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The first 24-hour sequence of Slough E-layer critical frequency measurements, made on 11-12 January 1931, and the corresponding 60th anniversary sequence, 11-12 January 1991. The differences are : [a] 1931 was near solar minimum, 1991 was near solar maximum; [b] foE could be recorded throughout the night in the quiet radio environment of 1931, but not in 1991. It might be mentioned that Sir Granville Beynon has expressed doubt as to whether someone actually got up every hour during the night to measure foE.
Figure 1 shows two Slough ionograms. These are believed to be the earliest Slough swept frequency ionograms, and were made on 27 December 1933 at 1030-1100 and 1130-1200 UT 1933 [before which, I think, they just measured criticals]. It appears that 30 minutes were required to record a complete ionogram. For comparison, the 60th anniversary ionogram, Figure 2, was made with a digital ionosonde at 1200 UT on 11 January 1991.
The comparison between the first 24-hour sequence of Slough foE on 11-12 Jan 1931 and its 60th anniversary on 11-12 Jan 1991 is shown in Figure 3.
All ionograms are credited to Daresbury & Rutherford
Appleton Lab
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