A coronal loop is a feature in the Sun's corona visible at X-ray, ultraviolet, and white-light wavelengths, consisting of an arch, extending upward from the photosphere for tens or hundreds of thousands of kilometers. Bright coronal loops, in the form of coronal condensations and bright spots, are common around the time of solar maximum. Larger faint ones, lasting days or weeks, are more typical of the quiet corona, when solar activity is low. The two ends of a loop, known as footprints, lie in regions of the photosphere of opposite magnetic polarity to each other.
Until recently, researchers had suspected that coronal loops were essentially static, plasma-filled structures. However, movies made from observations by the TRACE (Transition Region and Coronal Explorer) spacecraft have shown bright blobs of plasma racing up and down the coronal loops. SOHO data confirmed that these plasma blobs were moving at tremendous speeds, leading to the new view that coronal loops are hypervelocity currents of plasma blasted from the solar surface and squirted between the magnetic structures in the corona. Rather than being tubes of plasma enclosed within a magnetic container, the loops are jets of hot plasma flowing along in the alleys between the strong coronal magnetic fields. If coronal loops are indeed currents of plasma being propelled against solar gravity, they would have about the same density along their entire height, like an arc of water from a fountain. Plasma flows are seen in roughly half of all coronal loops visible by TRACE; flows may be present in the remainder but may be too faint for TRACE to detect. The plasma current that forms a coronal loop is probably caused by uneven heating at the bases of the loop, with plasma racing from the hotter end to the cooler end. The bases of a coronal loop are separated by many thousands of km, and there is no reason to assume that the environment at one end will be exactly the same, and input exactly the same amount of heat, as the environment at the other end. Although it isn't clear what causes coronal-loop heating in the first place, these new discoveries may help uncover the mechanism, shedding light on the long-standing mystery of why the corona is hundreds of times hotter than the solar surface.