Magnetogenesis via the canonical battery effect*

Magnetic fields are ubiquitous in the Universe, underpinning the dynamics of stars, planets, galaxies, accretion disks, neutron stars, and the intergalactic medium. These structures, composed of magnetized plasmas capable of supporting convective flows, typically exhibit large-scale magnetic fields attributed to turbulent dynamo effects. However, dynamo processes require an initial seed magnetic field, and the mechanisms generating these weak fields remain poorly understood.

 

In this talk, we demonstrate that the "canonical battery effect" is responsible for seed magnetogenesis. By taking the curl of the first moment of the Vlasov equation for any species, we derive an equation describing the time evolution of canonical vorticity. This equation reveals that, given an initially zero magnetic field and fluid vorticity, the only term capable of spontaneously generating magnetic fields is the canonical battery term, related to the species' pressure tensor.

 

We show that the canonical battery term generalizes popular magnetogenesis mechanisms. Firstly, it is shown trivially that the well-known Biermann battery is a specific case of the canonical battery for isotropic, scalar pressure. Secondly, particle-in-cell simulations demonstrate that the canonical battery also underlies the Weibel instability.

 

The canonical battery term enables the prediction of new pressure tensor configurations that induce spontaneous magnetogenesis. One such configuration, a 2D-localized pressure anisotropy, is shown to generate quadrupole magnetic fields which are important for various processes such as magnetic reconnection and flux rope generation. This mechanism is verified by particle-in-cell simulations. Various potential studies and extensions arising from this framework are discussed.