Effects of the Geomagnetic Superstorms of 10–11 May 2024 and 7–11 October 2024 on the Ionosphere and Plasmasphere

1. Introduction: Strongest Storms of Solar Cycle 25

In the present work, we compare the two strongest geomagnetic events of the solar cycle 25 in 2024: the Mother’s Day storm on 10-11 May 2024 and the successive storms on 8-11 October 2024. For these events, we compare the observations of solar wind parameters that generate the geomagnetic activity, ionosonde measurements (up to the maximum of electron density in the F layer of the ionosphere), Vertical Total Electron Content (VTEC) up to GNSS (Global Navigation Satellite System) orbit altitude, plasmapause measurements by SWARM satellites, and the BSPM plasmasphere model in order to determine the evolution in time and space of the plasma density variations. The observation methods are described in Section 2, results in Section 3 for 10-11 May 2024 Mother’s Day event and Section 4 for 8-11 October 2024 storms, while Section 5 discusses and summarizes the results.

2. Observation Methods

For both periods of geomagnetic disturbances, we describe in this section the models and the methods used to analyse the observations from different instruments that are used in this work.

2.1. Solar Wind and Geomagnetic Indices

OMNI is a multi-source data portal of near-Earth solar wind’s magnetic field and plasma parameters provided by NASA. We use it to obtain the solar wind parameters measured at 1 Astronomy Unit (UA) (i.e. the average distance between the Sun and Earth) and the geomagnetic activity indices.

2.2. Vertical Total Electron Content VTEC

One of the parameters used to characterize the ionospheric conditions is the vertical total electron content (VTEC). TEC is the total number of free electrons integrated along the raypath between the GNSS (Global Navigation Satellite System) satellites (located at an altitude between 19,100 km until approximately 23,300 km, thus a radial distance from the center of the Earth of approximately 4 Re) and the receiver on the ground. It is estimated in a unit called TECu in which 1 TECu = 10¹⁶ electrons/m². The VTEC is the vertical value of TEC projected from this raypath intersecting above the electron density peak at 450 km height. The VTEC is directly related to the GNSS signal propagation delay due to the ionosphere. These ionospheric conditions are closely monitored by the STCE in Brussels based on the GNSS observations of the EUREF Permanent Network (Bruyninx, 2019) and using the ROB-IONO software (Bergeot, 2014). With these 30-s GPS+Galileo+GLONASS real-time observations, VTEC maps are estimated every 5 min above Europe. The VTEC variability, reflecting the ionospheric state variations during the 5-min time span of the interpolation, is estimated. The VTEC time series provided in this paper are extracted from these maps at three locations in Figure 4 and Figure 11:

a) in the northern part of Europe (61°N, 5°E),

b) above Brussels, in mid-latitude Europe (50.5°N, 4.5°E),

c) in North Africa (36°N, 5°E).

2.3. Ionosonde Observations

Ionosondes employ high frequency (HF) radio transmissions of various frequencies to determine the electron density profile above the instrument. Frequencies below the plasma frequency $\omega =\sqrt{{\displaystyle \frac{N_e{e}^2}{{\epsilon }_{0}m}}}$ (where e is the electron charge, m its mass, ε₀ is the permittivity of free space) are reflected by the plasma in the ionosphere. Therefore, the electron density profile $N_e\left(h\right)$ can be reconstructed from the reflection of a range of different frequencies. The main limitation of this technique is that it allows observations only up to the height of the greatest electron density. This altitude varies significantly both during quiet conditions, and as a result of particular disturbances such as geomagnetic storms.

In the present study, we use data from three ionosondes in Europe (JR055, DB049, EB040), at different latitudes, as well as two ionosondes in other longitude sectors: IC437 in South Korea and MHJ45 in the USA. Ionograms were obtained from the DIDBase of the GIRO repository (Reinisch and Galkin, 2011). This allows us to compare the storm effects at different latitudes as well as the effect of the local time at the onset of the event. The coordinates of the observatories are listed in Table 1. Each of these observatories uses a similar Digisonde DPS-4D ionosonde, and produces regular soundings of the ionosphere at intervals of 5 (for the European observatories) or 7.5 min (for the American and South Korean observatories). All ionograms were manually inspected in order to derive the characteristics of the various layers (we use here the critical frequencies foF₁ and foF₂ of the two F-region density peaks), with only data points retained which could be obtained reliably according to the standard URSI (International Union of Radio Science) rules.

