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One of the most intense geomagnetic storms of recent times occurred on 10–11 May 2024. With a peak negative excursion of Sym‐H below −500 nT, this storm is the second largest of the space era. Solar wind energy transferred through radiation and mass coupling affected the entire Geospace. Our study revealed that the dayside magnetopause was compressed below the geostationary orbit (6.6 RE) for continuously ∼6 hr due to strong Solar Wind Dynamic Pressure (SWDP). Tremendous compression pushed the bow‐shock also to below the geostationary orbit for a few minutes. Magnetohydrodynamic models suggest that the magnetopause location could be as low as 3.3RE. We show that a unique combination of high SWDP (≥15 nPa) with an intense eastward interplanetary electric field (IEFY ≥ 2.5 mV/m) within a super‐dense Interplanetary Coronal Mass Ejection lasted for 409 min–is the key factor that led to the strong ring current at much closer to the Earth causing such an intense storm. Severe electrodynamic disturbances led to a strong positive ionospheric storm with more than 100% increase in dayside ionospheric Total Electron Content (TEC), affecting GPS positioning/navigation. Further, an HF radio blackout was found to occur in the 2–12 MHz frequency band due to strong D‐ and E‐region ionization resulting from a solar flare prior to this storm. Plain Language Summary: Life and a habitable atmosphere are sustained on Earth thanks to the protective and far‐stretched magnetic shield around the Earth, known as the Magnetosphere, which protecting the humanosphere from the hazardous solar wind particles that are continuously emanated from the Sun. However, massive solar wind ejections from the Sun often disturb the Earth's magnetosphere and affect mankind and critical space‐ and ground‐based technological infrastructure. When higher amounts of the solar wind traveling with supersonic speeds impact, the Earth's magnetosphere compresses significantly, and the crucial satellites in space become directly exposed to hazardous solar wind. Further, strong and rapid disturbances in the geomagnetic field and ionosphere could significantly affect the operations of various technical systems on ground, like electrical power grids, GNSS‐based precise navigation, etc. The strongest geomagnetic storm of the past three decades has recently occurred on 10–11 May 2024. This study investigates the solar sources and the possible mechanisms responsible for the occurrence of such an intense geomagnetic storm and also details the drastic reconfiguration of Earth's magnetosphere and significant ionospheric disturbances observed during this storm. Key Points: Strong SWDP caused a highly compressed magnetosphere with magnetopause pushed below geostationary orbit (6.6 RE) continuously for 6 hrThe highly compressed magnetosphere led an intense ring current at much closer distance to the EarthA super‐dense ICME with high SWDP (>15 nPa) and IEFY (>2.5 mV/m) sustaining for 409 min ‐ a key factor responsible for this intense storm [ABSTRACT FROM AUTHOR]

Interplanetary coronal mass ejections (ICMEs), characterized by their magnetic flux ropes, could potentially trigger geomagnetic disturbances. They have been attracting extensive investigations for decades. Despite numerous ICME models proposed in the past, few account for the curvature of the flux rope axis. In this study, we use conjunction observations from ACE, STEREO A and B, Juno and Solar Orbiter to analyze the evolution of the rope orientation of ICME flux ropes. Our findings indicate that the orientation of these ropes changes independently of the scale of the ropes or the distance they travel between spacecrafts. Furthermore, we estimate and compare the major radii of these flux ropes, uncovering a diverse range of distributions that do not seem to depend on the flux rope's width. These results provide fresh insights and constraints for global ICME models, thereby contributing to the advancement of space weather research. [ABSTRACT FROM AUTHOR]

Abstract One of the most intense geomagnetic storms of recent times occurred on 10–11 May 2024. With a peak negative excursion of Sym‐H below −500 nT, this storm is the second largest of the space era. Solar wind energy transferred through radiation and mass coupling affected the entire Geospace. Our study revealed that the dayside magnetopause was compressed below the geostationary orbit (6.6 RE) for continuously ∼6 hr due to strong Solar Wind Dynamic Pressure (SWDP). Tremendous compression pushed the bow‐shock also to below the geostationary orbit for a few minutes. Magnetohydrodynamic models suggest that the magnetopause location could be as low as 3.3RE. We show that a unique combination of high SWDP (≥15 nPa) with an intense eastward interplanetary electric field (IEFY ≥ 2.5 mV/m) within a super‐dense Interplanetary Coronal Mass Ejection lasted for 409 min–is the key factor that led to the strong ring current at much closer to the Earth causing such an intense storm. Severe electrodynamic disturbances led to a strong positive ionospheric storm with more than 100% increase in dayside ionospheric Total Electron Content (TEC), affecting GPS positioning/navigation. Further, an HF radio blackout was found to occur in the 2–12 MHz frequency band due to strong D‐ and E‐region ionization resulting from a solar flare prior to this storm.

This paper analyzes the Three events of Solar Parameters and Interplanetary Coronal Mass Ejections in the maximum phase of Solar Cycle 24 and focuses on the magnetic activity of interplanetary coronal mass ejection during the solar cycle 24. We investigate the magnetic field magnitude (B), Proton temperature (Tp), Proton density (Np). From this study, we find the highest peak of IP (Interplanetary) shocks on disk center MC (Magnetic cloud) events during the solar cycle 24 and also we investigate the magnetic activity of the solar cycle. In this study, we find that the ACE spacecraft shows the fastest coronal mass ejection and highest interplanetary shock wave on solar cycle 24. It is important to note that extreme events can happen at any time during a cycle. In solar cycle 24, from July 13, 2012 to July 15, 2012 largest storm occurred because the magnetic field was -52nT and linear speed was 1500 kms-1 observed. [ABSTRACT FROM AUTHOR]

