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The classification of X-ray solar flares is performed\udregarding their effects on the Very Low Frequency (VLF) wave propagation\udalong the Earth-ionosphere waveguide. The changes in propagation are\uddetected from an observed VLF signal phase and amplitude perturbations, taking\udplace during X-ray solar flares. All flare effects chosen for the analysis\udare recorded by the Absolute Phase and Amplitude Logger (AbsPal), during\udthe summer months of 2004–2007, on the single trace, Skelton (54.72 N,\ud2.88 W) to Belgrade (44.85 N, 20.38 E) with a distance along the Great Circle Path\ud(GCP) D≈2000 km in length.\ud\ud \ud\udThe observed VLF amplitude and phase perturbations are simulated by the\udcomputer program Long-Wavelength Propagation Capability (LWPC), using Wait's\udmodel of the lower ionosphere, as determined by two parameters: the\udsharpness (β in 1/km) and reflection height (H' in km). By varying the\udvalues of β and H' so as to match the observed amplitude and phase\udperturbations, the variation of the D-region electron density height profile\udNe(z) was reconstructed, throughout flare duration. The procedure is\udillustrated as applied to a series of flares, from class C to M5\ud(5×10−5 W/m2 at 0.1–0.8 nm), each giving rise to a different time\uddevelopment of signal perturbation.\ud\ud \ud\udThe corresponding change in electron density from the unperturbed value at\udthe unperturbed reflection height, i.e. Ne(74 km)=2.16×108 m−3\udto the value induced by an M5 class flare, up to Ne(74\udkm)=4×1010 m−3 is obtained. The β parameter is\udfound to range from 0.30–0.49 1/km and the reflection height H' to vary\udfrom 74–63 km. The changes in Ne(z) during the flares, within height\udrange z=60 to 90 km are determined, as well

Classification of X-ray solar flares regarding their effects on the lower ionosphere electron density profile