THEMIS satellite tracks mass electron escape

When scientists discovered two great swaths of radiation encircling Earth in the 1950s, it spawned exaggerated fears about “killer electrons” and space radiation effects.

The Cold War-era fears were soon allayed, as the radiation doesn’t reach Earth, though it can affect satellites and humans moving through the belts. 

 Nevertheless, numerous mysteries about the zone – dubbed the Van Allen Radiation belts – remain to this day. Filled with electrons and energetic charged particles, the radiation belts swell and shrink in response to incoming solar energy, but scientists are not quite sure how.

Indeed, what appears to be the same type of incoming energy has caused entirely different responses on different occasions, with increased particles in some cases and particle loss in others.

Unsurprisingly, there is no shortage of theories about what causes the belts to swell or shrink – with little hard evidence to distinguish between them.

To be sure, it was quite difficult to determine if, when the belts shrink, particles escape up and out into interplanetary space or down toward Earth.

However, a new study using multiple spacecraft simultaneously has managed to successfully track the particles and determine the escape direction for at least one event: up.

“For a long time, it was thought particles would precipitate downward out of the belts,” explained Drew Turner, a scientist at the University of California, Los Angeles. “But more recently, researchers theorized that maybe particles could sweep outward. Our results for this event are clear: we saw no increase in downward precipitation.”

While it may sound like a simple detail, such knowledge is hardly esoteric.  Yes, the study of particle losses in the belts has so far provided more mystery and potential theories than concrete information. But understanding the radiation belts – and how they change as particles and energy enters or exits – is a crucial part of protecting satellites that fly through the region.

The Van Allen belts fit into a larger system that stretches from the sun to Earth. The sun emits a constant stream of solar wind, not to mention the occasional larger bursts – such as explosions from the sun’s atmosphere called coronal mass ejections (CMEs) or shock fronts caused by fast solar winds overtaking slower winds known as corotating interaction regions (CIRs).

When these bursts of energy move toward Earth, they can affect Earth’s own magnetic environment, known as the magnetosphere, while creating a geomagnetic storm. Sometimes these storms can cause a sudden drop in the radiation belt particles, seemingly emptying in only a few hours. This “drop out” can last for days. What causes the drop out, why it lasts so long, and just how the particles even leave remain unanswered questions.

Solving such a mystery requires numerous spacecraft measuring changes at several points in space to determine whether an event in one place affects an event elsewhere. 

The Radiation Belt Storm Probes (RBSP), scheduled to launch in August 2012, are specifically geared for such observations, but in the meantime, a team of scientists have combined two disparate sets of a spacecraft to generate an early multipoint view of the radiation belts during an event when the belts experienced a sudden loss of particles.

“We are entering an era where multi-spacecraft are key,” said Vassilis Angelopoulos, a space scientist at UCLA. 

”Being able to unite a fleet of available resources into one study is becoming more of a necessity to turn a corner in our understanding of Earth’s environment.”

By harnessing the observation abilities of the three THEMIS, two GOES, and six POES spacecraft, researchers could observe events in the Van Allen radiation belts from numerous viewpoints simultaneously.

In this case, the team observed a small geomagnetic storm on January 6, 2011 using the three NASA THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft, two GOES (Geostationary Operational Environment Satellite), and six POES (Polar Operational Environmental Satellite).

The THEMIS and GOES spacecraft orbit around Earth’s equatorial region, while the POES spacecraft orbit at lower altitude near the poles and travel through the radiation belts several times per day. All are equipped to study the energetic particles in the region. The observations provided an unprecedented view of a geomagnetic storm from numerous viewpoints simultaneously – and the team determined (unequivocally) that particles escaped the radiation belts by streaming out into space, not by raining down toward Earth.

During this storm, electrons moving near the speed of light dropped out for over six hours. In that time period POES saw no increase in electrons escaping downward from the belts. On the other hand, the spacecraft did monitor a low-density patch of the belt that first appeared at the outer edges of the belts and then moved inward. 

This sequence is consistent with the notion that particles were streaming outward, just as the low density region of cars leaving from the front of a traffic jam moves backward over time as more and more cars are able to move forward and escape.

“This was a very simple storm… It’s not an extreme case, so we think it’s probably pretty typical of what happens in general and ongoing results from concurrent statistical studies support this,” added Turner.

And if electrons typically escape the radiation belts by streaming outward, it does seem quite likely that some kind of waves abet their outward motion – enabling them to reach the outer escape boundary.