Nighttime mid-latitude travelling ionospheric disturbances associated with mild spread-F conditions

Results of analyses of midlatitude nighttime traveling ionospheric disturbances on the night of July 19/20, 1990, when quiet conditions prevailed, are presented. Sunspot activity was near maximum in July 1990, with an average monthly R(Z) value of 147. It is shown that, although it seems likely that at least four separate traveling disturbances passed over the observing station on this particular night, the ionosphere remained relatively undisturbed. A number of well-defined diverging traces and one converging trace on the ionograms, resulting from the presence of these disturbances, are used to present these new results.