Temporal and altitudinal variability of the spread F observed by the VHF radar over Christmas Island

Abstract. The goal of this work is to study the time and altitude echo characteristics under different solar and seasonality conditions using the VHF radar range–time–intensity (RTI) images. The occurrence of equatorial spread F depends on the existence of conditions that can seed the Rayleigh–Taylor instability, and these conditions can change with solar flux, seasonality, longitude distributions, and day-to-day variability. So, the equatorial spread F is observed as its time and altitude occurrence. The VHF radar of Christmas Island (2.0∘ N, 157.4∘ W, 2.9∘ N dip latitude) has been operational in the equatorial region for some time, allowing long-term observations. The occurrence of echoes during solar minimum conditions is observed throughout the night since the post-reversal westward electric field is weaker than the solar maximum and the possibilities of the vertical plasma drift becoming positive are larger. On the other hand, echoes during solar maximum will be controlled by dynamics near the time of the pre-reversal peak (PRE). Our results indicate that the peak time occurrence of echoes along this period shows a well-defined pattern, with echoes distributed as closer to local sunset during solar maximum and around/closer to midnight during solar minimum conditions; meanwhile, the peak altitude occurrence of echoes shows a slightly regular pattern with higher-altitude occurrences during solar maxima and lower altitudes during solar minimum conditions.