The Time Delays in Reaction of the Ionosphere and the Earth’s Magnetic Field to the Solar Flares on 8 May and Geomagnetic Superstorm on 10 May 2024

Abstract

In the paper we consider the pulsed disturbances caused in the ionosphere by an extreme G5-level geomagnetic superstorm on 10 May 2024, and by the X1.0 and M-class solar flares on 8 May 2024, which preceded the storm. Particular attention is paid to the short-term delays and the sequence of disturbance appearance in the ionosphere and geomagnetic field during these extreme events. The results of a continuous Doppler sounding of the ionosphere on an inclined radio path with a sampling frequency of 25 Hz were used, as well as the data of a ground-based mid-latitude fluxgate magnetometer LEMI-008, and an induction magnetometer IMS-008, which operated with a sampling frequency of 66.6 Hz. Ionization of the ionosphere by the intense X-ray and extreme ultraviolet radiation of solar flares was accompanied by the equally sudden and similarly timed disturbances in the Doppler frequency shift (DFS) of the ionospheric signal, which had an amplitude of 2.0–5.8 Hz. The largest pulsed burst in DFS was registered 68 s after an X1.0 flare on 8 May 2024 at the time when the change of the X-ray flux was at its maximum. Following onto the effect in the ionosphere, a disturbance in the geomagnetic field appeared with a time delay of 35 s. This disturbance is a secondary one that arose as a consequence of the ionosphere response to the solar flare. It was likely driven by the contribution of ionospheric currents and electric fields, which modified the Earth’s magnetic field. On 10 May 2024, a G5-level geomagnetic superstorm with a sudden commencement triggered an impulsive reaction in the ionosphere. A response in DFS at the calculated reflection altitude of the sounding radio wave of 267.5 km was detected 58 s after the commencement of the storm. The sudden impulsive changes in Doppler frequencies showed a bipolar character, reflecting complex dynamic transformations in the ionosphere at the geomagnetic storm. Consequently, the DFS amplitude initially rose to 5.5 Hz over 86 s, and then its sharp drop to $-3.2$ Hz followed. Using the instruments that operated in a mode with a high temporal resolution allowed us to identify for the first time the impulsive nature of the ionospheric reaction, the time delays, and the sequence of disturbance appearances in the ionosphere and geomagnetic field in response to the X1.0 solar flare on 8 May 2024 as well as to the sudden commencement of the extreme G5-level geomagnetic storm on 10 May 2024.

1. Introduction

On 10–11 May 2024, an extreme G5-level geomagnetic storm took place, which attracted the attention of a wide range of researchers. Plenty of observations, from solar flares and geoeffective coronal mass ejections (CMEs) to the effects of the storm on the atmosphere, ionosphere, and geomagnetic field, were presented in comprehensive reviews and publications that appeared after this extraordinary event [1,2,3,4,5,6,7,8].

According to the GOES-16 satellite, several powerful flares of X1.0 class that occurred on 8 May 2024 were followed by the M8 class flares, at the time of which the intensity of X-ray and extreme ultraviolet (EUV) radiation from the Sun was suddenly increased [2]. The solar flares sharply increased the ionization in the ionosphere, which in turn changed the global electric fields and currents in the ionosphere, affecting its electrodynamics [9,10,11].

During the solar flares, there were several powerful CMEs that reached the Earth on 10 May 2024 and resulted in an extreme geomagnetic storm, which corresponded to the highest G5 level [12] since November 2003. It was a powerful super-intense geomagnetic storm with a peak $Dst$ value of $-412$ nT and the maximum $Kp$ index achieving $9.39$.

Arrival of the interplanetary shock wave caused a sharp compression of the Earth’s magnetosphere, which is a characteristic sign of a geomagnetic storm with sudden commencement (SSC). At the mid-latitude geomagnetic observatory of the Institute of Ionosphere “Almaty” (Kazakhstan, 43.176° N 76.951° E), this SSC was registered with an amplitude of 163 nT and a rise time of only 103 s, as it follows from the data of a LEMI-008 fluxgate magnetometer [13]. Compression of the magnetosphere under the influence of solar wind was so strong that the magnetopause did stay below the geostationary orbit ($6.6R_{\oplus }$) for 6 h [14].

