THE recent discovery(1) of coherent, large-scale magnetic spiral arms
located between the optical spiral arms of the nearby galaxy NGC6946 i
s difficult to reconcile with current galactic dynamo theory(2), The f
ormation of optical spiral arms is generally attributed to the role of
density waves in the gas of the galactic disk, which trigger the form
ation of massive, luminous stars; turbulent processes in the gas are a
lso believed to play a key role in the generation of the galactic magn
etic field, Here we present the results of magnetohydrodynamical (MHD)
calculations, which show that both fast (similar to 20 km s(-1)) and
slow (similar to 0.2 km s(-1)) MHD density waves can exist in a rotati
ng galactic disk, Slow MHD density waves occur over the portion of a g
alactic disk that rotates almost rigidly(3), and can give rise to a pa
ttern of polarized radio emission similar to that observed in NGC6946,
where the magnetic field is strong between the optical spiral arms. I
n contrast, fast MHD density waves produce spiral magnetic structures
that are roughly coincident with the optical spiral arms-and which ext
end into the differentially rotating disk-as observed in the Whirlpool
galaxy, NGC5194 (refs 4-6).