Aurora mystery explained

Mysterious aurora-causing high-speed electrons detected by space probes have finally been explained, thanks to a massive computer simulation.

The finding, say MIT researchers, could make it easier to predict high-energy electron streams in space that could damage satellites.

The simulation shows that an active region in Earth’s magnetotail  — a vast and intense magnetic field swept outward from Earth by the solar wind — is roughly 1,000 times larger than had been thought.

This explains the large numbers of high-speed electrons detected by a number of spacecraft missions, including the Cluster mission.

The problem was solved by using Oak Ridge National Laboratory’s Kraken supercomputer to follow the motions of 180 billion simulated particles in space over the course of a magnetic reconnection event.

As the solar wind stretches Earth’s magnetic-field lines, the field stores energy like a rubber band being stretched. When the parallel field lines suddenly reconnect, they release that energy all at once — like releasing the rubber band.

That release of energy is what propels electrons with great energy back toward Earth, where they impact the upper atmosphere and cause auroras.

What was confusing physicists was the number of energetic electrons generated in such events. According to theory, it should be impossible to sustain an electric field along the direction of the magnetic field lines, because the plasma in the magnetotail should be a near-perfect conductor. But such a field is just what’s needed to accelerate the electrons.

But according to the new simulation, the volume of space where such fields can build up can, in fact, be at least 1,000 times larger than the theorists had thought possible — and thus large enough to explain the observed electrons.

“People have been thinking this region is tiny,” says Jan Egedal, an associate professor of physics at MIT.

“But by analyzing the spacecraft data and doing the simulation, we’ve shown it can be very large, and can accelerate many electrons.”

The finding has important implications for the military and NASA, as these hot electrons can destroy spacecraft.