Solar Magnetic Field Telescope Research Articles

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.

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.

Automatic detection and correction algorithms for magnetic saturation in the SMFT/HSOS longitudinal magnetograms

A longitudinal magnetic field often suffers the saturation effect in a strong magnetic field region when the measurement is performed at a single-wavelength point and linear calibration is adopted. In this study, we develop a method that can judge the threshold of saturation in Stokes V/I observed by the Solar Magnetic Field Telescope (SMFT) and correct it automatically. The procedure is to first perform the second-order polynomial fit to the Stokes V/I vs. I/Im (Im is the maximum value of Stokes I) curve to estimate the threshold of saturation, then reconstruct Stokes V/I in a strong field region to correct for saturation. The algorithm is demonstrated to be effective by comparing with the magnetograms obtained by the Helioseismic and Magnetic Imager (HMI). The accuracy rate of detection and correction for saturation is ∼99.4% and ∼88% respectively among 175 active regions. The advantages and disadvantages of the algorithm are discussed.

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.

Method of controlling optical surface roughness based on stray light requirements

Scattering introduced by optical surface fabrication errors could degrade optical performance severely. Therefore, the optical designers are required to provide a roughness index for describing the specific surface or even all surfaces to ensure the final imaging performance. The surface root-mean-square (RMS) roughness is a common index to quantify surface topography. And there are also some available methods to acquire the surface RMS roughness based on bidirectional scattering distribution function theory or the angle spread function theory. However, the influence of the optical surface scattering on the optical system cannot be accurately revealed by the surface RMS roughness determined by these methods. On the one hand, the RMS roughness corresponds to an excessively wide spatial frequency range from 0 to 1/, where is the wavelength of the light. Consequently, it is difficult to measure the RMS roughness during manufacture. On the other hand, what really worsens the stray light performance of the system is only the surface profile located within a certain subinterval of the aforementioned frequency range, to put it in another way, the surface RMS roughness identified by the methods above is incompetent to quantify the amount of the energy that is surfacescattered to the detector. To address the issues above, in this paper we propose a novel approach to identifying the surface roughness. This method seeks to deduce the relation between optical surface RMS roughness and the stray light requirement of the system by dint of partial integrated scattering (PIS). In contrast to total integrated scattering, PIS counts the scattering light energy that could reach the detector. Hence, the RMS roughness identified in this way corresponds to the effective spatial frequency range that contributes to the stray light in the system. Firstly, the effective frequency range concerned with the system stray light level is identified through the analysis of the propagation path of the scattered light. Then, the surface RMS roughness would be measured within the established range according to the stray light requirement of the system and used to control the surface roughness as the roughness index during the optical manufacture process. The method not only considers the scattering as the surface characteristic, but also takes into account the influence of scattering on the system. Taking the solar magnetic field telescope (MFT) for example, the validity of the method is verified by comparing with the traditional methods. As manifested in the outcome, the effective frequency range of primary mirror is from 0 to 18 mm-1, and the surface RMS roughness identified in such a new way can stage the stray light performance of MFT in a more precise manner, which is more reliable to serve as a surface roughness index.

Improved magnetogram calibration of Solar Magnetic Field Telescope and its comparison with the Helioseismic and Magnetic Imager

In this paper, we try to improve the magnetogram calibration method of the Solar Magnetic Field Telescope (SMFT). The improved calibration process fits the observed full Stokes information, using six points on the profile of Fe i 5324.18 Å line, and the analytical Stokes profiles under the Milne–Eddington atmosphere model, adopting the Levenberg–Marquardt least-squares fitting algorithm. In comparison with the linear calibration methods, which employs one point, there is a large difference in the strength of longitudinal field Bl and transverse field Bt, caused by the non-linear relationship, but the discrepancy is little in the case of inclination and azimuth. We conclude that it is better to deal with the non-linear effects in the calibration of Bl and Bt using six points. Moreover, in comparison with Solar Dynamics Observatory/Helioseismic and Magnetic Imager (HMI), SMFT has larger stray light and acquires less magnetic field strength. For vector magnetic fields in two sunspot regions, the magnetic field strength, inclination and azimuth angles between SMFT and HMI are roughly in agreement, with the linear fitted slopes of 0.73/0.7, 0.95/1.04 and 0.99/1.1. In the case of pores and quiet regions (Bl < 600 G), the fitted slopes of the longitudinal magnetic field strength are 0.78 and 0.87, respectively.

