An Ultra-long Wavelength Sky Model with Absorption Effect
  The radio sky at frequencies below $\sim10$ 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
(ultra-long wavelength or ultra-low frequency) 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 $\sim1$ MHz. In
addition to the discrete HII objects, we take into account the free-free
absorption by thermal electrons in the Milky Way's warm ionized medium (WIM).
This absorption effect becomes obvious at $\lesssim10$ MHz, and could make the
global radio spectrum turn over at $\sim3$ MHz. Our sky map shows unique
features at the ultra-long wavelengths, including a darker Galactic plane in
contrast to the sky at higher frequencies, and the huge shadows of the spiral
arms on the sky map. It would be a useful guidance for designing the future
ultra-long wavelength observations. Our Ultralong-wavelength Sky Model with
Absorption (ULSA) model could be downloaded at
https://doi.org/10.5281/zenodo.4454153.
