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

5 The activity during the second half of the year is weaker than in the first one and recurrenoy is less significant. The maximum of activity, usually registered near the autumn equinox, did not occur during the studied year. Pig.2 shows that at the beginning of the year active days are arranged both in 27 and 28 day repetition periods, the sequences are presented both by horizontal and inclined lines on the plot in Pig.3. The sequences of active days in the second half of the year are denoted by horizontal lines only, the repetition period being 27 days. Previous investigations showed that in the first and second half of the year, respectively, the negative and positive sectors of IMF assisted to growth of the activity. The data of the studied year confirm , to some extend, this regulation, viz., rH level on the average is higher in the negative sector of the first half of the year than in the positive one. In the second half the, greater part of high activity days occurs in the positive IMP. Changes in illumination of the ionosphere determine electron density in D, E , F1 lower ionospheric layers; gradually changing density reaches its maximum nenr noon (10 (JT). These layers disappear during quiet winter nights. Disturbances creat additional ionization registered as appearance of sporadic E-layer as well as increase of absorption. Pig.3 presents foP-2 variations on quiet days averaged for seasons, whereas Pig 4 gives the same for months. The days without magnetic disturbances (r^j > 1 5 nT) with smooth foP-2 variation and.without continuous disturbances in the lower iono­ sphere were considered as quiet and are listed in Table 2. The increasing number of sunspots throughout the year was accompanied by growing of P-layer ionization. Daily foF-2 values at the end of the year are significantly greater than that at the beginning. In winter foF-2 variation itself is characterized by sharp increase of morning ionization, gaining the maximum near noon, which із followed by gradual fall towards night and morning hours. The number of days taken for determining variations was not large, hence the influence of disturbances was not eliminated completely, In particular, this influence is observed in growing of night ioniza­ tion marked on several plots in Pig.3,4. The width of day-time maximum increases together with growth of the duration of luminosity period towards summer. Daily foF-2 values reduce whereas night ones grow. In summer-time foF-2 diurnal variations of quiet days practically disappear. Increase of activity, appearance of geomagnetic disturbances lead to growth of electron density in P-layers during night hours as well as appearance of additional reflection (P-epread). Electron density is reduced during day hours at periods after prolonged magnetic storms and intensive nighttime substorms. Transformation of ordin­ ary conditions and gradual growth of doily ionization last for several days. In summer-time reduce of ordinary ionization by intensive disturbances, to some extend, restores diurnal variations and differences betv/een day and night, since disturbances appear mainly at night and are accompanied by reducing of ionization. The data of the studied year clearly show that appearance of evening disturbances is accompanied by sharp fall of foF-2. Gradual diminishing of electron density, typical for quiet variations, changes into an abrupt fall which is followed by a new rise connected with occurence of disturbances. The character of variations changes at the moment of the fall: the variation caused by the Earth's rotation is replaced by that connected with the substorm. The time of changing depends on the moment of appearance of the first positive magnetic bay. Pig.5 presents the relationship between the moment of the fall and the magnetic field. In accordance with f-plot of the present work the moment of electron density falls are denoted on magnetograms at January, February and October. It is evident that the fall occurs during various hours; for example, in October it happens between 10-19 UT, but in most cases close

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