A statistical study on property of spatial magnetic field for solar active region

Magnetic fields dominate most solar activities, there exist direct relations between solar flare and the distributions of magnetic field, and also its corresponding magnetic energy. In this paper, the statistical results about the relationships between the spatial magnetic field and solar flare are given basing on vector magnetic field observed by the Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station (HSOS). The spatial magnetic fields are obtained by extrapolated photosphere vector magnetic field observed by SMFT. There are 23 active regions with flare eruption are chosen as data samples, which were observed from 1997 to 2007. The results are as follows: 1. Magnetic field lines become lower after flare for 16(69%) active regions; 2. The free energy are decreased after flare for 17 (74%) active regions. It can conclude that for most active regions the changes of magnetic field after solar flare are coincident with the previous observations and studies.

MAGNETIC NONPOTENTIALITY IN PHOTOSPHERIC ACTIVE REGIONS AS A PREDICTOR OF SOLAR FLARES

Based on several magnetic nonpotentiality parameters obtained from the vector photospheric active region magnetograms obtained with the Solar Magnetic Field Telescope at the Huairou Solar Observing Station over two solar cycles, a machine learning model has been constructed to predict the occurrence of flares in the corresponding active region within a certain time window. The Support Vector Classifier, a widely used general classifier, is applied to build and test the prediction models. Several classical verification measures are adopted to assess the quality of the predictions. We investigate different flare levels within various time windows, and thus it is possible to estimate the rough classes and erupting times of flares for particular active regions. Several combinations of predictors have been tested in the experiments. The True Skill Statistics are higher than 0.36 in 97% of cases and the Heidke Skill Scores range from 0.23 to 0.48. The predictors derived from longitudinal magnetic fields do perform well, however they are less sensitive in predicting large flares. Employing the nonpotentiality predictors from vector fields improves the performance of predicting large flares of magnitude $\geq$M5.0 and $\geq$X1.0.

A Statistical Study on Force-Freeness of Solar Magnetic Fields in the Photosphere

AbstractIt is an indisputable fact that solar magnetic fields are force-free in the corona, where force-free fields mean that current and magnetic fields are parallel and there is no Lorentz force in the fields. While the force-free extent of photospheric magnetic fields remains open, in this paper, we give its statistical results. Vector magnetograms (namely, Bx, By, and Bz in heliocentric coordinates) employed are observed by the Solar Magnetic Field Telescope at Huairou Solar Observing Station. We study and calibrate 925 magnetograms calibrated by two sets of calibration coefficients, which indicate the relation between magnetic fields and the strength of the Stokes spectrum and can be calculated either theoretically or empirically. The statistical results show that the majority of active region magnetic fields are not consistent with the force-free model.

Changing photospheric vector magnetic fields associated with a B4.2-class solar flare

AbstractRecent observations have provided evidence that the solar photospheric magnetic fields could have rapid and permanent changes in both longitudinal and transverse components associated with large (X- or M-class) solar flares. However, few observations have been reported about small flares. In this paper we find the observational evidence of changing photospheric vector magnetic fields associated with a B4.2-class flare obtained with the Solar Magnetic Field Telescope (SMFT) installed at Huairou Solar Observing Station (HSOS) of Nation Astronomical Observatories of Chinese Academy of Sciences.

A Statistical Study on Photospheric Magnetic Nonpotentiality of Active Regions and Its Relationship with Flares During Solar Cycles 22 – 23

