  Based on CO(2-1) public data, we study the monoxide oxygen gas excitation
conditions and the magnetic field strength of NGC 2841, NGC 3077, NGC 3184, NGC
3351 spiral galaxies. For their galaxy outskirts, we found kinetic temperatures
in the range of $35 - 38 ~K$, CO column densities $10^{15} - 10^{16} ~cm^{-2}$
and H2 masses of $4 ~10^6 - 6 ~ 10^8 M_\odot$. An H2 density 1$0^3 cm^{-3}$ is
suitable to explain the 2 sigma upper limits of the CO(2-1) line intensity. We
constrain the magnetic field strength for our sample of spiral galaxies and
their outskirts, evaluating a simplified expression of the magneto-hydrodynamic
force equation. Our estimations provide values for the magnetic field strength
in the order of $6-31 ~\mu G$.
