Lecture Series on Geophysics and Astrophysics Fluid Mechanics

Report Title: New Perspectives on Large Scale Dynamos in Helsinki and Non Helsinki

Speaker: Dr. Zhou Hongzhe (Li Zhengdao Research Institute)

Report location: Conference room on the third floor

Report time: December 13, 2024 (Friday) at 10:00 AM

Tencent Meeting: 656-580-793

Report Summary:

Magnetic fields are ubiquitous in astrophysical systems, with notable examples including planets, the Sun, galaxies, and accretion disks in AGNs and X-ray binaries. The amplification of magnetic fields on scales larger than the turbulence scale is driven by a process known as the large-scale dynamo (LSD). Traditionally, LSDs rely on the presence of a net kinetic helicity, i.e., the alignment between the velocity and the vorticity fields. However, whether this mechanism remains efficient at the very high magnetic Reynolds numbers (Rm) relevant to astrophysical conditions has been a long-standing question. More recently, a novel type of LSD has been identified that operates without requiring kinetic helicity, though its underlying mechanism remains unclear. In this talk, I will present our recent findings on (i) the behavior of helical LSDs at high Rm, and (ii) the mechanisms driving nonhelical LSDs. I will also briefly discuss their implications for accretion disk magnetization and variability in AGNs.