Abstract
Total electron content (TEC) is an important parameter in the ionosphere that is extensively used to study the variability of the ionosphere as it significantly affects radio wave propagations, causing delays on GPS signals. Therefore, evaluating the performance of ionospheric models is crucial to reveal the variety of ionospheric behaviour in different solar activity periods during geomagnetically quiet and disturbed periods for further improvements of the IRI model performance over the equatorial region. This research aimed to investigate the variations of ionospheric VTEC and observe the improvement in the performance of the IRI-2016 (IRI-2001, IRI01-corr, and NeQuick). The IRI-2016 was evaluated with the IRI-2012 using NeQuick, IRI-2001, and IRI01-corr topside electron density options. The data were obtained using a dual-frequency GPS receiver installed at the Universiti Utara Malaysia Kedah (UUMK) (geographic coordinates 4.62° N–103.21° E, geomagnetic coordinates 5.64° N–174.98° E), Mukhtafibillah (MUKH) (geographic coordinates 6.46° N–100.50° E, geomagnetic coordinates 3.32° S–172.99° E), and Tanjung Pengerang (TGPG) (geographic coordinates 1.36° N–104.10°E, geomagnetic coordinates 8.43° S–176.53° E) stations, during ascending to high solar activity at the geomagnetically quiet and disturbed periods in October 2011, March 2012, and March 2013. The maximum hourly ionospheric VTEC was observed during the post-noon time, while the minimum was during the early morning time. The ionospheric VTEC modelled by IRI-2016 had a slight improvement from the IRI-2012. However, the differences were observed during the post-noon and night-time, while the modelled VTEC from both IRI models were almost similar during the early morning time. Regarding the daily quiet and disturbed period’s prediction capability of the IRI-2016 and IRI-2012, IRI-2016 gave better agreement with the measured VTEC. The overall results showed that the model’s prediction performance during the high solar activity period in 2013 was better than the one during the ascending solar activity period. The results of the comparison between IRI-2016 and IRI-2012 in high solar activity exhibited that during quiet periods, all the IRI models showed better agreement with the measured VTEC compared to the disturbed periods.
Highlights
The ionospheric total electron content (TEC) is a notable source of error that significantly affects the radio wave propagation, which disrupts the accuracy of the global positioning system (GPS) signals from the satellite to the receiver, causing the delay by changing the velocity of the propagated signals [1,2]
The overall VTEC data over Universiti Utara Malaysia Kedah (UUMK), MUKH, and Tanjung Pengerang (TGPG) stations indicated that the minimum value of VTEC was during sunrise, and gradually increased to attain its maximum value during the post-noon time
The results revealed that the daily maximum VTEC values were observed during the post-noon time (13:00–17:00 LT) and the minimum values were attained in the early morning time (4:00–6:00 LT)
Summary
The ionospheric total electron content (TEC) is a notable source of error that significantly affects the radio wave propagation, which disrupts the accuracy of the global positioning system (GPS) signals from the satellite to the receiver, causing the delay by changing the velocity of the propagated signals [1,2]. The TEC varies according to different geographical locations, such as high-latitude, mid-latitude, low-latitude, and equatorial regions [3]. Due to the low solar radiation, these variations occur frequently in low-latitude and equatorial regions such as Malaysia. In these regions, the plasma density in the ionosphere has shown significant variations in TEC based on various factors such as seasons, latitudes, longitudes, time of the day, solar cycles, and geomagnetic activity. The ionospheric TEC in equatorial regions is subject to day-to-day variability and represents a challenging problem for ionospheric modellers [4]. The study and evaluation the performance of ionospheric models in equatorial regions are crucial to provide a representative model error for further improvement of the IRI models
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