To understand the weaker geomagnetic activity in Solar Cycle 24, we present comparisons of interplanetary coronal mass ejections (ICMEs) fittings and in situ observation parameters in Solar Cycles 23 and 24. According to their in situ features, ICMEs are separated into two categories: isolated ICMEs (I-ICMEs) and multiple ICMEs (M-ICMEs). The number of I-ICMEs in Solar Cycles 23 and 24 does not show a strong difference, while the number of M-ICMEs, which have a high probability of causing intense geomagnetic storms, declines proportionally to the sunspot number in Solar Cycle 24. Despite no obvious variation in their distribution, the geoeffective ICMEs in Solar Cycle 23 have a larger average total magnetic field strength and a larger southern magnetic field than those of Solar Cycle 24. Since the average solar wind velocities of the two solar cycles differ, the geoeffective ICMEs in Solar Cycle 23 have a higher velocity and distinct speed distributions from those in Solar Cycle 24. The total magnetic flux and radius of I-ICMEs in Solar Cycle 23 are larger than those in Solar Cycle 24, while the axial magnetic field intensity is basically the same. We propose that geomagnetic activity in Solar Cycle 24 is lower than that of Solar Cycle 23, due to the smaller M-ICME number, the slower ICME speed, and absence of ICME events with significant southward magnetic field. [ABSTRACT FROM AUTHOR]

Magnetospheric magnetic flux dynamics is quantified in 29 geomagnetic storms between 2015 and 2019, using near‐equatorial Van Allen Probe, GOES, and Magnetospheric Multiscale satellites. For the first time, concurrent, multi‐probe observations are utilized to preserve magnetospheric time history, defined as the state of the magnetosphere leading up to an observation. It is revealed that, relative to pre‐storm conditions, (a) during the storm sudden commencement (SSC), magnetic flux uniformly increases ΔΨ = +15% throughout the magnetosphere, except in the nightside inner magnetosphere where ΔΨ = −30%, and (b) during storm main and recovery phases, ΔΨ = −30% and −15%, respectively, in the nightside magnetosphere, at radial distances 5 ≤ r [RE] < 8. It is found that a symmetric ring current is likely formed in the nightside, early in the storm process (localized during SSC), which then broadens during the main phase, before weakening during the recovery phase. The current system on the dayside shows a distinct dawn‐dusk asymmetry. Plain Language Summary: The Earth's magnetosphere serves as a shield against extreme space weather conditions, including interplanetary coronal mass ejections. Understanding the magnetospheric dynamics during geomagnetic storms is critical for a reliable and accurate space weather prediction capability. This study provides a novel statistical analysis technique for investigating the magnetospheric closed magnetic flux content during different storm phases, using magnetic field data from a multitude of satellite missions. The technique considers critical information, such as the state of the magnetosphere before a geomagnetic storm. This multi‐mission analysis technique, though challenging due to orbital differences, provides a comprehensive view of the storm‐time magnetosphere. Our analysis suggests that the magnetosphere is non‐uniformly impacted by different geomagnetic storm phases, paving the path for more reliable storm prediction models. Key Points: Concurrent, multi‐probe observations help preserve magnetospheric time history for a more reliable space weather prediction capabilityMagnetic flux is substantially reduced in the nightside inner magnetosphere during all storm phases, relative to the quiet periodMagnetic flux changes between different storm phases are most significant in the nightside inner magnetosphere, asymmetric on the dayside [ABSTRACT FROM AUTHOR]

Interplanetary coronal mass ejections (ICME) are large-scale eruptions from the Sun and prominent drivers of space weather disturbances, especially intense/extreme geomagnetic storms. Recent studies by our group showed that ICME sheaths and/or magnetic clouds (MC) could be transformed into a planar magnetic structure (PMS) and speculate that these structures might be more geo-effective. Thus, we statistically investigated the geo-effectiveness of planar and non-planar ICME sheaths and MC regions. We analyzed 420 ICME events observed from 1998 to 2017, and we found that the number of intense ( − 100 to − 200 nT) and extreme (< − 200 nT) geomagnetic storms are large during planar ICMEs (almost double) compared to non-planar ICMEs. In fact, almost all the extreme storm events occur during PMS molded ICME crossover. The observations suggest that planar structures are more geo-effective than non-planar structures. Thus, the current study helps us to understand the energy transfer mechanism from the ICME/solar wind into the magnetosphere, and space-weather events. [ABSTRACT FROM AUTHOR]

• A CME arrived on 19 December 2015 and caused changes in the ionosphere on 20 and 21. • A positive ionospheric storm was seen with longitudinal differences. • These are explained as combined effect of DD electric field and composition changes. • Morning overshoot in T e showed a response concurrent with the negative storm. This paper investigates the ionospheric storm of December 19–21, 2015, which was initiated by two successive CME eruptions that caused a G3 space weather event. We used the in situ electron density (N e) and electron temperature (T e) and the Total Electron Content (TEC) measurements from SWARM-A satellite, as well as the O/N 2 observations from TIMED/GUVI to study the ionospheric impact. The observations reveal the longitudinal and hemispherical differences in the ionospheric response to the storm event. A positive ionospheric storm was observed over the American, African and Asian regions on 20 December, and the next day showed a negative storm. Both these exhibited hemispheric differences. A positive storm was observed over the East Pacific region on 21 December. It is seen that the net effect of both the disturbance dynamo electric field and composition differences become important in explaining the observed variability in topside ionospheric densities. In addition, we also discuss the T e variations that occurred as a consequence of the space weather event. [ABSTRACT FROM AUTHOR]

