Observations of daytime topside ionospheric irregularities in the afternoon equatorial ionosphere

• Both daytime ionospheric irregularities and F3 layer were observed in the afternoon equatorial ionosphere. • Topside ionospheric irregularities in this study might not be associated with the nighttime and morning irregularities. • The combined effects of disturbed zonal electric field and velocity shears in plasma drift can play a significant role in forming daytime ionospheric irregularities. Previous observations show that the daytime irregularities (the fossil of nighttime irregularities) are mostly located in the low latitude region and at 10–14 LT. In this study, daytime topside ionospheric irregularities were observed along the longitude of ∼ 94 o E by Swarms satellite in the afternoon equatorial ionosphere (about 15:00 LT) on 18 June 2020. Simultaneously, the F3 layer were observed in the bottomside ionosphere. Occurrences of F3 layer shows that there are disturbances on the zonal electric field. Multi-satellite observations show that the topside ionospheric irregularities did not occur in the morning equatorial ionosphere. Results indicate that afternoon ionospheric irregularities might be associated with the electrodynamic process of the plasma in the equatorial ionosphere. Simulations show that the combined effects of the disturbed zonal electric field and velocity shears in zonal plasma drift can be favorable to increase the occurrence rate of daytime ionospheric irregularities in the topside equatorial ionosphere.