The local time versus day plot of ESF intensity, and the prereversal drift amplitude. The upper panel shows the local time versus day plot of the spread F (SF) intensity over Fortaleza. The SF intensity is quantified in terms of the parameter fop, the top frequency of the range spread F trace. The lower panel shows the amplitude of the prereversal vertical drift velocity peak, Vzp, that occurs near 21:00 UT (18:00 LT). The values near 20:50 UT/17:50 LT are also shown for comparison.