  HIFI GOT C+ Galactic plane [CII] spectral survey has detected strong emission
at the spiral arm tangencies. We use the unique viewing geometry of the
Scutum-Crux (S-C) tangency near i = 30degs to detect the warm ionized medium
(WIM) component traced by [CII] and to study the effects of spiral density
waves on Interstellar Medium (ISM) gas. We compare [CII] velocity features with
ancillary HI, 12CO and 13CO data near tangent velocities at each longitude to
separate the cold neutral medium and the warm neutral + ionized components in
the S-C tangency, then we identify [CII] emission at the highest velocities
without any contribution from 12CO clouds, as WIM. We present the GOT C+
results for the S-C tangency. We interpret the diffuse and extended excess
[CII] emission at and above the tangent velocities as arising in the
electron-dominated warm ionized gas in the WIM. We derive an electron density
in the range of 0.2 - 0.9 cm^-3 at each longitude, a factor of several higher
than the average value from Halpha and pulsar dispersion. We interpret the
excess [CII] in S-C tangency as shock compression of the WIM induced by the
spiral density waves.

Astrophysics of Galaxies