Supernova simulation shows source of cosmic rays and solar flares

Physicists have taken an important step towards creating a tiny stellar explosion in the laboratory, writes Cathal O’Connell

In an exploding star, plasma shockwaves can fire out particles at extraordinary speeds leaving behind a supernova remnant like this one.
In an exploding star, plasma shockwaves can fire out particles at extraordinary speeds leaving behind a supernova remnant like this one.

Shockwaves in plasma can whizz particles up to tremendous speeds, according to new computer simulations that confirm a decades-old theory. The effect may explain the constant rain of cosmic rays hitting our atmosphere as well as blackout-inducing solar flares that burst forth from the surface of the Sun.

Excitingly, the results indicate that physicists may soon be able to reproduce some of the effects of supernovae and solar flares in the laboratory, using high-energy laser blasts.

Cosmic rays are charged particles, such as protons, arriving from deep space at almost the speed of light. Although the atmosphere mostly protects us from cosmic rays, they are the reason airline pilots have an increased risk of cataracts. They can also fry electronics on satellites which fly outside the protective blanket of the Earth’s magnetic field.

In 2013, after decades of debate, data from the Fermi space telescope pinned down exploding stars as the source of most cosmic rays. But exactly how such a blast accelerates the particles is not well understood.

One idea, first proposed by the Italian physicist Enrico Fermi in 1949, describes how magnetic shockwaves in plasma – a gas of charged particles – can buffet the particles up to high speed.

The problem is, there’s been no way to directly test the idea.

“We want to reproduce the process in miniature,” says Will Fox, a physicist at the Princeton Plasma Physics Laboratory. His team is working towards creating a little piece of supernova in the lab, by using blasts of powerful lasers, fired into a plasma, as a miniature analogue of an exploding star.

In their latest work, published in Physics of Plasmas, Fox and his team ran computer simulations of their experiment on the US$97 million Titan supercomputer at Oak Ridge.

The simulation did produce accelerated particles just as Fermi predicted.

Their simulations were also able to reproduce another effect that occurs when magnetic field lines in plasma snap apart and reconnect – a phenomenon that can often precede a solar flare bursting from the sun.

The next step is to do the experiment for real. The team plan to use some of the world’s most powerful lasers – such as the OMEGA laser in Rochester,UK and the National Ignition Facility in Livermore, California, both of which can produce laser pulses using more power than the average power consumption of the entire rest of human society – albeit for the briefest fraction of a second.

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Cathal O'Connell is a science writer based in Melbourne.
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