Coronal loops are bright, curving structures that appear as arcs above the Sun's surface. Hot plasma causes these loops to glow. The electrified plasma flows along the curving lines of powerful magnetic fields, giving the coronal loops their characteristic shapes.
Coronal loops are often, but not always, associated with sunspots. Sunspots are actually the visible manifestations of places on the Sun's surface where powerful magnetic fields "break through" the photosphere (surface) and extend into the Sun's atmosphere. Coronal loops are often "rooted" in sunspots, arcing between pairs of sunspots with opposite magnetic poles.
Coronal loops come in many sizes. The larger loops extend upwards many thousands of kilometers above the photosphere into the solar corona, the Sun's upper atmosphere. Some loops are extremely hot, having temperatures well above a million degrees.
Coronal loops are more common around solar max, the time in the sunspot cycle when the Sun's magnetic field is highly disturbed and sunspots are numerous. They are most easily seen in images taken in the extreme ultraviolet and X-ray portions of the electromagnetic spectrum.
One mystery of the Sun that scientists are actively trying to solve is why the Sun's atmosphere is much hotter than the Sun's surface. Scientists suspect that energy trapped in magnetic fields somehow causes this intense heating. Coronal loops are probably one of the keys in this ongoing investigation.