The extreme event induced a significant influx of energy into the Earth’s upper atmosphere over the polar regions. Thermospheric heating on 10 May 2024 affected a multitude of interrelated processes in the Earth’s magnetosphere, ionosphere, and atmosphere [15,16]. Thus, immediately at the SSC on 10 May 2024, an ionospheric storm caused a negative effect all over across China, which was characterized by a significant and prolonged decrease in electron density, and it continued during the main phase and initial post-storm recovery phases on 11 May [3]. According to the observations carried out in Italy, a most noticeable ionospheric effect after the disturbance was a significant decrease in plasma density, which lead to a prominent negative ionospheric storm registered both by the critical frequency of the layer $F2$ ($foF2$) and by the total electron content (TEC) [2]. An interesting effect was revealed in the measurements of the vertical TEC and in the ionosonde observations made in Europe, the USA, and South Korea. It was shown that an initial response to the approaching CME on 10 May 2024 was a transient growth of the ionization in the upper atmosphere layer, which was followed by its rapid decrease at all latitudes. Next, the ionization level remained very low for more than a day [8].

The reduction in TEC, simultaneously on the entire dayside of the globe, correlates with the strength of the magnetic field during the geomagnetic storm, and for the mid-latitude ionosphere, this dependency corresponds to a negative correlation coefficient of at least $-0.8$ [17]. This effect is especially significant in SSC, when the delay in relation to the commencement of a magnetic storm is about 3–10 min. It should be noted that according to the statistics of the geomagnetic storms with gradual and sudden commencements acquired over five cycles of solar activity in 1950–2010, SSCs are three times more rare than the storms with gradual onset [18].

Even in their early studies, W. Appleton and M.A. Barnett noted the phenomenon of a delay in variation of the daily concentration of electrons in the ionosphere relative to the change in the flux of solar radiation [19]. Investigation of the ionosphere dynamics, in particular of the time delays between the peak X-ray flux at solar flares and the maximum of ionospheric response, was continued in [20,21,22,23].

To date, the question related to the time delays in the reaction of the ionosphere and magnetosphere to solar flares and geomagnetic storms requires further research. This is especially true when dealing with the fast impulsive ionospheric disturbances that follow a sudden impulse (SI) or SSC. Understanding these delays is important for the development of fundamental knowledge on the solar–terrestrial relationship.

In contrast to the mentioned works, our study is focused on the impulsive disturbances and delays in ionospheric response that were detected both at a sudden commencement of the geomagnetic storm and immediately at the arrival of the X-ray and EUV radiation from the solar flare to the Earth. This provides additional information on the dynamics of energy and particle transfer from the Sun to the Earth, revealing the complex mechanism of the interaction between different layers of the atmosphere and magnetosphere.

For studying the time delays, it is necessary to use equipment with a high time resolution and capacity of quick and continuous monitoring. Thus, for the registration of impulsive ionospheric disturbances, the method of Doppler sounding of the ionosphere in the decameter wavelength range is applied in the Institute of Ionosphere, while an induction magnetometer with a high time resolution is used to register the impulsive geomagnetic field.

This paper investigates the following topics:

-

The fast impulsive disturbances and time delays in the reaction of the ionosphere and the Earth’s magnetic field to the solar flares on 8 May 2024, which preceded a geomagnetic storm;

-

Peculiarities of the ionospheric response in the Doppler frequency shift (DFS) and establishment of its time delay relative to the sudden commencement of the extreme G5-level geomagnetic storm on 10 May 2024.

2. Method

2.1. Data Sources

Registration of the ULF electromagnetic signal in the frequency range of 0.0001–20 Hz was performed using an IMS-008 induction magnetic sensor developed in NIRFI (Nizhny Novgorod) in cooperation with VEGA Ltd. (St. Petersburg, Russia). The sensor operated with a sampling frequency of 66.6 Hz. The useful ULF signal was selected by a 12-order Chebyshev low-pass filter with a cutoff band of 20 Hz. For better rejecting the interference from the 50 Hz power main, a notch filter was additionally applied. The IMS-008 sensor is installed at an Geophysical Observatory which resides at the High-Mountain Station of the Institute of Ionosphere and the Tien Shan High-Mountain Research Station of the P. N. Lebedev Physical Institute, at a height of 3340 m above the sea level and far away from any sources of industrial noise.

The solar wind parameters measured by NASA’s Advanced Composition Explorer (ACE) spacecraft were taken at the website [24].

Information on the state of the magnetosphere and the level of the geomagnetic storm on 10 May 2024 was obtained from the website [12].

