  Recently Barrow, and Tsagas [Phys Rev D 77: 107302, (2008)] have shown that
slow decay of cosmological magnetic slowly decay in FRW universes on curvature
scales as $B\sim{a^{-1}}$ in the context of general relativity (GR). This helps
possible amplification of cosmic magnetic fields. In this paper starting from
dynamo equations in spacetimes with torsion we obtain also slow decay of
magnetic fields naturally on curvature-torsion scales of Riemann-Cartan
spacetime on a de Sitter universe. In this case the constant of proportionality
between the magnetic field and the curvature scale is the torsion in the
present universe. Thus the B-field becomes $B\sim{\frac{{\eta}}{H_{0}}Ta^{-1}}$
where T is torsion vector modulus, $H_{0}$ is the Hubble constant and $\eta$ is
the diffusive scale.