The exceptional ionospheric conditions during the events analysed here cause some particular difficulties in the interpretation of the ionograms. There are three reasons for occasional gaps in the time-series of the critical frequencies. First, during the night-time, spreading of the F₂ trace is observed (see the left panel of Fig. 1 for an example). This can render it impossible to determine accurately the critical frequency foF₂. During the first night of the 11 May storm, there were also sporadic E layers observed produced by particle impacts (which are associated with the visible auroras seen at those same times even at low latitudes). An example of this is shown in Fig. 1 on the right. In this example, the Es layer extends to above 4 MHz, making it impossible to detect the depleted F layer above.

Figure 1. Examples of spread-F condition (left, recorded by IC437 on 11 May 2024 at 20:15:00 UTC) and particle-induced Es-layer (right, recorded by DB049 on 11 May at 00:50:02 UTC). Note that the height (vertical axis) ranges from 0 to 800 km in both cases, but the frequencies (horizontal axis) from 1 to 12 MHz in the left panel and from 1 to 9 MHz in the right panel.

Figure 1. Examples of spread-F condition (left, recorded by IC437 on 11 May 2024 at 20:15:00 UTC) and particle-induced Es-layer (right, recorded by DB049 on 11 May at 00:50:02 UTC). Note that the height (vertical axis) ranges from 0 to 800 km in both cases, but the frequencies (horizontal axis) from 1 to 12 MHz in the left panel and from 1 to 9 MHz in the right panel.

Finally, there are some gaps in the data related to absorption of the ionosonde signals in the D region of the ionosphere during flares. On 11 May 2024, there was an X5.89 flare peaking at 01:23 UTC (Universal Time Coordinated) and an X1.54 flare peaking at 11:44 UTC. In addition, during the period of interest, there were some M-class flares as well. For most of 11 May, even the minima of the X-ray flux remained above the level of an M1 flare. Depending on the signal-to-noise ratio usually obtained by the different ionosondes, and the local time at each during the flares, this absorption in the lower ionosphere results in gaps in the observation of the F layer.

2.4. Plasmasphere Model and Data

The Belgian SWIFF Plasmasphere Model (BSPM) is a 3D-Kinetic semi-empirical model of the plasmasphere (Pierrard and Stegen, 2008), coupled to the ionosphere (Pierrard and Voiculescu, 2011). It has recently been improved to take into account the most recently identified relevant physical processes (Pierrard et al., 2021a). It can be run for any date to obtain every hour the number density and temperature of the electrons and protons inside and outside the plasmasphere, as well as the position of the plasmapause, as a function of the geomagnetic activity driven by the Kp index. It uses the kinetic approach for the particle densities and the mechanism of quasi-interchange instability for the formation of the plasmapause. The density in the plasma trough region has recently been improved using observations of Van Allen Probes (Botek et al., 2021). The results of the plasmasphere model have been compared to NASA’s IMAGE mission (2000-2005) global EUV images of the plasmasphere (Pierrard and Cabrera, 2005), Cluster (Verbanac et al. 2015) and Themis (Bandic et al., 2017), among others, allowing to validate the plasmasphere erosion and dynamical evolution.

Moreover, the magnetic and plasma observations of the low-Earth orbiting SWARM satellites allow us to derive the midnight plasmapause (Heilig et al., 2022). Launched on 22 November 2013 into a near-polar Low Earth Orbit (LEO), SWARM is a constellation of three identical satellites operated by the European Space Agency (ESA) with the purpose of mapping Earth’s magnetic field (Olsen et al., 2013). The initial altitude of the satellite pair SWARM A and SWARM C was about 490 km in April 2014 and it was about 510 km for SWARM B, both orbit altitudes do slowly decay with time. Based on in situ electron density and temperature (Knudsen et al., 2017), GPS-derived TEC, and auroral field-aligned current observations at the SWARM satellites, new SWARM products have recently been developed to characterize plasmapause-related boundaries in the topside ionosphere. This product also includes a plasmapause index, a proxy for the midnight plasmapause position. This index is derived from SWARM observations similarly as described in Heilig and Lühr (2018).

