Hydromagnetic Waves Generated at the Bow Shock by Tangential Discontinuities in the Solar Wind

The problem of interaction of an inhomogeneity frozen into the solar wind with the bow shock has been investigated. These frozen-in tangential discontinuities produce ripples that move along the shock, ripples that can cause magnetosonic waves in the downstream medium. There are five linearly independent types of frozen-in inhomogeneities. Of the five inhomogeneities, those involving changes in the magnitude of density or velocity produce magnetosonic waves of largest amplitude. Other inhomogeneities are substantially less effective in producing magnetosonic waves in the magnetosheath. For normal solar wind parameters, a 10% density inhomogeneity upstream typically produces a magnetosonic wave with |δB| = 0.03 Bs where Bsis the field strength in the magnetosheath. A 30° fluctuation in magnetic field direction upstream typically produces a magnetosonic wave downstream with magnetic amplitude ≃ 10−3 Bs. However, for certain angles between the upstream magnetic field and the shock, the efficiency of conversion into magnetosonic waves is infinite; i.e., the shock is unstable.