Double Coherent Scatter Radars Observations of the Daytime F-Region Irregularities in Low-Latitudes on 29 May 2017

In the morning on 29 May 2017, the Sanya very-high-frequency Radar and the Hainan COherent Scatter Phased Array Radar (HCOPAR) at Fuke were cooperated with similar operating frequencies to investigate the dissipation of the daytime F-region Field-Aligned Irregularities (FAIs). The two radars are located on south and north sides of the Hainan island of China, and the distance between them is about 140 km. The Sanya VHF radar recorded the FAIs between 00:12 and 02:42 UT (07:12 and 09:42 LT), and the FAIs observed by the HCOPAR appeared more than 10 min later at lower altitudes and suffered severe dissipation. The echoes emerged in all the beams simultaneously, and their Doppler velocities were close to zero, implying that the daytime FAIs were traveling along the magnetic field lines downward and northward with almost no zonal velocity. The front and bottom edges of the irregularities decayed earlier, and the parts in the FAIs with steeper density gradient lasted longer. The dissipation mechanism is attributed primarily to the photoionization and diffusing across the geomagnetic field in different period and gradient of the irregularities. Plain Language Summary Equatorial Plasma Bubbles, which cause Field-Aligned Irregularities (FAIs), are widely accepted as a nighttime phenomenon. They usually disappear before sunrise and are rarely recorded by radars during daytime. Especially the dissipation process of the daytime FAIs is seldom reported. The Sanya very-high-frequency radar located on the south of the Hainan Island and the Hainan COherent Scatter Phased Array Radar located on the north observe the daytime FAIs successively. The observation results show that while the daytime FAIs were traveling from Sanya to Fuke, the dissipation process first occurred in the front and bottom edges as well as the regions of the irregularities with small density gradient. The dissipation mechanism for the fossil F-region FAIs traveling along the magnetic field lines under the sun are discussed. This investigation results may deepen our understanding of the dissipation processes in the irregularity and their interaction with the ambient atmosphere.