Statistical Study on Plasma Velocities in the Bottom‐Side Ionosphere Over Low Latitude Hainan Station: Digisonde Measurement.

Data measured by the Digisonde at the low‐latitude station Hainan from 2003 to 2016 are statistically analyzed to specify the diurnal average variations of the bottom‐side F region ionospheric plasma velocity vector V. This is the first comprehensive analysis of Digisonde measurements of low latitude F region plasma velocities in the East Asian sector that use a database covering more than one solar cycle. The velocity components VN (Northward), VE (Eastward), and VZ (Upward) are analyzed for two levels of solar flux and two levels of geomagnetic activity, respectively. The diurnal variations of the average VZ show three positive peaks near the prereversal enhancement (PRE) period, pre‐midnight, and before sunrise, respectively, and a prominent valley in the early morning. The averaged VZ significantly increased with solar flux in the period of PRE during equinoxes, but it was only slightly affected by Kp. The VE component was westward in daytime and eastward in nighttime. The average eastward VE increased significantly with solar flux but decreased with Kp, whereas the average westward VE exhibited only a small variation with solar flux and Kp. The average VN was almost southward independent of solar flux and Kp. The plasma velocities over the Hainan station were mainly caused by the electric field and neutral wind. Our results show that the features of the vertical and meridional velocities over the Hainan station in the morning are associated with the formation of the equatorial ionization anomaly (EIA). Plain Language Summary: The ionospheric plasma drift has been studied with data from ground‐based observations for decades. However, there is no corresponding study for the east Asian sector. In 2002, a Digisonde named DPS‐4 was installed at the Hainan station which can directly measure the plasma velocities with Drift mode. Using the data from the DPS‐4 at Hainan, we perform a statistical study on the bottom‐side F region plasma velocities at two levels of F10.7 and two levels of Kp, respectively. The results show that the velocities are mainly caused by the electric field and neutral wind, and the VN and VZ components are associated with the EIA formation. We also compared our results with that measured by incoherent scatter radar at the Jicamarca station. Some statistical results are similar, but some are different in the Hainan station and Jicamarca station, though the locations, the observed altitudes, and the observation periods differ between the two stations. Key Points: Bottom‐side ionospheric plasma velocities in the East Asian sector are comprehensively analyzed using Drift mode data of the DigisondeStatistical features of ionospheric plasma velocities over the low latitude station Hainan for two levels of F10.7 and two levels of KpVelocities are mainly caused by the electric field and neutral wind. The VN and VZ components may be associated with the formation of EIA [ABSTRACT FROM AUTHOR]