Magnetogenesis by Ion-Acoustic Waves

For a planar electrostatic wave interacting with a single species in a collisionless plasma,  momentum conservation implies current conservation. However, when multiple species interact with the wave, they can exchange momentum, leading to current drive. A simple,  general formula for this driven current is derived [1-3]. For ion-acoustic waves, the wave results in momentum exchange between resonant electrons and nonresonant ions. The resulting force on the electrons can have a curl, and thus, as for the Biermann battery, give rise to compensating electric fields with curl on magnetohydrodynamic timescales. As a result, a magnetic field can be generated [4]. Surprisingly, in some astrophysical settings, this mechanism can seed magnetic fields with growth rates even larger than through the traditional Biermann battery, and thus could give rise to seed fields.

[1] K. Kato, Theory of current generation by electrostatic traveling waves in collisionless magnetized plasmas, Phys. Rev. Lett. 44, 779–781 (1980) [2] W. M. Manheimer, Energy flux limitation by ion acoustic turbulence in laser fusion schemes, Phys. Fluids 20, 265–270 (1977) [3] I. E. Ochs and N. J. Fisch, Momentum-Exchange Current Drive by Electrostatic Waves in an Unmagnetized Collisionless Plasma, Phys. Plasma, 27, 062109 (2020). [4] I. E. Ochs and N. J. Fisch, Magnetogenesis by Wave-driven Momentum Exchange, ApJ 905, 13 (2020).