Statistical Study of Geoeffective Fe XV 28.4 nm Emission Bursts and Their Influence on Ionospheric Total Electron Content During the Late Phase of Solar Flares

Recent studies have demonstrated that warm coronal emissions during the extreme ultraviolet late phase (ELP) of X-class solar flares can be sufficiently strong to also affect electron density in the Earth's ionosphere. The Fe XV 28.4 nm line was identified as one of the most geoeffective emissions produced during the ELP. In this study, we show that Fe XV 28.4 nm bursts with comparable amplitudes and growth rates may also follow significantly weaker flares. For this reason, a large statistical analysis of all Fe XV 28.4 nm irradiance enhancements observed from 2010 to 2014 was performed, resulting in the identification of 724 geoeffective events. The detected enhancements were associated with flare main phases (51%), long duration events (25%), late phases (15%), non-flare-related events (6%), or multiple-flare sequences (3%). The majority of ELP events (64%) were associated with M-class flares, while 8% of ELPs exhibited substantial Fe XV flux enhancements capable of producing ionospheric ionization comparable to that caused by the impulsive phase of X-class flares. In the second part of this study, the ionospheric total electron content response to the strongest and most isolated ELP events was evaluated using Fe XV emission measurements from several instruments covering the period 2010–2025. A total of 71 ELPs were identified. A strong positive relationship was obtained between the integrated (Formula presented.) TEC enhancement and the integrated Fe XV 28.4 nm flux increase, with a correlation coefficient of (Formula presented.). The empirical relationships provide a new quantitative tool for assessing the geoeffectiveness of warm coronal emissions and estimating the ionospheric response to ELPs.