The spiral arm-interarm interaction is an important process in the dynamics of galactic arms and the formation of interstellar clouds, as it initiates the transition from the highly ionized warm ionized medium (WIM) to neutral gas clouds. The far-infrared emission lines of [CII] and [NII] are important probes of these processes. We use high spectral resolution observations of [CII] 158-micron and [NII] 205-micron fine structure lines taken with the upGREAT and GREAT instruments on SOFIA, along with auxiliary HI and 13CO observations to study the ionized gas in the Scutum arm. The observations consist of samples in and out of the Galactic plane along 18 lines of sight between longitude 30 and 32 degrees towards the Scutum spiral arm tangency, which is a unique location to disentangle the spiral arm gas components using their velocity profiles. The observations and analysis show clear detections of the WIM along the inner edge of the spiral arm and the ionized boundary layers of the denser neutral clouds, and thus reveal the evolution of the WIM and the nature of the arm-interarm interaction.
Spiral Arm Structure:[NII] and [CII] in the Scutum Arm