Model-based reproduction and validation of the total spectra of a solar flare and their impact on the global environment at the X9.3 event of September 6, 2017

Abstract

We attempted to reproduce the total electron content (TEC) variation in the Earth's atmosphere from the temporal variation of the solar flare spectrum of the X9.3 flare on September 6, 2017. The flare spectrum from the Flare Irradiance Spectral Model (FISM), and the flare spectrum from the 1D hydrodynamic model, which considers the physics of plasma in the flare loop, are used in the GAIA model, which is a simulation model of the Earth's whole atmosphere and ionosphere, to calculate the TEC difference. We then compared these results with the observed TEC. When we used the FISM flare spectrum, the difference in TEC from the background was in a good agreement with the observation. However, when the flare spectrum of the 1D-hydrodynamic model was used, the result varied depending on the presence or absence of the background. This difference depending on the models is considered to represent which extreme ultraviolet (EUV) radiation is primarily responsible for increasing TEC. From the flare spectrum obtained from these models and the calculation result of TEC fluctuation using GAIA, it is considered that the enhancement in EUV emission by approximately 15–35 nm mainly contributes in increasing TEC rather than that of X-ray emission, which is thought to be mainly responsible for sudden ionospheric disturbance. In addition, from the altitude/wavelength distribution of the ionization rate of Earth's atmosphere by GAIA (Ground-to-topside Atmosphere and Ionosphere model for Aeronomy), it was found that EUV radiation of approximately 15–35 nm affects a wide altitude range of 120–300 km, and TEC enhancement is mainly caused by the ionization of nitrogen molecules.

Statistical analysis for EUV dynamic spectra and their impact on the ionosphere during solar flares

“…In fact, the photoionization and photo-dissociative ionization cross sections corresponding to the EUV emissions employed in GAIA are relatively large (see Fig. 5 of Watanabe et al 2021). Furthermore, the timescale of electron recombination in the D and E regions of the ionosphere ionized by soft X-ray emission and high-temperature coronal line emission is a few minutes, whereas the timescale of recombination in the F region ionized by low-temperature coronal line emissions is, on average, hours, suggesting that low-temperature coronal line emissions are important for TEC enhancement (Zhang et al 2011).…”

PSTEP: project for solar–terrestrial environment prediction

“…TEC distribution was obtained by inputting the solar flare spectrum obtained from the above study into GAIA. By comparing with the observed TEC distribution, it was found that the effective wavelength for increasing TEC is around 30 nm (Watanabe et al 2021).…”

Special issue “Solar–terrestrial environment prediction: toward the synergy of science and forecasting operation of space weather and space climate”

“…Nishizuka et al (2021) reported the operational solar flare prediction model using Deep Flare Net. Watanabe et al (2021) reported the model-based reproduction and validation of the total spectra of a solar flare and their impact on the global environment. Nishimoto et al (2021) examined the computed extreme ultraviolet emission spectra during solar flares.…”