Equipartition magnetic field ($B_{\mathrm{eq}}=\sqrt{{\mu }_0\rho }U$)

Equating magnetic energy density $B^2/(2{\mu }_0)$ to kinetic energy density $\rho U^2/2$ defines this order-of-magnitude field. It marks when a weak-field kinematic treatment of a given flow can fail, not a universal upper bound on concentrated fields. Gas-pressure confinement, compression, partial evacuation and dynamic pressure can allow stronger local fields.