In this study we propose the assimilation of topside in situ electron density data from the Republic of China Satellite (ROCSAT-1) along with the ionosonde measurements for accurate determination of topside ionospheric effective scale heights (H-T) using an alpha-Chapman function. The reconstructed topside electron density profiles using these scale heights exhibit an excellent similitude with Jicamarca incoherent scatter radar (ISR) profiles and are much better representations than the existing methods of Reinisch-Huang method and/or the empirical International Reference Ionosphere-2007 model. The main advantage with this method is that it allows the precise determination of the effective scale height (H-T) and the topside electron density profiles at a dense network of ionosonde/Digisonde stations where no ISR facilities are available. The demonstration of the method is applied by investigating the diurnal, seasonal, and solar activity variations of H-T over the dip-equatorial station Jicamarca and the midlatitude station Grahamstown. The diurnal variation of scale heights over Jicamarca consistently exhibits a morning time descent followed by a minimum around 0700-0800 LT and a pronounced maximum at noon during all the seasons of both high and moderate solar activity periods. Further, the scale heights exhibit a secondary maximum during the postsunset hours of equinoctial and summer months, whereas the postsunset peak is absent during the winter months. These typical features are further investigated using the topside ion properties obtained by ROCSAT-1 as well as Sami2 is Another Model of the Ionosphere (SAMI2) model simulations. The results consistently indicate that the diurnal variation of the effective scale height (H-T) does not closely follow the plasma temperature variation and at equatorial latitudes is largely controlled by the vertical E x B drift.
