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Article

Variation in Total Electron Content During a Severe Geomagnetic Storm, 23–24 April 2023

by
Atirsaw Muluye Tilahun
1,
Edward Uluma
2 and
Yohannes Getachew Ejigu
3,*
1
Department of Physics, College of Natural and Computational Science, Wachemo University, Hosaena 667, Ethiopia
2
Department of Physics, Masinde Muliro University of Science and Technology, Kakamega 190-50100, Kenya
3
Department of Built Environment, Geoinformatics, Aalto University, 2150 Espoo, Finland
*
Author to whom correspondence should be addressed.
Atmosphere 2025, 16(6), 676; https://doi.org/10.3390/atmos16060676
Submission received: 31 March 2025 / Revised: 15 May 2025 / Accepted: 24 May 2025 / Published: 3 June 2025
(This article belongs to the Special Issue Feature Papers in Upper Atmosphere (2nd Edition))
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Abstract

:
In this paper, we study the geomagnetic storm that occurred on 23–24 April 2023. We present variations in the values of interplanetary magnetic field (IMF-Bz), solar wind parameters (Vsw, Nsw, Tsw, and Psw), geomagnetic index (SYM-H), and vertical total electron content (VTEC) obtained from 18 GPS-TEC stations situated in equatorial, mid-latitude, and high-latitude regions. We analyze the variations in total electron content (TEC) before, during, and after the storm using VTEC plots, dTEC% plots, and global ionospheric maps for each GNSS receiver station, all referenced to universal time (UT). Our results indicate that GNSS receiver stations located at high latitudes detected an increase in ionospheric density during the main phase and a decrease during the recovery phase. In contrast, stations in equatorial and mid-latitude regions detected a decrease in ionospheric density during the main phase and an increase during the recovery phase. Large dTEC% values ranging from −80 to 190 TECU were observed a few hours before and during the storm period (23–24 April 2023); these can be compared to values ranging from −10 to 20 TECU on the day before (22 April 2023) and the day after (25 April 2023). Notably, higher dTEC% values were observed at stations in high and middle latitudes compared to those in the equatorial region. As the storm progressed, the TEC intensification observed on global ionospheric maps appeared to shift from east to west. A detailed analysis of these maps showed that equatorial and low-latitude regions experienced larger spatial and temporal TEC variations during the storm period compared to higher-latitude regions.

