Unexpected High Occurrence of Daytime F‐Region Backscatter Plume Structures Over Low Latitude Sanya and Their Possible Origin

Ionospheric F‐region irregularity backscatter plumes are commonly regarded as a nighttime phenomenon at equatorial and low latitudes. Using the Sanya (18.4°N, 109.6°E, dip lat. 12.8°N) VHF radar, F‐region backscatter echoes were observed at daytime during 0700–1800 LT, with an unexpected high occurrence in June solstice of solar minimum. Radar interferometry and ICON satellite in situ results show that the daytime F‐region echoes were from plume structures consisting of field‐aligned irregularities. The daytime echoing structures appeared mostly above 350 km altitude, extending up to 650 km or more with apparent westward drifts at times. We surmise that the daytime F‐region echoes were due to equatorial plasma bubble irregularities generated on the previous night around 100–125°E, where the irregularities could survive unexpectedly long time, beyond sunrise as vertically elongated fossil plumes. Under the ionospheric background dynamics, the fossil plumes could be elevated to high altitudes and drift zonally over Sanya. Plain Language Summary Equatorial plasma plumes, which are known as vertically elongated irregularity structures over the magnetic equator, have been widely accepted as being generated at nighttime. The plumes usually disappear before sunrise. At daytime, there are very few reported cases of F‐region backscatter echoes. It is still not clear what caused the daytime echoes. During November 2016 to August 2020, the Sanya VHF radar was operated for observing F‐region echoes at daytime. The observations show that the daytime F‐region echoing structures could appear at any time during 0700–1800 LT, with a maximum occurrence around 0900 LT. Radar interferometry and ICON satellite in situ results reveal that these daytime echoes were from field‐aligned irregularities, which are shown as plume structures in the topside ionosphere. It is suggested that the plume structures could be remnants of equatorial plasma bubble (EPB) irregularities generated on the previous night around 100–125°E. They rise to high altitudes and drift zonally together with background plasma, causing the daytime F‐region backscattering structure over Sanya. Our results indicate that the EPBs could maintain their vertically elongated structures and meter‐scale irregularities at F‐region topside for much longer time than previously thought and have important implications for understanding their dynamics. Key Points An unexpected high occurrence of daytime F‐region backscatter ec