Connecting nonlinear Alfvénic interactions to turbulent space plasma

 From the solar wind to more distant astrophysical plasmas, magnetohydrodynamic (MHD) turbulence is ubiquitous in our universe. Interactions between Alfvénic modes within these turbulent systems transfer energy from large to small spatial scales, but the relationship between the turbulence and the underlying Alfvénic fluctuations remains an active subject of research. In this seminar, I will first review the physics of nonlinear Alfvénic interactions in a simple incompressible MHD model and then discuss an important feature of MHD turbulence that is not consistent with linear Alfvén waves: excess energy in magnetic fluctuations compared to velocity fluctuations. The presence of this “residual energy” suggests a key role for nonlinear modes that do not satisfy the Alfvén wave dispersion relation but still retain some Alfvénic properties. We generate these Alfvénic quasimodes via the nonlinear interaction of counter-propagating Alfvén waves in a laboratory experiment, and our measurements demonstrate that the observed quasimodes can contain either excess magnetic or excess kinetic energy [Abler, et. al, in prep]. We show theoretically that net excess magnetic energy, as observed in the solar wind, can arise over many nonlinear interactions as a consequence of initial and boundary conditions [Dorfman, et. al. ApJ 2025]. The need to produce more astrophysicaly relevant conditions in the laboratory to open a new area of research for future turbulence studies will also be discussed [Dorfman, et. al. JPP 2025].