A study of the effect of seasonal and longitudinal variations of gravity wave on occurrence of ESF

Equatorial post-sunset ionospheric irregularity triggering mechanism is not yet certainly known. It has been proposed that the occurrence of ESF is high in the equinoctial months when the solar terminator is parallel to the geomagnetic field line; however, observations show that strong irregularities are observed in the months when it is not expected to occur. From theoretical investigations, it has also been suggested that atmospheric gravity wave (GW) could be the seeding mechanism for the occurrence of ESF. But detail investigations are still to be done to clarify the linkage between lower atmosphere activity and equatorial ionospheric irregularity. In this study simultaneous atmospheric temperature perturbation profiles that are due to GW obtained from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on-board TIMED (Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics) satellite and equatorial in-situ ion density and vertical plasma drift velocity observations obtained from C/NOFS (Communication/Navigation Outage Forecast System) satellites are used to investigate the effect of GW on the occurrence of ESF. Characteristics of GW with vertical wavelength less than or equal to 6 km are computed from temperature perturbations and compared with ionospheric parameters for the region between -20 to 20 ^oN and local time of 17 to 24. In the months that we observe mesospheric inversion we found significant decrease of potential energy of GW. The lowest and highest monthly mean of minimum GW potential energy, between 80 and 95 km altitudes, are found during September to April and May to August, respectively. Strong and weak occurrence of EPB (equatorial plasma bubble) are found for the months with low and high minimum potential energy of GW, respectively. Moreover, during the months with relatively higher minimum potential energy of GW vertical drift velocity and occurrence of EPB are found to be downward and very low respectively.