observing Station

AbstractThe regular solar observations are operated at Huairou Solar Observing Station (HSOS) since 1987, which make the construction of long-term magnetic field datasets available to understand solar magnetic field and cycles. There exist some inconveniences for solar physicist to use these data, because the data storage medium and format at HSOS experienced some changes. Additionally, the processes of magnetic field calibration are not easy to deal with for who are not familiar with these data. Here shows that the magnetic field of HSOS are further processed toward international standards, in order to explore HSOS observations data for scientific research.

Automatic detection of solar features in HSOS full-disk solar images using guided filter

A procedure is introduced for the automatic detection of solar features using full-disk solar images from Huairou Solar Observing Station (HSOS), National Astronomical Observatories of China. In image preprocessing, median filter is applied to remove the noises. Guided filter is adopted to enhance the edges of solar features and restrain the solar limb darkening, which is first introduced into the astronomical target detection. Then specific features are detected by Otsu algorithm and further threshold processing technique. Compared with other automatic detection procedures, our procedure has some advantages such as real time and reliability as well as no need of local threshold. Also, it reduces the amount of computation largely, which is benefited from the efficient guided filter algorithm. The procedure has been tested on one month sequences (December 2013) of HSOS full-disk solar images and the result shows that the number of features detected by our procedure is well consistent with the manual one.

The measurement of flat fields and polarization offset from the routine observation data of a solar rotation

The correction of flat fields and polarization offset is a necessary step in the reduction of the full-disk solar observing data. The general used methods of the measurement of flat fields are summarized as follows. The first one is realized by randomly moving the solar telescope and making an average of thousands of observed images of the quiet region near the disk center, which is adopted by the large aperture solar telescopes with a small field-of-view. The second one is to shift the solar images with slightly different pointings, which is mostly used in the full-disk solar images. The third method is to use a diffuser to generate the uniform light source and it can be used in the full-disk solar images, too. Observing the nearby continuum of the selected working spectral line is generally used to obtain the polarization offset. It is worth to mention that, all these methods need special calibration observing mode and occupy the valuable scientific observation time. In the paper, we propose a new method to derive the flat fields and polarization offset from the routine observing data. Assuming that the flat fields and polarization offset do not change during a solar rotation, we firstly got the formula which can be used in the calculation of flat fields and polarization offset in the case of the ideal and real telescope guiding and pointing accuracy. The core algorithm in the formula is median process. Moreover, we obtained the flat fields and the crosstalk from Stokes I to Q , U and V using the above mentioned formula from series of the routine observed monochromatic image and Stokes Q / I , U / I and V / I images belonging to the Carrington Rotation 2144 observed by Huairou Solar Observing Station. The derived flat fields could not only reflect the small-scale dirty points but also the large-scale non-uniform pattern from the whole telescope system, which can be removed after the flat-fielding process. The distribution of the resulting Stokes V / I offset from Stokes I to V seems to be a Saddle surface, while that of the Stokes Q / I and U / I from I to Q , U is an upside-down bell shape. In order to evaluate the validity of the correction of the polarization offset we choose a quiet region near the solar limb as an example. The averaged value of Stokes V / I , Q / I and U / I in the region after polarization offset correction is −0.8×10−4, 0.9×10−4, 1.3×10−4, while that before correction is −6.8×10−4, 2×10−3, 1.7×10−3, respectively. It is quite clear that the Stokes V / I , Q / I and U / I improved a lot after the correction of the polarization offset. The main disadvantage of the method is that it can only correct the crosstalk from Stokes I to Q , U and V , incapable of the correction of that from Stokes V to Q and U . Some other methods are needed so as to make up the disadvantages. We suggest that the above mentioned method can be considered as one of the calibration methods for the space-borne or remote unattended full-disk solar observing instrument because it only uses the routine observation data without any other special calibration observation.

Automated removal of stripe interference in full-disk solar images

The quality of full-disk solar Hα images is significantly degraded by stripe interference. In this paper, to improve the analysis of morphological evolution, a robust solution for stripe interference removal in a partial full-disk solar Hα image is proposed. The full-disk solar image is decomposed into a set of support value images on different scales by convolving the image with a sequence of multiscale support value filters, which are calculated from the mapped least-squares support vector machines (LS-SVMs). To match the resolution of the support value images, a scale-adaptive LS-SVM regression model is used to remove stripe interference from the support value images. We have demonstrated the advantages of our method on solar Hα images taken in 2001–2002 at the Huairou Solar Observing Station. Our experimental results show that our method can remove the stripe interference well in solar Hα images and the restored image can be used in morphology researches.

Comparison of potential field solutions for Carrington Rotation 2144

AbstractWe examined differences among the coronal magnetic field structures derived with the potential field source surface (PFSS) model for Carrington Rotation 2144, from 21 November to 19 December 2013. We used the synoptic maps of solar photospheric magnetic field from four observatories, the Huairou Solar Observing Station (HSOS), Global Oscillation Network Group (GONG), Helioseismic Magnetic Imager (HMI), and Wilcox Solar Observatory (WSO). We tested two smoothing methods, Gaussian and boxcar averaging, and correction of unbalanced net magnetic flux. The solutions of three‐dimensional coronal magnetic field are significantly different each other. An open‐field region derived with HSOS data agrees best with the corresponding coronal hole observed by Solar Dynamics Observatories/Atmospheric Imaging Assembly, while HMI data yielded best agreements with the near‐Earth OMNI database. The GONG data overall gave agreements as good as the HMI. The PFSS calculations using WSO data were least sensitive to the choices we examined in this work. Differences in PFSS solutions using different choices and parameters in smoothing imply that the photospheric magnetic field distributions with size of several degrees at midlatitude and low‐latitude regions can be decisive, at least, in the examined period. To better determine the global solar corona, therefore, further evaluation of influences from compact bipolar magnetic field is needed.