Map of ionization fraction across Orion B. We show the f_e at 120 arcseconds (0.24 pc) derived using W(C₂H)/W(HNC) in translucent gas and in regions of A_v between 6 mag and 10 mag, and W(C¹⁸O)/W(HCO⁺)in dense gas. In diffuse medium (A_v<2) and in region where we do not detect any W(C₂H)/W(HNC), we set the ionization fraction to be 10⁻⁴. The black contours show the boundaries of Orion B, defined from W(¹³CO(1−0)) emission.