Electrodynamic forcing of the Duskside ionosphere and the super fountain effect during the superstorm of May 10 to 11 2024

Abstract

During the extreme space weather event on May 10, 2024, a pronounced super-fountain effect was observed, driven by the prompt penetration electric field (PPEF) and pre-reversal enhancement (PRE) electric field. For this event, the SWARM-A satellite data provided unprecedented insights into the global impact of these electric fields due to its favorable orbit configuration which covered the local dusk sector. Observations revealed significant enhancements in the equatorial fountain, particularly over longitudes between $20^\circ$ E and 55$^\circ$ W. Notably, this study also detected the first in-situ observational evidence of electron temperature (T$_e$) increases in the evening equatorial ionosphere during an extreme geomagnetic storm, with T$_e$ enhancements aligning with the super-fountain effect’s extent. Additionally, westward electric fields were observed over a nearby longitude along with significant positive storm effects, highlighting the complex and localized nature of impact caused by extreme space weather.

Introduction

The response of the terrestrial thermosphere-ionosphere system to space weather events is determined by a complex and local time-dependent interaction of drivers showing significant variations, making it difficult to identify the processes involved. The equatorial fountain effect is known to be the basic mechanism for the formation of the equatorial ionization anomaly (EIA) [e.g^1,2,3], which is one of the most interesting large-scale features of Earth’s ionosphere. Since the electric field primarily drives this, it can become pronounced during geomagnetic storms when enhanced high-latitude electric fields in the eastward direction promptly penetrate to low latitudes^4,5,6. These electric fields lift the plasma to much higher altitudes than during quiet times, and hence the plasma diffuses along magnetic field lines to farther latitudes, often referred to as the super-fountain effect⁷. The super-fountain has been previously observed during Halloween storms⁸ and has been simulated using well-established models like the Thermosphere-Ionosphere-Mesosphere General Circulation Model (TIMEGCM) and the SAMI3 ionosphere model⁹. However, these studies were mainly confined to the daytime ionosphere. During the St. Patrick’s Day storm on 17 March 2015, the equatorial zonal electric field responses to prompt penetration of eastward electric fields (PPEF) to generate a rapid uplift of the equatorial F layer to more than 500 km and subsequent development of intense equatorial plasma bubbles (EPB) were observed¹⁰.

The pre-reversal enhancement (PRE) of the vertical plasma drift in the low-latitude ionosphere is a fundamental process in the terrestrial ionosphere^11,12,13, which affects the strength of the EIA. A strong eastward penetration of a high-latitude electric field during PRE would further strengthen it and hence cause an uplift of the F-layer, with increased vertical drifts. Along similar lines, the suppression of the EIA can be caused by a westward overshielding electric field even during the PRE period. However, these aspects are less studied compared to the daytime super-fountain effect.

In this work, we study the changes in the large-scale plasma structure of the Earth’s ionosphere during the initial phase of the extreme space weather event of May 10, 2024. The major factor contributing to the observed variability is the electric fields penetrating promptly from high to low latitudes, along with PRE presence during the space weather event. Multiple Coronal Mass Ejections (CMEs) were triggered by the intense activity from the region AR13664 on the Sun, which released a series of X-class flares and CMEs on May 8-9, 2024. The resulting geomagnetic storm was extremely intense, reaching NOAA G5 levels, with a minimum Dst index of approximately −412 nT. This storm is the most powerful since the Halloween storm of 2003 and has caused auroras even at much lower latitudes than usual. It may be noted that the extreme value theory calculations estimated the return periods for the May 2024 storm based on several geomagnetic indices and showed that while the storm magnitude was a 1 in 12.5 year event, its duration was a 1 in 41 year event¹⁴ which highlights the importance of this event.

Several aspects of this event have been investigated by both ground-based and space-based observationns. For instance, during this storm, the magnetopause location was modelled to be as low as 3.3$R_E$, and it was shown that that a unique combination of high Solar wind dynamic pressure with an intense eastward interplanetary electric field within a super-dense Interplanetary Coronal Mass Ejection led to the intensification of ring current much closer to the Earth causing such an intense storm, and even a new proton belt with energy 2-14 MeV was formed at low L-shells probably caused by solar proton penetration and subsequent sudden storm commencement^15,16. In this work, we foucs on the global dusk side response to this storm in terms of N$_e$, T$_e$, and TEC emphasizing on the electrodynamic effects. We show the effects of this event on the PRE electric field and the EIA structure, showing the super-fountain effect during the evening hours.

