The Global Navigation Satellite System (GNSS) applications serve as a dependable reference and a novel avenue for exploring natural dynamics sensing. Global Positioning System (GPS) is one of GNSS satellite which the quality of signals depends on electron diffusion conditions in the ionosphere (Chernyshov et al., 2020, p.1). The ionosphere layer is the propagation medium and slows down the propagation of the signal. These changes depend on the identity of ionosphere plasma density. When the ionosphere is disturbed, it has a detrimental impact on GPS satellites, reducing the accuracy and reliability of positioning results. Free electrons in the ionosphere can significantly influence signal characteristics, including phase and amplitude. The Total Electron Content (TEC) in the ionosphere influences the group delay and phase advance of GPS satellite signals. A higher TEC value along the path between the radio transmitter and receiver corresponds to a greater impact on the radio signal. Irregularities in the plasma density of the ionosphere cause distortion of electromagnetic waves during their propagation through the ionosphere. This can result in inaccurate parameter estimation or even a temporary interruption (slip) of the received GPS signal. The primary sources of ionospheric anomalies at different latitudes are disturbances in Earth's space environment caused by solar and geophysical factors. The near-Earth plasma is affected by the Interplanetary Magnetic Field (IMF), solar wind, and various physical phenomena, such as geomagnetic storms and sub-storms. These factors collectively contribute to the state of the near-Earth space environment in relation to solar activity (Hofmann-Wellenhof et al., 1992, pp. 90–97; Pi et al., 1997, p. 2283; Skone et al., 2000, p. 1067; Chernyshov et al., 2020, p. 1).
The positive ionospheric storms are associated with an enhancement in electron density caused by thermospheric winds, by compositional changes or by the transport of ionization or electric field changes whereas the negative storms are primarily due to compositional changes of the ionosphere (Serafimov et al., 1982; Chinmaya et al., 2016; Reddybattula et al., 2019, p. 283). The disturbance of TEC caused by the impact of storms varies depending on the latitude. The variation of TEC at low-latitudes is generally highly influenced by an electromagnetic field over the magnetic equator. This results in a significant increase in electrical conductivity over the equator (Li et al., 2019, p. 9380). During geomagnetic storms, the ionospheric currents and the influx of energetic particles into the ionosphere generate and release heat energy. This heat energy can result in an elevation of density and a redistribution of density within the upper atmosphere. As a result, satellites in low-Earth orbit experience increased drag. Geomagnetically, the global ionosphere can be classified into four regions: high-latitude, mid-latitude, low-latitude, and equatorial regions. The researchers reported the presence of ionospheric irregularities at different latitudes during the geomagnetic storm, including high, middle, low, and equatorial regions. The equatorial and low-latitude ionospheric regions are located within ± 20 degrees on either side of the geomagnetic equator. Those latitude regions have the highest peak electron density and show intricate ionospheric variations due to unique phenomena such as the Equatorial Ionization Anomaly (EIA), Equatorial Electrojet (EEJ), Fountain effects, and others. Therefore, various research works focusing on the equatorial ionospheric regions are frequently fascinating and crucial for gaining deeper insights into ionospheric morphology and improving modeling techniques (Reddy, 1986, pp. 247–263; Kumar et al., 2016, pp. 1755–1762; Jenan et al., 2021, p. 575). For example, Kenpankho et al. (2011, pp. 365–370) compared of GPS TEC measurements with IRI TEC prediction at the equatorial latitude station Chumphon, Thailand. Rao et al. (2013, pp. 1–6) studied the low-latitude TEC variability studied from magnetically conjugate locations along 73°E longitude. Nava et al. (2016, pp. 3421–3438) researched mid-latitude and low-latitude ionosphere response to 2015 St. Patrick’s Day geomagnetic storm. Zhou et al. (2016, pp. 9146–9163) studied the ionospheric storm effects and equatorial plasma irregularities during the 17–18 March 2015 event. Chen et al. (2017, pp. 3632–3639) extracted the geomagnetic activity effect from TEC data: a comparison between the spectral whitening method and 28 days running median. Reddybattula et