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.

“Physics o f Auroral Phenomena”, Proc. XL Annual Seminar, Apatity, pp. 86-88, 2017 © Polar Geophysical Institute, 2017 Polar Geophysical Institute SEMI-CENTENNIAL NORTH-SOUTH DISPLACEMENTS OF THE HCS BASED ON THE RECONSTRUCTED IMF SECTOR STRUCTURE M.V. Vokhmyanin, D.I. Ponyavin St-Petersburg State University, St-Petersburg, Russia A b stra c t. We present the analysis of the interplanetary magnetic field (IMF) sector structure reconstructed from geomagnetic data in the 19th and 20th centuries. During most of the 20th century the IMF polarity is inferred due to the Svalgaard-Mansurov effect using high latitude geomagnetic variations. The IMF polarity in the 19th century was inferred using mid-latitude observations. The latter is possible due to the ground magnetic effect of the field-aligned currents which are asymmetric during the IMF with non-zero By component. The reconstructed IMF sector structure reveals semi-centennial north-south displacements of the heliospheric current sheet (HCS). According to our results the dance of the “ballerina” was not bashful during 13(14)-19 solar cycles. In tro d u c tio n Due to the solar wind, the dipole magnetic field of the Sun extends into interplanetary space, forming the interplanetary' magnetic field (IMF). The IMF polarity is determined by the large-scale magnetic field of the Sun. The heliospheric current sheet (HCS) divides the IMF of opposite directions. Negative polarity is defined by the direction along the magnetic field lines toward (T) the Sun, and positive polarity - away (A) from the Sun. To reconstruct an alternation of the IMF polarity prior the satellite era, geomagnetic data in the past can be used (Svalgaard , 1972; Mansurov et al., 1973; Vennerstroem et al., 2001; Berti et al., 2006). The IMF controls magnetospheric and ionospheric currents, which cause different variations of the ground magnetic field. In case o f the IMF with non-zero By component, the high-latitude system of the field-aligned currents rotates either in clockwise, or in counterclockwise direction. This results in different variations o f midlatitude geomagnetic field, especially during the IMF with negative Bz component. These findings allows us to infer the IMF sector structure from the old geomagnetic records made at Saint-Petersburg, Helsinki, Ekaterinburg, Potsdam and other stations since the middle of the 19th century ( Vokhmyanin and Ponyavin, 2013, 2016). Analyzing the IMF data at distances of 0.7-1.5 AU to the Sun, in the range of latitudes ±7.3° (satellites Mariner 2, 4, 5, and OGO 5), Rosenberg and Coleman (1969) found the predominance of one or another polarity. Due to the inclination of the solar axis of rotation to the plane of the ecliptic, during periods near the equinox the IMF predominates with the polarity of the hemisphere of the Sun which is inclined toward the observer (the Rosenberg - Coleman effect). The excess of one polarity is pronounced in case of poloidal solar magnetic field, i.e. within minima of solar activity. But besides clear evidence for the R-C effect ( Vokhmyanin and Ponyavin, 2012, 2013) in fall and spring data, the reconstructed sector structure also show the consistent predominance o f one IMF polarity on annual scale. The difference in the widths of the two magnetic sectors was revealed in the simultaneous observations by Ulysses, Wind, and IMP-8. Smith et al. (2000) proposed a simple physical explanation of the offset of the sector structure in the ecliptic near solar minimum. The physical nature of this offset is a deflection of the HCS southward/northward, which makes it resemble a ballerina skirt. The average radial fields above and below the current sheet will be different depending on the solid angles that they occupy in the two hemispheres (Smith, 2011). In this paper, we analyse the excess of the IMF polarities recovered from the geomagnetic data. This, in turn, characterizes the north-south HCS displacements since 1844. N-S asymm etry in th e IM F d a ta The IMF By effect is seen even outside the auroral oval, at mid and low latitudes. This allows inferring the IMF By polarities far more back in the past than it was when only high-latitude geomagnetic data were used. In Vokhmyanin and Ponyavin (2016), we estimated the success rate of the IMF sector structure inferred from old geomagnetic observations in Europe (middle latitudes) to be about 65% before 1880, 75% in 1885-1901 and more than 80% for 1902-2010. This assumes the reliability o f the sector structure proxies. In this work, we divide the IMF polarity proxies in six groups according to the availability of the geomagnetic data in the past. Data set 1 is based on the geomagnetic observations in Saint-Petersburg and Helsinki. In Set 2 we add results from Ekaterinburg and Potsdam, in Set 3 - from De Bilt and Sitka, in Set 4 - from Sodankyla and Eskdalemuir, in Set 5 - from Godhavn and Lerwick, in Set 6 - from polar station Thule. For each set the daily IMF polarities P are calculated according to the following formula: P ^ s g n ^ H ' + DO) 8 6