Results

Fig. 1

The temporal variations of the solar wind parameters from 9 - 13 May 2024 at Sun-Earth L1 point; (a) Solar wind speed (black) and Dynamic Pressure (orange), (b) Z component of the interplanetary magnetic field near Earth, (c) Y component of the interplanetary electric field. Time variability in the (d) Sym-H, (e) AL, and (f) AE index values for the same period.

Fig. 2

The latitudinal variation in (a) electron number density, (b) electron temperature observed by SWARM-LP, and (c) SWARM TEC from 9-12 May 2024. The Arrow in each panel marks the point where SWARM starts to see the impact of the storm just after its commencement.

Fig. 1 shows the solar wind speed, dynamic pressure, the southward component of the Interplanetary Magnetic field (IMF B$_z$), and Y-component of the Interplanetary Electric Field (IEF$_y$) measurements near Sun-Earth L1 point, the Sym-H as well as the AL and AE (representing the auroral electrojet) variations at Earth for the period 9-12 May 2024. The arrival of the interplanetary shock is seen around 16:45 UTC on 10 May 2014, with an enhancement in solar wind velocity and dynamic pressure. The magnitude of the IMF-B$_z$ component became almost −50 nT in the early hours of 11 May 2024. The IEF$_y$ recorded a maximum of more than 20 mV/m, and several fluctuations are observed. The consequence of this vis-à-vis ionospheric electric field fluctuations is discussed later. The maximum Sym-H value of $\sim$ −412 nT was observed around 03:00 UTC on 11 May. The AE and AL indices show intense auroral activity up to the early hours of 12 May.

Fig. 2 shows the observations by SWARM-A for the period 9-12 May 2024. Fig. 2a and b show the in-situ observations of N$_e$ and T$_e$ by the SWARM-LP, and Fig. 2c shows the TEC observations. Fig. 2a shows the typical low-latitude enhancement in electron density due to EIA crests. However, as SWARM starts to see the storm effect (marked by the arrow), the crests are farther apart and the trough densities are much lower. Close to 22:40 UTC on 10 May, we see the strongest fountain. Fig. 2b shows the corresponding T$_e$ variation. We do see that a significant enhancement in T$_e$ in the 30–60$^\circ$ latitude region almost coincides with the arrival of the storm, with a hemispheric asymmetry. The T$_e$ enhancement lasts even to the mid-hours of 12 May. Another striking observation is the enhanced T$_e$ observed along the region of the enhanced fountain. These are the first-of-its-kind observations of T$_e$ variation associated with the enhanced fountain effect. Fig. 2c shows the TEC variation observed by SWARM-A. Prior to the storm, the TEC showed the typical low-latitude enhancement but not the EIA crest and trough structure. Since the SWARM-TEC data contain information about the integrated electron content from about 450 km, it is above the region of the maximum EIA crest to trough ratio³. In the topside F-region, the crest amplitude decreases with altitude³, and hence, it is expected that the EIA feature might get averaged out in the topside integrated electron content. Unlike this, coinciding with the arrival of the storm, the latitudinal pattern of TEC also starts to show two crests separated by a low-TEC region near the magnetic equator, at longitudes where enhanced EIA is seen in electron density.

Fig. 3

The SWARM observations of the global maps of (a and d) electron number density, (b and e) electron temperature, (c and f) TEC variation, observed on 10 and 11 May 2024, respectively. The cyan dots in (a) and (d) mark the equatorial crossings at those longitudes; the corresponding UTCs are also given. The dotted curve represents the position of the geomagnetic equator.

