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].

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.