Neutron detectors at the South Pole begin flashing, and the scientist on watch gets on the horn to astronauts at the International Space Station. Shut down your equipment and take cover in the shielded capsule, he says: A solar storm is coming.
That scenario is a bit closer to reality, thanks to a team of researchers that has found a way to estimate the intensity and arrival time of charged particles spewed toward Earth by strong solar storms. Such space weather could irradiate astronauts and fry satellites, and until now there hasn't been a good way to forecast it.
Not all space storms are the same. Some of the charged particles spewed by solar flares travel exceptionally fast and thus are extremely energetic, while others travel more slowly, says John Bieber, a space physicist at the University of Delaware, Newark. The more energy a particle carries, the more damage it can do. Because the less-energetic slower particles typically far outnumber fast ones, however, they do most of the overall damage. That delay offers the opportunity for an early warning before the most intense part of the solar storm strikes, he says.
The big problem is that solar storms are hard to predict. Sun-watching satellites can help monitor solar flares, but they can't provide accurate estimates of when the radiation will arrive or how strong it will be. Now, reporting in Space Weather, Bieber and his colleagues suggest a better way: neutron sensors at the South Pole. For decades, these sensors have been used to estimate the rate at which cosmic rays and other charged particles strike Earth's atmosphere. The sensors actually detect the neutrons created in the upper atmosphere when high-speed particles slam into the nuclei of atoms of gases, knocking them apart and sending the neutrons groundward. Some of those speeding particles are protons in solar flares.
The researchers analyzed data gathered by the Antarctic neutron sensors during 12 particularly strong solar storms between 1989 and 2005, and then compared the readings with data gathered by radiation sensors on board an Earth-orbiting satellite. They found that protons with energies between 165 million and 500 million electron volts (corresponding to speeds of between 53% and 76% the speed of light) arrived at the sensors, on average, about 95 minutes after the flares' first protons reached Earth. Slower-moving protons with energies between 40 million and 80 million electron volts (traveling from between about 29% and 39% the speed of light, respectively) arrived at Earth about 71 minutes later.
By comparing the numbers of neutrons detected by two different South Pole sensors—each tuned to detect neutrons of a different energy—Bieber and colleagues found they could estimate how many protons of various energies were striking the upper atmosphere. That number, in turn, enabled them to estimate the maximum amount of radiation damage that might be expected from the flare. If the expected damage exceeds a certain level, for example, scientists could warn astronauts to take cover or suggest that engineers temporarily shut down satellites.
"This is a very interesting and very intriguing piece of work," says Louis Lanzerotti, a space physicist at the New Jersey Institute of Technology in Newark. The new technique provides "one more arrow in the quiver" of scientists looking to predict space weather, he notes. "This lets scientists estimate reasonably well what peak [radiation] intensity will be."
If sufficiently accurate lightweight proton sensors can be developed, then the technique could even be used to predict the onset and intensity of solar flares that might strike crewed interplanetary craft, says Lanzerotti. The detectors now deployed in Antarctica are much too big to be used on spacecraft, he notes. Some sort of sensors would be crucial, however. That's because such a craft—which might carry minimal shielding to save on weight—would venture well outside Earth's protective magnetic field, where astronauts would be at the mercy of intense solar flares.