CAREER: Transmission and Reflection of Low-frequency Turbulence through Shock Waves

The interaction between turbulence and shocks is ubiquitous in heliophysics. Shocks are created in space plasmas when a supersonic flow interacts with an obstacle such as a coronal mass ejection (CME) or a planet. For example, interplanetary shocks are often driven by interplanetary CMEs propagating through the solar wind. The downstream region between the shock and the ejecta is commonly known as the sheath. Additionally, pairs of forward-reverse shocks can form at stream interaction regions (SIRs) due to the compressive interaction of fast and slow solar wind streams. These interplanetary shocks will interact with turbulent solar wind fluctuations. Another example is the Earth’s bow shock, which is created by the solar wind slowing down upon reaching the Earth’s magnetosphere. Strong turbulence is generated downstream of the bow shock, known as the magnetosheath. For the Earth’s bow shock, the upstream region is also affected by backstreaming particles, leading to a foreshock region where the fluctuation level is enhanced compared to the quiet solar wind. In both cases of interplanetary shocks and planetary bow shocks, the shocks interact with turbulent fluctuations in the solar wind. The complete problem of the interaction between low-frequency MHD wave modes and different shock geometries has not been systematically investigated. This project is to investigate this interaction; specifically, how upstream incident waves are transmitted and reflected upon interaction. The following objectives will be achieved: (1) To extend the present theoretical model of MHD wave-shock interaction to a more general case. Quantifying the differences caused by the shock geometry, shock strength, field and plasma parameters, and upstream incident wave mode. (2) To understand the relationship between the MHD waves observed upstream and downstream of heliospheric shocks through spacecraft data analysis guided by the theoretical framework of MHD wave-shock interaction. (3) To characterize the properties of low-frequency turbulence upon crossing the shocks, including cross helicity, compressibility, and anisotropy, based on the properties of the transmitted waves. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.