Observations on the field-aligned irregularities using Sanya VHF radar: 2. Low latitude Ionospheric E-region quasi-periodic echoes in the East Asian sector

The quasi-periodic (QP) radar echoes have been widely observed at middle latitudes. Analyzing the field-aligned coherent radar echoes observed on 6 February 2011 over Sanya (109.6 degrees E, 18.4 degrees N), we present first results of quasi-periodic echoes in the Chinese low latitude region. The observational results show that QP echoes occurred above 110 km around 2100-2200 LT, and coexisted with continuous radar echo layers. The altitude extent of the QP striations is in a range of about 5 similar to 20 km and the duration is about 5 similar to 15 minutes. The QP echoes first appeared at higher altitude and then descended to the height close to the continuous echoing region, with a descent rate of 20 similar to 30 m/s and a period of about 8 minutes. Further, the Doppler velocities of QP echoes change in time and range and are not related to the striation slope. The zonal drift velocities derived from radar interferometric analysis of QP echoes show apparent variations with altitude. On the basis of earlier theories and simulations related to QP echoes, we suggest that the generation of QP echoes over Sanya may not be induced by the isolated Es patches which periodically move in the radar-viewing region, but could be associated with the disturbed structures generated within Es layer.