LARGE-AMPLITUDE QUASI-PERIODIC FLUCTUATIONS ASSOCIATED WITH A MIDLATITUDE SPORADIC-E LAYER

For the first time a sounding rocket has been launched into a mid-lati tude sporadic E event which was shown to be the source of VHF radar ec hoes. The layer had a very high peak electron density (similar to 10(6 ) cm(-3)) and was thicker (similar to 5 km) than most events previousl y studied by rockets and incoherent scatter radars. The layer was modu lated in a remarkable quasi-periodic manner which has not been reporte d earlier. Twenty cycles of these structures were detected and they se em to be oriented horizontally rather than vertically with periods in the rocket frame in the rage 6-10 s. There is also some evidence that the modulation was detected below as well as above the peak in the ele ctron density, although the bulk of the flight was above the peak. Alt hough the VHF radar echoes were decaying at the time and place where t he rocket traversed the E layer, one burst of high amplitude short wav elength fluctuations was detected by the space-borne instruments and h ad a power spectrum similar to that of a secondary gradient drift mode . This burst occurred at the peak of one of the periodic electron dens ity fluctuations. We discuss two possible sources for the dominant flu ctuations: large-scale gradient drift waves and atmospheric acoustic w aves. The latter seem most consistent with the data.