Recent events from the Magneto-Fluid Dynamics Seminar are listed below. To see the complete list of events, visit the seminar's main page, here.
Tuesday, February 20, 201811AM, Warren Weaver Hall 905
Wave kinetic equation in a nonstationary and inhomogeneous medium with a weak quadratic nonlinearity
Daniel Ruiz, Sandia National Laboratory
In this talk, I present a systematic derivation of a wave kinetic equation describing the dynamics of a statistically inhomogeneous incoherent wave field in a medium with a weak quadratic nonlinearity. The medium can be nonstationary and inhomogeneous. Primarily based on the Weyl phase-space representation, the derivation makes use of the well-known ordering assumptions of geometrical optics and of a statistical closure based on the quasinormal approximation. The resulting wave kinetic equation describes the wave dynamics in the ray phase space. It captures linear effects, such as refraction, linear damping, and external sources, as well as nonlinear wave scattering. This general formalism could potentially serve as a stepping stone for future studies of weak wave turbulence interacting with mean fields in nonstationary and inhomogeneous media. In particular, I demonstrate how the general formalism can be applied to the study of interacting drift-wave turbulence and zonal flows in plasmas.
Tuesday, January 30, 201811AM, Warren Weaver Hall 905
Magnetic Reconnection in Three Dimensional Space
Allen Boozer, Columbia University
The breaking of magnetic field line connections is of fundamental importance in essentially all applications of plasma physics: laboratory to astrophysics. For sixty years the theory of magnetic reconnection has been focused on two-coordinate models. When dissipative time scales far exceed natural evolution times, such models are not realistic for ordinary three dimensional space. The ideal (dissipationless) evolution of a magnetic field is shown to in general lead to a state in which the magnetic field lines change their connections on an Alfvénic (inertial), not resistive, time scale. Only a finite mass of the lightest current carrier, the electron, is required. During the reconnection, the gradient in j_||/B relaxes while conserving magnetic helicity in the reconnecting region. This implies a definite amount of energy is released from the magnetic field and transferred to shear Alfvén waves, which in turn transfer their energy to the plasma. When there is a strong non-reconnecting component of the magnetic field, called a guide field, j_||/B obeys the same evolution equation as that of an impurity being mixed into a fluid by stirring. Although the enhancement of mixing by stirring has been recognized by every cook for many millennia, the analogous effect in magnetic reconnection is not generally recognized. An interesting mathematical difference is a three-coordinate model is required for the enhancement of magnetic reconnection while only two coordinates are required in fluid mixing. The issue is the number of spatial coordinates required to obtain an exponential spatial separation of magnetic field lines versus streamlines of a fluid flow.