Over‐the‐Horizon communication is strongly dependent on the state of the ionosphere, which is susceptible to solar flares. Trans‐ionospheric high frequency (HF, 3–30 MHz) signals can experience strong attenuation following a solar flare that lasts typically for an hour, commonly referred to as shortwave fadeout (SWF). In this study, we examine the role of dispersion relation and collision frequency formulations on the estimation of SWF in riometer observations using a new physics‐based model framework. The new framework first uses modified solar irradiance models incorporating high‐resolution solar flux data from the GOES satellite X‐ray sensors as input to compute the enhanced ionization produced during a flare event. The framework then uses different dispersion relation and collision frequency formulations to estimate the enhanced HF absorption. The modeled HF absorption is compared with riometer data to determine which formulation best reproduces the observations. We find the Appleton‐Hartree dispersion relation in combination with the averaged collision frequency profile reproduces riometer observations with an average skill score of 0.4, representing 40% better forecast ability than the existing D‐region Absorption Prediction model. Our modeling results also indicate that electron temperature plays an important role in controlling HF absorption. We suggest that adoption of the Appleton‐Hartree dispersion relation in combination with the averaged collision frequency be considered for improved forecasting of ionospheric absorption following solar flares.
The term “sluggishness” was coined by E. V. Appleton in the 1950s to describe the time delay between peak irradiance at solar noon and the resulting peak in ionospheric electron density. Sluggishness can be understood as an inertial property of the ionosphere that manifests as a lag of the ionospheric response to a solar driver. As shown by Appleton, estimates of sluggishness can be used to study the chemistry of the lower ionosphere, of the D‐region in particular. In this study, for the first time, we have examined ionospheric sluggishness in terms of the time delay between the peak irradiance during a solar flare and the resulting peak in ionospheric electron density using HF instruments. Estimates of the delay are obtained using HF observations from riometers and SuperDARN radars that are primarily sensitive to absorption in the D‐region. Two new methods for measuring delay are introduced. Sluggishness is shown to be anti‐correlated with peak solar X‐ray flux and positively correlated with zenith angle and latitude. The choices of instrument, method, and reference solar waveband affect the sluggishness estimation. A simulation study was performed to estimate the effective recombination coefficient in the D‐region. The coefficient was found to vary by orders of magnitude with peak flare intensity. We argue that the variation in effective recombination coefficient with peak flare intensity is highly sensitive to changes in the negative and positive ion chemistry of the D‐region, which is sensitive to the incoming solar X‐ray and EUV radiation.
more » « less- NSF-PAR ID:
- 10362034
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 126
- Issue:
- 4
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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