E region observations over Chung-Li during the SEEK campaign

During the Sporadic E Experiment over Kyushu (SEEK) campaign E-region field aligned irregularities (FAI) were observed with the Chung-Li VHF Radar (operated on 52 MHz) in Taiwan between August 15-22, 1996. The characteristics of the quasi-periodical echoes from the E-region about 80 km north of the Chung-Li radar are studied. During the same period ionogram records were taken with the Digisonde located at the Chung-Li radar site. Strong sporadic-E layers were detected with the Digisonde when the FAI, observed with the VHF radar, were also very intense. The variations of sporadic E-layer critical frequency and of the field-aligned irregularities were studied. The latter were estimated to be located at altitudes 95-115 km occurring in layers of 5-15 km thickness, moving downwards at a rate of about 2.5 km/h. There seemed to be a change over of the FAI echoes from lower to upper E-region every night approximately at 23 hour LT. The distinction between post-sunrise and post-sunset FAI echoes observed earlier by MU radar was not clearly seen in Chung-Li. A preliminary examination of the morphology of the fine structures within the layers indicates quasi-periodic features with 5-10 minutes period. Many of these periodic fine structures were observed at the Chung-Li VHF radar to have opposite slopes than those detected earlier with the MU radar in Japan, which is located 1500 km north of the Chung-Li VHF radar.