On the violation of the zeroth law of turbulence in space plasmas
Abstract
The zeroth law of turbulence states that, for fixed energy input into largescale motions, the statistical steady state of a turbulent system is independent of microphysical dissipation properties. This behaviour, which is fundamental to nearly all fluidlike systems from industrial processes to galaxies, occurs because nonlinear processes generate smaller and smaller scales in the flow, until the dissipation  no matter how small  can thermalise the energy input. Using direct numerical simulations and theoretical arguments, we show that in strongly magnetised plasma turbulence such as that recently observed by the Parker Solar Probe spacecraft, the zeroth law is routinely violated. Namely, when such turbulence is `imbalanced'  when the largescale energy input is dominated by Alfvénic perturbations propagating in one direction (the most common situation in space plasmas)  nonlinear conservation laws imply the existence of a `barrier' at scales near the ion gyroradius. This causes energy to build up over time at large scales. The resulting magneticenergy spectra bear a strong resemblance to those observed in situ, exhibiting a sharp, steep kinetic transition range above and around the ionLarmor scale, with flattening at yet smaller scales. The effect thus offers a possible solution to the decadelong puzzle of the position and variability of ionkinetic spectral breaks in plasma turbulence. The existence of the `barrier' also suggests that, how a plasma is forced at large scales (the imbalance) may have a crucial influence on thermodynamic properties such as the iontoelectron heating ratio.
 Publication:

Journal of Plasma Physics
 Pub Date:
 May 2021
 DOI:
 10.1017/S0022377821000489
 arXiv:
 arXiv:2009.02828
 Bibcode:
 2021JPlPh..87c5301M
 Keywords:

 space plasma physics;
 astrophysical plasmas;
 plasma nonlinear phenomena;
 Physics  Space Physics;
 Astrophysics  Solar and Stellar Astrophysics;
 Nonlinear Sciences  Chaotic Dynamics;
 Physics  Plasma Physics
 EPrint:
 doi:10.1017/S0022377821000489