3. Mother’s Day Storm: 10-11 May 2024

3.1. Observed Solar Wind and Geomagnetic Indices

An exceptionally strong geomagnetic storm occurred during the night from 10 to 11 May 2024, with auroras observed everywhere in Europe (Karan et al., 2024). It has been called the Mother’s Day event due to the date corresponding to Mother’s Day in many countries in the world. This was the strongest storm for the last twenty years with a minimum of Disturbed Storm Time index Dst < -400 nT. Due to the varying speeds and quick succession of different Coronal Mass Ejections (CMEs) coming from the active region NOAA 13664, several of them merged and interacted as they travelled through the interplanetary medium, leading to enhanced effects in the Earth’s space environment (Pierrard et al., 2024). These CMEs were associated to X2.2 flare on 9 May and three X1 flares on 8 May.

The characteristics of the solar wind when it reached 1 AU, obtained from OMNI, are shown in Figure 2. The bulk velocity of the solar particles reached more than 1000 km/s (see Figure 2, 2nd panel), and peaks of high temperature > 10⁶ K were observed (panel 4). The high-density peak (panel 1) at the arrival of the hot plasma shows the apparition of a shock. The pressure (panel 3) combines the density and velocity effects, and determines the position of the magnetopause. The downward peak of the Z component of the Interplanetary Magnetic Field Bz (panel 5) shows a southward direction that can explain the strong answer of the terrestrial magnetic field (Pierrard, 2024).

The arrival time of enhanced solar plasma at 1 AU with regard to density, velocity and pressure is 10 May 2024 at 17:07 UT. It directly caused the sudden commencement of the strong geomagnetic storm (see Figure 3), followed by the main phase when the Dst index decreased down to a minimum value of -412 nT (see panel 3) in the night of 11 May and the planetary geomagnetic activity index of Kp increased to the maximum value of 9 (panel 1). The geomagnetic activity indices Kp, Ap in nanoTesla (that reached 400 nT, see panel 2), and Dst (also in nT) are illustrated in Figure 3, together with the F10.7 solar radio flux at 10.7 cm (panel 4) and intensity of Lyman alpha line in the solar spectrum (panel 5) that are daily solar activity indices.

3.2. Ionospheric Vertical Total Electron Content

Figure 4 shows the time series of the VTEC (in red), the expected VTEC based on observations during the previous 15 days (in grey, with standard variation), and the 5-min VTEC variability (in blue) in the North of Europe, Middle of Europe (Brussels) and more Southward (North of Africa). The three near-real time VTEC time-series showed abnormal variations during the night of 10 to 11 May 2024 followed by a long depletion in VTEC until the 12 May 2024. This is due to the strong CME impact detected during the afternoon of 10 May. By comparing with Figure 3, these VTEC fluctuations (high variations during the storm and long depletion after) can not be explained by variations in F10.7 during these days.

Figure 4. VTEC time series at 3 locations in Europe from 9 to 14 May 2024. The left panel shows the time evolution of the Vertical Total Electron Content (VTEC) (in red) at three locations illustrated in the right panel on a map of Europe: a) in the northern part of Europe (61°N, 5°E), b) above Brussels (50.5°N, 4.5°E), c) in North Africa (36°N, 5°E).

Figure 4. VTEC time series at 3 locations in Europe from 9 to 14 May 2024. The left panel shows the time evolution of the Vertical Total Electron Content (VTEC) (in red) at three locations illustrated in the right panel on a map of Europe: a) in the northern part of Europe (61°N, 5°E), b) above Brussels (50.5°N, 4.5°E), c) in North Africa (36°N, 5°E).

Consequently, the geomagnetic storm had a strong impact on the ionospheric electron content, with sudden increases and decreases of 20 TECu from 18:00 UTC 10th May to 12:00 UTC 11th May in the North of Europe. At mid-latitude, the VTEC tends to decrease starting at 18:50 UTC with a minimum of -10 TECu w.r.t. expected quiet ionosphere and with a sudden peak of increase of 8 TECu (w.r.t. quiet ionosphere) at 22:20 UTC. At the same time, at low latitude, VTEC starts to decrease w.r.t. quiet ionosphere. For both mid- and low latitudes, the VTEC variability also remains high, until midnight on 11th May. These VTEC variations and rapid variability at these 3 latitudes are due to the injection of the particles associated to the storm and leading to the occurrence of auroras at unusually low latitudes.

The maximum variability in the VTEC is observed at 20:20 UTC (see Figure 5).

