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, https://dx.doi.org/10.1016/0021-9169(54)90029-X.
- Abraham, Saji, et al. “Ionospheric absorption on October 24, 1995 solar eclipse.” Geophysical research letters 25.15 (1998): 2945-2947. https://doi.org/10.1029/98GL01781