Abstract

Geomagnetic storms are generally the main source of interplanetary and geomagnetic disturbances, constituting a major natural hazard due to their potential to damage technological and electrical systems, on which our society is heavily dependent. On June 22 and 23, 2015, our magnetic shield was impacted by the second strongest storm during the solar maximum of solar cycle 24, propelling a series of interplanetary coronal mass ejections (ICME_1 and ICME_2) towards Earth. Based on observational data derived from ground-based magnetometers (BOX and DRV) and spacecraft (WIND, ACE, SDO, and SOHO), we are investigating the high-latitude dynamics of the Earth’s magnetospheric cavity as it was impacted by this series of extreme events. Our results show that the events of June 22-23, 2015, associated with violent storms with strong and prolonged main phases, radically altered the behavior of the inner magnetosphere. While ICME_1 was characterized by fairly calm weather upstream, ICME_2 produced strong storm effects at high latitudes. In addition, analysis of data collected at one-minute intervals highlights that the dynamics of magnetospheric plasma correlate with solar wind intensity depending on the period and phase of the storm. In general, while it appears that the East/West directions of the E_M field are associated with the orientation of the IMF-B_z, E_M field variability becomes more pronounced and direct in polar regions when solar winds interact with the geomagnetic field. During the main storm phase, E_M field intensifies in the dawn-dusk sector of the Earth’s magnetosphere for a south-facing IMF-B_z, while a north-facing IMF-B_z orientation is associated with a weakening of E_M field in the dusk-dawn sector of the magnetospheric cavity during the storm recovery phase. The results presented in this paper are likely to be important for Global Navigation Satellite System (GNSS) and weather forecasting applications.

Keywords

Superstorms, Interplanetary Coronal Mass Ejections, Magnetosphere, Geomagnetic Field, Solar Wind

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1. Introduction

Nowadays, the existence of various phenomena such as solar flares, coronal mass ejections (CMEs), and geomagnetic storms helps to understand Sun-Earth events. From previous studies [1] [2], geomagnetic storms are the most important component of space weather impacts on Earth. Observations of solar events on the ground and in interplanetary space reveal that a geomagnetic storm can be considered as an event in which disturbances are triggered by solar flares. A geomagnetic storm is defined as a large-scale, abrupt disturbance of the Earth’s magnetic field caused by the interaction of intense solar winds with the Earth’s magnetosphere. Geomagnetic storms can also generate ionospheric storms. It is well known that one of the possible causes of these geomagnetic events lies in solar flares, which are huge explosions of harmful electromagnetic radiation from the Sun’s active regions. In general, solar flares occur when the Sun’s intense magnetic fields become too entangled. These powerful blasts of magnetic plasma erupted by the Sun travel at millions of kilometers per hour through interplanetary space, interacting with our planet’s protective magnetic shield: the Earth’s magnetosphere. Violent eruptions can physically affect humans on Earth [3]-[5]. They can also cause potentially fatal power outages, damage to technological systems such as satellites, radio communication failures, and navigation problems [2] [6]-[8]. These disturbances, whether natural or man-made, can occur in both calm and disturbed geomagnetic conditions. However, the most significant magnetospheric alterations are caused by geomagnetic storms/storms when the magnetosphere is strongly disturbed on a global scale.

Since 1859, violent flares that could wreak havoc on Earth had been discovered before other active processes on the Sun. However, disturbances of the solar wind and Earth’s magnetosphere were linked only to solar flares. Later, in the early 1970s, other powerful processes of solar origin, such as coronal mass ejections (CMEs), were discovered. However, [9] significantly changed the situation, and today there is a broad consensus that geomagnetic disturbances can be caused by several sources, including corotation interaction regions (CIRs), magnetic clouds, and CMEs (see, for example, [10] [11]). CMEs often accompany solar flares, although scientists are still trying to determine exactly what the undeniable link is between these two magnetic phenomena. It is well known that CMEs significantly control space weather [12]-[14]. From sources located in the Western Hemisphere, CMEs originating near the central meridian of our star and heading towards Earth are the most geo-effective. Given their significant influence on our terrestrial environment in a variety of ways, it would be very useful to look into space weather forecasting (geomagnetic storms, effects on navigation/communication systems, risks for astronauts, etc.). That said, solar and interplanetary events play an important role in understanding the complex solar wind/magnetosphere system. While it seems that magnetosphere and ionosphere interact crucially during strong geomagnetic storms [15]-[17], to our current knowledge, recent work on the nature of this interaction only addresses the ionized—ionospheric—part of the terrestrial environment (see, for example, [18]-[21]). In this study, our particular focus will be on the magnetospheric cavity during violent storms associated with CMEs.

