Massive explosions on the sun unleash¹（释放，发动） radiation that could kill astronauts in space. Now, researchers from the U.S. and South Korea have developed a warning system capable of forecasting the radiation from these violent solar storms nearly three hours (166 minutes) in advance, giving astronauts, as well as air crews flying over Earth's polar regions, time to take protective action.

Physicists² from the University of Delaware and from Chungnam National University and Hanyang University developed the system and report on it in Space Weather: The International Journal of Research and Applications, published by the American Geophysical Union. 

Prof. John Bieber at UD's Bartol Research Institute, based in the Department of Physics and Astronomy, directed the scientific project. The article's lead author is Su Yeon Oh, a postdoctoral researcher from Chungnam National University, who worked with Bieber on the project at UD.

"Traveling nearly at the speed of light, it takes just 10 minutes for the first particles ejected from a solar storm to reach Earth," Bieber says. These sun storms can cover thousands of miles on the sun, like a wave of exploding hydrogen bombs.

The researchers used data collected by two neutron³ monitors installed years ago at the South Pole by UD -- one inside and one outside the Amundsen-Scott South Pole Station -- to determine the intensity⁴ of the high-energy, fast-moving particles that arrive to Earth first from solar storms. These particles can carry energies over 500 megaelectron volts⁵ (MeV) -- that's over 500 million electron volts.

By examining the properties of these first-arriving particles, the scientists can make useful predictions about the slower-moving, yet more dangerous particles to follow.

"These slower-moving particles are more dangerous because there are so many more of them. That's where the danger lies," Bieber explains.

When these firstcomer, positively⁶ charged particles, or protons, hit an air molecule⁷ in Earth's atmosphere, they blast apart into tiny pieces, which, in turn, slam into other air molecules⁸, and so on. Neutrons⁹, neutrally charged particles, are produced as part of this cascading¹⁰（级联） event.

From measurements of the neutrons produced in past solar events taken by UD's neutron monitors at South Pole, the scientists calculated the energy of the first-arriving protons and, from that, estimated the intensity of the later-arriving, more dangerous particles.

The authors compared their predictions for 12 solar events against observations made by geosynchronous satellites（地球同步人造卫星） , achieving good agreement for protons with energies higher than 40 to 80 megaelectron (million) volts.

Depending on the protons' energy, the system provides a warning time up to 166 minutes. That would give astronauts on deep space flights time to seek out an armored area in their spacecraft, Bieber says, and pilots flying in Earth's polar regions, where the planet's protective magnetic field is weaker, time to reduce their altitude.