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Abstract: The nature of plasma turbulence in the intracluster medium (ICM) of galaxy clusters is central to theories of thermal stability, particle acceleration, and magnetic-field amplification, with the viscosity of the ICM playing a particularly important role. Many estimates of viscosity in the ICM rely on the assumptions that Coulomb collisions determine the kinematic viscosity, and that viscous stresses are isotropic and Laplacian in nature. These assumptions, however, fail to explain X-ray observations of multi-scale density fluctuations, which suggest that ICM turbulence cascades to scales significantly smaller than the Coulomb-collisional viscous scale. This discrepancy has been interpreted as indicating a kinematic viscosity that is at least a factor of ten (if not more) smaller than the conventional Spitzer viscosity. Yet, the ICM is not a hydrodynamic fluid; it is a weakly collisional, magnetized plasma, and it is therefore subject to various non-equilibrium effects, such as anomalous scattering and anisotropic viscous stresses. I will present a theory of turbulence in the ICM that leverages these effects, in which ion-Larmor-scale instabilities regulate the plasma viscosity at the largest scales to mitigate viscous dissipation, while a self-organizational effect dubbed "magneto-immutability" organizes the flow at smaller scales to avoid motions that are viscously damped. The result is a robust, nearly conservative cascade that extends from scales spanning hundreds of kiloparsecs to the scale of individual particle Larmor radii. Results from a high-resolution, weakly collisional "Landau-fluid" simulation of trans-Alfvénic turbulence, with parameters chosen that approximately match the ICM of the Coma cluster, support this theory. This theory explains recent X-ray observations of "suppressed viscosity” in the ICM of many galaxy clusters, and holds consequences for thermal stability and the propagation and (re-)acceleration of cosmic rays. In particular, we explain how the same plasma microphysics responsible for reducing the thermal mean free path in the ICM enhances the confinement of sub-TeV cosmic rays.

Informal discussion with coffee, tea, and snacks to follow.

Hosted by Assistant Professor Muni Zhou

Please click the link below to join the webinar:

https://dartmouth.zoom.us/j/91888702369?pwd=aUlaVEFYNGZHNlZWL0R3cEVWQXg4UT09