5. Conclusions

[26]  The main results of our analysis can be summarized as follows:

[27]  1. An analysis of all available N_mF2 observations over Huancayo (American sector) and Kodaikanal (Indian sector) during geomagnetically quiet periods has revealed both positive and negative Q disturbances, their amplitude being comparable to F2 -layer storm effects resulted from increased geomagnetic activity.

[28]  2. Contrary to middle latitudes, negative Q disturbances are more numerous compared to positive ones at both stations and all levels of solar activity. The percentage of long (both negative and positive) disturbances is larger in the American sector. Long-duration ( 3 hours) disturbances are more numerous at solar minimum compared to solar maximum.

[29]  3. The majority of negative long-duration disturbances occur in the dark LT sector, and they are practically absent during daytime hours. At Huancayo, negative disturbances are clustering only in the postmidnight LT sector under solar maximum, but they cover the whole dark LT sector at solar minimum; the latter takes place at Kodaikanal for all solar activity levels.

[30]  4. Positive long-duration Q disturbances exhibit two occurrence maxima, the nighttime-early morning and the daytime ones. The nighttime peak is broad at Kodaikanal and covers the whole dark LT sector, while at Huancayo the peak localizes in the early morning LT sector similar to the negative disturbances occurrence.

[31]  5. Annual variations for the occurrence of both negative and positive Q disturbances at Huancayo exhibit a well-pronounced pattern with the occurrence maximizing in winter for all solar activity levels. On the contrary, daytime positive Q disturbances at Huancayo are observed only in summer. At Kodaikanal, annual variations for the occurrence are not distinct, although there is a tendency for a distribution with winter and summer maxima.

[32]  6. The revealed morphology of Q disturbances can be explained (at least qualitatively) by the observed EB vertical drifts, whose diurnal, seasonal, and solar activity variations are well documented especially in the American sector using Jicamarca observations. The situation at Kodaikanal is not that clear. Keeping in mind that variations of the F2 region at the geomagnetic equator is mostly controlled by EB vertical drifts, the morphological differences between Huancayo and Kodaikanal in Q disturbance occurrences should be attributed to corresponding differences in the vertical drift variations. However, the observed and modeled [e.g., Scherliess and Fejer, 1999] longitudinal (American/Indian) differences are not large enough to explain the revealed morphological differences. Further analyses using model calculations are required to clear up this problem as well as to answer the question if the observed EB drift variations are sufficient at a quantitative level to explain the revealed morphology of equatorial Q disturbances.