A study of VLF signals variations associated with the changes of ionization level in the D-region in consequence of solar conditions

Abstract

In this paper we confine our attention to the analysis of amplitude and phase data acquired by monitoring VLF/LF radio signals emitted by four European transmitters during a seven-year period (2008)(2009)(2010)(2011)(2012)(2013)(2014). All the data were recorded at a Belgrade site (44.85 0 N, 20.38 0 E) by the Stanford University ELF/VLF receiver AWESOME. Propagation of VLF/LF radio signal takes place in the Earth-ionosphere waveguide and strongly depends on ionization level of the D-region, which means that it is mainly controlled by solar conditions. Some results of amplitude and phase variations on GQD/22.10 kHz, DHO/23.40 kHz, ICV/20.27 kHz and NSC/45.90 kHz radio signals measurements at short distances (D < 2 Mm) over Central Europe and their interpretation are summarized in this paper. Attention is restricted to regular diurnal, seasonal and solar variations including sunrise and sunset effects on propagation characteristics of four VLF/LF radio signals. We study VLF/LF propagation over short path as a superposition of different number of discrete modes which depends on the variations of the path parameters. Although the solar X-ray flare effects on propagation of VLF/LF radio signals are well recognized on all paths, similarities and differences between them are defined under existing conditions over the paths. Statistical results show that the size of amplitude and phase perturbations on VLF/LF radio signal is in correlation with the intensity of X-ray flux. We present the calculations of electron density enhancements in the D-region caused by different classes of solar X-ray flares during the period of ascending phase and maximum of the solar cycle 24.

On the Use of VLF Narrowband Measurements to Study the Lower Ionosphere and the Mesosphere–Lower Thermosphere

“…That flare's magnitude was approximated to be X45 [Thomson et al, 2004], based on the conclusions of McRae and . Other studies have also examined the reflection height, electron density slope, and vertical profile modifications, due to various classes of solar flares, and obtained different parameter values, possibly due to different latitude range and lengths of the TRGCPs [e.g., Kaufmann and Mendes, 1968;Kamada, 1985;Žigman et al, 2007;Grubor et al, 2008;Schmitter, 2013;Grubor, 2014, 2015;Pandey et al, 2015;Šulić et al, 2016]. These calculated values had a large variance, and a few of them [e.g., Kamada, 1985;Grubor et al, 2008] reached magnitudes which were comparable to those deduced by Thomson et al [2005].…”

The effects of solar activity: Electrons in the terrestrial lower ionosphere

“…Locations of some transmitters and the receiving site are presented in the Figure 1. It should be noted that the signal characteristics strongly depend on relative positions of transmitters and receiver and their distance (Šulić, Srećković, & Mihajlov, 2016). The analysis and the comparison of VLF data have been carried out together with the examination of the corresponding solar X-ray fluxes.…”

“…It is now recognized that the plasma in the atmospheric Dregion is a very sensitive medium to external forcing like moderate solar influence (Nina & Čadež, 2014;Ilić et al 2017), stellar explosive radiation (see e.g. Šulić, Srećković, & Mihajlov, 2016;Bajčetić, Nina, Čadež, & Todorović, 2015;Nina, Simić, Srećković, & Popović, 2015), energetic particle intrusion (Radovanović, 2010). Processes like solar emission in far-UV and EUV regions (Mihajlov, Ignjatović, Srećković, & Dimitrijević, 2011, Mihajlov, Ignjatović, Srećković, Dimitrijević, & Metropoulos, 2013 strongly affect the Earth's atmosphere (Todorović Drakul et al, 2016;.…”

Special Issue on Astrophysics & Geophysics: Research and Applications

“…Earth and space are constantly exposed to the effects of numerous phenomena which highly differ in their properties [1,2]. In consequence, the detection of these events, the investigation of their influences, as well as modeling them must be based on data obtained in real observations [3,4]. Nowadays, the challenges related to data volume, data diversity, and data flow are similar in astro-and geo-observations.…”