Physics of auroral phenomena : proceedings of the 35th Annual seminar, Apatity, 28 Februaru – 02 March, 2012 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2012. - 187 с. : ил., табл.

*P h y sics o f A u roral Phenom ena", Proc. X X XV A n n u al Sem inar, A p a tity, p p . 1 1 9 - 1 2 2 , 2 0 1 2 © Kola Science Centre, Russian Academy o f Science, 2012 Polar Geophysical Institute ACOUSTIC EFFECTS OF AN UNDERWATER EXPLOSION IN THE D REGION OF THE POLAR IONOSPHERE V.D. Tereshchenko, S.M. Chemiakov, V.A. Tereshchenko, O.F. Ogloblina Polar Geophysical Institute, Kola Science Center, Russian Academy o f Sciences (PGI KSC RAS) Murmansk, Russia; e-mail :vladter@pgi.ru Abstract. Results of experimental researches of acoustic action of an underwater explosion by capacity 3-7 tons of trinitrotoluene (TNT) on the polar lower ionosphere are submitted. The experimental data were received with the medium wave radar (Tumanny, Murmansk region) by the method of partial reflections of radio waves. Parameters of the wave disturbances which were observable in the polar lower ionosphere during this event were determined. 1. Introduction An underwater explosion of chemical explosive charges represents, basically, a source of gas-dynamic influences on environment, resulting in seismic, hydroacoustic, acoustic, and other effects. The significant number of theoretical and experimental works is devoted to research of underwater explosions [Cole, 1950; Adushkin et al., 2004]. In the work, results of observations of the surface phenomena during explosion and registration of hydroacoustic waves in water, seismic waves on ground, and acoustic waves in air on various distances from the epicenter of an explosion are submitted. Parameters o f appearing waves, character of their attenuation with distance are listed, and their spectra are analysed. The characteristic feature of an underwater explosion is pulsations of the gas bubble produced by the explosion. The pulsations can be seen in wave forms of hydroacoustic, seismic, and acoustic waves and their spectra. Under certain conditions acoustic waves can be spread in the ionosphere to big distances causing in it disturbances, which are the strongest and longest in the polar regions. Despite of significant successes in research of atmospheric dynamics, now we have only enough general idea about distribution effects and interaction processes of atmospheric waves with the ionosphere [Gossard and Hooke, 1978; Gokhberg and Shalimov, 2008]. The purpose of the work is experimental researches of an acoustic impact of an underwater explosion on the polar lower ionosphere by the method of partial reflections (MPR) of radio waves. Studying of mechanisms of influence of atmospheric disturbances on weakly ionized ionospheric plasma is important for solution of the various scientific and practical problems touching the problem of interaction of the lithosphere, the atmosphere, and the ionosphere. 2. Equipment and measurement technique Reaction of the lower ionosphere to the underwater explosion was studied with MPR. At the heart of this method there is radiation of two wave modes as alternating pulses, or linearly polarized waves on frequencies in the range from 2 till 6 MHz [Rapoport, 1972]. At the first case we used separate reception of signals, which partially reflected by ionospheric irregularities, and measured their amplitudes. The amplitudes depended on time of the delay which determined the height of reflection. At the second case two orthogonal linear polarizations were received. Then signals of two circular components were formed by adding of the shifted phases by ±90°. In Russia it is only one radar of partial reflections which operates on a constant basis. It is near the village Tumanny, Murmansk region (69.0°N, 35.7°E) and belongs to PGI KSC RAS. Key parameters of the radar correspond to existing technological level of radars of the European countries. The radar for research of the lower ionosphere consists of the transmitter, the receiver, the transmit-receive phased array and the automated system of data acquisition. The radar working frequencies are 2.60-2.72 MHz; capacity of the transmitter at a pulse is about 60 kW; duration of the pulse is 15 jis; and the frequency of sounding is 2 Hz. Array consists of 38 pairs of the crossed dipoles, occupies the area of 105 m2, and has the width of the directional diagram about 20° at the level of half-power. In turn, two circular polarizations, which are amplified by the receiver of direct amplification with the band of 40 kHz, are received. Registration of amplitudes and phase differences of signals is carried out at the interval of heights of 50-240 km. The interval of recording of the data on height is from 0.5 till to 1.5 km. The detailed description of the equipment is submitted in the works Tereshchenko et al. [2003; 2011]. Amplitudes of ordinary and extraordinary components of the reflected signal were averaged for every minute at all of recorded heights. Using the average data of the amplitudes of reflections of radio waves the structures of the electron concentration Ne(h) by the method of differential absorption were calculated. Digital time series of parameters of the received signals were used to the spectral analysis by means of the short-time Fourier transform and wavelet transforms. 119