1. Introduction

Geomagnetic storms are disturbances in Earth’s magnetic field that result from fluctuations in the interplanetary magnetic field (IMF) [1,2,3]. Triggered by disruptions in the solar wind that interact with Earth’s magnetosphere, these storms significantly impact the near-Earth space environment [4]. The electrodynamics of Earth’s ionosphere are notably influenced during geomagnetic storms, which produce substantial ionospheric disturbances, manifested through marked changes in ionospheric density structures such as F-region electron densities [5] and total electron content (TEC) [6]. These disturbances cause scintillations in radio signals, which can severely affect navigation and communication systems [7,8]. Furthermore, these storms can cause absorption or scattering of radio waves, impacting long-distance communication. Severe geomagnetic storms also induce electric currents within the ionosphere, which pose risks to electrical infrastructure, such as power grids [9,10,11,12]. Moreover, they can locally heat the ionosphere, altering plasma density and affecting patterns of radio-wave propagation. Given that solar-wind parameters vary over the solar cycle, ionospheric and lower-atmospheric parameters may also be influenced over this cycle [13]. Monitoring these effects is critical for ensuring the reliability of communication and navigation systems.
TEC is defined as the total electron count in a 1 m2 column between a GPS satellite and receiver (1 TECU = 1016 electrons/m2) [14,15,16,17,18]. It is measured in total electron content units (TECU). It provides a vital parameter for characterizing charge densities across various ionospheric layers [19,20]. TEC can be expressed as an integral of electron density along the ray path between receiver and satellite, as follows:
T E C = ∫ l 1 l 2 n e l d l
where l1 and l2 represent the positions of the receiver and satellite, respectively; ne(l) is the electron density along the path; and dl represents incremental changes in distance between the receiver and satellite [21].
Numerous studies have examined the impact of geomagnetic storms on the ionosphere and the TEC. [22] analyzed ground-level geoelectric responses to this event using the GeoElectric Dynamic Mapping (GEDMap) code, which processed data from 29 magnetometers across northern Europe. Their results revealed strong spatial and temporal variability in the geoelectric field, especially during substorms. Modeled GICs at the Mäntsälä site showed strong correlations with observed values (r = 0.73–0.76), underscoring the risks posed to electrical infrastructure and the importance of accurate GIC modeling. The same storm produced pronounced low-latitude ionospheric responses. [23] reported enhanced equatorial ionization anomaly (EIA) crests, the formation of a one-sided F3 layer in the northern hemisphere, and post-midnight equatorial plasma bubbles (EPBs) extending up to 30° magnetic latitude. These effects were primarily driven by intensified transequatorial neutral winds and electric fields. A significant positive ionospheric storm was also observed over South America, where it occurred due to altered thermospheric composition and electrodynamic forcing. [24], analyzing both the February and April 2023 storms using GNSS-based TEC data from the American and Asian sectors, found that prompt penetration electric fields (PPEF) and disturbance dynamo electric fields (DDEF) were key drivers of regional TEC variability. Positive TEC changes over Pakistan and depletions over China during the April storm were attributed to changes in the O/N2 ratio. More broadly, [25] emphasized the importance of integrated, multi-domain observations by analyzing the 10–11 May 2024 superstorm (SYM-H = −518 nT), which generated extreme GICs and significant cosmic-ray modulation.
Ref. [26] utilized dual-frequency GPS data from the International Geodetic Survey (IGS) at the low-latitude Indian IGS Station IISC in Bangalore (13.02° N, 77.57° E) to investigate TEC during geomagnetic storms between 2014 and 2017. Their findings revealed that diurnal TEC variations were at their peak in 2014 and subsequently decreased from 2015 to 2017. The maximum diurnal TEC was observed in the afternoon (13:00–17:00 IST) in 2014–2017, showing asymmetry around noon. Additionally, they noted that seasonal TEC variations peaked during the March equinox. During storm periods, TEC values increased compared to quiet days, with both negative and positive storms observed. Negative storms were associated with a sharp decrease in TEC the day following the geomagnetic storm, with TEC changes on storm days ranging from 19–44%. [27] investigated TEC during three intense geomagnetic storms in 2015: the St. Patrick’s Day storm (16–21 March 2015), a storm that occurred from 21–24 June 2015, and another that occurred from 18–22 December 2015. They analyzed the variations in IMF-Bz, solar-wind parameters (Vsw, Nsw, and Psw), geomagnetic indices (AE and SYM-H), and VTEC using simultaneous data from 12 GPS-TEC stations across the Indian, Australian, Brazilian, and South African regions. Their study highlighted the global variations in TEC using ionospheric maps at 2 h intervals of UT during the storms, showing that equatorial and low-latitude regions were significantly affected. They suggested that disruptions in global winds and electric fields that were initiated by magnetosphere-ionosphere interactions severely modified TEC in equatorial and low-latitude regions. [28] examined the impacts of interplanetary coronal mass ejections (ICMEs) preceding a storm from 18–22 November 2003 on various space weather components. Their comprehensive study included parameters such as solar-wind speed, density, plasma temperature, plasma pressure, auroral electrojet, SYM-H, energy transfer, the components of Earth’s magnetic field, TEC, and cosmic-ray flux. The solar storm of November 2003 had a SYM-H value of −472 nT, with solar wind speeds reaching approximately 500 km/s and plasma pressure peaking around 20 nPa. The total energy dissipated in the magnetosphere during the storm was converted into ring current, joule heating, and auroral precipitation. The resulting joule heating altered thermospheric contents, leading to changes in the ionosphere–thermosphere system’s density, composition, circulation, and dynamics, causing fluctuations in TEC. Their study on TEC and SYM-H changes during the event suggested that significant alterations in space weather were caused by the solar superstorm of 18–22 November 2003.
The aforementioned studies demonstrate that the global distribution of effects of an ionospheric storm is highly complex and can vary considerably from one storm to another. The ionosphere’s response, as observed at various ionospheric stations, can exhibit significant differences even during the same geomagnetic storm. These differences are influenced by factors such as the location of the global positioning satellite (GPS) station, the local time of onset of the geomagnetic storm, and other relevant parameters [29,30]. Therefore, to elucidate the specific mechanisms through which solar phenomena influence TEC variations, it is crucial to investigate detailed temporal and spatial patterns of TEC fluctuations, explore methodologies for recognizing intervals between geomagnetic storms, and refine local empirical models for TEC prediction. Monitoring TEC during geomagnetic storms is essential for this purpose.
On 23 April 2023, at approximately 18:12 UT, a moderate solar flare (M1.7) erupted from the sun, expelling a substantial amount of superheated plasma. These coronal mass ejections (CMEs) resulted in the Earth experiencing a severe geomagnetic storm, rated as G4 on the (NOAA) scale, which is the focus of our current study. Geomagnetic storms significantly impact the TEC of the ionosphere. During such events, TEC undergoes considerable variations due to several factors. First, the influx of energetic particles from the solar wind enhances ionization in the ionosphere, leading to increased electron density and subsequently to higher TEC values. Second, geomagnetic storms induce disturbances in the ionosphere, causing irregularities and fluctuations in electron density, resulting in spatial and temporal variations in TEC along different paths. Finally, variations in the Earth’s magnetic field during these storms influence the motion of charged particles in the ionosphere, further affecting TEC distribution and magnitude.
In this paper, we present a case study on the variation of TEC over different regions of the globe during a severe geomagnetic storm that occurred on 23–24 April 2023, thereby contributing to the existing body of knowledge on the topic. Our analysis focuses on variations in the interplanetary magnetic field (IMF-Bz), solar wind parameters, geomagnetic index (SYM-H), global ionospheric maps, and vertical total electron content (VTEC) from 18 GPS-TEC stations across different latitudes. During the storm’s main phase, high-latitude stations detected increased ionospheric density, which then decreased during the recovery phase. Conversely, equatorial and mid-latitude stations showed the opposite trend. Significant dTEC% values, ranging from −80 to 190 TECU, were observed during the storm compared to surrounding days. The TEC intensification, shifting from east to west, was most pronounced in equatorial and low-latitude regions. The structure of the paper is as follows: Section 2 describes the data and methods employed in the study; Section 3 presents the results of our analysis; and Section 4 provides the conclusions drawn from our work.

