VHF scintillations and plasma drifts observed in southern Taiwan during the declining phase of solar cycle 24

Long-term observations of very high frequency (VHF) scintillations sounded at 244 MHz were conducted at one of the Scintillation Network Decision Aid (SCINDA) sites in Pingtung (22.6°N, 120.5°E; geomagnetic latitude 12.5°N), Taiwan, in 2015 - 2019, over the declining phase of solar cycle 24. Two VHF receivers are used to derive zonal drift velocity. The responses of S4 indices, occurrences and east-west drifts are investigated with respect to local time, season, and solar activity. The results show that strong scintillations (S4 > 0.5) appear frequently at early nighttime at equinoxes during the high solar activity year of 2015, while weak scintillations (S4: 0.2 - 0.5) occur often at early daytime and more frequently at early nighttime in summer without obvious correlation with solar activity levels. The peak drift of -50 - 60 m s-1 is observed eastward at night and westward during the day. Stronger scintillations and higher drift velocities are observed in spring and fall.