Global Ionospheric Response During Extreme Geomagnetic Storm in May 2024

Abstract

The main idea of the present study is to investigate in detail the time evolution of the spatial inhomogeneities connected with the ionospheric response to the geomagnetic storm registered in the period of 10–11 May 2024. The obtained ionospheric anomalies represented by the relative deviations of the global Total Electron Content (TEC) data have been utilized in the analysis. The used global TEC data have been converted to a coordinate system with a modip latitude and geographical longitude. In addition to the maps illustrating the global spatial distribution of the geomagnetically forced ionospheric anomalies, a presentation of the observed longitudinal structures by sinusoidal approximation has also been used. The resulting positive and negative responses have been studied depending on the magnetic latitude, local times and the behavior of the geomagnetic activity parameters during the considered event. The interpretation takes into account the known mechanisms for the effect of the geomagnetic storm on the electron density. A special attention is focused on the differences in the two hemispheres at high and mid latitudes, where a simultaneous direct impact of the particle precipitation and the change in the temperature regime of the neutral atmosphere has been assumed. The low-latitude response as a result of the Equatorial Ionization Anomaly (EIA) associated with Disturbed Dynamo Electric Fields (DDEFs) and its relationship with local time has also been considered.

1. Introduction

The phenomena known to scientists as geomagnetic storms, which are the result of enhanced solar wind–magnetosphere interaction, have a direct influence on the ionospheric electron density [1,2,3].

The main focus of the present study is on the characteristics of Global Scale Traveling Ionospheric Disturbances (GSTIDs) at high, mid and low latitudes. This type of disturbance is different from the known Large-Scale Traveling Ionospheric Disturbances (LSTIDs), which are explained by the presence of gravity waves [4,5,6]. The formation of this GSTID is situated at the auroral or subauroral region in the conditions of geomagnetic storms as a result of Joule heating and Lorentz forces triggered by the enhancement of the auroral electrojet and particle precipitation [7]. The explanation of these effects is due to changes in the solar wind, which affect the electron density by entering the auroral ovals.

It is well known that the auroral particle precipitation during geomagnetic storms causes ionization of the neutral air in the absence of sunlight (due to the fact that the particle precipitation occurs in the night sector of the Earth) and therefore an increase in electron density is observed [8,9]. The additional Joule heating along with the effect of particle precipitation causes an increase in temperature in the polar oval and a change in the O/N₂ ratio and therefore an increase in recombination. In auroral ovals, these two processes occur simultaneously and produce opposite Total Electron Content (TEC) responses, strongly dependent on geographical longitude and local time. The regions of the two auroral ovals form longitudinal structures of the ionospheric response during geomagnetic storms. These structures have the property of traveling zonally along with the displacement of the midnight region where particle precipitation occurs.

There is no direct effect of particle precipitation at mid and low latitudes, but the disturbed dynamo effect causes variations in the electron density that are no less significant than those in the auroral regions. The disturbed dynamo theory is presented by an appreciable number of studies. Its meaning is that the energy input to the thermosphere during geomagnetic disturbances leads to a change in the global thermospheric circulation and, consequently, to the generation of electric fields and currents at mid and low latitudes by ionospheric wind dynamo action [10,11,12,13]. The heating of the night sector of the polar oval causes the occurrence of a disturbed wind system, including a meridional equatorward air flow. This flow is deflected by Coriolis acceleration and induces additional current systems that interact with the Earth’s permanent magnetic field and create electric fields, which in turn induce ionospheric plasma drift. Created physical models [13] and measurement data [14,15] show the presence of geomagnetic disturbances induced by vertical drift of plasma related to local time. The connection of the disturbed dynamo with the particle precipitation zone in the auroral regions results in the movement of the ionospheric response at low and mid latitudes being synchronous with the midnight region.

Another interesting phenomenon occurring in the low- and mid-latitude ionosphere is the so-called equatorial ionization anomaly (EIA), which is formed as a result of the eastward electric field along the magnetic equator during the day [16,17,18]. The interaction of the magnetic field (B) and the electric field (E) generates an E × B drift that pulls the ionospheric plasma upward at the magnetic equator (the so-called Fountain effect), which then moves down along the magnetic field lines forming maxima at latitudes around ±20° [19,20,21]. The observed anomalies in EIA are explained by direct penetration of the magnetospheric electric fields and the thermospheric disturbances [22].

In their studies, Klimenko, M. V. and Klimenko, V. V (2012 [23]) and Karan et al. (2024 [24]) specifically consider the expansion of the EIA crest during geomagnetic storms, which is due to the disturbed electrodynamic conditions of the global ionosphere–thermosphere–magnetosphere system. The obtained results indicate the presence of the Fountain Effect and redistribution of the ionospheric plasma.

The present work investigates the global distribution of the ionospheric TEC response during the G5 Extreme Geomagnetic Storm that occurred on 10–11 May 2024. An attempt has been made to determine the amplitude and phase characteristics of the Traveling Ionospheric Disturbances (TIDs) at high, mid and low latitudes. The considered amplitudes and phases are presented as a zonal distribution with a close to sinusoidal shape of traveling waves. Special attention is paid to the time delay effects observed at low and mid latitudes, which are obtained in other investigations as well [25].