Fig. 3 shows the global map of SWARM observations for the fixed local time 19 LT, on May 10, and early 11, 2024 (the dates are mentioned according to UTC here). Fig. 3a and b show the map of N$_e$ and T$_e$ by the SWARM-LP, and Fig. 3c shows the map of TEC variations on 10 May. Similarly, Fig. 3d and e show the map of N$_e$ and T$_e$ by the SWARM-LP, and Fig. 3f shows the map of TEC variations on 11 May. It can be seen from Fig. 3a that on 10 May 2024, at 00:45 UT, the equatorial crossing occurred near −82.8$^\circ$ longitude and the next evening orbit crossed the equator over −106.4$^\circ$ longitude at 02:19 UTC. The SWARM continues to orbit, and the first encounter with the storm effect happens near 17.53$^\circ$ longitude at 18:00 UTC on 10 May. It can be seen that the maximum fountain effect (super-fountain) is observed near −54$^\circ$ longitude, at around 22:42 UTC of 10th May (equatorial crossing time). This enhancement is associated with the eastward penetration electric field effect seen west of 20$^\circ$E longitude and maximizes near −55$^\circ$ longitude. It is clear that in the region where the eastward penetration electric field-related super-fountain effect is seen (from 20$^\circ$E longitude towards the west and maximizes near −55$^\circ$ longitude), the TEC itself shows a double-peak structure (Fig. 3c).

Compared to this, a much reduced fountain is seen near −76.8$^\circ$ longitude, at 00:16 UT, and near −100.5$^\circ$ longitude at 01:50 UTC on 11 May, which also corresponds to $\sim$ 19 LT (Fig. 3d). Over these regions, the N$_e$ values over the trough region are significantly high compared to the regions where the super-fountain occurred. The width of the trough region is also much less. There is an overall positive storm effect compared to the previous day, both in N$_e$ and in TEC (Fig. 3d and f). However, the crest-to-trough ratio is significantly low compared to that seen over −55$^\circ$ longitude.

The mid-latitude T$_e$ shows an enhancement after the arrival of the storm and is seen throughout May 11 (Fig. 3b and e). Apart from this, there is an anomalous enhancement in T$_e$ over the equator, where the super-fountain effect is observed (Fig. 3b).

Fig. 4

The latitudinal variations of (a) Ne and (b) TEC correspond to −54$^\circ$, and −76$^\circ$ longitudes. The TEC values are smoothed, mean vertical TECs and the shades show the standard deviations, (c) The variations in the Y component of IEF (red line) and SYM-H (cyan line), (d) the temporal variation of the estimated zonal electric field, both climatological contribution (Q) and climatological added with prompt penetration contribution (Q+P), at −54$^\circ$ and −74$^\circ$ longitudes, on 10 May 2024.

Figs. 4a and b show the N$_e$ and TEC variations corresponding to −54$^\circ$ (red line) and −76.8$^\circ$ (blue line) longitudes. The N$_e$ variation over −54$^\circ$ longitude shows a very deep trough, extending to more than 30$^\circ$ latitude and very high crests on either side. In contrast, over −76.8$^\circ$ longitude, the crest values are much higher (Fig. 4a). The overall positive storm effect is seen at both trough and crest regions over −76.8$^\circ$ longitude, whereas over −54$^\circ$ longitude, the equatorial densities are transported to low latitude regions due to the enhanced fountain effect. Compared to previous days, the trough density is decreased (also clearly seen in Figure 2 and 3), and hence the positive storm effect is not seen over the equatorial region where the super-fountain has occurred. Figs. 4b also shows a similar picture. Though in both cases, the double-humped structure is seen, the trough is deeper over −54$^\circ$ longitude. In contrast, the trough is narrower and shallower over −76.8$^\circ$ longitude (Fig. 4a). Figs. 4c show the IEF$_Y$ (c, red line) and SYM-H variations (c, cyan line, right Y-axis) for 10 May 2024. The IEF$_Y$ starts to show enhancement concurrent with the storm arrival, and remains positive and high from $\sim$ 20 to 22 UTC on May 10, with a peak value of $\sim$ 30 mV/m and subsequently reverses to negative between 22:30 and 23:45 UT on May 10. Figs. 4d show the estimated zonal electric field, both the climatological contribution (Q, blue line) and the climatological values added to the prompt-penetration contribution predicted by a transfer-function model ¹⁷ applied to the interplanetary electric field (Q+P), for −54$^\circ$ and −74$^\circ$ longitudes. It can be seen that over −54$^\circ$ longitude, the normal PRE electric field is enhanced due to the mapping of prompt penetration electric field, which leads to the super-fountain effect. The typical PRE peak should have been later (as indicated by the blue curve) compared to that corresponding to the total contribution, including PPEF (red curve). In similar lines, over −74$^\circ$ longitude, the PRE peak is inhibited by the westward overshielding electric field, the consequence being the enhanced trough densities at these longitudes. The entire duration of the eastward and westward electric fields do coincide with the IEF$_Y$ variation, as expected.

