Solar Flares on Steroids
None of these things made headlines. The explosion was an "X-class" solar flare, and during years around solar maximum, such as 1998, such flares are commonplace. They happen every few days or weeks. The Aug. 24th event was powerful, yet typical.
Right: A solar flare blasts hot gas away from the limb of the Sun. [movie]
It came from outer space.
"The source of the blast was SGR 1900+14, a neutron star about 45,000 light years away," says NASA astronomer Pete Woods. "It was the strongest burst of cosmic x-rays and gamma rays we've ever recorded."
SGR 1900+14 is a special kind of neutron star called a magnetar. "Magnetars have the strongest magnetic fields in the universe: a million billion (1015) gauss," he says. For comparison, the magnetic field of the sun is less than 10 gauss in most places, and about 1000 gauss near sunspots.
Magnetism and solar flares go together. On the sun, flares happen when magnetic fields above sunspots become twisted and stretched. They're like rubber bands pulled too tightly. Snap! They recoil with explosive results. Physicists call this "magnetic reconnection."
Left: An artist's concept of a magnetar outburst. The red loops trace the star's intense magnetic field.
Reconnection events on the sun emit as much as 1032 ergs of energy. Flares from magnetars are about a million million times stronger, ~1044 ergs, befitting their more intense magnetic fields.
"They're solar flares on steroids," quips Woods.
When the blast wave from SGR 1900+14 arrived on August 27, 1998, it hit the night side of our planet--something flares from the sun never do--and scorched Earth's upper atmosphere. The radiation broke apart atoms and molecules into charged ions. Ions interact with radio signals, either absorbing or reflecting them, so radio listeners knew something had happened.
For instance, a registered nurse in Seattle was driving home from work at 2:00 a.m. listening to a local program on her car radio. The station faded--a blackout--and was moments later replaced by country music from Omaha, Nebraska. On the US east coast, where dawn was breaking at the time, hams chatting locally suddenly picked up voice transmissions from distant parts of Canada. Strange.
These propagation effects, so much like those experienced during ordinary solar flares, quickly subsided. No harm was done. Nevertheless, the event made a deep impression on astronomers. From halfway across the galaxy, SGR 1900+14 had "touched" our planet.
Inan leads the Very Low Frequency (VLF) Research Group at Stanford University. He and his colleagues operate a network of low-frequency radio stations in North America and Antarctica. When Earth gets hit by ionizing radiation, the network records telltale changes in radio propagation. "We saw the blast from SGR 1900+14 in 1998--it was very clear," he says.
Right: Stanford University's network of VLF receivers registered a fadeout of 21.4 kHz signals on August 27, 1998, when the magnetar burst reached Earth. The shaded area denotes the part of our planet illuminated by the burst. [more]
"Many things can change the ionization of Earth's atmosphere," adds Inan. "Lightning can do it. So can sudden bursts of auroras at high latitudes." But these things cause local ionization. Solar flares, on the other hand, have global effects, ionizing the top of Earth's entire dayside atmosphere. Flares from magnetars can ionize the nightside, too. These signatures--nightside vs. dayside, global vs. local--help Inan identify the source of the ionization.
His "unknown sources" are probably magnetars not yet discovered by astronomers.
"The best way to pinpoint a magnetar," says Woods, "is to catch it when it's bursting--but that's not easy because the bursts are unpredictable and brief. Oftentimes they come and go in less than one-tenth of a second." To date only ten of these stars are known. Many more await discovery, he believes.
Above: The distribution of magnetar candidates along the Milky Way. The red dot below the plane of the galaxy is located in the Large Magellanic cloud. Image credit: Rob Duncan.
Finding them is the job of the Interplanetary Network (IPN)--a flotilla of spacecraft scattered around the solar system. Members include Ulysses, 2001 Mars Odyssey, RHESSI and others. None of these missions are dedicated to magnetar research, but each one carries a gamma-ray or x-ray detector--usually for some unrelated purpose. The detector on 2001 Mars Odyssey, for instance, is used to hunt for subsurface ice on Mars. Catching magnetars is a bonus.
Here's how it works: When a wave of radiation sweeps through the solar system, it hits the different spacecraft at slightly different times. Astronomers can figure out where the burst came from by comparing the arrival times. "It's simple triangulation," says Kevin Hurley of UC Berkeley who leads the effort. "The Ulysses spacecraft is particularly important because of its long looping orbit around the Sun. Ulysses' great distance from the other spacecraft makes the triangulation precise."
As soon as the Interplanetary Network locates a burster, the coordinates are emailed to astronomers around the world so they can observe the magnetar using their own telescopes on the ground. NASA missions such as the Chandra X-ray observatory and the Rossi X-ray Timing Explorer sometimes join the effort, too. Magnetar candidates attract the attention of dozens of observatories.
That's understandable. "From a physics point of view," notes Woods, "the energy reservoir in the magnetosphere and crusts of magnetars is 10 to 100 times bigger than the energy released during the August 27, 1998, outburst. So there is the potential for much higher-energy events. It's a good idea to keep an eye on these things."
And an ear. The next time you're driving home in the middle of the night and, unexpectedly, a country tune blares out of your radio, you might wonder ... did a magnetar do that? The cosmos is full of the strangest surprises.
The Biggest Explosions in the Solar System -- (Science@NASA) They're not as powerful as magnetar bursts, but they're still impressive: solar flares.
Magnetars, Soft Gamma Repeaters & Very Strong Magnetic Fields -- (U. Texas) The definitive web resource for information about magnetars, written by astrophysicist Rob Duncan who along with colleague Chris Thompson invented the magnetar model.
The Interplanetary Timing Network -- (UC Berkeley) a flotilla of spacecraft strung across the solar system help pinpoint magnetar bursts. because of it's long looping orbit around the sun, Ulysses is one of the most important members.
Solar flare propagation effects -- (Windows on the Universe) Magnetar bursts can do many e things described at this web site.
Very Low Frequency Research Group -- (Stanford University) Umran Inan's group has detect 10 cosmic ionization events since 1998.
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