Mediterranean D‐Region Ionosphere Response to Moderate (M‐Class) and Strong (X‐Class) Solar Flare Events

Abstract Solar flares emit intense X‐ray and ultraviolet radiation, causing strong ionization in the neutral atmosphere and increasing the electron density in the ionospheric D‐region. These variations affect the propagation of very low frequency (VLF) radio signals, observed as perturbations in amplitude and phase. This study investigates the D‐region response to selected M and X‐class solar flares through VLF signal perturbation analysis. Data were recorded by a VLF receiver in Algeria (36.75N, 3.48E, Boumerdes), monitoring two transmitters (ICV and NSC) propagating over the Mediterranean Sea under similar conditions. The Long Wavelength Propagation Capability (LWPC) code was used to solve the inverse problem and derive Wait's parameters ( and ) and electron density variations. Nine flare events from the rising phase of Solar Cycle 24 (2011–2014) were analyzed, including eight M‐class and one X‐class flare. For the X2.8 flare on 13 May 2013, LWPC simulations showed that along the ICV–Algiers path, decreased from 74 to 54.71 km and increased from 0.3 to 0.485 . Along the NSC–Algiers path, decreased to 57.95 km and increased to 0.46 . These small differences are attributed to transmitter frequency. Averaging the results obtained from both paths improved electron density estimation. At 74 km, the electron density during the M1.0 flare increased from 216.10 to 2.9 × , while for the X2.8 flare it reached 91.3 × . Finally, ionization enhancement was simulated by solving the continuity equations using the Glukhov–Pasko–Inan model. The results are consistent with those derived from LWPC simulations.