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.

A,dB ?OHHz ABSORPTION, «84 FEB 16 Figure 1. Absorption measured w i t h riometers during polar cap event of 1984 F e b . 16. Using the radar at Chatanika, Alaska, Re agan and Watt (1976) studied the intense PCA of August 1972, the proton fluxes being measured on a polar orbiting satellite. Over a peri od of 5 days, satellite and radar data were compared on 28 occasions w h e n the satellite passed sufficiently close to Chatanika, and the behaviour of the effective recombination coefficient was investigated in some detail. The present paper reports measurements with the E uro pean Incoherent Scatter radar (EISCAT) duringjthe we a k e r PCA that occurred on February 16, 1984. Ionization rates were computed from the proton fluxes measured on GOES-5, and the effective recom bina tion coef ficient was evaluated as a function of altitude during the event. The results have been compared wi t h those of Reagan and Watt. THE POLAR CAP ABSORPTION EVENT OF FEBRUARY 16, 1984. The event started about 0915 UT; the proton fluxes at GOES-5 peaked between 1000 an d 1100 UT and thereafter decayed over the next two days. Riometers in Scandinavia began to register absorption increased to a peak about 1010 UT and decayed there after at most of the stations, though a flat second peak around 1100-1130 UT w a s seen at some. The ma x i m u m absorption was about 3-3.5 dB on the 30 MHz riometers. The absorption wa s quite small at each station b y 1500 UT, and no measurable absorption was detected on the following day. On February 16 the ionosphere over Tromso was illuminated by the sun between 0551 and 1604 UT at 50 km altitude and b etween 0517 a n d 1639 UT at 100 km. The absorption event was therefore entirely w i t h i n the sunlit period, and no sunrise or sun set effect is expected. Latitude profiles of absorption along the t wo longitude chains at vari ous times during the event (Figure 2) show the location of the equatorward edge a n d some spatial non-uniformally (which m a y be due in part to the use of different riometers). These profiles show that the EISCAT observations were sufficiently far fr o m the edge to be considered as fully wi t h i n the polar cap event. The possibility of minor spatial variations must be kept in mind, however. E I SCAT MEASUREMENTS. The measurements were made w i t h the EISCAT U HF system at 933 MHz in monostatic mode at Ramfjord, near Tromso (69.6C‘N, 19.2°E, L=6.2, invariant latitude 66.3°). Pr o g r a m code ESLA-14, wh i c h is d e signed according to the principles described by Turunen and Silen (1984) and Turunen et al.(1985) wa s operated between 1010 UT and 1357 UT, a period of nearly 4 hours. The b e a m was pointed to the zenith except for periods when it was pointed at zenith angle 45° to the west or to the south from 1 1 1 0 to 1150 UT and after 1345 UT..The program uses Ba r k e r coded pulses giving 6 0 0 m range resolution starting at 58.8 km. The basic time resolution was 1 0 seconds, but the data were Initially averaged over 1 minute and later I08