A structure in the Sun’s corona seen in X-ray, ultraviolet, or white-light images, consisting of an arch extending upwards from the photosphere. They appear particularly bright at extreme ultraviolet and X-ray wavelengths because of their higher density and temperature relative to the surrounding corona. Loops come in a wide range of sizes, from transequatorial loops up to 1 solar radius in length (700 000 km) that join active regions in opposite hemispheres, down to quiet region loops at around 5000 km long. The two ends of the loop, called the footpoints, are located in regions of the photosphere of opposite magnetic polarity to each other. This and other evidence suggests that coronal loops are bundles of magnetic flux tubes filled with hot plasma.