Princeton Center for Heliophysics Seminar

Abstract: Reconnection is regularly observed at current sheets in the solar wind at 1AU, and the scalings of these current sheets suggest that they are dynamically formed by the turbulence itself. Theoretical considerations suggest that reconnection onset in such sheets occurs when the growth rate of the tearing instability becomes comparable to the characteristic timescale of the turbulent fluctuations, leading to a dramatic change in the turbulent dynamics at a characteristic "disruption scale". On the other hand, in the near-Sun Alfvénic solar wind observed by Parker Solar Probe, ubiquitous small-scale current sheets produced by the turbulence are detected, but reconnection within these sheets appears to be extremely rare. Close to the Sun in the Alfvénic wind, the cross-helicity of the fluctuations is extremely high: every magnetic shear is accompanied by a corresponding velocity shear, with δu ∼ δB/√4πn_im_i; this also applies to the dynamically generated current sheets. We present new theoretical results showing that the tearing instability is suppressed in this case, in both resistive MHD and in a collisionless plasma, and discuss how this will affect the dynamics of the turbulence close to the Sun.