A statistical study is carried out on the photospheric magnetic nonpotentiality in solar active regions and its relationship with associated flares. We select 2173 photospheric vector magnetograms from 1106 active regions observed by the Solar Magnetic Field Telescope at Huairou Solar Observing Station, National Astronomical Observatories of China, in the period of 1988 – 2008, which covers most of the 22nd and 23rd solar cycles. We have computed the mean planar magnetic shear angle (\(\overline{\Delta\phi}\)), mean shear angle of the vector magnetic field (\(\overline{\Delta\psi}\)), mean absolute vertical current density (\(\overline{|J_{z}|}\)), mean absolute current helicity density (\(\overline{|h_{\mathrm{c}}|}\)), absolute twist parameter (|α av|), mean free magnetic energy density (\(\overline{\rho_{\mathrm{free}}}\)), effective distance of the longitudinal magnetic field (d E), and modified effective distance (d Em) of each photospheric vector magnetogram. Parameters \(\overline{|h_{\mathrm{c}}|}\), \(\overline{\rho_{\mathrm{free}}}\), and d Em show higher correlations with the evolution of the solar cycle. The Pearson linear correlation coefficients between these three parameters and the yearly mean sunspot number are all larger than 0.59. Parameters \(\overline {\Delta\phi}\), \(\overline{\Delta\psi}\), \(\overline{|J_{z}|}\), |α av|, and d E show only weak correlations with the solar cycle, though the nonpotentiality and the complexity of active regions are greater in the activity maximum periods than in the minimum periods. All of the eight parameters show positive correlations with the flare productivity of active regions, and the combination of different nonpotentiality parameters may be effective in predicting the flaring probability of active regions.

Magnetic Helicity of Solar Active Regions as Revealed by Vector Magnetograms and Coronal X-Ray Images

Abstract We have used photospheric vector magnetograms of 15 different solar active regions to calculate the current helicity parameter, $ \alpha _{\rm av}$ , and the linear force-free field (LFFF) parameter, $ \alpha _{\rm best}$ , that fits best the observed transverse field. The data were obtained with the Solar Magnetic Field Telescope at the Huairou Solar Observing Station, the National Astronomical Observatories of China, the Solar Flare Telescope of the National Astronomical Observatory of Japan, and the Haleakala Stokes Polarimeter at the Mees Solar Observatory, University of Hawaii, from 1997 to 2000. The agreement in sign of $ \alpha _{\rm av}$ between three vector magnetographs is better than 90%. For $ \alpha _{\rm best}$ , the agreement is 80%–90%. The line-of-sight magnetograms observed with the Michelson–Doppler Imager (MDI) on SOHO and coronal X-ray images observed with the Soft X-ray Telescope (SXT) on Yohkoh have been used to determine the constant $ \alpha _{\rm c}$ of the LFFF in the corona. The value of $ \alpha _{\rm c}$ corresponds to the extrapolated coronal field whose field lines best match, by visual inspection, the structure of coronal loops in X-ray images. It is found that the sign agreement between photospheric $ \alpha _{\rm av}$ or $ \alpha _{\rm best}$ and coronal $ \alpha _{\rm c}$ is lower (60%–85%). We consider the differences in measurements, observing conditions, data reduction methods, and limitation in LFFF extrapolation, and discuss their contributions to the dispersions in the hemispheric sign rule of helicity.

Quantifying solar superactive regions with vector magnetic field observations

The vector magnetic field characteristics of superactive regions (SARs) hold the key for understanding why SARs are extremely active and provide the guidance in space weather prediction. We aim to quantify the characteristics of SARs using the vector magnetograms taken by the Solar Magnetic Field Telescope at Huairou Solar Observatory Station. The vector magnetic field characteristics of 14 SARs in solar cycles 22 and 23 were analyzed using the following four parameters: 1) the magnetic flux imbalance between opposite polarities, 2) the total photospheric free magnetic energy, 3) the length of the magnetic neutral line with its steep horizontal magnetic gradient, and 4) the area with strong magnetic shear. Furthermore, we selected another eight large and inactive active regions (ARs), which are called fallow ARs (FARs), to compare them with the SARs. We found that most of the SARs have a net magnetic flux higher than 7.0\times10^21 Mx, a total photospheric free magnetic energy higher than 1.0\times10^24 erg/cm, a magnetic neutral line with a steep horizontal magnetic gradient (\geq 300 G/Mm) longer than 30 Mm, and an area with strong magnetic shear (shear angle \geq 80\degree) greater than 100 Mm^2. In contrast, the values of these parameters for the FARs are mostly very low. The Pearson \c{hi}2 test was used to examine the significance of the difference between the SARs and FARs, and the results indicate that these two types of ARs can be fairly distinguished by each of these parameters. The significance levels are 99.55%, 99.98%, 99.98%, and 99.96%, respectively. However, no single parameter can distinguish them perfectly. Therefore we propose a composite index based on these parameters, and find that the distinction between the two types of ARs is also significant with a significance level of 99.96%. These results are useful for a better physical understanding of the SAR and FAR