2. Data and Methods

Several types of data have been used to realize the main idea of the present study. This allows for tracing out both the solar and geomagnetic conditions and their related effects on the electron density.

2.1. Different Data Types

2.1.1. Indices Describing the Manifestation of Selected Geomagnetic Storms

For the consideration of the solar parameters, which include the Bz component of (IMF), solar wind speed, and density, data obtained from the world base of the Goddard Space Flight Center (available at https://omniweb.gsfc.nasa.gov/, accessed on 20 October 2024) were used. The first of the considered parameters, namely the Bz component of the IMF, is widely used for estimating geomagnetic disturbances. It is well known that geomagnetic storms, which are disturbances in the Earth’s magnetic field, occur as a result of a more significant transfer of energy from the solar wind to the Earth’s magnetosphere in cases of southern orientation of Bz (Bz < 0) [26,27,28]. In addition to the Bz component of IMF, according to some authors, the observed variations in solar wind density and speed could have a significant influence on solar wind–magnetosphere coupling [29,30].

The manifestation of geomagnetic conditions is described by the Dst index and planetary Kp index, also obtained from the Goddard Space Flight Center. These two parameters are also used to describe disturbances of the Earth’s magnetic field from high-energy fluxes from the Sun, known as geomagnetic storms [31]. The Dst index, which is derived from a network of near-equatorial geomagnetic observatories, shows the effect of the globally symmetrical westward-flowing high-altitude equatorial ring current, which causes the “main phase” depression worldwide in the H component of the Earth′s magnetic field during large geomagnetic storms [32]. The Kp index is used to characterize the magnitude of geomagnetic storms [33]. This index is calculated by taking the average value of local K indices at 13 ground magnetic field observatories. According to the accepted classifications, the range of the three-hour Kp index is from 0 to 9.

As an additional index providing information on an estimate of the energy entering the Earth’s polar regions, the so called Power Index obtained from the NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION (NOAA), Space Weather Prediction Center, at the following link is used: http://services.swpc.noaa.gov/text/aurora-nowcast-hemi-power.txt, accessed on 20 October 2024. The advantage of these types of data is that the discretization of the data is very small (time interval 5 min) and allows us to determine the exact time and initial phase of the geomagnetic storm that occurred on 10 May 2024.

2.1.2. Ionospheric Data

• Global TEC maps

The global ionospheric TEC response data with a time resolution of 1 h and a grid spacing of 5° × 2.5° in longitude and latitude, respectively, are used: https://www.izmiran.ru/ionosphere/weather/grif/Maps/TEC/, accessed on 10 October 2024. These types of TEC data are interpolated from two types of JPL data: (i) for the period 1994–1999, 2 h UT (00–23) JPL maps are linearly interpolated to 1 h UT maps; (ii) after 2000 to present, JPLR hourly maps are provided for 00:30, 01:30, …, 23:30. Additional sources of daily JPLR maps are available at https://sideshow.jpl.nasa.gov/pub/iono_daily/IONEX_rapid, accessed on 20 October 2024.

It is worth noting that the grid used in this paper is not appropriate for investigating scintillations; the latter need smaller grid spacing, ~1°, and a temporal resolution of about 5 min. This is important to highlight because of the frequent occurrence of scintillations during geomagnetic storms such as (i) plasma bubbles in the evening and nighttime equatorial and low mid latitude ionosphere, which drive scintillations, or (ii) scintillations at high latitudes, usually observed during intense geomagnetic storms and produced by patchy structures and tongue of ionization (TOI) ionosonde data for selected points.

In order to trace out the individual anomalies at the selected points in detail, a comparison of the ionospheric response is presented between the TEC data and the data from the vertical sounding of the ionosphere, obtained from the GLOBAL IONOSPHERE RADIO OBSERVATORY (GIRO): https://giro.uml.edu/index.html, accessed on 20 October 2024. The Digital Ionogram Data Base (DIDBase) by specification of station code, time interval and quantity, obtained by the ionospheric stations, which provide data from interpreted ionograms, is available at the indicated link. In a specific case, data on the critical frequency of the F2 region (foF2) and the height of the F2-region maximum (hmF2) from the ionospheric station Cachoeira Paulista (Station Code CAJ2M) with coordinates (22.7°S, 45.0°W, modip latitude 16.95°S) were used. These data were used for an additional analysis and comparison of the type of ionospheric response at the selected point with the obtained global spatial anomalies based on satellite data.

• Data for O/N₂

Data on the ratio of concentrations of atomic oxygen (O) and molecular nitrogen (N₂) in the thermosphere were used from the Global Ultraviolet Imager (GUVI) instrument, working on the TIMED spacecraft (http://guvitimed.jhuapl.edu/data_products, accessed on 10 October 2024).