CMEs are dynamically expelled and driven by coronal magnetic fields, which diminish as they pass through interplanetary space, where other processes can accelerate them. Thus, CME/solar wind interaction can provide the drag necessary for the acceleration/deceleration of CMEs as a function of their velocity, and the increase in this magnetic drag constitutes a major obstacle for spacecraft during their motion. Observations of CME by SOHO/LASCO and ground-based measurements of interplanetary CME (i.e., ICME) have highlighted the main causes of geomagnetic storms. If the counterparts of ICME have a significant meridional component of the interplanetary magnetic field (IMF), then after reaching the Earth’s magnetosphere, they can lead to geomagnetic storms [22] [23]. According to [24]-[26], a storm occurs after a very calm day preceded by a period of development during which the IMF turns southward (B_z < 0) with an increase in the energy density of the solar wind plasma near the Earth. Previous studies have classified storms according to the values of the horizontal component of the magnetic field (SYM/H) and their temporal variations in four geomagnetic conditions [27] [28]. More in-depth discussions of recent storm classification can be found in review articles by [29]-[33].

According to [34], geomagnetic disturbances due to the interaction between solar winds and the Earth’s magnetosphere are known to strongly influence electrodynamics during violent geomagnetic storms. The storm-time electrodynamics of the Earth’s magnetosphere are considerably altered compared to calm-weather behavior due to additional disturbances caused by magnetospheric convection electric fields (e.g., [35] [36]). During geomagnetic storms, the magnetospheric convection electric field—the interplanetary electric field mapped at high latitude—penetrates rapidly to the equator as a fast-penetrating electric field (PPEF) with eastward and westward polarities during the day and night, respectively (e.g., [35] [37]-[42]). Two mechanisms are responsible for the generation of electrodynamic effects on the magnetosphere during a thunderstorm. The first mechanism is PPEF penetrating electric fields [25] [40] [43]-[46] and the second, disruptive dynamoelectric fields [40] [45] [47]. According to several studies, such as [48], the main driver of storm weather effects in the upper ionosphere (i.e., magnetosphere) is the PPEF. PPEF is a local, transient disturbance, while a magnetospheric convection electric field (E_M) is a large-scale electric field resulting from the interaction of the solar wind with the Earth. The PPEF and E_M field are linked by the mutual influence of their effects on plasma and magnetospheric dynamics. During intense geomagnetic storms, the PPEF can disrupt and modify the E_M field. For example, a strong PPEF can lead to an increase in convection velocity, thus modifying the shape and dynamics of the magnetosphere. Conversely, the E_M field can influence the propagation and duration of PPEFs. The study of these phenomena is crucial to understanding the impact of geomagnetic storms and disturbances in the space environment. In this study, particular attention will be paid to the convective electric field that causes various types of global and local electrodynamic responses in the inner magnetosphere. Despite many efforts made in recent deployments of ground- and space-based instruments (see, for example, [18] [49]-[56], the behavior of the magnetosphere during violent storms is not yet well elucidated. The main challenge here is to present a global overview of the interplanetary electric/magnetic field conditions in order to understand the magnetospheric cavity response during the violent storm of June 22/23, 2015. Note that June 22/23, 2015, included a period of calm weather and a disturbed period associated with the arrival of two interplanetary shocks in the early morning and twilight of June 22. Clearly, the storm of June 22/23 2015 was not the strongest of solar cycle 24 compared with that of St. Patrick’s in March 2015, however, the choice of this storm is motivated by its notable effects on radio communications, its large amount of mass ejected by the solar corona, the intensity of its moderate to severe impact on the Earth’s magnetosphere, and so on.

