SPATIAL STRUCTURE OF THE E-REGION FIELD-ALIGNED IRREGULARITIES REVEALED BY THE MU RADAR

The MU radar (34.9-degrees-N, 136.1-degrees-E, geomagnetic latitude 25 .0-degrees-N) is utilized to observe the spatial structures of the fie ld-aligned irregularities in the midlatitude E region. The multibeam o bservation in 12 directions has found that the ''quasi-periodic'' echo es have a monochromatic wave structure propagating toward SSW. Much sm aller-scale structure of the irregularities has been observed by the i nterferometry techniques with three receivers. The results are consist ent with a model in which a southward propagating gravity wave modulat es the sporadic E layer to generate the quasi-periodic echoes of the i rregularities.