2. Data Acquisition and Methodology

2.1. Data on Geomagnetic Index and Solar Wind Parameters

In this study, data on the variations in IMF-Bz, solar wind parameters, velocity (Vsw), density (Nsw), temperature (Tsw), proton plasma pressure (Psw) and the geomagnetic index (SYM-H) for the 22–25 April 2023 were obtained from the Omniweb website: https://omniweb.gsfc.nasa.gov/form/dx1.html, accessed on 10 June 2023.

2.2. IGS Data

In this study, GPS-TEC data from 18 stations, which were evenly distributed, with six in equatorial, six in mid-latitude, and six in high-latitude regions (MSVG, ATQK, BAKE, TIDB, DUND, CHTI, CN41, YKRO, BELE, KANZ, HYDE, HKSL, KAZA, BJFS, OTMT, MADR, ZECK, and GODZ) were obtained from the University NAVSTAR Consortium (UNAVCO) which is a U.S. national organization that provides access to archived GPS data: https://www.unavco.org/data/gps-gnss/gps-gnss.html, accessed on 10 June 2023). The GPS-TEC data in the IGS receivers are saved in the zipped receiver independent exchange (RINEX) format and then converted to GPS observable files using appropriate software [31], in this case, GPS Gopi version 2.9.9 software. These GPS measurements are either code pseudoranges (P) or carrier phases (ϕ). The receiver receives the code time delay and carrier phase difference by cross-correlating the f1 and f2 modulated carrier signals, which are normally considered to travel along the same path through the ionosphere [32]. The estimates of the GPS-derived ionospheric TEC are obtained using dual-frequency GPS measurements [33,34,35,36]. GPS receiver data are useful for estimating the electron density along a ray path between a GPS satellite and a ground receiver [37,38]. Dual-frequency GPS receivers also offer integral information on the ionosphere and plasmasphere by computing the differential of the code and carrier-phase measurements and removing ionospheric inaccuracies when estimating the TEC [39,40]. In this case, the GPS-TEC computed by the dual-frequency receivers serves as an input to an ionosphere assimilation model [41]. For the present study, GPS-TEC data were obtained from more than 18 GNSS stations with dual-frequency receivers via pseudorange and carrier-phase measurements. Notably, the TEC calculated from the pseudorange measurement (slant TEC) is given by Equation (2), as follows:
S T E C = 1 40.3 f 1 2 f 2 2 f 1 2 − f 2 2 P 2 − P 1
Similarly, the TEC from carrier-phase measurement may be calculated using Equation (3), as follows:
S T E C = 1 40.3 f 1 2 f 2 2 f 1 2 − f 2 2 φ 2 − φ 1
where f1 and f2 are GPS satellite frequencies determined from the fundamental frequency; fo =10.23 MHz (f1 = 154, fo = 1575.42 MHz, (f2 = 120, fo = 1227.60 MHz); and the differential code and phase measurements are (P2 − P1) and (ϕ1 − ϕ2), respectively [42]. The VTEC is obtained in electrons per square meter using the relation described by Equation (4), as follows:
V T E C = S T E C × Cos χ ′
where the zenith angle χ ′ is given by Equation (5), as follows:
χ ′ = a r c s i n R E   C O S α R E + h s i n ⁡ ( χ )
The VTEC is therefore given by Equation (6), as follows:
V T E C = S T E C cos arcsin R E cos α R E + h sin χ
where α is the satellite’s elevation angle, RE is Earth’s mean radius, and h is the height of the ionospheric layer, which is considered to be 400 km.
Plots of the VTEC against universal time (00:00 to 24:00 UT) over each of the 18 GNSS receiver stations were plotted from 22–25 April 2023 and analyzed.
The positive and negative changes in TEC were also investigated using the percentage of deviation of TEC (dTEC), which was obtained using Equation (7), as follows:
d T E C % = T E C s − T E C q T E C q × 100 %
where T E C s represents the average TEC during geomagnetic-storm days (23 and 24 April 2023) and T E C q represents the average TEC during quiet days (22 and 25 April 2023).
Values of dTEC% against UT (00:000–24:00 UT) over each of the 18 GNSS receiver stations were also plotted from 22–25 April 2023 and analyzed.
The information for the 18 GNSS receiver stations used in this study (MSVG, ATQK, BAKE, TIDB, DUND, CHTI, CN41, YKRO, HYDE, BELE, KANZ, HKSL, KAZA, BJFS, OTMT, MADR, ZECK and GODS) is given in Figure 1.

2.3. Global TEC Map Data

Global TEC maps for 23 and 24 April 2023 were plotted using data from the Ionex database obtained from the NASA site (ftp://cddis.nasa.gov/pub/gps/products/ionex/, accessed on 10 June 2023). The generated hourly global ionospheric response maps were analyzed and discussed. The conversion of geographical latitude and longitude to magnetic latitude and longitude was processed by World Data Center for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/igrf/gggm/, accessed on 10 June 2023).

