The Interaction of Turbulence with Shock Waves

The heliosheath was expected to be turbulent, the result of upstream turbulence and disturbances (shock waves, pressure and density enhancements, structures, etc.) being transmitted across and interacting with the heliospheric termination shock (HTS). A turbulent heliosheath has indeed been observed downstream of the HTS, but the character of the turbulence is significantly different from that of the solar wind. Here, we discuss the transmission of waves and turbulence across the HTS, both analytically and numerically, in the large plasma beta approximation, and we investigate both small amplitude and large-amplitude cases. We find that the linear theory is a reasonable approximation for small amplitude waves incident on the shock. In the case of large amplitude entropy fluctuations incident on the shock, the downstream state is initially one of coherent wave forms, but this rapidly devolves to a highly disturbed state that evolves eventually to a state dominated by vortical structures. Of particular importance, we find that the HTS generates significant levels of downstream compressible turbulence, even in their absence upstream.