Parting Shots

Oct. 14, 2003, dawned with a virtually spotless sun. It turned out to be the calm before the storm. During the next few days, the number and size of sunspots increased. On Oct. 18, a massive solar flare spewed from a newly visible sunspot near the eastern edge of the sun. The sun’s rotation carried this Jupiter-size zone of intense magnetic activity across the solar face and, on Oct. 23, another hyperactive spot of similar size spun into view. A third blemish popped up on the sun’s left cheek on Oct. 27, tripled in size by the next day, and then quickly doubled. At their largest, these three dark pocks and their smaller satellite blotches covered enough of the sun’s surface to cause a dip in solar radiation measurable by Earth-orbiting satellites.

Over an exceptionally active 18-day period that ended Nov. 5, these sunspots spewed almost a year’s number of flares, ejecting massive torrents of charged particles into space to race outward along the sun’s magnetic field lines. Space probes throughout the solar system detected these gusts in the solar wind (SN: 6/5/04, p. 366: Available to subscribers at Huge solar flares hit far-flung craft).

Material from several flares swept past Earth, triggering geomagnetic storms that disrupted high-frequency radio communications, knocked out satellites, and caused a power outage in Sweden. Passengers on some high-flying aircraft received increased amounts of radiation, and astronauts on the International Space Station had to occasionally take shelter in protective areas of the facility’s service module. The northern lights, which usually unfold only at high latitudes, enthralled sky gazers as far south as Houston.

Nine months later, scientists are compiling their initial analyses of the record-setting barrage of flares. They’re beginning to present their findings at scientific meetings. A fleet of instrument-laden spacecraft recorded the reams of raw data. Staring unblinkingly into the sun’s glare, these probes collect information that’s vital for creating and refining models of solar activity.

Getting better at predicting how the sun will behave has become far more than a goal of academic solar physicists. Never before have so many people, services, and assets depended on the reliable operation of satellites. And better predictions of space weather may protect space travelers from solar tantrums, an important factor if people are to travel to the moon and Mars.

Solar wallop

Although they appear dark, sunspots glow brighter than an arc welder’s torch. At a temperature of around 3,500°C, they look dim merely in comparison with the rest of the sun’s surface, which radiates more energy because it’s about 2,000°C hotter.

Sunspots usually occupy only a minuscule portion of the sun’s surface. Not so for the three massive sunspots that appeared late last October and were dubbed 484, 486, and 488 by scientists who track such blotches and their activity. Together, the spots covered nearly 0.8 percent of the sun’s face, says Thomas N. Woods of the University of Colorado in Boulder. According to satellite data, total solar irradiance—the amount of the sun’s radiation reaching the top of Earth’s atmosphere—dropped as much as 0.34 percent, Woods and his colleagues report in the May 28 Geophysical Research Letters.

The sunspot trio also spawned an extraordinary number of large solar flares. During the previous two solar cycles, which together stretched from mid-1976 to mid-1996, one modest-size, or M-class, solar flare occurred every 2 days on average, and one extreme, or X-class, flare burst forth each month or so. During the most intense phase of last fall’s barrage, the sun belched out 44 M-class flares and 11 X-class ones in just 18 days.

“When the [solar] activity took off, it took off with a vengeance,” says Woods.

The flare that had the largest effect on Earth emanated from Sunspot 486, which was near the center of the sun’s disk when it erupted on Oct. 28. Although visible radiation from the sun didn’t rise appreciably, emissions at shorter, high-energy wavelengths increased substantially. At extreme ultraviolet wavelengths between 27 and 115 nanometers, radiation doubled. At UV wavelengths below 10 nm, irradiance was about 50 times the amount measured just before the flare. At X-ray wavelengths between 0.1 and 0.8 nm—the window of radiation that researchers traditionally analyze to rate solar flares—radiation skyrocketed to about 570 times the pre-flare value. That radiation intensity placed the Oct. 28 event in the category of X-17 flares. Only two flares in the two previous solar cycles had been that strong.