Vector magnetic field measurement and helicity of active regions

The Solar Magnetic Field Telescope (SMFT) at the Huairou Solar Observing Station of the National Astronomical Observatory of China has been obtaining photospheric vector magnetograms on a daily basis for more than 20 years. In this brief review, I will first introduce our magnetograph, together with some efforts on correcting for a few known problems in filter-type magnetograph measurement. Then, I will focus on what we can learn from these vector magnetic field measurements with an emphasis on the helicity of active regions. It is generally believed that magnetic helicity is produced by solar dynamo in the convection zone and its accumulation in the corona will influence coronal dynamics. Our observations of helicity in active regions can thus be used to constrain dynamo models and suggest favorable conditions for coronal eruptions.

Imbalance of magnetic field observed in vector magnetograms

The systematic investigation of the three components of the magnetic field is made on 6629 vector magnetograms obtained with the Solar Magnetic Field Telescope at Huairou Solar Observing Station over 18 years 1988–2005. The sign distribution of these values has been analyzed over the solar hemispheres and the solar activity cycle as follows: (1) In the 22nd cycle, 〈 B x 〉 in the northern (southern) hemisphere has been 59% (56%) positive (negative); 〈 B y 〉 in the northern (southern) hemisphere has been 66% (65%) negative (negative); while 〈 B z 〉 in the northern (southern) hemisphere has been 72% (71%) negative (positive). (2) In the 23nd cycle, 〈 B x 〉 in the southern (northern) hemisphere has been 73% (71%) positive (negative); 〈 B y 〉 in the both northern and southern hemisphere has been 72% (72%) positive (positive); 〈 B z 〉 in the southern (northern) hemisphere has been 70% (71%) negative (positive). The explanation needs further investigation.

A comparison of co-temporal magnetograms obtained with the Huairou magnetograph and the Spectro-Polarimeter on board Hinode

We have compared a set of co-temporal magnetograms obtained with the Solar Magnetic Field Telescope (SMFT) of the Huairou Solar Observing Station (HSOS) and with the Spectro-Polarimeter of the Solar Optical Telescope (SP/SOT) on board Hinode to check the linear calibrations of SMFT vector magnetograms. The comparison shows that the currently used calibration coefficients of the SMFT have under-estimated the flux density and that a center-to-limb variation of the calibration coefficients was not taken into account by previous calibrations.

The non-uniform pattern in full-disc vector magnetograms and its correction

The Solar Magnetism and Activity Telescope (SMAT) has been operational at Huairou Solar Observing Station since the end of 2005. Its scientific projects are the observational study of full-disc vector magnetic field and Hα chromospheric activities. In this paper, the Fourier low-frequency passing filter and the large-scale polynomials fit methods have been employed to analyse and remove the non-uniform pattern (NUP) in the full-disc Stokes V image. These methods are useful to effectively improve the quality of SMAT's data. The following main results have been obtained. (1) An intrinsic large-scale spatial distribution of NUP appears in SMAT's full-disc Stokes V image and its average amplitude accounts for 2 per cent of sunspots' field of kG. (2) NUP is testified as instrumental polarization through data reduction and observational explanation (passband shift). There are pseudo-passband shift in the wide field of view of SMAT due to NUP, and the maximum shifting value from the solar disc centre to the solar disc limb is 0.12 A. (3) As an example, after removing SMAT's NUP in the Stokes V image, the correlation coefficients of the magnetograms become 78 per cent between SMAT and MDI (Michelson Doppler Imager project) on the SOHO (Solar and Heliospheric Observatory) satellite, and 92 per cent between SMAT and the SMFT (Solar Magnetic Field Telescope) at Huairou.

