Altitude-Dependent Ionospheric Response Times to Solar Flare EUV and X-Ray Emission

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.