National Geographic : 1991 Oct
Earth Almanac Fierce Solar Storms Spawn Dazzling Auroras ast spring's giant flares from a stormy sun touched off spec tacular displays of earth's auroras near both Poles. With a ringside seat over Australia, the crew of the space shuttle Discovery photographed this aurora austra lis-the southern lights-worthy of a hallelujah chorus or two. Air Force engineers studied Dis covery's images for the Strategic Defense Initiative, or "Star Wars" program. They want to know how to detect an incoming missile that might be cloaked by an aurora. Solar storms emit blasts of charged particles-the solar wind. Fierce bursts disturb the planet's magnetic field, creating arcs of light in the upper atmosphere. During the spring solar storms the northern lights flared as far south as Georgia, jamming radio broadcasts and threatening power grids. Browsers Beware: Acacia Trees Can Kill Tannin is fine as a tea flavoring, but it can be deadly to wild life. The acacia tree uses tan nin to repel nibbling nuisances, according to South African zoolo gist Wouter van Hoven. He found that the trees have an astonishing defense system and that some 3,000 kudu died as a result. When these antelope browsers bite an acacia leaf, the tree steps up its tannin output and releases ethylene gas into the air. Acacias down wind sense the ethylene as a warning and increase their own tannin. Van Hoven's antelope-kudu fenced on game ranches during a drought-had little to eat but acacias and thus perished. ANIMALS,ANIMALS National Geographic, October1991 JEFF LEPORE,PHOTO RESEARCHERS,INC. (BELOW); NASA Deceptive Orchid Creates Plight of the Bumblebee queen bumblebee spies a lus cious pink orchid seemingly bursting with nectar. The bee lands on the inviting flower, called a pink lady's slipper, crawls inside, and can't back out. The imprisoned bee finds no nectar and can exit only through a narrow rear passage. Picking up pollen en route, most bees squeeze out with fire in their compound eyes and vow never to return. Only 23 bum blebees in 1,000 are dumb enough to go through this again, thereby transferring the pollen to a second orchid, says Maryland zoologist Douglas Gill. "The bee is duped into the flower and gets nothing for its troubles but a rough time," says Gill. Playing so hard to get, how does the orchid survive? With lon gevity-most plants live 25 years and fecundity-each pollinated orchid yields some 60,000 seeds.