Research Article

Estimation of the impact of solar flare spectra on the Earth’s ionosphere using the GAIA model

¹ National Defense Academy of Japan (NDA), Yokosuka 2398686, Japan
² National Institute of Information and Communications Technology (NICT), Koganei 1848795, Japan

^* Corresponding author: ed23002@nda.ac.jp; gekou.101.shine@gmail.com

Received: 21 February 2024
Accepted: 26 February 2025

Abstract

The rapid increase in X-ray and extreme ultraviolet (EUV) emissions owing to solar flares enhances ionization in the ionosphere, increasing radio wave attenuation. Among these phenomena, the shortwave communication disturbance caused by the increased electron density in the ionospheric D region is known as the shortwave fadeout (SWF). We investigated the relationship between SWF’s magnitude and solar flare emission, and evaluated the electron density variation in the ionospheric D region associated with flare. We defined the minimum frequency (fmin) observed in Japan’s ionograms as the SWF’s magnitude. We analyzed ionosonde data for 38 SWF events observed during daytime in Japan between May 2010 and May 2014. To investigate the relationship between flares and SWF, we compared the observed X-ray and EUV emissions during flares with the dfmin (background subtracted fmin). X-ray (0.1–0.8 nm) and EUV (11–14 nm) emissions correlate with dfmin. Then, using the GAIA model, a numerical model that treats the entire Earth’s atmosphere, we investigated the effect of the X-ray and EUV solar flare emissions on the ionosphere, which affects the SWF. The results showed that the main ionization source in the ionospheric D region was X-ray emission, and shortwaves were attenuated by ∼90%. In contrast, in the ionospheric E and F regions, the primary ionization source was EUV emission, with only ∼10% shortwave attenuation. Finally, we estimated the fmin values and blackout (total fadeout of the ionospheric echo observed in ionograms) and compared the simulated and observed fmin values. The hit rate of blackouts was 35% when we only used the GAIA calculations. Therefore, we estimated fmin using the electron density variation in the ionospheric D region corresponding to X-ray solar emission. As a result, the hit rate of the blackout was 68%, and the linear correlation coefficient between the simulated and observed fmin values was 0.85. The estimated magnitude of the SWF was improved by incorporating the effects of X-ray emissions into the ionospheric D region of GAIA. We are the first to implement a method for evaluating the electron density in the ionospheric D region using the fmin value.