2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/1911Using a periodic box calculation it is shown that, owing to helicity conservation, a large scale field can only develop on a resistive timescale. This behaviour can be reproduced by a mean-field dynamo with alpha and eta_t quenchings that are equally strong and `catastrophic'.3 Pages LaTex, to appear in proceedings of the 24th IAU General Assembly, Joint Discussion 14 "The Origins of Galactic Magnetic Fields"Astrophysicstext