Appleton anomaly increase following sunset; Its observed relation to equatorial F layer E x B drift velocity

The upward E x B drift velocity of the postsunset F layer at the dip equator, in response to the prereversal electric field, and the resulting enhancement of the fountain effect plasma at the Appleton anomaly are observed nearly simultaneously near the same geomagnetic meridian. Measurements, the first under such controlled conditions, are by a chain of ionospheric sounders recording ionograms at intervals of 5-15 min on two consecutive days during equinox at solar maximum, both having quiet magnetic conditions and high solar fluxes. Measured is the following sequence of cause and effect: the equatorial F layer electron density that persists through sunset; the solar E layer electron density conjugate to this F layer that decays rapidly through sunset; the bottomside equatorial F layer altitude, h'F, as it drifts upward in response to the F region dynamo electric field; and the anomaly F layer electron density that increases with the arrival of the resulting enhanced fountain effect plasma at 16.0 degrees and at 20.3 degrees dip latitude. At the equator the maximum drift velocity measured by dh 'F/dt is 70 m s(-1) on the first day and 50 m s(-1) on the second. The differences in drift velocity are reflected in the anomaly in latitudinal distributions of F layer maximum electron density between 0 degrees and 30 degrees dip latitude that show the anomaly crest to increase in magnitude and in latitude: rapidly on the first day, reaching a maximum of 55 x 10(5) el cm(-3) at 18 degrees dip latitude at 2100 LT, and more slowly on the second day, reaching greater than or equal to 38 x 10(5) el cm(-3) at less than or equal to 16 degrees dip latitude, also at 2100 LT. In addition to the qualitative relationship, maximum drift velocity and anomaly crest maximum electron density and its maximum rate of increase are all reduced by about the same factor on the second day relative to the first.