Examining the Role of Dispersion Relation and Collision Frequency
Formulations on Estimation of Shortwave--Fadeout
Abstract
Over–the–Horizon (OTH) communication is strongly dependent on the
state of the ionosphere, which is susceptible to solar flares.
Trans–ionospheric high frequency (HF) signals experience a strong
attenuation following a solar flare, commonly referred to as Short–Wave
Fadeout (SWF). In this study, we examine the role of dispersion
relation–collision frequency formulations on the estimation of
flare–driven HF absorption seen in Riometer observation using a data
assimilation framework. Specifically, the framework first uses modified
solar irradiance models (such as EUVAC, FISM), which incorporate
high–resolution solar flux data from GOES satellite X-ray sensors, to
compute the enhanced ionization produced during the flare events. The
framework then uses different dispersion relation–collision frequency
formulations to estimate enhanced HF absorption. Finally, the modeled HF
absorption is compared against the data to determine which combination
of dispersion relation–collision frequency formulation best reproduces
the Riometer observations. From the modeling work, we find that the
Appleton–Hartree dispersion relation in combination with Schunk–Nagy
collision frequency profile produces the best agreement with Riometer
data.