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Astron. Astrophys. 361, 759-765 (2000)

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2. Instrumentation

The optical data for this paper include narrow band H[FORMULA] filtergrams and Dopplergrams, Fe [FORMULA]5324 vector magnetograms in the photosphere. They were obtained by the Solar Magnetic Field Telescope (SMFT), a magnetograph system (Ai & Hu 1986) installed at Huairou Solar Observing Station (HSOS) of the Beijing Astronomical Observatory. The SMFT consists of a 35-cm refractor with a vacuum tube, a birefringent filter, a CCD camera and an imaging processing system controlled by computer. The birefringent filter is tunable between two setups, working either at the photospheric line, FeI 5324 Å, with a 0.150Å bandpass, or at the chromospheric H[FORMULA] line, with a 0.125 Å band pass.

A complete vector magnetogram is reconstructed from four narrow-band images of Stokes parameters transmitted by the birefringent filter. They are "I ", "V ", "Q ", "U ", respectively. Image V is the difference between the left and right circularly polarized images taken with the bandpass at 0.075 Å from the line center of Fe 5324 Å; Q and U are the differences between two orthogonal linearly polarized images for different azimuthal directions. When Q and U are taken, the filter bandpass is switched to the line center for achieving maximum sensitivity. I is always the "direct" intensity, derived from either the sum of two circularly polarized images for the line-of-sight field measurements, or the sum of two linearly polarized images for the transverse measurements. Both theoretical and empirical methods are used to calibrate the HSOS vector magnetogram. The relationship between the magnetic field specified by the atmospheric model and the fractional intensities of Stokes parameters is established as



where [FORMULA] G) and [FORMULA] G) are calibration coefficients for line-of-sight and transverse magnetograms (Wang et al. 1996), respectively. Observations presented in this paper were taken at good seeing. For the line-of-sight magnetogram with 256 integrated image pairs, used for this work, the noise level is about 15 G. For the transverse field measurements, the noise level is estimated from the standard deviation of the transverse field in weak field areas. We find the noise is about 100 G.

The H[FORMULA] Dopplergrams may contain many components such as solar rotation, steady flows, oscillation, convection, and evolutionary features. In this work, we are mostly interested in the evolutionary component. The solar rotation and steady flow are eliminated by subtracting a smooth background field, while the oscillation and convection are much smaller than the evolutionary component in magnitude. The spatial resolution for both magnetograms and Dopplergrams is 2 [FORMULA] 3 arcsec. Lacking full spectral information and adequate spatial resolution, the Doppler signals from filter-based observations may be considered as spectral and spatial averages of the true features.

The pixel size of the CCD for HSOS data is 0."613 and 0."425 in the east-west and south-north directions, at the time of this observation. To acquire 256 integrated image pairs takes approximately 41 seconds for the SMFT system. High-frequency distortion by seeing practically specifies the real angular resolution, which is approximately 2[FORMULA] for this set of magnetograms. This would certainly not enable one to resolve the intrinsic magnetic structures in active regions. In this case, the line-of-sight magnetogram measures the flux density in the resolved area.

The Solar and Heliospheric Observatory (SOHO), a joint European Space Agency (ESA) and National Aeronautics and Space Administration (NASA) effort, was launched late in 1995 and has provided unprecedented observations of the Sun and heliosphere. In this paper, we present observations made by two of the instruments on SOHO, the Michelson Doppler Imager instrument (MDI) and the Extreme Ultraviolet Imaging Telescope (EIT). A detailed description of these instruments is provided by Scherrer et al. (1995) and Delaboudinière et al. (1995). The YOHKOH Soft X-ray Telescope (SXT) (Tsuneta et al. 1991) observes the Sun in the wavelength range 3-50 Å with an instrumental pixel size of 2".5. All SXT images in this paper were taken through the thin aluminum (A1.1) filter, at various levels of resolution (full, half, quarter) corresponding approximately to 2".5, 5", or 10" pixel size, respectively.

To compare data from several instruments, data alignment is extremely important. For this study, the alignment is relatively easy, as HSOS H[FORMULA] filtergrams, SOHO/EIT, MDI and YOHKOH soft X-ray images were all full disk data. Alignment among these images was easily done by limb fitting. The HSOS H[FORMULA] images then are aligned with the HSOS full disk H[FORMULA] images, and the HSOS magnetogramsare are aligned with H[FORMULA] images, as their field-of-view are the same. The final coalignment error is estimated to be about 5" on the basis of the uncertainty in orientations.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001