Large amplitude quasi-periodic fluctuations associated with a mid-latitude sporadic E layer

For the first time a sounding rocket has been launched into a mid-latitude sporadic E event which was shown to be the source of VHF radar echoes. The layer had a very high peak electron density (∼10⁶ cm⁻³) and was thicker (∼5 km) than most events previously studied by rockets and incoherent scatter radars. The layer was modulated in a remarkable quasi-periodic manner which has not been reported earlier. Twenty cycles of these structures were detected and they seem 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 electron density, although the bulk of the flight was above the peak. Although the VHF radar echoes were decaying at the time and place where the rocket traversed the E layer, one burst of high amplitude short wavelength fluctuations was detected by the space-borne instruments and had a power spectrum similar to that of a secondary gradient drift mode. This burst occurred at the peak of one of the periodic electron density fluctuations. We discuss two possible sources for the dominant fluctuations: large-scale gradient drift waves and atmospheric acoustic waves. The latter seem most consistent with the data.