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.

“Physics o f Auroral P h en om en a ”, Proc. XXXVIII A nnu al Sem inar, A patity, pp. 40-43, 2 0 1 5 © Kola Science Centre, Russian Academy of Science, 2015 Polar Geophysical Institute THE SATURATION EFFECT OF THE POYNTING FLUX INTO THE MAGNETOSPHERE DURING SUPERSTORMS: RESULTS OF MIT AND THE GLOBAL PPMLR-MHD MODEL V.V. M ishin1, Yu. Karavaev1, J.P. Han2’3, C. Wang3 'ISTP SB RAS, Irkutsk 2CALT CASC, Beijing, China, 3NSSC CAS, Beijing, China Abstract. We study the effect of the slowing growth of the polar cap magnetic flux and the electromagnetic energy flux into the magnetosphere from the solar wind (SW) during the superstorm significant strengthening. This saturation effect is found not only depending on the SW electric field Eswand / or on the southward IMF component, but also on the SW dynamic pressure Pd. Analytically and numerically we have been shown the existence of another saturation effect- slowing the dayside magnetosphere compression upon reaching high values o f Pd and negative Bz. The both saturation effects are supposed to be associated with each other for superstorm conditions. Introduction Two important parameters characterize the efficiency of the SW- magnetosphere coupling: the cross polar cap voltage UpCand the polar cap magnetic flux 4f. A large number of papers were devoted to the saturation effect of the polar cap potential UPC and magnetospheric convection - slowing down their growth upon reaching high values Esw (Esw=3-5 mV/m) and its further increasing. This effect has been confirmed by various measurement techniques and modelling. UPCreaches an upper limit or saturates for increasing of large Esw values [e.g. Borovsky et al., 2009; Kan et al, 2010; Lyatsky et al., 2010; Gao et al., 2013]. Merkin et al. [2007] and Lopez et al. [2009] demonstrated that the polar cap magnetic flux also saturates (at around 1 GWb) as the polar cap potential saturates. Transpolar potential saturation is the limit on transpolar potential that corresponds to the ram-pressure limit on total region 1 current [Siscoe et al., 2002]. Because during superstorms both Esw and Pd reach unusually high values, there is a question about the influence of Pd on the saturation level during superstorms. However, all above studies including Siscoe et al. [2002] have not obtained the saturation effect depending on the SW dynamic pressure. Merkin et al. [2007] obtained the saturation as consequence of the nightside reconnection point motion towards the inner plasma sheet boundary. However, these authors as also Siscoe et al. [2002] supposed that the dayside magnetosphere can also play very important role in the saturation effect and wanted to study this possibility but did not do yet. This task was carried out in [Karavaev et a!., 2012 a, b], where by the method of magnetogram inversion technique (MIT) of ground-based magnetometers, we found the saturation effect of the polar cap magnetic flux, depending on the SW dynamic pressure Pd from data of the superstorms (24-25) September 1998 and 20 November 2003. In the present study, these results are generalized, involving also the (6-7) April 2000 superstorm data. In addition, we use the results of numerical modeling of the global PPMLR- MHD model of the magnetosphere for the 20 November 2003 superstorm. We compare the graphs of variations of the energy flux into the magnetosphere, depending on the SW parameters Esw, B2, and Pd obtained by the two methods. Moreover, we carried out an analytical and numerical analysis of the dependence of the subsolar magnetopause point position, on Pd as well as on the southward IMF component. It is shown a presence of saturation effects, not only of the magnetic flux VF and the corresponding energy flux through the polar cap, but also of the dayside magnetopause position - the suspension of its compression at strengthening SW. It is suggested that both effects are interrelated. We do not consider here other possible important effects in this study, which may also change the saturation level of UPC and VF, for example, the ionosphere and SW conductivity effects [e.g. Lyatsky et al., 2010; Kan et al., 2010, Gao et al., 2013]. The database and processing methods We use data of the superstorms: 24 September 1998, 6 April 2000 and 20 November 2003. The main input for the magnetogram inversion technique (MIT) [e.g. Mishin, 1990] are data of more than 110 ground magnetometers of the Northern Hemisphere. Table 1. SW conditions and geomagnetic indexes Date\ Para­ meters AE, nT SYM- H, nT Bz, nT By, nT Pd, nPa 24-25 Sept. 1998 7 3 - 2865 <-300 -23 - 2 3 -13 - 3 9 0 .5 - 28 6-7 Aug. 2000 22 - 2000 <-300 -30 - 21 -30 - 8 0 .7 - 25 20 Nov. 2003 1 3 - 3250 <-450 -53 - 30 -26 - 4 4 1 .5 - 26 Table 1 shows the range o f variations of the observed SW parameters and AE index, and also the minimum value of the SYM-H index for the three superstorms above. Abnormally strong values of IMF and Pd caused the appropriate activity of the aurora and ring current: AE- 40