Dimensionless ionization parameter, U, estimated for each radial profile plotted against its electron density. The correlation between density and U indicates that the filament properties depend on the incident radiation field. No correlation as a function of density would indicate that other (feedback) processes influence the physical properties (density) of the filamentary structure. Red (blue) data points are those that fall within the red (blue) regions defined in Fig. 7 and correspond to the filament profiles in locations that have (do not have) a local massive star that likely contributes to ionization. Typical error bars are about 0.3 dex.