A previously unseen ring of radiation formed within the Earth's Van Allen belt in September of 2012 and then vanished a month later. That is the finding of a team of researchers in the US, which analysed the first data available from the twin spacecraft of NASA's Van Allen Probes mission. The anomalous ring – made up of high-energy electrons – stayed largely unchanged, until it was disrupted and "virtually annihilated" by a powerful interplanetary shock wave. The new findings show how we need a better understanding of the underlying mechanisms of the Van Allen belts.

Ring of fire

Discovered by US physicist James van Allen more than 50 years ago, the Van Allen radiation belts are two concentric, "doughnut-shaped" rings that encircle our planet. They are held in place by the Earth's magnetic field and are filled with high-energy particles. The outer ring is mainly made up of MeV electrons that vary in intensity over a timescale of hours to days, depending on the solar wind. The inner ring consists of a mix of high-energy electrons and extremely energetic protons.

The belts are confined within the Earth's magnetosphere and extend from an altitude of about 1000 to 60,000 km above the Earth's surface. They tend to swell and shrink over time as they are a driven by solar wind and cosmic rays. The high amounts of radiation within the belts make them a threat to satellites in geostationary orbit, which must carry sufficient shielding if their orbit lies within the belts.

Probing plasmas

To better study the Van Allen belts, NASA launched the Van Allen Probes (formerly known as the Radiation Belt Storm Probes) mission on 30 August 2012 to investigate both rings. It comprises two spacecraft that are kitted out with energetic particle, plasma and magnetic-field instruments, plus plasma-wave sensors to investigate both rings. The mission's aims included understanding how particles are accelerated, transported and lost from the belts as well as determining how extreme space weather affects the region.

Unexpected ring

When Dan Baker from the University of Colorado and colleagues analysed the first data from the mission, however, they found the completely unexpected and surprising new "electro storage ring" nestled between the two known rings, which appeared after 2 September last year. According to a paper published in the journal Science, the "distinctive ring of highly relativistic electrons persisted, changing only gradually" until it abruptly disappeared on 1 October. While the inner ring of the Van Allen belt and the new ring showed very little change over the four weeks, the more distant part of the outer Van Allen belt seemed to be changing significantly throughout the same period. In the end, at the death of the new ring, almost the entire outer-zone electron population was diminished, according to sensors on board the probes.

Baker explains that the researchers have no idea whether the formation of this third ring is cyclic or occurs only very rarely. "This is one of the things we really want to study carefully with the Van Allen Probes over long time periods. We have not seen a recurrence in recent months," he says. The interplanetary shock wave, which is thought to have demolished the new ring, is a disturbance moving ahead of a "coronal mass ejection" event from the Sun's surface, which impacted the Earth's magnetosphere. "It produced a sudden increase in solar wind speed, density and magnetic-field strength," explains Baker. "We will be fascinated to see if this kind of event occurs again. We have not seen it in the last five months."

According to the researchers, previous studies have shown that the outer-zone electron populations are much more susceptible to space weather and would show a direct response to changes in the solar wind, the interplanetary magnetic field and geomagnetic activity. Indeed, they claim that the formation of the new "storage ring" itself could have its origins in another loss of outer-belt electrons that occurred on 3 September that was caused by a previous shock wave detected as a sharp increase in solar wind speed and an abrupt change in the interplanetary magnetic field.

According to Baker, this surprising finding "shows that some of our most 'treasured' beliefs about the radiation belt structure and dynamics have to be reconsidered at a fundamental level. We will have to see what this means for future spacecraft operations, etc."