## 1 Introduction  
The ionosphere, as a huge layer of the atmosphere, has physical and chemical properties that depend on the incident radiation and local energetic processes [1; 2]. In particular, the lower ionospheric region and the fluctuations of its parameters are very important for human life and many activities on Earth and require continuous measurements, observations and available information [3]. In-situ observations of the lower ionospheric layers are difficult and expensive and, consequently, remote sensing measurements are mainly used for investigating this region [4; 5]. High-energy events, triggered from an external source or inside the atmosphere, can induce ionospheric disturbances and as a consequence affect all processes within [6]. For example, during solar flares (SFs) the increase in the ionospheric electron concentration at all altitudes is noticeable. Solar flares are giant explosions on the surface of the Sun when a huge amount of electromagnetic energy is released across the entire electromagnetic spectrum [7]. The enhanced extreme ultraviolet (EUV) radiation is absorbed at higher altitudes ionizing the E (90-150 km above Earth's surface during daytime conditions) and F (160-400 km, same) regions of the ionosphere (see e.g., [8]). During SFs, electromagnetic radiation within soft X-rays in a wavelength range of 0.1-0.8 nm, significantly oversteps the ionization of the Lyman-a spectral line 121.6 nm and cosmic rays, becoming a major source of ionization at a range of altitudes corresponding to the D region (50-90 km, same), causing enhanced ionization and absorption of the EM waves that propagate within the Earth-ionosphere waveguide [9; 10]. As a result of radiation effects,the solar induced ionospheric disturbances and plasma irregularities cause perturbations in the amplitude and phase of radio signals. Furthermore, some processes create localised areas of increased electron density in the low ionosphere and thus change its properties. Radio waves, generated by atmospheric discharge and propagating along geomagnetic field lines into the plasmasphere, get into transversal cyclotron resonance with energetic electron beams (30-300 keV) moving in the opposite direction. As a consequence of such interaction, electron precipitation into the atmosphere takes place. Electron precipitation produces localised areas of increased electron density in the low ionosphere (on heights corresponding to D-region heights of 50-90 km [11; 12; 13]), known as localised ionization enhancement (LIE) [14]. Perturbations in very low frequency (VLF) propagation, known as trimpi events, can be explained by formation of such areas. Classic trimpi events [15] are phase delay and/or amplitude perturbations of VLF signals propagating in the Earth-ionosphere waveguide. Whistlers occur about one second after a corresponding atmospheric discharge and propagate in the magnetosphere along geomagnetic field lines [16]. Electron precipitation, caused by ducted whistlers, is explained in [17]. Since disturbances of VLF signals are a consequence of electron precipitation caused by atmospheric discharges, they are named as lightning-induced electron precipitation events (LEP). In addition to the above-mentioned phenomena, which can be detected as disturbances in VLF propagation, there are a number of other phenomena which can be optically detected in areas above thunderclouds. The formation of ducts of enhanced electroconductivity between thunderclouds and the ionosphere is connected with such phenomena [18; 19; 20]. Trimpi perturbations can be simultaneously monitored on multiple adjacent traces of VLF signals. Based on adjacent VLF signals recordings comparison, LIE dimensions and spatial position can be estimated by numerical modelling, depending on the transmitter frequency, VLF signal great circle path (GCP) length and position of LIE in relation to the VLF signal path.