The data of the ESPQuad ultraviolet spectrometer on the intensity of the 0.1–7 nm solar radiation were received at the website of the NASA’s Extreme ultraviolet Variation Experiment (EVE) [25], and the data on the X-rays in the range of 0.5–10 keV (0.05–0.8 nm wavelength), as registered by the GOES-16 satellite, were taken at [26].

Per-second measurements of the geomagnetic field made with the LEMI-008 fluxgate magnetometer developed at the Lviv Center of the Space Research Institute, National Academy of Sciences and National Space Agency of Ukraine, are presented at the website of the Geomagnetic Observatory of the Institute of Ionosphere [27].

The data of the International Real-time Magnetic Observatory Network (INTERMAGNET) were obtained from the website [28].

2.2. The Doppler Sounding Equipment

Monitoring of the DFS variation of the ionospheric signal at a radio frequency of 7.245 MHz was performed on a 970 km long inclined radio path “Dushanbe-orzu—Institute of Ionosphere (Almaty)”. The scheme of the radio path is shown in Figure 1, where the projection of the reflection point of the sounding radio wave onto the Earth’s surface (the sub-ionospheric point) is indicated with a red circle.

For the Doppler measurements, a software–hardware complex was applied, which is based on the phase-locked loop (PLL) operation principle and implements the capability to measure the DFS of a larger amplitude beam in multipath receiving conditions. In [29], a detailed description is presented of the complex and the method of Doppler sounding at an inclined radio path with a continuous carrier together with the PLL system and a software defined radio receiver (SDR). A broadband, full-featured 14-bit SDR receiver of RSPdx type [30] was used, which covers the spectrum of radio frequencies from 1 kHz to 2 GHz, providing the spectrum width of up to 10 MHz. Choosing an optimal radio frequency for the night- and daytime was made according to the BBC Frequencies and Sites catalog [31].

2.3. Calculation of the Trajectory and Reflection Height of Sounding Radio Wave

The propagation trajectory of the sounding radio wave and the height of its reflection in the ionosphere were determined on the basis of the profile of electron concentration ($Ne$).

The calculation of the $Ne$ profile for the middle point of the radio path (Figure 2a) was fulfilled in accordance with the IRI2020 model immediately at the website [32]. The model IRI2020 works fine mainly for the undisturbed ionosphere. Correspondingly, for the day of 8 May 2024, the calculation was made for the time of 01:35 UT (06:35 local time), i.e., 2 min 47 s before the arrival of the solar X-ray and EUV radiation, when the ionosphere still remained undisturbed. Analogously, for 10 May 2024, the profile of electron concentration was calculated in the time of undisturbed ionosphere at 17:05 UT (22:05 LT), 1 min 02 s before the SSC. Selection of that time is connected with the coming of solar radiation into the ionosphere.

The reflection height of the sounding radio wave (Figure 2b) was determined using a program we developed especially for trajectory calculation. The calculation was performed using the profile of electron concentration ($Ne$), and incorporating the IGRF-14 geomagnetic field model [33]. Note, that the radio wave propagation trajectory was calculated on the basis of the permittivity formulas for the ordinary component of the geomagnetic field derived in [34].

As shown in Figure 2, the calculation of radio wave trajectories resulted in the estimates of the radio wave reflection height of 150.9 km and 267.5 km for the dates 8 and 10 May 2024, correspondingly.

3. Results

Using the method of DFS measurement as a sensitive indicator of ionospheric disturbance, we studied the sudden impulsive disturbances in the ionosphere that occurred at the G5-level geomagnetic storm on 10 May 2024 and during the solar flares preceding this extreme event. Also, we traced the appearance sequence and the time delay between the ionospheric and geomagnetic disturbances, which evidences the different mechanisms of energy transfer from the solar flares and the disturbed geomagnetic field to the ionosphere.

3.1. The Effect of Solar Flares on the Ionosphere

Although the bursts of radiation flux at solar flares manifest themselves in all diapasons of electromagnetic spectrum, from radio waves to X-rays [35,36], a main agent of the disturbing impact to the ionosphere is X-ray and EUV emission. Ionization of the D and E layers of the ionosphere is mostly determined by X-rays [37], while the extreme flux of EUV radiation covers all layers of the ionosphere in dependence on the wavelength $\lambda$, and at $\lambda =91-30$ nm, it reaches the layers $F1-F2$ at the altitudes above 120 km [38,39].

