An analysis of four solar flare X-ray irradiance effects on VLF signal amplitude and phase delay variations on the NAA/24.0 kHz signal trace during the period from 2005 September to 2006 December was carried out. Solar flare data were taken from the GOES12 satellite one-minute listings. For the VLF data, recordings at the Institute of Physics, Belgrade were used. It was found that solar flare events affect VLF wave propagation in the Earth-ionosphere waveguide lowering the changes of the ionosphere electron density height profiles. This follows from the variation during the solar flare events of the following propagation parameters: the sharpness of the lower edge of the ionosphere and the reflection height.

An analysis of D-region electron density height profile variations, induced by four isolated solar X-ray flares during period from September 2005 to December 2006, based on the amplitude and the phase delay perturbation of 22.1 kHz signal trace from Skelton (54.72 N, 2.88 W) to Belgrade (44.85 N, 20.38 E), coded GQD, was carried out. Solar flare data were taken from NOAA GOES12 satellite one-minute listings. For VLF data acquisition and recordings at the Institute of Physics, Belgrade, Serbia, the AbsPAL system was used. Starting from LWPCv21 code (Ferguson, 1998), the variations of the Earth-ionosphere waveguide characteristic parameters, sharpness and reflection height, were estimated during the flare conditions. It was found that solar flare events affected the VLF wave propagation in the Earth-ionosphere waveguide by changing the lower ionosphere electron density height profile, in a different way, for different solar flare events.

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Comparative qualitative analysis of amplitude and phase delay variations was carried out along the trajectory of GQD/22.1 kHz and NAA/24.0 kHz VLF signal traces, propagating from Skelton (UK) and Maine (USA) toward Belgrade, induced by four isolated solar X-ray flare events occurred during the period from September 2005 to December 2006. For monitoring, recording and for storage of VLF data at the Institute of Physics in Belgrade, Serbia, the AbsPAL system was used. For modeling purposes of propagating conditions along GQD and NAA signal propagation paths, LWPCv21 program code was used. Occurred solar flare events induced lower ionosphere electron density height profile changes, causing perturbations in VLF wave propagation within Earth-ionosphere waveguides. As analyzed VLF signals characterize by different propagation parameters along trajectories from their transmitters to the Belgrade receiver site, their propagation is affected in different ways for different solar flare events and also for the same solar flare events.

The strongest X-class solar flare (SF) event in 24th solar cycle, X9.3, occurred on 6 September 2017, accompanied by earthward-directed coronal mass ejections (CMEs). Such space weather episodes are known to cause various threats to human activities ranging from radio communication and navigation disturbances including wave blackout to producing geomagnetic storms of different intensities. In this study, SFs’ ionospheric impacts and effects of accompanied heliospheric disturbances on primary cosmic rays (CR) are investigated. This work offers the first detailed investigation of characteristics of these extreme events since they were inspected both from the perspective of their electromagnetic nature, through very low frequency (VLF) radio waves, and their corpuscular nature of CR by multi-instrumental approach. Aside data recorded by Belgrade VLF and CR stations, data from GOES and SOHO space probes were used for modeling and analysis. Conducted numerical simulations revealed a significant change of ionospheric parameters (sharpness and effective reflection height) and few orders of magnitude increase of electron density. We compared our findings with those existing in the literature regarding the ionospheric response and corresponding parameters. In addition, Forbush decrease (FD) magnitude, corrected for magnetospheric effect, derived from measurements, and one predicted from power exponents used to parametrize the shape of energetic proton fluence spectra at L1 were compared and found to be in good agreement. Presented findings could be useful for investigation of atmospheric plasma properties, particles’ modeling, and prediction of extreme weather impacts on human activities.