MU-RADAR RECORDED FIELD-ALIGNED IRREGULARITIES IN THE F2 REGION AND ASSOCIATED SPORADIC-E DISTURBANCES

Recordings by the MU-radar of two ionospheric events involving F-2-layer field-aligned irregularities (FAIs) on one particular night in a sunspot-maximum period (July 24/25, 1991) have been examined and compared with ionospheric events deduced from the analyses of ionograms from the adjacent ionosonde stations of Akita and Kokubunji. Ionograms from three other Japanese stations have also been analysed. The analyses detected two medium-scale travelling ionospheric disturbances (MS-TIDs) which pass through the MU-radar observing region at times close to those of the FAT recordings. The MS-TIDs seem to be propagating at both F-2-layer and E(s)-layer levels. F-2-layer macroscale changes and E(s)-layer enhancement (particularly the E(s) enhancements) have been used to track the disturbances over significant distances from Wakkanai in the north to Yamagawa in the south, and even to the island of Okinawa (located some 2,500 km from Wakkanai). Spread-F is recorded during the passage of these MS-TIDs, but only at some stations. Quite apart from the apparent association of these MS-TIDs with the FAI recordings, these results would seem to reinforce other results which indicate an association between macroscale F-2-layer height changes and enhanced sporadic-E occurrence.