Effect of solar cycle on topside ion temperature measured by SROSS C2 and ROCSAT 1 over the Indian equatorial and low latitudes

Abstract. The effect of solar activity on the diurnal, seasonal and latitudinal variations of ion temperature T_i and its relationship with corresponding ion density N_i over the Indian low and equatorial topside ionosphere within 17.5° S to 22.5° N magnetic latitudes are being investigated, combining the data from SROSS C2 and ROCSAT 1 for the 9-year period from 1995 to 2003 during solar cycle 23. T_i varies between 800 K and 1100 K during nighttime and rises to peak values of ~1800 K in the post sunrise hours. Daytime T_i varies from 1000 K to 1500 K. The time of occurrence, magnitude and duration of the morning enhancement show distinct seasonal bias. For example, in the June solstice, T_i increases to ~1650 K at ~06:00 h and exhibits a daytime plateau till 17:00 LT. In the equinoxes, enhanced ion temperature is observed for a longer duration in the morning. There is also a latitudinal asymmetry in the ion temperature distribution. In the equinoxes, the daytime T_i is higher at off equatorial latitudes and lower over the Equator, while in the solstices, T_i exhibits a north–south gradient during daytime. Nighttime T_i is found to be higher over the Equator. Daytime ion temperature exhibits insignificant positive correlation with F_10.7 cm solar flux, while nighttime ion temperature decreases with increase in solar flux. Daytime ion temperature and ion density are negatively correlated during solar minimum, while nighttime T_i does not exhibit any correlation. However, during high solar activity, significant positive correlation of T_i with N_i has been observed over the Equator, while at 10° S and 10° N temperature and density exhibit significant negative correlation. The neutral temperature T_n derived from the MSISE 90 model is found to be higher than measured T_i during nighttime, while daytime T_i is higher than model T_n.