An instability that resembles the usual hydromagnetic mirror instability is demonstrated. It is caused by freshly created ions in a moving plasma. The source of free energy is associated with the ring distribution of the newborn ions which results in a high ''ion kinetic temperature'' in the direction transverse to the ambient magnetic field. The stability analysis is carried out based on the linearized Vlasov equations. The effect of electron Landau damping is also discussed. This instability can lead to the amplification of magnetosonic waves. ¿ American Geophysical Union 1988