4. Conclusions and discussions
From the analysis of 18 vector magnetograms of an emerging active region NOAA 7321 we have obtained the following results:
1. The total flux and total current grew synchronously, following a linear relationship, but such a linear relationship between the growth rate of the flux and that of the current is different for positive and negative regions.
2. The linear extrapolation of the electric current, , to the zero magnetic flux, , did not vanish, but produces a value of about 31012A and -51012A for positive and negative regions respectively.
3. The change in (as a measure of the total twist of the AR) was rather small; during 3 days of flux emergence, remained at about 610-8m-1.
Our first result coincides in general with that of Leka et al. (1996). The second result is more surprinsing. One possible explanation is that the photospheric magnetic field is not force-free (Metcalf et al. 1995; Abramenko & Yurchishin 1997), so the current may be not parallel to . Therefore, at the very beginning of the tube emergence, when the field in the tube is almost horizontal, the may have the vertical component. Besides, some pre-existing total current due to small-scale magnetic field in quiet regions (Abramenko et al. 1992) should be present in the area where the flux tube emerges. In any case, the non-vanishing total current at the vanishing total flux does not contradict the idea on the sub-photospheric pre-existing twist of the emerging flux tube.
According to our third result, the measured twist seems to keep constant during the emergence. This means that the topology complexity of the emerging flux system is an invariable. Moreover, this result is very important for the validity of Longcope et al. (1998) simulations of the twist evolution in the thin flux tube model. This model is restricted by sub-photospheric thin flux tubes in the depth not less than 30 Mm below the photosphere, since above this depth, the radius of the flux tube will expand rapidly so that the equations to describe the thin tube will no longer be valid. Nevertheless, such simulation shows a good agreement with the observations in both mean value and statistical dispersion of for 203 ARs (Longcope et al. 1998). The underlying cause for this agreement seems to be the small change of (twist) throughout the emergence.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000