Microscopic Current Sheets and Fast Tearing Modes in Plasma Turbulence

Abstract Since the seminal work by W. H. Matthaeus & S. L. Lamkin, a large amount of evidence has been collected over the years that magnetic reconnection can disrupt current sheets formed in turbulence. The details about how this happens, however, are not clear, yet. The observation of plasmoids suggests that tearing-type modes are involved, but their nature of spontaneous linear instabilities developing on a static (or at most steady) magnetic equilibrium poses strong constraints on their growth rate versus the timescale of the current sheet evolution. None of the tearing-based scenarios, which to date are most credited in literature, seems to fulfill both this constraint and other consistency requirements on the equilibrium profile. In revising them and the main hypotheses, which any tearing-based theory for 2D turbulent reconnection must satisfy, we propose a possible explanation—supported by numerical calculations—for why tearing modes may be relevant. This explanation is grounded on the microscopic thickness that current sheets attain in turbulence, which makes the growth rates of tearing modes large enough for the instability to possibly develop. At the same time, this implies that theoretical growth rates obtained from a boundary layer analysis cannot be applied in this case. We discuss a few implications of these elements in solar wind turbulence and in comparison with alternative models for tearing-based turbulent reconnection that are available in literature.