In March 2012, favorable positions of Mercury, Venus, Earth, Mars, and STEREO-B in the inner Solar System provided an opportunity to understand the global structure and the propagation of two coronal mass ejections (CMEs) across the inner Solar System. On 7 March 2012, the Sun ejected two very fast CMEs from the solar active region NOAA AR11489, which were accompanied by two X-class flares. Initialization and subsequent fast expansion from lower coronal heights of flux rope structures were detected as their early eruption signatures in Solar Dynamics Observatory (SDO) observations. White-light observations have been imaged using SOHO/LASCO and STEREO/SECCHI/COR2 and followed from 00:24 UT on 7 March 2012. We examined the kinematics of the reported CMEs and found a significant exchange of momentum and kinetic energy during the interaction, indicating that the collision was almost inelastic. Furthermore, we observed the arrival of this merged CME event at different distances in the inner Solar System and compared the arrival time with other models. The reported event arrived on Mercury at 04:30 UT; Venus, at 13:28 UT on 7 March 2012; and it took roughly 36 h to reach STEREO-B on 08 March, 03:36 UT. The arrivals at Mercury and Venus are observed in the magnetometer measurements onboard MESSENGER and Venus Express (VEx), respectively. A powerful interplanetary shock was observed on 08 March, 10:19 UT at Earth around 30 h after the two X-class flares and CMEs’ eruption. Subsequently, a south-directed interplanetary magnetic field (IMF) was observed on Earth, indicating the arrival of an interplanetary coronal mass ejection (ICME). This event caused the sudden storm commencement and development of one of the major intense geomagnetic storms of SC 24, with a minimum Dst value of −148 nT. The observations by the Mars Express (MEX) mission indicated the arrival of a merged CME ∼2.5 days after its initial observation at Sun. We have analyzed the evolution of these CMEs and their propagation in the inner heliosphere and arrival signatures at four planetary locations. The propagation and arrival signatures are compared to simulations using the WSA-ENLIL + Cone model and the drag-based model at various vantage points. The study showcases the importance of multi-vantage point observations in understanding the propagation of CMEs and their interactions.

Based on the OMNI database, the influence of the solar activity decrease in solar cycles (SCs) 23–24 on the behavior of the relative helium ions abundance Nα/Np inside interplanetary coronal mass ejections (ICMEs) is investigated. The dependences of the helium abundance on the plasma and interplanetary magnetic field parameters in the epoch of high solar activity (SCs 21–22) and the epoch of low activity (SCs 23–24) are compared. It is shown that Nα/Np significantly decreased in SCs 23–24 compared to SCs 21–22. The general trends of the dependences have not changed with the change of epoch, but the helium abundance dependences on some parameters (for example, the magnitude of the interplanetary magnetic field) have become weaker in the epoch of low activity than they were in the epoch of high activity. In addition, the dependence of the helium abundance on the distance from spacecraft to the ICME axis was revealed; the clearest dependence is observed in magnetic clouds. The Nα/Np maximum is measured at the minimum distance, which confirms the hypothesis of the existence of a helium-enriched electric current inside an ICME. [ABSTRACT FROM AUTHOR]

Abstract This review article summarizes the advancement in the studies of Earth-affecting solar transients in the last decade that encompasses most of solar cycle 24. It is a part of the effort of the International Study of Earth-affecting Solar Transients (ISEST) project, sponsored by the SCOSTEP/VarSITI program (2014–2018). The Sun-Earth is an integrated physical system in which the space environment of the Earth sustains continuous influence from mass, magnetic field, and radiation energy output of the Sun in varying timescales from minutes to millennium. This article addresses short timescale events, from minutes to days that directly cause transient disturbances in the Earth’s space environment and generate intense adverse effects on advanced technological systems of human society. Such transient events largely fall into the following four types: (1) solar flares, (2) coronal mass ejections (CMEs) including their interplanetary counterparts ICMEs, (3) solar energetic particle (SEP) events, and (4) stream interaction regions (SIRs) including corotating interaction regions (CIRs). In the last decade, the unprecedented multi-viewpoint observations of the Sun from space, enabled by STEREO Ahead/Behind spacecraft in combination with a suite of observatories along the Sun-Earth lines, have provided much more accurate and global measurements of the size, speed, propagation direction, and morphology of CMEs in both 3D and over a large volume in the heliosphere. Many CMEs, fast ones, in particular, can be clearly characterized as a two-front (shock front plus ejecta front) and three-part (bright ejecta front, dark cavity, and bright core) structure. Drag-based kinematic models of CMEs are developed to interpret CME propagation in the heliosphere and are applied to predict their arrival times at 1 AU in an efficient manner. Several advanced MHD models have been developed to simulate realistic CME events from the initiation on the Sun until their arrival at 1 AU. Much progress has been made on detailed kinematic and dynamic behaviors of CMEs, including non-radial motion, rotation and deformation of CMEs, CME-CME interaction, and stealth CMEs and problematic ICMEs. The knowledge about SEPs has also been significantly improved. An outlook of how to address critical issues related to Earth-affecting solar transients concludes this article.

The Saturn Kilometric Radiation (SKR) was observed for the first time during the flyby of Saturn by the Voyager spacecraft in 1980. These radio emissions, in the range of a few kHz to 1 MHz, are emitted by electrons travelling around auroral magnetic field lines. Their study is useful to understand the variability of a magnetosphere and its coupling with the solar wind. Previous studies have shown a strong correlation between the solar wind dynamic pressure and the SKR intensity. However, up to now, the effect of an Interplanetary Coronal Mass Ejection (ICME) has never been examined in detail, due to the lack of SKR observations at the time when an ICME can be tracked and its different parts be clearly identified. In this study, we take advantage of a large ICME that reached Saturn mid-November 2014 (Witasse et al., J. Geophys. Res. Space Physics, 2017, 122, 7865–7890). At that time, the Cassini spacecraft was fortunately travelling within the solar wind for a few days, and provided a very accurate timing of the ICME structure. A survey of the Cassini data for the same period indicated a significant increase in the SKR emissions, showing a good correlation after the passage of the ICME shock with a delay of ∼13 h and after the magnetic cloud passage with a delay of 25–42 h.

The intensity of the southward component of the magnetic field (Bs) carried by Interplanetary Coronal Mass Ejections (ICMEs) is one of the most critical parameters in causing extreme space weather events, such as intense geomagnetic storms. In this work, we investigate three typical ICME events with extremely intense Bs in detail and present a statistical analysis of the origins of intense Bs in different types of ICMEs based on the ICME catalogue from 1995 to 2020. According to the in-situ characteristics, the ICME events with extremely high Bs are classified into three types: isolated ICMEs, multiple ICMEs, and shock-ICME interaction events with shocks inside ICMEs or shocks passing through ICMEs. By analyzing all ICME events with Bs ≥ 10nT and Bs ≥ 20nT, we find that 39.6% of Bs,mean ≥ 10nT events and 50% of Bs,mean ≥ 20nT events are associated with shock-ICME events. Approximately 35.7% of shock-ICME events have Bs,mean ≥ 10nT, which is much higher than the other two types (isoloted ICMEs: 7.2% and multiple ICMEs: 12.1%). Those results confirm that the ICMEs interaction events are more likely to carry extreme intense Bs and cause intense geomagntic storms. Only based on the in-situ observations at Earth, some interaction ICME events, such as shock-ICME interaction events with shocks passing through the preceding ICME or ICME cannibalism, could be classified as isolated ICME events. This may lead to an overestimate of the probability of ICME carrying extremely intense Bs. To further investigate such events, direct and multi-point observations of the CME propagation in the inner heliosphere from the Solar Ring Mission could be crucial in the future.

