Correlation between τ₂₅₀ and with β = 1.8 for the main filament area shown in Fig. 2. τ₂₅₀ map is convolved to the same 60 resolution as map. The data are fitted with a straight line, using data in the ranges τ_J = 0–2 (black line) and τ_J = 2–4 (red line). The values of the slopes, k, are indicated in the frame with corresponding colours.