Magnetic Fields are not ignorable in the dynamics of disks

Abstract

Magnetic fields are considered to be dominant when $ε_{B}\geqε_{K}$, being $ε_{B}=B^{2}/8π$ the magnetic energy density and $ε_{K}=1/2 ρθ^{2}$ the rotation energy density, for a conventional moderate B= 1 $μ$G. They are considered to be negligible when $ε_{B}<ε_{K}$ for $B\sim 10 μ$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.