Observations of Evolution-Type Band-Like Structures of F Region Irregularities

Generally, equatorial spread-F (ESF) irregularities initiated at the bottom of the ionospheric F region and manifested as plume structures with an altitude extent more than 50 km in the RTI (range-time-intensity) plot. By using the Fuke and Sanya VHF radars located at low latitude of China, evolution-type band-like structures of ESF irregularities were observed near local sunset. One interesting aspect is that the ESF band-like structures initially appeared at high altitudes more than 400 km and then uplifted to higher altitude without zonal propagation. The lifetime of these ESF band-like structures, with a similar pattern in the RTI plot for each beam of radar, is less than 25 min. The band-like structures of F region irregularities were observed with big Doppler velocities and narrow spectral widths. During the presence of ESF band-like structures, apparent amplitude scintillations (S4 > 0.2), total electron content (TEC) fast fluctuations (ROTI > 1), and range-type spread F were detected. Possible mechanisms responsible for the initial generation of band-like ESF irregularities at high altitude are discussed.