Physics of auroral phenomena : proceedings of the 36th Annual seminar, Apatity, 26 February – 01 March, 2013 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2013. - 215 с. : ил., табл.

Thus, results presented in Fig. 2 show that projection of cloud axial magnetic field on the Earth's dipole varies depending on season. In solstice times greatest contribution to the geomagnetic activity should provide magnetic clouds with large values of inclination angle of cloud axis to ecliptic plane. In equinox periods in geomagnetic activity should contribute magnetic clouds any orientation. 3. Seasonal dependence of geomagnetic variations on the magnetic cloud structure Statistical testing of this hypothesis seasonal variation of geomagnetic activity performed on the data on orientation of 52 magnetic clouds, marked by OMNI system in near-Earth space from 1980 to 2004. All events were divided into three groups according to their inclination of axis cloud to ecliptic plane e: from 0 to 30°, 30 to 60 and from 60 to 90°. For more explicit manifestations of the contribution of each group o f clouds on angle values e , was considered the percentage of geoeffective (Dst < -10 nT) and geoeffective clouds, each o f angle selected range (Fig. 3, gray bars - geoeffective, black bars - not geoeffective clouds). Fig. 3 shows each range of values of e is nearly 100 %. Magnetic clouds configuration and seasonal dependence o fgeomagnetic activity 0-30 30-60 60-90 0-30 30-60 60-90 Range of variation £. deg Range o f variation £, deg Fig. 3 Percentages groups of angle £ from 0 to 30°, 30 to 60°, 60 to 90° geoeffective (gray columns) and not geoeffective (black columns) of magnetic clouds According to the distribution shown in Fig. 3, during equinox 40% of clouds with large inclination angles o f axis to ecliptic plane were geoeffective. During solstice geomagnetic storm caused 85% o f these clouds. Therefore, in solstice times clouds with large e are indeed more geoeffective stmctures. During equinox 100% clouds with small values of angle s caused storm, and in solstice - 78%. Thus, our study shows that the geomagnetic activity during equinoxes increases due to clouds of different orientation and especially clouds with low values o f inclination of axis clouds to the ecliptic plane, most frequently detected in Earth’s vicinity. 4. Conclusions Proposed and tested on real events hypothesis is the seasonal variation of geomagnetic activity, taking into account orientation in space of large-scale plasma flows, such as magnetic clouds. According to proposed hypothesis, magnetic cloud orientation in space should manifest itself in variations o f geomagnetic activity, depending on season of year due to changes in projection of cloud axial magnetic field on Earth's dipole. This study has shown that in solstice times sources of geomagnetic storms were 85% o f magnetic clouds with large tilt angles (60 to 90°), and during the equinox, only 40% o f such clouds. During the solstice is not geoeffective were 20% of magnetic clouds with small angles of cloud axis inclination to ecliptic plane. During equinox, all (100%) of such clouds caused the geomagnetic disturbances. Consequently, during periods o f geomagnetic activity equinox increases due to magnetic clouds with small angles of axis inclination to ecliptic plane most frequently detected in near-Earth space. In solstice times such clouds are not geoeffective structures by reducing the value of axial projection of clouds magnetic field on Earth’s magnetic dipole at such intervals, which is reflected in the reduction of level of geomagnetic activity in the summer and winter. A cknow ledgem ents. This work was supported by RFBR grant 12-05-00425 and by the project "Development of modern methods of forecasting the state of the magnetosphere-ionosphere in order to ensure successful communication by finding the fundamental regularities of solar activity" o f Ministry o f Education and Science. References Cortie A.L. Sunspots and terrestrial magnetic phenomena, 1898-1911: the cause of the annual variation in magnetic disturbances / /Monthly Notices of the Royal Astronomical Society. 1912. V.73. P.52. Chapman, S., and J. Bartels // Geomagnetism, chap. 11. Oxford University Press. New York. 1940. Boiler, B. R., and H. L. Stolov. Kelvin-Helmholtz instability and the semiannual variation of geomagnetic activity // J Geophys Res. 1970. V.75. P. 6073-7084. Russell C.T., McPherron R.L. Semiannual variation of geomagnetic activity// J. Geophys. Res. 1973. V.78. P.24. Bothmer V., Schwenn R. The structure and origin ofmagnetic clouds in the solar wind // Ann. Geophysicae. 1998. V 16 P 1-24 112