A pair of well-known radiation belts surrounding the Earth are sometimes a trio, a new study has found.

Scientists have known since the 1950s about a couple of donut-shaped rings of high-energy charged particles surrounding the Earth, known as the Van Allen belts. 

However, several months ago, they were astonished to witness that what they thought was relatively stable couple giving way to a temporary ménage à trois.

"It was so odd looking, I thought there must be something wrong with the instrument," said Dan Baker, who led the study published Thursday in Science Express.

The third ring was detected in September by a pair of NASA spacecraft called the Radiation Belt Storm Probes, launched last Aug. 30. Four weeks later a powerful shock wave from the sun "virtually annihilated" the outer two belts. Since then, the radiation has reformed into two belts.

"We have no idea how often this sort of thing happens," Baker said in a statement. "This may occur fairly frequently but we didn't have the tools to see it."

The surprising event, he added in a podcast interview with the journal Science, is "really telling us that what we thought we understood about how the Van Allen belts form, what it is that leads to the expected structure, really needs to be more deeply understood."

Space weather system

The Van Allen belts are part of a "space weather" system, driven by the interaction of charged particles blasting from the sun with the Earth's magnetic field, that can disrupt satellites, power grids and GPS. It could also potentially affect spacecraft passing through their region of space. Because of that, the Radiation Belt Storm Probes were sent up to study them in more detail and help improve space weather forecasts.

The space probes first detected two belts, as expected, when they were first turned on two days after launch.

Shortly after that, however, the outer part of the Van Allen belt was ripped away, likely by a blast of solar wind, leaving behind a ring of very high energy particles that seemed curiously "immune" to the solar wind, said Baker, director of the Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder.

On one side of it, the inner-most ring remained stable, and on the other side, an outer belt of electrons swelled and shrank repeatedly over the next few weeks. The inner-most ring remained stable during that time.

Four weeks later, a solar shock wave destroyed the two outer rings. Eventually, the familiar outer ring reformed in their place.

However, Baker said, there has been no recurrence of the third belt in the past five months.

Mona Kessel, a Van Allen Probes program scientist with NASA, said at the news conference that the researchers "don't completely understand the phenomena we're seeing."

"We're trying to piece this all together right now," she added.

The researchers said they were lucky to catch the formation of the third ring, because it took place days after the launch of the space probes, which were originally not scheduled to turn on until a month after launch.

The research team convinced NASA to turn them on early so their data would overlap with a satellite measuring the Van Allen belts that was soon expected to fail and stop collecting data for good.