2021ApJ...914..128C

The radio sky at frequencies below ∼10 MHz is still largely unknown; this remains the last unexplored part of the electromagnetic spectrum in astronomy. The upcoming space experiments aiming at such low frequencies (ultralong wavelengths or ultralow frequencies) would benefit from reasonable expectations of the sky brightness distribution at relevant frequencies. In this work, we develop a radio sky model that is valid down to ∼1 MHz. In addition to discrete H II objects, we take into account the free-free absorption by thermal electrons in the Milky Way's warm ionized medium. This absorption effect becomes obvious at <=10 MHz, and could make the global radio spectrum turn over at ∼3 MHz. Our sky map shows unique features at the ultralong wavelengths, including a darker Galactic plane in contrast to the sky at higher frequencies, and huge shadows of the spiral arms on the sky map. It will be a useful guide for designing future ultralong-wavelength observations. Our Ultralong-wavelength Sky Model with Absorption (ULSA) model can be downloaded at doi:10.5281/zenodo.4454153.