Electron density versus altitude as per section 2.2. This chart shows lines of criticality for different transmission frequencies on the Houston - Ft. Collins circuit. As the ionosphere ionizes in the morning or recombines in the evening, the density passes critical in the altitude range of 220 to 250 Km. Note that the F-Layer density never falls below critical for 5MHz, but does affect 10 through 20MHz. The November 1988 test was somewhere between solar minimum and maximum.
(from Tascione, 1988)