3. Results and Discussions

3.1. Time-Series Analysis Before, During, and After the Geomagnetic Storm

Figure 2 shows the variations in IMF-Bz, solar wind parameters (Vsw, Nsw, Tsw and Psw), and the geomagnetic index (SYM-H) for the 22–25 April 2024. On 22 April 2023, the IMF-Bz remained constant within a range between ~−15 and 15 nT. The solar wind also remained constant at ~300 km/s, indicating weak magnetic activity. On 23 April 2023, the IMF-Bz remained constant up to 18:00 UT, when it turned slightly southward before turning northward between 19:00 UT and 20:00 UT, at which time it reached a maximum magnitude of ~30 nT. The solar wind speed increased progressively from ~300 km/s to ~900 km/s within that same period, while the dynamic pressure of the solar wind increased to ~13 nPa. On the same day, between 20:00 UT and 24:00 UT, the IMF-Bz began turning southward, indicating the arrival of interplanetary shock in the magnetosphere. There was an increase in solar wind speed, which corresponded with a sharp increase in solar wind dynamic pressure. This signaled the commencement of the storm. According to [43], the sudden commencement of a storm is the result of an increase in solar wind dynamic pressure. The solar wind temperature also increased to 3 million K. An examination of the symmetrical ring current index (SYM-H) during the sudden onset of the storm revealed increased fluctuations. On 24 April 2023, the IMF Bz fluctuated sharply in the southward direction, reaching a value of −30 nT at 02:00 UT. The solar wind speed rose to ~800 km/s, with a corresponding solar wind pressure of ~9 nPa and SYM-H of ~−200 nT. Between 02:00 and 17:00 UT, the IMF-Bz sharply increased to ~22 nT and continued increasing northward. From 17 UT onward, the IMF-Bz fluctuated between ~−15 and 15 nT, indicating the onset of the recovery phase. The solar wind speed and solar wind pressure also decreased drastically during this period.

3.2. Variation in TEC Before, During, and After the Geomagnetic Storm

Figure 3 shows the variation in VTEC against universal time (UT) over MSVG, ATQK, BAKE, TIDB, DUND, CHTI, CN41, YKRO, BELE, KANZ, HYDE, HKSL, KAZA, BJFS, OTMT, MADR, ZECK, and GODS on 22–25 April 2023.

3.2.1. Variation in TEC over MSVG, ATQK, and BAKE

On the 22 April 2023, MSVG, ATQK, and BAKE GNSS receiver stations detected a gradual decrease in TEC from 00:00 UT to 06:00 UT before detecting a gradual rise, with a maximum TEC of 15 TECU at 12:00 UT for MSVG and 20 TECU at 20:00 UT for BAKE. However, ATQK detected increasing TEC values until 23 April 2023, on which day it detected its maximum TEC value of 27 TECU at 02:00 UT. On 23 April 2023, the TEC values at the three GNSS receiver stations decreased dramatically to approximately 3 TECU between 03:00 and 07:00 UT. The TEC values, however, began to rise, with MSVG and ATQK detecting maximum values of 21 and 8 TECU, respectively, at 12:00 UT. ATQK detected a maximum TEC value of 17 TECU at 20:00 UT. The TEC values over MSVG and BAKE decreased up to 24:00 UT. Conversely, the TEC value for ATQK increased and reached a maximum of 20 TECU at 24:00 UT. At the beginning of 24 April 2023, the TEC values at the three GNSS receiver stations detected fluctuations of between 5 and 10 TECU and reached maximum values at 24:00 UT. On 25 April 2023, the TEC values over the three GNSS receiver stations decreased slightly from 00:00 UT to 06:00 UT, reaching minima of 5 TECU at 06:00 UT before they began to rise and attaining maxima of 15 TECU at 15:00 UT over MSVG; 21 TECU at 20:00 UT over BAKE; and 12 TECU at 24:00 UT over ATQK. Notably, during the initial phase of the storm, large fluctuations in the TEC value were recorded over ATQK. MSVG and BAKE, however, detected smaller TEC fluctuations. The TEC fluctuations increased slightly toward the end of the main phase before starting to decrease during the recovery phase.

3.2.2. Variation in TEC over TIDB, DUND, and CHTI

On 22 April 2023, the TEC values of the TIDB, DUND, and CHTI GNSS receiver stations greatly decreased from 00:00 UT to 19:00 UT before increasing steadily, reaching maxima at 04:00 UT on 23 April 2023, with TIDB, DUND, and CHTI detecting values of 38 TECU, 28 TEC, and 27 TECU, respectively. The TEC values over all three receiver stations began dropping at 18:00 UT, attaining minima of 2 TECU. The TEC values over the three GNSS receiver stations began to rise and reached maxima on 24 April 2023. TIDB detected a maximum TEC value of 48 TECU at 06:00 UT; DUND detected a maximum TEC value of 39 TECU at 03:00 UT; and CHTI detected a maximum TEC value of 28 TECU at 03:00 UT. However, the TEC values over the three GNSS receiver stations began to decrease, reaching their minima between 12:00 and 18:00 UT on 24 April 2023. This increase was followed by a steady increase in TEC values for all three GNSS receiver stations, with maximum values detected on 25 April 2023 between 00:00 and 06:00 UT. Notably, during the main phase, the TEC values across the three GNSS receiver stations detected negative storm effects, whereas during the recovery phase, they detected positive storm effects.

