By Ron Cowen
Several times a day, the seething cauldron known as the sun undergoes a major eruption, shooting billions of tons of electrically charged gas into interplanetary space. Some of these parcels of gas, called coronal mass ejections, strike Earth and can damage sensitive satellite instruments and knock out power grids. Such temper tantrums are expected to peak in frequency this year as the sun reaches the maximum of its 11-year activity cycle, yet astronomers understand precious little about the origin of these explosive events.
This week, new clues emerged from movies made by Yohkoh, a Japanese satellite that records X rays from the corona, the sun’s hot outer atmosphere. Reviewing a sequence of Yohkoh images from May 1998, Josef I. Khan of University College London noticed something strange. Magnetically confined loops of material that straddle the sun’s equator and arc far out into the corona disappeared just after the sun emitted a flare, a concentrated outpouring of X rays and other radiation. Khan saw the same pattern on three occasions—May 6, 8, and 9, 1998.