Coronal loop

Coronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle; it is for this reason coronal loops are often found with sunspots at their footpoints. The upwelling magnetic flux pushes through the photosphere, exposing the cooler plasma below. The contrast between the photosphere and the solar interior gives the impression of dark spots, or sunspots.

A coronal loop is magnetic flux fixed at both ends, threading through the solar body, protruding into the solar atmosphere. They are ideal structures to observe when trying to understand the transfer of energy from the solar body, through the transition region and into the corona.

Many scales of coronal loops exist, neighbouring open flux tubes that give way to the solar wind and reach far into the corona and heliosphere. Anchored in the photosphere (a rigid, line-tied, anchor is assumed where the high-β, external plasma holds the loop footpoints in place), coronal loops project through the chromosphere and transition region, extending high into the corona.

Also, coronal loops have a wide variety of temperatures along their lengths. Loops existing at temperatures below 1 MK are generally known as cool loops, those existing at around 1 MK are known as warm loops, and those beyond 1 MK are known as hot loops. Naturally, these different categories radiate at different wavelengths.

Coronal loops populate both active and quiet regions of the solar surface. Active regions on the solar surface take up small areas but produce the majority of activity and are often the source of flares and Coronal Mass Ejections due to the intense magnetic field present. Active regions produce 82% of the total coronal heating energy. Coronal holes are open field lines located predominantly in the polar regions of the Sun and are known to be the source of the fast solar wind. The quiet Sun makes up the rest of the solar surface. The quiet Sun, although less active than active regions, is awash with dynamic processes and transient events (bright points, nanoflares and jets). As a general rule, the quiet Sun exists in regions of closed magnetic structures, and active regions are highly dynamic sources of explosive events. It is important to note that observations suggest the whole corona is massively populated by open and closed magnetic fieldlines.

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Wikipedia