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This study explores the impact of the radial interplanetary magnetic field (IMF) on the strength and latitude of peak field-aligned currents (FACs). FACs are derived through vector magnetic field observations of the Swarm satellite mission. The analysis examines how the responses of FACs to radial IMF vary according to local time, season, and hemisphere. In the dawn and noon–midnight sectors, which are primarily influenced by westward auroral electrojets, the Northern Hemisphere (NH) exhibits stronger poleward FACs (FACp) when the IMF cone angle is ≥135° and weaker FACp when the cone angle is ≤45°. In contrast, the Southern Hemisphere (SH) shows the opposite response to the IMF Bx polarity. The effect of IMF Bx is more pronounced during summer than winter, especially in the noon-to-midnight sector, while its influence on FACs is more significant during the dawn period in winter. The latitude of FACs is most strongly affected by IMF Bx around noon and midnight. A relationship is observed between FAC density and latitude in response to IMF Bx, with stronger FACp occurring at lower latitudes.

The recent superstorm of 2024 May 10–11 is the second largest geomagnetic storm in the space age and the only one that has simultaneous interplanetary data (there were no interplanetary data for the 1989 March storm). The May superstorm was characterized by a sudden impulse (SI+) amplitude of +88 nT, followed by a three-step storm main-phase development, which had a total duration of ∼9 hr. The cause of the first storm main phase with a peak SYM-H intensity of −183 nT was a fast-forward interplanetary shock (magnetosonic Mach number M ms ∼ 7.2) and an interplanetary sheath with a southward interplanetary magnetic field component B s of ∼40 nT. The cause of the second storm's main phase with an SYM-H intensity of −354 nT was a deepening of the sheath B s to ∼43 nT. A magnetosonic wave (M ms ∼ 0.6) compressed the sheath to a high magnetic field strength of ∼71 nT. Intensified B s of ∼48 nT were the cause of the third and most intense storm main phase, with an SYM-H intensity of −518 nT. Three magnetic cloud events with B s fields of ∼25–40 nT occurred in the storm recovery phase, lengthening the recovery to ∼2.8 days. At geosynchronous orbit, ∼76 keV to ∼1.5 MeV electrons exhibited ∼1–3 orders of magnitude flux decreases following the shock/sheath impingement onto the magnetosphere. The cosmic-ray decreases at Dome C, Antarctica (effective vertical cutoff rigidity <0.01 GV) and Oulu, Finland (rigidity ∼0.8 GV) were ∼17% and ∼11%, respectively, relative to quiet-time values. Strong ionospheric current flows resulted in extreme geomagnetically induced currents of ∼30–40 A in the subauroral region. The storm period is characterized by strong polar-region field-aligned currents, with ∼10 times intensification during the main phase and equatorward expansion down to ∼50° geomagnetic (altitude-adjusted) latitude.

Strong interaction of CMEs with Earth’s environment causes serious space weather effect through the coupled magnetosphere system. A geomagnetic storm is a global disruption of the earth’s magnetic field, and usually occurs in response to abnormal conditions in the IMF and solar wind. In this paper we present the consequences on interplanetary causes of strong geomagnetic storms (Dst ≤ -200nT), that occurred during solar cycle 23 and 24. It was found that the positive correlation with correlation coefficient (0.46) between the magnitude of strong geomagnetic storm and magnitude of solar wind plasma temperature, (0.54) between the magnitude of strong geomagnetic storm and magnitude of solar wind plasma velocity, (.58) between the magnitude of strong geomagnetic storm and magnitude of southward component of interplanetary magnetic field (IMF Bz) and (0.54) between strong geomagnetic storm and magnitude of interplanetary magnetic field (IMF B total ).The correlation between magnitude of strong geomagnetic storm and magnitude of interplanetary magnetic field (with positive correlation coefficient 0.58) is higher, as compared with magnitude of strong geomagnetic storm and magnitude of solar wind plasma temperature (0.46). It has been verified that strong geomagnetic storm intensity is correlated well with the interplanetary magnetic field IMF (B total and B z ) better than with its solar parameters.

