Электродинамические процессы в высоких широтах: материалы междунарордного симпозиума «Полярные геомагнитные явления», 25-31 мая 1986 г., Суздаль, СССР / Междунар. геодез. и геофиз. союз, Междунар. ассоц. геомагнетизма и аэрономии ; Акад. наук СССР, Кол. фил. им. С. М. Кирова, Поляр. геофиз. ин-т ; [редкол.: О. М. Распопов (отв. ред.). и др.]. - Апатиты : Кольский филиал АН СССР, 1988.

occuring during such global quiet periods from June 72 to December 75. The geophysical situation around these events hea bee n described by magnetic indices as well as by data concerning the interplanetary magnetic field and the solar wind. For the case studies records have bee n used fro m riometer stations (see Table 1) as well as fr o m magnetic stations (see Table 2) in high latitudes. W i t h i n the interval June 72 to December 75 16 AA-events have be e n sele cted disturbed over all local time sectors. S ix events w i t h large amplitudes w e r p "slowly varying absorption" - (SVA)-events (see e.g. B r o w n , 1964} Bewers- dorff et al.,1966) occuring in the late m o rning sector. As a base for future studies the data for five different events have been w orked up. These events will be discussed in short below. The data presentation is the same in all cases. Fart (a) of the figure describes the geophysical situation. I n the upper left h a n d c o m e r the daily m e a n values of Dst, the A p and the I Kp-values are shown for five days wit h the diay of the event wider study in the center. Be l o w me a n h ou r l y values are gi ve n for the Ae- and Det-index, the de n s i t y a n d the velocity of the solar wind, the amplitude and the Bz--component of the interplanetary magnetic field in dependence on universal time (UT). The duration of the AA-event in Scandinavia is ma r k ed b y vertical lines. Part (b) of the figure gives the magnetic variations recorded at a n umber of stations around the auroral zone, w h i c h are close to corresponding r iometer stations or to magnetic midnight. All the variations are shown in the same scale. In the center of that figure a m ap (geographic coordinates a nd L=const.) is given w h i c h has b e e n oriented in such a w a y that (magnetic) local midnight is downwards. The location of the stations have b e e n indica ted. Part (c) of the figure presents the auroral absorption in the same manner. In order to avoid overloading in this pioture the absence of a bsorp tion at a station has b e e n marked by a dark dot in the m a p at the location of the station. 4. D e s cr iption of the selected events: 4.1. Example for SVA-events: August 16,1974) 4.00 - 10.00 UT, The event occured in a globally v e r y quiet period (see Fig.1a). Dst has been positive at least for 3 days, AE is ne a r zero, and Bz(IMF) directed no r t h wards. At about 1.00 UT it turns southward and turns b a c k to more northward direction between 4.00 U T and 5.00 UT, At 4.45 U T a magnetic substorm starts (see Fig.1b n e a r midnight). The riometer stations Godthfib and Narssarssuaq (about 2,00 15LT) see a v e r y small a bs orption Increase beginning at 5.00 UT, some indications are also seen at Thorshavn (6.00 MLT'), but at the same time (5,00-5,03 UT) starts a strong SVA-event, seen b y the F i n nish stations (8.00 MLT), The premidnight station Churchill (22.00 MLT) sees a sharp onset at 5.12 UT and the stations in the evening sector detect absorption still later. P re c i pitation of h i g h energy electrons is founded only o n a latitudinal very n a r r o w region, a nd only i n the SVA-region it occurs as a more widespread l arge-scale phenomenon. H aving i n mind, that we are studying globally very quiet periods it is astonishing that the SVA-event amplitudes are of the same order than those occurring during storms. The time delay be t ween the onset of the magnetic substorm In the midnight sector and the onset of the SVA- event is in the same range as found for more active conditions (Bewersdorff et al.,1968; Kangas et al.,1975). But using riometer da^a in the post- 91