South Pole Neutron Detectors Offer Predictions of Damaging Solar Radiation

By Peter Rejcek, Antarctic Sun Editor — One of the most frigid places on the planet appears to be an ideal location to help protect humans living and working in the cold of outer space against radiation bursts from the sun.

Scientists recently reported in the journal Space Weather that neutron detectors at the U.S. Antarctic Program’s South Pole Station appear to offer a reasonably reliable early-warning system to detect damaging radiation associated with high-energy particles that sometimes accompany what’s called coronal mass ejections (CMEs), a massive blast of low-energy plasma from the sun.

The high-energy protons and other subatomic particles ejected during such events blast through space near the speed of light. The particles that hit the Earth, called primary cosmic rays, are destroyed when they hit the atmosphere, producing a cascade of secondary subatomic particles.

Neutron detectors at the South Pole are particularly sensitive to the highest and rarest of the high-energy particles, which arrive before a slower but more intense “wave” of high-energy particles capable of delivering hazardous doses of radiation to humans in space. The researchers used the measurements from a pair of ground-based detectors at the South Pole to predict the peak intensity at different particle energies.

“What we’re predicting is a particle storm, which is a high-energy, high-intensity burst of particle radiation,” explained Paul Evenson , a co-author on the study with the Bartol Research Institute at the University of Delaware . “By using our comparatively simple technique — measuring the energy spectrum of these particles — we actually can make a prediction that’s worth something for the lower-energy and more damaging particles.”

The solar storm that produces such a blast of high-energy particles usually arrives about two days later, with the potential to disrupt satellites and the planet’s energy grid. Such an event crashed into Earth’s magnetic field in mid-July, doing no damage but producing some of the most intense auroral displays seen in years, including at the South Pole. These sorts of sun-generated storms are outside the scope of the South Pole early-warning system.

The team validated its method against data collected from satellites that are part of the Geostationary Operational Environmental Satellite System (GOES) . However, the instruments aboard the satellites aren’t capable of detecting particles much higher in energy than those associated with the peak radiation dose, according to Evenson.

“The instruments on the spacecraft are just too small to detect the faster, high-energy particles. They are set up to detect the particles that are most damaging,” he explained.

While the method presented in the Space Weather paper — “South Pole neutron monitor forecasting of solar proton radiation intensity” — offers an average warning time of 166 minutes, Evenson warned that it’s not foolproof. The Earth has to be in the proper orientation to observe the highest-energy particles, he said.

“It’s very effective when it works, but it’s possible an event could slip through without much warning,” he said.

The South Pole neutron observatory is one of a dozen that make up an international neutron-monitoring network called Spaceship Earth , which also includes a neutron monitor at McMurdo Station . Neutron detectors were first installed at the South Pole in 1964.

Only the South Pole Station neutron monitors are ideally configured and located to make the high-energy measurements needed to predict the peak intensity of particle storms, according to Evenson.

“South Pole is really very unique. It has both the very high altitude and the very low geomagnetic cutoff, so that particles that reach the surface at Pole carry much more information about the shape of the spectrum at the top of the atmosphere than any other station on Earth,” he said.
Instruments embedded in tub of ice.

A newer experiment at the South Pole Station called IceTop , which is part of the larger neutrino observatory called IceCube , should help researchers learn more about the high-energy solar particle spectrum. IceTop uses the same photomultiplier modules as those buried within the ice sheet to detect neutrinos indirectly from events such as supernova explosions, but at the surface of the ice sheet and targeting secondary particles.

“By adding the detectors of IceTop, we can actually get a more accurate representation of the spectrum,” Evenson said. “In the future, we’ll be able to get more details of the spectrum.”

He added that currently the data processing time for the IceTop data would preclude its use for the early-warning system. The heavy and bulky ground-based neutron detectors also make it unlikely that the technology would be used aboard spacecraft.

“That’s why we’re still in business in the space age, because it’s impossibly expensive to fly tons of instrumentation on a spacecraft,” Evenson said. “The bottom line is that there’s very little danger to people on the surface of the Earth, and it’s an annoyance to aircraft. It’s very critical to understand these things for spacecraft, especially for something like a lunar base.”

NSF-funded research in this story: Paul Evenson, John Bieber and John Clem, University of Delaware, Award No. 0838839 .