Conditions for MAC-wave stability in the Earth’s core

The condition of stability for the model of MAC-wave propagation in the toroidal region with an infinite radius and the numerical results in the toroidal region with a finite radius have been obtained analytically, taking into account a nonuniform distribution of the magnetic field and compressibility. It has been indicated that the frequency of unstable MAC-waves depends on the azimuthal velocity. Instability mostly develops in the Earth’s outer core with a decreased value of the azimuthal magnetic field. It has been qualitatively indicated that close drift velocities are typical of multiple-frequency variations. The MAC-wave frequencies for instabilities that develop near the outer boundary decrease if compressibility of a substance in the Earth’s outer core is taken into account.