3.2.3. Variation in TEC over CN41, YKRO, BELE, KANZ, HYDE, and HKSL

The TEC values over CN41, YKRO, and BELE exhibited similar trends. On 22 March 2023, the TEC values decreased and reached a minimum value of 0 TECU between 06:00 and 09:00 UT. The TEC values then rose steadily and reached a maximum value of approximately 70 TECU between 14:00 and 21:00 UT. The TEC values then decreased and reached a minimum of 0 TECU on 23 April 2023 between 00:00 and 10:00 UT. From 10:00 UT, the TEC values began to rise over YKRO and BELE, reaching a maximum of 70 TECU at 18:00 UT. However, CN41 attained a higher maximum TEC value of 130 TECU at 23:00 UT. On 24 April 2023, the TEC values decreased and reached the lowest value of 0 TECU at 06:00 UT. The TEC values then steadily increased, reaching maxima of 50 TECU for BELE and CN41 at 18:00 UT and 75 TECU for YKRO. The TEC values then decreased to the minimum TEC value of 00:00 UT between 06:00 and 08:00 UT. During the main phase of the storm, the TEC value increased, reached its maximum, and then decreased to a minimum value at the start of the recovery phase.
The TEC values over KANZ, HYDE, and HKSL also exhibited similar trends. On 22 March 2023, the TEC values rose steadily and reached maxima of approximately 80 TECU between 09:00 and 13:00 UT. The TEC values then decreased and reached minima of 0 TECU between 20:00 UT on 23rd April 2023 and 03:00 UT on 24th April 2023. On 24 April 2023, the TEC values at the three GNSS receiver stations increased steadily and reached maxima of 70 to 90 TEC before starting to decrease again. During the main phase, there was a reduction in the TEC values across the three GNSS receiver stations, which was followed by an increase just before the recovery phase; this increase continued until the end of the recovery phase.

3.2.4. Variation in TEC over KAZA, BJFS, OTMT, MADR, ZECK, and GODS

The TEC values over KAZA, BJFS, and OTMT increased on 22 March 2023 and reached maxima of approximately 25 to 35 TECU between 03:00 and 12:00 UT. The TEC values then decreased and reached minima at 24:00 UT. On 23 April 2023 at 06:00 UT, the TEC values began to rise, attaining maxima between 27 and 38 TECU between 06:00 and 12:00 UT. The TEC values then decreased and reached minima at 24:00 UT. On 24 April 2023, the TEC values increased, reaching maxima ranging from 25 to 38 TECU. The TEC values then decreased to their minima between 00:00 and 03:00 UT. During the main phase, there was a reduction in the TEC across the three GNSS receiver stations. This was followed by an increase just before the recovery phase; this increase continued until the end of the recovery phase.
The TEC values over MADR, ZECK, and GODS also exhibited trends similar to those seen over KAZA, BJFS, and OPMT. On 22 March 2023, the TEC values rose steadily and reached a maximum value of approximately 25 TECU between 09:00 and 18:00 UT. The TEC values then decreased and reached a minimum of 0 TECU from 20:00 UT on 22nd April 2023 to 03:00 UT on 23rd April 2023. On 23 April 2023, the TEC values over the three GNSS receiver stations increased steadily and reached maxima of 27 to 39 TEC before starting to decrease again to minima at 24:00 UT. On 24 April 2023, the TEC value increased steadily from 00:00 UT to approximately 12:00 UT, at which time it reached a maximum of approximately 39 TECU. The TEC value then decreased until it reached its minimum value of 0 TECU between 24:00 on 24 April and 09:00 UT on 25 April 2023. Notably, during the main phase of the storm, the TEC reached peak values over MADR and ZECK. However, the value over GODS was at its minimum. The TEC values over MADR and ZECK, however, began to decrease with the commencement of the main phase and then rose steadily during the recovery phase of the storm. Conversely, the TEC values over the GODS began increasing with the commencement of the main phase, started to decrease at 24:00 UT, and then began increasing along with the values above the MADR and ZECK stations.
Generally, the GNSS receiver stations MSVG, ATQK, BAKE, TIBD, DUND, and CHTI detected increases in ionospheric density during the main phase and decreases in ionospheric density during the recovery phase. However, the GNSS receiver stations KAZA, BJFS, OPMT, MADR, ZECK, GODS, CN41, YKRO, BELE, KANZ, HYDE, and HKSL detected decreases in ionospheric density during the main phase [44] and increases in the ionospheric density during the recovery phase. Increases in the ionospheric density and TEC are referred to as positive storm effects, whereas decreases are referred to as negative storm effects. The occurrence and magnitude of positive and negative storm effects largely depend on the local time, the latitude, and the phase of the storm [45,46] and the competing effects of prompt penetration electric fields (PPEF) and disturbance dynamo electric fields (DDEF). Storm-time ionospheric electrodynamics are driven by a combination of DDEF [47] and PPEF [48,49]. DDEF is caused by the dynamic action of storm-time winds resulting from Joule heating, whereas PPEF occurs during the southward turning of the Z-component of the IMF (IMF-Bz) [47,50]. The PPEF affects both high and low latitudes, as well as equatorial latitudes, depending on its orientation (either southward or eastward). The PPEF affects the E × B drift, leading to an increase or decrease in the TEC and uplift of the postsunset F-layer to higher altitudes. Low latitudes can be penetrated by PPEF for several hours during rapid southward turning of the IMF-Bz [51]. On the other hand, the DDEF, which reaches the equator several hours after the beginning of a storm, lasts for a day or even several days during the recovery phase [52]. In equatorial and low-latitude regions, during geomagnetic storms (when IMF-Bz is southward), the PPEF is eastward during local daytime, leading to enhancement of the plasma fountain [53,54]. Similarly, the westward orientation of the PPEF reduces the number of plasma fountains [5], leading to a reduction in ionospheric plasma density around the equatorial ionization anomaly (EIA) crest [55].

