Jan. 31, 2011

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 on a February 2011 Nature Physics paper 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."

Substorms originate opposite the sun on Earth's "night side," at a point about a third of the distance to the moon. At this point in space, energy and particles from the solar wind store up over time. This is also a point where the more orderly field lines near Earth -- where they look like two giant ears on either side of the globe, a shape known as a dipole since the lines bow down to touch Earth at the two poles – can distort into long lines and sometimes pull apart and "reconnect." During reconnection, the stored energy is released in explosions that send particles out in all directions. But reconnection is a magnetic phenomenon and scientists don't know the exact mechanism that creates speeding particles from that phenomenon.