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.

The response of electric field intensity of VLF radio atmospherics during two tropical cyclones Fani (May 2019) and Amphan (May 2020) has been presented in this paper. VLF radio atmospherics (or VLF sferics) received at Coochbehar (CHB) and Kolkata (CUB) at three discrete frequencies (4 kHz, 7 kHz, and 9 kHz) showed clear amplitude anomalies with respect to the reference level during the two cyclonic storms. This is explained using the electrical structure and distribution of cloud-to-ground lightning associated with the cyclones. Effects of ‘local lightning’ and ‘distant lightning’ have been identified for both the CUB and CHB receivers. Field intensity of VLF sferics at CUB station was found to get enhanced for both types of lightning events. But the intensity of VLF sferics at CHB station was found to be reduced for ‘distant lightning’ and enhanced for ‘local lightning’, possible reasons of which are also explained.