Seasonal and Latitudinal Distributions of the Dominant Light Ions at 600 km Topside Ionosphere During Years of Solar Maximum

Data taken by ROCSAT-1 from 1999 to 2002 during years of solar maximum has been analyzed for the observations of the dominant light hydrogen or helium ions at 600 km topside ionosphere. The results reveal some interesting seasonal and latitudinal distributions of the dominant light ion species in the topside ionosphere even during years of high solar activity. The hydrogen ions are observed as the dominant ion species only during the nighttime. Except for the March spring, the distribution for such transition shows a strong hemispheric asymmetry. Furthermore, the transition that occurs in the winter hemisphere during the solstice seasons seems to be bounded along a low dip-latitude that circumscribes most of the transition occurrences in one hemisphere. No significant seasonal dependence of this latitude bound is noticed during high solar activity years. On the other hand, the occurrence of dominant helium ions is very rare but it shows the seasonal and latitudinal distributions similar to that of dominant hydrogen ions. Finally, a good correlation between the observed field-aligned ion flow and the hemispheric asymmetry for the transition of dominant hydrogen ions has been found as the cause for the asymmetry inferred from the existence of interhemispheric plasma flow in relation to the hemispheric asymmetry of the F peaks and to the enhancement or retardation of the nocturnal re-distribution of the light ions along the field line.