The role of the postsunset vortex in the creation of turbulence in the bottomside F region

The bottomside of the F region at the magnetic equator becomes turbulent at night, and the turbulent region appears as a coherent backscattering in radar observations. The bottomside turbulent region is known to develop near the center of the postsunset vortex which represents the vortical plasma motion at premidnight in the time and altitude frame. However, our investigation of the plasma motion during the last solar maximum using the incoherent scatter radar observations at Jicamarca in Peru shows the development of the bottomside turbulent region at the upper part of the postsunset vortex. This observation indicates that the vertical shear in the zonal plasma motion associated with the postsunset vortex may not be the cause of the turbulence in the bottomside. Because the postsunset vortex is associated with the prereversal enhancement (PRE) in the vertical plasma motion, presumably PRE is more important factor for the creation of the turbulence than the shear is. We further analyze the radar data acquired since 1996 and investigate the relationship between the postsunset vortex and bottomside turbulence. Our study extends to the investigation of the similarity of the turbulences in the bottomside and bubbles.