Yearly variations of global plasma densities in the topside ionosphere at middle and low latitudes

In this paper, the 10-year (1996-2005) measurements of total ion density (N-i) from the Defense Meteorological Satellite Program ( DMSP) spacecraft at 0930 and 2130 LT have been analyzed to investigate the yearly variations of global plasma densities in the topside ionosphere at magnetic latitudes from 60 degrees S to 60 degrees N. Results indicate that there are strong yearly variations in the DMSP Ni at 840 km. The annual components of longitude-averaged N-i dominate at most latitudes with maxima around the June solstices in the Northern Hemisphere and the December solstice in the Southern Hemisphere. In contrast, seasonal anomaly ( maxima Ni around the December solstice) exists in the northern equatorial zone. Moreover, the differences in N-i at the two solstices are not symmetrical about the magnetic equator, being generally higher in the Southern Hemisphere than in the Northern Hemisphere. Conjugate-averaged N-i is substantially greater at the December solstice than at the June solstice. This annual asymmetry is modulated by solar activity effect and has latitudinal and longitudinal structures. The longitude effects of the annual asymmetry depend on local time, being stronger in the evening sector than in the morning sector. The solstice differences and annual asymmetry are more marked with increasing solar activity. The annual asymmetry appears not only in the rising phase of the solar cycle but also in the declining phase. Thus the solar condition differences between the two solstices do not account for the N-i asymmetry. The concentration of neutral oxygen [O], provided from the NRLMSIS model, shows a similar pattern of annual and hemispheric asymmetries. Moreover, effects of the HWM model neutral winds are also constituent with the change patterns of N-i. Therefore, considering the principal processes in the topside ionosphere, the changes of [O] and the rates of thermospheric winds should contribute to the annual asymmetry in N-i at 840-km altitude.