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The thickness of current sheets is extremely important, especially as itrelates to the onset of fast magnetic reconnection. Onset determines how muchmagnetic free energy can build up in a field before it is explosively released.This has implications for many phenomena on the Sun and throughout theuniverse, including the heating of the solar corona. Significant effort hasbeen devoted to the question of whether equilibrium current sheets in realisticgeometries have finite or zero thickness. Using a simple force balanceanalysis, we show why current sheets without a guide field (2D) and with aguide field that is invariant in the guide field direction (2.5D) cannot be inequilibrium if they have both finite thickness and finite length. We thenestimate the conditions under which the tension of a curved line-tied guidefield can facilitate equilibrium in 3D sheets that are finite in alldimensions. Finally, we argue that some quasi-statically evolving currentsheets undergoing slow stressing (e.g., when the coronal magnetic field issubjected to photospheric boundary driving) may reach a critical shear, atwhich point they lose equilibrium, spontaneously collapse, and reconnect. Thecritical shear is generally consistent with the heating requirements of solaractive regions.

The Thickness of Electric Current Sheets and Implications for Coronal Heating