Observations of nighttime equatorial-upper-E-valley region irregular structures from São Luís radar and their occurrence statistics: A manifestation of vertical coupling between E and F regions

Radar observations of nighttime irregular structures in the equatorial upper-E-valley region are presented. These structures are observed strictly during occurrences of an ascending irregular bottomtype or bottomside F layer, during or prior to the appearance of a high-rising F region plume and a momentarily intensification of E-region irregularities. The slope and the altitude coverage of these structures are proportional to the ascending rate of the irregular bottomtype/bottomside F layer and the altitude coverage of the plume respectively. These structures are found to consist of substructures separated in times suggesting the presence of quasi-periodic striations within these irregular structures. On the basis of occurrence statistics during October 2001–December 2008, the occurrence rate of these structures is found to be 3.5%, indicating the nature of rare occurrence of these structures over São Luís. Moreover, their occurrence remains confined to the maximum solar-flux period (2001–2003) and to the summer months (October–January). The presence of these structures is a manifestation of the vertical coupling of E and F regions owing to the ambient electric fields of the ionosphere and the fringe-field associated with the F region plume.

► The presence of upper-E-valley region irregular structures.
► The dynamical coupling between the equatorial F and E regions owing to the ambient electric field.
► The dynamical coupling between the equatorial F and E regions owing to the fringe-field of the equatorial F region plume.
► The importance of ambient electric field in the valley region to drive interchange instability considering smoke.