Turbulence in counter-flowing partially-ionized plasma may help explain astrophysical systems

Much of the plasma in space is subject to instabilities caused by interactions between fluids with sharply contrasting properties, which can lead to turbulent behavior. In a new paper, Andrew Hillier studies this instability by simulating the dynamics between a neutral fluid and an ionized plasma in a magnetic field.

Hillier optimized his simulations to allow the largest possible turbulent behavior in a uniform initial magnetic field. He found at varying timescales, different types of non-linear behavior and vortex formation emerge in the velocity flow and density structures of the fluids. The magnetic field acts to suppress the instability in the magnetized fluid, but the neutral fluid is unaffected. The ionized plasma initially appears to only behave as though it is undergoing typical hydrodynamic evolution due to thermal coupling. However, at larger timescales, vortex-like density structures begin to form in the fluids and are squashed and elongated by the magnetic field.

To Hillier, the most surprising result was the effect of the instability on the heating of the system. As the width of the instability increases, the neutral fluid begins to couple with the magnetic field, decreasing the drift velocity and frictional heating rate. Because small vortices generate smaller drift velocities, the energy dissipation from frictional heating of ions slipping past each other is predominantly driven by larger vortices.

“It’s typically the smallest scale of a system that determines how much heating happens, but the frictional heating is determined by the larger vortex scale instead of the small scale,” Hillier said. “It’s different from heating in other turbulent systems.”

Because most of the plasma in the universe is not fully ionized, Hillier’s simulations have potential applications in many astrophysical systems.

Source: “Ion-neutral decoupling in the nonlinear Kelvin—Helmholtz instability: Case of field-aligned flow,” by A. Hillier, Physics of Plasmas (2019). The article can be accessed at https://doi.org/10.1063/1.5103248 .