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The spiral arm tangencies are ideal lines of sight in which to determine thedistribution of interstellar gas components in the spiral arms and study theinfluence of spiral density waves on the interarm gas in the Milky Way. Wepresent a large scale (~15deg) position-velocity map of the Galactic plane in[CII] from l = 326.6 to 341.4deg observed with Herschel HIFI. We use [CII] l-vmaps along with those for Hi and 12CO to derive the average spectral lineintensity profiles over the longitudinal range of each tangency. Using the VLSRof the emission features, we locate the [CII], HI, and 12CO emissions along across cut of the spiral arm. In the spectral line profiles at the tangencies[CII] has two emission peaks, one associated with the compressed WIM and theother the molecular gas PDRs. When represented as a cut across the inner toouter edge of the spiral arm, the [CII]-WIM peak appears closest to the inneredge while 12CO and [CII] associated with molecular gas are at the outermostedge. HI has broader emission with an intermediate peak located nearer to thatof 12CO. The velocity resolved spectral line data of the spiral arm tangenciesunravel the internal structure in the arms locating the emission lanes withinthem. We interpret the excess [CII] near the tangent velocities as shockcompression of the WIM induced by the spiral density waves and as the innermostedge of spiral arms. For the Norma and Perseus arms, we estimate widths of ~250pc in [CII]-WIM and ~400 pc in 12CO and overall spiral arm widths of ~500 pc in[CII] and 12CO emissions. The electron densities in the WIM are ~ 0.5 cm^-3,about an order of magnitude higher than the average for the disk. The enhancedelectron density in the WIM is a result of compression of the WIM by the spiraldensity wave potential.

Internal structure of spiral arms traced with [CII]: Unraveling the WIM, HI, and molecular emission lanes