The response in DFS during solar flare depends on the variation rate of the electron concentration in the reflection region of radio wave in the ionosphere. On 8 May 2024, the GOES-16 satellite detected several powerful solar flares: two extreme flares of X1.0 class and a series of moderate M-class flares up to M8.6. During these events, the flux of X-ray and EUV emission from the Sun was sharply increasing [2,11].

Consider more precisely the flares which found a distinct response of the ionosphere in the DFS. Such events are presented in Figure 3 as a combination of the variation plots of X-rays, EUV radiation, and the DFS of the ionospheric signal registered during the solar flares on 8 May 2024, which preceded the extreme G5-level geomagnetic storm of 10 May 2024. Figure 3 clearly demonstrates the flares with sharp increases in the intensity of X-ray and EUV radiation, which were repeatedly occurring throughout the day of 8 May 2024. The periods of strong ionization of the ionosphere were accompanied by the equally sudden and similar in duration disturbances of the Doppler frequency with an order of 2–6 Hz.

The first class X1.0 X-ray flare, which started at 01:37 UT, produced the largest response in DFS with an amplitude of 5.8 Hz. The disturbance of Doppler frequency lasted about 660 s, which approximately corresponds to the duration of the burst of ionizing radiation in the X-ray and EUV spectrum ranges.

At 04:37 UT, the second X1.0 X-ray flare began, which had a more gradual and prolonged increasing phase of X-ray and EUV emission, and a slower return to the initial level. This time, the duration of the DFS disturbance was 7212 s. There was a noticeable absence of any clear peak in the DFS record, which indicates a prolonged deterioration in the passage of radio waves at the decameter frequencies range.

It is seen in Figure 3 that the Doppler ionosonde has distinctly detected the ionospheric disturbances also at a series of M-class flares. An exception was the M8.6 flare, which occurred in the evening time and was not reflected in the DFS. Previously, we demonstrated the possibility of using a PLL principle-based Doppler ionosonde for detection of the ionospheric response to even weaker C-class solar flares [40].

3.2. The Response of the Ionosphere in the DFS to the X1.0 Solar Flare

Already in early studies, E. V. Appleton and M. A. F. Barnett have discovered that the ionosphere does respond to disturbances, such as a change in solar activity, not instantly, but with a delay [41]. Consider more narrowly in this context an X1.0 solar flare that had commenced at 01:37:47 UT on 8 May 2024, as shown in Figure 3. The continuous monitoring of the DFS with a sampling frequency of 25 Hz in our experiment allowed us to trace with a per-second accuracy the delay in the response of the ionosphere to the X-ray and EUV bursts on 8 May 2024. This effect is clearly demonstrated in Figure 4. At that time, the reflection height of the sounding radio wave was of 150.9 km, as determined by the profile of electron concentration in the ionosphere (see Figure 2).

It is seen in the upper panel of Figure 4 that the delay of the ionospheric reaction reflected in the DFS (top panel) is equal to 68 s in relation to the beginning of the X-ray flash in the bottom plot and to 50 s relative to the rise of EUV emission in the middle panel. The largest increase in the Doppler shift with reaching its peak value occurred, when the changes rate of the X-ray intensity, and thus the rate of ionospheric ionization, were at their maximum. At the same time, the decline in the DFS amplitude has started against the background of still continuing growth of EUV radiation, as indicated with a vertical dotted line in Figure 4. Thus, according to the data of the DFS monitoring, it may be assumed that it was the X-ray radiation that made the main contribution to the ionization of the ionosphere during the considered X1.0 solar flare.

3.3. The Time Delay Between the Reaction of the Ionosphere and Geomagnetic Field to a X1.0 Solar Flare

While the direct impact of a solar flare on the ionosphere comes almost instantly [42], the subsequent reaction of the geomagnetic field is more complex and proceeds with a certain delay, which is associated with the different mechanisms of disturbance transmission from the ionosphere to the geomagnetic field, and with the role of the ionospheric current system in this interaction [43]. In the present study, the issue of time delays between the response of the ionosphere to the solar flare on 8 May 2024 and subsequent changes in the geomagnetic field was addressed based on a comparative analysis of the data of continuous DFS monitoring, the measurements with the IMS-008 induction magnetometer, and the data of the INTERMAGNET system of ground-based fluxgate magnetometers [28].

