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.