Comparison of Total Electron Content with International Reference Ionosphere Model Predictions near the Northern Crest of EIA at Bangladesh

Total electron content (TEC) observations extracted from a Global Positioning System (GPS) receiver station in Dhaka, Bangladesh (geographic latitude $23.8^{\circ}N$, longitude $90.4^{\circ}E$, geomagnetic latitude $14^{\circ}N$) under the northern crest of equatorial ionospheric anomaly (EIA) is investigated. Eight years of TEC measurements from 2004-2011 constitutes an extensive dataset, which provides an excellent opportunity to perform such a study for the first time over this area. The analysis reveals that the diurnal maximum of TEC occurs around 13:00-14:00BST. Peak TEC values during noontime are maximum during March and April ($\sim 70$ TECU) and autumnal equinox ($\sim 50-60$ TECU), and lowest during winter ($\sim 35-43$ TECU) at high solar activity period. During low solar activity period, the peak TEC values reduce to $\sim 35-50$ TECU during equinoxes and to $\sim 24$ TECU during winter. TEC values minimise ($\sim 2-10$ TECU) after sunset to predawn hours. Observed monthly median TEC values are compared with predictions from the International Reference Ionosphere (IRI-2012) model. Results indicate that during high solar activity period, IRI-2012 underestimates TEC values ($\sim 8-17$ TECU) at vernal and autumnal equinoxes during noon. IRI predictability is better as the values are underestimated by $\sim$4 TECU from after sunset to pre-dawn hours and overestimated by 4 TECU during early hours of the day. IRI predictions are more accurate in summer and winter months than in equinoctial months. IRI exhibits improved prediction accuracy during low solar activity period with discrepancies as low as $\sim 0.13$ TECU.