IONOSPHERIC F2 LAYER SEASONAL AND SEMIANNUAL VARIATIONS

An extensive series of computations, using the Coupled Thermosphere-Io nosphere-Plasmasphere model (CTIP), has been undertaken to investigate the semiannual variation in peak noontime electron density, a common feature of the Fa-layer, particularly at low latitudes and in the sout hern hemisphere at mid-latitudes. Results from the model reveal such a variation, most prominently, at mid-latitudes, in the South American sector. An analysis of this phenomenon shows that it is intimately rel ated to the large offset of the geomagnetic axis from Earths spin axis in the southern hemisphere. Because of this offset, a given geographi c latitude in the South American sector corresponds to a lower magneti c latitude than in other sectors and is thus farther from the energy i nputs associated with the auroral regions. As a result, the compositio n changes are much smaller during the winter months than at other long itudes, the mean molecular mass being essentially constant for a 4-mon th period centered on the winter solstice. This result is understood i n terms of the global thermospheric circulation. In the absence of any composition changes, noon ionospheric density is influenced primarily by the solar zenith angle. This angle reaches a maximum at the winter Solstice, leading to diminished ion production, a minimum in N(m)F2, and therefore a semiannual variation overall. On the basis of the mode l results, the semiannual variation is seen as a feature of the midlat itude ionosphere at geographic longitudes opposite to the location of the geomagnetic pole. This phenomenon is seen in both northern and sou thern hemispheres, though the effect is much larger in the southern he misphere as a result of the greater magnetic offset.