In Figure 5, it can be observed that increase of TEC w.r.t to the quiet ionosphere occurs at high and low latitudes while this is negative at mid-latitude. Over Scandinavia, the VTEC variability reached a maximum 5 TECu for a 5-min interval at 20:20 UTC. This could be linked with the hight values of Rate Of TEC change Index (ROTI) observed in Spogli et al. (2024) supporting the fluctuations implying disturbances in GNSS applications.

The other effect of the storm is the depletion of the VTEC down to 30 TECu at low latitudes until the 12th May around 05:00 UTC. As mentioned in Mendillo (2006) and Bergeot et al. (2013), the persisting depletion in TEC a few days after the onset is due to the contraction of the plasmasphere implying disturbances in the thermosphere circulation and a chemical loss in the ionosphere. This contraction of the plasmasphere is coherent with the observation in Figure 9 Section 3.4 of the present paper.

3.3. Ionosonde Observations

Figure 6 shows the peak electron densities for the F₁ (in blue) and F₂ (in red) layers for the three European ionosondes for a three-day period covering 10 to 12 May 2024. The E-layer (not shown) was found to be not affected by the geomagnetic storm. This is to be expected, because the E-layer is known to be driven entirely by solar irradiance.

At the onset of the storm, a very rapid depletion of the F₂-layer density can be observed. The depletion appears to be more rapid at the higher latitudes, although of course the peak density before the storm is lower at higher latitudes as well. For the most part of 11 May, the F₂ layer was not detected, because the electron density in the F₂ region was lower than in the F₁ layer and thus not visible to the ionosondes. This is referred to as “G-condition” in the ionosonde community, and is often seen during major storms (see for instance Mošna et al. (2024) for some recent examples). The density of the F₁ layer is also seen to be lower than during the quiet day before the storm, but the depletion is much less severe than in the F₂ layer because transport processes and chemical changes are less important at the F₁ layer altitude compared to the influence of solar irradiance.

The strong depletion of the F-region lasts throughout 11 May and morning of 12 May. The reappearance of the F₂ layer on 12 May and its return to quiet time peak density is almost as rapid as the depletion at the onset of the event. This indicates that the refilling of the F₂ layer was very quick. It is also evident that this refilling occurred earlier at the EB040 observatory, which is at the lower latitude. Here, the F₂ layer was detected again around 06 UTC. At the DB049 and JR055 ionosondes at higher latitudes, the F₂ layer was not detected until around 09 UTC, and the peak density reached its climatological values only around noon.

Although the reappearance of the F₂ layer happens earlier at lower latitudes, it can be seen that the peaks of highest electron density reached on 12 May occur earlier at the highest latitude. The highest density is seen on 12 May at 12:20 UTC by JR055, at 12:50 UTC by DB049, and at 13:45 UTC by EB040. This seems to indicate large scale travelling ionospheric disturbances (LSTIDs) moving from the auroral region to lower latitudes. Such LSTIDs are usually seen during the beginning of a storm (e.g. Tsagouri et al., 2023) but seem to occur here also at the time of refilling of the F₂ layer.

Figure 7 shows the critical frequencies of the F₁ and F₂ layers for the Millstone Hill (MHJ45) and I-Cheon (IC437) ionosondes. The behaviour of foF₂ at MHJ45 (top panel) can be seen to be very similar to what was observed by the European ionosondes (see Fig. 6). There is a sudden depletion starting at 18:00 UTC on 10 May, and lasting until 12 May. G-condition was observed for the entirety of 11 May. Around 09:00 UTC on 12 May, there is a sudden increase again of the F₂-layer density. At MHJ45, this corresponds to the normal morning time density increase (as is evident by comparing to the same time on 10 May, which is still before the onset of the storm). This sudden refilling thus occurs at the same time in the European and American sectors, despite the local time difference. The pattern of the F₁ layer at MHJ45 is of course delayed in time compared to the European observatories as a result of this local time difference.

The observations at IC437 (bottom panel of Fig. 7) show a different pattern. A sudden depletion at the storm onset is again evident, but at this location, 18:00 UTC corresponds to nighttime. During the local daytime on the two succeeding days, there are multiple gaps in the data due to the various difficulties explained above. Nevertheless, from those ionograms that could reliably be scaled, it is clear that severe depletion persisted throughout these days, and even during the local night-time on 12 May —the end of the time-series shown in the Figure—, the electron density has clearly not recovered yet to the quiet time level seen before the storm onset. The peaks in foF₂ seen during the local nighttime on 11 May correspond to periods of strong spread-F associated with auroral oval structures expanding to the latitude of this observatory.

