Analysis of Vector Magnetic Fields in Solar Active Regions by Huairou, Mees and Mitaka Vector Magnetographs

ANALYSIS OF VECTOR MAGNETIC FIELDS IN SOLAR ACTIVE REGIONS BY HUAIROU, MEES AND MITAKA VECTOR MAGNETOGRAPHS

H. ZHANG1, B. LABONTE2,J.LI2 and T. SAKURAI3 1National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China (e-mail:[email protected]) 2Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, U.S.A. 3National Astronomical Observatory of Japan, Mitaka, Tokyo 181, Japan

(Received 13 March 2002; accepted 11 October 2002)

Abstract. We analyze the vector magnetograms in several well-developed active regions obtained at Huairou Solar Observing Station, National Astronomical Observatories of China, at Mees Solar Ob- servatory, University of Hawaii, and at National Astronomical Observatory of Japan. It is found that there is a basic agreement on the transversal fields among these magnetographs. The observational error (mutual difference) for the transversal magnetic fields is estimated. In addition to comparison of transversal fields among different instruments, we used the morphological configurations of sunspot penumbrae in white-light and EUV 171 Å images obtained by the TRACE satellite as a reference of the orientation of transversal magnetic fields.

1. Introduction

The vector magnetic fields within active regions contain crucial information of total magnetic flux and energy in the photosphere, as well as electric current density. These important magnetic properties can be used to study the evolution of active regions and to predict magnetically-caused eruptions. The measurements of solar vector magnetic fields are based on the pioneering works by Unno (1956) and Rachkovsky (1962), and have been carried on for three decades. Among the solar vector magnetographs built, those still in operation include the Haleakala Stokes Polarimeter (HSP) (Mickey, 1985), and Imaging Vector Magnetograph (IVM) (Mickey et al., 1996) at Mees Solar Observatory (MSO); the NASA/Marshall Space Flight Center (MSFC) Vector Magnetograph (Hagyard, Cumings, and West, 1985); the vector magnetograph with a magneto- optic filter at Big Bear Solar Observatory (BBSO) (Cacciani, Varsík, and Zirin, 1990); the Solar Magnetic Fields Telescope (SMFT) at Huairou Solar Observing Station (HSOS)/Beijing (Ai and Hu, 1986); the Solar Flare Telescope of National Astronomical Observatory of Japan at Mitaka (Sakurai et al., 1995); the Advanced Stokes Polarimeter at NSO/Sacramento Peak (Lites et al., 1993) and Vector Mag- netographs at Sayan (Grigoryev et al., 1985), Potsdam (Staude, Hofmann, and

