Solar Activity and Geomagnetic Storm Effects on GPS Ionospheric TEC over Ethiopia

Ionospheric GPS total electron content (TEC) is an important parameter to monitor for possible Space Weather impacts. The effects of solar activity on TEC at low latitude stations with geographic locations (latitude, longitude) of Addis Ababa (9.04 N, 38.77 E) and Bahir Dar (11.6 N, 37.36 E) in Ethiopia, East Africa in the year of 2015 around peak of solar cycle 24 has been carried out. The data from the two stations was used to study the diurnal, monthly and seasonal variations of TEC and its dependence with solar activity and space weather effects. These observations were investigated and further discussed with an analysis of Disturbance Storm Time (Dst) and Ap indices, solar radio flux (F10.7cm) and sunspot number during the period of 2015. During the period of low or high sunspot number, that provided GPS ionospheric TEC builds up slowly or quickly. The obtained results reveal TEC undergoes diurnal and seasonal variations, daily variation of TEC value at both stations sharply increases to its peak from 0900 -1500 UT and decreases around 1600 0700 UT. Seasonal variations showed that TEC maximizes during the equinoctial months and least in summer over the two stations. In all seasons the maximum value of TEC in Addis Ababa is higher. The effects of geomagnetic storms on TEC values have been found negative and positive output.

Where, f 1 and f 2 are GPS signal frequencies and are equal to 1.57542 GHz and 1.2276GHz respectively, and K=80.62 is a constant that relates plasma frequency to electron density.
Where, λ 1 and λ 2 are wavelengths corresponding to f 1 and f 2 . In addition to that if we assume there are no lateral or horizontal electron density gradients, the vertical TEC (VTEC) can be simply mapped to the Slant TEC(STEC) as follows.

= ℵ
Where,ℵ is the satellite zenith angle at the point of intersection of the line of sight with that of spherical single layer atmosphere Piercing point (Abdullah et al., 2009).
The TEC measurements obtained from dual frequency GPS receivers are one of the most important methods of investigating the Earth's ionosphere. Because of the sensitivity of the science as stated above and the insufficient studies about ionospheric variability in Ethiopia, East Africa, we are motivated to investigate effects of solar activities and geomagnetic storms on GPS ionospheric TEC over Ethiopia by using Addis Ababa and Bahir Dar GPS stations in the year of 2015, which is around peak of solar cycle 24.
Due to insufficient ionospheric devices the physics of the ionosphere in the Ethiopian sector has not been studied well. Now-a-days few ground-based GPS receivers have been installed and as a result it gives a good opportunity for us to observe effects of solar activities on GPS ionospheric TEC. The computation of reliable vertical total electron content (VTEC) of the Ionosphere is at the same time a useful and challenging goal. It is useful because, in both Science and Technology fields, they can provide valuable information concerning space weather events, empirical model predictions and user navigation improvement, among others. Because of the sensitivity of the science and the insufficient studies about ionospheric variability in Ethiopia, East Africa, we are motivated to observe the effects of solar activities and space weather events by using ground-based GPS receivers over this sector with recent data. The main objective of this study is thus, to investigate effects of solar activities and geomagnetic storms on GPS ionospheric TEC over Ethiopia by using Addis Ababa and Bahir Dar GPS stations in the year of 2015, around peak of solar cycle 24.

DATA AND METHODOLOGIES
One of the parameters that are used to study the ionosphere is the Total Electron Content (TEC). After all the data are received from the source, MATLAB programming with its code has been developed by importing the data. Using the MATLAB programming code the value of TEC in Addis Ababa and Bahir Dar stations for all days of each month and season including selected disturbed and undisturbed days and Dst index, solar radio flux and sunspot number data with the obtained Matlab program has been plotted.

