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Astron. Astrophys. 336, 359-366 (1998)

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2. Observation

The active region NOAA7321 was an emerging flux region (EFR), which was born on October 24, 1992 and disappeared from the western limb on November 2. During the few days, it underwent a continuous change in configuration and frequent flares haunted this region. Preliminary studies on the magnetic features and flares in this active region have been published (Kawakami 1993; Takakura et al. 1994; Zhang 1995) and in the present paper we will investigate in detail the evolution of the magnetic configuration from October 26 to October 27 as a clue to the interpretation of the 1N/M1.1 flare at 01:44UT on October 27.

The flare at 01:44UT on October 27 was observed by the [FORMULA] spectrometer in the solar tower in Nanjing University and the Yohkoh instruments SXT, HXT and BCS. The flare underwent the onset, impulsive phase and the decaying phase, evolving rapidly within half an hour. It was made up of a low-lying, compact flaring loop with its two feet emitting intensely in both HXR and [FORMULA] bands, and some other coronal bright structures (loops). The analysis of the co-ordinated observations was done by Qiu et al. (1997 Paper I). In Sects. 5 and 6, we will refer to some of the main results in Paper I for further investigation in this paper.

For this active region, a series of photospheric vector magnetograms (FeI 5324.19Å) and [FORMULA] filtergrams (4861.34Å) were taken by the Solar Magnetic Field Telescope (SMFT) in Huairou Solar Observing Station (HSOS), Beijing Astronomical Observatory. The field of view is [FORMULA], and the angular resolution is around [FORMULA]. The noise level of the line-of-sight magnetograms is [FORMULA] 25G, while for the transverse magnetograms, it is less than 190G, estimated from the average value of the transverse field strength in areas far away from flux concentrations in the active region. The 180o ambiguity in the transverse field is resolved by using the empirical method in a multi-step procedure (Canfield et al. 1993; Wang 1994).

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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998