In June 2015, a series of interplanetary coronal mass ejections (ICMEs) appeared to originate from the Sun’s active AR 2371 region (Figure 1(a)). These were subsequently recorded by NASA’s Advanced Composition Explorer (ACE) satellite, located on the Sun-Earth line at a distance of around 1% of the Earth-Sun distance, as abrupt enhancements in solar wind speed and density. Note also that there have been episodes of large-scale disturbances on the western side of AR 2371 (more precisely in the AR 2367 region) that have not had associated CME, probably because they were not large enough to become halos. The impact of these ICMEs on the Earth’s magnetosphere resulted in a violent G4-class geomagnetic storm (i.e., M6.5-class solar flare) on June 22/23, 2015, as shown in Figure 1(b). In addition to large auroras and a radio signal blackout in several Nordic countries, the June 2015 storm produced a significant compression of the upstream of the Earth’s magnetosphere, reducing it to 11 Earth radii R_T (1R_T = 6371 km). Although solar flares can be visible in white light, they are often more easily noticed thanks to their luminous X-ray and ultraviolet emissions. Fortunately, the unsuspected June 22/23 event lasted only a few hours and had only minor consequences on our planet Earth. This example is a reminder of how important it is to study the impact of geomagnetic storms so as to be able to prevent solar flares and, in this way, prepare for them to avoid planetary chaos in our societies, which have become highly dependent on electricity, radio exchanges, satellites, and so on. This article is organized as follows: first, the data set and methodology adopted are introduced in section 2, followed by the choice of empirical model organized according to the E_y electric field frozen in solar winds. Next, Section 3 describes our results and various interpretations. And finally, a conclusion is presented in Section 4.

(a) (b)

Figure 1. Flare and sunspot of June 22, 2015. (a) Sunspot AR2371. Credit: SDO/HMI; (b) M6.5-class solar flare on June 22, 2015, at 18:43 UT. Credit: NASA/SDO.

2. Data and Methodology

In this study, high-throughput data from solar events on June 22 and 23, 2015, include observations of solar wind variables and magnetic indices. On the one hand, geophysical and interplanetary (solar geocentric and magnetospheric coordinate system (GSM)) parameters from the WIND spacecraft orbiting the L1 Lagrange point are compiled by the space science community and made available via the OMNIWeb system. On the other hand, it is well known that ground-based magnetic observations offer excellent spatial and temporal coverage for statistical studies. Ground-based magnetic data are therefore crucial for a number of applications, including monitoring the evolution of geomagnetic storms and space weather. In addition to the geomagnetic indices (Dst, Kp, and AE) extracted from the International Magnetic Index Service ISGI “http://isgi.unistra.fr/”, definitive ground observation data “http://www.bcmt.fr/data_download.php” come from two magnetometers close to the Earth’s magnetic poles and located at high geomagnetic latitudes (latitudes > 50˚). Since key geomagnetic indices such as Dst and SYM/H are derived from low-latitude magnetometer stations, the choice of high latitudes (both BOX and DRV) is justified by the fact that the strong interactions of the solar wind/magnetosphere system are particularly evident at auroral latitudes. An overview of the location of the BOX and DRV observatories is shown in Figure 2. The BOX magnetometer is located in the Northern Hemisphere (58.07˚N, 38.23˚E) at Borok, Russia, and the DRV magnetometer is located in the Southern Hemisphere (66.665˚S, 140.007˚E) at Dumont-d’Urville on the Antarctic coast of France. The scalar FS and vector FV 1-minute cadence functions (values) of BOX and DRV are calculated from the 5-second numerical values using a minute-centered Gaussian function (http://www.bcmt.fr/). FV, due to convection currents in the Earth’s liquid outer core, is characterized by intensity, direction, and sense at each point in space, while FS is mainly represented by the total intensity of the geomagnetic field.

Figure 2. Observatory locations (source: taken from data catalogue N˚27 (2005), published by the World Data Center for Geomagnetism, Kyoto).

