A theoretical model of the magnetic field of the Galaxy based on the density-wave theory is proposed. In the coordinate system rotating at the pattern speed of our Galaxy, the magnetic line of force and the streamline of the interstellar gas are parallel to each other, and both of them form closed circuits. The difficulty of the winding up of an arm and the continual stretching of magnetic lines of force is completely avoided. Both the linear analysis and the nonlinear analysis of the hydromagnetics are carried out. It is found that the magnetic field inside the arm is nearly parallel to the direction of the galactic spiral, and the strength there is of the order of 5 microgauss, which corresponds to a mean field strength of about 2-3 microgauss in the base state. Large-scale hydromagnetic shock waves are found to be possible in the nonlinear gas-flow picture and are suggested as the triggering mechanism for star formation in narrow spiral strips lying along a grand design of spiral structure. The compatibility of a two-armed spiral shock pattern with magnetic field is confirmed. The results are found to be in good agreement with observational studies.
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W. et al. (1970) studied this question.