Physics of auroral phenomena : proceedings of the 38th annual seminar, Apatity, 2-6 march, 2015 / [ed. board: A. G. Yahnin, N. V. Semenova]. - Апатиты : Издательство Кольского научного центра РАН, 2015. - 189 с. : ил., табл.

FIELD-ALIGNED CURRENT DYNAMICS IN TWO SELECTED INTERVALS OF THE 6 APRIL 2000 SUPERSTORM "P hysics o f Auroral P hen om ena ”, Proc. XXXVJII A n n u a l Sem inar, Apatity, pp. 24-27, 2 0 1 5 Geophysical © Kola Science Centre, Russian Academy of Science, 2015 Vv*/ Institute V.M. Mishin, M.A. Kurikalova, V.V. Mishin (JSTP SB RAS, Russia ) C. Wang, J.Y. Wang (NSSC CSSAR CAS, Beijing, China) Abstract. We investigate two intervals of the 2000 April 6 superstorm with the expansion phase (EP) signatures. Obtained were the maps of the field-aligned current (FAC) distribution in the polar ionosphere of the Northern Hemisphere. When analyzing the maps, mesoscale cells with the local maximum of their FAC density in each Iijima and Potemra (I-P) Region were taken into account, which two- or threefold increased the spatial resolution. We describe two EP types (termed "summer" and "winter") for the equinox season. The FAC spatial distribution and dynamics during the expansion phase differ dramatically within these two types. We propose a scenario, in which the winter-type EP starts with the collapse (decrease) of the FAC that flows into the winter hemisphere ionosphere in the R1 premidnight sector. Simultaneously, and almost at the same rate, there occurs the FAC increase in the adjacent R1 postmidnight sector. Thus, in the proposed scenario, the substorm EP starts simultaneously in two hemispheres, but in different MLT-sectors of the nightside I-P Region 1. The magnetosphere-ionosphere (M-I) feedback global instability in the summer hemisphere premidnight sector serves as an initiator and organizer of the global EP. 1. Introduction The paper addresses the common problem of modeling the current system in the disturbed magnetosphere- ionosphere (M-I) system [McPherron et al., 1973; Lui and Kamide, 2003; Akasofu, 2015]. Studying the global distribution and dynamics of field-aligned currents (FACs) also refers to this problem [Weimer, 2001; Papitashvili et al., 2002; Kabin et al., 2004; Korth et al., 2011; Anderson et al. 2014]. Versions of modeling electric circuits during substorms [e.g., Lyatsky et al., 1972; Sugiura, 1975; Siscoe, 1982; Kamide and Baumjohann, 1993; Cowley, 2000; Sojko et al, 2013; Sandholt et al, 2014; Ohtani et al., 2014] have been addressed in the literature. The problem of substorm asymmetry in two hemispheres [Ostgaard et al., 2012; Reistad et al; 2014; 2015; Laundal and Ostgaard, 2009] has been also actively investigated. The results of this study are original (see Abstract), but have a direct relation to each of the listed topics. In this paper, we use the data on the solar wind, AE indices [] , and the data from the global ground-based magnetometer network (see Acknowledgements). The latter were processed through the magnetogram inversion technique (MIT-ISTP) [Mishin, 1990]. From the data for the 6 April 2000 two intervals [(02-*-04) and (12+15) UT], we calculated the time series of the maps for the FAC density distribution in the ionosphere. Kamide and Baumjohann [1993] performed a comparative analysis for different MIT techniques. They noted that the MIT-obtained ionospheric current distribution is not sensitive to the selection o f the ionosphere conductivity model, unless the conductivity auroral maximum is displaced in the ionosphere. Here, we apply the ionosphere conductivity model adapted to the addressed events [Mishin et al., 1986; Shirapov et al., 2000]. One peculiarity of this empirical model is the positive feedback o f the computed ionosphere conductivity with the FAC density FAC in the considered point. The other MIT-ISTP peculiarity is the method for choosing an optimal spectrum of the spherical harmonics, whose series approximates the potential o f the magnetic field under consideration [Mishin, 1990]. The third feature of this paper is calculating and analyzing the intensities of mesoscale inhomogeneities within the 1-P Regions. These original peculiarities o f the applied techniques are one of the reasons, why the principal results o f this paper have not been obtained in the literature earlier. Paragraph 2 addresses the technique to determine the FAC intensity within individual ionospheric cells. Paragraphs 3+5 deal with the analysis of temporal series of the maps of the FAC density and intensity distribution in the polar ionosphere during the two selected EP intervals. Paragraph 6 presents discussion and conclusions. 2. Determining FAC intensity in cells within every I-P Region Fig. 1 provides examples of the FAC density distribution maps in corrected geomagnetic coordinates. Upon determining the boundaries, we find the full FAC value (i.e., FAC intensities) in a cell. To interpret the results and estimate the errors of the operation, we use the schematical model for the electric circuit o f the M-I nightside disturbed system in Fig. 2. The model is not identical, but generally it is similar to the known corresponding models [see references in Introduction]. In Fig. 2, generator G is formed in the tail current disruption, and feeds directly the FAC ionospheric cells of Region 1 in two hemispheres. In the Northern Hemisphere, the cells of downward and upward FAC are denoted as R1+ and R1-. The FAC intensities in these cells are IRi+ and IR1.. The Region 2 pair [R2+, R2-] and the Region 0 24