In addition, the high-latitude dynamics of magnetospheric plasma have been interpreted via the frozen electric field E_y in solar winds. E_y (E_y = −V_sw × B_z) is the main factor determining the structure of the inner magnetospheric electric field and the associated plasma convection processes. In this study, the magnetospheric convection electric field E_M (E_M = 0.13E_y + 0.09) was determined by the E_y electric field transformation law frozen in the solar wind. For more details on the determination of E_M field, please refer to [57] [58]. During geomagnetic storms, E_M field plays a crucial role in transporting charged particles (from solar wind) from magnetosphere’s tail to the front and injecting energy, which can cause significant disturbances. Because of the persistent statistical approach adopted in this manuscript, each geophysical and interplanetary data item is carefully examined to remove any aberrations related to the surprisingly large discrepancies. Only cases where solar and magnetic parameters were available simultaneously were taken into account in this study, i.e., 87% of the raw data for the period June 22-23, 2015.

3. Results

3.1. Geomagnetic Storms of June 22 and 23, 2015

In this section, we discuss the time-corrected variations over one-minute cadences of various interplanetary plasma parameters of the unusual event dated June 22 to 23, 2015, during the maximum phase of solar cycle 24. Indeed, from June 22 (white background in Figure 3) to June 23 (orange-ish background in Figure 3) of 2015, a series of geomagnetic storms was broadcast by large polar aurorae, causing a radio signal blackout on Earth. Mainly caused by solar flares, these extraordinary events eject charged particles into space in the form of coronal mass ejections (CMEs) or their counterpart, interplanetary coronal mass ejections (ICMEs). As ICMEs cross the Earth’s magnetosphere and interact with the solar wind in the background [59] [60], it would be necessary to monitor the evolution of solar wind parameters and magnetic indices. Panels (a)-(c) in Figure 3 show temporal variability in 1-minute steps of near-Earth solar wind parameters in Geocentric Solar Magnetospheric (GSM) coordinates at L1 Lagrangian point and magnetic indices from June 22 to 23, 2015. These panels show how the onset of an intense geomagnetic storm took place in the early morning of June 22, when two asymmetrical full-halo interplanetary shocks struck Earth’s magnetosphere, and how magnetospheric conditions were abruptly altered. According to Figure 3(a), BOX (in the Northern Hemisphere) and DRV (in the Southern Hemisphere) magnetometers on the ground recorded a sudden jump in the magnetic field (Storm Sudden Commencement: SSC) that heralded the arrival of two interplanetary shocks (IS) on Earth at around 05:45 UT and 18:40 UT on June 22, 2015. [61] report that these IS were associated with a series of interplanetary coronal mass ejections (ICME_1 and ICME_2). According to Figure 3(b), these arrivals caused concomitant increases in dynamic pressure (red curve) and solar wind velocities (black curve). As a result of these synchronized increases across the shock, the ram pressure of the solar wind exerted on the Earth’s magnetosphere abruptly compressed the upstream magnetosphere while generating eastward magnetopause electric currents and positive jumps in the horizontal component of the Earth’s magnetic field (SYM/H), as can be seen in Figure 3(d), red curve. It is well known that magnetic field variations observed on the ground are highly dependent on the latitude of the magnetic observatory. Low-latitude observatories clearly see the SSC and main phase of a magnetic storm. On the other hand, those at high latitudes see the first SSC, followed by a series of substorms. Figure 3(a) shows the difference in vector FV and scalar FS values of the total intensity of the Earth’s magnetic field from BOX and DRV located at high latitudes 58.07˚North and 66.67˚South, respectively. FV-FS difference shows that the event of June 22/23, 2015 was felt more on the northern side of the hemisphere, with a not sufficiently clear response (i.e., lower amplitude) from DRV and a latency of around 62 min from BOX to the arrival of a second ICME (ICME_2). These findings suggest that the source of this series of solar flares is located in the Northern Hemisphere.

