Nighttime Mid-Latitude Travelling Ionospheric Disturbances Associated with Mild Spread-F Conditions.

Several diverging duplicate traces and one converging duplicate trace for mild mid-latitude spread-F events (on one particular night) have been used to investigate medium-scale travelling ionospheric disturbances (TIDs) during night hours. Swept-frequency and fixed-frequency sonograms (which have phase information added) show spread-F characteristics similar to those recorded for more intense spread-F events. These include modulated height rises in the isoionic contours and electron-density depletions. The phase-path information on the fixed-frequency recordings allowed the detection of the TID wavetrains responsible for the modulation. Usually for these mild events one tilt in the isoionic contours produced by one cycle of a wavetrain is sufficiently large to allow significant off-vertical reflections which in turn are responsible for a diverging trace. Speeds of travel have been determined by considering both the rate of change of group-range for the duplicate traces as well as the rate of change of phase-path indicated on each fixed-frequency recording. The speeds of travel of these nighttime TIDs are compared with the speeds of daytime TIDs.