Impacts of atmospheric ultrafast Kelvin waves on radio scintillations in the equatorial ionosphere

We present a statistical analysis of the amplitudes of GPS scintillations (S4 index) observed throughout 2008\textendash2010 using the satellite radio occultation measurements of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC). Here, for the first time, periodic variability in the occurrence of S4 is investigated using these data. Significant variations of S4 with periods of 2.5\textendash4 days (quasi-3 days) are identified from the observations during postsunset hours (1900\textendash2400 local time) between 15\textdegreeS\textendash 15\textdegreeN magnetic latitude during this 3 year interval. Coherence analyses of these variations with the geomagnetic Ap index, solar EUV irradiance, and atmospheric wind measurements from an equatorial mesosphere meteor radar at Thumba, India ( 8.5\textdegreeN, 77\textdegreeE) are performed, providing a measure of the relationship between variations in the scintillations and potential drivers. The quasi-3 day variations in S4 are found to covary with the variations of the three drivers examined. In particular, the S4 signatures are found to be coherent with the atmospheric ultrafast Kelvin (UFK) planetary waves characterized by the zonal wind measurements of the radar. This study shows that these UFK waves are as important as the solar and geomagnetic drivers in forcing the day-to-day variations of the occurrence of equatorial spread F.