During the first event (ICME_1), B_z component of the interplanetary magnetic field (IMF-B_z) shifted from north to south, with peaks of 8.99 nT and −9.77 nT, respectively (Figure 3(c), black curve). On average, the dynamic pressure of solar flux was only 7.21 nPa, with a corresponding 12% increase (381 km/s to 427.1 km/s) in velocity. While the E_y electric field frozen in solar wind showed small variations upstream of ICME_1 (Figure 3(c), red curve), dynamic pressure (Figure 3(b), red curve), auroral activity characterized by AE index (Figure 3(d), grey bars) and SYM/H index (Figure 3(d), red curve), all of almost linear characteristics in constant evolution, fluctuated respectively with vigors less than 0.24 Pa, 13.09 nT and 1.07 nT from their mean. However, downstream of this event, the quantities in question showed more moderate variations.

Figure 3. Variation in interplanetary and geophysical parameters during the solar events of June 22-23, 2015. (a) FS scalar and FV vector functions of the ground magnetic field; (b) Velocity (V_sw) and pressure (P_sw) of the solar wind; (c) Interplanetary magnetic and electric fields; (d) Auroral activity index (AE) and north-south component of the geomagnetic field (IFM-B_z). Dotted vertical lines indicate the arrival times of the two interplanetary coronal mass ejections (ICME_1 and ICME_2) that struck the Earth.

Furthermore, on the same date (i.e., June 22, 2015), a second event (ICME_2) was observed at 18:40 UT with significant storm-time changes in solar wind parameters resulting in a positive disturbance of about 88 nT in SYM/H. The event began with a fairly intense IMF-B_z with a predominantly southerly orientation (for around 90 min) of minimum intensity −38.98 nT at 19:23 UT, which is the most negative excursion measured in 2015, before turning sharply northwards. This event is accompanied by a remarkable ~64% variation in solar wind speed and ∆P = 55.75 nPa increase in dynamic pressure to the southwest. This sudden increase in solar wind pressure compresses Earth’s magnetosphere, producing SSC: this is the initial phase of the geomagnetic storm. This increase, according to [62] and [63], is caused by a temporal storm penetration electric field. Downstream of ICME_2, while solar flux dynamic pressure dropped from 92% of its initial value on June 23 at 03:49 UT to remain constant in the ±5.2 nPa range over the rest of the time, solar particle velocity slowly peaks at 781.8 km/s at 03:56 TU, then slowly decays to 516.8 km/s at 13:07 TU. IMF-B_z underwent various storm effects: that is, multiple positive (North) and negative (South) orientations between 01:30 UT and 12:00 UT on June 23, 2015. We consider this effect in our results to be due to the effect of rapid penetration of interplanetary electric fields IEF E_y, which is positive on the day side and negative on the night side (see, for example, [64]). Southern orientations of IMF-B_z (B_z < 0) confirmed by the ACE spacecraft led to an interconnection between IMF and geomagnetic field lines. The consequence of this interconnection was the significant depression to −208 nT of the SYM/H geomagnetic index around 04:24 UT. This intense storm episode in June 2015 caused spectacular magnetospheric (and therefore ionospheric, since the two systems are closely coupled) variability, which impacted global navigation satellite systems (GNSS) and degraded the performance of the European geostationary overlay navigation service [19] [48] [65]-[69]. This shows how magnetospheric electrodynamics contributes to improving GNSS performance (e.g., attenuation of ionospheric scintillation) or space weather forecasting models [70]-[72].

Figure 4. Hourly average solar wind velocities from June 22 to 23, 2015.

Additionally, while the AE index reached the limit of 1298 nT at 07:24 TU downstream of ICME_1, AE exceeded 2000 nT during ICME_2 (Figure 3(d), grey bar). Similarly, solar flux velocity amplitudes during ICME_1 and ICME_2 on June 22 were less significant than those downstream of ICME_2 on June 23, as shown in Figure 4. Indeed, in Figure 4, by setting the bar at 450 km/s, which would be the minimum amplitude of fast solar wind speeds [60] [73]-[75], we can see that 28.60% (June 22) of the solar flux versus 92.44% (June 23) have minimum amplitudes of 450 km/s. As a result, variations in electrojet index AE (Figure 3(d), grey bar) show that the storm was accompanied by fairly intense auroral activity associated with strong fluctuations between −21.09 and +26.57 mV/m in IEF E_y. According to [43] [76] [77], such amplitudes are comparable to extreme storm values. In addition, [78] shows that large IEF E_y values are responsible for violent and intense geomagnetic storms. Moreover, with a predominantly southerly orientation of the IMF-B_z accompanied by strong solar flux surges associated with oscillatory behavior of auroral activity AE reaching 2698 nT on June 22, the series of storms observed on June 22/23, 2015 remains the second most important storm in the 24^th solar cycle after the St. Patrick’s storm that occurred in March 2015 [18] [61] [79]-[83].

