A MORPHOLOGICAL-STUDY ON MIDLATITUDE E-REGION FIELD-ALIGNED IRREGULARITIES OBSERVED WITH THE MU-RADAR

The MU radar located at Shigaraki, Japan (34.9-degrees-N, 136.1-degrees-E) can observe field-aligned irregularities in the E-region by steering its antenna beam perpendicular to the geomagnetic field. First, this paper investigates daily and seasonal variations of occurrence frequency of echoes from the irregularities. The irregularities were most frequently observed in June, near the summer solstice. they appeared in the periods after sunset and sunrise of the E-region, and the variance of the signal intensity was larger for the echoes in the post-sunset period than those in the post-sunrise period. The frequency occurrence showed similar behavior both in June 1989 and June 1990. In November and December, near the winter solstice, the irregularities were least active in all seasons, and the echoes appear earlier in time of day. The limited number of data in October showed fairly active irregularities though they were less active than in summer months. Second, by using the data on 20-23 June 1990, the time-height distributions of the echoes were compared with the activity of sporadic-E layers observed by the ionosonde at the MU radar site. It was shown that the activity of the irregularity echoes was associated with that of the sporadic-E layers in the post-sunset period. In the post-sunrise period, on the other hand, the echoes did not show clear correlation with the sporadic-E layer. Although the sporadic-E layers were very active in the daytime, no irregularity echoes were found by the MU radar in the afternoon. Finally, as for other features of the irregularity echoes, it was found on 25 October 1989 that echo power showed fluctuations with a short period of 2-3 min, which was much shorter than the Brunt-Vaisala period in the E-region.