OUR STORMY SUN
What do refrigerator magnets,
and solar flares have in common?
By: Charles Liu
L ate last year a huge solar flare erupted from a massive sunspot,
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 ot potential solar disaster, though good news copy, are at best unreliable.
Although the understanding of solar activity---- including the origins and life histories ot 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 Grcenbelt, 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.
M ost 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, supe-rheated 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 tip by the twisting of the plasma is then 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 (C ME). The cloud of magnetized gas typically gets launched into space at several million miles au hour, fast enough to reach Earth in a day or so.
Huge amounts of charged matter swarming over our planet can damage delicate satellite electronics and overload unprotected power grids. Usually, though, the solar particles released by a CME only produce a harmless light show; deflected by the Earth’s magnetic field, they crash into gas particles in the upper atmosphere and set them aglow----creating auroras----the northern and southern lights.
Sunspots and coronal mass ejections are localized phenomena. Although often large enough to swallow several Earths, they still erupt across just a small fraction of the Sun’s surface. . But the Sun’s internal motions cause another, larger-scale magnetic effect: the reversal of the Sun’s magnetic poles. Over a roughly eleven- year period, coinciding with the solar sunspot cycle, the Sun’s north magnetic pole gradually diminishes in its “north-ness7 Then, after a period ot seeming indecision, it becomes the south magnetic pole, increasing its south-ness to maximum strength. During the next eleven years, the now-sotith pole switches back again into the north pole. The same process happens in reverse at the Sun’s south magnetic pole. (The process is not really as bizarre as it sounds—the Earths magnetic poles flip too, albeit on much longer timescales; the planet’s most recent polar reversal took place 780,000 years ago.)
Since both coronal mass ejections and polarity reversals are aspects of solar magnetism, astronomers have long suspected that the two phenomena are somehow fundamentally connected. Now Gopalswamy and his colleagues have uncovered souse hard evidence to support that idea. Analyzing data gathered by Sun—watching satellites in the past quarter century, they found that the number of CMEs generated near the solar poles increases dramatically just before the polarity reverses. Then, after the reversal is completed, the CM F rate drops to near zero. Gopalswaniy’s group hypothesizes that CMEs are how the Sun expels the last bits of the old “north-ness” or “south-ness” from a magnetic pole, making way for the new, opposite magnetic orientation. For those who like comparisons, think of the Sun as a cosmic kitty, throwing up CME hairballs to regulate it’s internal magnetic balance.
A s you’ve probably noticed by now, the solar storms of 2003 didn’t amount to much here on Earth. Only one of the many CMEs launched toward our planet caused significant disruption to anyone’s routine—leaving 50,000 people without power in southern Sweden for about an hour. The other CMES last year did nothing more than generate colorful auroras. But what about a future barrage of record—setting CM Es? Will their effects be just as benign, and if not, how should we prepare?
Not too long ago, forecasting violent storms on Earth was more art than science. But decades of patient research have helped meteorologists understand, and then predict, their behavior. Simlarly, the work of Gopalswainy, his collaborators, and other solar astronomers may one day lead to a deeper understanding of solar storms—and, maybe, what to expect the next time the Sun sends a blob of itself hurtling toward our planet.
Charles Liu is a professor of astrophysics
at the City University of New York and an
associate with the American Museum of
February 2004. (Pgs. 64`65)
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