  Ionized interstellar gas is an important component of the interstellar medium
and its lifecycle. The recent evidence for a widely distributed highly ionized
warm interstellar gas with a density intermediate between the warm ionized
medium (WIM) and compact HII regions suggests that there is a major gap in our
understanding of the interstellar gas. Here we investigate the properties of
the dense warm ionized medium (D-WIM) in the Milky Way using spectrally
resolved SOFIA GREAT [NII] 205 micron line emission and Green Bank Telescope
hydrogen radio recombination lines (RRL) data, supplemented by Herschel PACS
[NII] 122 micron data, and spectrally resolved 12CO. We observed eight lines of
sight in the 20deg <l < 30deg region in the Galactic plane. We derived the
kinetic temperature, and the thermal and turbulent velocity dispersions from
the [NII] and RRL linewidths. The regions with [NII] 205 micron emission are
characterized by electron densities, n(e) ~ 10 to 35 cm(-3), temperatures from
3400 to 8500 K, and column densities N(N+) ~ 7e16 to 3e17 cm(-2). The ionized
hydrogen column densities range from 6e20 to 1.7e21 cm(-2) and the fractional
nitrogen ion abundance x(N+) ~1 to 3e-4, implying an enhanced nitrogen
abundance at ~ 4.3 kpc from the Galactic Center. The [NII] 205 micron emission
coincides with CO emission, although often with an offset in velocity, which
suggests that the D-WIM gas is located in, or near, star-forming regions, which
themselves are associated with molecular gas. These dense ionized regions are
found to contribute > 50% of the observed [CII] intensity along these LOS. The
kinetic temperatures we derive are too low to explain the presence of N+
resulting from electron collisional ionization and/or proton charge transfer of
atomic nitrogen. Rather, these regions most likely are ionized by extreme
ultraviolet radiation.
