Theoretical analysis of magnetoacoustic waves driven unstable by a neutral charged-particle beam suggests that such waves can be generated in the plasma sheet at the onset of a substorm. It is further suggested that the magnetohydrodynamic waves observed to occur at the onset of substorms (often called Pi2 pulsations) may be attributed to the coupling of the magnetoacoustic waves to shear Alfv¿n waves on locally-resonant field lines on the nightside of the earth. Experimental determinations of the distribution of resonant field lines for various frequency components of the waves, together with the theoretical concepts, can ultimately yield information on the inward velocity of hot plasma particles at substorm onsets.