Improving the twilight model for polar cap absorption nowcasts

During solar proton events (SPE), energetic protons ionize the polar mesosphere causing HF radio wave attenuation, more strongly on the dayside where the effective recombination coefficient, αeff, is low. Polar cap absorption models predict the 30 MHz cosmic noise absorption, A, measured by riometers, based on real‐time measurements of the integrated proton flux‐energy spectrum, J. However, empirical models in common use cannot account for regional and day‐to‐day variations in the daytime and nighttime profiles of αeff(z) or the related sensitivity parameter, m=A/J. Large prediction errors occur during twilight when m changes rapidly, and due to errors locating the rigidity cutoff latitude. Modeling the twilight change in m as a linear or Gauss error‐function transition over a range of solar‐zenith angles (χl