Ионосферные явления-79: материалы наблюдений Поляр. геофиз. ин-та по проекту Междунар. исслед. магнитосферы за 1979г. (оперативно-информационный материал). Апатиты, 1983.

6 appearance of the positive bay. The falle either coincide with the beginning of the bay or slightly precede it. Two mechanisms can be considered as the reasons of an abrupt fall of electron density. In the evening sector the equatorial boundary of tha auroral oval coincides with the region of field-aligned currents flowing into the ionosphere and, hence, the region where electrons leave the ionosphere along the field line. The effect of plasma drift caused by the poleward electric field increased during the disturbances can be proposed as the second mechanism. The westward plasma drift accelerates daily variations and leads to fast decrease of electron density. Three PCA events ?/ere observed during the year. The first one accompanied the magnetic storm with Sc at 19*16 UT on June 06, At Loparskaya station this storm was registered as moderate, though significant auroral absorption was observed since its beginning. On June 07 it transformed into PCA and lasted until June 08 when the iono- sond registered no reflections. The second event occured on August 20. It was ■ characterizes by anomalous absorption prior to the beginning of the magnetic storm, though absorption diminished after the commencement of the storm. Intensive riometric absorption and ionosond blackout were registered since the beginning of 20.08 event. A magnetic storm with Sc appears a little later at 06.25 UT on 20.08. At that moment absorption diminished, the ionosond registered reflections. Repeated growth of absorption occured 3 hours after the beginning of the storm and 5 hours later it led to blackout. Diminishing of absorption was accompanied by sharp compression of the magnetosphere which was manifested by changing of the equatorial Dst value from -20 nT before the magnetic storm to +46 nT after its beginning. High day-time absorption similar to PCA aocompanied an intensive but short-lasted magnetic storm which appeared on September 18. The Russian text presents the analysis of the ionospheric behaviour for each month of the year.