Chromospheric Network

The chromospheric network is highly co-spatial with the photospheric supergranulation and dynamic over a timescale of hours. Evolution occurs by the appearance of new field in the centre, migration to the boundaries, further migration along the boundaries and eventual disappearance (Schrijver, Hagenaar, & Title 1997; Hagenaar et al. 1999). The cell is dominated the appearance of many small, upflowing centres of magnetic field, known as the internetwork (Section 1.4.2). Lites, Rutten, & Kalkofen (1993) point out that the network viewed is not a mosaic of regular cells, instead it is a system of irregular patterns without actual well-defined insides. As such, the use of the terms `intranetwork', along with `cell boundary' and `cell interior', are not appropriate. However the use of these terms is still in common practice. On the granulation scale, granular buffeting moves these magnetic regions in a random walk. Across the supergranules the flow patterns have a longer lifetime, hence the magnetic regions are eventually collected in the network and move along it. Along the network these regions may interact; opposite polarities may meet and reconnect, and so vanish; some may split into two or more regions (fragmentation); two regions of the same polarity may meet and merge.

The precise co-spatiality with the photospheric magnetic network can be used to find the polarity of the network regions in the chromosphere. They tend to be mixed in the quiet Sun and unipolar at the poles, near active regions, or near previous active regions. The K index (the strength of the central K₃ line in a 1 Å band) is a measure of the intensity of regions in the chromosphere. In quiet regions the K index increases linearly with magnetic flux density. In the network regions (highly magnetized) the K index increases with magnetic flux density with an index less than unity. This is an indication of these regions spreading out horizontally with increasing height. (Figure 1.6). Eventually the field lines form a canopy over internetwork regions at a height of 2000 km (Section 1.3.3).