Arc widths were calculated for 3126 stable auroral arcs observed by an all-sky camera located in Gillam, Manitoba. The camera is filtered to accept 5577-¿ emissions and has a single-pixel spatial resolution of 1.7 km at zenith. The measured mean width of stable mesoscale arcs located within ¿ 5¿ of magnetic zenith is 18 km with a standard deviation of 9 km. The width distribution exhibits a steep cutoff below 8 km; when combined with studies of small-scale auroral structure this cutoff suggests a gap in the occurrence of arcs with widths of order 1 km. This feature of the arc width spectrum argues against the notion of a turbulent cascade of energy from larger to small scales. Residuals from the Gaussian fits are only about 3% of the arc amplitude on average, indicating little sub-structure within arcs at scales down to the measurement resolution. ¿ 2001 American Geophysical Union