Semidiurnal behavior of quasi-periodic echoes in the mid-latitude Es region observed with the Chung-Li VHF radar

The range rate of the so-called quasi-periodic (QP) echoes from nighttime mid-latitude sporadic-E regions observed with the Chung-Li VHF radar during the SEEK (Sporadic-E Experiment over Kyushu) campaign displayed semidiurnal characteristics. The time periods before ~2200 LT and after ~0400 LT were dominated by the negative range rate QP (NQP) echoes. In contrast, a preponderance of positive range rate QP (PQP) echoes occurred between 2200 and 0400 LT. The slopes of NQP echoes found during the evening period steepened with time and became positive around 2200 LT. Similar behavior was also found prior to ~0400 LT when PQP echoes were replaced by NQP echoes during the SEEK campaign. The sign of the range rate changed around 2200 and 0400 LT on the six consecutive nights. Semidiurnal neutral-wind variations are proposed to explain the slope sign reversals in the observations. These results, together with previous findings, suggest that semidiurnal tides and long-period waves (with 30 minute periodicity) may be playing a role, in addition to atmospheric gravity waves, in the formation of QP echoes.