Longitudinal features of day- and night-time ionospheric annual variations during the solar cycles 23 and 24

We present a study concerning the ionospheric annual variations (IAV) using the global ionospheric maps (GIMs). In this regard, the corresponding regional electron content (REC) is computed in four longitudinal sectors, namely Asia, Africa, America and Pacific. The features of day- and night-time IAV are investigated using (i) band-pass filters and (ii) summer to winter (SW) ratio of REC. The results indicate that IAV are stronger in the southern hemisphere as compared to its northern counterpart. The asymmetry in the amplitude of IAV is maximum for the Pacific region, followed by America, Africa and Asia. The IAV in southern hemisphere are regular ionospheric variations (RIAV) with positive and negative peaks occurring in the months of December and June, respectively. The northern hemisphere and latitudinal regions exhibit a day-time anomalous ionospheric annual variations (AIAV), during the peak years of the considered solar cycles, in which maxima (minima) occurs in local winter (summer). In this regard, the phase of AIAV is found to vary with solar activity. The night-time IAV show anomalous behavior in the northern hemisphere during the solar minimum. The SW ratio of day-time REC is less than unity at northern hemisphere during the peak of a solar cycle. However, the said ratio is always greater than one in southern counterpart. The fitting curve for day-time SW ratio has a strong negative slope in the northern hemisphere. The threshold of SSN (sunspot number) indicating the presence of the daytime winter anomaly at low- latitudes is found to be 25, 80, and 115 in the Africa, Asia-Pacific, and American regions, respectively. Additionally, this anomaly becomes observable in the mid-latitudes region of America and the Pacific region for SSN values exceeding 100 and 150, respectively. On the other hand, the correlation and fitting analysis demonstrate that night-time SW ratio enhances with increase in the solar activity. This result predicts that night- time winter anomaly can only be present during the periods of minimum and low solar activity. However, our analysis revealed that the night-time winter anomaly is observable only in the African low-latitude region for SSN < 85.