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. – 150 с.

few minutes between the SI and the onset of pulsation activity. One of the moat important results reported in above-mentioned papers which is of interest in the context of present study is an observation that there is often a step-like increase of the frequency of ongoing pulsations due to the SI. Pig. 1 shows that the SI may generate a new frequency band. There are pulsations at about 0.3 Hz when the SI occurs at 06.07 UT. After the SI three types of pulsations appear: Pel pulsations at about 0.4 and 1.2 Hz and a broad-band burst between these two. The low-frequency band is more intense at the low-altitude station Nurmijarvi whereas the 1.2 Hz pulsations are seen only at the high-latitude station Kevo. Thus the SI induces only minor changes in the pulsation spectrum at low latitudes but a totally new band appears at high latitudes. It is mo3t remarkable that the pulsations at 1.2 Hz are not saen or they are very weak at Hurmijarvi. Ordinary Pci's are typically observed with the same frequency aoross the meridional chain of stations (see e.g.Baransky et al.,1981). Pig. 2. Dynamic spectra of SI associated magnetic pulsations recorded at the Finnish north—south chain of stations on 22 M a r c n f 1 9 7 9 . 97