3.2. Interplanetary Conditions and Magnetospheric Convection

When ICMEs arrive and interact with the Earth’s magnetic field, a series of mechanisms occur in the Earth’s magnetosphere, leading to the disturbances observed in the geomagnetic field and magnetosphere [45] [69] [84] [85]. The importance of these mechanisms varies from case to case and phase to phase of a geomagnetic storm. The magnetospheric convection electric field, noted here as E_M [mV/m], is one of the mechanisms causing these strong disturbances, leading to the formation of complex magnetospheric structures, such as the magnetic tail and the aurora borealis. Details of these disturbances are examined in this section. In fact, solar cycle 24 (January 2008-December 2019) saw fewer sunspots than average, but major solar events did occur. For example, as shown in Figure 5, on June 22, 2015, two solar corona ejections (ICME_1 and ICME_2) bombarded the Earth’s protective cavity. Ring current and response of high-latitude geomagnetic activity, as represented by SYM/H indices and AE auroral electrojet, were discussed in the previous section. Examination of Figure 3(b) showed a sudden increase in V_sw velocity associated with a sharp jump in plasma dynamic pressure P_sw at the shock level at ~18:40 UT. At the same time, flux density (Figure 5(a)) increased drastically by ~41% at the sheath (downstream of the shock), and IMF-B_z turned sharply southwards. This instant of abrupt change marks the initial phase of a magnetic storm associated with a sudden decrease in the magnetospheric convective electric field E_M (Figure 5(b)) due to the northern rotation of the IMF-B_z [40] [86] [87].

Immediately after ICME_2 at 18:40 UT on June 22 marked by an exceptional SSC with an amplitude of SYM/H = ~2084 nT, while the auroral electrojet continued to progress in a strong increase, a major geomagnetic storm began. During its main phase, storm underwent rapid development in two stages: a first moderate peak was recorded on June 22 at 20:17 UT with an intensity of SYM/H = −138

Figure 5. Variation of solar and magnetic parameters during the solar events of June 22/23, 2015: (a) Solar flux density; (b) Magnetospheric convection electric field; (c) Magnetic indices Dst and interplanetary Kp.

nT; and a second peak of SYM/H = −208 nT was observed at 04:24 UT the following day (see Figure 3(d), red curve). We believe that two stages in the main phase of the storm are justified by the fact that magnetic fields were oriented towards the south both in the sheath for ICME_1 (B_z very close to 0) and in the solar ejecta for ICME_2. According to several studies [88]-[91], such events are quite common and are caused by two southward-facing field events of roughly equal strength, with the Dst base of the second event (i.e., ICME_2) much lower than that of the first (ICME_1). During this main phase on June 22, a decrease in the horizontal component of the geomagnetic field (SYM/H) was observed. This decrease implies an intensification of the ring current, which controls the electric field inside the magnetosphere at dawn and dusk: magnetospheric convection electric field E_M. Figure 3(c), red curve, and Figure 5(b), black curve, show large fluctuations in E_y electric field frozen in solar winds and in E_M field at the beginning of storm’s initial phase. These fluctuations become even more pronounced during the storm’s main phase. Increasing E_M field fluctuations are clearly associated with negative Dst indices (Figure 5(c), red curve). Indeed, as the Dst index and horizontal component of the Earth’s magnetic field SYM/H become more negative during the main phase of the magnetic storm, the convective electric field E_M becomes more fluctuating. BOX magnetometer (Figure 3(a), red curve) is more sensitive to these fluctuations than DRV (Figure 3(a), black curve). As BOX is at high latitude (close to the Earth’s magnetic poles, where the lines are oriented vertically), BOX consequently shows more pronounced effects, indicating greater sensitivity of high-latitude regions to solar wind interactions. We can therefore suggest that during the main phase of geomagnetic storms, the variability E_M field becomes more pronounced and direct in high-latitude regions as solar winds interact with the Earth’s magnetic field. [92] reveals that E_M field varies as a function of magnetic latitude, local magnetic time, IMF orientation, and storm season. According to [93], the strongest manifestations of solar wind/magnetosphere system interactions are particularly evident at auroral and polar latitudes. It is important to note that the storm’s main phase lasted almost half a day and is characterized by significant electrodynamic effects. The second peak of SYM/H = −208 nT ends the main phase of the storm [18] [61] [80] [94] [95].

