Linear dependence of the postsunset equatorial anomaly electron density on solar flux and its relation to the maximum prereversal E × B drift velocity through its dependence on solar flux

The postsunset equatorial ionization anomaly, with maximum F layer electron density, Nemax, occurring near 2100 LT, has been found during solar maximum to be a linear function of the maximum prereversal E × B drift velocity (E × B drift). In order to examine this relation at all levels of solar flux, Nemax is measured during 13 years of an entire solar cycle by eight ionospheric sounders located in the anomaly in both north and south dip latitudes and in eastern Asia, the Pacific, and South America. At each location the monthly median Nemax increases linearly with the monthly average solar flux, Sa, over the range from 70 to 285 sfu. The linear function varies markedly with location and by month at each location. The relation to E × B drift, which is also a linear function of Sa, is determined using measurements of Nemax versus Sa measured at Bogota in the anomaly plotted as a function of E × B versus Sa measured at Jicamarca at the dip equator. The result is that Nemax is a linear function of E × B, which is in agreement with that found previously during solar maximum. Accordingly, the Nemax versus E × B relation is independent of Sa. The fact that Nemax is linear in Sa at each site implies Nemax is linear in E × B at each but with a functional dependence that varies with latitude and longitude.