During the main phase of the storm, the electron density and the height of the F region decreased and increased, respectively, indicating an upward displacement of the ionosphere (depression and uplift), irregularities (plasma bubbles) due to stable aurora red arc convergence of low and high latitude effects.

The electron density in the F₂ layer decreased due to the expansion of the atmosphere that modified the recombination time of the ionized particles. During the strong post-storm depletion, F₂ disappeared behind the F₁ layer. The strong spread-F initiated during the main phase was still present during the next nights and caused scintillation of satellite signals. After the arrival of the CME, the electron density decreased and remained significantly lower than during the quiet days, for more than a full day, thus including during the recovery phase of the geomagnetic storm. This meant that for High Frequency (HF) communication, the portion of higher HF frequencies was not available for communication, and so several advisories were sent to civil aviation via PECASUS (https://pecasus.eu/) to warn for this so-called post-storm depression (Kauristie et al., 2021).

Spogli et al. (2024) also showed unusual high spatially distributed values of the Rate Of TEC change Index (ROTI) on the nights of 10 and 11 May. The ROTI enhancements on 10 May might be linked to stable auroral red arcs and an equatorward displacement of the main ionospheric trough. Instead, the ROTI enhancements on 11 May might be triggered by a joint action of low-latitude plasma pushed poleward by the pre-reversal enhancement in the post-sunset hours and wave-like perturbations propagating from the North.

3.4. Plasmasphere

While ionosonde measurements are not affected by the density of electrons in the plasmasphere, this is not the case of VTEC. The sudden but short increase of density observed at the onset of the storm, in VTEC but not by the ionosondes, indicates that this additional ionization appears above the F2-peak. The plasmaspheric density is thus very important to determine. The sharp density decrease measured by both techniques shows that it takes already place at low altitudes in the F layer. Considering also the plasmasphere erosion appearing during storms (e.g. Pierrard and Cabrera, 2006), VTEC observations allow us to determine the plasmaspheric part in comparison to the ionospheric reduction.

The plasmasphere is the extension of the ionosphere at higher altitude at low and middle latitude (Darrouzet et al., 2009), and its erosion can also explain the reduction of VTEC during and after the storm. Indeed, the extension of the plasmasphere is strongly reduced after storms, with a sharp plasmapause appearing at lower radial distances, as it is illustrated in Figure 8 using the BSPM plasmasphere–ionosphere model (Pierrard et al., 2021a). The top panel shows the extended plasmasphere (orange region with high electron density) before the storm, with an average plasmapause position above 4 Re at all Magnetic Local Time (MLT). The plasmasphere is seen in the geomagnetic equatorial plane (left panel) and in the meridian plane (right panel), with the Sun (thus noon 12:00 MLT) on the left.

The bottom panels show the eroded plasmasphere after the storm, which is very strong due to the Kp reaching the maximum value of 9. The new plasmapause is located around 3 Re in average, but is as low as 2.3 Re in the post-midnight sector, and as high as 5 Re at the endpoint of the plume in the afternoon MLT sector. The plume co-rotates with the Earth after its formation. It takes typically 2 or 3 days after the storm to progressively refill the outer shells of the plasmasphere (Pierrard et al., 2021a).

The plasmasphere erosion after the storm can thus at least partially contribute to the observed VTEC. When the plasmaspause comes closer to the Earth in the equatorial plane (see Figure 8 left bottom panel), the ionospheric trough appears also at lower latitudes (see right bottom panel), due to the motion of the particles along the magnetic field lines (Pierrard and Voiculescu, 2011).

The midnight plasmapause proxy derived from the magnetic and plasma observation of the low-Earth orbiting SWARM satellites also confirms a very low plasmapause after 11 May 2024 reaching less than 2 Re, as illustrated in Figure 9 (top panel) with the Kp index (bottom panel). Such plasmapauses lower than 2 Re are very rare. The green line corresponds to the midnight plasmapause position and the black lines give the position +/- the standard variation. The squares correspond to the SWARM plasmapause observations. The plasmapause is directly related to the mid-latitude ionospheric trough observed by SWARM (Heilig et al., 2022).

