Correlations between ion density and temperature in the topside ionosphere measured by ROCSAT-1

From the ROCSAT-1 satellite plasma data at an altitude of 600 km, the correlation between ion temperature (Ti) and density (Ni) was investigated. The data were obtained in a magnetic dip latitude (MLAT) of less than ±40° in 2000–2004. Positive and negative correlations between Ni and Ti were observed around the magnetic dip equator, while weak positive correlations were observed in |MLAT| > 25° during daytime (10:00–16:00 local time). These variations were found in all longitudes, seasons, solar flux (F_10.7) levels, and magnetic disturbance levels, although the minimum value of Ti clearly increased with increasing solar flux levels. The results suggest that the solar flux dependence of Ti arises from the solar flux dependence on neutral temperature (Tn). Since Ti is determined by heating through Coulomb collision with electrons and cooling through elastic collision with neutral species, the ratio of ion density to neutral density is an important factor. The ratio reaches its maximum value around the magnetic dip equator and decreases with increasing MLAT. The correlation between Ni and Ti in the topside ionosphere can be explained by electron temperature (Te) and Tn as well as the ratio because Ti follows Te variation when the ratio is high, while it follows Tn when the ratio is low.