# TEC and ROTI Measurements from a New GPS Receiver at BOWEN University, Nigeria
## 4 Results and Discussion  
Figures 4 and 5 show the first results of the monthly TEC and ROTI observations from the CASES GPS receiver installed at Bowen University, Iwo, Osun State. The x and y axes of the contour plots show the days of the month and the universal time, respectively. The unfilled (white background) points represent unavailable data points due to unknown  
Figure 4: Monthly ROTI variation over the Bowen CASES GPS during 2021.  
sources. Unavailable data points can be due to electric power supply or instrument errors which cannot be corrected during the CASES GPS's recording. The monthly TEC from the GPS data, as described in the methodology, has been computed for all months in 2021, a low-solar-activity year. These monthly observations are shown in Figure 5 from January to November 2021. The right-handed bar shows the intensity of the monthly TEC. The monthly TEC is characterized by a minimum in magnitude at pre-sunrise (04:00 LT-06:00 LT, \(\sim\)4 TECU\(\rightarrow\)5 TECU), maximum in magnitude at daytime (11:00 LT-15:00 LT, \(\sim\)30 TECU--50 TECU), decrease in magnitude at post-sunset (17:00 LT-20:00 LT, (\(\sim\)25 TECU--35 TECU), and a further decrease at nighttime (20:00 LT-24:00 LT, \(\sim\)7 TECU--15 TECU). Our observations agree with the previous investigations of [1; 22; 23; 24; 25; 26; 27]. They reported the weakest TEC during the pre-sunrise hours. Around the equatorial station, they reported maximum TEC during the daytime, which was associated with the greater ionization of electrons due to the solar extreme ultra-violet (EUV) production, coupled with the upward vertical E X B drift [1; 22; 23; 27; 28; 29]. According to references [24; 30; 31], the decrease in TEC magnitude at post-sunset and nighttime hours is mostly due to the cessation of solar EUV production. This is in combination with the downward E x B drift velocity that lowers the ionosphere to an altitude where chemical losses are larger. In comparison to the year 2021, a low-solar-activity period having a solar flux of \(\sim\)82 sfu, is the year 2010, having a closer solar flux unit of 80. References [32; 33] reported a minimum and maximum TEC magnitude at the June solstice and September equinox, respectively. Generally, the increase in electrons in the ionosphere is majorly controlled by solar flux production (photoionization) and recombination processes [33; 34; 35]. Therefore, plasma is expected to be higher around the geomagnetic equator during the equinoctial months due to the abundance of the peak photoelectron and intense eastward electric field within the subsolar region. The plasma at the equator, associated with a decrease in photoelectron peaks during the solstice months, is due to the movement of the subsolar point to higher latitudes. In addition, we observed a semi-annual variation characterized by two maxima in equinoctial months. As reported by [30], this was associated with the combined effect of zenith angle and magnetic field geometry. All of these results from CASES GPS that compared well with other literature reports are good indicators as they confirm that the CASES GPS receiver at Bowen University  
Figure 5: Monthly TEC variation over the BOWEN CASES GPS during 2021.  
has the required potential and capacity to capture reliable information about the varying equatorial ionosphere over Nigeria at any point in time. Figure 4 displays the monthly variations in the ROTI. Similar to the TEC data in Figure 5, the white background in Figure 4 reaffirms the missing TEC data points. Our observations over all months show total suppression of irregularities. The suppression of these ionospheric irregularities may be ascribed to the decrease in the zonal neutral wind which was unable to generate the F-region dynamo near the sunset. In order to further validate our results, we compared the TEC and ROTI measurements at BJCO with our results. Both results showed similar monthly TEC characterized by a diurnal pattern having a minimum pre-sunrise and a maximum daytime, post-sunset, and midnight decrease.