Vladimir A. Srećković

Altitude distribution of electron concentration in ionospheric D-region in presence of time-varying solar radiation flux

In this paper, we study the influence of solar flares on electron concentration in the terrestrial ionospheric D-region by analyzing the amplitude and phase time variations of very low frequency (VLF) radio waves emitted by DHO transmitter (Germany) and recorded by the AWESOME receiver in Belgrade (Serbia) in real time. The rise of photo-ionization rate in the ionospheric D-region is a typical consequence of solar flare activity as recorded by GOES-15 satellite for the event on March 24, 2011 between 12:01 UT and 12:11 UT. At altitudes around 70 km, the photo-ionization and recombination are the dominant electron gain and electron loss processes, respectively. We analyze the relative contribution of each of these two processes in the resulting electron concentration variation in perturbed ionosphere.

The non-symmetric ion–atom radiative processes in the stellar atmospheres

The aim of this research is to show that the processes of absorption charge exchange and photoassociation in A + B + collisions together with the processes of AB + photodissociation in the case of strongly non-symmetric ion–atom systems, significantly influence the opacity of stellar atmospheres in ultraviolet (UV) and extreme UV (EUV) region. In this work, the significance of such processes for solar atmosphere is studied. In the case of the solar atmosphere the absorption processes with A = H and B = Mg and Si are treated as dominant ones, but the cases A = H and B = Al and A = He and B = H are also taken into consideration. The choice of just these species is caused by the fact that, of the species relevant for the used solar atmosphere model, it was only for them that we could determine the necessary characteristics of the corresponding molecular ions, i.e. the molecular potential curves and dipole matrix elements. It is shown that the efficiency of the examined non-symmetric processes within the rather wide corresponding quasi-molecular absorption bands in the far-UV and EUV regions is comparable and sometimes even greater than the intensity of the known symmetric ion–atom absorption processes, which are included now in the models of the solar atmosphere. Consequently, the presented results suggest that the non-symmetric ion–atom absorption processes also have to be included ab initio in the corresponding models of the stellar atmospheres.

The ion-atom absorption processes as one of the factors of the influence on the sunspot opacity

As a continuation of the previous investigations of the symmetric and strongly nonsymmetric ion-atom absorption processes in the far UV region within the models of the quiet Sun photosphere, these processes are studied here within a model of the sunspot. Here we mean the absorption processes in the H(1s)+H + and H(1s) + X + collisions and the processes of the photo-dissociation of the H + and HX + molecular ions, where X is one of the metal atoms: X =Na, Ca, Mg, Si and Al. Obtained results show that the influence of the considered ion-atom absorption processes on the opacity of sunspots in the considered spectral region (110 nm . � . 230 nm) is not less and in some parts even larger than the influence of the referent electron-atom processes. In such a way, it is shown that the considered ion-atom absorption processes should be included ab initio in the corresponding models of sunspots of solar-type and near solartype stars. Apart of that, the spectral characteristics of the considered non-symmetric ion-atom absorption processes (including here the case X = Li), which can be used in some further applications, have been determined and presented within this work.

The influence of solar spectral lines on electron concentration in terrestrial ionosphere

Abstract One of the methods of detection and analysis of solar flares is observing the time variations of certain solar spectral lines. During solar flares, a raise of electron concentration occurs in Earth’s ionosphere which results in amplitude and phase variations of the recorded very low frequency (VLF) waves. We compared the data obtained by the analysis of recorded VLF signals and line spectra for different solar flares. In this paper we treated the DHO VLF signal transmitted from Germany at the frequency of 23.4 kHz recorded by the AWESOME system in Belgrade (Serbia) during solar flares in the period between 10:40 UT and 13:00 UT on 2011 April 22.

Detection of short‐term response of the low ionosphere on gamma ray bursts

In this paper, we study the possibility of detection of short term terrestrial lower ionospheric response to gamma ray bursts (GRBs) using a statistical analysis of perturbations of six very low or low frequency (VLF/LF) radio signals emitted by transmitters located worldwide and recorded by VLF/LF receiver located in Belgrade (Serbia). We consider a sample of 54 short lasting GRBs (shorter than 1 min) detected by the SWIFT satellite during the period 2009-2012. We find that a statistically significant perturbations can be present in the low ionosphere, and reactions on GRBs may be observed immediately after the beginning of the GRB event or with a time delay of 60 s - 90 s.

Diagnostics of plasma in the ionospheric D-region: detection and study of different ionospheric disturbance types

Abstract Here we discuss our recent investigations of the ionospheric plasma by using very low and low frequency (VLF/LF) radio waves. We give a review of how to detect different low ionospheric reactions (sudden ionospheric disturbances) to various terrestrial and extra-terrestrial events, show their classification according to intensity and time duration, and present some methods for their detections in time and frequency domains. Investigations of detection in time domain are carried out for intensive long-lasting perturbations induced by solar X-ray flares and for short-lasting perturbations caused by gamma ray bursts. We also analyze time variations of signals used in the low ionospheric monitoring after earthquake events. In addition, we describe a procedure for the detection of acoustic and gravity waves from the VLF/LF signal analysis in frequency domain. The research of the low ionospheric plasma is based on data collected by the VLF/LF receivers located in Belgrade, Serbia. Graphical abstract

THE EFFECTS OF SOLAR ACTIVITY: ELECTRONS IN THE TERRESTRIAL LOWER IONOSPHERE

Solar flare X-ray energy can cause strong enhancements of the electron density in the Earth’s atmosphere. This intense solar radiation and activity can cause sudden ionospheric disturbances (SIDs) and further create ground telecommunication interferences, blackouts as well as some natural disasters and caused considerable material damage. The focus of this contribution is on the study of these changes induced by solar X-ray flares using narrowband Very Low Frequency (VLF, 3–30 kHz) and Low Frequency (LF, 30–300 kHz) radio signal analysis. The model computation and simulation were applied to acquire the electron density enhancement induced by intense solar radiation. The obtained results confirmed the successful use of applied technique for detecting space weather phenomena such as solar explosive events as well for describing and modeling the ionospheric electron density which are important as the part of electric terrestrial-conductor environment through which external-solar wind (SW) electrons can pass and cause natural disasters on the ground like fires.