A comparative study of measured amplitude and phase perturbations of VLF and LF radio signals induced by solar flares

Very Low Frequency, VLF and Low Frequency, LF signal perturbations were examined to study ionospheric disturbances induced by solar X-ray flares. The aim was to understand processes in propagation VLF/LF radio signals over short paths, and to estimate specific characteristics of each short path. The receiver at Belgrade station continuously monitor the amplitude and phase of coherent and subionospherically propagating LF signal operated in Sicily, NSC at 45.90, kHz and VLF signal operated in Isola di Tavolara ICV at 20.27 kHz, with great circle distances of 953 km and 976 km, respectively. Geographical locations of transmitters and receiver site result that these short paths have many similarity. The main difference is in transmitter frequencies. In period from 2008 to February 2014 were selected around 200 events for further examination. In all selected examples amplitude and phase on VLF and LF signals were perturbed by occurrence of solar X-ray flares. This six years period covers minimum and maximum of solar activity. Also, simultaneous measurement of amplitude and phase on the VLF/LF signals during occurrence of solar flare were employed for constructing electron density profile versus altitude and also in a function of time over middle Europe.Comment: published in Serb.Astron.

The influence of the radiative non-symmetric ion-atom collisions on the stellar atmospheres in VUV region

The aim of this work is to draw attention to the processes of radiative charge exchange in non-symmetric ion-atom collisions as a factor of influence on the opacity of stellar atmospheres in VUV region. For that purpose calculations of the spectral absorption coefficients for several ion-atom systems, namely: He + H$^{+}$ and H + X$^{+}$, where X = Na and Li have been performed. On chosen examples it has been established that the examined processes generate rather wide molecular absorption bands in the VUV region, which should be taken into account for the interpretation of data obtained from laboratory measurements or astrophysical observations. In this paper the potential significance is discussed of the considered radiative processes for DB white dwarfs and solar atmospheres, as well as for the atmospheres of the so-called lithium stars

Modeling of the Continuous Absorption of Electromagnetic Radiation in Dense Hydrogen Plasma

In this work is examined a new modeling way of describing the continuous absorption of electromagnetic (EM) radiation in a dense partially ionized hydrogen plasmas with electron densities about $5\cdot10^{18}$ cm$^{-3}$ - $1.5\cdot10^{19}$cm$^{-3}$ and temperatures about $1.6 \cdot10^{4}$ K - $2.5 \cdot 10^{4}$ K in the wavelength region $300 \textrm{nm} < \lambda < 500 \textrm{nm}$. The obtained results can be applied to the plasmas of the partially ionized layers of different 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 non-symmetric 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$_{2}^{+}$ and H$X^{+}$ 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 $\lesssim \lambda \lesssim$ 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 \emph{ab initio} in the corresponding models of sunspots of solar-type and near solar-type 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.Comment: Accepted in Monthly Notices of the Royal Astronomical Societ

Excitation and deexcitation processes in atom-Rydberg atom collisions in helium-rich white dwarf atmospheres

We aim to show the importance of non-elastic excitation and deexcitation processes in $\textrm{He}^{*}(n)+ \textrm{He}(1s^{2})$ collisions with the principal quantum number $n \ge 3$ for helium-rich white dwarf atmospheres. We compare the efficiencies of these processes with those of the known non-elastic electron-$\textrm{He}^{*}(n)$ atom processes in the atmospheres of some DB white dwarfs. We show that in significant parts of the considered atmospheres, which contain weakly ionized layers (the ionization degree $\lesssim 10^{-3}$), the influence of the studied atom-Rydberg atom processes on excited helium atom populations is dominant or at least comparable to the influence of the concurrent electron-$\textrm{He}^{*}(n)$-atom processes.Comment: Astronomy & Astrophysics, Accepted: 14 February 201