Discussion and conclusions

The super-fountain effect during the geomagnetic storm has been reported for the interplanetary electric field enhancement during 5–6 November 2001, corresponding to a storm arrival with minimum Dst of −275 nT⁷. Large dayside TEC enhancements occurred due to the penetration of the interplanetary dawn-to-dusk directed electric field, and TEC enhancements were observed at $\sim$ 10:30 LT and $\sim$15:00 LT, by spacecraft-based observations and near 14:00 LT by ground-based observations. They reported that the ‘super-fountain effect’ could have transported electrons from lower to higher magnetic latitudes. The enhanced fountain effect in the daytime during the ‘Halloween’ storms of October 29–30, 2003 was reported⁸. It was concluded that the enhanced electric field lifts the low latitude plasma upward due to enhanced $\vec {E}\times \vec {B}$ drift, (or the daytime super-fountain effect associated with the space weather event), so that a net increase of plasma density occurred above 400 km (Challenging Minisatellite Payload satellite altitude)⁸. However, they have studied only TEC observations from the region near the North American west coast, during daytime (12:30–13:30 LT).

For the present event, apart from SWARM-TEC, we also investigated the global maps of both N$_e$ and T$_e$ in-situ measurements by the SWARM-LP with fixed LT. The SWARM data have been widely used to study the impact of geomagnetic storms on the ionosphere^{18,19,20,21,22}. For this event, the SWARM observations coincided with the dusk local time sector, which hosts a very important process in the equatorial ionosphere, viz the pre-reversal enhancement, which provided a unique opportunity to observe the ‘pre-reversal and PPEF induced super-foutain’ effect in both TEC and N$_e$. It can be seen that over the region from 20$^\circ$ W, the enhanced fountain effect is seen, which maximizes near 55$^\circ$ W longitude (Fig. 3a). For this extreme storm, it has been shown using GOLD observations that the night time EIA crests between −70$^\circ$ and −35$^\circ$ longitudes were shifted poleward compared to quiet times, with northern and southern crests reaching $\sim$38$^\circ$ N and $\sim$35$^\circ$ S magnetic latitudes, during the strong IMF Bz and $\frac{d(Dst)}{dt}$ conditions on 10 May 2024²³. GOLD observations have also shown the more poleward location of the EIA crests, which indicate a super-fountain effect²³. They also used the in-situ N$_e$ measurements from Swarm-A satellite to estimate the speed of the poleward motion of the EIA to be $\sim$450 m/s near $\sim$55$^\circ$ W longitude²³. We also show that the PRE electric field is enhanced in this region, as seen in Fig. 4d, which has enhanced the vertical $\vec {E}\times \vec {B}$ drift, which transported plasma from lower to higher magnetic latitudes. This is evident in N$_e$ observations around $\sim$450 km (Fig. 4a and b). The presence of two well-defined crests separated by a deep and wide trough in the in-situ N$_e$ (Fig. 4a) observations clearly shows that the PPEF-related vertical drift during the PRE time could enhance the crest amplitudes in the topside ionosphere so that it could be seen even in the integrated electron content. This is clear evidence of the super-fountain effect. The peak eastward vertical drift observed by the Jicamarca radar was $\sim$ 95 m/s between 17:00 and 19:00 UTC (12:00 - 14:00 LT)²⁴. In the equatorial ionosphere, generally, $\sim$ 1 mV/m electric field corresponds to a drift in the range from about 33 to 52 m/s²⁵. A poleward motion of crest with a maximum velocity of $\sim$450 m/s, near 55$^\circ$W longitude, is reported which can be interpreted as a vertical drift near the magnetic equator with an electric field more than 8 mV/m²³. This can indeed generate the observed enhancement in the fountain effect. As mentioned, the SWARM EIA observations correspond to $\sim$ 22:42 UTC at −54$^\circ$ longitude. The first peak in the prompt penetration electric field enhancement occurred $\sim$ 2 hours prior to this, which has contributed to the observed poleward shift in the EIA crests, considering the typical transport time of plasma from the equator to EIA crests³. However, in this calculation, the poleward motion is completely ascribed to the electric field, whereas the asymmetry in the EIA crests indicates the possible wind effects.

