First‐Order Orbit Theory of the Rotational Discontinuity

A study is made of the propagation properties and internal equilibrium structure of a large‐amplitude Alfvén wave which conserves the magnitude of the magnetic‐field component tangential to the wave front but changes its direction. The propagation properties are found to be identical with those of small‐amplitude sinusoidal waves. The equilibrium wave structure is shown to involve an electric field along the wave normal. The sign and magnitude of this field are related to the rate and sense of rotation of the tangential magnetic field component as well as to the particle distributions. In certain cases the electric field may be sufficiently large to cause nonnegligible variations in particle and magnetic pressures within the wave. Observed polarizations of the magnetopause current layer may be explained by the theory.