Two giant swaths of radiation, known as the Van Allen Belts, surrounding Earth were discovered in 1958. In 2012, observations from the Van Allen Probes showed that a third belt can sometimes appear. The radiation is shown here in yellow, with green representing the spaces between the belts.

(NASA/Van Allen Probes/Goddard Space Flight Center)

A pair of NASA probes has discovered a previously unknown ring of radiation blanketing the Earth, upending a long-standing scientific theory about how charged particles coalesce around the planet, scientists reported Thursday.

Just four days after the twin Radiation Belt Storm Probes were launched in August, NASA scientists looked on in amazement as instruments revealed a third belt of high-energy particles between the planet's inner and outer radiation belts, known as the Van Allen belts.

The new belt held steady for four weeks before a solar flare blasted it to pieces. It has not been seen since.

"The sun giveth, and the sun taketh away," said Nicola Fox, a deputy scientist on the mission based at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

The twin probes were the first built by NASA to study the radiation belts, which were discovered in 1958 by astrophysicist James Van Allen. The inner belt is between 6,000 and 8,000 miles out and the outer belt forms at about 15,000 miles.

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Scientists have found charged particles hovering between the inner and outer belts before, in 1991 and 1992. But they were never this stable or remained as long as a month.

"There are a host of different processes that occur in radiation belts," Fox said. "What we do not yet know is why sometimes one is more dominant than another. It is like knowing all the ingredients but not the proportions."

The so-called Van Allen probes gave scientists their best glimpse yet at those proportions. Equipped with identical particle, plasma, magnetic field and plasma wave sensors, the probes measured charged particles in and around the planet's magnetosphere between September and October.

Most NASA spacecraft take several months to power up, but the Van Allen probes began taking measurements early -- a last-minute decision by scientists who wanted to get data that would overlap with readings from an older satellite set to go out of service in November. That timing revealed an amazing show in progress.

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A third radiation belt had begun to form after a solar eruption blasted some -- but not all -- of the outer belt's particles away. The remaining particles settled closer to Earth, between 11,900 and 13,900 miles above the planet, according to a report published online by the journal Science.

Study leader Dan Baker of the University of Colorado's Laboratory for Atmospheric and Space Physics said the solar eruption was a "Goldilocks phenomenon." The force of the eruption was not too strong and not too weak, but just right to trigger the formation of a third belt.

The Van Allen probes also showed that different belts of particles had different reactions to the same event, a discovery that came as a surprise to scientists. A second solar event in early October delivered particles that shored up the outer belt but left the inner belt undisturbed.

Saturn, Uranus and Neptune all have radiation belts. "The same thing happening here is happening everywhere," Fox said.

While they pore over their data, the researchers wait for another Goldilocks moment to see whether a third belt will appear again.

"The frequency will depend on what the sun throws at us," Fox said. "The more active the conditions, the more we are likely to see it."

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The Sun's Innermost Atmosphere

This picture is made of images taken far away as well as close to the sun, which allows scientists to compare what happens at both locations. Here you see a coronal mass ejection moving away from the sun in the upper right corner. (Image: ESA/NASA)