Diurnal variations of TEC
The diurnal variation of the electron density can be studied using TEC parameter. Figure    resulted due to the activity of the Sun. However, the modifications of the atmosphere from below cannot be neglected (Maruyama, 2003;Abdu and Brum, 2009). The diurnal patterns of TEC exhibits a steady increase starting about sunrise to an afternoon maximum and falls to attain a minimum just before sunrise. The diurnal characteristic of TEC has seasonal, solar activity, geomagnetic activity and latitudinal dependence.
As clearly indicated in figure  As observed from figure 2 due to the incompleteness of the data at Addis Ababa station during certain days of months, we presented the diurnal variations observed at Bahir Dar station dual frequency GPS-TEC data. The diurnal variation in TEC at Bahir Dar, Ethiopia exhibits many characteristics typical to low (equatorial) latitude ionosphere such as a TEC minimum at predawn and gradual increase with the time of day attaining a maximum in the afternoon and a gradual decrease after sunset. The daily peak occurs at about 0900 -1500 UT (around 1200 UT) around low latitude (equatorial) regions. In equatorial regions or low latitude the daily variation peak TEC greatly depends on Sun's activity. The day to day variability of TEC is contributed by the various parameters like EUV, solar flares and CME which is consistent with previous results obtained earlier (e.g. Rama Rao et al., 2006). UT. Generally this temporal variation is depending on intensity of radiation coming from the Sun, since the intensity increases starting from sunrise and become maximum when Sun is overhead and comes to zero when sunset. As observed from figure 3, concentration of TEC is greater between 0900 -1500 UT, maximum TEC is observed during January 30 and during UT. In the morning and evening the amount of TEC is similar and less than the peak value. On the other hand maximum TEC around 80 TECU is measured in April.      we observed its effect on TEC at the end of the next section. As shown from figure 6, the least peak TEC observed from the station during the days in the two months namely July and August.
Based on figure 7 we can generalize that, the derived GPS ionospheric TEC value is maximum during late days of September and October as compared to June, July and August. In In all the figures 3-8, the TEC value of one day is different from the TEC value of another day of the same month. In all these contour plots the diurnal variations show a maximum occurring of TEC is found between 0900 -1500 UT and short-lived minimum TEC occurring around 0000 -0800 UT and 1600 -2300 UT. These day to day variations of TEC may be attributing to the change in activity of the Sun itself. This means that, sunspot number, geomagnetic activity and different particles coming from the Sun are different from day to day so that the TEC value varies accordingly. Moreover, the maximum VTEC is obtained at the day time near noon. As it is known, the intensity of the sun's radiation is influenced by the zenith angle of the Sun's rays striking the atmosphere. At noon time the intensity of light from the Sun is relatively large compared to the other hours of the day. Thus, photo-ionization processes are maximum and maximum TEC is obtained at noon time. During the night time the value of TEC is very small because electrons are lost as a result of the recombination processes. Figure 9 shows that there are significant VTEC differences in both magnitude and pattern over 12 months period in the two stations in the buildup region, afternoon plateau and decay region.

Monthly Variation of Ionospheric TEC
In all months the TEC value is maximum near noon and of all months the highest TEC is obtained around noon. In Addis Ababa station, the month of March recorded the highest peak  The day-to-day or month-to-month variability in TEC, which has also been observed by some authors (Yizengaw et al., 2007;Tyagi, 1974), may be due to the changes in the activity of the Sun itself and to the associated changes in the intensity of the incoming radiations and the zenith angle at which they are incident on the Earth's atmosphere. The reason for this phenomenon is that the hourly values of TEC can be altered by intensity of solar electromagnetic radiation and emission of particles from the Sun.

Seasonal Variation of Ionospheric TEC
To study the seasonal variability of the ionospheric TEC, Lloyd's seasons as cited in (Rabiu et al., 2007;Rastogi et al., 2008  It is important to note that Earth does not move at a constant speed in its elliptical orbit.
Therefore the seasons are not of equal length. The relative position of the Earth's axis to the Sun changes during the cycle of seasons. This phenomenon is the reason why the Sun's height above the horizon changes throughout the year. It is also responsible for the seasons through controlling the intensity and duration of sunlight received at various locations around the planet. So during solstice the Sun is not overhead at the equator, that means the light intensity that pass through the ionosphere above the equator is small compared to the equinox which leads to small value of TEC in the ionosphere which in turn produces different navigational and communication problems.  figure 10. The study of TEC variation at both stations has been grouped into three seasons as in the case stated before. Figure 10 shows the variation of TEC for the whole seasons for the two stations. As observed from this figure the TEC variations attained maximum during the afternoon with equinoctial months followed by winter for the two stations.
Summer months recorded the least TEC amplitudes both in Addis Ababa and Bahir Dar.
The position of the Sun relative to the equator affects the ionization process in the ionosphere because the intensity of ionization depends on the solar zenith angle. That is when the sun is at different positions the solar zenith angle changes so the rate of production will be changed.
When the Sun is at horizon the intensity reaching the equatorial ionosphere is relatively small compared to the intensity reaching at the equatorial ionosphere from the overhead Sun. The results of variation in median TEC over the low-latitude stations considered in this study showed that the variation of median TEC is dependent on seasons and local time of occurrence in both stations of Ethiopian low latitude regions. As shown in figure 11 in the whole seasons the maximum value of TEC leads first in Addis Ababa. The difference in VTEC over Addis Ababa and Bahir Dar is due to differences in geographical latitude. Figure 11. The dependence of seasonal variation of TEC on location.

