The Third Van Allen Belt
The Van Allen radiation belt is supposedly composed of two torus-shaped layers of energetic charged particles (plasma) around the planet Earth, held in place by its magnetic field. The belt extends from an altitude of about 1,000 to 60,000 kilometers above the surface, in which region radiation levels vary. On Aug. 30, NASA launched the Radiation Belt Storm Probes mission, since renamed the Van Allen Probes mission, to learn more about the belts, which are known to be hazardous to satellites and astronauts. Each probe carries a Relativistic Electron-Proton Telescope, or REPT, designed and built at CU-Boulder’s Laboratory for Atmospheric and Space Physics, known as LASP. When CU-Boulder scientists turned on the instruments, just a few days after launch, they were shocked by what they found: a third storage ring radiation belt.
It is generally understood that the inner and outer Van Allen belts result from different processes. The inner belt, consisting mainly of energetic protons, is the product of the decay of so-called albedo neutrons which are themselves the result of cosmic ray collisions in the upper atmosphere. The outer belt consists mainly of electrons. They are injected from the geomagnetic tail following geomagnetic storms, and are subsequently energized through wave-particle interactions. The new third belt has no theory yet.
"It was so odd looking, I thought there must be something wrong with the instrument," said LASP Director Dan Baker, REPT principal investigator and lead author of the study published online today in the journal Science. "But we saw things identically on each of the spacecraft. We had to come to the conclusion that this was real."
The data sent back to Earth from the REPT instruments during the month of September initially showed two Van Allen belts, as expected. But after a few days, the outer ring appeared to compress into an intense, tightly packed electron band and a third, less compact belt of electrons formed further out, creating a total of three rings. The middle storage ring persisted as the belt furthest away from Earth began to decay away in the third week of September, until, finally, a powerful interplanetary shock wave traveling from the sun virtually annihilated both the storage ring and the rest of the outer belt.
Scientists have known that the outer Van Allen belt can fluctuate wildly, at times swelling with charged particles before letting them slip away again, depending on space weather. In the months since the storage belt and the outer belt virtually disappeared, the Van Allen radiation zones have re-formed into the originally expected two-belt structure.
"We have no idea how often this sort of thing happens," Baker said. "This may occur fairly frequently but we didn’t have the tools to see it."
The fact that NASA’s new tools observed the events at all was somewhat serendipitous. When NASA launches a new spacecraft, instruments onboard are turned on, tested and calibrated in a prescribed order. CU-Boulder’s REPT instruments were originally scheduled to be turned on about a month after launch, when the third Van Allen radiation belt would have already dissipated. But Baker and his colleagues lobbied to jump the REPT instrument to the front of the instrument commissioning line.
"Had we not done so, we would have missed this," Baker said. "It’s good to be in the right place at the right time with the right instrument."
The two NASA probes, which are flying around Earth in an elliptical orbit, are able to send back observations for the first time from the heart of the two belts as each probe passes through. The information gathered by the twin, octagonal spacecraft will help researchers better understand how space weather affects near-Earth phenomena by interacting with, feeding and stripping away the Van Allen belts.
A better understanding of belt formation, including the number of belts, will help researchers refine their understanding of how and when solar storms can wreak havoc on Earth.
"We can offer these new observations to the theorists who model what’s going on in the belts," said Shri Kanekal, the deputy mission scientist for the Van Allen Probes at NASA’s Goddard Space Flight Center in Greenbelt, Md. and a co-author of the new study. "Nature presents us with this event — it’s there, it’s a fact, you can’t argue with it — and now we have to explain why it’s the case. Why did the third belt persist for four weeks? Why does it change? All of this information teaches us more about space."
For further information see Third Belt.
Van Allen Belt image via Wikipedia.