Coupled Solar–Ionospheric Dynamics: EUV Pulsations and Ionospheric Response Timescales During Flares

The extreme ultraviolet (EUV) and X-ray radiation emitted during solar flares cause considerable increases in the electron density of the Earth's ionosphere. During flares, quasi-periodic pulsations (QPPs) in coronal X-ray flux 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 have been detected in solar EUV emission and ionospheric total electron content (TEC) measurements. QPPs were identified in chromospheric EUV emission lines (304Å, 972Å and 977Å) during the impulsive phase of the X5.4 flare on 7 March 2012 using SDO/EVE. These lines contribute to ionisation in the ionospheric E- and F-regions, producing corresponding variations in electron density detectable in TEC with delays of ~30 seconds. Building on this analysis, we extend the time-delay investigation to a sample of ten powerful solar flares to quantify the characteristic F-region response timescales. We assess the measured delays in relation to multiple solar and geophysical factors. The results show that the ionosphere responds rapidly and measurably to both small-scale EUV fluctuations and the overall flare-driven increase in EUV irradiance, highlighting the diagnostic potential of QPP-driven ionospheric signatures and their applications in atmospheric modelling, solar–terrestrial coupling, and ionospheric recombination studies.