Lower Ionospheric Disturbances Due To Geomagnetic Storms of March and April 2023: Inferred From VLF Navigational Signals and Numerical Simulations

D-region effect of intense storm of 23 March (D_st = −163 nT) and super storm of 23 April 2023 (D_st = −212 nT) have been investigated using VTX and NWC transmitter signals recorded at low latitude station Dehradun (DDN; 30.31°N, 78.03°E), India. During March storm on 23 March, amplitude anomalies of +2.92 (NWC) and +1.91 dB (VTX) were observed. April storm showed amplitude anomalies of −9.41 (NWC) and −2.68 dB (VTX) on 23 April. Long Wave Propagation Capability code (LWPC) has been used to model signal anomalies to obtain D-region Very low frequency (VLF) reference height (H′) and electron density gradient (β). During March storm, NWC-Dehradun path showed a decrease in H′ by 12.04 km and an increase in β by 0.301 km⁻¹, while VTX-Dehradun path showed an increase in H′ by 0.07 km and β by 0.032 km⁻¹ on 23 March. During April storm, VTX-Dehradun path showed increase in H′ by 0.315 km and an increase in β by 0.024 km⁻¹, while H′ increased by 2.91 km and β by 0.23 km⁻¹ for NWC—Dehradun path on 23 April. Wavelet analysis of VLF anomalies showed signatures of atmospheric gravity waves of periods between 30 and 100 min during both storms associated with joule heating in the high/auroral latitudes. Results suggest that storm time prompt penetration (PP) of electric fields has contributed to VLF signal anomalies during April storm's main phase, which is also supported by PP Equatorial Electric Field Model run for the location of DDN station.