Intriguing aspects of low latitude night-time F region irregularities over East and South-East Asia

Generally, the occurrence of F region irregularities associated with Equatorial Plasma Bubbles is considered difficult to predict due to its day-to-day variability. Recent investigation shows that the dominant period of variability of EPB can be > 25 days (quasi 27 day), presumably associated with solar rotation period. VHF scintillation recorded from Pingtung, Taiwan in 2015 indicated the coexistence of planetary scale variability (4-8 days), 10-15 days variability and quasi 27 day variability. Lomb-Scargle power spectrum of S4 time series indicated that the larger period variability (quasi 27 days) is much more dominant than the other scales (which includes day-to-day variability). Interestingly, the dominant period of S4 variability corresponded exactly with the dominant period of high-latitude geomagnetic variability. Continuous operation of Equatorial Atmosphere Radar (EAR) in 2012 also indicated similar observation. Dominant period of variability of EPB in EAR observations corresponded with the high-latitude geomagnetic variability. These observations reveal that EPB are controlled predominantly by geomagnetic activity through large period variability (quasi- 27 days).