Hybrid Simulation Study of Waves Supported by Tangential Discontinuities

In the solar wind, multiple spacecraft have observed abrupt field rotations, called directional discontinuities. Surprisingly, single spacecraft misidentify many discontinuities as rotational ones. Timings from three or more spacecraft reveal that the discontinuity actually has a small magnetic field normal, more in agreement with a tangential discontinuity (TD) which has zero normal field component. The discrepancy at a single spacecraft is interpreted as a direct sign of the TDs supporting surface waves. Hollweg [1982] predicted the existence of linear noncompressive magnetohydrodynamic (MHD) surface wave solutions on solar-wind TDs where the magnetic field rotates across the layer. Because these waves are noncompressive, they would propagate unattenuated by collisionless resonant particle damping. Hollweg showed that such a wave on a TD causes the inferred normal component from a single spacecraft to appear large. Hollweg's analysis was limited to a cold plasma and linear MHD equations wherein the TDs are true discontinuities. We present numerical hybrid simulation results with particle ions and fluid electrons of the nonlinear behavior of finite-amplitude surface waves, in a warm plasma, and on finite-width TDs. We find that the initial surface wave can steepen when ducted between two TDs and generate body waves supported by the TDs. We examine how wave solutions supported by TDs can contribute to observed solar-wind fluctuations.