al. (2019, pp.283–292) analyzed the ionospheric TEC from GPS, Global Ionosphere Map (GIM), and global ionosphere models during moderate, strong, and extreme geomagnetic storms over Indian region. de Paula et al. (2019, pp. 1–15) analyzed the ionospheric irregularity behavior during the September 6–10, 2017, magnetic storm over Brazilian equatorial-low-latitudes. Dugassa et al. (2019, pp. 1161–1180) investigated of the relationship between the spatial gradient of TEC between two nearby stations and the occurrence of ionospheric irregularities. Mansilla (2019, pp. 26–36) studied the behavior of TEC over the Arctic and Antarctic sectors during several intense geomagnetic storms. Jenan et al. (2019, pp.131–137) studied the influences of solar activities on TEC variations of equatorial ionosphere over Sri Lanka. Chernyshov et al. (2020, pp. 1–13) reported on relationship between TEC jumps and auroral substorm in the high-latitude ionosphere. Jenan et al. (2021, pp. 575–587) presented about ionospheric total electron content response to September-2017 geomagnetic storm and December-2019 annular solar eclipse over Sri Lankan region. Nitta et al. (2021, pp.1–53) reported about the understanding the origins of problem geomagnetic storms associated with “Stealth” coronal mass ejections. Iyer and Mahajan (2021, pp.1–53) studied the granger causality analysis of deviation in total electron content during geomagnetic storms in the equatorial region. Zong et al. (2021, pp. 1–34) studied about the shock induced strong substorms and super substorms: preconditions and associated oxygen ion dynamics. Hammou et al. (2021, pp. 4857–4871) studied the transient variations of Vertical Total Electron Content (VTEC) at low latitude during the period 2013–2017. Ratovsky et al. (2022, pp. 1–15) investigated the relation of extreme ionospheric events with geomagnetic and meteorological activity. Klimenko et al. (2022, pp. 923–938) reported on similarity and differences in morphology and mechanisms of the foF2 and TEC disturbances during the geomagnetic storms on September 26–30, 2011. Adolfs et al. (2022, pp. 1–17) experimented on storm-time relative TEC modelling using machine learning techniques. Bojilova et al. (2023, pp. 1–23) compared and analyzed a global and regional ionospheric responses during two geomagnetic storms on February 3–4, 2022.
After thoroughly studying and reviewing relevant research, we conceived an idea to investigate, analyze, and compare low-latitude TEC disturbances over Bangkok and Chumphon, Thailand, to the severe geomagnetic storms on March 24, 2023, and April 23–24, 2023. By collecting TEC data from three sources: GPS, International GNSS Service (IGS), and International Reference Ionosphere (IRI), we can conduct a comprehensive analysis on low-latitude TEC disturbances over Bangkok and Chumphon, Thailand. In the related research, Chen et al. (2017, pp. 3632–3639) investigated the impact of geomagnetic activity on TEC data. They compared two methods: the Spectral Whitening Method (SWM) and the 28 days Running Median Centered (RMC). They found that the ionospheric fluctuations during both quiet and disturbed conditions were analyzed using both the SWM and the RMC techniques. The results from the RMC technique, when compared with those from the SWM, show an overestimation of the disturbance occurrence by approximately 5–20% during the geomagnetic storms and up to 35% during the quiet time. Nava et al. (2016, pp. 3421–3438) conducted a study on the ionospheric response at mid-latitude and low-latitudes during the 2015 St. Patrick's Day geomagnetic storm. For the Asian sector, they compared two mid-latitude stations: Wuhan (Northern Hemisphere) and Yar2 (Southern Hemisphere), and the low-latitude station: Cusv (Northern Hemisphere). They observed a significant decrease in TEC for several days after the storm at the mid-latitude stations. However, at the low-latitude station, they found an increase in TEC on the day of the storm and no decrease in the following days. These observations align with the maps of the Asian sector, which clearly show that in the days following the storm, ionization is concentrated at low-latitudes near the equator, while it diminishes significantly at mid-latitudes.
Based on a detailed review and analysis of relevant studies, we have developed research aimed at investigating, analyzing, and comparing the occurrence of TEC disturbances at low-latitudes, especially in Bangkok and Chumphon, Thailand, to the severe geomagnetic storms on March 24, 2023 and April 23–24, 2023 by using TEC data from three sources: GPS, IGS, and IRI.