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Abstract. The classification of X-ray solar flares is performed regarding their effects on the Very Low Frequency (VLF) wave propagation along the Earth-ionosphere waveguide. The changes in propagation are detected from an observed VLF signal phase and amplitude perturbations, taking place during X-ray solar flares. All flare effects chosen for the analysis are recorded by the Absolute Phase and Amplitude Logger (AbsPal), during the summer months of 2004-2007, on the single trace, Skelton (54.72 N, 2.88 W) to Belgrade (44.85 N, 20.38 E) with a distance along the Great Circle Path (GCP) D≈2000 km in length.The observed VLF amplitude and phase perturbations are simulated by the computer program Long-Wavelength Propagation Capability (LWPC), using Wait's model of the lower ionosphere, as determined by two parameters: the sharpness (β in 1/km) and reflection height (H in km). By varying the values of β and H so as to match the observed amplitude and phase perturbations, the variation of the D-region electron density height profile N e (z) was reconstructed, throughout flare duration. The procedure is illustrated as applied to a series of flares, from class C to M5 (5×10 −5 W/m 2 at 0.1-0.8 nm), each giving rise to a different time development of signal perturbation.The corresponding change in electron density from the unperturbed value at the unperturbed reflection height, i.e. N e (74 km)=2.16×10 8 m −3 to the value induced by an M5 class flare, up to N e (74 km)=4×10 10 m −3 is obtained. The β parameter is found to range from 0.30-0.49 1/km and the reflection height H to vary from 74-63 km. The changes in N e (z) during the flares, within height range z=60 to 90 km are determined, as well.

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A simultaneous analysis of solar are X-ray irradiance and VLF signal amplitude on the GQD/22.1 kHz trace was carried out. Solar are data were taken from GOES 12 satellite listings. The VLF amplitude data were recorded by means of the AbsPAL (Absolute Phase and Amplitude Logger) at the Institute of Physics, Belgrade, Serbia. It was found that solar are events from class C to class X affect the VLF signal amplitude in various ways and can be classified according to the type of effect produced in the Earth-ionosphere waveguide on the VLF propagation.

In this paper, we present a model for determination of a weakly time dependent effective recombination coefficient for the perturbed terrestrial ionospheric D-region plasma. We study consequences of a class M1.0 X-ray solar flare, recorded by GOES-15 satellite on February 18, 2011 between 14:00 UT and 14:15 UT, by analyzing the amplitude and phase real time variations of very low frequency (VLF) radio waves emitted by transmitter DHO (located in Germany) at frequency 23.4 kHz and recorded by the AWESOME receiver in Belgrade (Serbia). Our analysis is limited to ionospheric perturbations localized at altitudes around 70 km where the dominant electron gain and electron loss processes are the photo-ionization and recombination respectively.