Bachmann, 1991), Crimea (Stepanov and Severny, 1962), etc. The comparison between vector magnetograms obtained at different observatories is a basic study, because it can be used to analyze the distribution of photospheric vector magnetic fields and confirm the accuracy in measurements of the fields. A comparison between magnetograms with two very different vector magnetographs, HSP at MSO and Vector Magnetograph at MSFC, was made by Ronan et al. (1992). A good agreement with line-of-sight field components between two magnetograms was found. Because of the poor seeing at MSFC, while seeing is generally good at MSO, the agreement between transversal field measurements was reached within the uncertainty caused mostly by MSFC’s image quality. A comparison among vector magnetographs at three observatories, HSOS, BBSO and MSO, was made by Wang et al. (1992). The SMFT at HSOS is very similar to the vector magnetograph at BBSO, while both of them are very different from the Stokes polarimeter at MSO. The comparisons included morphology, azimuth of transversal fields and magnetic strength. The general conclusion from this work was that the longitudinal fields agree better than transversal fields among magnetograms from the three observatories. The agreement of vector fields is better between BBSO and HSOS than between BBSO and MSO. As the vector magnetic field measurements in the photosphere grow mature, they are used to investigate the magnetic helicities in the active regions (Pevtsov, Canfield, and Metcalf, 1994). The observations over decades using vector magnetograms enable researchers to study the magnetic and current helicity distribution in the solar surface with hundreds of active region samples. This scenario is re- flected in the series of papers about helicities in the active regions through solar cy- cles (Pevtsov, Canfield, and Metcalf, 1995; Bao and Zhang, 1998; Zhang and Bao, 1998, 1999; Pevtsov, Canfield, and Latushko, 2001). Since helicity reveals the magnetic generator underneath the photosphere (Seehafer, 1990; Longcope, Fisher, and Pevtsov, 1998), the comparison of magnetograms between HSOS and HSP/MSO was made with particular emphasis on helicity calculations for one active region (Bao et al., 2000). The study shows the basic agreements of the vector magnetograms obtained with different instruments, except for the slight differences of azimuthal angles of transverse fields, such as about 10◦ azimuthal angle difference of transverse fields between HSOS and HSP/MSO vector magnetograms. In this paper, we conduct a comparison among the vector magnetograms taken with four different instruments/telescopes in two active regions (AR 8525 and AR 9114). The four instruments are the Solar Magnetic Field Telescope at Huairou Solar Observing Station (SMFT/HSOS), Imaging Vector Magnetograph (IVM) and the Haleakala Stokes Polarimeter (HSP) at Mees Solar Observatory (MSO), and Solar Flare Telescope at Mitaka, National Astronomical Observatory of Japan (SFT/MTK). The comparison between vector magnetograms at different observatories allows us to estimate the uncertainties of the measured photospheric vector magnetic field. To ensure the best measurement of transversal magnetic orientations, we will use VECTOR MAGNETIC FIELDS 89 high-resolution images taken by TRACE as an independent reference for transversal fields. It is normally believed that the fine structures of solar active regions provide some information of the magnetic field direction as a result of the frozen- in condition in the solar atmosphere (Zirin, 1972). Looking for the transversal field orientations from high-spatial-resolution intensity observations has not been done in the previous comparisons, but it was realized long ago that the Hα fibrils could give the direction of transversal fields before the vector magnetograph era (cf., Bray and Loughhead, 1964).

2. Instruments

First of all, we brieﬂy describe the instruments measuring magnetic ﬁelds used in the comparison.

2.1. SMFT/HSOS

The Solar Magnetic Field Telescope at Huairou Solar Observing Station (SMFT/HSOS) in Beijing is equipped with a birefringent filter for wavelength ∗ selection and KD P crystals to modulate polarization signals. The Fe I λ5324.19 Å line is used at the Huairou vector magnetograph. It is a normal triplet in the magnetic field and the Landé factor g = 1.5, the excitation potential of the low energy level of this line is 3.197 eV. The equivalent width of the line is 0.33 Å and the residual intensity at the core is 0.17 (Kurucz et al., 1984). The bandpass of the birefringent filter of the Huairou magnetograph with three sets of KD∗P crystal modulators is about 0.15 Å. The center wavelength of the filter can be shifted and is normally at −0.075 Å for the measurements of longitudinal and at the line center for the transversal magnetic fields (Ai and Hu, 1986).

2.2. SFT/MTK

The Solar Flare Telescope at Mitaka (SFT/MTK) in Japan has the similar design to the SMFT/HSOS in term of measuring the magnetic fields. The birefringent filter has the bandpass 0.125 Å and the transmission peak is set at the blue wing −0.08 Å of Fe I λ6302.5 Å line (Landé factor g = 2.5) (Sakurai et al., 1995). In the recent analysis for the transverse magnetograms, it is found that some effect of Faraday rotation (FR) exists in the Mitaka data in strong field regions, where the longitudinal field is larger than 1000 G (Sakurai, 2002).

2.3. HSP/MSO

The Haleakala Stokes Polarimeter (HSP) at Mees Solar Observatory is probably the oldest polarimeter of its kind (Mickey, 1985). Its modulator is a rotating 90 H. ZHANG ET AL. waveplate, which is ‘of the fixed retardence, variable orientation type’. The output of the modulator includes four Stokes parameters that are de-convolved by software. The spectrometer is an echelle grating which provides ‘high angular dispersion and efficiency, while maintaining a low scattered light level’ (Mickey, 1985). The Fe I λ6301.5 Å (Landé factor g = 1.667) and Fe I λ6302.5 Å (Landé factor g = 2.5) are used by the Stokes Polarimeter (Ronan, Mickey, and Orrall, 1987). The Stokes Polarimeter Magnetogram (SPM) data are normally analyzed by two different methods: (1) a least-squares profile fitting (Skumanich and Lites, 1987) and (2) an integral method. The profile fitting includes the effects of Faraday rotation, while the integral method suffers from these effects, which was analyzed by Ronan, Mickey, and Orrall (1987). The data reduction is a combination of integral and least-squares (LS) line profile fitting. The LS method fails for weak polarization. For pixels with B<1000 G, the integral method has been used and for pixels with B>1000 G, the LS method has been used.

