Further findings on flare phenomena.

Further findings on flare phenomena

To the earth-bound, the sun lends an appearance of being a docile, warm, friendly sort of star that keeps flowers in bloom and summer days languid. But to the solar astronomer, that same sun is a massive ball of hot swirling gases prone to complex atomic reactions and violent explosions that rise thousands of miles above the surface. And though the sun's proximity allows researchers a blazing laboratory in their own backyard, much about the 5-billion-year-old star remains shrouded in mystery.

One such enigma is the solar flare, which surges out from the sun in a fiery blast that may last a few minutes to an hour, but is capable of spewing forth the equivalent amount of solar energy the earth gets in a year. Large flares can cause temporary radio blackouts, auroral displays and loss of satellite communications. No one quite knows why these flares occur, but current theories tie them in with he interaction and distortion of magnetic fields surrounding sunspots (SN: 6/28/80, p.404).

As much of a mystery is the timing of these flares, shown to occur every 152 to 154 days. Some astronomers think this timing occurs because of interactions between various phenomena, such as the coinciding of hotspots or other rotating features of the sun. Now, Taeil Baiand Peter A. Sturrock of Stanford University report in the Jue 18 NATURE that the periodicity of solar flares is a product of a global, rather than local, phenomenon.

The researchers studied the 152-day periodicity of 442 major flares recorded from February 1980 to December 1983 by the Hard X-Ray Burst Spectrometer, one of several devices on board the Solar Maximum Mission that began studying the sun in 1980 (SN: 9/6/80, p.152). The spectrometer monitored X-ray emission by the amount of energy across different spectral ranges.

Earlier studies have shown that flares occurring each 152 days might result from the coinciding of the sun's hotspots, or active zones, with each other. To check this, Bai measured the rotation period of the sun's hotspots in the Northern Hemisphere and found it to be 26.75 days. But observations showed that the alignment of these hotspots, as well as alignment of other active zones rotating at different periods, failed to produce most of the flares seen at the 152-day period.

Bai also tested a theory that previously showed that flares occurred during the coupling of "active bands" inside the sun produced by gravity-mode oscillations, a type of distorting force in the sun. It is believed that when these bands overlap they generate excess energy and induce convection in the sun, followed by the creation of sunspots and the production of flares. But Bai's analysis of the rotation period of two bands thought to do this showed that that the occurrence of flares every 152 days didn't match up to the times the bands overlapped.

Although Bai's study showed that almost half of all flares studied occurred in the prominent active zones of each hemisphere, he also found that flare activity outside active zones correspnded with the 152-day cycle, indicating that the periodicity occurs because of some mechanism involving the entire sun. In addition, the flares in the Northern and Southern Hemispheres that showed the 152-day periodicity independently also peaked at the same tme. Although Bai attributes the timing of the flares to something that involves the whole sun, he won't hazard a guess as to what that might be.

Bai is equally mystified by findings in a recent study of 443 flares occurring during the sun's 19th solar cycle, from 1954 to 1964. "We found a periodicity of 51 days [within that cycle], which is one-third of what we found in cycle 20 and 21," he says. "I'm puzzled why the period was reduced by a factor of three." That problm, Bai says, is one he'll leave the theorists to solve.