Cosmic-rays (CRs) are relativistic particles that pervade galaxies and intergalactic space. They likely play a dynamically important role in a wide range of astrophysics systems by interacting with thermal gas via magnetic fields. The consequence of this interaction at macroscopic scales is known as CR feedback, including driving galactic outflows that crucially affect the cosmic ecosystem. The key to understanding CR feedback lies in how well the gas and CRs are coupled, which is often described by some arbitrary CR transport coefficient, leading to dramatic uncertainties. At microphysical level, the coupling is mediated by resonant wave scattering, with waves primarily excited by the CR gyro-resonant instabilities, yet the multi-scale nature of the problem precludes numerical studies over decades. I will describe the first effort studying such instabilities from first principles using the novel magnetohydrodynamic-particle-in-cell technique. By properly driving the instabilities balanced by wave damping over a semi-macroscopic box while resolving the microscopic length scale, we are starting to self-consistently characterize the CR transport coefficients under certain regimes, thus offering faithful sub-grid prescriptions for future studies of CR feedback.

报告人简介：Prof. Xuening Bai graduated from Tsinghua University with a B.S. in mathematics and physics in 2007, and obtained his PhD in astrophysics from Princeton University in 2012. He was a Hubble Fellow and Institute for Theory and Computation (ITC) fellow at the Harvard-Smithsonian Center for Astrophysics from 2012-2017. He joined the faculty as a research professor at the Institute for Advanced Study, Tsinghua University in 2017, and is jointly affiliated with Department of Astronomy. He is a theoretical and computational astrophysicist. His research group studies protoplanetary disks and planet formation, several areas of plasma astrophysics especially on cosmic-ray acceleration and transport, and develops computational tools for related applications.