Traveling from the sun at the speed of light, the X-ray and extreme ultraviolet radiation resulting from the flare from sunspot 486 took a little more than 8 minutes to reach Earth. Unlike visible light, photons at those wavelengths don’t penetrate very far into the atmosphere.

The flare was “like the flashbulb on a camera,” dumping much energy in a short time, says Woods. That energy stripped electrons from atoms of high-altitude gases, heating the atmosphere, causing it to expand farther into space, and adding to the drag on low-orbiting satellites. On Earth’s sunlit side, the density of free electrons in the ionosphere—the atmospheric layer that lies at altitudes above 75 kilometers or so—abruptly jumped about 25 percent, thereby disrupting high-frequency radio communications whose signals travel over the horizon by bouncing off this layer.

The large blob of charged particles flung from the sun during the flare accelerated into space along the sun’s magnetic field lines. Called a coronal mass ejection because the material originated in the lower levels of the sun’s atmosphere, or corona, this material traveled at more than 7 million km/hr. Nineteen hours after leaving the sun, many of those particles approached Earth, where they spiraled down Earth’s magnetic field lines to pummel the upper atmosphere.

For several days, many aircraft scheduled for flight in polar regions were routed farther southward than normal—not only to retain radio contact but to avoid exposing passengers and crew to increased radiation. Even aircraft flying more southerly routes couldn’t avoid encountering a boost in radiation. Quantas flight 107, a Boeing 747 en route from Los Angeles to New York on the afternoon of Oct. 29, is a case in point. Ian L. Getley of the University of New South Wales in Sydney document that plane’s experience in the May Space Weather.

During the middle portion of the plane’s journey, while the aircraft flew between Utah and Ohio at a cruising altitude of about 11,300 meters, onboard instruments measured radiation doses normal for that latitude and altitude, about 3 microsieverts (µSv) per hour. Just as the aircraft began a climb to 11,900 m, however, a barrage of charged particles emitted by an X-10 solar flare earlier that day was arriving at Earth.

Radiation doses during the final hour of the Oct. 29 flight peaked at 4.65 µSv/hr, Getley says. Even so, total radiation exposure for passengers on the flight added up to only 12 µSv—about one-quarter of what patients typically receive during a chest X ray.

Collateral damage

The billion-ton burps of material from the sun last autumn didn’t blast just Earth. On the basis of previous studies, NASA scientists calculated that shock waves from solar flares temporarily compressed the atmosphere on the sunward hemisphere of Mars, which has no planetwide magnetic field to protect it from such salvos.

As the flares swept past Jupiter, their interactions with that gas giant’s intense magnetic field triggered low-frequency radio emissions for a day or so. At Saturn, similar emissions lasted for a week, says Thomas H. Zurbuchen of the University of Michigan in Ann Arbor.

In the next 10 months, when the shock waves from last fall’s flares reach the heliopause—the boundary between the sun’s solar wind and interstellar space—they’ll push the surface of that bubble outward as much as 300 million miles, says Edward C. Stone of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Low-frequency radio emissions from that boundary, like those generated at Jupiter and Saturn, should alert scientists to the encounter, he notes.

The largest coronal mass ejection now hurtling toward the edge of the solar system thankfully missed Earth altogether. On Nov. 4, 2003, as Sunspot 486 was rotating out of view on the western edge of the sun, it launched a coronal mass ejection that overwhelmed space-based instruments viewing the event. For 12 minutes, their measurements of radiation in various wavelengths were off the charts. However, by analyzing data trends just before and after that gap, scientists estimate that the flare peaked at a record-setting strength: X-27.

In tonnage, the mass ejection on Nov. 4 was the biggest in at least 20 years, says Howard A. Garcia of the National Oceanic and Atmospheric Administration in Boulder, Colo. Material from the flare raced away from the sun at 8 million km/hr—the fastest speed ever recorded. The flare was also the hottest on record, a whopping 41 million °C. The charged particles that spewed into space missed Earth because of sunspot 486’s off-center location.

“We were lucky,” says Alan L. Kiplinger, also at NOAA in Boulder. In March 1989, a flare about half the size of the Nov. 4 flare knocked out electrical power across large portions of Quebec and damaged power transformers there, he notes.

