Fantastic fortnight of active region 5395.

Fantastic fortnight of active region 5395

Scientists at first thought the huge solar flare detected on March 6 was "merely" one of the largest in the last decade. In subsequent days, however, it turned out to have signaled the appearance of a spectacular active region on the solar disk, setting records at every turn.

Scientists have compiled detailed records of the last 22 solar cycles, each cycle lasting 11 years. "I've been living this cycle just about from beginning to end, and it's sort of an inspirational experience," says Patrick S. McIntosh of the National Oceanic and Atmospheric Administration's Space Environment Laboratory in Boulder, Colo. The previous two solar cycles, according to McIntosh, appear feeble by comparison. "I've compiled the levels of solar activity since we've been keeping satellite records, and [the present active region, designated AR 5395 and the source of the big flare] is off the top of the scale."

The flare, spotted by an X-ray instrument aboard the Solar Maximum Mission satellite, was "one of the largest X-ray events ever recorded," says Judith J. Nelson of ST Systems Corp. in Lanham, Md. In fact, she adds, it was "the largest ever observed by [Solar Max]." Nelson is in charge of forecasting solar conditions for Solar Max scientists at the NASA Goddard Space Flight Center in Greenbelt, Md.

Richard Schwartz, also of ST Systems, notes that between March 6 and 19 -- the time required for the active region to cross the sun -- the satellite's Hard X-Ray Burst Spectrometer recorded 447 hard X-ray flares, a rate of about 32 per day. This exceeded the previous high by 50 percent. During one five-day span within that stretch, the instrument detected more than 250 flares, also a record. Schwartz says the active region also produced the most intense "single-spike event" of X-rays ever measured by the device. Furthermore, it identified three flares that were turning out X-rays faster than all but 10 other flares in the history of the satellite's mission, which began in 1979.

Besides the X-rays, Nelson says, radio telescopes observed radio events unprecedented both in intensity and in duration. Moreover, active region 5395 triggered major disturbances of Earth's magnetic field. On March 13, an index of geomagnetic activity known as the AFR reached a level of 248, the highest it had been since Nov. 13, 1960. The effects showed up at an unusually low latitude, where such disturbances are produced only by intense solar activity. Auroras were reported at the time in night skies as far south as the Bahamas, Nelson says.

Ironically, the tumult also hastened the demise of Solar Max, notes Chris St. Cyr at Goddard. The increase in solar activity has heated and thus raised the height of Earth's atmosphere, increasing the drag on the satellite. So during the two weeks when the active region was crossing the sun, the low point of the satellite's altitude dropped about 3 miles, says project scientist Joseph B. Gurman of Goddard. Goddard's Flight Dynamics Branch now predicts Solar Max will be impossible to control from the ground after Aug. 3, and that by Oct. 9 it will reenter the atmosphere and burn up.