Physics of auroral phenomena : proceedings of the 40th annual seminar, Apatity, 13-17 March, 2017 / [ed. board: N. V. Semenova, A. G. Yahnin]. - Апатиты : Издательство Кольского научного центра РАН, 2017. - 143 с. : ил., табл.

М. V. Vokhmyanin and D.I. Ponyavin A (away from the Sun) is the number of days with positive IMF polarities and T — with negative (toward the Sun). In Fig. 1, the cause of inequality between T and A is shown schematically. As Smith et al. (2000) explain, if southern polar field dominates (minima Ti and T 2 on scheme), the HCS shifts southward and we observe more IMF with polarity of the northern hemisphere. At the ecliptic plane this is more clearly seen when solar magnetic field has poloidal and axisymmetric form, i.e. within the solar activity minima. In case of consistent HCS displacement and due to regular reversals of the solar magnetic field, the ТА ratio has opposite signs within two consecutive minima o f solar activity. Positive maxima between odd and even cycles indicate southern displacements of the HCS (due to negative polarity of the solar magnetic field in the northern hemisphere). In the opposite case, the ТА indicates northern HCS displacements. In Fig. 1, the change occurs in minima T 0 , resulting in phase shift o f the ТА wave. The ТА values obtained from polarity proxies are shown in Fig. 2 using different colors for different data sets. For satellite period 1967-2013, we use the IMF data from the OMNI data base (purple curve). It is seen that all polarity proxies are able to reproduce actual ТА ratio fairly closc. The approximate wave of the ТА ratio during 1844-2016 is indicated with dashed grey curve. Red vertical lines denote changes in phase of the ТА wave. Our results suggest the HCS is coned southward during cycles 9-12 and 20-24, and northward in cycles 13-19. Hiltula and Mursula (2006, 2007) also investigate reconstructed sector structure to find the HCS displacement. They use the polarity proxies obtained by Svalgaard (1972) and Vennerstrom (2001) and found that the HCS was shifted southward for the entire period of study, 1926 - 2006. We suggest that this result is wrong due to lower quality of the above polarity proxies. Besides, our assumption on the HCS displacements is supported by other studies o f the north-south asymmetries: in differential rotation ( Zhang et al. 2013, Pulkkinen and Tuominen, 1998) and in solar activity ( Verma 1992, 1993). Conclusions The use of the midlatitude geomagnetic data allows us to infer the IMF sector structure and track the evolution o f the north-south asymmetry of the solar magnetic field. We find that the HCS was shifted northward in cycles 13-19, and southward in cycles 9-12 and 20-24. The same N-S asymmetry is found in other solar data. We suggest that this asymmetry changes with the period o f Gleisberg cycle where northern solar magnetic field dominates on the ascending phase and southern on the descending. Acknow ledgements. This work was partially supported by the RFBR research projects No. 15-02-06959-a and No. 16-02-00300-a. References Berti R., Laurenza М., Moreno G., Storini, М., (2006). J. Geophys. Res., I l l , A06I09, doi: 10.1029/2005JA011325 Hiltula, Т., Mursula, K. (2006). Geoph. Res. Lett., 33(3), L03105, doi.org/10.1029/2005GL025198 Hiltula, Т., Mursula, K. (2007). Advances in Space Research, 40(7), 1054-1059, doi.org/10.1016/j.asr.2007.01.068 Mansurov S. М., Mansurova L. G., Heckman G. R. et al. (1973). IAGA Bulletin 34, 610 Pulkkinen, P., Tuominen, I. (1998), Astronomy and Astrophysics, 760, 755-760. Rosenberg R. L., Coleman P. J., (1969). J. Geophys. Res., 74, 5611, doi: 10.1029/JA074i024p05611 Smith E. J., Jokipii J. R., Kota J., Lepping R. P., Szabo A., (2000). Astrophys. J., 533, 1084, doi: 10.1086/308685 Smith E. J., (2011). JASTP, 73, 277, doi: 10.1016/j.jastp.2010.03.019 Svalgaard L., (1972). J. Geophys. Res., 77, 4027, doi: 10.1029/'JA077i022p04027 Venncrstroem S., Zieger B., Friis-Christensen E., (2001). J. Geophys. Res., 106, 16011, doi: 10.1029/2000JA000103 Vokhmyanin М. V., Ponyavin D. I., (2012). J. Geophys. Res., 117, 2156, doi: 10.1029/2011JA017060 Vokhmyanin М. V., Ponyavin D. I., (2013). Geoph. Res. Lett., 40, 3512, doi: 10.1002/grl.50749 Vokhmyanin М. V., Ponyavin D. I., (2016). J. Geophys. Res., 121, 11943, doi: 10.1002/2016JA023138 Verma, V. K., (1992), ASP Conf. Ser., 31, 429 Verma, V. K„ (1993), ApJ, 403, 797 Zhang, L., Mursula, K., Usoskin, I. (2013), Astronomy & Astrophysics, 552, A84. doi.org/10.1051/0004- 6361/201220693 8 8