Weibel Instability-Driven Seed Magnetic Fields during Reionization

TL;DR

This paper explores how the Weibel instability can generate seed magnetic fields during the epoch of reionization by analyzing anisotropies in electron velocity distributions caused by photoionization fronts.

Contribution

It demonstrates that anisotropies in reionization fronts can lead to rapid growth of seed magnetic fields via the Weibel instability, providing a potential mechanism for cosmological magnetogenesis.

Findings

Fractional anisotropy can reach 6×10⁻³ within the ionization front.

Linear growth timescale of the Weibel instability is much shorter than the ionization front crossing time.

The study suggests a plausible pathway for seed magnetic field generation during reionization.

Abstract

Cosmological reionization was a highly out-of-equilibrium event that affected every parcel of the intergalactic medium, making it a candidate for astrophysical generation of intergalactic magnetic fields. During reionization, the first stars and galaxies ionized the surrounding, largely neutral, medium in ever-expanding envelopes. Photoionization from sources on one side of the front, combined with the quadrupolar angular dependence of the photoionization cross section, leads to an anisotropic electron velocity distribution. We investigate instabilities in these reionization fronts as a mechanism to generate seed magnetic fields. The Weibel instability has the potential to create a magnetic field from these anisotropies. We calculate the magnitude of the isotropic and anisotropic distribution within a simulated reionization front. We find that the fractional anisotropy can grow to $6\times 10^{-3}$ toward the middle of the ionization front. We show that the linear growth timescale of the Weibel instability is fast compared to the crossing time of the ionization front ($\sim 2\times 10^5$ seconds). We briefly speculate on the possible non-linear evolution of the instability and the implications for cosmological magnetogenesis.