Kiloparsec-scale turbulence driven by reionization may grow intergalactic magnetic fields

The intergalactic medium (IGM) underwent intense heating that resulted in pressure disequilibrium in the wake of ionization fronts during cosmic reionization. The dynamical relaxation to restore pressure balance may have driven small-scale turbulence and, hence, the amplification of intergalactic magnetic fields. We investigate this possibility for the first time using a suite of $\approx 100$ pc resolution radiation-hydrodynamics simulations of IGM gas dynamics. We show that as the spatial resolution improves beyond that achieved with most prior studies, much of the IGM becomes turbulent unless it was pre-heated to $\gg 100~$K before reionization. In our most turbulent simulations, we find that the gas energy spectrum follows the expected $k^{-5/3}$ Kolmogorov scaling to the simulation’s resolution, and the eddy turnover time of the turbulence is $< 1$ Gyr at $k \approx 1 ~$kpc$^{-1}$. Turbulence will grow magnetic fields, and we show that the fields grown by reionization-driven turbulence could explain lower limits on the strength of volume-filling B-fields from observations of TeV blazars. As reionization sweeps over the cosmos, this mechanism could create turbulence throughout the cosmic volume with a character that only depends on the amount of IGM preheating.

💡 Research Summary

This paper investigates a previously unexplored mechanism for generating turbulence and amplifying magnetic fields in the intergalactic medium (IGM) during cosmic reionization. Using a suite of radiation‑hydrodynamics simulations with unprecedented spatial resolution (≈100 pc, up to 2048³ cells in a 250 h⁻¹ kpc box), the authors show that the sudden heating and resulting pressure disequilibrium created by ionization fronts drive vigorous, small‑scale turbulence throughout the IGM, provided the gas has not been pre‑heated to temperatures far above ~100 K before reionization.

The simulations begin at redshift z = 300 and evolve to z = 4. A planar ionization front sweeps the box at z ≈ 7, instantly raising the temperature of low‑density gas to ~30,000 K while over‑dense filaments and mini‑halos expand, cool adiabatically, and compress surrounding gas to >50,000 K. The resulting pressure gradients trigger Rayleigh‑Taylor‑like shear instabilities and generate a turbulent cascade. By z ≈ 6 the gas shows clear signs of mixing; by z ≈ 5.5 a fully developed turbulent field is visible in both density and temperature maps. The kinetic energy spectrum follows the Kolmogorov scaling k⁻⁵ᐟ³ down to the simulation’s resolution limit, and the eddy turnover time at k ≈ 1 kpc⁻¹ is less than 1 Gyr. Convergence tests demonstrate that turbulence disappears when the cell size exceeds ~0.5 kpc, indicating that prior cosmological studies (typical resolution 1–2 ckpc) missed this phenomenon due to numerical viscosity.

The authors also explore the impact of X‑ray pre‑heating. Imposing a temperature floor of 100 K at z < 15 still allows turbulence, though it becomes more localized around the most massive filaments. Raising the floor to 1000 K suppresses turbulence entirely, because the pre‑heated IGM lacks the small‑scale density contrasts that later generate overlapping pressure‑driven flows. Thus, the amount of pre‑heating is the sole parameter controlling the strength and volume‑filling fraction of the turbulence.

Having established the existence of reionization‑driven turbulence, the paper turns to magnetic field amplification via the turbulent dynamo. Assuming a fraction f_B of the turbulent kinetic energy is transferred to magnetic energy, the authors derive a present‑day field strength B ≈ 1.5 × 10⁻⁹ G × f_B^{1/2} for a driving velocity V_dr ≈ 20 km s⁻¹ at z_re ≈ 7. The field decays as (1+z)⁻² due to cosmic expansion, while its coherence length expands by the same factor, yielding a present‑day coherence scale of order 10 kpc. This magnitude comfortably exceeds the lower limits inferred from TeV blazar observations, which require B > 3 × 10⁻¹⁶ G for coherence lengths ≳10 kpc (or equivalently stronger fields for smaller coherence scales). The authors argue that, given the high magnetic Prandtl number (Pr ≈ 10¹⁵) expected for the IGM, the dynamo saturates near the Batchelor scale, producing magnetic structures with coherence lengths ≈10⁵ km and energy fractions f_B ∼ 10⁻², still sufficient to meet the blazar constraints when the turbulence fills most of the volume.

In summary, the study demonstrates that (1) reionization inevitably generates kiloparsec‑scale turbulence throughout the IGM, provided the gas is not strongly pre‑heated; (2) this turbulence follows a Kolmogorov cascade and persists for several hundred Myr before dissipating; (3) the turbulent dynamo can amplify seed magnetic fields to ≥10⁻¹⁶ G on ∼10 kpc scales, thereby explaining the magnetic field lower limits derived from high‑energy gamma‑ray observations; and (4) the only controlling factor is the pre‑heating temperature, making the mechanism robust and universal. The authors suggest that future work should incorporate larger simulation volumes, more realistic X‑ray heating histories, and explicit magnetohydrodynamics to refine predictions of the magnetic field topology and its observational signatures.