## 3. Observations## 3.1. Active region NOAA 7321The active region NOAA 7321 was a new emerging flux region (EFR), passing across the solar disk from October 24 to November 1 in 1992. During this period, it evolved from an explicit main bipole, nearly resembling the potential field, into a complicated configuration with strong shear around the magnetic neutral line when several new satellite spots continuously emerged from between the two main poles (Zhang 1995). On October 27, the active region (then located at S23W18) was fully developed, and a series of flares occurred in the flux-emerging area characterized by strong shear (Wang, Qiu & Zhang 1998). We hence take the observed fields on that day to investigate the 2D topological properties. The photospheric magnetic field in AR7321 was observed by a vector
magnetograph system (Ai & Hu 1986) installed at the Huairou Solar
Observing Station (HSOS) of Beijing Astronomical Observatory. To
reduce the noise, and to match the practical spatial resolution, the
smoothing on the Stokes parameters V,
Q and U is made. After the ambiguity
is resolved for the transverse field components with a multistep
method (Wang et al. 1994), the vector magnetograms are transformed
into the heliospheric plane. In addition, we also obtained
H filtergrams for a 1N/M1.1 flare on
Oct. 27 from HSOS and observations of the SXR emission during
00:48UT-07:58UT from ## 3.2. Data reductionIn order to accurately determine 2D singular points in the observed transverse field, we take a conventional lowpass filter technique to process the raw data, reducing the influence of the measurement noise. In the Cartesian coordinate system, and represent the observed field components and their Fourier components, respectively. Then the filtered components, , are where is a filter function. We take as the butterworth form, i.e. where , and k where is the relative cutoff frequency, defined as and are typical sizes of the active region. The loss of signals in the filtering can be estimated with a parameter, , where .
and
(s= Table 1 lists the values of ,
and
in different filtered cases.
According to Expr. (3), the cutoff frequencies,
=20, 10 and 5, correspond to the
effective resolutions of 43,
86 and
1611 pixels. The comparisons show
that for the raw data the noise level of B
© European Southern Observatory (ESO) 1999 Online publication: February 23, 1999 |