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.

MAGNETIC CLOUDS CONFIGURATION AND SEASOIS DEPENDENCE OF GEOMAGNETIC ACTIVITY N.A. Barkhatov E.A. Revunova A.E. Levitin2. 1Nizhny Novgorod State University o fArchitecture and Civil Engineering, Nizhniy Novgorod, Russia 2Pushkov Institute o f Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Moscow-Troitsk, Russia Abstract. Proposed and tested on experimental data a new model o f geomagnetic activity increase in intervals equinox associated with Solar wind magnetic clouds. It assumes that axial magnetic field in clouds at different times will be different projection on Earth dipole and hence have different geomagnetic effectiveness. As result o f the arrangement of magnetic clouds Solar sources are primarily formed clouds with small angles o f inclination of axial magnetic field in ecliptic plane. With this in mind, during equinox geomagnetic activity caused by such clouds will increase. 1. Introduction Long-term follow geomagnetic activity revealed the existence o f its semi-annual variations that occur in spring and autumn in form of peaks in long-term average values of various indices o f geomagnetic activity [Russell and McPherron, 1973]. Now to explain the seasonal variations of the geomagnetic activity there are two main groups of hypotheses: axial hypothesis and equinox hypothesis. Semiannual variation axial hypothesis of geomagnetic activity is related to changes in the Earth's latitude helioprojection on the Solar disk, depending on the season [Cortie, 1912; Chapman and Bartels, 1940]. Due to the tilt Sun rotation axis to the Sun-Earth line at ~ 7°, spring and autumn projection o f angular distance o f Earth from the Sun's equator is maximum. In these periods, Earth is more closely related to active zones, which are grouped together in redistribution of 10 to 30 degrees north and south heliographic latitudes (royal latitude). Thus, during equinox, when Earth’s helioprojection latitude has maximum value, it is most likely to aim in geoeffective Solar flows from active regions. Due to this geomagnetic activity is increased (Fig. 1). “Physics o f Auroral Phenomena", Proc. XXXVI Annual Seminar, Apatity, pp. 110 -1 1 2 , 2013 © Kola Science Centre, Russian Academy of Science, 2013 Fig. 1 Change o f latitude on Earth helioprojection on Sun disk, depending on season December June March September Equinox hypothesis of geomagnetic activity semiannual variation associated with peculiarities o f impact on magnetosphere magnetized Solar wind. There are two main models of this impact - Kelvin-Helmholtz instability (Boller-Stolov model) and Russell-McPherron effect [Boiler and Stolov, 1970; Russell and McPherron, 1973]. Kelvin-Helmholtz instability occurs on magnetosphere flanks and its intensity depends on daily and annual changes in angle between Earth’s dipole and Solar wind flow. The maximum o f the wave instability falls on the equinoxes (when the Earth's dipole is perpendicular to Solar wind flow), and the minimum - for periods solstice. It is assumed that Kelvin-Helmholtz instability observed at magnetopause, initiates geomagnetic disturbances, which are defined as semi-annual changes in geomagnetic activity [Boiler and Stolov, 1970]. According to hypothesis proposed by Russell and McPherron, interplanetary magnetic field (IMF) should be considered objectively in the solar equatorial coordinate system (GSEQ). In turn, interaction o f IMF with Earth’s magnetosphere be seen in Solar magnetospheric coordinate system (GSM). These coordinate systems have a common axis X, which indicates on Sun, and Y axis and Z axis are different rotation about X axis. Since all systems have common X axis only. Consequently, value of geoeffective Bz component will change during the transition from one coordinate system to another. Thus, this hypothesis explains the semi-annual variation o f geomagnetic activity as result of additional values of southern component of the IMF in terms o f GSEQ in GSM [Russell and McPherron, 1973]. All of hypotheses seasonal dependence of geomagnetic activity take into account relative orientation o f the Sun, Solar wind and the Earth's dipole only, without affecting the internal structure o f the flows and their characteristics! Such geoeffective magnetic structure as clouds, unlike other plasma flows have a particular orientation in space during year and must observe a change in the projection of the axial magnetic field o f cloud on the Earth's dipole. That is why in times of equinoxes and solstices magnetic clouds can cause magnetic storms of varying intensity. Polar Geophysical Institute JL 110