Table 2: Magnetic phenomena in NGC 6946 and their probable origins.
Total magnetic energy density $\simeq$ turbulent energy density in inner galaxy Field amplification by turbulent gas flows

Total magnetic energy density dominates in outer galaxy Magneto-rotational instability ?

Low degrees of polarization in gas spiral arms Field tangling due to star-formation activity

Large-scale pattern in Faraday rotation Coherent fields

Magnetic arms with regular, almost completely aligned fields Efficient dynamo action in interarm regions

Two inner magnetic arms Superposition of dynamo modes m=0 and m=2

No large-scale field reversals Higher dynamo modes weak or absent in inner galaxy

Outer magnetic arms Higher dynamo modes or shear flows in outer galaxy

Alignment of ordered fields along spiral structure Interaction between gas flow and magnetic field

Magnetic arms are phase-shifted images of gas arms Magnetic field may trigger the formation of gas spiral arms

North-south asymmetry in Faraday rotation Spiral field points inwards in both inner magnetic arms

Enhanced magnetic field along inner edge of inner gas spiral arm Compression or shear by density waves

Smooth turning of field lines into inner gas spiral arm No shock

Mild field compression in inner gas spiral arm Magnetic field connected to warm, diffuse gas

Anti-correlation of RM and field pitch angle Helical fields

RM structure function flat until 6 kpc scale Uncorrelated field structures below 6 kpc scale

RM structure function rising for scales larger than 6 kpc Large-scale coherent fields

Depolarization rings at $\lambda20~{\rm cm}$ Differential Faraday rotation in ${\rm H~\scriptstyle II}$ complexes

Strong Faraday depolarization at $\lambda18~{\rm cm}$ and $\lambda20~{\rm cm}$ Faraday dispersion by random fields

Large-scale asymmetry in Faraday depolarization Helical fields ?

**Table 2:** Magnetic phenomena in NGC 6946 and their probable origins.
Total magnetic energy density $\simeq$ turbulent energy density in inner galaxy	Field amplification by turbulent gas flows
Total magnetic energy density dominates in outer galaxy	Magneto-rotational instability ?
Low degrees of polarization in gas spiral arms	Field tangling due to star-formation activity
Large-scale pattern in Faraday rotation	Coherent fields
Magnetic arms with regular, almost completely aligned fields	Efficient dynamo action in interarm regions
Two inner magnetic arms	Superposition of dynamo modes m=0 and m=2
No large-scale field reversals	Higher dynamo modes weak or absent in inner galaxy
Outer magnetic arms	Higher dynamo modes or shear flows in outer galaxy
Alignment of ordered fields along spiral structure	Interaction between gas flow and magnetic field
Magnetic arms are phase-shifted images of gas arms	Magnetic field may trigger the formation of gas spiral arms
North-south asymmetry in Faraday rotation	Spiral field points inwards in both inner magnetic arms
Enhanced magnetic field along inner edge of inner gas spiral arm	Compression or shear by density waves
Smooth turning of field lines into inner gas spiral arm	No shock
Mild field compression in inner gas spiral arm	Magnetic field connected to warm, diffuse gas
Anti-correlation of RM and field pitch angle	Helical fields
RM structure function flat until 6 kpc scale	Uncorrelated field structures below 6 kpc scale
RM structure function rising for scales larger than 6 kpc	Large-scale coherent fields
Depolarization rings at $\lambda20~{\rm cm}$	Differential Faraday rotation in ${\rm H~\scriptstyle II}$ complexes
Strong Faraday depolarization at $\lambda18~{\rm cm}$ and $\lambda20~{\rm cm}$	Faraday dispersion by random fields
Large-scale asymmetry in Faraday depolarization	Helical fields ?

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