Equatorial ionospheric irregularities using GPS TEC derived index

We have used the rate of change of TEC (ROTROT) derived fluctuation index to study irregularities in the ionosphere at Franceville in Gabon (Lat.= −1.63°, Long.=13.55°, Geomag. Lat.= −0.71°), an equatorial station in the African sector. Based on a preliminary study at two equatorial stations at different longitude an average ROTIROTI index which gives the fluctuation level over half an hour at a particular station was put forward. This index eliminates the noise spikes or extreme value usually present in ROTIROTI index estimate. The new index ROTIAVEROTIAVE was used to study ionospheric irregularity occurrence at Franceville. As far as we know, this is the first time irregularity occurrence study is being done at this station using GNSS data. The results obtained showed that ionospheric irregularity season at Franceville is from March to November and that there is a kind of minimum around June. Very low irregularities activity is also observed around January. Pre-midnight fluctuation is observed to be more pronounced at Franceville during the period studied.

► Ionospheric irregularity was studied using rate of change of GPS TEC derived index.
► A new index based on the idea of two of the previously used indices was proposed.
► The new index was used to study phase fluctuation occurrence at Franceville, Gabon.
► Irregularities season at Franceville is found to be from March to November.
► Pre-midnight fluctuation is observed to be more pronounced for the period studied.