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

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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 opera- tion 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 Solar Physics 213: 87–102, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 88 H. ZHANG ET AL. 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 be- tween 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 magneto- graph 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 longitudi- nal 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 magne- tograms 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 mag- netic 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 magne- tograms 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 transver- sal 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 mag- netic 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 proba- bly 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 out- put 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 simi- lar setup with the final output as real-time polarization images.
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