2.4. IVM/MSO

The Imaging Vector Magnetograph (IVM) at Mees Solar Observatory is yet an- other Stokes profile analyzing magnetograph. It has been in operation since 1992. The magnetograph includes a dedicated 28-cm aperture telescope, a polarization modulator, a tunable Fabry–Pérot filter, CCD cameras and control electronics. It takes images of areas on the Sun, and records the polarization and wavelength in sequences (Mickey et al., 1996). The data reduction was described by LaBonte, Mickey, and Leka (1999). The typical spectral line used by IVM is Fe I λ6302.5 Å. Among these instruments, SMFT/HSOS, IVM/MSO, and SFT/MTK have similar setup with the final output as real-time polarization images. The HSP/MSO has the setup of spectral line scanning images, which has low tem- poral resolution. The SMFT/HSOS and SFT/MTK use birefringent filters to select specific wavelengths, and KD∗P crystals to modulate polarization signals, while IVM/MSO uses a polarization modulator for analyzing polarization signals and a Fabry–Pérot for wavelength adjustment. The magneto-optical effects in the measurement of the solar vector magnetic field can not be completely determined, even when theoretical analysis has been used. It is normally believed that the influence of magneto-optical effects for the measurements of transverse fields is insignificant in the far wing of the magnetic sensitive lines. For comparison, we will use a magnetogram by SFT/MTK as a template for AR 8525 and IVM/MSO for AR 9114, due to the lower sensitivity to magneto-optical effects. VECTOR MAGNETIC FIELDS 91

Figure 1. The white-light and 171 Å images observed by the TRACE satellite on 5 May 1999 in active region NOAA 8525. The size of the images is 1. 85 × 1. 85. North is the top and east is at the right.

3. Observations

The AR NOAA 8525 and 9114 showed a relatively simple magnetic conﬁguration and were located near the center of the solar disk. We chose these two regions for magnetogram comparisons.

3.1. ACTIVE REGION NOAA 8525

Figure 1 shows the white-light and 171 Å images of the active region NOAA 8525 observed by the TRACE satellite on 5 May 1999. It is found that the active region consisted of a main sunspot and some small pores located northwest of the main spot. The active region is located at N22, E06 near the center of the solar disk. It is clearly seen that fibrils extended out from the center of the main sunspot in 171 Å. These provide some basic morphological information on the magnetic field in the active region atmosphere. Figure 2 shows three sets of the photospheric vector magnetograms overlaid by the white-light and 171 Å images. We found a basic consistency of the vector magnetic field among the magnetograms of Huairou (HRM), Mitaka (MTK), and HSP/Mees (SPM). This active region was an αp region. The direction of transverse magnetic fields is roughly parallel to 171 Å fibril features and is also consistent with that of penumbral features in the white- light image in Figures 1 and 2. Figure 3 shows the relationship between HRM and MTK magnetograms in the active region NOAA 8525. For the stronger transverse magnetic field (larger than 200 G) the mean error angle of the transverse magnetic field is −3.6◦. The similar case is found in the comparison of vector magnetograms between SPM and HRM. The mean error angle of the transverse field (larger than 200 G) between SPM and HRM is −12.8◦ (see Figure 4), while that between SPM and MTK the mean error angle is −17.5◦ in Figure 5. It means that the transverse 92 H. ZHANG ET AL.

Figure 2. The vector magnetograms observed at Huairou (HR), Mitaka (MTK), and with HSP/Mees (SPM) in active region NOAA 8525. The arrows mark the directions of transverse magnetic ﬁeld. The solid (dashed) contours correspond to positive (negative) ﬁelds of ±50, 200, 500, 1000, 1800, 3000 G. VECTOR MAGNETIC FIELDS 93

TABLE I Active region 8525.

