Physics of auroral phenomena : proceedings of the 33rd Annual seminar, Apatity, 02 - 05 March, 2010 / [ed.: A.G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 206 с. : ил.

“P hysics o f Auroral P henom ena”, Proc. XXXIII A n n u a l Sem inar, A patity, pp. 5 5 - 58, 2011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute THE SOLAR WIND ^-EVENTS AND THEIR EFFECTS ON THE GEOMAGNETIC ACTIVITY L.A. D rem ukhina 1 , A.E. L ev itin1, N.M. Rudneva2, L.I. G rom ova 1 'Pushkov Institute Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Troitsk, Russia 2 Fedorov Institute o fApplied Geophysics, Moscow, Russia Abstract, Southern vertical component of the Interplanetary Magnetic Field (IMF Bz < 0) is usually regarded as a predictor o f the magnetospheric electromagnetic state disturbance. If IMF Bz < -5 nT for 3 hours consecutively, this component indicates the occurrence of a magnetic storm and its development. So far, this criterion is commonly accepted. As a result, a geomagnetic forecast turns out to be reduced to the IMF Bz variation forecast, insufficiently reliable, for the indistinct role o f the solar activity in the generating IMF Bz sign and sign change during the solar wind moving from the Sun to the Earth. The geomapetic activity caused by solar plasma ejections depends on the Earth’s moving trajectory through the body of a solar plasma ejection. Meanwhile the Earth’s should cross the ejection area during interval when amplitude of IMF B: is great but positive and these conditions do not cause a magnetic storm generation. We have examined hourly average \Bz\ measured by satellites in near space and found Sr-events when \Bz\ > 5 nT during 4-hour interval. The events when the Earth, while crossing a solar ejection, experiences positive and negative Bz-events during a year, are compared. Moreover, the results of statistical analysis of the influence of Bz-events on the geomagnetic activity are presented. Introduction The southern vertical component o f the Interplanetary Magnetic Field (IMF Bz < 0) is the most geoeffective parameter. [ Tsurutani, at al., 1989]. The most intense geomagnetic storms with peak Dst < -100 nT are primarily caused by strong negative vertical IMF component with duration greater than 3 hours. [ Gonzalez. at al., 1994]. Therefore the modem prediction of the geomagnetic activity bases on forecast of occurrence of IMF Bz < 0 in the solar wind region close to the magnetosphere. Meanwhile question about origin of increasing this geoeffective parameter amplitude is leaving open, and we don’t know whether occurrence o f the southern vertical IMF component is caused by the solar magnetic field or great amplitudes of negative IMF Bz occur during the solar wind moving from the Sun to the Earth in the interplanetary space [Tsurutani et al., 1989] As noted, with increasing of statistic data for coronal plasma solar mass ejection (CME) condition for occurrence of intense geomagnetic disturbances could be created both in the solar ejecta and as in the region of interaction of the mass ejection with the solar wind during in the interplanetary space. Results o f modelling o f the magnetic field within magnetic cloud show a magnetic storm could be caused by magnetic cloud i f : - the solar mass ejections occur in the specific solar region from which they probability direct towards the Earth and reaching it; - the negative vertical IMF Bz component is observed in this magnetic cloud; - the Earth should move through the body of the cloud where magnetic filed o f the negative vertical IMF Bz component is observed. Therefore it is rather possible that duration o f phases and amplitude characters of magnetic storms depend on the Earth’s trajectory moving though the body of the solar mass ejection only. Depending on the Earth’s trajectory mass ejection should cause magnetic storms with different intensity and duration or could not originate a magnetic strom at all. Correspondingly, it is important to examine proportion between number of positive and negative Bz-events observed by satellites. The geomagnetic activity is estimated by DsMndex. From study o f magnetic storms and variation of negative IMF Bz during these storms it has been established relationship between magnetic storm classes and observed IMF Bz with duration equal to 3 hours (See Table 1). Table 1. Classification of magnetic storms Magnetic storm class Dst peak, nT IMF Bz (nT) with duration equal to 3 hours in the beginning of a storm Weak -30 - -50 -5 Moderate -50 - -100 - 10 Intense -100 - -200 - 15 Major -200 - -350 - 20 55