The Magnetic Field Structure in M81
M81 (NGC 3031) is a nearby galaxy (d = 3.25 Mpc ) of Hubble type Sab with a bright central region and tightly wound spiral arms of low luminosity. The HI spiral arms follow a logarithmic spiral pattern of pitch angle −15°  close to the pitch angles of the optical spiral arms  and those of the blue light and Ha (−12°±2°, ). The theory of density waves has been applied successfully to explain the clear two-armed spiral structure of M81 [2,5–8].
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