This study investigates the impact on the Earth’s ionosphere of a severe geomagnetic storm (Dst $\sim -212$ nT) that began on 23 April 2023 at around 17:37 UT according to very low-frequency (VLF, 3–30 kHz) or low-frequency (LF, 30–300 kHz) radio signals and ionosonde data. We analyze VLF/LF signals received by SuperSID monitors located in mid-latitude (Europe) and low-latitude (South America, Colombia) areas across nine different propagation paths in the Northern Hemisphere. Mid-latitude regions exhibited a daytime amplitude perturbation, mostly an increase, by ∼3–5 dB during the storm period, with a subsequent recovery after 7–8 days post April 23. In contrast, signals received in low-latitude regions (UTP, Colombia) did not show significant variation during the storm-disturbed days. We also observe that the 3-hour average of foF2 data declined by up to 3 MHz on April 23 and April 24 at the European Digisonde stations. However, no significant variation in foF2 was observed at the low-latitude Digisonde stations in Brazil. Both the VLF and ionosonde data exhibited anomalies during the storm period in the European regions, confirming that both D- and F-region ionospheric perturbation was caused by the severe geomagnetic storm.

Sudden Stratospheric Warmings (SSWs) are among the strongest manifestations of vertical coupling between the lower and upper atmosphere, yet their influence on the lower ionosphere through Very Low Frequency/Low Frequency (VLF/LF) propagation remains incompletely understood. This study investigates the response of the upper mesosphere–lower ionosphere to the major 2018–2019 Northern Hemisphere SSW using VLF/LF observations from five propagation paths spanning short, medium, and long distances across Europe. The polar stratospheric temperature at the 10 hPa pressure level began to rise continuously from 23 December 2018, reached a peak warming by approximately 55 K on 29 December 2018, and was followed by zonal wind reversal on 1 January 2019 under geomagnetically quiet conditions. Distinct path-dependent ionospheric responses were observed. The short GQD propagation paths exhibited daytime amplitude reductions of up to ∼8 dB, whereas the medium-length NRK paths and the long NAA-Germany path showed amplitude enhancements of ∼3-8 dB during the disturbed period, demonstrating that SSW-induced VLF/LF signal perturbations depend strongly on propagation geometry. Complementary TIMED/SABER observations reveal significant stratopause warming (> +3σ) accompanied by mesospheric cooling (< −3σ), with a two-day lag between the stratopause temperature maximum and the mesospheric temperature minimum. Notably, the mesospheric temperature minimum occurred three days before the temperature peak at 10 hPa. Nitric oxide (NO) exhibited pronounced enhancements exceeding the +3σ level in the stratopause region for a longer duration. However, NO exhibited short-duration enhancements exceeding the +3σ level for approximately two days centered on and following the central date of the SSW, followed by a second enhancement after the main SSW period. These episodic increases of NO in the stratosphere and upper mesosphere indicate that thermal restructuring plays the dominant role during the SSW, while subsequent elevated stratopause evolution contributes to later chemical changes.