Brilliant Hashes of light in the solar atmosphere, lasting less than an hour, or even perhaps only a few seconds, are named SOLAR. FLARES. The brightest are visible in white light, but they are easier to see in the light of hydrogen or calcium lines. A flare is a highly concentrated, explosive release of energy, usually in the vicinity of an active region. Ha photographs typically show a starlike brightening in the lower atmosphere, within a plage region, which spreads rapidly to cover areas the size of the Earth (figure 8.27). Most of the energy in a flare is released in about five minutes across the entire electromagnetic spectrum from radio waves to X-rays.

Flares spring up in active regions where the magnetic field has been stressed into a strong, unstable, configuration. A major puzzle in solar physics is the reason for the sudden release of energy through the flare mechanism. Once they have started to release energy at a particular place they may flash on and off several times. Violent shock waves tear through the photosphere and chromospheres at velocities of 2000 km s-1. Simultaneously X-ray and ultraviolet detectors register intense blasts. In the hard X-ray region, the total emission from the Sun may rise by 100 times during a flare. For this reason it is considered hazardous to send astronauts into space at times when strong flares are likely to erupt on the Sun. Flares also eject copious bursts of energetic charged particles, such as protons and electrons, from the Sun. After a major outburst it is not un¬common to see material crashing back to the Sun in the form of a loop prominence.

Flare studies expanded considerably once engineers had developed instruments that could be launched in rockets and satellites. The copious X-ray emission can only be monitored outside Earth’s atmosphere. When a flare occurs, a sharp burst of hard X-rays (wavelengths shorter than 0.1 nm) is detected, followed by a gradual rise and fall of soft X-rays (wavelengths between 0.1 and 2nm). In the soft spectrum are emission lines from highly ionized species, such as Fe24+ and Fe25+. The continuum is thermal radiation with a characteristic temperature 2 — 3 x 107K. On the other hand the hard X-rays have a non-thermal spectrum. This may be produced when intense streams of relativistic electrons collide with other particles. The same electron beams may also create the synchrotron radiation from flares which is detected by microwave radio telescopes.

Flares have observable consequences for Earth. The energetic particles streaming into our atmosphere about two days after a flare excite atoms and electrons, causing them to emit light when they deexcite (i.e. return to their normal state); the result is the auroral displays, which are strongest at high latitudes where the magnetic field emerges vertically from the Earth’s surface, and at solar maximum. The X-ray bursts indirectly cause the fadeout of shortwave radio communications on the sunlit side of Earth and cause geomagnetic disturbances. Powerful surges in the Sun’s radio emission also occur.

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