On the role of the magnetic field in a wave theory of the spiral structure of galaxies

The spiral waves in a model galaxy consisting of the differentially rotating interstellar gas and Population I are considered. The instability of spiral waves in the presence of differential rotation and magnetic field is found. This instability may lead as well as the Landau-instability found by Marochnik and Suchkov (1968, 1969) to the formation of an observable spiral pattern in a short time. It may play a definitive role in the spiral structure formation in galaxies with weak Population II.