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Context. The spiral arm tangencies are ideal lines of sight in which to determine the distribution of interstellar gas components in the spiral arms and study the influence of spiral density waves on the interarm gas in the Milky Way. [C ii] emission in the tangencies delineates the warm ionized component and the photon-dominated regions and is thus an important probe of spiral arm structure and dynamics. Aims. We aim to use [C ii], H i ,a nd 12 CO spectral line maps of the Crux, Norma, and Perseus tangencies to analyze the internal structure of the spiral arms in different gas layers. Methods. We used [C ii] l-V maps along with those for H i and 12 CO to derive the average spectral line intensity profiles over the longitudinal range of each tangency. Using the VLSR of the emission features, we located the [C ii], H i ,a nd 12 CO emissions along a cross cut of the spiral arm. We used the [C ii] velocity profile to identify the compressed warm ionized medium (WIM) in the spiral arm. Results. We present a large-scale (∼15 ◦ ) position-velocity map of the Galactic plane in [C ii] from l = 326. ◦ 6 to 341. 4 observed with Herschel HIFI. In the spectral line profiles at the tangencies, [C ii] has two emission peaks, one associated with the compressed WIM and the other the molecular gas photon-dominated regions. When represented as a cut across the inner to outer edge of the spiral arm, the [C ii]‐WIM peak appears closest to the inner edge while 12 CO and [C ii] associated with molecular gas are at the outermost edge. H i has broader emission with an intermediate peak located nearer to that of 12 CO. Conclusions. The velocity-resolved spectral line data of the spiral arm tangencies unravel the internal structure in the arms locating the emission lanes within them. We interpret the excess [C ii] near the tangent velocities as shock compression of the WIM induced by the spiral density waves and as the innermost edge of spiral arms. For the Norma and Perseus arms, we estimate widths of ∼250 pc in [C ii]‐WIM and ∼400 pc in 12 CO and overall spiral arm widths of ∼500 pc in [C ii ]a nd 12 CO emissions; in H i the widths are ∼400 pc and ∼620 pc for Perseus and Norma, respectively. The electron densities in the WIM are ∼0.5 cm −3 , about an order of magnitude higher than the average for the disk. The enhanced electron density in the WIM is a result of compression of the WIM by the spiral density wave potential.

Internal structure of spiral arms traced with [C II]: Unraveling the warm ionized medium, H I, and molecular emission lanes