Data obtained with the Applied Physics Laboratory HF radar located in Goose Bay, Labrador, have been used to determine the characteristic features of high-latitude ionospheric irregularities at decameter wavelengths. In this paper, we describe a set of four events exhibiting particular characteristics. These observations took place in the postmidnight sector at E region altitudes. The scanning capabilities of the radar indicated that arclike regions of irregularities were moving approximately along L contours with a drift velocity of the order of 200 m/s or less. For periods of a few minutes to a few tens of minutes, localized regions of irregularities exhibiting high Doppler velocities (350 to 650 m/s) and large signal to noise ratios appeared within the radar arcs. Among the high Doppler velocity signals, two distinct types have been identified. Both types can be present simultaneously. One type is distributed between 320 and 550 m/s and has an average value of 445 m/s, while the other is distributed between 500 and 650 m/s and has an average value of 580 m/s. If one assumes the lower of the high-velocity signals to be the ion acoustic velocity Cs, the higher velocity can be interpreted as electrostatic ion cyclotron (EIC) waves produced by NO+ ions. These EIC waves follow perfectly the dispersion relation established from the fluid approximation ωr = (Ωi² + k² Cs²)½. The radar echoes with low Doppler velocities are associated with irregularities produced by the gradient drift instability which presumably was operative at the top of the E layer. We suggest that magnetic field-aligned drifts combined with the subcritical perpendicular electron drifts are responsible for the production of both the EIC waves and the ion acoustic waves that were observed.
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