Abstract We investigate the development of tearing-mode instability using the highest-resolution 2D magnetohydrodynamic simulations of reconnecting current sheets performed on a uniform grid, for Lundquist numbers of 10 3 ≤ S ≤ 5 × 10 5 , reaching up to 65,536 2 grid cells. We demonstrate a Sweet–Parker scaling of the reconnection rate V rec ∼ S −1/2 up to Lundquist numbers S ∼ 10 4 . For larger values of Lundquist number, between 2 × 10 4 ≤ S ≤ 2 × 10 5 , plasmoid formation sets in, leading to a slight enhancement of the reconnection rate, V rec ∼ S −1/3 , consistent with the prediction from linear-tearing-mode-induced reconnection, indicating that reconnection remains resistivity-dependent and therefore slow. In this range of S -values, the plasmoids do not undergo a merger cascade, as they are rapidly advected out of the reconnection layer. Only for S > 2 × 10 5 , we observe the nonlinear development of the tearing-mode instability, with plasmoid coalescence and a saturation of the reconnection rate at V rec / V A ∼ 0.01. At such high S , however, the corresponding Reynolds number is large, reaching Re > 2000 even on scales comparable to the current-sheet thickness. We therefore conclude that, in astrophysical systems, it is essential to account for the dominant influence of turbulence and 3D effects in the reconnection process.
Vicentin et al. (2026) studied this question.