A Wave-Coherent Structure Duality in Plasma Turbulence: Are They Two Sides of the Same Coin?

Gregory_G. Howes
Department of Physics and Astronomy, University of Iowa, 505 Van Allen Hall, Iowa City, IA 52242, USA
The dynamics and dissipation of turbulence in weakly collisional space plasmas throughout the heliosphere remains a controversial topic at the forefront of space physics research. Both fluid and kinetic simulations of plasma turbulence ubiquitously generate coherent structures---in the form of current sheets---at small scales, and the locations of these current sheets appear to be associated with enhanced rates of dissipation of the turbulent energy. The quest to understand the physical mechanisms by which the energy of turbulent fluctuations is converted to particle energy or plasma heat has driven vigorous debate about the relative roles of wave damping processes vs. localized dissipation mechanisms associated with current sheets, such as magnetic reconnection. A major unanswered question is how these coherent structures arise in the first place. Recent analytical and numerical work has demonstrated that strongly nonlinear interactions among counterpropagating Alfvén wavepackets---known as Alfvén wave collisions---naturally generate current sheets self-consistently. Subsequent work has shown that the dissipation of the turbulent energy is localized near these current sheets but is clearly mediated through the process of collisionless Landau damping. Together, these results suggest that framing the debate as a choice between waves or coherent structures may be a false dichotomy. Rather, is there a duality between wave or coherent structure descriptions of the turbulence? Are they merely alternative descriptions of the same dynamics? I will close with the question about whether there exist aspects of the turbulence that cannot be described as either waves or structures.