Title: Decay Laws of Helical and Nonhelical Magnetically-Dominated MHD Turbulence

Speaker: Dr. Hongji Zhou (Shanghai Jiao Tong University)

Time: 14:00pm, October 19, 2023

Location: 3-402, PMO Xianlin Campus

Abstract: The absence of GeV halos around TeV blazars implies a lower bound of the inter-galactic magnetic fields which could be a result of decaying primordial fields from the early Universe. However, the inferred decay rate is slower than what theorists have expected. A refined theory of decaying MHD turbulence by Hosking & Schekochihin (Phys. Rev. X, vol. 11, issue 4, 2021, 041005) potentially resolves the issue by proposing a new conserved quantity called the Hosking integral. In this talk I'll first introduce the general concepts of decaying turbulence, and then explain the role played by the Hosking integral and how the decay rate is derived. Our high-resolution simulations supports Hosking's theory that (i) the Hoksing integral is an ideal invariant, and (ii) the decay is governed by the reconnection time scale rather than the Alfvén time scale, justifying its relevance in understanding primordial magnetic fields.