Solar Activity Dependence of TEC
The Sun emits a wide spectrum of radiation along with high energy particles. When, radiation is absorbed, intensity is decreased. If intensity were not decreased or radiation were not absorbed one cannot exist over the Earth. This radiation increases during high solar activity and this has its In general from figures 13 to 16, maximum TEC value is formed during spotted days while minimum TEC value is formed during spotless days. For the day exhibits maximum sunspot numbers and solar radio flux, the TEC value is also maximum. It is highly relevant for ionospheric effects. Jee et al. (2005) reported the direct control of the solar activity on the ionization level, with higher values during a high solar activity period and low value during a low solar activity period. We deduce that, during the period of low or high sunspot number, the provided GPS ionospheric TEC builds up slowly or quickly.

Variation of TEC during geomagnetic storms
The Earth's ionosphere responds dramatically to geomagnetic storms coupling the neutral thermosphere with the overlying magnetosphere. Geomagnetic disturbances are caused either when the interplanetary magnetic field (IMF) is directed south ward or when the magnetosphere of the Earth is stroked by solar wind. On sun spotted day more solar electromagnetic radiation and energetic particles such as CME, solar are, X-ray, EUV are bombarding the ionosphere to have large magnitude of TEC. Strong solar flares could be, associated with cosmic ray and it disrupts radio communication on Earth, the intense X-ray and EUV radiation emitted during the flares heats the atmosphere, disrupting communication and altering Satellites orbits (Schunk and Raitt, 1980) The size of a geomagnetic storm is classified as moderate when Dst is between -50 nT and -100 nT, intense storm between -100 nT and -250 nT and super storm below -250 nT. From these different SSC we took some days from international disturbed ( IDD ) and undisturbed ( IQD ) days at which strong geomagnetic storm is formed and no geomagnetic storm was happened respectively depending on Dst indices. Diurnal and disturbance-dependent change in the ionospheric TEC depends on Space Weather. Therefore, there is strong relationship between geomagnetic storm and variation in the ionospheric TEC.        Generally, we observe that in both stations the value of TEC is maximum during disturbed days as compared to undisturbed once. In a different case the value of TEC is maximum on undisturbed days of March and November, 2015 as compared to disturbed days as observed from the study areas found in Ethiopia, East Africa. Geomagnetic storms may be positive or negative in regarding to their effects on TEC that is the value of TEC increases due to positive while decreases due to negative geomagnetic storms. So we showed that the storms occurred during March 17 -18 and November 25 are negative storms while the rest days of geomagnetic storms are positive storms since they increase the value of TEC as compared to other undisturbed days of the observed months. When TEC is maximized /minimized by storms in the ionosphere the radio communication will also be disrupted. In all aspects of the observation we saw that the variation of TEC of the ionosphere results due to Space Weather effects.

CONCLUSIONS
The results show that the median TEC varies from a pre-dawn minimum to an afternoon The value of TEC shows increment and decrement in line with the values of solar activity parameters (sunspot number and solar radio flux). We deduce that, during the period of low or high sunspot number and solar radio flux, the provided GPS ionospheric TEC builds up slowly or quickly. Geomagnetic storm has occurred on June 23, 2015. The Dst values for the day shows that it is an intense intensity geomagnetic storm. We observed the value of TEC is greater during disturbed days than undisturbed once. In a different case the value of TEC is maximum on undisturbed day of March and November, 2015 as compared to disturbed days. We showed that the storms occurred during March 17 -18 and November 25 are negative storms while the rest