Energetic electron precipitation is a source of both ionospheric and geomagnetic disturbances. The resultant increased ionization in the auroral oval leads to the absorption of high frequency radio waves in the auroral zone, or auroral absorption. Auroral absorption is typically characterized by global geomagnetic activity indices, such as the Kp index. In this paper the hourly range of the magnetic field (HR) is examined as an alternative to the 3-hour Kp index for describing the dynamic and localized features of auroral absorption represented by the hourly range of absorption (HRA). Kp, magnetometer, and riometer data were examined for a 3-year period for stations spread across typical auroral latitudes. A general linear relationship was shown to exist between Kp and LOG(HRA) for Kp<4; for Kp≥4 the correlation was weaker. A stronger linear correlation was demonstrated between LOG(HRA) and LOG(HR) for HR >50 nT, characterized by a correlation coefficient of R=0.63. Increased variability in the relationship between HRA and Kp was attributed to the following factors: the variability of the magnetic field within the 3-hour window characterized by the Kp-index which was better represented by a 1-hour HR; the dependence of the Kp index on sub-auroral magnetic data which is not subject to the energetic electron precipitation experienced within the auroral region; and reduced statistics for Kp>6.

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.

The term “ionospheric sluggishness” is used to describe the time delay between maximum radio absorption in the ionosphere following the time of maximum irradiance during a solar flare. Sluggishness is one of the characteristic properties known to be maximized around D-region heights and can be used for studying lower ionospheric (D-region) and mesospheric chemistry. This article is our first attempt to estimate ionospheric sluggishness using high frequency (HF, 3 – 30 MHz) instruments. Specifically, we report on first estimates of sluggishness from riometer and SuperDARN observations following a solar flare and propose two new methods to estimate sluggishness. Sluggishness is shown to be anti-correlated with the peak solar X-ray flux and positively correlated with solar zenith angle and geographic latitude. The choice of instrument, method, and reference solar waveband effects the sluggishness estimation. A simulation study was performed to estimate the effective recombination coefficient, which was found to vary between 4-5 orders of magnitude. We suggest that the effective recombination coefficient is highly sensitive to D-region’s negative and positive ion chemistry.