Measurements of ƒ°F2 at 13 North American ionospheric stations are used to study the geographical distribution of ƒ°F2 deviations from normal and the changes with time during four ionospheric disturbances. In two of these, storm centers can be identified which appear to move across the maps during the disturbance. In the other two, the contours of percentage deviation are roughly directed east and west, and show relatively little change with time.
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Robert S. Lawrence (1953) studied this question.