Statistical study of anomalous nighttime maximums in the NmF 2 diurnal variations in the region of appearance of the equatorial anomaly northern crest

The occurrence probabilities of the first and second anomalous nighttime local maximums in the diurnal variations in the electron density at a maximum of the ionospheric F ₂ layer (NmF2) in the region where the crest (hump) of the equatorial anomaly originates in the northern geographic hemisphere have been studied using the data of the stations for vertical sounding of the ionosphere (Paramaribo, Dakar, Quagadougou, Ahmedabad, Delhi, Calcutta, Chongoing, Guangzhou, Taipei, Chung-Li, Okinawa, Yamagawa, Panama, and Bogota) from 1957 to 2004. It has been demonstrated that the anomalous nighttime NmF2 maximums are least frequently formed at ～53° geomagnetic longitude. The calculations have indicated that the studied probabilities are independent of solar activity. Geomagnetic activity weakly affects the rate of occurrence of the first nighttime NmF2 maximum at geomagnetic longitudes of approximately 140° to 358°. At geomagnetic longitudes of approximately 16° to 70° (i.e., in the longitudinal zone of a decreased occurrence frequency of anomalous nighttime maximums), the occurrence probability of the first anomalous nighttime NmF2 maximum under geomagnetically quiet conditions is pronouncedly lower than under geomagnetically disturbed conditions. The dependence of the occurrence probabilities of the first and second anomalous nighttime NmF2 maximums on the month number in a year has been studied.