The keograms based on CMONOC in 2014 showed that the irregularities had highest monthly occurrence rate in March, as much as 100%, then 80% in September, 48% in December and 27% in June. The high occurrence in both equinoxes and low occurrence in two solstices are consistent with the previous results (Su et al, 2008). It is worth mentioning that the occurrence rate in December was higher than in June. This is rarely reported in Southeast Asia (Li et al., 2021). Vyas and Dayanandan (2011) reported that the occurrences of VHF scintillations at (24.6°N, 73.7°E) showed a clear seasonal behavior with equinoctial maxima followed by winter and a summer minima during both high and low sunspot activity years. The seasonal occurrence rate of the irregularities in this work was consistent with these results. The seasonal characteristics of occurrence rate showed good correspondence with the vertical drift velocities of the ionospheric plasma (Vyas and Dayanandan, 2011; Su et al., 2008).
The keograms in March showed the irregularities had long lifetime. They usually lasted several hours or till early morning. At the same time, eastward drift was observed at a speed of 90-160m/s. Previous results also showed that the typical eastward drift velocity of the ionospheric irregularities is on the order of ~100-200m/s (Valladares et al., 1996; Kinter et al., 2004). Chapagain et al. (2012) pointed that the observed ionospheric irregularities velocity is consistent with the ambient plasma drift velocity. In the nighttime the equatorial F region plasma drifts eastward (Fejer et al., 1991). Chandra et al. (1993) showed VHF scintillations at the station close to the magnetic equator were strong and lasted till early morning in single patch during March-April 1991 in India, however for the station in the anomaly crest region or beyond, scintillations occurred in small patches with periods of no scintillations in between. The long lifetime and large patch in March 2014 are in agreement with the VHF scintillation observed by Chandra et al. at the station close to the magnetic equator. The irregularities in March may be associated with the EPBs. Figure 8 and 9 showed the typical sTEC and ROTI in March and June 2014. sTEC minus some fixed values are presented to show the relative variation in the same scale. The relative change is concerned in studying the ionospheric irregularities. From figure 8, it can be seen that the sTEC suddenly dropped when the satellite encountered an ionospheric irregularity. This is the typical characteristic of the EPBs.
In June, the irregularities with large range had a westward drift at a speed of 80~130 m/s. This was different from the equinox equatorial irregularities. Figure 9 presented the typical sTEC and ROTI in June. The sTEC began to undulate when it encountered the ionospheric irregularities, quite different from the sharp dropping in March. The longitude-time keograms showed some characteristics of the irregularities, but the latitude information is lost. In order to understand the characteristics of the irregularities, the latitude ranges of the irregularities based on the latitude-time keograms are summarized in Figure 10. It showed that the irregularities in June often appeared at ~30°N. The irregularities in June with westward drift may be related to the medium-scale traveling ionospheric disturbances (MSTIDs). And the sTEC undulation is similar to the pattern of MSTIDs. Previous study pointed that the nighttime MSTIDs are mainly observed in middle latitude and their propagation direction is primarily southwest in the northern hemisphere (Otsuka et al. 2004; Takahashi et al., 2018). They are more active during June solstice and they can propagate to lower latitude (Sivakandan et al., 2019).
In September, the occurrence rate, the lifetime, the ROTI values and the longitude scale of the irregularities were weaker than those in March. The occurrence time and the eastward drift were similar to those in March, different from those in solstice months. Previous study also showed the equinoctial asymmetry in the occurrence, which was greater in the spring equinox than in the autumn equinox. The asymmetry may be attributed to differences in plasma densities and meridional winds during two equinoxes [Nishioka et al., 2008; Maruyama et al., 2009; Otsuka et al., 2006; Sripathi et al., 2011].
In China, the irregularities are rarely reported in December. The GPS scintillation in south of China occurred mainly in the equinox months with particularly low solar activity during winter and summer months (Huang et al., 2014; Deng et al., 2013). The higher occurrence in winter than in summer is different from the previous results. There are two reasons for this phenomenon. The first one is the different latitude. In this work, the latitude ranges of the receivers are below 30°N, higher than those in Huang and Deng’s study. Figure 9 also showed the winter irregularities mainly appeared at 20~30°N. Besides higher latitude range, 2014 is the solar maximum year; the occurrence rate in high solar activity year is different from the low and moderate solar activity. Deng et al. (2013) also pointed that the occurrence of GPS scintillation increased in winter months in enhanced solar activity years (January 2011 to March 2012). Some researchers found that the occurrence rate of VHF scintillation or spread F in the low-latitude of Indian peaked in the equinoxes and winter during the high solar activity period; and during the low solar activity period the occurrence peaked in equinoxes and summer (Kumar et al., 2000; Singh et al., 2004; Vyas and Dayanandan, 2011; Sahithi et al., 2019; Swapna et al., 2015). In this work the results were based on the data in high solar activity year 2014, and the winter irregular mainly appeared in low-latitude (20~30°N). Another characteristic of the winter irregularities is patchy and discrete. Vyas and Dayanandan (2011) pointed that patchy and discrete are the features of scintillations over equatorial anomaly region.