Observations on the field-aligned irregularities using Sanya VHF radar: 1. Ionospheric E-region continuous echoes

The occurrence characteristics of continuous radar echoes from low-latitude ionospheric E-region field-aligned irregularities are investigated, using the data from Sanya (109. 6 degrees E, 18. 4 degrees N) VHF coherent radar. The observational results show that the Doppler velocity and spectral width of daytime continuous echoes range from -50 to 25 m/s, and from 20 to 70 m/s, respectively. The echoes descend slowly with a descent rate of 1 km/h, and are highly correlated with the descending of the bottom height of Es trace (h(b)Es). This probably indicates that the background electric field provides a favorable condition for the gradient-drift instability below the density layer producing the continuous echoes. While at nighttime, the Doppler velocity and spectral width of continuous echoes range from -50 to 50 m/s, and from 20 to 110 m/s, respectively. The E-region multiple echo layers, echo layer disruption and upper E-region echo layer generation are often observed during the post-sunset hours, which could be associated with the tidal ion layers, and with the mapping of polarization electric fields generated within the equatorial plasma bubbles, respectively. Additionally, the meridional winds derived from continuous echoes (coherent radar) were generally consistent with those estimated from specular meteor trails (all-sky meteor radar) below 100 km.