The daytime F layer trough is studied by means of an extensive network of ground-based ionospheric sounders in the northern hemisphere under conditions of solar maximum near winter solstice. The trough is observed to be a continuous band having an instantaneous extent of thousands of kilometers consisting of depletions in the daytime electron density, often by an order of magnitude. It lies in regions of sunward ionospheric-magnetospheric convection, an afternoon sector corresponding to the dusk cell, a morning sector corresponding to the dawn cell, and morphology and activity dependence consistent with convection. As detected in the diurnal distributions of f0F2, the trough is a persistent feature at high latitudes, appearing on each day of a 31-day period of continuous observation, and, although highly variable from day to day, is apparent in the monthly medians. The afternoon trough, which is detected independently by at least five and as many as 17 stations on each day, is generally continuous and stationary for a duration of many hours in magnetic latitude/magnetic local time coordinates. The trough contracts during quiet conditions so as to lie above 70° magnetic latitude but expands during disturbed conditions so as to extend from 75° to 52° magnetic latitude. The trough has a pronounced dependence on longitude, appearing principally in the afternoon in eastern magnetic longitudes but in the morning in western magnetic longitudes, an effect so prevalent that it produces large east-west local time asymmetries in the diurnal distributions of median daytime F layer electron densities throughout a wide range of latitudes. The longitudinal dependence is found to result from the relation between the two principal coordinate systems of the ionosphere-magnetosphere interaction: solar geomagnetic coordinates in which the convection pattern and the resultant daytime trough reside, and solar terrestrial coordinates in which solar ion production and the undisturbed daytime F layer in general reside; as a consequence of the fact that these coordinate systems vary with respect to one another with longitude, the trough varies within the daytime F layer with longitude.
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