Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

Clasification o fionospheric disturbances at middle latitudes based on long-term observations The first disturbance class. This class corresponds to the quiet state of the ionosphere. In the year of Solar activity minimum (1975) for each month were considered international undisturbed days within Kp index values were near zero (Q-days). Each month has 5 such days [http://wdc.kugi.kyoto-u.ac.jp/qddays/index.html] . During this period the average positive and negative CCF deviations from the zero level were 0.75 Hz and -0.7 Hz respectively. The minimum value of Dst index in the considered undisturbed 60 days during the year of minimum Solar activity was -25 nT. The second disturbance class. This class corresponds to the state of the ionosphere with small disturbance. In the years of minimum Solar activity were considered international disturbed days (D-days). Average positive and negative CCF deviations from the zero level in these days were 0.8 Hz and -0.75 Hz respectively. The minimum value of Dst index in the studied disturbed days was -110 nT. The third disturbance class. This class corresponds to increased ionospheric disturbance. For each month of maximum Solar activity year (1982) examined international undisturbed days (Q-days). Average positive and negative CCF deviations from the zero level were 1.2 Hz and -1.2 Hz respectively. The minimum value of Dst index in the studied days was —83 nT. Note that the level of global geomagnetic disturbances measured by the value of Dst is lower than in the second class. However during the ionospheric disturbances classification it should consider not only the level of geomagnetic activity but also the overall Solar activity level. It is known that during Solar maximum the radio emission flow is greatly increased which leads to ionospheric ionization increase and hence to increase of critical frequencies variation amplitudes. The fourth disturbance class. It corresponds to a high ionospheric disturbance. Here we used to consider international disturbed days in the year of Solar activity maximum. Average positive and negative values of CCF deviations from the zero level were 1.5 Hz and -1.5 Hz respectively. The minimum value of Dst index during considered period was -325 nT. Note that for a significant increase in the geomagnetic disturbance compared to the second and third classes, the mean CCF amplitude increased slightly. This may be due to an absence of direct connection between midlatitude ionosphere and magnetospheric currents. The fifth disturbance class. It corresponds to the state of ionosphere with extreme disturbance. This class includes events with CCF deviation amplitudes from the zero level more than 1.5 Hz or less -1.5 Hz. Selected ionospheric disturbances classes were form the basis for the index of ionospheric activity LAI. These index values take into account both positive and negative CCF variations within each of five disturbance classes. In Table 1 to each of disturbance class values conferred values of index IAI. Table 1. Ionospheric disturbance class Intervals of CCF values Values of ionospheric activity index IAI Class 1 -0.7 < CCF < 0.75 0 0.75 < CCF <0.8 1 Class 2 -0.75 < CCF < -0 .7 -1 0.8 < CCF < 1.2 2 Class 3 -1.2 < CCF <-0.75 -2 1.2 < CCF < 1.5 3 Class 4 -1.5 < CCF < -1 .2 -3 CCF > 1.5 4 Class 5 CCF < -1 .5 -4 IAI index values were computed for all seasons in years of minimum and maximum Solar activity and compared with the global geomagnetic disturbances characterized by Dst index and disturbances in the auroral region characterized by index AE. The top panel of Fig. 3 shows examples of IAI variations for April 1975 and June 1982, the middle panel shows the corresponding change in the index Dst, the bottom panel - the variation of AE index. Gray color indicates increased values o f index LAI observed within the intense magnetic storms and auroral disturbances. Comparison of IAI index values with values of indices Dst and AE (see Fig. 3) shows high negative values of IAI in times o f geomagnetic disturbances. For two geomagnetic disturbances in April 1975 the maximum negative value of IAI = -2 is marked. Note what during this period there is activity burst in the auroral region - AE index value are more than 1000 nT. In June 1982 there are several geomagnetic storms of low intensity. During the first storm the maximum negative value o f index IAI = -4 is marked which corresponds to a class of extreme ionospheric disturbance. Note that in this period took place a sequence of intense auroral substorms and each of them has exceeded values of index AE - up 1000 nT. For second magnetic storm minimal value of index IAI = -3 is marked. Within this interval the activity in auroral region is also high - index AE ~ 700 nT. 127