Fine Structure

The most useful lines for imaging the chromosphere are Hα and Ca II H and K. As explained in Section 1.3.2, moving from line centre to the wings of these lines corresponds to imaging different heights in the atmosphere (Figure 1.8). Images in K₃ show a series of bright and dark patches over the surface. The bright features are known as mottles. These are areas of large brightness contrast which link together to form a quasi-regular pattern around and above supergranules. The larger features are coarse mottles, also known in literature as flocculi. They combine to form the chromospheric network (Section 1.5.3). Each mottle consists of several bright fine mottles, around 7000 km long and 700 km in diameter. Images in H contain additional contribution from higher levels (due to higher excitation energy). In H line centre, fine mottles can be bright (low-lying) or dark (height ~3000 km), the latter sometimes showing fragmentation. In the wing of this line (Figure 1.8), fine mottles are always dark. Rosettes are formed when alternate dark and bright fine mottles radiate from a common centre, where the common centre makes up part of the network. Typical fine mottles have a lifetime of 12-20 minutes. They are sites of upward and downward motions which can be regarded as cylinders less than 1000 km thick, with a temperature in the range of 5000 - 15000 K. The dark/bright contrast is due to an increase in gas pressure. Spicules are limb structures of ~10,000 K, up to 1500 km across which extend 10,000 km into the corona, with a typical upward velocity of 20 km s. This upflow could fill the corona in three hours, and so requires a mass flow back down into the chromosphere. Often spicules, and the downflow corresponding to them, are attributed to mottles on the limb. In the centre of each cell smaller features can be found. The H violet wing shows small (1000 km) dark features associated with upward flow. In Ca II lines, bright grains are found which increase and decrease in brightness periodically over a few minutes. In the network, these are found to be spatially coherent with small magnetic elements which collide and reconnect. The brightness may be due to heating after magnetic reconnection, where magnetic flux is turned into heat (Chapter 2).