The role of the F-region vertical drift on the onset time of the equatorial spread F over Ilorin, Nigeria

Abstract

The equatorial ionosphere often shows the occurrence of plasma-density irregularities and velocity fluctuations with a broad range of scale sizes and amplitudes. These irregularities in the F-region are commonly referred to as the equatorial spread F (ESF) and they are predominantly a night-time phenomenon. The vertical drift of ionisation to higher altitudes post-sunset hours has been suggested to be responsible for the occurrence of these irregularities. We have therefore used all the available ionograms at Ilorin, Nigeria ( $$\hbox {latitude} = 8.48^{\circ }\hbox {N}$$ , $$\hbox {longitude}= 4.67^{\circ }\hbox {W}$$ and $$\hbox {dip}= 4.1^{\circ }\hbox {S}$$ ) to study the occurrence of ESF and the effect of the F-region drift on the occurrence of ESF. The available data at Ilorin, though not continuous, cover a period of 6 yrs (i.e., 2002, 2005, 2006, 2007, 2008 and 2010). The F-region vertical drift velocity was estimated starting from around local sunset until the onset of ESF, and the maximum value for a night ( $$V_{Z,\mathrm{max}})$$ is used to represent the strength of the $$E\times B$$ drift for that night. Results obtained showed that the percentage occurrence of ESF increases with the increase in solar activities. A seasonal trend is also observed in the occurrence frequency; occurrence seems to be more frequent during and around the equinox periods. The onset time or the time of commencement (TOC) of ESF was observed to vary from day to day. Also, $$V_{Z,\mathrm{max}} $$ seems to increase with the increase in solar activities, which in turn influences the TOC of ESF. It was found that the higher the $$V_{Z,\mathrm{max}} $$ , the closer the TOC was found to be to the local time $$V_{Z,\mathrm{max}}$$ . In a few cases, there was no noticeable movement of ionisation prior to the commencement of the ESF, and this might be pointing to the fact that the drifting of ionisation to a higher altitude is not the only condition for the occurrence of ESF. Other conditions that have been identified to play a role in ESF occurrence are thermospheric neutral dynamics, equatorial thermodynamic meridional winds and the post-sunset base height of the F-layer ( $$h^{\prime }F)$$ .