Dif. mag. ϕσϕ σT Tnum Snum ◦ ◦ MTK–HR −3.6 20.5 213.6 G 3600 1100 ◦ ◦ HR–SPM −12.8 14.5 199.9 G 255 29 ◦ ◦ MTK–SPM −17.5 10.2 177.2 G 255 30

field observed by the SPM tends to be rotated counter-clockwise, as one compares with that of MTK and HRM. The relative intensity of white-light images is about 50 in the umbra and 250 in the quiet-Sun region near the sunspot in Figures 3–5. It is found that the scatter distribution of stronger magnetic field normally occurs where the relative intensity of white light is about 200; this is just the penumbral region of the sunspot in active region NOAA 8525. As one notices that the sunspot relates to the strong longitudinal magnetic field, this actually reflects the correlation between the transverse and longitudinal field in the active region. The relationship between the transverse and longitudinal magnetic field, and also the transverse field observed at different wavelengths from the Fe I λ5324.19 Å line center, were analyzed by Zhang (2000) for this active region. The observational result by HRM is consistent with the interpretation that the magneto-optical effect causes counter- clockwise rotation of the linear polarized light of the spectral lines in the positive polarity regions. It is noticed that one cannot exclude the influence of Faraday effects on the measurement of MTK vector magnetograms, which are observed at −0.08 Å of Fe I λ6302.5 Å line, even if it is weaker than that obtained at the line center. The statistical results on the comparison of transverse magnetic field in active region 8525 observed by different vector magnetographs can be found in Table I ∗.

3.2. ACTIVE REGION NOAA 9114

Figure 6 shows the white-light and 171 Å images of active region NOAA 9114 observed by the TRACE satellite on 8 August 2000. The active region is located at N11, W2 near the center of solar disk. The 171 Å fibril features extend out from the center of the sunspot. Figure 7 shows vector magnetograms obtained by HRM ∗ Tables I and II show the statistical results on comparing vector magnetograms observed by different vector magnetographs, where Dif. Mag. is the vector magnetograms observed by different magnetographs; ϕ is the mean error angle of transverse magnetic field; σϕ is the root mean square of the error angle; σT is the root mean square of error intensity of the transverse field; Tnum is the total point numbers of the transverse field and Snum is the statistical point numbers of the strong transverse field in the both transverse magnetograms of active regions. Please notice that, for the convenience of the comparison of the vector magnetic field, the spatial resolution of some of vector magnetograms is reduced in getting the same data pixels. 94 H. ZHANG ET AL.

Figure 3. The relationship between the vector magnetograms observed by Solar Flare Telescope at Mitaka (MTK) and Huairou Vector Magnetograph (HR) in active region NOAA 8525, where the transverse magnetic field is larger than 200 G. (a) The relationship between azimuthal angles of the transverse magnetic field observed by MTK and HR. (b) The azimuthal angle differences as a function of the intensity of the white-light image. (c) The relationship between intensity of both transverse magnetic field. (d) The black (white) arrows mark the MTK (HR) transverse field. The white (black) areas mark the positive (negative) polarity of the longitudinal magnetic field by HR and solid (dashed) contours correspond to positive (negative) fields by MTK. at Huairou and IVM and SPM at Mees Solar Observatories overlaid by white-light and 171 Å images. The morphological configuration of magnetic field is relatively simple. The relationship between the HRM and IVM magnetograms is shown in Fig- ure 8. It is found that the mean errors of the azimuthal angles of the transverse magnetic field is 3.0◦, while the mean error of the azimuthal angles between SPM and IVM is 20.6◦ in Figure 9. It means that the observed SPM transverse field in active region 9114 tends to be rotated clockwise, with respect to IVM and HRM. The scatter distribution of the intensity, azimuthal angles of transverse magnetic VECTOR MAGNETIC FIELDS 95

Figure 4. The relationship between the vector magnetograms observed by Huairou (HR) Vector Mag- netograph and Mees Stokes Polarimeter (SPM) in active region NOAA 8525, where the transverse magnetic field is larger than 200 G. (a) The relationship between azimuthal angles of the transverse magnetic field observed by HR and SPM. (b) The azimuthal angle differences as a function of the intensity of the white-light image. (c) The relationship between intensity of both transverse magnetic fields. (d) The black (white) arrows mark the HR (SPM) transverse field. The white (black) areas mark the positive (negative) polarity of the longitudinal magnetic field by SPM and solid (dashed) contours correspond to positive (negative) fields by HR. In the scatter correlation of the transverse field, a 1.5 factor has been applied to the SPM transverse field due to the difference calibration parameters of the transverse field.

