Supercomputer clues to violent magnetic events

Researchers are a step closer to understanding the violent magnetic events that cause the storms on the sun’s surface and fling clouds of hot gas out into space, thanks to colossal computer simulations at Princeton University in the US.

The disruptions in the magnetic field, known as magnetic reconnections, are common in the universe – the same process causes the aurora in high latitude skies – but existing models are unable to explain how they happen so quickly.

A team led by Jackson Matteucci decided to investigate by building a full three-dimensional simulation of the ejected hot gas, something that required enormous computing power. The results are published in the journal Physical Review Letters.

The researchers modelled more than 200 million particles using Titan, the biggest supercomputer in the US. They discovered that a three-dimensional interaction called the Biermann battery effect was at the heart of the sudden reconnection process.

Discovered in the fifties by German astrophysicist Ludwig Biermann, the Biermann battery effect shows how magnetic fields can be generated in charged gases, known as plasma.

In such plasmas, if a region develops in which there is a temperature gradient at right angles to a density gradient, a magnetic field is created that encircles it.

Astrophysicists propose that this effect might take place in interstellar plasma clouds, such as nebulae, and generate the cosmic magnetic fields that we see throughout the universe.

In contrast with the huge scale of cosmic plasma clouds, magnetic reconnection happens at a scale of microns when two magnetic fields collide, says Matteucci.

He likens the process to collisions between two sizable handfuls of rubber bands. In stable circumstances the magnetic field lines are loops, like the bands. But sometimes turbulence in the plasma pushes these band analogues together so forcefully that they sever and reconnect to different ones, thus forming loops at different orientations.

Some of the new loops are stretched taut and snap back, providing the energy that ejects material so violently, and causes magnetic storms or glowing auroras.

The Princeton simulation showed that as the fields collide there is a sudden spike in the temperature in a very localised region, which sets off the Biermann battery effect, suddenly creating a new magnetic field in the midst of the collision. It’s this newly-appearing field that severs the lines and allows them to reconfigure.

Although Matteucci’s simulations are for tiny plasma clouds generated by lasers hitting foil, he says they could help us understand large-scale processes in the atmosphere.

“If you do a back of the envelope calculation, you find it could play an important role in reconnection in the magnetosphere, where the solar wind collides with the Earth’s magnetic field,” he says.