Similar wave-like velocity modulations within the irregular bottom-type F layer and irregular E layer prior to the F region plume

In present study, São Luís equatorial radar observation of 5 meter irregularities on December 01, 2002 are presented. On this night, a distinct bottom-type F layer is observed followed by a well developed plume. The focus of the present study is to examine Doppler variations within this irregular bottom-type F layer and simultaneously observed underlying irregular E layer. Within the irregular bottom-type F layer, three distinct Doppler envelops are identified: a positive Doppler envelope during initial phase, a negative Doppler envelope during intermediate phase and a positive Doppler envelope during the final phase of the irregular bottom-type F layer. This suggests the wave-like modulation or oscillatory nature of the electric field within the irregular bottom-type F layer. In the simultaneously observed underlying irregular E layer in 90-105 km, three Doppler envelops similar to those in the irregular bottom-type F layer are noted. These characteristics suggest that the irregular bottom-type F layer and the irregular E layer are influenced and coupled by similar kind of wave dynamics. On the basis of generation of wave-like electric field mechanism proposed by Keskinen [J. Geophy. Res., doi:10.1029/2010GL042963, 2010], it is suggested that the observed Doppler envelops within the irregular bottom-type F layer are owing to the combined action of gravity wave seeding and wind-shear flow associated with the gravity wave. Since similar Doppler envelops are found in the underlying irregular E layer, it is suggested that the gravity wave may be the likely cause for the wave-like electric field within irregular E layer. On this basis, it may be said that the present observation identifies the coupling of irregular bottom-type F layer and irregular E layer via gravity wave prior to the occurrence of the F region plume.