Scherrer and his colleagues at Stanford watch for echoes of these drumbeats. Every drumbeat will echo numerous times, as the sound waves go into the sun and come back out and bounce off the surface and go back in, over and over. Eventually, after several bounces, they may come out close to their starting point again. A roomful of computers watch for these reverberations 24 hours a day. Amid the cacophony of a million drums, they try to pick out the sound of a single drum and its echo from roughly a million miles away on the opposite side of the sun.
When they do pick up an echo, physicists can learn a great deal about the sun’s interior. Sound waves do not travel equally fast at all temperatures. The hotter the gases, the faster they go. So a sound wave that passes deep through the interior of the sun to the opposite side and bounces back will return sooner than one that makes lots of little bounces close to the surface. The difference in transit time allows physicists to take the temperature of the regions the sound waves pass through. The technique is even sensitive enough to measure the temperature under a single sunspot. Scherrer and his colleagues have showed that although a sunspot is cooler on the sun’s surface, it traps a layer of hotter gas beneath it.
The most spectacular consequence of this discovery is the ability to detect sunspots on the unseen side of the star—a possibility first broached by Douglas Braun and Charles Lindsey, solar researchers at NorthWest Research Associates in Boulder, more than 10 years ago. If there is an active sunspot region on the other side of the sun, Braun and Lindsey argued, the sound speed should increase underneath it because of the hotter gas under the spots. That speeds up the round trip of a drumbeat by a few seconds.
At first “we heard a lot of skepticism about the idea,” Braun says. Lindsey is blunter: “Some people thought it was pie-in-the-sky craziness.” And it didn’t work when Lindsey and Braun looked at the sun through Earth-based telescopes. “All we got was noise,” Braun says. “We really needed 24 hours of observation time to reduce the noise.”
When SOHO was launched in 1995, it provided the perfect round-the-clock viewing platform. So in 1999, Braun and Lindsey decided to try their idea again. This time it was a stunning success: Their computer program produced fuzzy yet unmistakable images of sunspots on the far side. Five years later, this amazing feat has become routine. (The far side of the sun can be observed every day at www.spaceweather.com.)
Far-side imaging even provided plenty of advance warning of the gamma-ray flare in 2002 that pleased Lin. The active region it came from, 10039, had grown larger throughout its transit across the far side. Similarly, region 10486, the main source of last fall’s storms, drew researchers’ attention long before the sun rotated and the region came around to the front. Besides showing up like a big red bruise on the helioseismological maps of the far side, it had released coronal mass ejections that could not be traced to any region on the near side. The eruption had also emitted ultraviolet light that reflected off interplanetary hydrogen gas and back into SOHO’s cameras. “When you see all three of these things together, it gives you the picture that something spectacular is happening on the far side,” says St. Cyr.
While solar physicists refine their forecasting techniques, some people are just getting acclimated to the concept of space weather. “Blizzards, a miserably cold and wet spring, a weirdly dreary June, a nasty hurricane, and now the weather has turned terrible in space,” wrote columnist Joel Achenbach in The Washington Post after hearing of the October 28 flare.
But the people who run power grids and satellites have wanted this information for a long time. The eye-opener was a blackout in Quebec on March 13, 1989, that plunged some 6 million people into darkness for more than nine hours. The cause was surges of electricity induced by a geomagnetic storm that burned out a few key transformers. Last fall’s storm didn’t cause any blackouts in the United States, but it did create power surges in long-range transmission lines. If the flares had happened at a time when there was more demand on the power grid, the consequences could have been more severe.
For satellites, the danger comes from energetic particles, which can confuse computers by changing bits of data in memory chips. Also, solar particles can short-circuit electronic instruments, and drag induced by the particles heating the atmosphere around a spacecraft can alter its orbit. In one of the great ironies of the space age, Skylab—the first manned U.S. spacecraft to observe the sun—was dragged to Earth prematurely by this effect.
Although SOHO, RHESSI, and other satellites have improved our ability to prepare for solar hazards, they are only a start. SOHO can see an eruption lift off from the sun, but it can’t tell when it will get to Earth or whether it has the proper magnetic orientation to pose a threat. By the time particles actually hit SOHO, they are only an hour away from Earth. “All it can say is, ‘Look, here comes a geomagnetic st—!’ ” says Scherrer.
Several upcoming space missions may fill in the blanks. The mission of the Solar Terrestrial Relations Observatory, scheduled for launch in 2005, is to send two identical satellites into solar orbit, one ahead of Earth and one behind. That will give astronomers their first three-dimensional view of coronal mass ejections. Now they have to make do with views that cannot easily distinguish a coronal mass ejection headed toward Earth from one headed in the opposite direction. The Solar Dynamics Observatory, planned for 2008, will allow more complete study of the sun’s interior and perhaps enable the prediction of sunspots before they appear.
Still, the list of what scientists don’t understand about the sun is daunting. For example: What drives the 11-year sunspot cycle, what makes the corona so hot, and how big do solar flares get? After the unexpected intensity of the November 4 flare, some ideas about the scale of solar fury are likely to change. St. Cyr notes that we do know what a 100-year hurricane is like, “but on the sun, we don’t know what the 100-year events are. We don’t really have a clue.”
Late 2006 will mark the close of a solar cycle that began in 1996. The graph below tracks the frequency of sunspots during the current cycle, the 23rd since astronomers have been monitoring solar activity. The large orange solar image—created from helium emissions—was taken in 2000 as the cycle neared its peak. The green images—created from iron emissions—show the trend toward increased solar activity. Bright areas indicate intense magnetic activity. During active periods, up to 250 sunspots may form, but only a handful occur during the quiet periods. German astronomer Samuel Schwabe recorded sunspots over 17 years and reported in 1843 that sunspot activity is cyclic. A typical solar cycle is about 11 years, although some have been as long as 17 years or as short as 8 years. Solar physicists believe that waxing and waning magnetic activity within the sun drives the solar cycle, but they do not know why the cycle occurs. Some climatologists suspect that changes in solar activity can provoke climate change on Earth. For example, Europe’s “little ice age” coincided with a period of minimal solar activity between 1640 and 1710. But other researchers believe changes in current solar activity will have a minimal effect, if any, on Earth’s climate.
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