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Topside ionospheric parameters-total ion density (Ni) and ion temperature (Ti) have been analysed at low latitude region with changing solar activity (years 1999 to 2003). The Ni and Ti data collected from ROCSAT-1 satellite has been compared with the estimated values of IRI-2016 model. The annual diurnal features observed for Ni (measured by ROCSAT-1) are: a minimum value just before local sunrise (~04:00/05:00 LT), a day-time peak (~13:00/14:00 LT) and then a gradual decrement throughout the evening and nighttime. The annual diurnal variation of Ti (measured by ROCSAT-1) shows that Ti exhibits a morning peak (morning overshoot, ~07:00 LT), a day-time trough, a secondary peak (evening enhancement) followed by nighttime minima and a minimum value before the sunrise. The distinct annual diurnal feature observed by the IRI model is the presence of a secondary evening peak in Ni which is absent in Ti, which is ex- actly opposite to the trend measured by ROCSAT-1. Some other discrepancies observed in the model are:overestimation of Ni during all the years, specifically in the morning and evening time; overestimation of Ti, during the entire day except in the morning peak hours of the year 1999, 2000 and 2003.For each year, the hourly averaged ROCSAT-1 measured value of Ni and Ti has been correlated with the estimated value of IRI-2016 model. The correlation coefficient factor R2 is ~ 0.8 for Ni and ~ 0.9 for Ti respectively. The variations of Ni and Ti with changing solar flux have also been studied. The ionospheric parameters are found positively and linearly correlated with solar- flux (F10.7). The correlation coefficient factor R2 for Ni and Ti with F10.7 is ~ 0.8 and ~ 0.9 respectively.
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