Lehnert number squared against the normalised radius in the magnetostrophic (dark red) and equipartition (green) regimes, for a 1 M_⊙ star. The reader is referred to the body of Sect. 4 for the new definition of Le that involves a small-scale magnetic field. The Lehnert number squared is plotted at different epochs: amid the pre-main sequence (midPMS; ∼30 Myr), at zero-age main sequence (ZAMS; ∼60 Myr), amid the main sequence (midMS; ∼5.3 Gyr), and toward the end of the main sequence (TAMS; ∼10.5 Gyr).