Аскаплоты советских станций за 1981 год : (оперативно-информационный материал) / Акад. наук СССР, Кол. фил. им. С. М. Кирова, Поляр. геофиз. ин-т. – Апатиты : Кольский филиал АН СССР, 1989.

12 Auroral PA index. The index of auroral activity calculated on the baais of ascaplots was suggested in /9/. It equals to: PA = H*K (Ф.Л ), where N is the number of blackened squares within the time interval, and К is the coefficient depending on the corrected geomagnetic coordinates. The connection between N and the auroral magnetic index AL for midnight hours was determined in /9/ from asca plots of 35 stations during IGY /1,2/. Using these data the dependence of K- coefficient on the latitude was obtained for two longitudinal intervals. This provides adjusting the data of the stations located at different latitudes to a common value. Since the К-index was calculated for midnight auroras the PA index is calculated only for a 4 hour interval in the region of the local geomagnetic midnight. AL scales are chosen so that the corresponding values of PA index is to be lineary proportional to the energy of auroral electrons of the auroral oval /9/. This permits to make arithmetic operations with PA index, such as averaging, without losing the physical content. Table 2 includes PA values for each day in 1981. For a separate station PA can be determined only for a 4 hour interval which may not coincide with the maximum of auroral activity. That is why Table 2 presents PA values obtained by averaging the indices of all stations operated during that day. The numeral in brackets following every index gives the number of operating stations the data of which were averaged in calculating the index. The trough in the Table denotes absence of observations on that day at local midnight. Auroras registered at cloudy conditions were taken into account in calculation of PA index. This change increases the representativity and statistical veracity of the data but, on the other hand, this leads to a rise of the PA average value. The correlation coefficient of PA value obtained according to different procedures is 0.7 . According to the new procedure, on the average, the value of PA index is higher by a factor of 1.3 . Auroral activity in 1981. Figure 3 presents PA index variations for each day in 1981. The points denoting gaps of observations are linked by dashed lines. The same figure contains corresponding diurnal changes of the Kp index sum (the scale downwards). The year of 1981 befalls with the period of high solar activity. A character istic feature of PA index variations is a sharp modulation of auroral activity, i.e. alternation between quiet intervals and periods of significant rise of magnetic activity with mean PA values often exceeding 5. A tendency of recurrence of activity with a 12 day quasi-period is observed. Figure 4 displays a histogram of distribu tion of time intervals between auroral activity rises with PA 4. The histogram displays a distinctly manifested maximum for 11-15 day recurrence and the distribution itself is not similar to the Hauss one. The obtained mean quasi period is approximately equal to a half of 27 day recurrence of appearence of solar spots. Figure 5a is a sketch of seasonal variation of auroral, magnetic and solar activity. The solid horizontal line marks the mean value of the corresponding activity taken as a point of reading. Auroral activity has no distinct seasonal variation ; in 1981 PA-index rises only in December. As usually seasonal variation