Experiments support theory of cosmic magnetic field growth
Cosmic magnetic fields permeate galaxies and galaxy clusters throughout the universe. Scientists can explain how those fields might be seeded: Weak magnetic fields emerge in astrophysical plasmas via misaligned temperature and density gradients. But they have not yet demonstrated how seed fields in distant stars and protogalaxies grow to their observed astronomical values of several microgauss. Theory suggests that chaotic motion in an intragalactic plasma amplifies tiny magnetic fields by converting kinetic energy into magnetic energy. Now Gianluca Gregori (Oxford University) and Petros Tzeferacos (University of Chicago) have measured the amplification of a magnetic field in a turbulent laboratory plasma, providing the first physical demonstration of the turbulent dynamo mechanism.
At the Omega Laser Facility at the University of Rochester, Tzeferacos and colleagues used a series of laser pulses to irradiate two targets of penny-sized pieces of foil, fixed on a frame and spaced 8 mm apart. The laser energy quickly stripped and ionized the atoms from each foil to generate two hot plasma jets directed toward each other. When the opposing jets passed through grids in the frame and collided, they generated a strongly turbulent region, which is shown in the x-ray image. By imaging protons fired through the plasma, the team tracked the magnetic field in the turbulent region and found that in less than 10 ns, the field became 25–30 times stronger (see video below).
Numerical simulations confirmed that density and temperature gradients led to the emergence of tiny magnetic fields in the precollision plasmas. As a consequence of turbulent motion, the fields grew until the plasma’s kinetic energy and magnetic energy reached equilibrium, as predicted by simulations.
The study shows that the turbulent dynamo is a viable mechanism for the observed magnetization of galactic structures and offers an experimental platform for future astrophysical research. (P. Tzeferacos et al., Nat. Commun. 9, 591, 2018 .)