Figure 9. Top panel: Midnight plasmapause proxy derived from the magnetic and plasma observations of the low-Earth orbiting SWARM satellites from 9 to 12 May 2024. Bottom panel: Observed Bartels geomagnetic activity Kp index. (https://swe.ssa.esa.int/elte-plasma-federated).

Figure 9. Top panel: Midnight plasmapause proxy derived from the magnetic and plasma observations of the low-Earth orbiting SWARM satellites from 9 to 12 May 2024. Bottom panel: Observed Bartels geomagnetic activity Kp index. (https://swe.ssa.esa.int/elte-plasma-federated).

The position of the plasmapause is crucial because different waves are generated inside and outside the plasmasphere and they affect the particles trapped in the radiation belts (Ripoll et al., 2023; 2024). Several unusual electron belts were observed after the 11 May 2024 geomagnetic storm, as well as a strong increase of the protons from 9.5 to 13 MeV trapped in the South part of the South Atlantic Anomaly (Pierrard et al., 2024).

4. Comparison with Event of 10 to 11 October 2024

4.1. Solar Wind and Geomagnetic Activity Indices

The second most important storm (so far) of the present solar cycle 25 started on 10 October 2024 and reached a minimum of Dst=-335 nT on 11 October 2024. It was preceded by another storm with a minimum Dst=-153 nT on 8 October, as illustrated in Figure 10 (bottom panel) together with the Kp index (5th panel) and the solar wind characteristics that generated these storms: density (1st panel), bulk speed (2nd panel), and temperature (3rd panel). The 8 October storm, associated with a X9 flare on 3 October, is mainly due to the arrival of a high density but low energy particles, while the 11 October storm (associated to a X1.8 flare on 9 October and a X2.1 flare on 7 October) is due to a density peak of energetic particles, as can be seen in the temperature and bulk speed observations. The velocity of the solar wind particles exceeded 700 km/s for this last event. The negative Bz of the interplanetary magnetic field (see 4th panel) explains the high geomagnetic perturbations induced on the terrestrial magnetosphere.

In Figure 10, the arrival of the energetic solar wind particles initiating the strong geomagnetic storm is well visible in the evening of 10 October 2024 (Dst = -335 nT, Kp=9-), preceded by a smaller event (Dst = -153 nT, Kp=7) starting on 7 October due to a peak of solar wind density. Both give effects visible in the ionosphere.

4.2. Ionospheric VTEC

Figure 11 shows that the two geomagnetic storms led, at the three different latitudes, to high variations of VTEC during the main phase of the storm (see red line). The VTEC variability reach 2 TECu/5min in the North and South. However, the storm impact is different in the southern location. Indeed, while the variability is high, the VTEC does not show rapid variations, but a smooth abnormal VTEC increase (red line, Figure 11c) compared to the expected behaviour with a maximum of differences at 21:10 UTC up to +35 TECu.

Figure 11. VTEC time series at the same 3 locations as in Figure 4 from 7 to 13 October 2024 included.

Figure 11. VTEC time series at the same 3 locations as in Figure 4 from 7 to 13 October 2024 included.

Figure 12 shows the state of the ionosphere during this maximum VTEC difference observed in the southern location at low latitude. As we can see, the VTEC observed is more than 30 TECu higher than the expected values. This is not seen ionosonde electron density observed by the ionosonde in Spain (see Figure 13, EB040) where no significant increase is observed. This difference between VTEC and electron density from ionosonde can be interpreted as an increase of the electron density above the F2 layer up to the plasmapause. This density peak is followed by a sharp decrease, leading to a value lower than the averaged density of the quiet times (gray line) during the night and the full next day and night. This after-storm depletion stands ~1.5 days (until the 12th 08:00 UTC) as seen for the mother storm day, with a minimum of -55 TECu in the low latitude the 11 May at 13:50 UTC.

4.3. Ionosonde Observations

Figure 13 shows the equivalent observations as Figure 6 but for the storm of 11 October 2024. The climatological conditions in October are different from those during the May event. The foF₁ peak is not as pronounced in October as it is in May. On the other hand, the foF₂ is slightly higher during the October event than during the May storm.

Note that there are again some gaps in the time series, in particular during the night of the storm at the highest latitude observatory (JR055). These are primarily due to the presence of particle induced sporadic layers precluding observing the F region of the ionosphere. Blanketing of observations by regular Es layers is not important for this event as such layers are more rarely present at these observatories in October as compared to May.

