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 - São Luís (SL: 44.2° W, 2.33° S; dip angle: -6:9°) and Fortaleza (FZ: 38.45° W, 3.9° S; dip angle: -16°). The irregularity structures observed from midnight to postmidnight hours of moderate solar activity (F10.7<97 sfu, where 1 sfu=10^-22Wm^-2 s^-1) have characteristics different from typical post-sunset equatorial spread F. The spread-F traces first appeared at or above the Flayer 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° W, 6.87° S; dip angle: -21:4°). 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-Pérot interferometer (FPI) installed at CZ and discuss its possible role in the development of the irregularities.