Структура и динамика полярных токовых систем : материалы международного симпозиума «Полярные геомагнитные явления», 25-31 мая, Суздаль, СССР / Акад. наук СССР, Кол. фил. им. С. М. Кирова, Поляр. геофиз. ин-т. – Апатиты : [б. и.], 1988.

statistical analyses (Poster et al.,1971; Caan et al.,1978; Dmitrieva and Sergeev,1983; Troshichev et al.,1985). Fig. 9 shows the results of the epoch superposition analysis carried out by Troshichev et al.(1985) on the basis of the 5-minute data on the IMF and 2.5-minute data on AU and AL indices for 1978-79. To define the relative rola of various components of the IMF on substorm dynamics the following limita tions are taken in selecting the events in this analysis: the "examined" IMF component should be negative (positive) for an hour before moment TQ and positive (negative) for half an hour after while the "background" IMF compon** ent should be invariable in sign for this period, the events with TQ falling within the substorm active phase are excluded from the analysis. The north ward reversal of B„ following" the period of its southward orientation proves to be generating the increase of magnetic activity (AL-index) being the most effective when By component is negative. The similar analysis carried out for the cases of short positive impulses in B z shows their inefficiency. The role of the IMF azimuthal component in the dynamics of subatorms has not been taken into account until recently though the first indication of it • was made by Pudovkin et al.(1970). Fig. 10 gives some examples of the substorm explosive development affected by a turning of the azimuthal component from the positive to the negative direction. Such reversal of By may increase the magnetic activity even under the northward component (Fig.10c) provided the amplitude of By change is sufficiently large. The radial IMF component (Bx ) was invariable in these cases and hence just By component effects are observed. The results of statistical treatment make it possible to evaluate the efficiency of the reversal of By from the positive to negative direction: AL enhances up to — 50 nT in winter and ~ 15 nT in summer per 1 nT change in B y (provided Вz < 0) (Fig.11). а ь с Fig. 10. Response of magnetic activity to the reversal of the IMF В component from the positive to the negative direction while В and В y components do not change their signs: a - Jan.7,1979; b - Jan. 6,1978'? с- June 5, 1978 (Troshichev et al.,1985). I26