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

4. Vertical current system

The non-potential feature of the magnetic field, specifically, the strong shear of transverse field along the inversion line, implies that the active region could well be a current-carrying system, which was formed by emerging flux and spots motions. In this section, we will examine the evolution of the major vertical current channels (regions of high vertical current density) from October 26 to October 27.

4.1. Distribution of vertical current

In order to calculate the vertical current, we first pretreat the transverse field data with a Fourier low-pass filter. This method is proved efficient for minimizing the high-frequency measurement noises without losing the major structures in the field (Wang et al. 1997). Hence the vertical current can be readily deduced with the differencing method and the accuracy of such calculation is directly determined by the resolution of the filter. For our data, we obtain the filtering resolution as about , when the relative cutoff frequency, , is given as 15. is a dimensionless quantity normalized to , where and are the characteristic sizes of the active region. The major vertical current channels can be illustrated in this resolution (Fig. 2) and the noise level in the vertical current is estimated from the standard deviation of currents measured in areas where the transverse field is weak (Canfield et al. 1993).

Fig. 2. Vertical current distributions on a  Oct. 26 and b  Oct. 27 with the magnetic longitudinal field superposed on (solid and dashed contours). The bright (dark) color indicate the current flowing out of (into) the photosphere, or upward (downward). The grey scale at the bottom gives the intensity of the current and the levels are -2.4, -1.8, -1.2, -0.6, 0, 0.6, 1.2, 1.8, 2.410-2A/m2 from dark to bright colors. The magnetic longitudinal inversion line is drawn in white, thick lines in both figures. The FOV is the same as that in Fig. 1.

Fig. 2 shows the distributions of the vertical current density in gray-scale maps with the contours of the line-of-sight magnetograms superposed on. The vertical currents above the 2 level are drawn in the maps. The current upflowing from the photosphere (the positive footpoint) is shown in bright colors, while that flowing down into the photosphere (the negative footpoint) is shown in dark colors.

In order to check the balance between the upflowing and downflowing currents, in Table 1, we list and compare the total positive/negative vertical current I⁺/I^- calculated at different current cutoff levels (J_c=, 2, and 3). The minimum and maximum vertical current densities J_min and J_max are also given in Table 1. From the imbalance measurement (defined as ), it is seen that the total currents are basically balanced when the cutoff J_c is taken as , or 2, whereas the upflowing current dominates in the active region when the cutoff is taken as 3; this is especially true on October 27, when the imbalance measurement exceeds 22%. This can be understood with respect to the fact that the upflowing current is strong and concentrated, while the downflowing current is relatively weak and disperse, and this trend becomes more obvious on October 27. The distinct imbalance of the total currents at 3 level also means that this selection of the current cutoff value is probably unsuitable for this active region.

Table 1. Parameters of the vertical current distribution

4.2. Evolution of the vertical current system

On October 26 (Fig. 2 a), the pair of current footpoints and - were just located along the zone of the emerging pattern L1, and , were co-spatial with the emerging poles N1 and S1, implying that L1 might be a current-carrying pattern. Similarly, the dominant current pair and were also located in the area where the dominant emerging pattern L2 was present and the maximum of positive current footpoint was co-spatial with the pole N2, the most drastic emerging feature on this day.

On October 27 (Fig. 2 b), the total currents increased with the enhancement of the emerging poles. The current pair - became significant. The positive footpoint extended to a larger area and its maximum turned to be associated with the newly emerging pole N3; on the other hand, the negative footpoints became more stronger (addressed as in the figure) while disappeared. Such change was associated with the enhanced pole S1 and the new pole S3. Due to the motions of the dominant emerging poles, there was some change in the morphology of the current pair -. In addition, it is noteworthy that the sections M1 and M2 of the magnetic inversion line were nearly coincident with the reverse lines of the vertical currents, and the vertical current density bore a strong gradient on both sides of the current reverse lines.

The analysis of the vertical current system reveals the following features in its evolution: (1) the major current pairs are associated with the dominant emerging patterns; (2) the peak of the current footpoints appears to be nearly located in the areas with the most drastically emerging poles; and (3) the sections of the magnetic inversion line are coaligned with the current reverse lines, where there is a strong vertical current gradient and perhaps horizontal currents.

© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998
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