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‘Dropout’ Electrons Get Pushed out of Van Allen Belt

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Judging by the many flares erupting from the sun at the moment, it is well on track to reach its next peak in activity early next year. As this peak approaches, we can expect many more huge bursts of energy that erupt from the sun and send lots of energetic particles, and sometimes magnetic fields, our way. These in turn will lead to more of the fantastic light displays, which you might have seen (or at least heard about) lately, creeping down from the North Pole towards the equator.

Aurora as seen from the International Space Station as it crossed over the southern Indian Ocean on September 17, 2011. Credit: NASA

These light shows are the visible sign that a geomagnetic storm is raging overhead. But there’s another phenomenon that happens alongside the northern lights that you won’t have noticed at all. Surrounding our planet, way up above the atmosphere, is a doughnut shaped ring of charged particles held in place by Earth’s magnetic field. In fact, there are two of them. They’re called the inner and outer Van Allen belts.

The Van Allen belts were found in 1958 and were the first major scientific discovery of the space age. During geomagnetic storms, electrons in the Van Allen belts have been known to vanish – only to return a few hours later. This strange phenomenon was first spotted in the 1960s, and has puzzled physicists ever since.

Surely, they thought, at the height of a geomagnetic storm in which many energetic particles from the sun hit Earth’s atmosphere, there would be more electrons in the Van Allen belts, not less?

A new paper published online at Nature Physics seems to have the answer: the electrons are swept away by particles from the sun.

Drew Turner, from the University of California, Los Angeles, and his colleagues (also at UCLA) used data from three different spacecraft for this research: THEMIS, GOES and POES spacecraft.

THEMIS stands for Time History of Events and Macroscale Interactions during Substorms (and is also the name of a Greek goddess, something that I’m guessing wasn’t entirely coincidental) and was a NASA mission that investigated what causes auroras to go from moving slowly across the sky to dancing rapidly within minutes. It consisted of five identical satellites that lined up over North America once every four days to witness auroras.

The original THEMIS mission ended in 2009. Now two of the satellites have been sent off to orbit the moon and only three remain close to Earth. Those three were teamed up with two GOES (Geostationary Operational Environment Satellite) and six POES (Polar Operational Environmental Satellite) spacecraft, both run by the National Oceanic and Atmospheric Administration (NOAA), with the POES also jointly run by European Organization for the Exploitation of Meteorological Satellites, to witness a small geomagnetic storm on 6th January last year.

THEMIS and GOES both orbit Earth near the equator, with POES taking on the polar regions at a lower altitude, and pass through the Van Allen belts several times a day.

A solar flare accompanied by a coronal mass ejection (CME) erupted from the sun on January 23rd 2012. Credit: NASA/SDO

There are several solar phenomena that can cause geomagnetic storms. Coronal mass ejections, or CMEs, are one that we hear about a lot, possibly because of the amazing images that NASA mission Solar Dynamics Observatory (SDO) has been talking of them lately. But what caused the storm on 6th January 2011 was something called a co-rotating interaction region (CIR). CIRs are created because there are two different streams of particles coming from the sun: fast and slow. The fast stream taking over the slow one causes turbulence at the boundary between the two and creates a CIR.

During the 6th January storm, several satellites in the outer Van Allen belt noticed a ‘dropout’ of electrons – they appeared to go missing, but reappeared again around six hours later.

Turner and his colleagues looked at the data from the THEMIS, GOES and POES satellites and found that, while some electrons at lower energies did appear to have been replaced by electrons coming in with the CIR that caused the storm, ones with higher energies were pushed out from the Van Allen belts and away from Earth.

Some physicists thought that the electrons might have fallen downwards out of the belts during geomagnetic storms, but this new research is clear evidence that they seem to be pushed up and away instead. It might seem like a small distinction, but information on how the Van Allen belts work is important if we are to properly protect satellites flying around in them.

An upcoming NASA mission, Radiation Belt Storm Probes (RBSP) should be able to help give a fuller answer to what happens to the Van Allen belts during these storms. It’s due to launch this August – just in time to witness the many solar storms that will come our way in the run up to the next solar maximum.

Turner, D., Shprits, Y., Hartinger, M., & Angelopoulos, V. (2012). Explaining sudden losses of outer radiation belt electrons during geomagnetic storms Nature Physics DOI: 10.1038/nphys2185

Kelly Oakes About the Author: Kelly Oakes has a master's in science communication and a physics degree, both from Imperial College London. Now she spends her days writing about science. Follow on Twitter @kahoakes.

The views expressed are those of the author and are not necessarily those of Scientific American.

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