χ² values for a grid of isochoric ISMDB models compared to the observed fine structure line intensities in the G1 filament. Blue circle and the elongated red area show best fits, respectively, to the [C II]/[O I] 145 µm data at positions M–O and [C II]/[O I] 63 µm data at positions G–L. The elongated magenta area shows the best fit to all data with [O I] 145 µm line intensities scaled down by a factor of 3, as described in the text. All models assume normal incidence. Increasing the incidence angle to 60° provides comparable fit, but shifts the best-fit solution to higher densities and lower χ values (magenta area).