The extreme ultraviolet (EUV) and X‐ray radiation emitted during solar flares has been shown to significantly increase the electron density of the Earth's ionosphere. During flares, quasi‐periodic pulsations in X‐ray flux originating in the corona have previously been linked to subsequent pulsations in the Earth's ionospheric D‐region. Similar pulsations have been detected in chromospheric EUV emission, although their impact on the Earth's ionosphere has not previously been investigated. Here, for the first time, synchronous pulsations were detected in solar EUV emission and ionospheric Total Electron Content (TEC) measurements. Using wavelet and periodogram analysis, we detect QPPs with approximately 85 s periods in chromospheric EUV emission lines (He ii 304 Å, C iii 977 Å, and H i 972 Å) from the Solar Dynamics Observatory Extreme Ultraviolet Variability Experiment during the impulsive phase of an X5.4 flare on 7 March 2012. These lines contribute to ionization in the ionospheric E‐ and F‐regions, resulting in subsequent variations of electron density with the same periodicity, which was detected in TEC measurements. This work demonstrates that the Earth's ionosphere is responsive to fine‐scale fluctuations in EUV emission during flares, with a time delay of approximately 30 s found. These findings may have applications in atmospheric modeling and solar‐terrestrial studies, including the calculation of ionospheric recombination rates.

X-ray and EUV solar flare emission cause increases in the Earth’s dayside ionospheric electron density. While the response of the lower ionosphere to X-rays is well studied, the delay between EUV flare emission and the response of the ionospheric F-region has not been investigated. Here, we calculate the delays between incident He II 304 Å emission, and the TEC response for 10 powerful solar flares, all of which exhibit delays under 1 min. We assess these delays in relation to multiple solar and geophysical factors, and find a strong negative correlation (∼−0.85) between delay and He II flux change and a moderate negative correlation (∼−0.55) with rate of increase in He II flux. Additionally, flare magnitude and the X-ray-to-He II flux ratio at peak He II emission show strong negative correlations with delay (∼−0.80 and ∼−0.75, respectively). We also identify longer delays for flares occurring closer to the summer solstice. These results may have applications in upper-ionospheric recombination rate calculations, atmospheric modelling, and other solar–terrestrial studies. We highlight the importance of incident EUV and X-ray flux parameters on the response time of the ionospheric electron content, and these findings may also have implications for mitigating disruptions in communication and navigation systems.

Since its launch in 2010, the Solar Dynamics Observatory (SDO) has provided continuous, high-cadence, multi-wavelength observations of the Sun, capturing thousands of solar flares and offering new insights into coronal dynamics. Among the discoveries enabled by SDO is the EUV late-phase (ELP), characterised by a secondary enhancement in warm coronal emission occurring tens of minutes after the main flare. While recent work has demonstrated the relevance of the ELP for space weather, the statistical behaviours and physical origins are not fully understood. Here, we present the most comprehensive statistical analysis of the ELP to date, based on 15-years of Fe xvi (335 Å) observations from the Atmospheric Imaging Assembly onboard SDO (SDO/AIA). From a sample of 5335 isolated flares between 2010 and 2025, we identify and validate 467 ELP events. The overall ELP occurrence rate was found to be 9%, with no significant dependence on the solar cycle and only a modest enhancement in the low-mid M-class range. The ELP typically exhibited an onset delay of 19 minutes, a peak-to-peak delay of 88 minutes, and a duration of 93 minutes. Strong correlations were found between ELP rise and decay rates ( $\rho =0.76$ ρ = 0.76 ), and between flare and ELP impulsivity ( $\rho =0.61$ ρ = 0.61 ). However, a comprehensive pairwise analysis revealed no significant correlation between the flare and ELP phases ( $|\rho |<0.3$ | ρ | < 0.3 ). A Principal Component Analysis of flare and ELP properties identified several semi-independent axes of variability, broadly associated with late-phase temporal scale, impulsive heating characteristics, and the relative prominence of flare and late-phase intensity measures. These results highlight the continuing importance of SDO’s long-term, high-resolution observations for uncovering new aspects of solar flare evolution and improving understanding of the Sun-Earth connection.

Since its launch in 2010, the Solar Dynamics Observatory (SDO) has provided continuous high-cadence, multi-wavelength observations of the Sun, capturing thousands of solar flares and offering new insights into coronal dynamics. Among the discoveries enabled by SDO is the EUV late-phase (ELP), characterised by a secondary enhancement in warm coronal emission occurring tens of minutes after the main flare. While recent work has demonstrated the relevance of the ELP for space weather, its statistical behaviour and physical origin remain poorly constrained. Here, we present the most comprehensive review of the ELP to date, based on 15 years of Fe XVI observations from the Atmospheric Imaging Assembly onboard SDO (SDO/AIA). From a sample of 5335 isolated flares between 2010 and 2025, we identify and validate 467 ELP events. The overall ELP occurrence rate is 8 percent, with no significant dependence on the solar cycle and only a modest enhancement in the low-to-mid M-class range. The ELP typically exhibits and onset delay of 19 minutes, a peak-to-peak delay of 88 minutes, and a duration of 93 minutes. Strong correlations are found between ELP rise and decay rates (p=0.76), and between flare and ELP impulsivity (p=0.61), while no significant correlation is observed between flare and ELP phases. A Principal Component Analysis revealed three dominant axes of variation, corresponding to a timescale component, an energy-release intensity axis, and a partitioning of energy between the flare and ELP. These results suggest that ELP evolution is governed by both flare loop properties and reconnection-driven energetics, likely modulated by a finite magnetic energy budget, and highlight the importance of SDO's long-term observations for understanding flare evolution and the Sun-Earth connection.

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 TEC enhancement and the integrated Fe XV 28.4 nm flux increase, with a correlation coefficient of . The empirical relationships provide a new quantitative tool for assessing the geoeffectiveness of warm coronal emissions and estimating the ionospheric response to ELPs.

Enhanced radiation during flares increases ionisation in the Earth's ionosphere, yet different ionospheric layers absorb different emissions because they differ in composition and plasma characteristics. To investigate the relative timings of the responses at different altitudes, we analyse 66 X-class flares from 2024–2025 using EUV and X-ray irradiance with simultaneous GNSS vertical total electron content (VTEC), very-low-frequency (VLF) phase measurements and magnetometer observations of the horizontal geomagnetic perturbation (\(\Delta B_{\mathrm{h}}\)). Response lags were measured by cross-correlating time derivatives and feature matching. Against the same EUV drivers, \(\Delta B_{\mathrm{h}}\) response lags by \(\approx\)13 s and VTEC by \(\approx\)33 s, consistent with slower electron loss in the F region than the E region. \(\Delta B_{\mathrm{h}}\) responses reproduce unique EUV and X-ray features, indicating the influence of both wavelength ranges on E-region ionisation. In the D region, VLF phase lags the X-ray rise by +51, +30 and +12 s relative to progressively softer channels, and phase maxima precede softer X-ray peaks, indicating that recombination begins to prevail over ionisation before flux reaches its maximum. These results, for the first time, empirically demonstrate the altitude difference in recombination rates of the ionosphere and the noticeable contribution of both EUV and X-ray emission to E-region dynamics during flares.