Ogo 6 observations of small‐scale irregularity structures associated with subtrough density gradients

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Abstract
The high‐resolution Ogo 6 retarding potential analyzer (RPA) data obtained during November 1969 to February 1970 have been utilized to demonstrate the association of small‐scale ionospheric irregularities (several kilometers to hundreds of meters) with sharp F region cross‐L density gradients in the L range of approximately 2–4. These subtrough irregularity structures which often appeared in near‐conjugate locations were observed during periods of quieting following magnetospheric storms and were found to be associated with sharp gradients of O+ ions or of the light ions depending on the height of the satellite. Similar structures in O+ ions were observed at perigee (400 km) altitudes during periods of extended quiet geomagnetic conditions. During such quiet periods the small‐scale density irregularities associated with O+ gradients were found to be correlated with low‐frequency electrostatic fluctuations but not with any appreciable temperature variations. Since gradient scale lengths as small as 1 km were often observed, the drift wave plasma instability is suggested as a possible mechanism for the formation of the small‐scale irregularities. A study of 137‐MHz scintillations at a point L = 2.8 together with model computations based on Ogo 6 irregularity parameters strongly indicates that at least a part of the quiet time mid‐latitude scintillations is caused by the plasma instabilities associated with the subtrough density gradients.