Effects of solar flare X-ray, EUV, and Lyman-alpha emissions on the Dellinger phenomenon

Solar flares cause a rapid increase in the intensity of multi-wavelength electromagnetic emissions. The increase in X-rays to Extreme Ultraviolet (EUV) emissions promotes ionization and it can cause a rapid increase in electron density in the ionosphere. It is believed that long-wavelength ultraviolet radiation, especially Lyman-alpha emission, affects the lower atmosphere, however, short-wavelength EUV and X-rays can also affect the ionosphere (e.g., Woods et al., 2000). The communication failure caused by the absorption of the radio waves, due to the variations in electron density in the ionosphere D region (60-90 km) is called the Dellinger phenomenon (Dellinger 1937). The occurrence of the Dellinger phenomenon can be known from the minimum reflection frequency (fmin) observed by the vertical incident ionosonde. The fmin fluctuation depends on the peak X-ray intensity of flare and the solar zenith angle (e.g., Tao et al., 2020). However, the effects of flare emission spectra on the Dellinger phenomenon is not clearly understood. In this study, we compared flare emissions observed by GOES/X-ray Sensor (XRS), Extreme Ultraviolet Sensors (EUVS), the Solar and Heliospheric Observatory (SOHO)/Solar EUV Monitor (SEM) and the Solar Dynamics Observatory (SDO)/ EUV Variability Experiment (EVE) with the fmin operated by the National Institute of Information and Communications Technology (NICT) in Japan. We studied 38 flare events which were larger than M3-class occurred during the daytime (JST) between 2011 and 2014. First, we compared the fmin with the GOES/XRS, EUV and SOHO/SEM data. We found good correlation with shorter wavelength flare emission. However, Lyman-alpha emission did not contribute to the increase of fmin. Then, to identify the flare emission wavelengths that affect the increase in fmin, we compared the variation of fmin with the EUV spectra using the SDO/EVE data. We will report on these results in detail.