As seen in Fig. 3 d-f, and in Fig. 4 a-b, during the PRE period the fountain is much reduced near the equatorial region of −76$^\circ$ (i.e near 00:00 UTC on 11 May) longitude regions compared to $\sim$ −55$^\circ$ W longitude, which is indicated by elevated N$_e$ at the trough and decreased and T$_e$, due to the prevailing westward overshielding electric field (Fig. 4 c). This is opposite to the eastward penetration observed just 20$^\circ$ away, which demonstrates that the space weather event can cause different effects in very nearby longitudes, especially over the equatorial region. For instance, the equatorial trough densities are much lower (negative storm-like) over the super-fountain region, whereas over −76$^\circ$ longitude, significant positive storm prevailed from the equator to mid-latitudes.

The effects of this storm observed by the Jicamarca (76.8$^\circ$ W) was also studied using incoherent scatter radar and associated instruments²⁴. It is shown that a strong westward penetration electric field suppressed pre-reversal enhancement, leading to downward plasma drift ($\sim$ −96 m/s) at around 24:00 UT (19:00 LT) over Jicamarca, which is basically caused by the over-shielding electric field. The observations show similar oscillations in IEF$_{Y}$ and vertical drift, which suggest the presence of penetration electric fields. The ionosonde and VTEC data over the American sector were analyzed for this event²⁶. A negative storm effect over the equatorial region, and a positive storm in the low-mid latitudes during the main phase (with GNSS TEC showing drastic enhancements of 50–100% in a few hours²⁷), accompanied by the uplift of the F-layer to altitudes exceeding 1024 km²⁶  indicative of enhanced vertical drifts.

Previous investigations also suggest that during intense geomagnetic storms, the penetration electric field effect dominates the main phase, followed by a combined effect of PPE and disturbance dynamo in the early recovery phase ²⁸. For this event, the storm’s main phase is characterized by two major SYM-H peaks on May 10, which are −183 nT at 19:21 UT and −354 nT at 23:12 UT, as well as a third peak of −518 nT at 02:14 UT on May 11²⁹. Therefore, though the observed hemispheric asymmetry in the EIA crest latitudes suggests an interplay between the electric field and meridional wind, we expect the dominant contribution to the enhanced (reduced) fountain effect is from eastward (westward) penetration of electric fields up to early UTs of May 11. However, the asymmetry in crests indicates that wind effects were possibly present. It is also shown that subsequently on May 11, counter electrojet (CEJ) was observed with a peak amplitude of −240 nT, which could be the combined effect of disturbance dynamo electric field (DDEF) and over-shielding²⁴. A negative storm effect was observed in GPS TEC over the American sector during the recovery phase on May 11 in daytime, probably due to O/N$_2$ ratio changes²⁶.

Another important point to be noted is that the electric fields generated by the model are much smaller than those estimated by other studies²³, and it does not consider the effects arising from the disturbance-dynamo electric field. There are also reports on the role of IMF-By component during the time when IMF-Bz is northward in aiding observed eastward PPEF effects over this sector³⁰. Therefore, the actual electric field magnitudes could have been much larger²³.

