Researchers have long been puzzled about the origins of cosmic rays -- high energy particles which move very close to the speed of light.
Now a team of scientists from the UK and Sweden think that an idea for a particle accelerator first put forward twenty years ago might explain how high energy cosmic ray electrons are produced close to the remnants of exploded stars ("supernovae").
These very high speed electrons betray their presence by emitting "synchrotron" radiation as they gyrate in a magnetic field. Until now, however, it has been far from clear why the electrons are accelerated to such high energies.
Researchers from the University of Warwick, the Culham Science Centre in Oxfordshire and Linköping University in Sweden used computer simulations to investigate the behavior of electrons in the presence of a magnetic field and a wave consisting of an oscillating electric field, and found that, depending on the intensities of the magnetic field and the wave, and the direction in which the wave is moving, it is possible for a charged particle such as an electron to be accelerated indefinitely by the wave electric field.
This idea was proposed by physicists Tom Katsouleas and John Dawson at the University of California Los Angeles (UCLA) in 1983 as a novel method of producing high energy charged particles in the lab, with the wave being provided by a laser.
The term "surfatron" was coined to describe this type of accelerator, because the particles ride across the wave front like surfers riding across an ocean wave.
There are no lasers in supernova remnants, but the Warwick, Culham and Linköping researchers believe that the rapid expansion of a supernova remnant into space creates shock waves that accelerate ions. These ions then generate waves which can play a role similar to that of the laser in the surfatron concept.
The analogy is not exact. For example, whereas the surfatron laser has only a single wavelength, in the case of the supernova remnant it is impossible to avoid the generation of waves with a range of wavelengths: this makes it more likely that particles will eventually stop gaining energy.
However, the Culham-Warwick-Linköping team has shown that acceleration of electrons to speeds approaching that of light is still possible in these circumstances. The research will be published in the journal Physical Review Letters in early December.
The researchers acknowledge funding from the PPARC, the Department of Trade and Industry, the Commission of the European Communities and Naturvetenskapliga forskningsrådet (NFR). - By Peter Dunn
The University of Warwick
Space and Astrophysics Group at the University of Warwick
Scientific Visualization Group of the Institute of Science and Technology (ITN, University of Linköping)
[Contact: Professor S. C. Chapman, Dr. K. G. McClements, Dr. M. E. Dieckmann, Peter Dunn]