MODEL STUDIES OF THE LATITUDINAL EXTENT OF THE EQUATORIAL ANOMALY DURING EQUINOCTIAL CONDITIONS

The latitudinal extent of the equatorial anomaly has been studied for equinoctial conditions using a theoretical model of the ionosphere which incorporates measured values of vertical E x B drift at the Earth's magnetic equator. Realistic values of neutral winds are also included. The equatorial anomaly region, typically between +/- 20-degrees magnetic latitude, is that part of the world where the highest values of electron density and total electron content (TEC) normally occur and hence is very important to high-frequency propagation and to transionospheric propagation effects. During the daytime, upward E x B drift at the magnetic equator drives the ionization across field lines to higher latitudes, causing crests in ionization to occur at approximately +/- 15-degrees dip latitude. The E x B drift mechanism is explained in detail by Hanson and Moffett (1966). The latitude range over which the anomaly makes a significant difference in values of f(o)F2 and TEC is calculated as a percent departure from the case with no equatorial electric field. Results from the model studies with different values of realistic electric fields show that the effects of the anomaly can be more highly variable and widespread in latitude and local time than is generally assumed.