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Solar Active Regions

Solar active regions form where the tops of loops of magnetic flux, shaped like the Greek letter omega ( ), emerge into the solar atmosphere where they can be seen. These loops are thought to be generated by the solar dynamo, located deep within the Sun (see DYNAMOS: SOLAR AND STELLAR). The appearance of active regions depends greatly on the radiation used to image them, as well as the angle at which they are observed. When seen projected against the dark sky at the edge of the Sun (called the LIMB), the magnetic field of an active region is traced by glowing arches of gas confined within CORONAL LOOPS (figure 1). When projected against the solar disk the appearance of active regions is much different. In hydrogen Hα, at low telescopic resolution, one sees bright SOLAR CHROMOSPHERIC PLAGE (figure 2). At higher resolution, the relatively cool (chromospheric) loops of the active region appear (figure 7). In x-rays, the much hotter (coronal) loops of the active region are seen (figure 3). All of these active region loops, whatever their temperature, trace out magnetic field lines. Plage appears at their footpoints, where the loops pass through the chromosphere and photosphere. At one footpoint of each loop the magnetic field comes out of the Sun (north polarity), and at the other footpoint it goes back into the Sun (south polarity). Active regions are formed of many such bipolar pairs of footpoints connected by coronal loops, as if a large magnet was buried beneath the visible solar surface at the site where the active region appears. The high density at which magnetic loops are packed together at their photospheric footpoints is the distinctive feature that makes active regions different from the surrounding quiet atmosphere. The effects of the magnetic field on the temperature and density structure of an active region depend on its strength, the relative fraction of the active region it fills (compared with non-magnetic regions) and its topology, i.e. how each point is connected to another. In chromospheric plage—the bright areas that can be seen in figure 2 both scattered around the quiet Sun (two examples shown) and within the active region itself—magnetic loops are packed 10–100 times more densely than in the quiet Sun. In SUNSPOTS, loops are packed so tightly that there is no significant amount of unmagnetized gas, which inhibits the flow of heat by convective motions and makes sunspots cooler and darker than their surroundings. Finally, irregular dark strands of gas called solar FILAMENTS form where the magnetic field lines dip to form valleys in the corona that can hold ionized gas (which cannot easily move across field lines) in clouds above the photosphere. Several such filaments can be seen in figure 2, both inside the active region and outside it, in the quiet solar atmosphere. When filaments are seen in profile at the solar limb, they are called SOLAR PROMINENCES. Active regions come and go, and their lifetime depends to a large extent on their size. Active regions have no preferred size; they form a continuous distribution, with many more small ones than large ones. The smallest Figure 1. A small solar active region near the edge of the solar disk, seen in emission from a highly ionized Fe line emitted by gas at about 1.5 × 106 K. Photo from the NASA TRACE satellite courtesy Lockheed Martin.