On Jan. 25, the four spacecraft encountered an electron diffusion region, a boundary just a few kilometers thick that occurs at an altitude of approximately 60,000 kilometers (37,000 miles) above Earth's surface. It marks the frontier between the planet's magnetic field and that of the Sun, which is carried to Earth by the electrically charged solar wind.

An electron diffusion region is like an electrical switch: When it is flipped, it uses energy stored in the Sun's and Earth's magnetic fields to heat the electrically charged particles in its vicinity to large speeds. It is how auroras are created in the skies, as fast-moving charged particles collide with atmospheric atoms and make them glow.

The accelerated particles also can damage satellites by colliding with them and causing electrical charges to build up, which can short-circuit and destroy sensitive equipment.

Nineteen times in one hour-long period, the Cluster quartet became engulfed in an electron diffusion region, caused because the solar wind was buffeting the boundary layer, causing it to move back and forth.

Each crossing of the electron diffusion region lasted just 10-20 milliseconds for each spacecraft, but an instrument aboard each spacecraft, called the Electron Drift Instrument, was fast enough to measure the accelerated electrons.

The observation provides the most complete measurements yet of an electron diffusion region.

"Not even the best computers in the world can simulate electron diffusion regions; they just don't have the computing power to do it," said lead investigator Forrest Mozer of the University of California, Berkeley.

The data will help scientists understand the process of magnetic reconnection. The phenomenon occurs throughout the universe on many different scales, but in all cases tangled magnetic fields occasionally will collapse into more stable configurations, reconnecting and releasing energy through electron diffusion regions.

On the Sun, magnetic reconnection drives the solar flares that occasionally release enormous amounts of energy above sunspots.

The research could help nuclear physicists trying to build fusion generators, because they need to create stable magnetic fields in their reactors, but reconnection events tend to ruin their configurations. If the process of reconnection can be better understood, perhaps scientists can establish ways of preventing it in nuclear reactors.

"We need to do a lot more science before we fully understand reconnection," Mozer said, adding that his team's aim is to understand which solar wind conditions trigger the reconnection events and the associated electron diffusion regions seen by Cluster.

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