Total magnetic energy amplification as a function of time (a) for different values of the initial magnetic field (upper panel) B_IGM = 10^-7   μG (solid), B_IGM = 10^-6   μG (dotted), B_IGM = 10^-5   μG (dashed) and B_IGM = 10^-4   μG (dash-dotted), and (b) for different values of the Lagrangian refinement criterion with B_IGM = 10^-7   μG (bottom panel): m_res = m₀ is the reference run (solid line), m_res = 5 × m₀ (dotted line), m_res = 25 × m₀ (dashed line) and m_res = 100 × m₀ (dash-dotted line). In each plot, the simple Ω amplification model is overplotted in red for Ω_G = 19   Gyr^-1 as average angular velocity.