The solar wind energy input and dissipation in the magnetospheric–ionospheric systems of 17 supersubstorms (SSSs: SML < −2500 nT) triggered by interplanetary shocks during solar cycles 23 and 24 are studied in detail. The SSS events had durations ranging from ∼42 minutes to ∼6 hr, and SML intensities ranging from −2522 nT to −4143 nT. Shock compression greatly strengthens the upstream interplanetary magnetic field southward component (B s), and thus, through magnetic reconnection at the Earth’s dayside magnetopause, greatly enhances the solar wind energy input into the magnetosphere and ionosphere during the SSS events studied. The additional solar wind magnetic reconnection energy input supplements the ∼1.5 hr precursor (growth-phase) energy input and both supply the necessary energy for the high-intensity, long-duration SSS events. Some of the solar wind energy is immediately deposited in the magnetosphere/ionosphere system, and some is stored in the magnetosphere/magnetotail system. During the SSS events, the major part of the solar wind input energy is dissipated into Joule heating (∼30%), with substantially less energy dissipation in auroral precipitation (∼3%) and ring current energy (∼2%). The remainder of the solar wind energy input is probably lost down the magnetotail. It is found that during the SSS events, the dayside Joule heating is comparable to that of the nightside Joule heating, giving a picture of the global energy dissipation in the magnetospheric/ionospheric system, not simply a nightside-sector substorm effect. Several cases are shown where an SSS is the only substorm that occurs during a magnetic storm, essentially equating the two phenomena for these cases.

The southward component of the interplanetary magnetic field, often originating from solar coronal mass ejections (CMEs), plays a crucial role in driving geomagnetic storms. Accurate prediction of the flux rope orientation of CMEs as they arrive at Earth requires a clear understanding of how the orientation of magnetic flux ropes evolves from the solar corona to 1 au. In this study, we investigated six geoeffective CMEs, initiated either from active regions (ARs; three events) or quiet-Sun (QS) filament eruptions (three events). The orientation prior to the eruption is determined by the eruptive filament or the magnetic flux rope near the solar surface. During the CME propagation away from the Sun, the graduated cylindrical shell model and the Grad–Shafranov technique are used to estimate the orientation of the CME magnetic field structure. Our results show that the orientation of flux ropes associated with QS eruptions did not change from the Sun to 1 au. For three rotating events initiated from ARs, the direction of rotation remains consistent during the propagation from the Sun to 1 au. The trend indicates that the heliospheric current sheet has a relatively limited influence on these events. For rotating events, the direction of rotation basically follows the prediction of Lynch’s model.

In situ observations by Parker Solar Probe (PSP) suggest that the heliospheric current sheet (HCS) undergoes near-continuous magnetic reconnection close to the Sun, in stark contrast to scarce observations of this phenomenon in the HCS at 1 au. Situated at the boundary between sectors of opposite interplanetary magnetic field polarity, reconnection in the HCS has important consequences for magnetic topology and plasma dynamics in the slow solar wind. We report observations of a reconnection outflow in the HCS near the Alfvén transition region in PSP’s 17th solar encounter, featuring plasma jetting, proton temperature enhancement, and electron heat flux dropout. Embedded within the exhaust is a non-force-free flux rope plasmoid exhibiting counterstreaming strahl electrons, indicating connection at both ends to the Sun in an otherwise disconnected region of the magnetic field. The flux rope features diminished isotropic proton temperature and lower bulk speed compared to the remainder of the HCS exhaust. Its oblique orientation and different plasma properties imply that the flux rope originates from a different reconnection site to the HCS exhaust, suggesting PSP has intercepted a flux-rope-like streamer blob produced at the helmet streamer. Remote observations show several comparable blobs traveling in a distant coronal ray, demonstrating the possibility that the in situ flux rope is a streamer blob. The combination of in situ and remote observations demonstrates the role of magnetic reconnection in HCS dynamics, contributing to a growing understanding of this fundamental mechanism and its impact on the young solar wind.