Technology is vulnerable to massive solar flares because coronal mass ejections that strike Earth can cause rapid fluctuations in our planet’s magnetic field, which in turn can induce huge electrical currents in long metal objects. The phenomenon was first noted in the 1840s, when equipment connected to long-distance telegraph lines sometimes sprang to life even though they weren’t connected to batteries, says Frank Jansen of Ernst-Moritz-Arndt University in Greifswald, Germany.

Today, such currents can plague many components of the modern infrastructure, says Jansen. He and Risto Pirjola of the Finnish Meteorological Institute in Helsinki describe those vulnerabilities in the June 22 Eos.

Flare-induced electrical surges in power lines disrupted power grids in southern Sweden following the flare last Oct. 28. Although many long-distance communications cables are now made of fiber-optic material and therefore aren’t susceptible to currents induced by solar flares, the signal-amplifying equipment used on such networks receives its power via long metallic wires that could carry such currents, Jansen and Pirjola note.

Also vulnerable are the metal pipes in oil and gas pipelines, where coronal mass ejections can cause voltage differences between the pipes and the surrounding soil, which accelerate corrosion.

Accurately forecasting space weather could protect power lines and pipelines, electronic equipment on satellites and aircraft, and astronauts orbiting Earth or traveling to and from the moon or Mars.

For example, consider what happens when utility managers don’t have enough time to prepare for a major geomagnetic storm caused by solar flares. If power plants don’t disconnect from the utility grid, the storm can knock out dozens of high-capacity transformers. Some of the transformers are so large and complex that manufacturers can make only a few units per month. Therefore, it may take months, if not years, to repair all the damaged grids.

Weather forecast

The same spacecraft that provide data to solar researchers are being used to predict solar weather. Satellites have been sun gazing since the 1960s, says Woods. Early probes lasted just a few months and observed flares and other solar activity in just one wavelength of radiation. Instruments on more-recent spacecraft look at the sun in many wavelengths and take measurements on a daily basis. One room-size craft—the Solar and Hemispheric Observatory (SOHO), a joint mission of NASA and the European Space Agency—hovers 1.5 million km sunward of Earth and therefore can observe the sun without interruption.

SOHO instruments, for example, directly measure the speed and strength of clouds of charged particles headed for Earth, providing researchers with early warnings of approaching space storms. The Space Environment Center, operated in Boulder, Colo., by NOAA and the U.S. Air Force, was particularly busy last fall distributing warnings worldwide about the solar disturbances that had the potential to affect people, satellites, and infrastructure.

The swarms of charged particles spewing from the sun generally travel along the sun’s magnetic field lines. The sun’s rotation, which takes 27 days, makes these lines curve through space like water from a spinning sprinkler. The coronal mass ejections most likely to smack Earth originate from solar longitudes between 10° and 50° west of the sun’s center, says Woods. Flares typically lift straight off the solar surface, so scientists can roughly estimate where they’re headed, but accuracy plummets when, for instance, the shock waves from several flares interact with each other.

To better predict space weather, researchers have developed a technique to quickly characterize fresh coronal mass ejections. Using three or more satellite images of the flare’s cloud of charged particles taken through a polarizing filter, which can be rotated to different angles to block waves of light along different directions, scientists construct a three-dimensional image of the flare and estimate its speed and direction. Collecting the data and producing the image take just a few minutes, says Thomas G. Moran, a solar physicist at NASA’s Goddard Space Flight Center in Greenbelt, Md. He and his colleague Joseph M. Davila describe the technique in the July 2 Science.

Previous techniques for analyzing coronal mass ejections sometimes failed or were too slow, says John C. Raymond, an astrophysicist at Harvard University. Besides providing scientists with a better idea of when clouds of charged particles will arrive at Earth, the new 3-D views of solar flares could contribute to models of how solar flares behave, he notes.

Many other new visualization techniques could result from a European search for better ways to monitor, simulate, and predict space weather. By coincidence, that 17-nation, 4-year program was launched just as last fall’s barrage of superflares waned.

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