2.2. Methods

• Modified dip (modip) latitude of ionospheric data

The introduction of this modification was introduced by Rawer in order to improve the accuracy of the representation of the ionospheric response at low and mid latitudes [34]. The study of the variability of F2 parameters depending on modip shows that the errors obtained with this methodology increase from mid latitude toward the auroral zone [35].

• Relative deviations of the ionosphere

The relative deviation of the ionospheric parameters (TEC, foF2 and hmF2) was introduced to filter out the diurnal and seasonal behavior as well as the long-lasting variations in solar activity, which strongly affect the ionosphere [36]. The following formula was used:

$$rValue\left(t\right)={\displaystyle \frac{{Value\left(t\right)-Value}_{med}\left(UT\right)}{{Value}_{med}\left(UT\right)}}$$

(1)

where Value(t) denotes the value at the moment t, while Value_med(UT) is the median value of the same quantity obtained from the values for the universal time (UT) coinciding with the universal time at moment t during a 27-day period centered on the current day.

• Method of stationary amplitudes and phases

In the present investigation, components of Stationary Planetary Wave 1 (SPW1) have been utilized. The advantage of this method is that the obtained amplitudes provide information about the maximum positive deviation from the zonal mean. In this decomposition, the phase describes the longitude where the positive maximum is registered. The phase at a given hour in universal time allows the local time at which the response is maximal to be calculated. The presentation of the amplitudes and phases over time makes it possible to follow the variability of longitudinal distribution of the ionospheric response in latitude hour-by-hour [37].

3. Results

This section presents in detail the results which illustrate the spatial distribution of the ionospheric response under the conditions of the extreme geomagnetic storm on 10 May 2024. In its first part, this section reveals the solar and geomagnetic conditions that influence the electron density. The found spatial structures of the TEC anomalies allow us to propose the physical mechanisms by which the solar processes affect the Earth’s ionosphere. Additionally, a detailed analysis of the latitudinal structure of the ionospheric response is included, which shows the hemispheric asymmetry of the electron density behavior as a result of the geomagnetic storm. The SPW1 amplitudes and phases are also presented in order to examine the place and time of maximum manifestation of the effects of the storm on the ionosphere.

3.1. Solar and Geomagnetic Parameters

3.1.1. Solar Wind Indices

Figure 1 presents the variability of solar wind parameters that include the Bz component of the IMF, solar wind speed, and density for the period from 12:00 UT on 10 May 2024 to 12:00 UT on 12 May 2024, with a time resolution of four hours. The behavior of the Bz component of the IMF is shown in Figure 1a. It is well known that geomagnetic storms are produced as a result of the significant transfer of energy from the solar wind to the Earth’s magnetosphere, provided that the Bz component has a southern orientation (Bz < 0). From the graph, it can be seen that such behavior is present during almost the entire selected period of two days. According to Figure 1a, the first significant decrease in the Bz component was recorded around 17–18 UT on 10 May 2024 when Bz was around −8 nT. This is followed by a gradual increase in values until around 19:30 UT, when the quantity shows a sharp decrease and its values reach their first minimum (Bz component is about −20 nT) at around 21 UT on 10 May. The negative values of the parameter persist, showing a second significant minimum around 01 UT on 11 May when the Bz component is about −23 nT. It can be seen from the figure that several more minima follow, namely the following: around 6 UT, when the Bz component was about −21 nT, and in the hours between 14 and 15 UT, the last more significant minimum of the Bz component, which was about −13 nT, was registered. The above observations reveal essential moments in which the interaction of the interplanetary magnetic field and the Earth’s magnetosphere occurred. It is worth noting that the Bz component is negative most of the time in the selected period.

The middle and bottom panels of Figure 1 show the speed and density of the solar wind. From Figure 1b, it can be seen that after 16 UT on 10 May 2024, the speed of the solar wind starts to increase, and from about 400 km/s, it reaches a speed of over 730 km/s (midnight on 11 May 2024). The graph shows a continuous increase in the solar wind speed, which reaches over 930 km/s (maximum value) in the early hours of 12 May 2024 (00–02 UT).

The solar wind density clearly shows a sudden increase in the hours after 14 UT on 10 May 2024 (see Figure 1c). The first maximum of the quantity was registered around 20 UT, when solar wind density had values about 55 cm⁻¹. A slight drop in density follows, lasting until the early hours of 11 May 2024 (around 00 UT). Three more peaks weaker than the first one were registered in the behavior of the parameter obtained at (i) 02 UT on 11 May 2024 (density about 43 cm⁻¹), (ii) 07 UT on 11 May 2024 (density is about 37 cm⁻¹), and (iii) 11 UT on 11 May 2024 (density is about 38 cm⁻¹). In the hours after noon (12 UT) on 11 May, stabilization in the behavior of the quantity and values close to quiet conditions was observed.

Examining these two parameters together with the Bz component allows us to supplement the analysis and obtain a more detailed view of the overall picture of the state of the solar wind, which has a significant impact on the electron density.

