X-rays offer early warning for solar flares

Outpourings of X-rays can reveal when the sun is prepping a big outburst, new research suggests.

Researchers analyzed the X-rays shot out by the sun before and during roughly 50,000 solar explosions, called flares. The researchers discovered that the flood of X-rays preceding a solar flare can be used to determine the flare’s ultimate intensity. The most intense flares fling particles toward Earth at nearly the speed of light; such particles can disable satellites, impair radio communications and pose a health hazard to astronauts. By monitoring solar X-rays, astronomers can identify an upcoming X-class flare, the most intense designation, about 6.7 minutes before the flare reaches its peak, the researchers report in a paper to be published in Space Weather.

“We can’t forecast exactly when flares are going to occur yet, so we’re really excited to even have a few minutes lead time on how powerful a flare will be,” says lead author Lisa Winter, an astrophysicist at Atmospheric and Environmental Research in Superior, Colo.

This new way to foresee a flare’s ferocity is worth pursuing, says space weather scientist Rodney Viereck, director of the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center Testbed in Boulder, Colo. While flare forecasters already use X-rays to predict flare intensity, they do so without an objective understanding of how the two relate. “This could enhance our current procedures by providing a quantitative element,” he says. “We just use a rule of thumb, and this could provide us with more precision.”

The sun’s outer layers are jumbles of hot plasma and strong magnetic fields. As the magnetic fields twist and coil, energy becomes pent up just like in a compressed spring. When two magnetic fields cross and reconnect, that stored energy can spark a solar eruption with the explosive force of millions of 100-megaton hydrogen bombs. Depending on the flare’s location on the sun, destructive protons and electrons accelerated by the blast can reach Earth just 30 minutes after the flare starts.

Advanced warning that an incoming solar flare will be particularly violent can provide more time for astronauts to move to a more shielded area of the International Space Station and for satellite operators to temporarily turn off crucial systems to avoid damage. Flare forecasters, however, currently can’t predict exactly how big a future flare will be or when the flare will start. They can only provide forecasts of how likely the sun is to shoot off a certain sized flare within the next few days.

When a flare kicks off, it shines brightly in the X-ray spectrum. Winter and coauthor Karatholuvu Balasubramaniam, a solar physicist at the Air Force Research Laboratory in Albuquerque, gathered nearly four decades of solar flare observations recorded by Earth-orbiting satellites. The pair fed the data into a computer program that hunted for relationships between X-ray emissions and flare intensities.

Winter and Balasubramaniam discovered that the final intensity of a solar flare depends on how fast the erupting solar region lit up with X-rays and how intensely the region radiated X-rays before the flare started. Both of those measurements relate to how much bottled-up energy is available to power the flare, Winter says.

Applying their findings to the most recent bout of solar flares that began in January 2008, the researchers successfully predicted the intensities for all X-class flares and 76 to 81 percent of weaker flare designations. The researchers now plan to implement their forecasting technique in real time and predict flare intensities even further in advance.

Although the ultimate goal of space weather forecasting is to provide advance warnings of exactly when big flares will strike, gauging a flare’s intensity faster is important, says solar physicist David Hathaway of the NASA Ames Research Center in Moffett Field, Calif. “The flare tips its hand early on and lets you know how big it’s going to be,” he says. “It’s still only a few-minute warning, but a few minutes can go a long way.”