3.3. Variation in Percentage Deviation in TEC (dTEC%) Before, During, and After the Geomagnetic Storm

Figure 4 shows that MSVG, ATQK, BAKE, TIDB, DUND, CHTI, CN41, YKRO, BELE, KANZ, HYDE, HKSL, KAZA, BJFS, OTMT, MADR, ZECK, and GODS all detected both positive and negative changes in the TEC between the 22 and 25 of April 2023. On 22 April 2023, most of the GNSS stations detected positive TEC changes of up to 20%. On 23 April 2023, there were positive TEC changes between 00:00 and 12:00 UT over MSVG (up to 50%), ATQK (up to 100%), BAKE (up to 50%), TIDB (up to 180%), BJFS (up to 120%), QTMT (up to 110%), GODS (up to 50%), and YKRO (up to 40%) and negative TEC changes between 12:00 and 24:00 UT over MSVG (up to −30%), ATQK (up to −50%), BAKE (up to −60%), TIDB (up to −20%), BJFS (up to −20%), QTMT (up to −30%), GODS (up to −50%), DUND (up to −40%), and YKRO (up to −90%). However, DUND detected a negative TEC change of up to −50%, whereas MADR detected a positive TEC change of up to 90% between 00:00 and 24:00 UT. On 24 April 2023, a negative TEC change of up to 50% occurred between 00:00 and 24:00 UT over MSVG. However, ATQK, BAKE, ZECK, HYDE, BELE, and HKSL presented positive TEC changes of up to 190% between 00:00 and 12:00 UT and negative TEC changes of up to −80% between 12:00 and 24:00 UT. On 25 April 2023, both positive and negative TEC changes (20% and −10%, respectively) were detected.
As shown in Figure 4, a large dTEC% ranging between −80 and 190 TECU was detected a few hours before the storm and during the storm period (23 and 24 April 2023) compared with values from the day before the storm (22 April 2023) and the day after the storm (25 April 2023), which ranged between −10 and 20 TECU. However, larger dTEC% values were observed at stations situated in the southern, northern, and middle latitudes than at stations in the equatorial region during the study period. Studies by [56,57] have shown that positive or negative TEC changes, either in the prestorm period or in the late recovery period, imply that changes in the geomagnetic field are not directly connected with ionospheric changes. However, they attributed the changes to the local effects brought from the polar regions by neutral winds, which spread toward the equator during geomagnetic activity, and to the direct entry of plasma into low and middle latitudes.