3.1.2. Geomagnetic Indices

The second type of quantities that are included in the analysis are the geomagnetic parameters of the Power Index, Kp index and Dst index, presented in Figure 2, which are essential in tracking the variability of geomagnetic activity and the related solar–terrestrial interactions [38].

Figure 2a shows the changes in the above-mentioned Power Index, which is a characteristic of the incoming energy in the polar regions. It can be seen from the figure that around 17 UT on 10 May, the Power Index started to increase sharply and reached its first peak, with a value above 650 GW at around 22 UT on 10 May 2024. The initial moment of the registered increase in the Power Index (marked with a green square) coincides in time with the obtained significant variations in two of the considered solar parameters, namely the Bz component of the IMF and solar wind speed, shown, respectively, in Figure 1a,b. It can also be seen from Figure 2a that after this moment there is a slight decrease and a repeated and more significant peak in the index around 720 GW in the hours between 02 and 03 UT on 11 May 2024. Again, there was a decrease followed by two more increases in the Power Index in the hours between 09 and 10 UT (Power Index is about 625 GW) and at 16 UT (Power Index is about 650 GW) on 11 May 2024.

Figure 2b shows the variability of the geomagnetic activity, represented by the three-hourly planetary Kp index, which can be used to characterize the magnitude of geomagnetic storms. The parameter has values between 0 and 9, and NOAA presents a classification of the Kp index, which describes the significance of effects of a geomagnetic storm to the public and those affected by the space environment [39]. According to the mentioned classification, geomagnetic storms can be referred to as being in any of the following categories: (i) Kp < 4 (quiet conditions); (ii) 4 ≤ Kp < 5 (active); (iii) Kp = 5 (minor G1); (iv) Kp = 6 (moderate G2); (v) Kp = 7 (strong G3); Kp = 8 (severe G4); Kp = 9 (extreme G5).

From Figure 2b, it can be seen that for most of the time during the examined time interval of two days, the Kp index values are marked in red color, which indicates that there is a geomagnetic storm during this whole time. In the hours after 14 UT on 10 May, the values of the Kp index increase sharply and reach values around 8. In the selected period, the Kp index has maximum values (Kp = 9) in the early hours of May 11 and before noon on the same day. High values of the parameter were registered until the early hours of May 12 (when the Kp index decreased to just around 6), after which a slight increase followed for several hours to about 7. In the early hours of May 12, the Kp index values were below 5, which also indicated the end of the storm.

Figure 2c shows the behavior of the Dst index for the selected period. The graph shows that at around 17 UT on 10 May, there is a slight rise (Dst is around 70 nT), after which a sharp decrease in the index is observed. A similar reaction to enhanced Dst that follows the storm sudden commencement (SSC) is defined as the storm initial phase and has a duration of a few hours [40,41,42]. The time interval when the Dst index decreases is called the main phase of the storm, which is a consequence of the interaction of the southward interplanetary field and the Earth’s magnetosphere. In the considered event illustrated in Figure 2c, the Dst index begins to decrease in the hours after 18 UT on 10 May, reaching a minimum of Dst around −410 nT in the early hours on 11 May, which, according to the accepted classification, is of the great storm type (Dst < −350 nT) [43]. After this moment, the recovery phase of the storm begins, in which the trapped particles in the ring current area start to dissipate through several mechanisms (such as wave–particle interactions, Coulomb scattering, Joule heating) and the Dst index slowly returns to quiet conditions [44].

According to the behavior of the considered solar and geomagnetic parameters, it can be summarized that the beginning of the geomagnetic storm occurs at around 17 UT on 10 May 2024 and has a maximal manifestation in the hours shortly after midnight on 11 May 2024.

3.2. Spatial Distribution of Relative TEC Response

To analyze the global manifestation of the ionospheric response, spatial rTEC maps are presented for the period 12 UT on 10 May 2024 to 10 UT on 12 May 2024 with a time resolution of 2 h.

Figure 3 demonstrates the spatial distribution of rTEC for the period from 12 UT on 10 May 2024 to 02 UT on 11 May 2024, which includes a quiet period, as well as the onset of the geomagnetic storm at 17 UT on 10 May and the hours after the event began. The figure shows that at around 18 UT, there is an increase in rTEC in the Southern Hemisphere at high latitudes and longitudes between 30° and 60°. This increase can be explained by particle precipitation in the auroral region and additional direct ionization, which leads to an additional electron density that is added to the existing one [45]. At 20 UT on 10 May, the positive response at high latitudes (around 60°S) in the Southern Hemisphere (winter conditions) and the corresponding negative response in the Northern Hemisphere (summer conditions) are clearly visible. In the Northern Hemisphere, at the same time, at latitudes around 60°N, a weakly positive response is obtained. The next panel showing the ionospheric response at 22 UT on 10 May illustrates the splitting of the positive TEC response in the Southern Hemisphere and the resulting region with two maxima at latitudes between 30°S and 60°S. Between the hours of 22 UT and 00 UT on 11 May, the positive response in the Southern Hemisphere, with its double maximum manifestation, is the most significant. For the analogous latitudes in the Northern Hemisphere at around 22 UT, a slightly positive response is also observed, which has a less pronounced double maximum character. But at 00 UT on 11 May, due to the difference from the Southern Hemisphere, the TEC response at latitudes between 30°N and 60°N shows its seasonal difference through negative values. A peculiar Northern Hemisphere response is a weak positive region at latitudes between 20°N and 50°N, which coincides with the maximum positive response in the Southern Hemisphere and has synchronous movement of the two maxima in both hemispheres.