Statistical removal of Faraday rotation in vector magnetograms taken by filter-type magnetographs

Based on the spectral scan data of NOAA AR 10325, 10484 and 10377 obtained with the Solar Magnetic Field Telescope at Huairou Solar Observing Station, we analyse the azimuthal differences (ΔδΦ) between the line centre and the line wing -0.12 A of Fe 1 λ 5324.19 caused by Faraday rotation. Grids of the longitudinal field B z and inclination ψ for intervals of 100 G and 3° are made. It is found that the azimuthal rotations are of the same magnitude when B z is in a 100-G interval. The amplitudes of azimuthal differences obviously increase with increasing B z and decrease with increasing ψ. Averaging ΔδΦ in each 100-G interval of B z and making a linear fit between and B z , then we obtain the correlation between the observed azimuthal difference and B z depicted by the linear fitting formula (1), namely ΔδΦ° = 0.0114 |Β z |. In addition, through the simulation we also obtain the azimuthal rotation at the line centre of Fe I λ5324.19 described by the fitting formula (2), namely δΦ 0.00 = 0.0254 |B z | - 3.975 x 10 -6 B 2 . We also present a third fitting formula (3), namely ΔδΦ t = 0.0149 |B z | - 1.455 x 10 -6 B z 2 , which describes the azimuthal differences between the line centre and the line wing -0.12 A. Formula (3) is consistent with the observational results of the three active regions. We correct the vector magnetograms of the three active regions with formulae (1) and (3) and find that the electric current J z , the current helicity He, and the average force-free field factor α av calculated from the correct magnetograms are nearly equivalent to those measured at the line wing -0.12 A. Furthermore, we calculate J z with 393 corrected vector magnetograms and find that formulae (1) and (3) can remove part of the false current whereas formula (2) can remove most of it.

The Effects of Polarization Crosstalk and Solar Rotation on Measuring Full‐Disk Solar Photospheric Vector Magnetic Fields

Observations of full-disk solar photospheric vector magnetic fields are very important for the studies of the global properties and evolution of solar magnetism. Such observations can allow us to investigate the magnetic connectivities between active regions and the nonpotentiality of magnetic fields with a much wider field of view (FOV) than do local-region observations. Therefore, we developed a full-disk vector magnetograph for the Solar Magnetism and Activity Telescope (SMAT) at Huairou Solar Observing Station. In this paper, we point out two problems that affect measurement accuracy of full-disk vector magnetic fields: circular-to-linear polarization crosstalk produced by the instrument, and wavelength shift due to solar rotation. An attempt is made to correct such polarization crosstalk and to reconstruct full-disk vector magnetic fields from polarization data. We also give a superficial discussion about the problem of an incompletely wide field of view (IWFOV) of the Lyot filter in the measurements of full-disk solar vector magnetic fields. In addition, the polarization data of the SMAT are compared with those of the Solar Magnetic Field Telescope (SMFT), which operates the local-region observations. Although the SMFT is 2-3 times as sensitive as the SMAT, there is a basic agreement on the polarization data of the two instruments.

Helicity comparison among three magnetographs

We compare vector magnetograms of 228 active regions observed by Solar Magnetic Field Telescope (SMFT) at Huairou (HR) Solar Observing Station and the Solar Flare Telescope (SFT) at Mitaka (MTK) of the National Astronomical Observatory of Japan from 1992 to 2005 and 55 active regions observed by SFT and Haleakala Stokes Polarimeter (HSP) at Mees Solar Observatory, University of Hawaii from 1997 to 2000. Two helicity parameters, current helicity density h c and α ff coefficient of linear force free field are calculated. From this comparison we conclude: (1) the mean azimuthal angle differences of transverse fields between HR and MTK data are systematic smaller than that between MTK and Mees data; (2) there are 83.8% of h c and 78.1% of α ff for 228 active regions observed at HR and MTK agree in sign, and the Pearson linear correlation coefficient between these two data sets is 0.72 for h c and 0.56 for α ff. There are 61.8% of h c and 58.2% of α ff for 55 active regions observed at MTK and Mees agree in sign, and the Pearson linear correlation coefficient between these two data sets is 0.34 for h c and 0.31 for α ff; (3) there is a basic agreement on time variation of helicity parameters in active regions observed at HR, Mees, and MTK.