The sun, in a periodic show of force, is shooting off powerful solar flares that disrupt the Earth's magnetic force and could cause power shortages in the northern United States this week and spark a display of the northern lights. This is a sun storm, and scientists monitoring it in Boulder, Colo., have classified it as "severe," the category designating the highest degree of force.
The last such severe solar storm, in March 1989, caused power blackouts in New York and Canada. Damage, however, is hard to predict, experts say, and there could be almost no noticeable effects. But an alert is on for this week as the sun rotates and its most active region turns to face the Earth more directly.
Scientists don't know why the sun goes through this cyclic pattern, said Joseph W. Hirman, manager of the Space Environmental Services Center in Boulder. "But you can tell what's happening. We can measure it."
Hirman is responsible for monitoring the large solar flares _ sudden bursts of energy released by sun spots during periods of high activity _ and trying to forecast the effects those solar flares can have on the Earth.
The sun's 11-year cycle has a summer and winter, Hirman said, and the sun is now in the summer phase, the most active, when storms are most likely.
The center in Boulder, part of the National Oceanic and Atmospheric Administration, measures the number of sun spots with radiotelescopes.
The last large flare occurred Wednesday evening, and its effects were reaching the Earth late Friday.
Much of the time the sun is quiet, said Dr. Mario H. Acuna, an astrophysicist at NASA's Goddard Space Flight Center. But when it develops an active region, it resembles a pot of boiling spaghetti sauce that spits out high-energy particles into space.
The sun emits a so-called solar wind made up of ionized gas. This solar wind exerts strong pressure on the earth's magnetic field, a huge area extending into space far beyond the thin layer of atmosphere. The solar wind cannot penetrate the Earth's magnetic field, but there are two "funnels" at the north and south poles through which it can pass.
When the high-energy particles of the solar wind whip through those funnels, they reach the atmosphere. There they excite oxygen and nitrogen atoms, causing them to glow in the green, blue and gold glow of the aurora borealis, the northern lights seen in the northern hemisphere, Acuna said.
The same phenomenon occurs in the southern hemisphere, he said.
Such disruption of the earth's magnetic field can cause large currents of energy to circulate through power lines, overwhelming transformers. The buildup of energy either can trip circuit breakers, shutting down a transformer, or destroy it, the scientist said.