Tracking the origins of speedy space particles

NASA - This is an artist's rendition of the five THEMIS space spacecraft traveling through the magnetic field lines around Earth.

NASA's Time History of Events and Macroscale Interaction during Substorms THEMIS spacecraft combined with computer models have helped track the origin of the energetic particles in Earth's magnetic atmosphere that appear during a kind of space weather called a substorm. Understanding the source of such particles and how they are shuttled through Earth's atmosphere is crucial to better understanding the Sun's complex space weather system and thus protect satellites or even humans in space.

The results show that these speedy electrons gain extra energy from changing magnetic fields far from the origin of the substorm that causes them.

THEMIS, which consists of five orbiting satellites, helped provide these insights when three of the spacecraft traveled through a large substorm on February 15, 2008. This allowed scientists to track changes in particle energy over a large distance. The observations were consistent with numerical models showing an increase in energy due to changing magnetic fields, a process known as betatron acceleration."The origin of fast electrons in substorms has been a puzzle," says Maha Ashour-Abdalla, the lead author of a Nature Physics paper that appeared online on January 30, 2011 on the subject and a physicist at the University of California, Los Angeles. "It hasn't been clear until now if they got their burst of speed in the middle of the storm, or from some place further away.

... "When the team looked at their models they saw that electrons near the reconnection sites didn't gain much energy. But as they looked closer to Earth, where the THEMIS satellites were located, their model showed particles that had some ten times as much energy – just as THEMIS had in fact observed.This is consistent with the betatron acceleration model. The electrons gain a small amount of energy from the reconnection and then travel toward Earth, crossing many changing magnetic field lines. These fields produce betatronic acceleration just as Kivelson predicted in the early 1980s, speeding the electrons up substantially. ...

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