2022ApJ...940..180C

The free-free absorption of low-frequency radio waves by thermal electrons in the warm ionized medium of our Galaxy becomes very significant at ≲10 MHz (ultralong wavelength), and the absorption strength depends on the radio frequency. Upcoming space experiments such as the Discovering Sky at the Longest Wavelength and Farside Array for Radio Science Investigations of the Dark Ages and Exoplanets will produce high-resolution multifrequency sky maps at the ultralong wavelength, providing a new window to observe the universe. In this Paper we propose that from these ultralong-wavelength multifrequency maps, the 3D distribution of the Galactic electrons can be reconstructed. This novel and robust reconstruction of the Galactic electron distribution will be a key science case of those space missions. Ultralong-wavelength observations will be a powerful tool for studying the astrophysics relevant to the Galactic electron distribution, for example, the impacts of supernova explosions on electron distribution, and the interaction between interstellar atoms and ionizing photons escaped from the H II regions around massive stars.