Northβsouth asymmetry of the equatorial anomaly: A model study Original Research Article

While Appleton (1946) originally linked the Equatorial Anomaly with the real magnetic inclination in the ionosphere the radio communication forecasters, misled by a name, replaced “dip” by “geomagnetic latitude”. As late as 1967 the Comite Consultatif International des Radiocommunications (CCIR) finally decided to replace that misleading mathematical construct by the modified dip (Modip) latitude which has an equator that is identical with the dip equator. In order to check the equatorial symmetry properties low and mid-latitude peak electron densities as given by CCIR or URSIʹs numerical maps were plotted against geomagnetic and Modip latitudes. For low and high solar activity NorthβSouth symmetry is found not around the geomagnetic equator but around the equator common to Modip and dip. This is true in the ionosphere including the position of both crests. Electron Content up to 1000 km, ECion, was determined with the International Reference Ionosphere (IRI) while the content of the plasmasphere (above 1000 km), Ecpl, was determined with an extension of IRI up to the plasmapause using the Standard Model of the Ionosphere (Chasovitin et al., 1998). Data from the ISSb topside sounder were compared with the IRI model. NorthβSouth symmetry was found for Ecpl (Ecion) with respect not to the dip equator but to the geomagnetic one. This finding reflects the fact that the magnetic field acts like a dipole at greater distances, but not near Earth. Since the plasmasphere contribution to the total electron content is small, symmetry appears not with geomagnetic but with Modip latitude.