In

Table 1. The mid-latitude magnetometer stations of the INTERMAGNET network.

#	Geomagnetic Observatory	Geographical Coordinates	Time Zone	Local Solar Time	Presence of Effect
1	AAA (Almaty, Kazakhstan)	43.250° N 76.920° E	UT+5	6:46	+
2	ARS (Arti, Russia)	56.433° N 58.567° E	UT+5	5:34	+
3	NVS (Novosibirsk, Russia)	54.850° N 83.230° E	UT+7	7:11	+
4	IRT (Irkutsk, Russia)	52.170° N 104.450° E	UT+8	8:36	+
5	MMB (Memanbetsu, Japan)	43.910° N 144.190° E	UT+9	11:15	−
6	PET (Paratunka, Russia)	52.971° N 158.248° E	UT+12	12:11	−
7	HLP (Hel, Poland)	54.604° N 18.811° E	UT+2	2:49	−
8	LVV (Lviv, Ukraine)	49.900° N 23.750° E	UT+3	3:13	−

, the information is listed on the mid-latitude magnetometers INTERMAGNET for the time of the X1.0 class solar flare that occurred on 8 May 2024 at 01:37:47 UT. Mid-latitude magnetometers are of particular importance, since, although situated outside polar electrojets, they are sensitive enough to the changes in globally propagating ionospheric currents.

According to Table 1, the effects in the geomagnetic field were detected only when the solar flare happened to be on the daytime side of the Earth and in morning hours. Thus, geomagnetic disturbances were fixed between the moments 5:34 and 8:36 of local time at the points AAA, ARS, NVS, and IRT. In Figure 5, it is clearly seen that the violations of the horizontal X-component of the field in these points were arising nearly at the same time. Simultaneously, a geomagnetic disturbance was detected by the IMS-008 induction sensor, as shown in the bottom frame of Figure 5.

Taking into account that the Doppler shift is one of the most early indicators of an impact of solar flare on the ionosphere, we could consider in more detail the sequence and mechanism of energy transfer from the solar flare to the ionosphere and geomagnetic field.

As shown in Figure 6, an impulsive response to the solar flare did first appear in the ionosphere; then, 35 s later, the IMS-008 and LEMI-008 magnetometers detected a sharp positive increase of the magnetic field.

Thus, using the instruments, which operated in the mode with high time resolution, made it possible to trace the sequence and time delay of the response of the ionosphere and the Earth’s magnetic field to the extreme solar flare. As the increase in ionization caused by the X-ray and EUV radiation was accompanied by a change in the electron density of the ionosphere, that change, as a consequence, was registered in the DFS. We determined that at first, the disturbance in the ionosphere was realized 68 s after the impact of ionizing radiation from the solar flare, as seen in Figure 4. Next, with a delay of 35 s, the ionospheric effect was followed by a response in the geomagnetic field. The last is seemingly a consequence of an amplification of ionospheric currents, which results in the generation of additional fields superimposed on the basic stationary magnetic field of the Earth. The data obtained allowed us to conclude that the effect in the geomagnetic field was a secondary one in relation to the ionospheric disturbance.

3.4. The Geomagnetic Superstorm on 10 May 2024

On 10 May 2024, one of the strongest geomagnetic storms in the last 67 years has occurred, which reached the extreme G5-level, as confirmed by the $Dst$ value of $-412$ nT (see

Table 2. Five great geomagnetic storms since 1957 [7].

Date	Year	$\mathit{Dst}$, nT	Class
10–13 May 2024	2024	$-412$	Great
9–13 March 1989	1989	$-589$	Great
15–16 July 1959	1959	$-429$	Great
11 February 1958	1958	$-425$	Great
13–14 September 1957	1957	$-427$	Great

).

At 16:38 UT, 10 May 2024, the ACE spacecraft detected a sharp increase in the speed and energy of the solar wind, resulting in the formation of the front of an interplanetary shock wave that achieved the Earth’s magnetosphere. The geomagnetic superstorm began after arrival to the Earth of a cloud of cosmic plasma ejected from the Sun at the extreme solar flares of 8–9 May 2024. The storm immediately exceeded the G4 level and intensified itself up to the maximum G5 level. According to the space weather scale, the maximum level of geomagnetic activity remained over two days, which is when the global index of geomagnetic disturbance reached its highest values of $Kp=9$ (see Figure 7). The descent of geomagnetic activity finished on 13 May 2024 [44].

