Publication Date:
2016-10-15
Description:
During Solar Proton Events (SPE), energetic protons ionize the polar mesosphere causing HF radiowave attenuation, more strongly on the dayside where the effective recombination coefficient, α eff , is low. Polar cap absorption (PCA) 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 day- and nighttime profiles of α eff ( z ) or the related sensitivity parameter, . 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 ( χ l 〈 χ 〈 χ u ) provides a better fit to measurements than selecting day or night α eff profiles based on the Earth-shadow height. Optimal model parameters were determined for several polar cap riometers for large SPEs in 1998-2005. The optimal χ l parameter was found to be most variable, with smaller values (as low as 60°) post-sunrise compared with pre-sunset, and with positive correlation between riometers over a wide area. Day and night values of m exhibited higher correlation for closely spaced riometers. A nowcast simulation is presented in which rigidity boundary latitude and twilight model parameters are optimized by assimilating age-weighted measurements from 25 riometers. The technique reduces model bias, and root-mean-squared errors are reduced by up to 30% compared with a model employing no riometer data assimilation.
Print ISSN:
1539-4964
Electronic ISSN:
1542-7390
Topics:
Geosciences
,
Physics
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