Astrophysics Seminar

Energetic charged particles, or cosmic rays (CRs), are major contributors to the nonthermal energy budget of the universe. These CRs are believed to be accelerated in collisionless shocks, which are ubiquitous in astrophysical environments and are now a subject of growing interest in laboratory plasma experiments. While the primary acceleration mechanism -- diffusive shock acceleration -- is well established, the processes that inject thermal particles into the diffusive shock acceleration mechanism and convert them into nonthermal particles remain poorly understood. Understanding these processes is crucial, as nonthermal particles are not only responsible for radio, X-ray, and gamma-ray emission but also provide pressure that can reshape their accelerators. Despite significant efforts over the past decades, many fundamental questions in this field remain unsolved.

In this talk, I will present our recent efforts to address these challenges using first-principles kinetic plasma theory, simulation, and observation. I will discuss the journey from thermal to nonthermal particles at shocks, highlight the critical role of the plasma instabilities in particle acceleration, and demonstrate how CRs can regulate their acceleration sites. This research is essential for developing subgrid models of kinetic processes and for interpreting the phenomenology of nonthermal sources, from heliospheric shocks to galaxy clusters. I will conclude by highlighting promising future directions and the exciting frontiers awaiting exploration.