The F region T$_e$ is determined by the balance between the heating and cooling processes, which are governed by plasma and neutral density, neutral and ion temperatures, as geomagnetic conditions, and is a proxy for the changes in thermal electron energy sources and sinks³¹. Even on quiet days, the physical processes dominating the relation between N$_e$ and T$_e$ are very complex³². Though in general it is observed that T$_e$ shows anti-correlation with N$_e$, there are several studies which reveal positive correlation between the N$_e$ and T$_e$ in the F-region^33,34. Over EIA region also, the general anti-correlation between the N$_e$ and T$_e$ is mostly not evident³⁵. Moreover, the impact of space weather events on T$_e$ has not been well-studied compared to TEC and N$_e$, except for a few recent studies^36,37. For the present event, we see two important effects in T$_e$. Firstly, we see T$_e$ enhancements over the trough of EIA over the region where the super-fountain is prominent (Fig. 3b). These are the first in-situ observations of highly enhanced T$_e$in the evening equatorial topside ionosphere, during an extreme geomagnetic storm. The previous reports related to the super-fountain effect during the Halloween storms (e.g^7,8.), are mostly based on TEC observations. In this context, our exclusive report on in-situ T$_e$ and N$_e$ observations during this event provide additional new insight. Previously, it was shown that anomalous T$_e$ increases in the evening equatorial ionosphere over Jicamarca using radar observations³⁸. They have shown an event where the T$_e$ increase occurred during a moderate solar event and reached peak values of about 1000 K near 350 km and attributed it to the sharp reduction in plasma densities over the equator following large upward plasma drifts. Further, using simulations they showed that this is due to the decrease in the electron-ion cooling³⁸. In the present case, we show the latitudinal and longitudinal extent of the equatorial enhancement in T$_e$ for an extreme space weather event, for the first time. The latitudinal extent of T$_e$ enhancement is maximum near −55$^\circ$ longitude (Fig. 2b), where the super-fountain effect is seen to maximize. The second striking feature in T$_e$ is the enhancement seen coinciding with the storm, above $\sim$28-30$^\circ$ latitude in the northern hemisphere and $\sim$ 32-35$^\circ$ latitude in the southern hemisphere. The southern hemispheric enhancement is of shorter duration, while the northern hemispheric enhancement is observed even on 12 May (Fig. 2b). It may be noted that while this enhancement is seen from the beginning of the storm to the recovery phase, previous studies have shown enhancements in the T$_e$ in the afternoon sector during the recovery phase of the storm³⁷. This could have contributions from the auroral hot electrons as, during this superstorm, the auroral emissions were observed even at lower mid-latitudes^23,39. The heat transfer from the plasmasphere and magnetosphere are important in deciding the electron temperature enhancement. For instance, the overlap of the ring-current with the outer plasmasphere is thought to play a major role in the observations of storm-time related increases in the ambient topside electron temperatures⁴⁰. On May 10, 2024, SSUSI aboard DMSP (Figure 5 top panels) recorded significant changes in auroral emissions over the northern polar region during the main phase of intense geomagnetic storm. It is observed that in Pre-Storm Condition (17:00 UT), auroral activity was weak and concentrated near the magnetic pole, indicating minimal particle precipitation. During the main phase of the storm (20:30 UT and 22:15 UT) intense auroral emissions were observed, expanding equatorward. The low latitude red auroras are ascribed to the low energy electrons from the inner part of the plasma sheet precipitating⁴¹ at the top side of the ionosphere. This intensification and expansion of aurora suggests enhanced particle precipitation and increased Joule heating, leading to the possible heating of the upper atmosphere and enhancement in electron temperature. The enhancement in T$_e$ continues to even to pre-midnight sector⁴². The SWARM- B satellite observed medium scale plasma irregularities over 40$^\circ$ N–50$^\circ$ N latitudes over the Mediterranean sector, which were associated with small fluctuations inplasma density and the increase of the plasma temperature.

Fig. 5

(a)-(c) Auroral images recorded by SSUSI for 17:00 UT, 20:30 UT and 22:15 UT on 10 May 2024. (d) Auroral boundary derived using Field Alligned Currents (FAC) for northern hemisphere (e) The dual SWARM satellite-based FAC variation, observed from 9 to 13 May 2024.

Fig. 5d shows the equatorward boundary of aurora derived using Field Alligned Currents (FAC) for the northern hemisphere, whereas Fig. 5e shows the intensity of FACs during the storm period based on dual satellite observation of SWARM. These observations indicate the auroral boundary moved to the mid-low latitude region, almost reaching $45^\circ$ N geographic latitude, and the presence of intense FACs during the peak time of the storm on 11th May 2024. Satellites and ground-based observations have detected the aurora extending to mid and low geomagnetic latitudes during this storm²³. Moreover, the merging of the southern crest of the EIA and Aurora has been reported²³. The range of northern latitudes covered by the citizen science auroral reports spans from $32.43^\circ$($38.70^\circ$ MLAT) to $17.81^\circ$($25.99^\circ$ MLAT)^39,43, indicating the significant equatorward movement of the auroral boundary. However, it is also seen that the TEC over the region of T$_e$ enhancement is low compared to the pre-storm values (Fig. 2b). Therefore, this could also indicate the decreased heat conduction and collision cooling, which might have enhanced the T$_e$. However, the reduction in N$_e$ is less prominent compared to the enhancement in T$_e$. Another important process is the downward conduction of energy along the magnetic field lines from the plasmasphere, which might have been enhanced due to the storm. Using three GNSS receivers in the northern, and mid latitude parts of Europe and North Africa and ionosonde observations along with a plasmasphere model, the role of the plasmasphere, near the mid-latitude trough region has been studied for this event⁴⁴. It is shown that the erosion of the plasmasphere led to a plasmapause located at exceptionally low radial distances, probably favouring heat transfer. Both these processes might have contributed and could produce the observed effects, and there is a significant interhemispheric asymmetry in these processes.

