Özet:

The occurrences of the disturbed geomagnetic activities (RA, SA and FA) according to both classifications (AC and NC) have been studied at Dakar ionosonde station (Lat: 14.8°N Long: 342.6°E). RA occurs more during equinoctial months than during solstices thus there is a semi-annual asymmetry during RA (which is more marked during the AC). The maximum negative , observed during summer, is  and the maximum positive one, observed during spring, is . SA of the AC occurs more during equinoctial times than during solstice periods. Therefore, there is a semi-annual asymmetry and a winter anomaly is also observed but, for the NC the SAs occurrence dont differ considerably with the seasons. The maximum negative , observed during summer, is  and the maximum positive one, observed during spring time is . During spring and summer, the ACs FA occurrences are greater than those of the NC and we observe the opposite during autumn and winter. The winter and equinoctial anomalies are obviously observed with the FA of the AC but for those of the NC, we observe an absence of equinoctial anomaly and a slight winter asymmetry. The maximum negative , observed during autumn, is  and the maximum positive one, observed during summer, is . The ionosphere F2 layer critical frequency foF2 diurnal profiles throughout seasons concerning both classifications have been compared. The FA diurnal profiles dont show difference over seasons but the RAs and the SAs profiles show an obviousness difference between both classifications during summer time and a non-considerable difference at spring time.

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