Geophysical & Astrophysical Fluid Dynamics Seminar Series

Title：New perspectives on helical and nonhelical large-scale dynamos

Speaker：Dr. Hongzhe Zhou (Tsung-Dao Lee Institute)

Location: Middle conference room, 3rd floor

Time：10:00 am Dec. 13, 2024 (Fri)

Tencent meeting: 656-580-793

Abstract:

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.

Bio:

Hongzhe Zhou obtained his B.S. degree at Fudan University and Ph.D. degree at University of Rochester. From 2020 to 2022, he was a postdoc fellow at Nordita. Since 2022, he has been a Siyuan postdoc fellow at Tsung-Dao Lee Institute. Hongzhe's research interest includes magnetohydrodynamics and plasma physics in accretion flows around compact objects, and also extends to broader astrophysical contexts like stars, galaxies and cosmic magnetic fields.