Twenty-two theoretical mechanisms for auroral arcs are examined (12 electron-acceleration mechanisms and 10 generator mechanisms) and a characteristic auroral-arc thickness is worked out for each mechanism except one. The arc thicknesses are then mapped down to the ionosphere along the terrestrial magnetic-field lines: near the Earth a dipole magnetic-field model is used and farther from the Earth the mapping includes the effects of magnetic-field-line draping. The predicted thicknesses are compared with published ground-based measurements of the optical thicknesses of auroral arcs in the ionosphere, which typically find arcs to be 100 m wide. The 22 theoretical models all predict auroral-arc thicknesses that are at least an order of magnitude wider than the optically observed arcs. As an alternative explanation of the optical observations of narrow auroral arcs, the acceleration of ionospheric electrons to produce airglow in electrical-discharge mechanisms is explored: these electrical-discharge mechanisms are found to be improbable. Also explored is the possibility that the observed narrow auroral arcs are caused by interference effects when Alfven waves reflect off the ionosphere: this is found to be an unlikely explanation. Suggestions are made for future ground-based auroral-arc measurements. ¿American Geophysical Union 1993