Magnetic Fields are not ignorable in the dynamics of disks

Magnetic fields are considered to be dominant when ${ϵ}_B\ge {ϵ}_K$, being ${ϵ}_B=B^2\mathrm{/}8\pi$ the magnetic energy density and ${ϵ}_K=1\mathrm{/}2\rho {\theta }^2$ the rotation energy density, for a conventional moderate B= 1 $\mu$G. They are considered to be negligible when ${ϵ}_B<{ϵ}_K$ for $B\sim 10\mu$G. With no assumption and no theoretical calculation, we show that magnetic fields cannot be ignored in the outer parts of a galaxy like the Milky Way and in the whole disk of a dwarf galaxy.