Geomagnetic disturbances can generate various types of waves (acoustic–gravity waves, Alfven waves) that, when propagating in the ionosphere, cause local changes of its parameters [45,46]. Figure 8 shows an ionospheric response to the geomagnetic superstorm of 10 May 2024 in the variation in the dynamic power spectrum of the DFS.

It is seen in Figure 8 that almost simultaneously with the change in the $Kp$ and $Dst$ indices characterizing the disturbance level of the geomagnetic field, medium-scale traveling ionospheric disturbances (TIDs) become visible in the dynamic power spectrum calculated for the DFS in an interval of time periods between 30 and 120 min. The maximum spectral power of TID was achieved on 10 May 2024, which was 142 min after the SSC. The reflection height of the sounding radio wave at the frequency of 7.245 MHz was 267.5 km, as calculated with the special program for the time of 17:05 UT and using the profile of the electron concentration in the ionosphere (see Figure 2).

As it is seen in the bottom panel of Figure 8, the geomagnetic storm on 10 May 2024 was so strong that the readings of the IMS-008 induction magnetometer went off the scale until 13 May 2024. Such effects are especially characteristic for SSC events.

3.5. Reaction of the Ionosphere to the Sudden Commencement of the Geomagnetic Storm on 10 May 2024

At the impact of the interplanetary shock wave on the Earth’s magnetosphere on 10 May 2024, both the IMS-008 and LEMI-008 magnetometers detected the spikes in the variation of the geomagnetic field, which corresponded to the sudden commencement of a magnetic storm. In the top panel of Figure 9, a sharp increase in the solar wind velocity is presented, which was fixed by the ACE spacecraft at 16:38 UT on 10 May 2024. According to the ACE data, in the moment when the solar wind reached the Earth’s magnetosphere, its speed in vicinity of the Earth was of 671.6 km·s⁻¹ [24]. In Figure 9, this moment is marked with a solid vertical line, and an appearance of the disturbances in the magnetometer data and in the DFS is distinctly visible after that time.

A high monitoring rate of the DFS variation allowed us to determine the delay of the response and the characteristics of the reaction of the ionosphere to the SSC on 10 May 2024. Consider in more detail the appearance sequence of the disturbances in the geomagnetic field and in the DFS of the ionospheric signal. For the purpose, a fragment of these disturbances from Figure 9 is shown in Figure 10 in an enlarged scale.

According to the data of the induction magnetometer IMS-008 operating with a high sampling frequency of 66.6 Hz, the sudden commencement of the storm was registered on 10 May at 17:06:02 UT. No more than 3–5 s later, the effect of SSC was detected by the fluxgate magnetometer LEMI-008 operating with a 1 Hz sampling frequency at the Geomagnetic Observatory of the Institute of Ionosphere. The rising time of the X-component to its maximum was 108 s, starting from the SSC. At the same time, the strength of the geomagnetic field increased from $-32$ to 162 nT [27].

Then, 58 s after the SSC moment, a reaction of the ionosphere was detected in the variation in the DFS at the height of 267.5 km, as calculated for the reflection of the sounding radio wave (see Figure 2). Over the course of 86 s, the DFS amplitude did first increase to 5.5 Hz; then, a sharp decrease followed to $-3.2$ Hz.

Also noteworthy is an appearance in the record of the induction magnetometer of a wave packet with the frequencies of 0.15–0.25 Hz, 30 s ahead of the response to the SSC. Similar effects in the form of a packet of oscillations in the frequency range of 0.2–7 Hz before a magnetic pulse were considered in [47] as a precursor to SSC.

Thus, the SSC on 10 May 2024 caused an impulsive reaction of the ionosphere. The DFS of the ionospheric signal was changing stepwise, and its variation had a bipolar shape: at first, there was an abrupt shift of the DFS toward higher and then toward lower frequencies, reflecting the complex nature of dynamic transformations in the ionosphere that followed the SSC.