This review article summarizes the advancement in the studies of Earth-affecting solar transients in the last decade that encompasses most of solar cycle 24. It is a part of the effort of the International Study of Earth-affecting Solar Transients (ISEST) project, sponsored by the SCOSTEP/VarSITI program (2014–2018). The Sun-Earth is an integrated physical system in which the space environment of the Earth sustains continuous influence from mass, magnetic field, and radiation energy output of the Sun in varying timescales from minutes to millennium. This article addresses short timescale events, from minutes to days that directly cause transient disturbances in the Earth's space environment and generate intense adverse effects on advanced technological systems of human society. Such transient events largely fall into the following four types: (1) solar flares, (2) coronal mass ejections (CMEs) including their interplanetary counterparts ICMEs, (3) solar energetic particle (SEP) events, and (4) stream interaction regions (SIRs) including corotating interaction regions (CIRs). In the last decade, the unprecedented multi-viewpoint observations of the Sun from space, enabled by STEREO Ahead/Behind spacecraft in combination with a suite of observatories along the Sun-Earth lines, have provided much more accurate and global measurements of the size, speed, propagation direction, and morphology of CMEs in both 3D and over a large volume in the heliosphere. Many CMEs, fast ones, in particular, can be clearly characterized as a two-front (shock front plus ejecta front) and three-part (bright ejecta front, dark cavity, and bright core) structure. Drag-based kinematic models of CMEs are developed to interpret CME propagation in the heliosphere and are applied to predict their arrival times at 1 AU in an efficient manner. Several advanced MHD models have been developed to simulate realistic CME events from the initiation on the Sun until their arrival at 1 AU. Much progress has been made on detailed kinematic and dynamic behaviors of CMEs, including non-radial motion, rotation and deformation of CMEs, CME-CME interaction, and stealth CMEs and problematic ICMEs. The knowledge about SEPs has also been significantly improved. An outlook of how to address critical issues related to Earth-affecting solar transients concludes this article. [ABSTRACT FROM AUTHOR]

Coronal mass ejections (CMEs) are one of the most energetic explosions in the solar system. It is generally accepted that CMEs result from eruptions of magnetic flux ropes, which are dubbed as magnetic clouds (MCs) in interplanetary space. The composition (including the ionic charge states and elemental abundances) is determined prior to and/or during CME eruptions in the solar atmosphere and does not alter during MC propagation to 1 AU and beyond. It has been known that the composition is not uniform within a cross section perpendicular to the MC axis, and the distribution of ionic charge states within a cross section provides us an important clue to investigate the formation and eruption processes of flux ropes due to the freeze-in effect. The flux rope is a three-dimensional magnetic structure intrinsically, and it remains unclear whether the composition is uniform along the flux rope axis as most MCs are only detected by one spacecraft. In this study, we report an MC that was observed by Advanced Composition Explorer at ∼1 AU during March 4–6, 1998, and Ulysses at ∼5.4 AU during March 24–28, 1998, sequentially. At these times, both spacecraft were located around the ecliptic plane, and the latitudinal and longitudinal separations between them were ∼2.2° and ∼5.5°, respectively. It provides us an excellent opportunity to explore the axial inhomogeneity of flux rope composition, as both spacecraft almost intersected the cloud center at different sites along its axis. Our study shows that the average values of ionic charge states exhibit significant difference along the axis for carbon, and the differences are relatively slight but still obvious for charge states of oxygen and iron as well as the elemental abundances of iron and helium. Besides the means, the composition profiles within the cloud measured by both spacecraft also exhibit some discrepancies. We conclude that the inhomogeneity of composition exists along the cloud axis.

In situ measurements of interplanetary coronal mass ejection (ICME) composition, including elemental abundances and charge states of heavy ions, open a new avenue to study coronal mass ejections (CMEs) besides remote-sensing observations. The ratios between different elemental abundances can diagnose the plasma origin of CMEs (e.g., from the corona or chromosphere/photosphere) due to the first ionization potential (FIP) effect, which means elements with different FIPs get fractionated between the photosphere and corona. The ratios between different charge states of a specific element can provide the electron temperature of CMEs in the corona due to the freeze-in effect, which can be used to investigate their eruption process. In this review, we first give an overview of the ICME composition and then demonstrate their applications in investigating some important subjects related to CMEs, such as the origin of filament plasma and the eruption process of magnetic flux ropes. Finally, we point out several important questions that should be addressed further for better utilizing the ICME composition to study CMEs. [ABSTRACT FROM AUTHOR]

