2 Observations and data reduction

Active region NOAA 9077 was the target of an international ground-based observing campaign at Huairou Solar Observing Station (HSOS) which was coordinated with TRACE and SOHO. A detailed analysis of the photospheric magnetograms of this active region is discussed in Liu & Zhang (2001), who show that it is a very complex and highly dynamic region. Along the highly sheared magnetic neutral line, Yan et al. (2001) propose the existence of a magnetic flux rope. A description of the major flare and an associated CME is given by Zhang et al. (2001). Manoharan et al. (2001) study the interplanetary effects caused by the CME. In the present paper, we study the evolution of two small magnetic features in this region. Figure 3 shows the development of the photospheric vector magnetic field, in which the magnetic features of interest (P6 and F4) are near the magnetic neutral line. Obviously, P6 and F4 are moving toward each other and they form a $\delta$-configuration on 13 July. Because of their shearing motion, the neutral line of this active region becomes very curvy at this site. A filament is lying over the right part of the neutral line with one end also rooted at the $\delta$-configuration. Before the major flare of 14 July, the filament is activated violently. During the flare, P6 and F4 separate rapidly. P6 disappears late on 15 July while F4 remains until 17 July.

The vector magnetograph of HSOS operates at two Fraunhofer lines: FeI $\lambda$5324.19 Å for photospheric and H$\beta$ for chromospheric observations, respectively (Ai & Hu 1986). For the photospheric magnetic field observations, the passband is tuned -0.075 Å off the FeI $\lambda$5324.19 Å line center so as to measure the longitudinal component, and at the line center to measure the transverse component. In the measurement, we increase the signal-to-noise ratio by integration of 255 frames and the $4\times 3$ average. After this, the image scale of the magnetograms is about 2'' pixel^-1.

Some factors may cause potential problems in the calculation of current helicity with a filter-type magnetograph (Hagyard & Pevtsov 1999; Bao et al. 2000). We have to consider Faraday rotation, magnetic saturation, and $180\hbox{$^\circ$ }$ ambiguity. In this work, we avoid these negative effects for several reasons: (1) the observed $\delta$-configuration is of medium size and the maximum of its magnetic field is no more than 2000 G, i.e., there is no magnetic saturation in this $\delta$-configuration; (2) it is because of the median field strength in the umbrae and off-center line measurements that magneto-optical effects (Faraday rotation) are not serious (see also, Bao et al. 2000; Zhang 2000); (3) the projection effect is not important, since the active region was at the central meridian at a low latitude (N17 $\hbox{$^\circ$ }$) during the period from 13-15 July; (4) we resolve the $180\hbox{$^\circ$ }$ambiguity of the transverse field by a multi-step method (Wang et al. 1994); (5) we calculate the current helicity with B_z above 60 G and B_tabove 200 G (3$\sigma$ levels); (6) to examine the simple relation to large flares, we only emphasize the obvious variation of the helicity sign.

TRACE (Handy et al. 1999) white-light images have $1024\times 1024$ pixel, with an $8.5'\times8.5'$ field of view, 1'' spatial resolution, and about 0.5 hour temporary resolution. The data let us trace the evolution of important spots conveniently and without ambiguity. We co-aligned these images and used movie programs to show them. Readers can obtain an MPEG movie from the URL (http://sun.bao.ac.cn/staff/lyu/spot.mpe), or see it directly on line by (http://sun.bao.ac.cn/staff/ lyu/gifmovie/gifmovie.html).

We judge the spots' magnetic polarities by superimposing longitudinal magnetograms of HSOS on the corresponding white-light images. It is easy to locate one spot in a three-dimensional coordinate system and calculate its proper motion using a sequence of full-disk SOHO/MDI data and TRACE white-light images.