Occurrence pattern of large-scale ionospheric irregularities over Ilorin, Nigeria-During solar cycle 24

Ionospheric plasma density irregularities are regular features of the nighttime equatorial ionosphere especially around solar maximum. These irregularities exist in different scales ranging from small sizes of meters to very large scales of hundreds of kilometers and are responsible for the scintillation of Global Navigation Satellite Systems (GNSS) signals. Occurrence of these irregularities poses serious danger to GNSS signals and their applications. It is therefore necessary to continuously study their occurrence pattern and the severity of their effects on radio signals when new data are available. GNSS data for the whole of solar cycle 24 for Ilorin, Nigeria (Lat. = 8.48 degrees N, Long. = 4.67 degrees E, Dip = 4.1 degrees S) acquired using dual frequency GPS receiver (GPStation-2) and Novatel GISTM receiver (GPStation-6) were used to study the trend of large-scale irregularities. The indices used are the rate of change of TEC Index (ROTI) and ROTI average (ROTIAVE) obtained from 30 s RINEX data. The results showed two peaks in the level and frequency of occurrence of large-scale irregularities at Ilorin - one in March and the other in September. The level and frequency of occurrence of irregularities widened around these 2 months as the level of solar activity increases. It was observed that the frequency of occurrence is higher in March equinox than in September for all the years. Similarly, the spread of irregularities in terms of the monthly average number of satellites affected with irregularities was found to be higher in March equinox than in September during the increasing phase of the solar cycle; while during the decreasing phase, the spread for September was found to be higher than that of March. In terms of the local time of occurrence, irregularities are more pronounced between 1900 LT and 2400 LT. The level of irregularities was found to be correlated with solar indices (i.e. EUV and F10.7). A correlation study between solar indices (i.e. EUV and F10.7) with irregularities showed consistency pattern during the scintillation season i.e. during equinox. A slightly higher correlation values were however obtained between the level/spread of irregularities and EUV compared to that with F10.7.