Dissipation

F. Califano, G. Arrò Dipartimento di Fisica "E. Fermi", Università di Pisa, Pisa, Italy S.S. Cerri Department of Astrophysical Sciences, Princeton University, Princeton, 08544 USA

It has been observed experimentally the occurrence of a new process, namely electron-only reconnection, where the reconnection dynamics is driven only by electrons (e-rec-only) [1]. Recently, a theoretical study in the context of plasma magnetized turbulence has given evidence about the possibility to drive e-rec-only by fluctuations at scales of the order of the ion scale length [2] (see Faganello abstract, this Conference). By considering two Vlasov simulations of magnetized plasma turbulence where “standard” reconnection or e-rec-only separately occur, we make a compared study of the turbulence statistical properties, in particular of the structure functions in order to separate the contribution of the ions at the so-called dissipative scale. We found, in agreement with experimental [3] and theoretical [4] studies a non-Gaussian statistics in both the fluid and sub-ion range with a transition from an intermittent to a self-similar behavior. Our main finding here is that the transition is observed at a scale length of the order of several de instead that around di independently from the ion dynamics. The transition seems to be driven mainly by the small scale electron dynamics around the reconnection structures where the electron inertial terms become non-negligible.

[1] T.D. Phan et al., Electron magnetic reconnection without ion coupling in Earth’s turbulent magnetosheath, Nature, 2018

[2] F. Califano, S.S. Cerri, M. Faganello, D. Laveder, M. W. Kunz, Electron-only magnetic reconnection in plasma turbulence, Astrophys. Journal, submitted

[3] Kiyani et al., Global Scale-Invariant Dissipation in Collisionless Plasma Turbulence, Phys. Rev. Lett., 2009

[4] E. Leonardis et al., Multifractal scaling and intermittency in hybrid Vlasov-Maxwell simulations of plasma turbulence, Physics of Plasmas, 2016

• This contribution has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 776262 (AIDA, www.aida-space.eu)

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.