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.

can be a continuation of the Pel activity. Such a situation ie illustrated in Fig.3 where the dynamic spectra of pulsations recorded at Kevo and Oulu on 15 December, 1984 are shown. Strong Pel activity ocours before the onset of subatorm at 20.30 UT. The onset can be identified by a PiB pulsation burst which is seen in the dynamic spectrum as a vertical line (see, e.g. Bosinger et al.,1981). Pel pulsations continue at nearly constant frequency to the onset of substorm at Oulu. The amplitude analysis of all stations shows that the most intense pulsations have been recorded all the time at Oulu. This indicates thet the ionospheric source of the Pel pulsations has not been far from Oulu latitude. Immediately after the onset IPDP type magnetic pulsations have been recorded at Oulu. A gradual increase of the frequency of pulsations occurs and a pronounced intensification of pulsations takes place after the second PiB at about 20.40 UT. It is usually believed that the upward sweep of the frequency is related to the radial displacement of the IPDP source due to the S i f drift, especi ally near the midnight sector (Heacock et al.,1976). But no such direct meas urements have been presented so far. It occurred that a meridional scanning experiment was run by EISCAT on 15 December,1984. The experiment was started at 19.35 UT and it ended at 20.40 UT. The scan time was 30 minutes and thus data is available from two scans. Measurements have been made at 15 positions of the radar beam at the Tromsfrf meridian. The common volume for multistatic measurements was at 325 km. As the scan started from the north, the six last positions at 325 km cover approximately the latitudes between Sodankyla and Oulu. Only Troms^ and Sodankyla receiving stations were in operation during the experiment. In Pig. 4 we show the westward component of electric field at 250 km deduced from Tromsii measurements of plasma drift velocity. We notice that the field enhances after the onset of substorm and later a new intensification occurs. FiK 4. Westward component of the electric field at 250 km measured by the EISCAT radar on 15 December, 1984. Data from Tromsrf receiving station is used. 99