  Based on ray tracing in a smooth ionosphere described by the IRI-2016 model
we have infered the seasonal-diurnal dynamics of radio noise observed by four
mid-latitude HF radars. In the calculations, noise is assumed to propagate from
the radar dead zone boundary. Noise absorption along the ray path is simulated
from the IRI-2016 electron density, and from the molecular nitrogen density and
electron temperatures obtained from the NRLMSISE-00 model. Model results are
compared with experimental radar data, and good agreement between the two is
demonstrated. The model makes it possible to estimate the amount of absorption
in D- and E- layers under average undisturbed conditions. This is important for
the retrieval of long term variations in the electron density in the lower
ionosphere. The model also makes it feasible to interpret vertical absorption
in experimental data, thereby significantly expanding the capability of HF
radars to monitor the lower ionosphere.