3.4. Global Ionospheric Maps Showing the Storm

Figure 5, Figure 6 and Figure 7 show ionospheric maps as a function of geographic longitude and latitude over a global map; these maps were generated using 18 GNSS stations (MSVG, ATQK, BAKE, TIDB, DUND, CHTI, KAZA, BJFS, OPMT, MADR, ZECK, GODS, CN41, YKRO, BELE, KANZ, HYDE, and HKSL) on 23 and 24 April 2023. The global contour plot for each region shows alterations in TEC, with a time resolution of 2 h between 00:00 UT and 24:00 UT. As shown in Figure 5, on 23 April 2023 at 08:00 UT, the TEC marginally intensified over the Asian equatorial regions. At 10:00 UT, the TEC value further increased over the African equatorial region. At 12:00 UT, during the commencement of the storm, the TEC values increased toward the South American equatorial region. At 14:00 UT, the intensity of the TEC increased over the South American and African equatorial regions, whereas a mild increase in TEC occurred over the Asian equatorial region experiences. At 16:00 UT, the TEC values increased further in the South American and African equatorial regions, whereas the TEC values over the Asian equatorial region decreased drastically. Over time, the high TEC values appeared to shift westward such that by 22:00 UT, the high TEC values had spread further toward the Pacific Ocean. The TEC values revealed that strong activity began over the South American and African equatorial regions, with mild increases over the Asian equatorial region. This was reflected by the values of the TEC over South American and African equatorial regions from 16:00 to 22:00 UT, which ranged between 80 and 100 TECU, whereas those over the Asian equatorial region ranged between 60 and 80 TECU. The middle-latitude and higher-latitude regions presented TEC values below 40 TECU. Notably, as time progressed from 08:00 to 22:00 UT, the difference in the TEC values between the storm period and the quiet period increased, as shown by the red contours on the global TEC map.
In Figure 6 and Figure 7, on 24 April 2023, at 00:00 UT, the TEC values shifted farther past the South American equatorial region toward the Pacific Ocean. As time progressed, the TEC intensification, shown by the yellow contour on the global map, appeared to shift westward toward the Asian equatorial region. Between 04:00 and 06:00 UT, there was an abrupt increase in the TEC over the Asian equatorial region. In our study, this was the time when the recovery phase of the storm began. The TEC values continued to shift westward, reaching the African equatorial region between 08:00 and 10:00 UT. At 12:00 UT, the TEC values intensified toward the South American equatorial region. Between 18:00 and 22:00, the intense TEC values shifted to the South American equatorial region. These results are consistent with the results obtained by [27]) on the TEC response to intense geomagnetic storms of solar cycle 24 over equatorial and mid-latitude stations. Moreover, the TEC differences between the storm period and the quiet period tended to decrease with time. A close analysis of the ionospheric maps in Figure 5, Figure 6 and Figure 7 revealed that, compared with the high-latitude regions, the equatorial and low-latitude regions presented the largest spatial and temporal variations in TEC. Studies by [51,58,59] attributed the increased TEC values to the intense disruption caused by global winds moving toward the equator from higher latitudes and electric fields resulting from magnetosphere–ionosphere interactions during storms.
Ref. [60] attributed the large TEC values in equatorial and low-latitude regions to electrodynamic changes that develop rapidly during storm periods in response to high-latitude forcings as a result of the interaction between zonal winds and meridional winds.
To summarize, the geomagnetic storm of 23–24 April 2023, which followed a moderate solar flare (M1.7) and a CME-induced G4-level disturbance, resulted in substantial ionospheric variability across different latitudinal regions, as revealed by the VTEC and dTEC% analysis in our study. Our findings are consistent with the broader literature on geomagnetic storm effects but also contribute novel observations regarding the storm-time dynamics of TEC, particularly in equatorial and high-latitude sectors. Several previous studies, including those by [4,6], established that geomagnetic storms significantly disturb ionospheric density, especially in the F-region, resulting in fluctuations in TEC. Our results affirm this, showing intense TEC depletions in equatorial and mid-latitude regions during the main phase and increases during the recovery phase. This behavior is indicative of negative and positive ionospheric storms, which are commonly linked to electrodynamic disturbances such as prompt penetration electric fields (PPEF) and prompt penetration electric fields (DDEF), as also reported by [24]. Additionally, the large dTEC% values (ranging from –80% to +190%) observed during the storm period in our analysis surpass typical quiet-day fluctuations (–10% to 20%), emphasizing the storm’s severity. Similar ranges of TEC disturbances were noted by Mishra et al. (2020) [27] during the 2015 superstorms, who also pointed to electric field-induced restructuring of ionospheric plasma, particularly over equatorial and low-latitude regions.
Our observation of spatial east-to-west progression of TEC changes aligns well with the findings of [23], who reported enhanced EIA crests and equatorial plasma bubbles (EPBs) during the same April 2023 storm. These phenomena were attributed to intensified transequatorial winds and electromagnetic forcing, which likely explain the high spatial and temporal TEC variations we observed over equatorial and low-latitude stations. High-latitude responses in our study, characterized by increases in ionospheric density during the main phase followed by decreases in the recovery phase, agree with observations by [22], who reported intense variability in the geoelectric field during substorms in the same event. These are typically associated with increased particle precipitation and auroral activity, consistent with the patterns we noted at polar GNSS stations. Our use of global ionospheric maps, revealing intensified TEC shifts and localized disturbances, supports the multi-domain, multi-instrument perspective advocated by [25]. Their emphasis on integrated observations is validated by our results, which underscore that local TEC responses are highly variable and depend on both geophysical location and local time of storm onset. Moreover, the asymmetry in TEC variations across latitudes, as well as the higher sensitivity of mid- and high-latitude stations to storm-time dTEC%, reinforce earlier conclusions by [7,26], who stressed the importance of the input of solar wind energy and thermospheric composition (e.g., changes in the O/N2 ratio) in shaping TEC responses.

4. Conclusions

In this study, we analyzed the variations in total electron content (TEC) during the geomagnetic storm of 23–24 April 2023, using vertical TEC (VTEC) data derived from 18 GPS stations evenly distributed across equatorial, mid-latitude, and high-latitude regions. Our results reveal that GNSS stations at high latitudes detected increases in ionospheric density during the storm’s main phase, followed by decreases during the recovery phase. In contrast, stations located in equatorial and mid-latitude regions experienced decreases in ionospheric density during the main phase and subsequent increases during the recovery phase.
Significant relative changes in TEC (dTEC%) were observed during the storm period, ranging from −80% to +190%, in stark contrast to the more stable values (−10% to +20%) recorded on quiet days (22 and 25 April 2023). These variations were especially pronounced at mid- and high-latitude stations, which recorded higher dTEC% values compared to equatorial stations. As the storm evolved, TEC changes observed in global ionospheric maps exhibited a westward progression, with changes initially affecting the South American equatorial region, then shifting toward the Asian sector. During the recovery phase, the Asian equatorial region showed marked TEC increases, which then propagated westward toward Africa and intensified over the South American equatorial sector. A detailed examination of ionospheric maps confirmed that equatorial and low-latitude regions experienced greater spatial and temporal variability in TEC compared to higher-latitude regions. Overall, this study highlights the complex latitudinal dependence and dynamic behavior of ionospheric TEC in response to geomagnetic storm activity, emphasizing the importance of spatially distributed GNSS observations for space weather monitoring and forecasting.

