Radio recombination lines from inner galaxy diffuse gas .2. The extended low-density warm ionized medium and the ''worm-ionized medium''

We have searched for 1.4 GHz radio recombination lines (RRLs) at 583 positions, mostly toward the galactic interior, and achieved detections at 418 positions. These data characterize the extended low-density warm ionized medium (the ELDWIM). We derive an electron temperature of 7000 K from a comparison of RRL and radio continuum, and estimate that non-LTE effects increase the line intensities by a factor of similar to 1.3. We examine the distribution of the ELDWIM with a velocity-longitude diagram and find some concentration into spiral arms. The ELDWIM is not particularly well correlated with strong H II regions. The azimuthally symmetric inner Galaxy ring component of Taylor & Cordes does not appear in RRL emission, and we propose a revised model in which its electrons are located farther out in spiral arms. We derive the ELDWIM filling factor phi(WIM) similar to 0.01 and an electron density in the emitting regions of similar to 5 cm(-3). A fraction of the diffuse radio continuum and 100 mu m IR emission is characterized by vertical structures that correspond well to the ''worm'' and ''chimney'' models, in which clustered supernovae blow large cavities in the gaseous disk that, for chimneys, connect to the gaseous halo. We interpret the RRL emission from these structures in terms of the ''worm-ionized medium'' (W-IM), in which the thermal radio emission arises in the worm walls; the walls are ionized by photons from hot stars in the cluster whose supernovae originally produced the cavity. The nearest example of a worm is the Orion/Eridanus cavity. The previously best-studied example is the Stockert chimney, which we argue is part of a much larger structure. The worms that have well-defined distances are closely confined to spiral arms. We discuss ionization requirements for worms and their associated H II regions and define three classes that describe the ionization of worm walls. These classes depend on the state of star formation in the central molecular cloud, which eventually dissipates. The global ionization requirement for all worm walls is small (similar to 20%) compared to the total requirement for the Galactic ELDWIM. High-\z\ CO may be associated with at least two worms.