Magnetic Field Vector Structure of NGC6946

We studied large-scale magnetic field reversals of a galaxy based on a magnetic vector map of NGC6946. The magnetic vector map was constructed based on the polarization maps in the C and X bands after the determination of the geometrical orientation of a disk with the use of an infrared image and the velocity field, according to the trailing spiral arm assumption. We examined the azimuthal variation of the magnetic vector and found that the magnetic pitch angle changes continually as a function of the azimuthal angle in the inter-arm region. However, the direction of the magnetic field had $180^\circ$ jumps at the azimuthal angles of $20^\circ, 110^\circ, 140^\circ, 220^\circ, 280^\circ$, and $330^\circ$. These reversals seem to be related to the spiral arms since the locations of the jumps are coincident with those of the spiral arms. These six reversals of the magnetic field were seen only in the inner region of NGC6946 whereas four reversals can be identified in the outer region.