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Astron. Astrophys. 363, 279-288 (2000)

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4. Network bright points (low brightness)

In the course of our measurements we noticed emission points located unmistakably on the network boundaries that look very much like bright INBPs. We call these network bright points (low brightness) and abbreviated as NBPs (low brightness) to distinguish them from the conventional elements that form the quiet sun network which we call network bright points (high brightness) or NBPs (high brightness). We show in Fig. 1 (boxes A & B) examples of these NBPs (low brightness). Bappu & Sivaraman (1971) noticed such emission points in some of the excellent quality Kodaikanal K-line spectroheliograms during the years of deep solar minimum and coined the term "network bright points (low intensity)" to distinguish them from the conventional quiet sun network elements. They also found that these "network bright points (low intensity)" possess the same value of the K2V emission width as the inner network bright points (INBPs). We have measured from the co-aligned K-line SHG - magnetic scan pairs the brightness of all such NBPs and the magnetic fields of the magnetic elements with which they spatially coincide, the same way as we did for the INBPs. In Fig. 4 we present the scatter plot of the brightness vs magnetic field for these NBPs. It can be seen that the fields associated with NBPs are higher than for the INBPs. Our interpretation is that the NBPs (low brightness) are those INBPs (and the corresponding IN magnetic elements too) that had a long lifetime during which the corresponding IN magnetic fields coalesced with consequent increase in K emission. Nindos & Zirin (1998) have provided examples of such events. Originally located in the cell interior, they have been swept towards the cell boundaries during their lifetimes by the radial flows within the cell. According to Martin's estimates (1990) about 90% of the network concentrations (mixed polarity network) originate from ephemeral active regions and the rest, 10%, from merged clusters of intranetwork fields. Wang et al. (1995) and Zhang et al. (1998b) have also provided many instances of merging of the IN magnetic elements. In view of this it would be reasonable to infer that the network elements (low brightness) are those INBPs associated with the IN magnetic elements that merged during the time they spent within the network.

[FIGURE] Fig. 4. Scatter plot of the maximum brightness of the NBPs (low brightness) vs. the maximum absolute value of the magnetic field of the cospatial magnetic elements over an area of 2 [FORMULA] 2 pixels. Notice that the fields are higher than for the INBPs of Fig. 2.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000