This paper reports a new technique of observing the high-latitude ionospheric trough which defines its continuity in space and time on a macroscopic scale not previously possible. Vertical ionospheric soundings provide the observations in the midday to evening sector where the signature of the trough is found to be a ''bite-out'' of a factor of 3--10 in the diurnal distribution of the solar produced electron density at the F layer maximum. The data base employed is that produced by the extensive array of ionospheric sounders operating at high latitudes in the northern hemisphere during the International Geophysical Year period of December 1958, hence at solar maximum. The worldwide extent, high density, simultaneity, and continuity of these measurements make possible the description of the trough for the first time as an integral, spatially continuous entity in its entire daytime extent and UT duration. The technique is demonstrated in studies of 4 days which span the range of activity for the month. On the most disturbed day the daytime trough is continuous in magnetic latitude, magnetic local time between (75¿, 1030) and (51¿, 1630) and remains nearly stationary in this frame of reference for a duration of 14.5 hours UT. A ''snapshot'' constructed from multiple simultaneous observations on this day shows the trough to have a continuous length of 6¿103 km and an area of >4¿106km2. The trough, which is also stable for similar durations on the other 3 days, is found to rotate clockwise to earlier magnetic local time by 5.5 hours and to expand equatorward in magnetic latitude by 20¿ as activity increases from quiet to disturbed. Activity dependence and morphology are consistent with other observations that the trough locates rapid westward convection within the dusk convection cell. These properties of the trough appear on each of the 31 days studied but only in a sector which lies primarily in the eastern magnetic longitudes. Elsewhere, the daytime trough can occur principally in the morning to midday sector. ¿American Geophysical Union 1987