Plasma Physics Seminar

Structure formation in magnetohydrodynamic turbulence as a modulational instability

Speaker Name: Suying Jin, Princeton University

Abstract :
Structure formation in magnetohydrodynamic (MHD) turbulence can be described as an modulational instability (MI) of the fluctuations comprising the turbulent background. In the first part of this talk, I will discuss a framework in which the turbulent fluctuations are modeled as an effective plasma of quasi-particles, and coherent structures self-consistently emerge as mean- fields with and through which these quasi-particles interact. By treating this fluctuation/mean-field system with a quantum-like kinetic theory (Wigner-Moyal formalism), scale separation between the fluctuations and mean-fields need not be invoked. Although this approach provides a powerful analytical framework in regimes where the underlying quasilinear approximation (QLA) holds, this popular and often reliable closure must be carefully examined for MHD. In the second part of this talk I will discuss an extended quasilinear theory (XQLT) that considers modulations of a simple background such that the full chain of modulational harmonics can be retained. This allows for a systematic examination of the validity of simple closures, while revealing the rich modulational dynamics beyond the QLA. In particular, we find propagating spectral waves (PSWs) which carry energy ballistically to higher harmonics, thereby driving deviations from the QLA. Finally, I will show that corrections to ideal incompressible MHD suppress PSWs and reinstate the validity of the QLA.