A clear understanding of storm‐time magnetospheric dynamics is essential for a reliable storm forecasting capability. The dayside magnetospheric response to an interplanetary coronal mass ejection (ICME; dynamic pressure Pdyn > 20 nPa and storm‐time index SYM‐H < −150 nT) is investigated using in situ OMNI, Geotail, Cluster, MMS, GOES, Van Allen Probes, and THEMIS measurements. The dayside magnetic flux content is directly quantified from in situ magnetic field measurements at different radial distances. The arrival of the ICME, consisting of shock and sheath regions preceding a magnetic cloud, initiated a storm sudden commencement (SSC) phase (SYM‐H ~ +50 nT). At SSC, the magnetopause standoff distance was compressed earthward at ICME shock encounter at an average rate ~−10.8 Earth radii per hour for ~10 min, resulting in a rapid 40% reduction in the magnetospheric volume. The "closed" magnetic flux content remained constant at 170 ± 30 kWb inside the compressed dayside magnetosphere, even in the presence of dayside reconnection, as evident by an outsized flux transfer event containing 160 MWb. During the storm main and recovery phases, the magnetosphere expanded. The dayside magnetic flux did not remain constant within the expanding magnetosphere (110 ± 30 kWb), resulting in a 35% reduction in pre‐storm flux content during the magnetic cloud encounter. At that stage, the magnetospheric magnetic flux was eroded resulting in a weakened dayside magnetospheric field strength at radial distances R ≥ 5 RE. It is concluded that the inadequate replenishment of the eroded dayside magnetospheric flux during the magnetosphere expansion phase is due to a time lag in storm‐time Dungey cycle. Plain Language Summary: A clear understanding of Earth's magnetospheric dynamics is essential for a reliable space weather forecasting capability. To achieve this, we take advantage of the Heliophysics System Observatory's (HSO) multitude of in situ observations in order to, for the first time, quantify the amount of magnetic flux stored in the dayside magnetosphere. The stored magnetic flux shields our ground‐based and space‐borne assets from adverse space weather events. We examine the dayside magnetic flux content during an encounter with an interplanetary coronal mass ejection (ICME). ICME is a large‐scale bundle of magnetic flux and charged particles originating from the Sun. Upon arrival, the ICME which occupied nearly one third of the space between the Sun and Earth forced the dayside magnetosphere to rapidly shrink down to geosynchronous orbit where most communications and weather satellites are located. Though the dayside magnetosphere significantly shrunk, its magnetic flux content remained constant. It was only when the dayside magnetosphere started to expand that the dayside magnetospheric flux content gradually reduced by 35%. It is concluded that, during large ICME encounters, the rate at which dayside magnetic flux is transported to the magnetotail is faster than the rate at which magnetic flux is recycled, via a process known as the Dungey cycle. In addition to the observed loss in magnetic flux, this time lag in Dungey cycle can further cause magnetopause shadowing, wherein significant population of magnetospheric charged particles is lost to solar wind. Key Points: Dayside closed magnetic flux is quantified during an interplanetary coronal mass ejection encounter using cross‐scale observationsClosed magnetic flux remains constant inside the reconnecting dayside magnetosphere compressed by 70% in storm sudden commencement phaseDayside closed magnetic flux is reduced by 35% in storm main phase, indicating a time lag in storm‐time Dungey cycle [ABSTRACT FROM AUTHOR]

Abstract We present a case study on the long‐period ( >105 s) induced geoelectric field disturbance during magnetic storms. A set of continuous 33‐day measurements of the geoelectric field, geomagnetic field, geomagnetic indices, and interplanetary magnetic field are analyzed. In the studied 33 days, two magnetic storms occurred after 10 geomagnetic quiet days. To exclude the effects from the different underground electrical structures, geomagnetic and geoelectric field measurements from the same ground observatory enrolled in Chinese Meridian Project are studied. Besides, Space Weather Modeling Framework is adopted to calculate the global geomagnetic field disturbances and the contribution from different current systems. The wavelet power spectra analysis reveals that the long‐period geoelectric field disturbance appears only during magnetic storms. Especially, stronger magnetic storm generates weaker geoelectric disturbances at the same observatory. The opposite‐direction eastward geomagnetic field disturbance, which is generated mainly by the field‐aligned current at different magnetic local time, significantly changes the magnitude and direction of the induced geoelectric field via the underground impedance tensor. Therefore, both the effects caused by ring current and field‐aligned current should be considered in analyzing the ground induced geoelectric field. Excluding the effect from the geospace current source, the more accurate electrical conductivity in the upper mantle would be obtained in future.

Coronal mass ejections (CMEs) are intense solar explosive eruptions and have significant impact on geomagnetic activities. It is important to understand how CMEs evolve as they propagate in the solar-terrestrial space. In this paper, we studied the coalescence of magnetic flux ropes embedded in five interplanetary coronal mass ejections (ICMEs) observed by both ACE and Wind spacecraft. The analyses show that coalescence of magnetic flux ropes could persist for hours and operate in scale of hundreds of earth radii. The two merging flux ropes could be very different in the axial orientation and the plasma density and temperature, which should complicate the progress of coalescence and have impact on the merged structures. The study indicates that coalescence of magnetic flux ropes should be an important factor in changing the magnetic topology of ICMEs.

Coronal mass ejections (CMEs) are intense solar explosive eruptions. CMEs are highly important players in solar‐terrestrial relationships, and they have important consequences for major geomagnetic storms and energetic particle events. It has been unclear how CMEs evolve when they propagate in the heliosphere. Here we report an interplanetary CME consisting of multiple magnetic flux ropes measured by WIND on 25–26 March 1998. These magnetic flux ropes were merging with each other. The observations indicate that internal interactions (reconnections) within multiflux rope CME can coalesce into large‐scale ropes, which may improve our understanding of the interplanetary evolution of CMEs. In addition, we speculated that the reported rope‐rope interactions may also exist between successive rope‐like CMEs and are important for the space weather forecasting. Plain Language Summary: Coronal mass ejections (CMEs), most of which have rope structures, are an important cause of extreme space weather. Therefore, it is important to understand how CMEs evolve when they propagate in the heliosphere. We report an interplanetary coronal mass ejection consisting of three magnetic flux ropes. The three magnetic flux ropes were merging with each other. The observations provide definite understanding of CME evolution and have important implications for space weather forecasting. Key Points: An interplanetary coronal mass ejection consisting of multiple magnetic flux ropes was observedThese internal magnetic flux ropes were merging with each otherThe interactions of substructures may have significant effect on the evolution of CMEs [ABSTRACT FROM AUTHOR]