The interplanetary magnetic field (IMF) is one of the primary factors influencing the Martian plasma environment. In this study, a multifluid magnetohydrodynamic model is adopted to investigate how variations in IMF affect planetary ion escape, particularly the tailward escape flux. Our results reveal that for nominal IMF direction ( 56° Parker spiral), as IMF intensity increases, the ion escape rate decreases considerably. This reduction is primarily due to the decrease in planetary ion density in the plume and the magnetotail, which is caused by the lower ion production rate through the charge exchange process under high IMF conditions. With high IMF conditions, the dynamo at the bow shock is significantly enhanced, leading to a more severe deceleration of solar wind protons and fewer protons entering the magnetosheath. Consequently, intensified electromagnetic fields create a stronger induced magnetosphere, which shields the Martian ionosphere and atmosphere. Although the enhanced loading process for planetary ions results in higher ion escape velocities, the overall ion escape fluxes decrease due to the significant reduction in planetary ion density.

Magnetic field intensity increases when solar wind compresses a planet’s magnetosphere. The compression can be measured using the ratio of compressed magnetic fields to purely dipolar magnetic fields just inside the magnetopause. For Earth, the ratio is proportional to the subsolar standoff distance of the magnetopause. Data from in-orbit observations by the MESSENGER spacecraft indicate an opposite ratio for Mercury; the compression ratio is inversely proportional to the subsolar standoff distance. The additional magnetic fields induced by currents at the top of Mercury’s core enhance the total magnetic field strength. We also evaluated differences in the subsolar standoff of Mercury’s magnetopause according to the north–south polarity of the interplanetary magnetic field (IMF). Previous studies have not identified meaningful differences in subsolar standoff distance between those in northward versus southward IMF polarities for Mercury; however, we found that the difference is statistically significant at a large IMF B Z (15–20 nT). The magnetic reconnection that occurs behind the cusp for a large northward IMF transfers the magnetic flux to the dayside and increases the subsolar standoff distance. The eroded magnetic flux for a large southward IMF is compensated by the induced magnetic fields.

The gradient and curvature of the Parker spiral interplanetary magnetic field give rise to curvature and gradient guiding-center drifts on cosmic rays (CRs). The plasma turbulence present in interplanetary space is thought to suppress the drifts; however, the extent to which they are reduced is not clear. We investigate the reduction of the drifts using a new analytic model of heliospheric turbulence where the dominant 2D component has both a wavevector and magnetic field vector normal to the Parker spiral, thus fulfilling the main criterion of 2D turbulence. We use full-orbit test-particle simulations of energetic protons in the modeled interplanetary turbulence, and analyze the mean drift velocity of the particles in heliolatitude. We release energetic proton populations of 10, 100, and 1000 MeV close to the Sun and introduce a new method to assess their drift. We compare the drift in the turbulent heliosphere to drift in a configuration without turbulence, and to theoretical estimates of drift reduction. We find that drifts are reduced by a factor 0.2–0.9 of that expected for the heliospheric configuration without turbulence. This corresponds to a much less efficient suppression than what is predicted by theoretical estimates, particularly at low proton energies. We conclude that guiding-center drifts are a significant factor for the evolution of CR intensities in the heliosphere, including the propagation of solar energetic particles in the inner heliosphere.

The solar wind and the interplanetary magnetic field (IMF) are of significant importance, as they affect the plasma environment and dynamic processes in the planetary magnetosphere. This study statistically analyzes the solar wind parameters upstream of Earth (ACE), Mars (MAVEN), Jupiter (Ulysses, Galileo, Juno), and Saturn (Cassini) and their solar activity phase (solar maximum, descending, minimum, and ascending phases) dependence. The heliolatitude variations are validated using Ulysses observations. Results show that the radial magnetic field decays more gradually than expected (r−2), and the IMF intensity (B) decreases more rapidly at higher heliolatitudes. The solar wind proton density (Np) near Mars is significantly higher than expected, while the solar wind speed (V) is lower. The IMF spiral angle (SA) aligns well with the Parker SA upstream of planets and at high heliolatitudes, except for Mars. Martian anomalies may stem from mass loading by pickup ions (contributing to the increased Np and lower V) and the IMF bending effect caused by Martian current systems. The B and solar wind dynamic pressure are higher during solar maximum compared to solar minimum. Additionally, the V, plasma beta, and Alfvén Mach number exhibit larger values during the solar descending phase compared to the solar ascending phase. The higher V results in a smaller IMF SA, and this effect is more pronounced near Earth and Mars but less noticeable near Jupiter and Saturn. Our statistical survey provides a reference for the upstream solar wind conditions at these planets, benefiting solar wind studies and planetary space environment research.