Figure 4 and Figure 5 show the behavior of the global distribution of ionospheric TEC until the end of the considered geomagnetic storm. It can be seen from Figure 4 that the two positive maxima in the Southern Hemisphere begin to slowly decrease and a larger area is covered by the negative response. The analogous positive response in the Northern Hemisphere continues to move synchronously with the maximum in the Southern Hemisphere. An interesting effect in the polar oval of the Southern Hemisphere is observed around 14 UT when the most significant negative values are obtained (see Figure 4). This effect is followed by a positive anomaly around 18 UT on 11 May in the polar region of the Southern Hemisphere. This could be a result of the interaction between the solar wind and the Earth’s magnetosphere seen by a decrease in the Bz component of the IMF (see Figure 1a).

Figure 5 demonstrates the gradual transition of the spatial distribution of rTEC to quiet conditions. The figure shows that after 00 UT on 12 May, the two positive formations at latitudes 30°N–50°N and 30°S–50°S completely decrease and disappear, which coincides in time with the normalization of solar and geomagnetic parameters.

3.3. Latitudinal Structure

In order to analyze in detail the ionospheric response to the geomagnetic storm on 10 May 2024, the latitudinal behavior of rTEC is presented.

Figure 6a demonstrates the latitudinal distribution of zonal mean rTEC from 12 UT on 10 May 2024 to 12 UT on 12 May 2024. This parameter characterizes the predominant response at corresponding modip latitudes and its evolution during the event. In the Northern Hemisphere, at the beginning of the storm, a negative response of the summer season is manifested, which spreads to 30°N at the end of 11 May and gradually subsides at the end of the studied time period. The almost instantaneous onset of the negative response in the Northern Hemisphere indicates the presence of a strong meridional wind from the pole to the equator, which causes the penetration of the warmed high-latitude neutral air with a reduced ratio of O/N₂. As a result, the increased recombination in the ionospheric F region causes a decrease in the electron density and respectively in the TEC. In the Southern Hemisphere (winter conditions) at latitudes south of 50°S, at the beginning of the storm, an area with a predominant positive response is formed, which, after midnight on 11 May, turns into a negative one.

In order to compare the changes in the O/N₂ ratio during quiet geomagnetic activity shown in Figure 6b and during the geomagnetic storm (see Figure 6c), the latitudinal and longitudinal distribution of the available O/N₂ ratio data from GUVI on 9 May 2024 is shown.

The latitudinal and longitudinal distribution of the O/N₂ ratio shown in Figure 6b at the same local time, around 8:30 UT, is almost uniform at latitudes higher than 30°N throughout the day. This result indicates that the transport of neutral air with an increased recombination coefficient in the daytime region occurs without significant delay.

Seasonal differences in the ionospheric response are illustrated in Figure 7 through the behavior of absolute TEC and median TEC for two symmetrical points situated at the Greenwich meridian at 60°N and 60°S, respectively. The values of median TEC at local noon (12 UT) are approximately the same, but the midnight values in the summer hemisphere are much higher (about 20 TECU) than in the winter (about 5 TECU). The impact of particle precipitation into the polar oval is particularly noticeable at 60°S (see Figure 7c). The maximum of the positive response is at local time (coinciding with universal time) around 22 LT. The difference between the responses in the two hemispheres is due to the fact that two types of reactions operate simultaneously in the area of particle precipitation—ionization and increased recombination due to the heating—which causes an absolute decrease in the electron density similar to the one existing before the onset of the geomagnetic storm. It can be seen from the figure that at a high value of electron density in nighttime conditions at 60°N, the decrease in electron density is more significant compared to 60°S and can compensate for the increase due to ionization, which is approximately the same in both hemispheres.

The longitudinal variations and their evolution during the storm are illustrated in Figure 8 and Figure 9. The deviation of rTEC from the zonal mean is revealed in both figures, as the left/right panel of Figure 8 shows the longitudinal/local time distribution of the considered deviation. In the right panel of Figure 8, the longitude has been replaced by the corresponding local time at the corresponding universal time.

