What were you doing last September? The charged particles that dance around Earth were busy. Unbeknown to most earthlings, a previously unseen ring of radiation encircled our planet for nearly the whole month – before being destroyed by a powerful interplanetary shock wave.
We already knew that two, persistent belts of charged particles, called the Van Allen radiation belts, encircle Earth. The discovery of a third, middle ring by NASA's twin Van Allen probes, launched in August 2012, suggests that these belts, which have puzzled scientists for over 50 years, are even stranger than we thought. Working out what caused the third ring to develop could help protect spacecraft from damaging doses of radiation.
Charged particles get trapped by Earth's magnetic field into two distinct regions, forming the belts. The inner belt, which extends from an altitude of 1600 to 12,900 kilometres, is fairly stable. But the outer belt, spanning altitudes ranging from 19,000 to 40,000 kilometres, can vary wildly. Over the course of minutes or hours, its electrons can be accelerated to close to the speed of light, and it can grow to 100 times its usual size.
No one is sure what causes these "acceleration events", although it seems to have something to do with solar activity interacting with the Earths' magnetic field.
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