Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

Study o fradiation related with atmosphericprecipitations summer and exclude radon origin o f one. Also lack of contamination of the charged particle component in the X-ray events has been shown. The detector is sensitive both to the electromagnetic component of radiation and to the charged one. To clarify the relative contribution of the charged component radiation to the increases is possible with the help of the charged particle detector based on Geiger counters. It is known that sensitivity of these counters with regard to the charged component of radiation is two orders of magnitude higher than to gammaray quanta. Connection of two layers of counters on coincidence, as described in Section 2, ensures detecting the charged component of radiation only. There are no charged components (electrons and muons) in the radiation causing the increase on the detector. 5. Discussion As it was shown above, the increases in the count rate of the gamma-ray spectrometers during precipitations are not related with radioactivity of the atmosphere, including radon. They also are not caused by charged particles. Increases should be caused by photons which upper energy limit is less than 1 MeV. The probable source of these particles can be the X-ray bremsstrahlung produced by electrons accelerated in a rain (snowfall) cloud. Near the Earth's surface the electric field in quiet weather is about 100 volts per meter. Inside the rain clouds the electric field strength is much higher and may reach kilovolts per meter and even tens of kV/m [7]. The intensity of the photons is determined by the generation of energetic electrons and positrons in the form of bremsstrahlung and the alternative process of absorption in the air. When passing through the matter, the electron loses energy due to ionization and radiation losses. In our low-energy energy domain (Ee < IMeV) a decisive contribution to the energy losses gives the ionization process. Radiation losses become significant at much higher energies [8]. 6. Spectra of increases and interpretation of experimental data At the instrumental complex in Apatity we also obtained energy spectra of photons during increases with the help of the 4096-channel pulse-height analyzer. The calibration of the spectrometer was carried out with the help of radioactive gamma-sources Am241 (lines of 26 and 60 KeV) and Cs 37 (662 KeV). The spectrum of photons causing an increase was obtained by a subtraction from the measured spectrum during the increase of a spectrum of a quiet background before the increase. In Fig.2 by a thick line is shown one of the experimental spectra obtained in the event on March 8, 2010 (Fig.lb). The spectrum has a gradual form. There are not narrow peaks characters for lines of radionuclides. We tried to simulate the observed X-ray spectra in the frames of our above suggestion about the bremsstrahlung origin. The energy spectrum o f electrons in the our energy range can be nearly approximated by an exponential law [8] dN^ _ N >е-ед (1) dE 0 where E0 is characteristic energy, which varies in the range from tens to hundreds keV. At simulation we use expressions for conversion of an electron flux in a X-ray bremsstrahlung in view of simultaneous losses on ionization in air [8]. dN{hv) =a togif ( h v x ^ d(h v) Eo у ( 2 ) dE where ---------- - is spectrum of photons for a given electron spectrum with an exponent E q . d(hv) a = 1.63, p = 3.48-10Л It should be noted that one must account effects of absorption in the air of both electrons and produced by them X- ray photons. Due to the strong absorption of electrons with energies of tens or hundreds KeV in the air, it is expected that only particles produced no higher than 1000 m can reach the ground level. This is also confirmed by the fact that almost all registered increases we observed in the overcast with the lower edge of the clouds from 200 to 600 m. According to balloon studies [7] in nimbostratus clouds, which we usually observe during the X-ray increase events, there are electric fields with intensity of vertical component: Ez =1*12 kV/m and a layer width (LC) of a few hundred meters. Intensity of a horizontal component is EH = 0 .2 * 2 8 kV/m. We suppose that presence of such electric fields in clouds in rainy weather gives extra energy AE = EZ -LC to electrons, which results in the increased flux of X-ray bremsstrahlung on the ground level. We obtain the expression that describes the spectrum of X-ray radiation at the earth's surface after accounting for losses due to absorption of both electrons and gamma rays: 187