Postmidnight equatorial plasma irregularities on the June solstice during low solar activity - a case study

We present a case study of unusual spread-F structures observed by ionosondes at two equatorial and low-latitude Brazilian stations - Sao Luis (SL: 44.2 degrees W, 2.33 degrees S; dip angle: -6.9 degrees) and Fortaleza (FZ: 38.45 degrees W, 3.9 degrees S; dip angle: -16 degrees). The irregularity structures observed from midnight to postmidnight hours of moderate solar activity (F10.7 < 97 sfu, where 1 sfu = 10(-22) W m(-2) s(-1)) have characteristics different from typical post-sunset equatorial spread F. The spread-F traces first appeared at or above the F-layer peak and gradually became well-formed mixed spread F. They also appeared as plasma depletions in the 630.0 nm airglow emissions made by a wide-angle imager located at the nearby low-latitude station Cajazeiras (CZ: 38.56 degrees W, 6.87 degrees S; dip angle: -21.4 degrees). The irregularities appeared first over FZ and later over SL, giving evidence of an unusual westward propagation or a horizontal plasma advection. The drift-mode operation available in one of the ionosondes (a digital portable sounder, DPS-4) has enabled us to analyze the horizontal drift velocities and directions of the irregularity movement. We also analyzed the neutral wind velocity measured by a Fabry-Perot interferometer (FPI) installed at CZ and discuss its possible role in the development of the irregularities.