A real-time model of HF radio propagation conditions is being developed as a service for aircraft communications
at high latitudes. An essential component of this is a real-time map of the absorption of HF (3-30 MHz) radio signals
in the D-region ionosphere. Empirical, climatological Polar Cap Absorption (PCA) models in common usage
cannot account for day-to-day variations in ionospheric composition and are inaccurate during the large changes
in recombination rate at twilight. However, parameters of such models may be optimised using an age-weighted
regression to absorption measurements from riometers in Canada and Scandinavia. Such parameters include the
day- and night-time sensitivity to proton flux as measured on a geostationary satellite (GOES). Modelling the twilight
transition as a linear or Gauss error function over a range of solar-zenith angles (χl < χ < χu) is found to
provide greater accuracy than ‘Earth shadow’ methods (as applied in the Sodankylä Ionospheric Chemistry (SIC)
model, for example) due to a more gradual ionospheric response for χ < 90°. The fitted χl parameter is found to
be most variable, with smaller values (as low as 60°) post-sunrise compared with pre-sunset. Correlation coefficients
of model parameters between riometers are presented and these provide a means of appropriately weighting
individual riometer contributions in an assimilative PCA model.
At times outside of PCA events, the probability of absorption in the auroral zones is related to the energetic electron
flux inside the precipitation loss cone, as measured on the polar-orbiting POES satellites. This varies with magnetic
local time, magnetic latitude and geomagnetic activity, and its relation to the real-time solar wind – magnetospheric
coupling function [Newell et al., 2007] will be presented.