Abstract We investigate the propagation of the 2015 June 21 CME-driven shock as revealed by the type II bursts at metric and longer wavelengths and coronagraph observations. The CME was associated with the second largest geomagnetic storm of solar cycle 24 and a large solar energetic particle (SEP) event. The eruption consisted of two M-class flares, with the first one being confined, with no metric or interplanetary radio bursts. However, there was intense microwave burst, indicating accelerated particles injected toward the Sun. The second flare was eruptive that resulted in a halo CME. The CME was deflected primarily by an equatorial coronal hole that resulted in the modification of the intensity profile of the associated SEP event and the duration of the CME at Earth. The interplanetary type II burst was particularly intense and was visible from the corona all the way to the vicinity of the Wind spacecraft with fundamental-harmonic structure. We computed the shock speed using the type II drift rates at various heliocentric distances and obtained information on the evolution of the shock that matched coronagraph observations near the Sun and in-situ observations near Earth. The depth of the geomagnetic storm is consistent with the 1-AU speed of the CME and the magnitude of the southward component. Highlights • The solar source of the 2015 June 21 CME clarified. • Sun-to-Earth evolution of the CME-driven shock characterized using radio data. • SEP time profile and ICME duration are affected by CME deflection by coronal hole. • Coronal flux rope from constructed from eruption data is consistent with ICME. [ABSTRACT FROM AUTHOR]

We reexamine the magnetic cloud (MC) event during the period of 21–23 May 2007. In this event, the axis of the MC has a high inclination to the ecliptic plane and the heliospheric current sheet happens to be on the ecliptic plane. Therefore, we can use the feature of zero north‐south component of interplanetary magnetic field to identify the MC boundaries. Inside the MC, there is an enhanced pressure/density region enclosed by two discontinuities. We verified these discontinuities through multiple spacecraft in situ observations. The front one is a forward fast shock, which is a quasi‐perpendicular shock at STEREO B but a quasi‐parallel shock at Wind location. The discontinuity at the rear part of the enhanced pressure region resembles a reverse slow shock. However, we verify it is a tangential discontinuity (TD) using multispacecraft observations. Furthermore, we analyze the successive TDs inside the MC based on the TD signature of no normal magnetic field component to estimate the magnetic field morphology along the spacecraft trajectories. A novel method to evaluate the uncertainties of those TDs in this study has been given. It is found that the errors of the TD normal are much smaller than that calculated by conventional methods. Key Points: The interval of the magnetic cloud is identified by means of the heliospheric current sheet, which happens to be in the ambient regionThe nature of the studied discontinuities is verified through multiple spacecraft observationsA novel method to evaluate the uncertainties of interplanetary tangential discontinuities is provided [ABSTRACT FROM AUTHOR]

Abstract: Owing to the heritage of previous missions such as the Pioneer Venus Orbiter and Venus Express, the typical global plasma environment of Venus is relatively well understood. On the other hand, this is not true for more extreme driving conditions such as during passages of interplanetary coronal mass ejections (ICMEs). One of the outstanding questions is how do ICMEs, either the ejecta or sheath portions, impact (1) the Venusian magnetic topology and (2) escape rates of planetary ions? One of the main issues encountered when addressing these problems is the difficulty of inferring global dynamics from single spacecraft obits; this is where the benefits of simulations become apparent. In the present study, we present a detailed case study of an ICME interaction with Venus on 5 November 2011 in which the magnetic barrier reached over 250 nT. We use both Venus Express observations and hybrid simulation runs to study the impact on the field draping pattern and the escape rates of planetary O+ ions. The simulation showed that the magnetic field line draping pattern around Venus during the ICME is similar to that during typical solar wind conditions and that O+ ion escape rates are increased by approximately 30% due to the ICME. Moreover, the atypically large magnetic barrier appears to manifest from a number of factors such as the flux pileup, dayside compression, and the driving time from the ICME ejecta. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we study the variations of the radiation belts electron fluxes induced by the interaction of two types of solar wind structures with the Earth magnetosphere: the corotating interaction regions and the interplanetary coronal mass ejections. We use a statistical method based on the comparison of the preevent and postevent fluxes. Applied to the National Oceanic and Atmospheric Administration‐Polar Operational Environmental Satellites data, this gives us the opportunity to extend previous studies focused on relativistic electrons at geosynchronous orbit. We enlighten how corotating interaction regions and Interplanetary Coronal Mass Ejections can impact differently the electron belts depending on the energy and the L shell. In addition, we provide a new insight concerning these variations by considering their amplitude. Finally, we show strong relations between the intensity of the magnetic storms related to the events and the variation of the flux. These relations concern both the capacity of the events to increase the flux and the deepness of these increases. [ABSTRACT FROM AUTHOR]

A statistical analysis is undertaken on ICME's efficiency in producing the geomagnetic and ionospheric storms. The mutually-consistent thresholds for the intense, moderate and weak space weather storms and quiet conditions are introduced with an analytical model based on relations between the equatorial Dst index and geomagnetic indices AE , aa , ap , ap ( τ ) and the ionospheric Vσ indices. The ionosphere variability Vσ index is expressed in terms of the total electron content ( TEC ) deviation from the −15-day sliding median normalized by the standard deviation for the 15 preceding days. The intensity of global positive ionospheric storm, Vσp , and negative storm, Vσn , is represented by the relative density of anomalous ± Vσ index occurrence derived from the global ionospheric maps GIM-TEC for 1999–2016. An impact of total 421 ICME events for 1999–2016 on the geomagnetic and ionospheric storms expressed by AE , Dst , aa , ap , ap ( τ ), Vσp , Vσn indices and their superposition is analyzed using ICME catalogue by Richardson and Cane (2010) during 24 h after the ICME start time t 0 . Hierarchy of efficiency of ICME → storm relation is established. The ICMEs have a higher probability (22–25%) to be followed by the intense ionospheric and auroral electrojet storms at global and high latitudes as compared to the intense storms at middle and low latitudes (18–20%) and to moderate and weak storms at high latitudes (5–17%). At the same time ICMEs are more effective in producing the moderate storms (24–28%) at the middle and low latitudes as compared to the intense and weak storms at these latitudes (13–22%) and to moderate storms at high latitudes (8–17%). The remaining cases when quiet conditions are observed after ICMEs present higher chance for a false alarm. The risk factor for a false alarm can vary from 18% if the superposition of all indices is considered, to 51–64% for individual AE, Vσp and Vσn indices. The analysis indicates that the mutually-consistent thresholds can be successfully applied to the external sources of the geomagnetic and ionospheric storms other than ICME which present challenge for the further investigation. [ABSTRACT FROM AUTHOR]

