Low-latitude valley region irregularities studied using the Gadanki radar

A study of valley region irregularities (VRI) and structures utilizing a database of observations obtained by the Gadanki radar is presented. It is shown that VRI occur both in the presence and absence of equatorial spread F (ESF) activity. VRI are found to occur initially near 2000 LT at an altitude as high as 180 km and then descend with time. Notably, the occurrence pattern of VRI resembles closely to that of the low-latitude intermediate layers. The above occurrence pattern is observed irrespective of the type of ESF event present. During ESF plume events, however, significant temporal structures in VRI with close relation to the ESF structures are observed, which are otherwise missing. VRI velocities in relation to ESF are in the range of -70 m s-1 to 100 m s-1 while in the absence of ESF, VRI velocities are in the range of ± 50 m s-1. Further, SNR and spectral width are considerably higher when they are associated with ESF plumes. It is suggested that VRI are generated on the intermediate layers possibly through gradient drift instability. It is also suggested that the valley region ambient fields are quite capable of generating these irregularities. The role of the unstable F region electric fields on VRI is found to be important for structuring in VRI and may be critical for the generation of irregularities when the valley region is stable by itself