Physics of auroral phenomena : proceedings of the 33rd Annual seminar, Apatity, 02 - 05 March, 2010 / [ed.: A.G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 206 с. : ил.

“P hysics o f Auroral P henom ena”, Proc. XXXIII A n n u a l Sem inar, A patity, pp. 155 -1 5 8 , 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute STUDY OF HIGH-LATITUDE D-REGION IONOSPHERE CHARACTERISTICS DURING PERIODS OF POLAR MESOSPHERIC ECHO (PME) APPEARANCES V.D. Tereshchenko, A.P. Osepian, V.A. Tereshchenko (Polar Geophysical Institute, Murmansk, 183010, Russia; E-mail: vladter@pgi.ru) Abstract. Based on partial reflections data and theoretical profiles of the electron density the structure of region D of an ionosphere is determined during appearance the Polar Mesospheric Summer and Winter Echoes (PMSE and PMWE). Methods of determination o f the charged dust and metal ions concentration are offered during these events. Introduction The presence o f mesospheric reflections on the medium and shorter radio waves was first noticed more than half a century ago. However, systematic studies of this phenomenon started only in 80th years of the last century [Bremer et a l, 1996]. The main attention was spared to the summer of reflections in the VHF band, named Polar mesospheric summer echoes (PMSE). Polar mesospheric echo are quite strong scattered signals in excess of the noise level by 20-30 dB. It is believed that the cause of PMSE is formation at altitudes of 80-90 km of charged aerosols (particles of ice), which are observed visually as noctilucent clouds. For the first time the structures of PMSE in the data of medium frequency radar at 2.7 MHz were found research workers Polar Geophysical Institute (PGI) in 1991 during the international experiment "Noctilucent cloud-91» (NLC-91) [Vlaskov et a l, 1994]. This result was confirmed in paper [Bremer et a l, 1996]. Later it was found that such reflections exist in all seasons [Tereshchenko et al, 2002]. Intensive mesospheric reflections, which were observed in winter, are called Polar mesospheric winter echoes (PMWE). These reflections occur at altitudes of 55- 80 km of the mesosphere, which were considered free of aerosols. Typically, PMWE observed during the intrusion of solar protons with energies from several MeV up to hundreds of MeV, and high-energy electrons from the magnetosphere [Tereshchenko et al, 2008]. Radio echoes at high latitudes, obtained in different seasons have different properties. The nature of these reflections up to the end is not understood. It is possible that the formation of medium-reflections play decisive role sharp gradients of electron density and high levels o f atmospheric turbulence. Mechanisms of formation in the mesosphere irregularities o f electron concentration with the size needed for the scattering signals in different wavelengths are currently the subject o f further study. This paper presents the characteristics o f the ionospheric D-region during the registration of the polar mesospheric echoes. The necessary conditions for the appearances of intense mesospheric reflections are determined. Facility characteristics and methods of study Observations of the method o f partial reflections (PR) were made on facility of the PGI with the following parameters: operating frequency - 2.65 ■*- 2.78 MHz, the transmitter power in a pulse - 60 kW, pulse duration - 15 ms, repetition frequency - 2 Hz. Reception of scattered signals were receiving-transmitting antenna with a beam width at half power 19°x22°. Alternately, take two circular polarizations, which amplified receiver with a bandwidth of 40 kHz. The registration of the signal amplitudes was carried out in the altitude range 50-146 km. The step of the data reading was 0.5-1.5 km. A detailed description of the equipment is presented in paper [Tereshchenko et al, 2003]. On average data by means of differential absorption of radio waves was calculated electron density profile Ne(h) as described in paper [Belikovich et al, 2003]. For an explanation o f the physical processes occurring at heights of D-region, the model of lower ionosphere PGI [Smirnova et a l, 1988] is used. The model allows to calculate density of four positive ions 0 +2 , NO+, Cl*, C lJ, four negative ions 0~2, O ', CO 3 , NO j , and electrons N e , as the ion production rate q, effective recombination coefficient a eff, differential and integrated absorption of radiowaves. The initial ion clusters O ^ H jO ^ , and NO+(H jO ) , where n= l, 2, 3 . . , and most complicated secondary ion clusters H+(H 2 0 ) n are denoted Cl| and Cl j , respectively. 155