Simone Benella — INAF - IAPS #
Application of the stochastic thermodynamics to space plasma physics: new insights on turbulence #
Turbulence is ubiquitous in space plasmas and arise from nonlinear dynamics as emergent collective behavior, from the largest scales of the energy injection to the smallest scales where dissipation occurs. The turbulent dynamics of velocity and magnetic field fluctuations in nearly collisionless plasmas, such as the solar wind, can be envisioned as a scale-to-scale Langevin process. This allows us to embed the statistics of magnetic field fluctuations in the framework of stochastic process theory, and then to resort to fundamental concepts of the recent theory of stochastic thermodynamics. Magnetic field increments as a function of the scale define the cascade trajectories (viz., the stochastic process) over which we have calculated the stochastic entropy variation. The total stochastic entropy produced along a trajectory can be expressed as the ratio between the path probability of the forward trajectory divided by the path probability of its reversal. Thus, the production of entropy expresses, on average, the imbalance of forward with respect to backward processes, which, in the case of turbulence, are proxies for direct and inverse cascades. By using the stochastic entropy we are able to identify two different regimes where fluctuations exhibit contrasting statistical properties. In the inertial range a net production of entropy is linked to an increase of the flatness, thus indicating the occurrence of  intermittency in the sample of fluctuations. On the other hand, cascade trajectories associated with a decrease of entropy are assimilated to a global scale invariance. In the transition region between inertial and ion scales the scenario reverses: trajectories characterized by ΔS<0 exhibit a sudden increase of the flatness due to small-scale intermittency, whereas trajectories with ΔS>0 show a constant flatness. Results suggest how the broad framework of stochastic thermodynamics can provide new insights in the field of space plasma turbulence, allowing us to perform a precise classification of cascading trajectories with opposite behavior based on their stochastic entropy production/consumption only.