Speaker
Description
Magnetic fields in cosmic voids could be of primordial origin, generated by early universe phase-transitions. During their turbulent decay, primordial magnetic fields (PMFs) might undergo inverse energy transfer, increasing their correlation length over time. Understanding this process is crucial for constraining PMFs, yet its physical mechanism is still debated, particularly in systems where magnetic helicity is zero on average.
We study how energy is transferred across different scales in the magnetic and velocity fields by measuring shell-to-shell energy transfer functions from numerical simulations of decaying helical and non-helical magnetohydrodynamic turbulence.
Independent of magnetic net-helicity, growth of large magnetic scales is predominantly sourced by integral-scale structures in the magnetic and kinetic reservoir, leading to partially non-local inverse transfer. In the case of vanishing net-helicity, transfer functions between the positively helical and negatively helical parts of the field are computed. We find that inverse transfer is helicity-segregated: energy flows to large scales only within each helical sector, not across them.
These findings are consistent with the theory underlying the conservation of the Hosking integral which is thought to control the decay of magnetic fields in the case of vanishing net-helicity. Beyond power-spectra and global quantities, helicity-decomposed shell-to-shell energy transfer functions provide a more detailed diagnostic for the evolution of PMFs and their interaction with the primordial plasma.