At the onset of the storm, a steep drop in the F₂-layer density can again be observed, indicating the sudden depletion of the ionospheric plasma. The main phase and recovery, on the other hand, look different for this storm. The cases of G-condition being observed are limited, in particular in the southernmost observatory, and the background conditions were restored already in the morning of 12 October.

Figure 14 shows the critical frequencies observed by the American and Korean ionosondes. Once again, it should be noted that the F₁-peak is not as clearly observed as during the May event, and that the foF₂ values are slightly higher. At these observatories, no G-condition was observed. This is due to the local time of the onset of the storm. As can be seen from Figure 13, the period of G-condition observed by the European ionosondes was relatively short, compared to what was seen during the May event, from around 06:00 to around 12:00 UTC at JR055. This coincides with a period when no F₁ layer is detected at MHJ45 and IC437, so there can be no G-condition.

Some depletion of the F₂ layer during the first day after the storm can still be seen at both observatories though. In both cases, the foF₂ at the end of the period has not yet recovered entirely to the values seen before the storm. In addition, the presence of large-scale travelling ionospheric disturbances can be seen. Especially during 11 October at IC437, there are periodic oscillations evident in foF₂.

4.4. Plasmasphere

The BSPM model shows that on 10 October 2024 12:00 UT, before the superstorm, the plasmasphere is already anisotropic in the geomagnetic equatorial plane (see Figure 15 top left panel) due to the previous storm of 8 October. On 10 October 22:00 UT, during the storm, a double plume that extends up to 6 Re appears in the afternoon MLT sector (see Figure 15 bottom left panel). Like for all storms, the plumes rotate with the Earth (Pierrard et al., 2021b). The plasmasphere is eroded and a plasmapause close to the Earth around 2.4 Re is found at after midnight MLT ~ 3.

A lower position of the plasmapause at 2 Re is even observed by SWARM at midnight MLT (see Figure 16), at least on 11 October, while no measurements are accessible on 10 October 2024. These exceptional observations of Kp>8+ allow us to improve the BSPM model, by increasing the extrapolated intensity of the electric field model for such high Kp values, since the empirical E5D model (McIlwain, 1986) had been validated only for lower values of Kp due to insufficient strong events.

5. Discussion and Conclusions

In the present work, we show that a study of the ionospheric observations as a function of the latitude and the Magnetic Local Time using VTEC, ionosondes and plasmaspheric observations is crucial for understanding the causes of the density variations associated with the magnetic storms. We have analysed the electron density measured by ionosondes in the F₁ and F₂ layers, the VTEC measured from the ground to GNSS satellites, and the position of the midnight plasmapause measured by SWARM during the two biggest geomagnetic storms of the present solar cycle 25, in May and October 2024. The observations at different latitudes and longitudes allow us to determine the spatial and temporal effects, and to study the fraction of electron density due to the ionospheric peak and to the plasmaspheric storm response.

The observations during the two analysed storms show that, while the ionization increases during the main phase of the storms, the density of electrons decreases for at least one day after the storms. The VTEC depletion is not only due to a decrease of the ionization in the F₂ layer, but also to a closer plasmapause. This was also observed in previous studies. For instance, Gallagher et al. (2021) found that the plasmasphere can lose 40% or more of its total mass during massive erosions. The relative contribution of the plasmasphere to the nighttime (i.e. locally) total electron content (TEC) can easily go beyond 80% during severely disturbed periods (Klimenko et al., 2015).

Nevertheless, clear differences could be observed in the response of the ionospheric layers to both storms. The storms were similar in strengths and in local time of the onset. However, the background conditions of the ionosphere in May and October are very different, at least in the lower ionosphere. The F₁ peak is more pronounced in May than in October, but the F₂ peak is more compact in October, with a peak density somewhat higher than in May. These differences in the structure of the ionospheric layers lead to the effects of the storm being visible for a longer time for the May event, with the F₂ layer only becoming visible again during the second day after the storm. G-condition (i.e. when F₂ layer is not detected because the density is higher in the F₁ layer than in F2) is observed for the entirety of 11 May while it is almost absent in October 2024. Such seasonal differences are smaller at higher altitudes.

The spectacular loss of F₂-layer ionization observed during both storms can be due to an increase of the recombination rates, associated to a higher temperature and density caused by the injection of particles, in combination with outflow of ionization.