During daylight hours (and other more occasional events) D-region absorption increases immensely for VLF, LF, MW, and lower HF frequencies. The frequency of interest passing through the D-region is very important; the daytime D-region attenuation is hundreds of dB .
 Sir Edward Appleton, W.R. Piggott, Ionospheric absorption measurements during a sunspot cycle, J. Atm. Terr. Phys., (5)1, 1954, 141-172, http://dx.doi.org/10.1016/0021-9169(54)90029-X.