Sky & Telescope October 1983 [antikvár]

Solar Flares and Shock WavesAlan Maxwell, Harvard-Smithsotiian Center for AstrophysicsFlares on the sun are among the most dramatic events in the solar system. a flare appears with hardly any warning as a small, bright patch on the solar surface and sends material exploding out\vard through the Sun's atmosphere into space. Sometimes the blast is powerful enough to sweep through the solar system for days afterward. When the disturbance impinges on the Earth's magnetosphere it gives rise to auroras, disrupts radio communications, and may induce severe electric power surges. a solar flare is one of the very few astronomical events that directly disturbs the earthly environment.In just a few minutes, a large flare releases as much as 10" ergs of energy the explosive power of 100 million hydrogen bombs with a yield of 100 megatons each. The source of this energy and the manner in which it is released have been intensely studied for several decades but are still poorly understood. Solar astronomers now believe that the energy is somehow derived from the magnetic fields associated with sunspots.The initial stages of flares in the solar chromosphere and lower corona were described by M. Kundu in Sky Telescope for July, 1982, page 6. This article shall concentrate on the spectacular, large-scale phenomena that are subsequently observed in the outer corona, and also on how solar physicists have developed models that attempt to simulate these events.The energy of a solar flare is released in a relatively small volume over the course of about 30 seconds to five minutes. This explosive phase intensely heats local areas of the chromosphere and lower corona, and as a result, a large amount of material is suddenly ejected. If this matter is expelled with a velocity of greater than about 500 kilometers per second, it will generate a shock wave that expands outward ahead of it.The solar atmosphere has a relatively low density and a strong magnetic field. Therefore, the shock takes the form of a fast-moving magnetic disturbance, rather than a more familiar pressure wave such as a thunderclap or sonic boom in the Earth'sA flare spray ejects material faster than any other event on the Sun. Velocities up to 750 km per second were recorded in this event, observed in hydrogen-alpha light with a coronagraph at Haleakala, Hawaii, on November 24, 1980, at 20:05 Universal lime. The spray originated from a flare just behind the limb. A Type II radio burst occurred at the same time. Photograph courtesy Marie McCabe.atmosphere. The shock wave and the material behind it will tend to expand almost as a hemisphere, but nearby magnetic fields may influence the motion.CORONAL TRANSIENTS Photographs of the Sun taken in the light of various spectral lines often reveal matter moving rapidly from the vicinity of a flare. If the outburst occurs near the solar limb, we may see matter being expelled in the form of a fast-moving "flare spray" like the one illustrated below. These have velocities of up to 1,000 km per second. At other times, the ejection may be in the form of eruptive prominences, seen as exploding arches moving outward at a few hundred km per second. On occasion, matter may be expelled in the form of slow-moving surges traveling atabout 200 km per second. This is less than the solar escape velocity, so the ejected matter subsequently falls back to the Sun's surface.Earthbound telescopes normally can examine the solar atmosphere only to about two solar radii from the Sun's center. Sat-ellite-borne equipment, however, has made it possible to track the outward-moving disturbance from a flare to 10 radii. In the outer corona these features often take the form of loops (as illustrated on the next page) with leading edges that may be expanding at 1,000 km per second.Many of these so-called coronal transients seem to be two-dimensional arches, but it is often impossible to determine their true, three-dimensional forms. Recent data on the polarization of their light, together with a striking observation of aOctober. !983, Sky & Telescope 285