The High-resolution Ogo 6 retarding potential analyzer (RPA) Data obtained during 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 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 point L=2.8 together with model computations based on Ogo 6 irregularity parameters strongly indicates that at least a part of the quiet mid-latitude scintillations is caused by the plasma instabilities associated with the subtrough density gradients.