We discuss observations of the journey throughout the Solar System of a large interplanetary coronal mass ejection (ICME) that was ejected at the Sun on 14 October 2014. The ICME hit Mars on 17 October, as observed by the Mars Express, Mars Atmosphere and Volatile EvolutioN Mission (MAVEN), Mars Odyssey, and Mars Science Laboratory (MSL) missions, 44 h before the encounter of the planet with the Siding-Spring comet, for which the space weather context is provided. It reached comet 67P/Churyumov-Gerasimenko, which was perfectly aligned with the Sun and Mars at 3.1 AU, as observed by Rosetta on 22 October. The ICME was also detected by STEREO-A on 16 October at 1 AU, and by Cassini in the solar wind around Saturn on the 12 November at 9.9 AU. Fortuitously, the New Horizons spacecraft was also aligned with the direction of the ICME at 31.6 AU. We investigate whether this ICME has a nonambiguous signature at New Horizons. A potential detection of this ICME by Voyager 2 at 110-111 AU is also discussed. The multispacecraft observations allow the derivation of certain properties of the ICME, such as its large angular extension of at least 116 °, its speed as a function of distance, and its magnetic field structure at four locations from 1 to 10 AU. Observations of the speed data allow two different solar wind propagation models to be validated. Finally, we compare the Forbush decreases (transient decreases followed by gradual recoveries in the galactic cosmic ray intensity) due to the passage of this ICME at Mars, comet 67P, and Saturn. [ABSTRACT FROM AUTHOR]

This overview paper presents/discusses the major solar, interplanetary, magnetospheric, and ionospheric features of the CAWSES II interval of study: 7–17 March 2012. Magnetic storms occurred on 7, 9, 12, and 15 March with peak SYM-H intensities of −98 nT, −148 nT, −75 nT (pressure corrected), and −79 nT, respectively. These are called the S1, S2, S3, and S4 events. Although three of the storm main phases (S1, S3, and S4) were caused by IMF Bsouth sheath fields and the S2 event was associated with a magnetic cloud (MC), the detailed scenario for all four storms were different. Two interplanetary features with unusually high temperatures and intense and quiet magnetic fields were identified located antisunward of the MCs (S2 and S3). These features are signatures of either coronal loops or coronal sheaths. A high speed stream (HSS) followed the S4 event where the presumably southward IMF Bz components of the Alfvén waves extended the storm “recovery phase” by several days. The ICME-associated shocks were particularly intense. The fast forward shock for the S2 event had a magnetosonic Mach number of ~9.4, the largest in recorded history. All of the shocks associated with the ICMEs created sudden impulses (SI+s) at Earth. The shocks preceding the S2 and S3 magnetic storms caused unusually high SI+ intensities of ~60 and 68 nT, respectively. Many further studies on various facets of this active interval are suggested for CAWSES II researchers and other interested parties.

We examine solar sources for 20 interplanetary coronal mass ejections (ICMEs) observed in 2009 in the near-Earth solar wind. We performed a detailed analysis of coronagraph and extreme ultraviolet (EUV) observations from the Solar Terrestrial Relations Observatory (STEREO) and Solar and Heliospheric Observatory (SOHO). Our study shows that the coronagraph observations from viewpoints away from the Sun-Earth line are paramount to locate the solar sources of Earth-bound ICMEs during solar minimum. SOHO/LASCO detected only six CMEs in our sample, and only one of these CMEs was wider than 120. This demonstrates that observing a full or partial halo CME is not necessary to observe the ICME arrival. Although the two STEREO spacecraft had the best possible configuration for observing Earth-bound CMEs in 2009, we failed to find the associated CME for four ICMEs, and identifying the correct CME was not straightforward even for some clear ICMEs. Ten out of 16 (63 %) of the associated CMEs in our study were 'stealth' CMEs, i.e. no obvious EUV on-disk activity was associated with them. Most of our stealth CMEs also lacked on-limb EUV signatures. We found that stealth CMEs generally lack the leading bright front in coronagraph images. This is in accordance with previous studies that argued that stealth CMEs form more slowly and at higher coronal altitudes than non-stealth CMEs. We suggest that at solar minimum the slow-rising CMEs do not draw enough coronal plasma around them. These CMEs are hence difficult to discern in the coronagraphic data, even when viewed close to the plane of the sky. The weak ICMEs in our study were related to both intrinsically narrow CMEs and the non-central encounters of larger CMEs. We also demonstrate that narrow CMEs (angular widths ≤ 20) can arrive at Earth and that an unstructured CME may result in a flux rope-type ICME. [ABSTRACT FROM AUTHOR]

We present the results of a statistical data analysis of the geo-effectiveness of non-magnetic-cloud interplanetary coronal mass ejections (ICMEs) and compare them with those of magnetic-cloud (MC) interplanetary coronal mass ejections observed during solar cycle 23. (The term ICME as used here will refer to a non-MC ICME.) The starting point of this investigation is the set of intense geomagnetic storms (Dstmin ≤ -100 nT) of solar cycle 23 between 1996 and 2005. We also compare the solar source locations of the ICMEs with those of the MCs. The source locations of the solar disturbances are, on average, closer to the Sun-Earth line for the MCs than for the ICMEs. There is an anomaly for the location of the related solar sources: no event came from the region between the solar equator plane and 10°S (south) of that plane. The primary results are listed as follows. The average duration of these MCs is slightly longer (7%) than that of ICMEs. The average geomagnetic storm intensity for the MCs is higher than that for the ICMEs and CIRs formed by high-speed streams from coronal holes, especially for the events associated with X class flares. The relevant average magnetic field component, i.e., Bzmin, is more intense within the MCs than within the ICMEs. The average solar wind speed is similar for both MCs and ICMEs. Maximum solar wind speed is higher within ICMEs than within MCs. Maximum solar wind proton density is higher for MCs than for ICMEs. [ABSTRACT FROM AUTHOR]

