Speaker
Description
The Universe was born without magnetic fields, yet in the modern Universe dynamically important magnetic fields are ubiquitous. The standard model of magnetogenesis explains this with small-scale dynamo (SSD) amplification of weak primordial fields to the levels we see today, followed by larger-scale coherence generated through large-scale dynamos (LSDs). This model, however, faces challenges in explaining large-scale ($\sim$ kpc) magnetic structures observed in merging and young (z $>$ 2) galaxies, where LSDs are absent. Incompressible SSD theories predict magnetic fields in these environments to be correlated on $\ll$ pc scales, significantly smaller than the fields we observe. We address this discrepancy by using direct numerical simulations to establish clear relationships between the magnetic field morphology produced by SSDs, and the global properties of these plasma environment, leading us to a resolution of the scale problem: supersonically-moving shocks within compressible plasmas violently reorganise magnetic fields into structures associated with denser regions of plasma. While magnetic fields naturally concentrate their energy near their dissipation scale ($\ll$ pc), turbulent, shock-filled environments like young galaxies, reorganise these fields to significantly larger-scales ($\sim$ kpc), and thus we can explain the ordered magnetic fields observed.
