A new laboratory experiment suggests that protons can gain energy by traveling along shock waves in space, like surfers on waves. But the speed of protons increases not due to ocean waves, but due to shock waves in plasma, a mixture of electrically charged particles. Such shock waves are disturbances similar to a sonic boom. They are characterized by a sharp increase in density, temperature and pressure.
The study authors are confident that the experiment will help to better understand the behavior of some of the high-energy particles that permeate space. Shock waves in space are believed to set in motion charged particles. But it is still not entirely clear how they get energy.
In an experiment that mimicked certain types of cosmic shock waves, protons reached energies of up to 80,000 electron volts, the scientists report in an article for Nature Physics. In space, similar shock waves occur where the outflow of charged particles from the Sun meets, for example, the Earth’s magnetic field, and also where they slow down sharply as they approach the edge of the solar system.
Scientists have used powerful lasers to recreate the physics of such cosmic shock waves on a smaller scale. In the experiment, a laser beam vaporized the target, sending a plasma explosion that rushed into a cloud of hydrogen gas. As the measurements showed, when the plasma flowed through the gas, a shock wave was formed and the protons from the gas accelerated.
Physicists have predicted that protons can be accelerated by a process that occurs in the presence of a magnetic field. The particle is pushed by the electric field of the shock wave, and the magnetic field helps it to “stay on course.” If the particle escapes from the shock wave, the magnetic field twists its trajectory to return it to the wave. As a result, the proton can again move along its surface.
Scientists have carried out a computer simulation of the experiment. Comparison of artificially created and real data suggests that the protons moved precisely along the shock wave.