4. Discussion and Conclusions

On 10 May 2024, an extreme G5-level geomagnetic storm has occurred, one of the strongest in the last 67 years. The storm commenced after coming to the Earth of a cloud of cosmic plasma thrown out the Sun at the extreme solar flares on 8–9 May 2024. Upon interaction of the interplanetary shock wave with the Earth’s magnetosphere, the ground-based magnetometers detected a sudden commencement of a geomagnetic storm. The storm has at once exceeded the G4 level and amplified up to the maximum G5 level, the planetary $Dst$ index dropped to $-412$ nT, and the $Kp$ index raised to a maximum of 9. Geomagnetic storms of such intensity are very rare events. The sluggishness of the ionosphere and the time lags in response to solar flares, as well as the ionospheric response to SSC, were the subject of a wide range of research studies [1,2,3,4,5,6,7,8].

We had a unique opportunity to focus on the study of the impulsive disturbances of the ionosphere and of the delays in the ionospheric response both to the SSC and directly to the arrival of the X-ray and EUV radiation from the solar flares to the Earth. Since the solar X-ray and EUV radiation induce instantaneous ionization increase and change the electron density in the ionosphere, those changes were clearly detected in the variation in the DFS. Particular attention was paid to the appearance sequence of the disturbances in the ionosphere and geomagnetic field at these extreme events, which required using complex equipment capable of high temporal resolution. This is why, for registration of the impulsive ionospheric disturbances, we used the Doppler method of the ionosphere sounding in the decameter range with a sampling frequency of 25 Hz. To register the pulsed geomagnetic disturbances, an induction magnetometer with a high time resolution and a sampling frequency of 66.6 Hz was applied. Such formulation of the goal and objectives of our study principally differs from the works [20,21,22,23], where the sluggishness of the ionosphere is investigated mainly through a comparison of the time delay between the peak of the X-ray flux and the maximum of ionospheric response. The studying of time delays gives additional knowledge on the processes of energy and particles transfer from the Sun to the Earth, thus disclosing particularities of complex interaction between the atmosphere, ionosphere and magnetosphere.

Summarizing the results of the current investigation, the following conclusions could be made:

1.

Ionization of the ionosphere by the extreme X-ray and EUV radiation of solar flares was accompanied by the sudden and similar in duration disturbances of Doppler frequency with an amplitude of 2–5.8 Hz. The greatest pulsed burst of the DFS was registered, with a 68 s long delay, during an X1.0 flare, when the changing rate of the X-ray flux was the highest, and the calculated reflection altitude of the sounding radio wave was 150.9 km. The duration of the disturbance in Doppler frequencies was about 660 s, and they generally corresponded to the length of the burst of ionizing radiation in the X-ray and EUV ranges of the electromagnetic spectrum.

2.

The disturbance in the geomagnetic field, as registered by the induction magnetometer, followed the ionospheric response to the solar flare with a 35 s delay. Seemingly, the reaction of the geomagnetic field is connected with an increase in the conductivity of the ionized medium in the ionosphere at the time of solar flare, and it originates from the contribution of ionospheric currents and electric fields that modify the Earth’s magnetic field. Thus, our data showed that the response of the geomagnetic field is secondary in relation to the ionospheric disturbance caused by the solar flare.

3.

On 10 May 2024, at 17:06:02 UT, the ground-based magnetometers IMS-008 and LEMI-008 registered the bursts in the variation of the magnetic field, which corresponded to a sudden storm commencement. The impulsive response of the ionosphere at the 267.5 km reflection height of the sounding radio wave was detected 58 s after the SSC moment.

4.

The Doppler frequency did vary suddenly, and its change had a bipolar character, reflecting complex dynamic transformations in the ionosphere at the SSC. During 86 s, the amplitude of the DFS was firstly rising up to $5.5$ Hz; then, its sharp decrease was followed by a decline down to $-3.2$ Hz.

5.

Practically simultaneously with an increase in the $Kp$ and $Dst$ indices, which characterize the disturbance level of the geomagnetic field, the Doppler ionosonde detected an appearance of the medium-scale TIDs in a 30–120 min interval of time periods. The maximum TIDs level was achieved at night on 10 May, 142 min after the SSC.

Using the instruments which operated in a mode with a high temporal resolution allowed us to identify for the first time the impulsive nature of the ionospheric reaction, the time delays, and the sequence of disturbance appearances in the ionosphere and geomagnetic field in response to an X1.0 solar flare on 8 May 2024 as well as to the sudden commencement of the extreme G5-level geomagnetic storm on 10 May 2024.

This research is planned to be continued during the 2026–2028 years in the frames of the Fundamental Scientific Research project funded by the Committee of Science of the Ministry of Science and Higher Education of the Republic of Kazakhstan.