The SWARM space-based measurements in the dawn-dusk orbital plane have made it possible to have unique observations of the evening time ionosphere during the 10 and 11 May 2024 extreme geomagnetic storm. The most important results are as follows:

• The PRE-time super-fountain effect was observed over the region westside of 20$^\circ$ longitude and maximized near −55$^\circ$ longitude. The presence of a deep and wide equatorial trough extending even to low latitudes is a clear indication of this.

• This region also showed an anomalous enhancement of equatorial T$_e$, which showed maximum latitudinal extent near −55$^\circ$ longitude. These are the first observations showing the full latitudinal and longitudinal extent of T$_e$ enhancement associated with a superstorm event for the dusk sector.

• The mid-latitude T$_e$ enhancements show strong hemispheric asymmetry and continued for more than a day.

• The aurora expanded to mid-low latitudes ($45^\circ$ N), coinciding with intense Field-Aligned Currents (FACs) at the storm’s peak on May 11, 2024.

• The effect of the westward overshielding electric field is seen near −76$^\circ$ longitude, with reduced fountain effect, as evident from the enhanced trough densities.

These observations open up several compelling questions about the mechanisms underlying extreme geomagnetic storm-induced T$_e$ enhancements, contrasting longitudinal variations in EIA, and pronounced hemispheric asymmetries, which need to be addressed by future modeling and observational studies.

Methods

The SWARM-Alpha (A) satellite orbits Earth in a near-polar orbit with an inclination of 87.5$^\circ$⁴⁵, at an altitude of about 450 km and the equator crossing times were $\sim$7:00 LT and 19:00 LT (close to the time of PRE in the ionosphere) during May 2024. We used in situ measurements of the electron density (N$_e$) and electron temperature (T$_e$) by the Langmuir Probe onboard SWARM-A. The Total Electron Content (TEC) measurements from SWARM-A are also used in this study. These contain information about the integrated electron content from the orbital altitude of SWARM to the altitude of the Global Navigation Satellite System (GNSS). The signals transmitted by the GNSS satellites are received by the receivers onboard the SWARM satellite and the integrated electron content along the line of sight is estimated from the observed delays. These are subsequently converted into Vertical TEC values. The details are available in the Swarm Level 2 Processing handbook https://earth.esa.int/eogateway/missions/swarm/product-data-handbook/level-2-product-definitions. The SWARM data used in this study are downloaded using the https://vires.services/ interface. The solar wind parameters, the geomagnetic SYM-H index, as the auroral AL and AE indices for the period 9-13 May 2024 are obtained from the NASA Space Physics Data Facility (SPDF) OMNIWeb data center (https://omniweb.gsfc.nasa.gov/). The Defense Meteorological Satellite Program (DMSP) satellites orbit Earth at an altitude of approximately 830 km. DMSP-F17 and 18 are equipped with the Special Sensor Ultraviolet Spectrographic Imager (SSUSI), these satellite image far-ultraviolet (FUV) emissions, in the Lyman-Birge-Hopfield short (LBHS) band emissions (140-150 nm). These emissions are indicative of auroral activity resulting from energetic particle precipitation in the upper atmosphere. The data were obtained from https://ssusi.jhuapl.edu. The equatorial ionospheric eastward electric field (EEF), including the penetration component, is estimated using the model provided in the https://geomag.colorado.edu/real-time-model-of-the-ionospheric-electric-fields. This model estimates the EEF for all longitudes, which uses a transfer function model to predict the EEF variation from solar wind data and a climatological model to account for the quiet day variations of EEF¹⁷.