26 January 2012

Lasers shine light on galactic magnetic fields

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The mystery of how galaxies, including our Milky Way, have become magnetised has been solved by experiments using high-power lasers.
magnetic seed

Physicists have successfully reproduced the shock waves found in developing galaxies, a possible source of galactic magnetic fields.
On the left: an image of a laser-produced shock wave. Brighter colours correspond to regions of higher density or temperature (i.e. a shock). On the right: A simulation of a collapsing shock wave arising in a protogalaxy.
Credit: A. Ravasio, A. Pelka, J. Meinecke and C. Murphy (LULI) / F. Miniati (ETH)

SHROPSHIRE: The mystery of how galaxies, including our Milky Way, have become magnetised has been solved by experiments using high-power lasers.

By reproducing the conditions found in developing galaxies, physicists have shown that shock waves can generate tiny ‘seed’ magnetic fields, which might eventually grow into the large-scale magnetic fields we observe today. The research is described in this Nature today.

“Observations indicate that magnetic fields are ubiquitous in galaxy clusters, galaxies, and even in voids,” said Gianluca Gregori, lead author of the study from the University of Oxford in England.

“To explain this large-scale magnetisation, magnetic fields must have existed for a long time. But where have these magnetic seeds come from? Our experiment indicates that the generation of seeds by shocks is a plausible explanation, as initially suggested in numerical simulations.”

Magnetic galaxies

It is thought that every galaxy and galaxy cluster in our universe has a magnetic field. Although much weaker than the Earth’s magnetic field, these galactic fields play an important role in star formation, as well as helping to prevent galaxies from gravitational collapse.

Scientists believe that galactic magnetic fields are generated from weaker seed fields. These seeds are then amplified via a dynamo mechanism, in which the rotation and turbulence of the galaxy’s interstellar medium – the gas and dust between stars – acts to reinforce the original magnetic field.

However, this mechanism doesn’t explain how the seed fields themselves come into being. One of the proposed methods for creating seeds is via shock waves generated by collapsing matter in developing ‘protogalaxies’. According to this hypothesis, the shocks pass through the protogalaxy’s plasma (a gas composed of charged particles), creating an electric current, and hence a magnetic field. This process is known as the ‘Biermann battery’.

Simulating shock waves

Now, an international team of physicists has successfully simulated this Biermann battery process by using a high-power laser to create shock waves comparable to those in a protogalaxy.

The pulsed laser was directed at a small carbon rod inside a chamber filled with helium gas. “This mimicked an explosion,” said Gregori. “A big laser was required as a starting point because we wanted to make the explosion as strong as possible. The one we used at LULI (Laboratoire pour l’Utilisation de Lasers Intenses) in Paris is one of the largest in Europe.”

This explosion created a shock wave ahead of the expanding material. As the shock wave moved through the plasma in the chamber, a current loop was created, which in turn generated a magnetic field.

Tiny magnetic seeds

Having measured the strength of this lab-created magnetic field, the researchers then used simple scaling relations to estimate the strength of a hypothetical galactic magnetic field.

Their calculations gave a galactic magnetic field strength of around 1 zG (zeptogauss). To put this into perspective, the Earth’s magnetic field is approximately one hundred, billion, billion times stronger.

But although it may be miniscule, this magnetic field strength agrees with the values predicted by previous numerical simulations. The researchers propose that this seed field can then be amplified to larger values by turbulent motions, over timescales of hundreds of millions of years.

Not the only way

“These types of explorations – studying exotic astrophysical phenomena in the lab – are great fun and potentially very interesting,” commented Larry Widrow, an astronomer and magnetic field expert from Queen’s University in Ontario, Canada.

But Widrow went on to emphasize that lab experiments such as these can only tell us so much: “What we really require is a better understanding of the early stages of galaxy formation, and this is something that can only be achieved through detailed theoretical modelling and simulations.”

He also added that the Biermann process is not the only suggested mechanism for creating seed fields. “Two other examples are the early generation of stars, which can explode and expel their magnetic fields into the interstellar medium, and active galactic nuclei, which similarly expel their fields through winds and jets,” said Widrow.

“There are indeed other mechanisms that have been proposed,” said Gregori. “Experiments are planned in even larger laser facilities to study these scenarios.” One of these experiments will be carried out at the National Ignition Facility in Livermore, California – the largest laser system in the world.

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