Furthermore, the recovery phase (the longest phase of all) of the storm, characterized by an increase in SYM/H due to magnetospheric currents associated with a southerly orientation of IMF-B_z, began on June 23 at 04:25 UT and then progressed slowly over the rest of the day. This slowness is due to the fact that all solar wind conditions were constant during June 23, 2015 (see, for example, [19]). Note that IMF-B_z turned north ~01:15 UT after the start of the recovery phase. Earlier estimates by [96] indicated delays of ≈1 - 2 hours. Just before the start of the recovery phase, there is a rapid decrease in E_M field (Figure 5(b)) due to the northward orientation or weakening of the southern component of the IMF-B_z (Figure 3(c), black curve). This result is consistent with previous observations [40] [97]. Inversion of IMF-B_z (and therefore of the E_M field since both evolve in phase opposition, [98]) has been identified as an over-shielding electric field [99]-[101]. During the recovery period, over-shielding of the E_M field is driven by the northward orientation of IMF-B_z (e.g., [40] [43]). An important E_M field remark must be made about the dawn-dusk sectors downstream of ICME_2. Indeed, according to Figure 3(c) and Figure 5(b), E_M field intensifies in the dawn and dusk sectors in the inner magnetosphere on June 23 for a south-facing IMF-B_z. However, for a north-facing IMF-B_z, E_M field intensity weakens around 04:20 UT and 04:30 UT, marking a clear difference between the main and recovery phases of the storm. According to [102], when E_M suddenly increases due to a southward rotation of IMF-B_z, the result will be a dawn-to-dusk E_M field in the inner magnetosphere. On the other hand, if IMF-B_z rotates northwards, convection diminishes, and there is suddenly a reversed E_M field (from dusk to dawn). [103] and [104] have shown that in the dawn sector, E_M field turns westwards at night, while in the dusk sector, it turns eastwards during the day. While the westward E_M field in the midnight sector was reported by [105], we report here that the eastward and westward E_M field is associated with the orientation of IMF-B_z. More clearly, East/West orientations of the dawn-dusk/dusk-dawn electric field during the day/night are associated with a sudden intensification/decrease of the E_M field due to South/North rotations of the IMF-B_z. Note that during the storm’s recovery period, several peaks in auroral activity are evident, with its maximum value of around 1795 nT reached around 12:15 UT, as can be seen in Figure 3(d), grey bar. However, after the arrival of ICME_2, there was a notable increase in auroral activity, reaching its highest point of ~2698 nT on June 22 at 20:09 UT. This increase is due to the North/South fluctuations of IMF-B_z, indicating sub-storm activity. The presence of two peaks in the AE index indicates that the most intense auroral activity occurred after ICME_2. Intensification of AE suggests a strong, rapid penetration of the auroral electric field towards low latitudes due to substorm activity during the recorded geomagnetic storm. Magnetic substorming occurs when ICME/magnetosphere interaction leads to repeated cycles of magnetic field buildup in magnetotail lobes, cross-current enhancement, and disruption [106] [107].

