The Most Sensitive Radio Recombination Line Measurements Ever Made of the Galactic Warm Ionized Medium

Abstract Diffuse ionized gas pervades the disk of the Milky Way. We detect extremely faint emission from this Galactic warm ionized medium (WIM) using the Green Bank Telescope to make radio recombination line (RRL) observations toward two Milky Way sight lines: G20, ( ℓ , b ) = (20°, 0°), and G45, ( ℓ , b ) = (45°, 0°). We stack 18 consecutive Hn α transitions between 4.3 and 7.1 GHz to derive 〈Hn α 〉 spectra that are sensitive to RRL emission from plasmas with emission measures EM ≳ 10 cm −6 pc. Each sight line has two Gaussian-shaped spectral components with emission measures that range between ∼100 and ∼300 cm −6 pc. Because there is no detectable RRL emission at negative LSR velocities, the emitting plasma must be located interior to the solar orbit. The G20 and G45 emission measures imply rms densities of 0.15 and 0.18 cm −3 , respectively, if these sight lines are filled with homogeneous plasma. The observed 〈Hn β 〉/〈Hn α 〉 line ratios are consistent with LTE excitation for the strongest components. The high-velocity component of G20 has a narrow line width, 13.5 km s −1 , that sets an upper limit of ≲4000 K for the plasma electron temperature. This is inconsistent with the ansatz of a canonically pervasive, low-density, ∼10,000 K WIM plasma.