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.

LONGITUDINAL VARIATIONS OF THE ELECTRON DENSITY ENHANCEMENTS IN THE SUMMER NIGHT-TIME IONOSPHERIC F2-LAYER: NUMERICAL MODELING Yu.V. Zubova, M.A. Knyazeva, A.A. Namgaladze (Murmansk State Technical University, Murmansk, Russia; y-zubova@yandex. ru) Abstract. The longitudinal variations of the enhanced electron density regions (EEDRs) under the quiet conditions of the December solstice are investigated using the global numerical Upper Atmosphere Model (UAM) and the empirical ionospheric model IRI-2001. It has been proved that the Weddell Sea Anomaly in the southern hemisphere under the summer conditions is reproduced qualitatively by the UAM. The numerical experiments have showed that the phenomenon is caused by the non-coincidence of the geomagnetic and geodetic axes which produces the difference in the vertical ion velocities due to the thermospheric wind action at different longitudinal sectors. 1. Introduction The numerous papers described the anomalous night-time enhancements of the F2-layer electron density at the geomagnetic middle-latitudes. The morphology and physical mechanism were investigated by the global 3D Upper Atmosphere Model in details [Knyazeva and Namgaladze, 2008a], In the late 1950s in the Antarctica region the ionosonde measurements detected the anomalous diurnal electron density variations with the night-time values exceeding the day-time ones under summer conditions [Bellchambers and Piggott, 1958; Penndorf, 1965]. The phenomenon was called the Weddell Sea Anomaly although the anomaly covers rather the Bellinshausen Sea [Horvath and Essex, 2003]. Later satellite observations verified the WSA existence and revealed the analogous phenomenon in the Northern Hemisphere also in summer [Thampi et al, 2009]. The electron density enhancements are observed in the region from 40°S to 80° in latitude. The phenomenon covers longitudes of about 255°-315° in the summer Southern Hemisphere and longitudes of about 75°-135° in the summer Northern Hemisphere. Various hypotheses of physical mechanisms forming the WSA and analogous phenomenon in the Northern Hemisphere were advanced. The most discussed are the effects of solar ionization, neutral winds, geomagnetic field geometry, energetic particles precipitations, electric fields and plasma convection [Karpachev et al, 2010]. Almost all investigations were performed using ionosonde and satellite data. No WSA reconstruction was produced using global theoretical models. 2. Model calculations We performed the investigation of the EEDRs and in particular Weddell Sea Anomaly using the global numerical Upper Atmosphere Model which was described in details by [Namgaladze et al, 1998]. One of the UAM most important features is the ability to use empirical models alternative to theoretical calculation. So the NRLMSISE-00 model [Picone et al, 2002] can be used for neutral composition and temperature calculation; the HWM-93 [Hedin et al, 1996] can be employed for thermospheric wind velocity calculation. Such combined simulations allow estimating an influence of a single physical mechanism on the upper atmosphere behavior. We performed numerical experiments using the following UAM configurations: 1) marked as UAM-T - the fully self-consistent UAM version which calculates neutral composition, temperature neutral gas velocity by solving of the continuity, heat balance and momentum equations; 2) marked as UAM-MSIS - the configuration using the NRLMSISE-00 model for neutral composition, temperature and pressure gradients calculations; 3) marked as UAM-MSIS-HWM - the configuration using horizontal neutral gas velocities calculated by the HWM- 93 model and neutral composition and temperatures by the NRLMSISE-00 model. The numerically calculated distributions of the F2-layer critical frequency (foF2), diurnal variations of the F2-layer peak electron density (NmF2) and height (hmF2) were compared with the empirical IRI-2001 model [Bilitza, 2001] results. 3. Simulation results The foF2 distribution at the 285° geodetic meridian was simulated using the UAM-T, UAM-MSIS and UAM-MSIS- HWM versions for the December solstice (23.12.1985) under lower solar activity. The meridian of the 285° geodetic longitude is crossing the Weddell Sea Anomaly region approximately in the middle. The calculation results are shown in Fig.l in comparison with the IRI-2001 data. Fig.l demonstrates that the IRI-2001 reproduces the WSA (marked by the solid black circles) at 45°-70° of southern geomagnetic latitudes. In this latitude region the night-time foF2 values exceed the daytime ones. The UAM-MSIS version gives the most evident phenomenon at higher latitudes with the difference between daytime and nighttime 143 "Physics ofAuroral Phenomena", Proc. XXXIV Annual Seminar, Apatity, pp. 143-1462011 £ / T \ Polar © Kola Science Centre, Russian Academy of Science, 2011 Geophysical W У Institute