Revenge of the Magnetar

Solar flares that scorch Earth's atmosphere are commonplace. But scientists have discovered a few each year that are not like the others: they come from stars thousands of light years away.

On August 24th, 1998, there was an explosion on the sun as powerful as a hundred million hydrogen bombs. Earth-orbiting satellites registered a surge of x-rays. Minutes later they were pelted by fast-moving solar protons. Our planet's magnetic field recoiled from the onslaught, and ham radio operators experienced a strong shortwave blackout.

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 August 24th event was powerful, yet typical.

A few days later - no surprise - another blast wave swept past Earth. Satellites registered a surge of x-rays and gamma-rays. Hams experienced another blackout. It seemed like another X-class solar flare. Except for one thing: this flare didn't come from the sun.

"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.

An artist's concept of a magnetar outburst. The red loops trace the star's intense magnetic field.

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."

Physicist Maxim Lyutikov of McGill University thinks the same thing happens on magnetars. "I imagine that the atmosphere of a magnetar is similar to the solar corona - filled with plasma and complicated magnetic fields," he says. "Reconnection on the sun is often caused by a plasma instability called the 'tearing mode.' Detailed calculations show that a similar instability may develop in the strongly magnetized plasma of a magnetar."

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.

When the blast wave from SGR 1900+14 arrived on August 27th, 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.

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.

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.

It happens more often than most people know. Since 1998, Earth has experienced "about 10 similar ionization events," says Umran Inan of Stanford University. "Five of them were caused by SGR 1900+14, and the rest from unknown sources."

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.

"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.

"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.

Image credit: Rob Duncan

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.

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."

"Each year we pinpoint dozens of magnetar outbursts this way," he says. Most are from already-known objects like SGR 1900+14, but sometimes a new magnetar reveals itself. (Note: the majority of the bursts detected by the IPN are faint; only the strongest few ionize Earth's atmosphere.)

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.