Statistical study on plasma velocities in bottom-side ionosphere over low latitude Hainan station: Digisonde measurement

In this paper, the data measured with Digisonde at the low-latitude Hainan station 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 vector components V_N, V_E, and V_Z are analyzed for two levels of solar flux and two levels of geomagnetic activity, respectively. The diurnal variations of the average V_Z 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 V_Z significantly increased with solar flux in the period of PRE during equinoxes, but it was only slightly affected by Kp. The V_E component was westward in daytime and eastward in nighttime. The average eastward V_E increased significantly with solar flux but decreased with Kp, whereas the average westward V_E exhibited only a small variation with solar flux and Kp. The average V_N 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 associated with the formation of the equatorial ionization anomaly (EIA).