Our Stormy Sun

By Liu, Charles | Natural History, February 2004 | Go to article overview

Our Stormy Sun


Liu, Charles, Natural History


What do refrigerator magnets, northern lights, and solar flares have in common?

Late last year a huge solar flare erupted from a massive sunspot, with a blast of X rays so intense that detectors aboard the Earth-orbiting Geostationary Operational Environmental Satellite (GOES) went off scale for more than eleven minutes. A day later, astronomers confirmed that the flare was by far the most powerful ever recorded, obliterating the previous record set in 1989 and matched in 2001.

The flare put an exclamation point on a fortnight of unprecedented storminess on the Sun. It also drew unprecedented media attention to the Sun, just 93 million miles away, and to the electromagnetic disruptions its violent surface can cause on Earth: power outages, disrupted communications, satellites lost through damaged electronics. But dire warnings of potential solar disaster, though good news copy, are at best unreliable.

Although the understanding of solar activity-including the origins and life histories of solar storms-remains poor, solar scientists are making steady progress in unraveling the mysteries of the Sun. A recent example is a study led by Natchimuthuk Gopalswamy, a solar astronomer at NASA's Goddard Space Flight Center in Greenbelt, Maryland, which makes a new connection between two great solar puzzles: what gives rise to coronal mass ejections-vast clouds of ionized gas thrown outward during solar storms-and why the Sun's vast magnetic field periodically flips polarity.

Most everyone who has used a compass knows that our planet has a magnetic field; at Earth's surface, it is slightly weaker than that of a typical refrigerator magnet. Our star's magnetic field is just a bit stronger than Earth's, but like any other magnet, its strength and direction can be represented as the density and direction of field lines that run between its north and south magnetic poles. The difference is that those field lines are enmeshed within the electrically charged, superheated plasma that comprises the body of the Sun.

Unlike a solid refrigerator magnet, the Sun has a restless interior. Heat gets transferred from deep inside the Sun to its surface as hot gas swells outward, cools off, and then plunges inward again, back and forth in a cycle. The Sun also spins unevenly: a "day" on its surface lasts about thirty-one Earth days near the Sun's poles, but only about twenty-seven Earth days at its equator. Huge gobs of Sun-stuff are pushed in and out, then pulled round and round in the chaotic turbulence. The magnetic field lines get dragged around as well, stretching, twisting, and tangling within the plasma into dense, messy knots.

When the knots emerge at the surface, they appear as sunspots; when too many field lines get tangled up in one spot, conditions become highly unstable. The tangled field lines act like a coiled steel spring too tightly wound, which can suddenly snap and realign. All the energy built up by the twisting of the plasma is suddenly released, as if millions of atomic bombs exploded in just a few seconds. Such an eruption of energy manifests itself as a solar flare. Often such magnetic realignments cause billions of tons of solar matter to be ejected outward through the solar corona-an event called, unsurprisingly, a coronal mass ejection (CME). The cloud of magnetized gas typically gets launched into space at several million miles an hour, fast enough to reach Earth in a day or so. …

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