Nonlinear Steepening of Shock-like Structures in the Solar Wind: Wave-Particle Interaction

It has been reported that nonlinear steepening of waves and fluctuations frequently occurs in the solar wind upstream of Earth's bow shock. The observation of nonlinear steepening provides important clues how shocks form in collisionless space plasmas. Previously, using Cluster multi-spacecraft measurements we reported that a compressional pulse formed at the edge of a density hole can steepen and develop into a shock-like structure. In that study we found that ion thermalization is insignificant during the nonlinear steepening and suggested that electrons may play an important role in the steepening. In this study we will focus on the interaction between electrons and waves. Investigation of electron phase space distributions shows that electron distributions in the downstream region of the steepened shock-like edge still have similar shape with the upstream solar wind distribution instead of becoming flat-top as observed in the downstream region of Earth's bow shock. On the other hand, the distributions show strong enhancement along the field-aligned direction in high energies, which implies there exists strong acceleration along the field-aligned direction. Wave observations show that broad-band whistler mode waves are strongly enhanced at the steepened structure. We will analyze the interaction between the electrons and waves and investigate the role of the wave-particle interaction in the steepening process.