The features of the longitudinal TEC response distribution in the polar oval are especially distinct at 60°S in accordance with the seasonal peculiarities (see Figure 8g). Longitudinal distribution at a given moment has a positive maximum and a negative minimum. The corresponding minimum and maximum migrate westward in time. The latitudinal distribution shown in Figure 8h at local time demonstrates that the positive response during the storm at this latitude (60°S) remains during the night hours, with a maximum persisting around 1 h before midnight. The assumption that the positive maximum coincides with the particle precipitation region is confirmed by its westward migration along with the movement of the midnight meridian. Figure 8a,b show the latitudinal distribution at 60°N where similarity is observed, but the low values of the response cause some noise. Both the longitudinal distribution and its time evolution are almost completely similar to the symmetry about the magnetic equator modip latitudes of 40°N (see Figure 8c,d) as well as the analogous ones for the Southern Hemisphere of 40°S (see Figure 8e,f). The only difference is that the response at the northern latitude (40°N) is weaker. In this case, the movement to the west is observed, but the local time distribution shows that the speed is significantly smaller, about two times, than the phase speed of the Earth’s rotation. This indicates that there is a noticeable lag behind the westward movement of the midnight meridian. The same phenomenon was noted in the paper of Bojilova and Mukhtarov, 2024 [37], where three geomagnetic storms that occurred in different seasons were considered.

A similar dependence was found by Zhang et al., 2019 [46], and presented by the authors in Figure 9 of their paper, where the deviation of TEC from the reference level for latitude of 40N on 09 June 2017 as a function of universal time and latitude was illustrated. A positive response is observed in the night hours, distributed by longitude analogously to Figure 8f in the present research. The longitudinal variations at the magnetic equator, shown in Figure 9, demonstrate the following peculiarities: (a) the negative response during the storm is localized in the night sector, for example, from 22 LT to 6 LT, and no significant deviation from the Earth’s rotation speed is observed; (b) the positive response remains in the daytime sector.

The results from Figure 8 and Figure 9 show a longitudinal distribution with a single maximum, which allows the hourly representation of the amplitudes and phases of a quasi-stationary wave with wavenumber 1 to be used for its description. This representation will be denoted GTID1 (Global Scale Traveling Ionospheric Disturbance with zonal number 1) for convenience. The time change of the obtained amplitudes and phases of the quasi-stationary waves with wavenumber 1 is shown in Figure 10.

In order to verify the results obtained by independent TEC data, we obtained vertical TEC data from the Institute for Space-Earth Environmental Research (ISEE), Nagoya University (https://stdb2.isee.nagoya-u.ac.jp/GPS/GPS-TEC/index.html, accessed on 20 October 2024). An analogous approach was used in the processing of this data type as for the other TEC data considered. These data is not interpolated. Figure 10 shows latitude of 40°N, at which a sufficient number of values are available. Comparison of Figure 10 with Figure 8c,d shows a good coincidence. A very good coincidence can be seen in Figure 10b analogous to Figure 8d, namely a movement of the maximum towards larger local time values.

From Figure 11a, it can be seen that increased values of the amplitudes are observed at latitudes of 40°N, 40°S and 60°S, which coincide with the regions of noticeable longitudinal inhomogeneity of the response visible in the maps obtained on the basis of the raw rTEC data (see Figure 3, Figure 4 and Figure 5). The phase behavior (showing the local time of the positive maximum) illustrates the migration of the positive response region during the storm (see Figure 11b). In the southern polar oval (60°S), the phase remains in the night sector and the values are close to midnight. This means that the region of positive feedback follows the movement of the midnight meridian where particle precipitation from the solar wind takes place. The region of negative response also migrates in a westward direction, remaining in the daytime sector of the Earth, as can also be seen in Figure 8. The phase in local time at 40°N and 40°S is displaced from daily values at the very beginning of the storm in the increasing direction. This shows that the movement of the positive anomaly region migrates westward, but at a lower speed than the Earth’s rotation speed. This effect is also seen in Figure 8. The variations in the amplitudes and phases at both latitudes, 40°S and 40°N, are very similar, as it is the behavior of the longitudinal inhomogeneity shown in Figure 8, confirming the convenience of using the quasi-stationary representation.

3.4. Stationary Amplitudes, Phases and Phases Velocity

This subsection presents a detailed analysis of amplitudes, phases and phase velocities of GTID1 for selected latitudes and global distribution of GTID1 in the night hemisphere of the Earth for 11 May 2024.

To obtain more detailed information about the ionospheric response at selected latitudes, Figure 12, Figure 13 and Figure 14 present the amplitudes and phases at the magnetic equator, a comparison between 40°N and 40°S, and a comparison between 60°N and 60°S of the GTID1 characteristics. In each of Figure 12, Figure 13 and Figure 14, the top panel (namely Figure 12a, Figure 13a and Figure 14a) shows the changes in incoming energy at the polar oval represented by the Power Index. This plot allows one to compare the ionospheric response at different latitudes with the variations in the Power Index.

