Observation of Low-Latitude Ionospheric Irregularities using Rate of Change of Total Electron Content over the Philippine Sector

When radio signals encounter ionospheric irregularities while being transmitted, they appear to be rapid signal fluctuations. These rapid fluctuations in radio signals are called scintillation and the irregularities that cause them are found in the ionosphere around 300-500 km altitude. In this paper, the occurrence of low-latitude ionospheric irregularities over the Philippine sector using 8 Global Navigation Satellite System receivers from the Philippine Active Geodetic Network (PageNET) on March 2015 was observed through the rate of change of total electron content index (ROTI). The 5-minute ROTI was calculated from raw GNSS data from the PageNET receivers. High ROTI values were mostly observed during post-sunset (1200 UT or 8:00 PM local time) and lasted for about 3-4 hours. This shows that generally, ionospheric irregularities are present during this time of the day. However, when a strong geomagnetic storm hit Earth, the usual occurrence of these irregularities appears to be suppressed. According to Nayak et.al. (2017), the suppression was due to the reduced pre-reversal enhancement (PRE) electric field which caused a condition detrimental to the formation of ionospheric irregularities. Moreover, ROTI values correlated very well with the station's latitude where higher latitudes exhibited higher ROTI than latitudes near the geomagnetic equator. This corresponds to the presence of the equatorial ionization anomaly.