Moreover, analysis of Figure 3(b) shows that ICME_2 caused very large velocity amplitudes in the solar wind on June 23. According to Figure 5(b), these strong amplitudes caused an increase in magnetospheric plasma convection, thus enhancing the SSC signature [108]. Solar wind has an average velocity of 625.80 km/s, and the embedded magnetic field is around 1.57 nT. IMF-B_z intensity of 1.92 nT was predominantly meridional. [19] [109] [110] have proven that the magnetic storm triggered on June 22, 2015 was major. During its main phase, magnetic (B_z) and electric (E_y) fields exhibited very high amplitudes. [96] describes these high magnetic fields as components of fast ICMEs (speeds > 500 km·s⁻¹) originating from the Sun. Strong electric fields within Earth’s magnetosphere also provide evidence of strong magnetospheric plasma convection, as can be seen both in Figure 5(b) and in [111]. These interplanetary signatures are responsible for sharp decreases in the Dst index (Figure 5(c), red line). As a reminder, the Dst index provides an overview of the effectivity of magnetic storms while excluding auroral phenomena [112]. In this study, Dst recorded observations similar to those of the SYM/H component (Figure 3(d) and Figure 5(c), red curves) with a high correlation of 97.79%. Although it seems that Dst and SYM/H are calculated in similar ways, these two indices are totally different in terms of the number of stations used and their temporal resolution. The high correlation found here shows that their meteorological phenomena are not dissociated, as both measure geomagnetic field variations due to ring currents. This storm had a −Dst maximum of 198 nT from 04:00 UT to 04:59 UT, achieved on June 23, 2015. The high intensity (198 nT) of the storm can be explained by the two stages of development during the main phase discussed above. According to Figure 5(c), the magnetic activity index Kp reached a value of 8, i.e., one less than the maximum, which is 9 on the logarithmic scale. Consequently, the June 22-23 storm would be slightly smaller than the St. Patrick’s Day storm of March 17, 2015.

4. Conclusion

During the filamentary eruptions of June 22 and 23, 2015, one-minute cadence data from ground-based magnetometers and satellite observations were analyzed in this manuscript to further our understanding of the response of Earth’s magnetosphere to violent geomagnetic storms. Several interesting features were recorded when two interplanetary coronal mass ejections ICMEs (ICME_1 and ICME_2) bombarded the Earth at 05:45 UT and 18:40 UT on June 22. While it appears that the dynamics of the Earth’s magnetosphere were significantly affected by the interaction with the selected ICMEs, ICME_2 showed more significant effects on the magnetosphere on both the day and night sides. Analysis of the one-minute cadence data highlighted the important role played by electric field orientations associated with ICME-B_z polarities during the period/phase of extreme storms. Thus, the East/West directions of the dawn-dusk/dusk-dawn electric field during the day/night are associated with a sudden intensification/decrease of the convection electric field due to the South/North rotations of the IMF-B_z. While the main phase of the storm is characterized by an electric convection field that becomes strong and positive towards the east during the daytime hours and towards the west during the night hours, the recovery phase is highlighted by a negative component of the electric convection field towards the west during the day and towards the east during the night. The configuration of the magnetospheric convection electric field during the main phase of the storm could be related to the established dawn-dusk convection cycle. Between the two storm phases, our results show a transition characterized by a weakening of the magnetospheric electric field. However, some complex areas remain to be studied, where the same weaknesses in the electric convection field have been observed but do not seem to mark any transition, which could be one of the perspectives of this study. The results obtained in this work are important because, primarily, geomagnetic storms modulate ionospheric electron density and affect the global atmospheric electrical circuit, which has an impact on radio signal propagation. Furthermore, this study could contribute to refining space weather models by improving our understanding of the influence of electric fields on the magnetosphere during geomagnetic storms. However, the main limitation of this work lies in the small sample size considered or the particular structure of this event with two ICMEs. This limitation makes it difficult to generalize our results to other studies. We believe that extending the sampling of severe storms to a larger scale could yield more meaningful results for meteorological decision-making purposes.

Acknowledgements

The authors would like to thank OMNIWeb “https://omniweb.gsfc.nasa.gov/ow_min.html”, ISGI “http://isgi.unistra.fr/”, and BCMT “http://www.bcmt.fr/data_download.php” for the data.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

References