ﬁeld and their difference with the relative intensity of the white light between magnetograms obtained by different magnetographs can also be found in Figures 8 and 9. The statistical results on the comparison of transverse magnetic ﬁeld in active region 9114 observed by different vector magnetographs can be found in Table II also. The magneto-optical effects in HRM vector magnetograms have been discussed by Wang et al. (1992), Bao et al. (2000), and Zhang (2000). 96 H. ZHANG ET AL.

Figure 5. The relationship between the vector magnetograms observed by Mitaka (MTK) Vector Magnetograph and Mees Stokes Polarimeter (SPM) in active region NOAA 8525, where the transverse magnetic field is larger than 200 G. (a) The relationship between the azimuthal angles of transverse magnetic field observed by MTK and SPM. (b) The azimuthal angle differences as a function of the intensity of the white-light image. (c) The relationship between intensity of both transverse magnetic fields. (d) The black (white) arrows mark the MTK (SPM) transverse field. The white (black) areas mark the positive (negative) polarity of the longitudinal magnetic field by SPM and solid (dashed) contours correspond to positive (negative) fields by MTK.

4. Discussions and Results

The comparison of vector magnetic fields in the active regions, which relates to determination of the transverse field by the linearly polarized light of spectral lines in the solar atmosphere, is not straightforward because of different sensitivities of telescopes for the solar polarized light. It is not surprising that we found differences among magnetograms of Huairou, Mees and Mitaka based on various reasons. The magneto-optical effect is one of notable causes for the measurement errors of vector magnetic field (Landolfi and Landi Degl’Innocenti, 1982; West and Hagyard, VECTOR MAGNETIC FIELDS 97

Figure 6. The white-light and 171 Å images observed by the TRACE satellite on 8 August 2000 in active region NOAA 9114. The size of the images is 1. 85 × 1. 85. North is the top and east is at the right.

1983; Bao et al., 2000). In principle, the influence of magneto-optical effects on the HRM, MTK and IVM vector magnetograms probably is more significant than on SPM, because a Stokes profile analysis has been used in the data reduction of SPM vector magnetograms of NOAA 8525 and 9114. However, the real cases probably are more complex. One can notice that the mean error angle of transverse magnetic field in active region NOAA 8525, measured by SPM of Mees Observatory, rotates in the direction of the magneto-optical effects relative to the observational results by Huairou and Mitaka. The rotation rule of the azimuthal angles of transverse magnetic field in active regions 8525, due to the magneto-optical effects, was discussed by Zhang (2000). We found that in active region NOAA 9114 the azimuthal angles of transverse fields are different between SPM and IVM observed at Mees Solar Observatory. The transverse field in active region 9114 observed by SPM rotates clockwise relative to IVM and HRM, which shows an opposite rotation tendency relative to that of NOAA 8525. The influence of the magneto-optical effects on the integral method of the Stokes profiles of the magnetically sensitive lines has been analyzed by Ronan, Mickey, and Orrall (1987). As one believes that the transverse magnetograms obtained in the wing of the magnetically sensitive lines are less affected by the magneto-optical effects such as that of IVM and MTK, the obvious difference of the mean angle for transverse field with the SPM magnetograms becomes questionable. The transverse field inferred by SPM shows more twist than IVM and MTK ones, and the TRACE 171 Å fibrils. As one removes the differences of the calibration of the transverse magnetic field at different observatories and compares the correlation of the transverse components of vector magnetograms obtained by different vector magnetographs, the good statistical correlations can be found between the vector magnetograms obtained by different magnetographs, such as SPM and IVM, SPM and MTK, HRM 98 H. ZHANG ET AL.