The GTID1 amplitude at the equator reaches a maximum around 00 UT on 11 May and persists for about 5 h (see Figure 12b). The phase of GTID1 illustrated in Figure 12c (the positive maximum) at the beginning of the storm practically coincides with local noon; by 12 UT it shifts to 18 LT, and then returns to local noon again. The phase velocity at the beginning of the storm is close to the phase velocity of the Earth (−15°/h), gradually decreasing to zero around 12 UT (see Figure 12d). After a second increase in amplitude around 00 UT on 12 May, the phase velocity again approaches the Earth’s rotation velocity. The phase behavior of the equatorial response shows that during the storm, the movement of the noon (respectively) midnight meridian on the Earth’s surface follows; i.e., the equatorial GTID1 can be represented as a zonal wave with wavenumber 1, which shows a direction of rotation from east to west, and the phase velocity is approximately coincident with the phase velocity of the Earth’s rotation. The positive maximum is in the daytime hemisphere, while the negative minimum is in the night hemisphere. The obtained characteristics completely coincide with the variability in the longitudinal distribution of the rTEC deviation from the zonal mean value shown in Figure 9.

The dynamics of GTID1 at the other two characteristic latitude zones 40°S and 40°N are shown in Figure 13. Analogous to those shown in Figure 8, the two selected latitudes demonstrate a distinct symmetry. The course of the amplitudes is completely similar despite the larger amplitudes in the Southern Hemisphere (see Figure 13b). The course of the phases at 40°S and 40°N is also similar, illustrated in Figure 13c. This gives reason to assume the same origin of GTID1 for both 40°S and 40°N. At the beginning of the storm, the phase of GTID1 coincides with local noon, and during the storm it migrates toward the nighttime value and reaches 0 LT around 12 UT. The time (in universal units) for the passage of all hours of the local day is greater than 24 h, which means that GTID1 migrates in a westward direction but at a speed less than the speed of the Earth’s rotation. The phase velocity variation shown in Figure 13d is, on average, half of the Earth phase velocity.

Figure 14 shows the dynamics of GTID1 at the two latitudes coinciding with the auroral ovals in both hemispheres, namely 60°N and 60°S. The amplitude in the Northern Hemisphere (see Figure 14b, red color) is insignificant; therefore the phase and phase velocity cannot be considered reliable. In the Southern Hemisphere, where the amplitude is strong, a practically constant phase (around midnight local time) and almost zero phase velocity is observed, which means that the positive response follows the movement of the midnight meridian.

Figure 15 shows the global GTID1 distribution of the Earth’s night hemisphere from 00 UT to 15 UT on 11 May 2024. The positive response in the Southern Hemisphere remains centered on the midnight meridian until its attenuation, which means that it moves synchronously with the midnight meridian on the Earth’s surface. At the beginning of the geomagnetic storm, around the magnetic equator, an extensive area with a negative response is formed, which slowly migrates in an eastward direction, which means a delay from the movement of the midnight meridian. The two symmetric positive mid-latitude responses also shift eastward, with the local time change decreasing by about 6 h for 15 UT. This means a movement with an average phase velocity about half of that of the midnight point.

4. Discussion

The analysis of the global distribution of the ionospheric TEC during the geomagnetic storm on 10–11 May 2024 allows us to make hypotheses about the possible mechanisms which could have some impact on the ionospheric electron density depending on the magnetic latitude. The main attention in the present study is paid to the existing Traveling Ionospheric Disturbances caused by the impact of the solar wind on the Earth’s upper atmosphere. The advisability of the presentation of GTID1 by the method of quasi-stationary wave amplitudes and phases is substantiated in detail. For the specific geomagnetic storm, it turned out that the quasi-stationary zonally distributed wave (denoted as GTID1) sufficiently representatively describes the existing Traveling Ionospheric Disturbances. The main reason for this effect is the fact that particle precipitation in the two polar ovals and, accordingly, their energetic impact is concentrated in the midnight sector at modip latitudes around 60°.

Direct ionization under the action of particle precipitation manifests as a strong positive ionospheric response in the winter hemisphere due to low TEC values in the night hours. In the daytime, due to the absence of such a direct impact, the effect of heating appears, causing a redistribution of gases and an increase in recombination, as a result of which a negative response is observed. The quasi-stationary wave formed in this way migrates in a westward direction with a speed almost coinciding with the speed of the movement of the midnight point.

The processes in the equatorial and mid-latitude regions are related to the formation of a disturbed dynamo. In the study of Blanc and Richmond, 1980 [10], the main regularities of the occurrence of disturbed neutral wind under the action of the heating of the polar oval and the creation of a system of horizontal currents are described. When these currents interact with the permanent magnetic field of the Earth, they cause plasma drift perpendicular to the magnetic field. The presence of an upward vertical drift of the ionospheric plasma can lead to a significant reduction in the ion, and therefore the electron, density in the F region [47]. In the area around the magnetic equator, there is a so-called “fountain effect”, which is expressed in the fact that the plasma raised under the action of the vertical drift flows along the lines of force of the magnetic field to the north and south of the equator and accumulates there. In the study of Mukhtarov and Pancheva, 2012 [48], it was shown that such an effect occurs not only in quiet conditions, but also under the influence of geomagnetic disturbances.