We studied the relation between the near-Earth signatures of the interplanetary coronal mass ejections (ICMEs) shocks such as sudden storms commencement (SSC), and their counterparts of coronal mass ejections (CMEs) observed near-Sun by solar and heliospheric observatory (SOHO)/large angle and spectrometric coronagraph (LASCO) coronagraph during 1996-2008. Our result showed that there is a good correlation between the travel time of the ICMEs shocks and their associated radial speeds. Also we have separated the ICME shocks into two groups according to their effective acceleration and deceleration. The results showed that the faster ICME shocks (with negative accelerations which decelerated by solar wind plasma) are more correlated to their associated travel time than those with positive accelerations. [ABSTRACT FROM AUTHOR]

The generally low interplanetary magnetic field magnitude around the minimum between Solar Cycles 23 and 24 (SC 23/24 minimum) allows us to identify weak and small solar wind structures. We use observations from near-Earth and twin STEREO spacecraft to study solar wind conditions from January 2007 through December 2010. In addition to 84 clear interplanetary coronal mass ejections (ICMEs), we identified 58 ICME-like transients, which exhibit some classical ICME signatures but have weak magnetic fields (<7 nT) and/or short durations (<10 hours). The number of ICME-like transients peaked during the SC 23/24 minimum, while the ICME rate increased with increasing solar activity. The magnetic structures of flux rope type ICMEs and transients show similar solar cycle variation trends, suggesting that ICMEs and transients originate from similar polarity regions at the Sun. We observed a gradual transition from ICME-like structures to ICMEs. The identified events display continuous distributions in duration and magnetic field magnitude ranging from a few hours to several days and from a few nanoteslas to a few tens of nanoteslas, respectively. Our ICME-like transient rate (less than one event/month) is considerably smaller than that suggested by solar observations of narrow CMEs. This implies that the majority of small coronal ejections are merged as a part of the solar wind by the time they reach 1 AU. We found that ICME-like transients generally occur closer to stream interaction regions (SIRs) than ICMEs, and the majority of the events we identified in declining parts of fast solar wind streams were ICME-like structures. This suggests that ICME-like transients tend to arise close to coronal hole boundaries and thus may have an important role in coronal hole dynamics. Diverse solar wind transients presumably manifest the variation of solar eruptions from small-scale blobs to wide CMEs. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is an important universal plasma dissipation process that converts magnetic energy into plasma thermal and kinetic energy, and simultaneously changes the magnetic field topology. In this paper, we report the first observation of energetic electrons associated with asymmetric reconnection in the sheath of an interplanetary coronal mass ejection. The magnetic field shear angle was about 151°, implying guide-field reconnection. The width of the exhaust was about 8×10 km. The reconnection rate was estimated as 0.044-0.08, which is consistent with fast reconnection theory and previous observations. We observed flux enhancements of energetic electrons with energy up to 400 keV in this reconnection exhaust. The region where energetic electron fluxes were enhanced is located at one pair of separatrices in the higher density hemisphere. We discuss these observation results, and compare with previous observations and recent kinetic simulations. [ABSTRACT FROM AUTHOR]

Abstract: Interplanetary coronal mass ejections (ICMEs) and their subset, magnetic clouds (MCs), are important manifestations of solar activity which have substantial impact on the geomagnetic field. We re-analyze events already identified in Wind and Voyager 2 data and estimate changes of their geometry along the path from the Sun. The analysis is based on the thickness of the sheath between a shock and a particular ICME or MC which is proportional to the apparent curvature radius of ICMEs/MCs. We have found that this apparent radius of curvature increases with the Mach number and this effect is attributed to the larger deformation of the fast ICME/MC. Further, the relative sheath thickness that is proportional to the flux rope oblateness decreases with the magnetic field intensity inside the ICME/MC and increases with the heliospheric distance. [Copyright &y& Elsevier]

We compare the number and characteristics of interplanetary coronal mass ejections (ICMEs) to those of magnetic clouds (MCs) by using in-situ solar wind plasma and magnetic field observations made at 1 AU during solar cycle 23. We found that ≈ 28% of ICMEs appear to contain MCs, since 103 magnetic clouds (MCs) occurred during 1995 - 2006, and 307 ICMEs occurred during 1996 - 2006. For the period between 1996 and 2006, 85 MCs are identified as part of ICMEs, and six MCs are not associated with ICMEs, which conflicts with the idea that MCs are usually a subset of ICMEs. It was also found that solar wind conditions inside MCs and ICMEs are usually similar, but the linear correlation between geomagnetic storm intensity ( Dst) and relevant solar wind parameters is better for MCs than for ICMEs. The differences between average event duration (Δ t) and average proton plasma β (〈 β〉) are two of the major differences between MCs and ICMEs: i) the average duration of ICMEs (29.6 h) is 44% longer than for MCs (20.6 hours), and ii) the average of 〈 β〉 is 0.01 for MCs and 0.24 for ICMEs. The difference between the definition of a MC and that for an ICME is one of the major reasons for these average characteristics being different ( i.e., listed above as items i) and ii)), and it is the reason for the frequency of their occurrences being different. [ABSTRACT FROM AUTHOR]

Abstract: To improve the physical understanding of the Forbush decreases (FD) and to explore the Space Weather drivers, we need to measure as much geospace parameter as possible, including the changing fluxes of secondary cosmic rays. At the Aragats Space Environmental Center (ASEC) are routinely measured the neutral and charged fluxes of secondary cosmic rays. Each of species has different most probable energy of primary “parent” proton/nuclei. Therefore, the energy range of the Galactic Cosmic Rays (GCR) affected by Interplanetary Coronal Mass Ejection (ICME) can be effectively estimated using data of the ASEC monitors. We presented relations of the magnitude of FD observed in different secondary particle fluxes to the most probable energy of the primary protons. We investigate the correlations between the magnitude of FD with the size, speed, density and magnetic field of the ICME. We demonstrate that the attenuation of the GCR flux incident on the Earth’s atmosphere due to passing of the ICME is dependent on the speed and size of the ICME and the magnetic field strength. [Copyright &y& Elsevier]