Yearly variations in the low-latitude topside ionosphere

Observations made by the Hinotori satellite have been analysed to determine the yearly variations of the electron density and electron temperature in the low-latitude topside ionosphere. The observations reveal the existence of an equinoctial asymmetry in the topside electron density at low latitude s, i.e. the density is higher at one equinox than at the other. The asymmet ry is hemisphere-dependent with the higher electron density occurring at th e March equinox in the Northern Hemisphere and at the September equinox in the Southern Hemisphere. The asymmetry becomes stronger with increasing lat itude in both hemispheres. The behaviour of the asymmetry has no significan t longitudinal and magnetic activity variations. A mechanism for the equino ctial asymmetry has been investigated using CTIP (coupled thermosphere iono sphere plasmasphere model). The model results reproduce the observed equino ctial asymmetry and suggest that the asymmetry is caused by the north-south imbalance of the thermosphere and ionosphere at the equinoxes due to the s low response of the thermosphere arising from the effects of the global the rmospheric circulation. The observations also show that the relationship be tween the electron density and electron temperature is different for daytim e and nighttime. During daytime the yearly variation of the electron temper ature has negative correlation with the electron density, except at magneti c latitudes lower than 10 degrees. At night, the correlation is positive.