The study of Fejer et al., 2008 [15], shows that the average equatorial disturbance dynamo vertical plasma drifts have positive values in the interval 22 LT–8 LT (i.e., in night conditions) and are negative during the rest of the day. According to the current study of a selected geomagnetic storm, the maximum negative response in the magnetic equator region is observed at local time close to midnight. This type of response persists all the time in the night hemisphere (see Figure 9), which is in good agreement with theory and measurement results. According to the study of Fejer et al., 2008 [15], the longitudinally averaged equatorial prompt penetration vertical plasma drifts have a diurnal course opposite to the course of the disturbance dynamo vertical plasma drifts; therefore, the equatorial TEC response in the selected storm cannot be related to these effects.

The detailed symmetric response of 40°S and 40°N has the character of an extension of the Equatorial Ionospheric Anomaly. This gives reason to assume that the region with a positive TEC response is due to an accumulation of the plasma raised from the equatorial region under the action of upward vertical drift generated by disturbed dynamo currents. Karan et al., 2024 [24], reported that the position of the EIA crests during the considered geomagnetic storm extend to ±40° magnetic latitudes.

Figure 16 shows a comparison between the variations in the measured values of foF2 and hmF2, their monthly medians, and their relative deviations measured by the ionospheric station at Cachoeira Paulista (22.7°S, 45.0°W, modip latitude 16.95°S). The figure shows that in the hours around midnight on 11 May, the foF2 values are close to the medians, but there is a strong increase in the height of the F2-region maximum (hmF2), which can be interpreted as the result of an upward vertical drift. Figure 16c shows the GTID1 map at 0 UT on 11 May 2024 coinciding in time with the maximum manifestation of the geomagnetic storm. The location of the ionospheric station CAJ2M is marked by a rhombus and coincides with the transition of the negative equatorial response to the positive response at 40°S. A possible explanation for the different longitudes of the negative and positive response is that they are a result of the presence of not only vertical but also horizontal drift.

An interesting result obtained in the present study is the delay of the mid-latitude GTID compared to the equatorial one and that in the auroral ovals. A similar effect has been observed in other geomagnetic storms occurring in different seasons [37]. Bojilova and Mukhtarov, 2024 [37], hypothesize that it is possible that the particle precipitation heated region of the polar ovals delays the movement of the midnight meridian due to the inertia of the heating. In addition, it is possible to manifest the effect of the Conservation of Angular Momentum, which also applies to a closed rotating system, such as the Earth’s atmosphere. The disturbed meridional wind near the polar oval rotates around the Earth along with movement of the midnight meridian. Passing into regions close to the equator, the radius of this rotation increases and its angular velocity must decrease, which means a decrease in its phase velocity. As a result, there is a delay in the eastward direction, which is observed in the longitudinal distribution of the ionospheric response at 40°N and 40°S. These hypotheses, however, cannot explain the differences between the equatorial and mid-latitude regions. The impact of the disturbed meridional winds on the electron density is indirect through electric fields, with the possible influence of prompt penetration electric fields, as well as plasma transfer through the “fountain effect”, which makes the processes complex enough to explain with simplified assumptions.

An interesting result of this study is the negative response in mid latitudes caused by the spread of warmed air from the polar oval to low latitudes. Analysis of this response shows a longitudinal structure that sums up with the structure induced by the disturbed dynamo effect and can induce a phase shift. The map of the global distribution of O/N₂ from the GUVI satellite shows that, at fixed local time on 11 May, the mid-latitude region in the Northern Hemisphere is covered almost evenly by a region of very low O/N₂ values. This gives reason to assume that during the storm, if there are longitudinal inhomogeneities, they are relatively small. In the Southern Hemisphere, for which no data are available, the penetration of warmed air is generally limited in latitude, which is reflected in the significantly larger amplitudes of GTID1 at 60°S relative to 60°N with a nearly identical phase structure.

5. Conclusions

The main results of the present study are summarized as follows:

• The presentation of GTID1 by the method of quasi-stationary wave amplitudes and phases is discussed in detail to describe the positive TEC anomaly in the two polar ovals in the midnight sector at modip latitudes around 60°.
• During the day, as a result of the redistribution of gases and an increase in recombination, a negative ionospheric response is observed. The quasi-stationary wave formed in this case migrates in a westward direction with a speed almost coinciding with the speed of the movement of the midnight point.
• The results show the delay of the mid-latitude GTID compared to the equatorial one and that in the auroral ovals.
• An interesting result is the negative response in mid latitudes caused by the spread of warmed air from the polar oval to low latitudes.
• The observed symmetric ionospheric response at 40°S and 40°N is explained by the extension of the Equatorial Ionospheric Anomaly.
• According to the selected geomagnetic storm, the maximum negative response in the magnetic equator region is observed at local time close to midnight. This type of response persists all the time in the night hemisphere.
• A comparison is presented between the global distribution of the ionospheric response and the CAJ2M ionospheric station data. The obtained differences in the negative and positive responses in the two types of data are explained by the presence of not only vertical but also horizontal drift.