Statistics of the apex expansion in the elongated (cyan) and shortened (orange) directions. The top plot shows the number of flux tubes that fall in the a _apx and b _apx bins and the bottom plot shows the total flux in each bin. The black curve for each plot corresponds to what the expansion would have been if the cross section was circular, that is, it corresponds to . The cross-sectional radius at the base is r ₀ = a ₀ = b ₀ = 0.5 pixels and is shown with a dashed vertical line.