Author Contributions

Conceptualization, A.M.T., E.U. and Y.G.E.; Methodology, E.U.; Software, A.M.T. and Y.G.E.; Formal analysis, A.M.T.; Investigation, A.M.T., E.U. and Y.G.E.; Data curation, A.M.T. and Y.G.E.; Writing—original draft, A.M.T.; Writing—review & editing, A.M.T., E.U. and Y.G.E.; Visualization, A.M.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors thank the University of NAVSTAR Consortium: https://www.unavco.org/data/gps-gnss/gps-gnss.html, accessed on 10 June 2023 for the GNSS data, Omniweb for the solar-wind parameters: https://omniweb.gsfc.nasa.gov, accessed on 10 June 2023.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Geographic locations of the 18 GNSS receiver stations. The blue line represents the geomagnetic equator.
Figure 1. Geographic locations of the 18 GNSS receiver stations. The blue line represents the geomagnetic equator.
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Figure 2. The time-series plot, displaying OMNI parameters for 22–25 April 2023, effectively visualizes the progression of solar wind conditions and their impact on Earth’s magnetosphere. Panel (a) illustrates the interplanetary magnetic field’s z-component, BzGSM, revealing critical southward deflections during April 23–24 that are instrumental in instigating geomagnetic activity. Simultaneously, Panel (b) shows a significant surge in both solar wind speed (VSW) and dynamic pressure (PSW) peaking around April 24, indicating the arrival of a faster and more energetic solar wind stream. Complementing these observations, Panel (c) displays the solar wind proton density (NSW) and Panel (d) the solar wind proton temperature (TSW), both exhibiting sharp increases that align with the heightened speed and pressure, which are characteristic signatures of disturbed solar wind. Finally, Panel (e) highlights the SYM-H index, a crucial measure of geomagnetic storm intensity, which undergoes a dramatic negative excursion following these solar wind disturbances, unequivocally signifying the occurrence of a major geomagnetic storm.
Figure 2. The time-series plot, displaying OMNI parameters for 22–25 April 2023, effectively visualizes the progression of solar wind conditions and their impact on Earth’s magnetosphere. Panel (a) illustrates the interplanetary magnetic field’s z-component, BzGSM, revealing critical southward deflections during April 23–24 that are instrumental in instigating geomagnetic activity. Simultaneously, Panel (b) shows a significant surge in both solar wind speed (VSW) and dynamic pressure (PSW) peaking around April 24, indicating the arrival of a faster and more energetic solar wind stream. Complementing these observations, Panel (c) displays the solar wind proton density (NSW) and Panel (d) the solar wind proton temperature (TSW), both exhibiting sharp increases that align with the heightened speed and pressure, which are characteristic signatures of disturbed solar wind. Finally, Panel (e) highlights the SYM-H index, a crucial measure of geomagnetic storm intensity, which undergoes a dramatic negative excursion following these solar wind disturbances, unequivocally signifying the occurrence of a major geomagnetic storm.
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Figure 3. TEC variations at different stations on 22–25 April 2023.
Figure 3. TEC variations at different stations on 22–25 April 2023.
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Figure 4. Plots of dTEC comparison for 22–25 April 2023 over MSVG, ATQK, BAKE, TIDB, DUND, CHTI, CN41, YKRO, HYDE, BELE, KANZ, HKSL, KAZA, BJFS, OTMT, MADR, ZECK, and GODS.
Figure 4. Plots of dTEC comparison for 22–25 April 2023 over MSVG, ATQK, BAKE, TIDB, DUND, CHTI, CN41, YKRO, HYDE, BELE, KANZ, HKSL, KAZA, BJFS, OTMT, MADR, ZECK, and GODS.
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Figure 5. TEC during the storm day (left panel) and on a quiet day (middle panel) and the difference (right panel) between 08:00 UT and 22:00 UT during geomagnetic storm of 23 April 2023.
Figure 5. TEC during the storm day (left panel) and on a quiet day (middle panel) and the difference (right panel) between 08:00 UT and 22:00 UT during geomagnetic storm of 23 April 2023.
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Figure 6. TEC during a storm day (left panel) and on a quiet day (middle panel) and the difference (right panel) between 00:00 UT and 14:00 UT during the geomagnetic storm on 24 April 2023.
Figure 6. TEC during a storm day (left panel) and on a quiet day (middle panel) and the difference (right panel) between 00:00 UT and 14:00 UT during the geomagnetic storm on 24 April 2023.
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Figure 7. TEC during the storm day (left panel) and on a quiet day (middle panel) and the difference (right panel) between 16:00 UT and 22:00 UT during the geomagnetic storm of 24 April 2023.
Figure 7. TEC during the storm day (left panel) and on a quiet day (middle panel) and the difference (right panel) between 16:00 UT and 22:00 UT during the geomagnetic storm of 24 April 2023.
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Tilahun, A.M.; Uluma, E.; Ejigu, Y.G. Variation in Total Electron Content During a Severe Geomagnetic Storm, 23–24 April 2023. Atmosphere 2025, 16, 676. https://doi.org/10.3390/atmos16060676

AMA Style

Tilahun AM, Uluma E, Ejigu YG. Variation in Total Electron Content During a Severe Geomagnetic Storm, 23–24 April 2023. Atmosphere. 2025; 16(6):676. https://doi.org/10.3390/atmos16060676

Chicago/Turabian Style

Tilahun, Atirsaw Muluye, Edward Uluma, and Yohannes Getachew Ejigu. 2025. "Variation in Total Electron Content During a Severe Geomagnetic Storm, 23–24 April 2023" Atmosphere 16, no. 6: 676. https://doi.org/10.3390/atmos16060676

APA Style

Tilahun, A. M., Uluma, E., & Ejigu, Y. G. (2025). Variation in Total Electron Content During a Severe Geomagnetic Storm, 23–24 April 2023. Atmosphere, 16(6), 676. https://doi.org/10.3390/atmos16060676

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