Figure 7. The vector magnetograms observed at Huairou (HR) and Mees (Imaging Vector Mag- netograph (IVM) and Stokes polarimeter (SPM)) Observatories in active region NOAA 9114. The arrows mark the directions of the transverse magnetic ﬁeld. The solid (dashed) contours correspond to positive (negative) ﬁelds of ±50, 200, 500, 1000, 1800, 3000 G. VECTOR MAGNETIC FIELDS 99

Figure 8. The relationship between the vector magnetograms observed by Imaging Vector Magne- tograph (IVM) and Huairou (HR) Vector Magnetograph in active region NOAA 9114, where the transverse magnetic field is larger than 200 G. (a) The relationship between the azimuthal angles of the transverse magnetic field observed by HR and IVM. (b) The azimuthal angle differences as a function of the intensity of the white-light image. (c) The relationship between intensity of both transverse magnetic fields. (d) The (white) arrows mark the IVM (HR) transverse field. The white (black) areas mark the positive (negative) polarity of the longitudinal magnetic field by IVM and solid (dashed) contours correspond to positive (negative) fields by HR.

TABLE II Active region 9114.

Dif. mag. ϕσϕ σT Tnum Snum ◦ ◦ IVM–HR 3.0 32.1 206.9 G 3600 385 ◦ ◦ IVM–SPM 20.6 50.1 208.6 G 324 44 100 H. ZHANG ET AL.

Figure 9. The relationship between the vector magnetograms observed by Imaging Vector Mag- netograph (IVM) and Mees Stokes Polarimeter (SPM) in active region NOAA 9114, where the transverse magnetic field is larger than 200 G. (a) The relationship between the azimuthal angles of the transverse magnetic field observed by IVM and SPM. (b) The azimuthal angle differences as a function of the intensity of the white-light image. (c) The relationship between intensity of both transverse magnetic field. (d) The black (white) arrows mark the IVM (SPM) transverse field. The white (black) areas mark the positive (negative) polarity of the longitudinal magnetic field by IVM and solid (dashed) contours correspond to positive (negative) fields by SPM. and MTK in Figures 3, 5, and 8. Even if the values of the root mean square of the transverse magnetic fields obtained by different magnetographs are roughly the same in Tables I and II, the fine features of the transverse field are slightly different in these vector magnetograms. This means that the comparison of vector magnetograms obtained at different observatories requires more caution because the seeing conditions are different and the observing time differences which may allow real solar changes in the magnetic field. As an example, a locally strong transverse magnetic field can be found in the southern part of the active region in the SPM magnetogram (below the sunspot) in VECTOR MAGNETIC FIELDS 101

Figure 2 relative to HRM and MTK ones. This probably is caused by the evolution of the magnetic field. This difference also probably brings a possibility that some difference of the transverse field in the active regions is caused by the variation of the field in the lower solar atmosphere. In the umbrae of sunspots in Figure 7, the difference of the distribution of the transverse field in the vector magnetograms obtained by different magnetographs is more significant. This is probably because of the lower intensity of the light with high noise in the measurement of vector magnetic field and because the field is almost vertical to the solar surface. Of course, this difference probably also contains some information about the magneto-optical effect on the measurement of magnetic field. On the other hand, we only analyzed a few vector magnetograms of two active regions by different vector magnetographs. This means that the basic properties of these instruments for the measurement of the vector field have not yet been fully understood and the analysis of more data samples is needed. The main results are as follows: (1) The measurement of the vector magnetic field by vector magnetographs (or Stokes polarimeters) still is a notable problem. This concerns not only the measurement methods (filters vs. Stokes polarimeters) but also the instruments that are used at different solar observatories. (2) Even if a difference of vector magnetograms obtained at different observatories is found, this also provides a basic estimation on the distribution of photospheric magnetic field, and the error range of the measurement of vector magnetic fields. (3) The morphological features (such as EUV 171 Å) in the solar atmosphere generally provide a basic frame of reference on the direction of magnetic field, if one believes that these features relate to the magnetic field due to the frozen-in condition in the solar atmosphere.

Acknowledgements

The authors thank the staff at Huairou, Mees and Mitaka for their observations, and Dr A. Pevtsov for discussions and the SPM data reduction of Mees Solar Obser- vatory. The authors also thank Dr D. Mickey for some comments and the referee for comments and suggestions for the improvement of the manuscript. This study has been supported by the Chinese Academy of Sciences and National Scientiﬁc Foundation of China. 102 H. ZHANG ET AL.

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