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. 1 1 7 -1 2 0 , 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute MAGNETOSPHERIC AND THERMOSPHERIC ORIGIN ELECTRIC FIELDS INFLUENCE ON THE ENHANCED ELECTRON DENSITY REGIONS IN THE NIGHT-TIME IONOSPHERIC F2-LAYER M.A. K nyazeva1, A.A. Nam galadze1 1 Murmansk State Technical University, 13 Sportivnaya Str., Murmansk, 183010,e-mail: mariknyazeva@yandex.ru, namgaladzeaa@mstu. edu. ru Abstract. The electric fields influence on the equatorial sides of the enhanced electron density regions (EEDR’s) in the night-time middle-latitude ionospheric F2-layer has been investigated by using the global Upper Atmosphere of the Earth Model (UAM) and the empirical model of the ionosphere IRI-2001. It has been shown that the strengthening o f the magnetospheric electric field results in the decrease of the steepness of the equatorial sides of the EEDR’s. The seasonal and solar activity variations of the latitudinal location o f the EEDR’s equatorial sides are formed by corresponding variations of the thermospheric dynamo electric field. Introduction The middle-latitudinal enhanced electron density regions (EEDR’s) appear in the night-time F2-layer of the ionosphere in all seasons under different solar activity levels. These regions have been detected in the observed diurnal and latitudinal variations of the F2-layer critical frequency (foF2), maximal electron density ( NmF2 ) and total electron content ( TEC) [Gilliland, 1935; Bertin and Lepine, 1970; Balan et a l, 1991; Richards et a l, 1994, 2000; Horvath and Essex, 2000; Mikhailov et a l, 2000; Farelo et a l, 2002; Brunini et a l, 2003]. The mechanism of the EEDR’s forming is based on the joint action of the plasma flows from the plasmasphere and the wind induced transportation o f the ionospheric plasma along the geomagnetic field lines [Knyazeva and Namgaladze, 2005; Knyazeva, 2009]. The electromagnetic drift influences on the latitudinal location of the high-latitude sides o f the EEDR’s, moving them to lower latitudes due to the equatorward displacement of the main ionospheric trough under the action of the magnetospheric electric fields [Knyazeva and Namgaladze, 2005; Knyazeva, 2009]. In this work we have presented the results of the investigation of the magnetospheric and thermospheric (dynamo) electric fields influence on the equatorial sides o f the EEDR’s for different helio-geomagnetic conditions including quite conditions, equinox and solstice under the low and high solar activity. The global numerical model of the Upper Atmosphere of the Earth Model (UAM) has been used in this investigation [Namgaladze et al, 1998]. Model calculations 1. Magnetospheric electric field influence on the EEDR’s The global distributions o f the electron density have been calculated by using the version of the UAM with the empirical model o f the thermosphere NRLMSISE-00 [Picone et a l, 2002] (UAM-MSIS in abbreviated form) in which the across polar cap potential A<p was constant and equal to 20 kV and 60 kV. The global pattern of the electromagnetic drift velocity has been calculated from the distribution of the electric field intensity E — gradcp. The selected day (16.04.2002) represents quiet condition (near equinox) under the high solar activity. The calculated geomagnetic latitude-altitude distributions o f the electron density (ne) along the night-time geomagnetic meridian 01:30 MLT for the altitude range from 800 to 3000 km (top row) and the corresponding latitude- longitude distributions of the eastward electric field at the altitude 175 km (bottom row) at the night longitudinal sector (18:00-06:00 MLT) are presented in Fig. 1. The results of the model calculations with constant across polar cap potential A<p=20 kV are presented at the left column and with A^=60 kV ones are presented at the right column. The midday-midnight geographic meridian, the terminator line and geographic equator are drawn on the maps. The geomagnetic force field lines are drawn on the meridional cuts. The increase o f A (p results in strengthening o f the magnetospheric origin electric field. As it was noted before this effect results in compressing of the EEDR’s from poles because of main ionospheric trough displacement to lower latitudes. The model calculation with constant across polar cap potential A^=20 kV has shown that at middle and low latitudes the zonal electric field is eastward in pre-midnight hours and it reverses (to westward) near 21:30-22:30 MLT. The increase of A <p influences weakly on the magnitude of the middle-latitude electric field. At low latitudes it leads to disappearance of the eastward electric field area, i.e. the zonal electric field is westward in all night-time longitudinal sector. It determines the night-time variations o f the low-latitudinal and equatorial ionospheric F2-layer. The model calculation with A<p=60 kV has shown that the process of the equatorial anomaly disappearance by 117