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The radio sky at frequencies below $\sim10$ MHz is still largely unknown,this remains the last unexplored part of the electromagnetic spectrum inastronomy. The upcoming space experiments aiming at such low frequencies(ultra-long wavelength or ultra-low frequency) would benefit from reasonableexpectations of the sky brightness distribution at relevant frequencies. Inthis work, we develop a radio sky model that is valid down to $\sim1$ MHz. Inaddition to the discrete HII objects, we take into account the free-freeabsorption by thermal electrons in the Milky Way's warm ionized medium (WIM).This absorption effect becomes obvious at $\lesssim10$ MHz, and could make theglobal radio spectrum turn over at $\sim3$ MHz. Our sky map shows uniquefeatures at the ultra-long wavelengths, including a darker Galactic plane incontrast to the sky at higher frequencies, and the huge shadows of the spiralarms on the sky map. It would be a useful guidance for designing the futureultra-long wavelength observations. Our Ultralong-wavelength Sky Model withAbsorption (ULSA) model could be downloaded athttps://doi.org/10.5281/zenodo.4454153.

An Ultra-long Wavelength Sky Model with Absorption Effect