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.

Table 2 gives the values of f needed to explain the observed height va r i a tion of о£ if o£Bo2x10- ^cm^a- ^ (as for О and о^^=10- ^ст^з-1 (ae for H + (H20)g). The average values of Ы- were used in this estimate. While the actual values of f are speculative, they are consistent wi t h a transition from simple tq h y d r a t e d . i o n s » the ratio of their concentrations wo u l d be unity at about 78 km altitude. To explain such behaviour in detail it is necessary to reeort to mathematical models of the D region photochemistry, and this will be attempted in due course. Since the incoherent scatter spect r u m contains information about ion mass and temperature, and can indicate the presence of negative ions, it is important to include spectral measurements in future observations of polar cap absorption events. Table 2 POSSIBLE VALUES OP THE RATIO O F HYDRATED T O SIMPLE IONS h (km) 3 — 1 oie (om -sec ) f 85 6.5x10"7 0.05 80 2 .8X10"6 0.36 75 9. 4 x 1 0 ~ 6 15 70 9.1X10-6 11 CONCLUSIONS. 1. The EISCAT UHP r a d a r is able to make significant measurements of the D region electron d ensity at least down to 65 km during polar cap absorp tion events. 2. The m e asured electron density profiles account satisfactorily for the cosmic noise a b s orption observed w i t h riometers during the event. 3. Ionization production rates, computed fr o m proton fluxes measured on the GOES-5 satellite, were a m aximum at 65 k m altitude during most of th« event of F ebruary 1984,✓'but the proton spectrum was harder in the growth phase w h e n production rates maximized as low as 25 km. Ground-level effects were detected around the time of PCA onset. 4. The effective recombination rates determined from the calculated p roduction rates and the measured electron densities are generally consistent w i t h previous estimates. They show a gradual decrease during the day, wh i c h w as also observed b y Re a g a n and Watt (1976) and presumably indicates a p r o g ressive change in D region photochemistry. 5. The gradual change of effective recombination coefficient between 90 k m and 75 k m is consistent wi t h a transition from simple (Og) to hydrated ions, though a n accurate explanation will require the application of m a t h e matical models. 6. Fuiure observations of PCA events by incoherent-scatter radar should include the spectrum of the scattered signal to pi’ovide information on ion mass and temperature, and on negative io n concentration. It is re c o m mended that the high-latitude radars should prepare programs for P C A observ ations that can be put into operation at short notice wh e n an event occurs. II4