Daikou Shiota

We studied a coronal mass ejection (CME) structure that propagated through the inner heliosphere in October 2021, which was observed by BepiColombo, Solar Orbiter, PSP, STEREO‐A, and DSCOVR spacecraft located at different radial distances and heliospheric longitudes. Notably, four of the five spacecraft detected a dip‐like signature in the magnetic field embedded within the CME. We investigated the cause of the dip‐like signature on the basis of both in situ solar wind data and solar surface data. We examined the flare‐triggering structure of the M1.6 flare, which probably caused the CME; however, there was no conclusive evidence found linking the flare to the dip‐like signature observed in the ejected CME. We applied the minimum variance analysis (MVA) to the in situ magnetic field data to determine whether the dip‐like signature could represent a planar magnetic structure (PMS) embedded in the CME‐driven shock sheath. Our MVA results confirmed that the magnetic field associated with the dip‐like structure lies along a plane perpendicular to the minimum variance direction, consistent with the characteristics of the PMS previously reported in the literature. Additionally, we examined the shock parameters and found that the CME‐driven shock developed more rapidly than usual, probably because of the passage of a stream interaction region (SIR) during the initial phase of its propagation. This fact suggests that the interaction between the CME and the SIR played a significant role in the formation of the PMS, resulting in the observation of dip‐like structure downstream at most of the spacecraft.

WArning System for AVIation Exposure to Solar energetic particles (WASAVIES) is a physics‐based nowcasting system designed to support warning operations for aviation exposure to solar energetic particles (SEPs) by providing near‐real‐time global SEP dose‐rate estimates. On 11 November 2025, ground level enhancement GLE77 occurred and successfully triggered the SEP mode of WASAVIES, demonstrating fully autonomous initiation of global SEP dose‐rate nowcasts during an extreme space‐weather event. The real‐time analysis, however, was partially degraded during parts of the event due to irregular behavior in some neutron monitor (NM) data streams and evolving background conditions. We therefore performed a post‐event reanalysis using a patched version of the operational algorithm to mitigate these issues. Our analysis indicates that GLE77 consisted of multiple peaks and maintained an unusually low rigidity spectral index (as low as 4) for several hours, suggesting that it was the hardest‐spectrum GLE since 2000. The analysis also indicates that the estimated global maximum effective dose rate (including the galactic cosmic‐ray contribution) briefly exceeded the International Civil Aviation Organization Moderate advisory threshold (30 μSv/h) at altitudes of 14 km (∼FL460) and above, but the exceedance was short‐lived (∼10 min) and limited in magnitude and geographic extent. This finding confirms that GLE77 did not warrant major operational responses such as route or altitude changes. Overall, these results validate the operational potential of WASAVIES and identify key upgrades needed to ensure stable, reliable nowcasts for future extreme events.

WASAVIES (WArning System for AVIation Exposure to Solar energetic particles) is a physics-based nowcasting system designed to support warning operations for aviation exposure to solar energetic particles (SEPs) by providing near-real-time global SEP dose-rate estimates. On 11 November 2025, ground level enhancement GLE77 occurred and successfully triggered the SEP mode of WASAVIES, demonstrating fully autonomous initiation of global SEP dose-rate nowcasts during an extreme space-weather event. The real-time analysis, however, was partially degraded during parts of the event due to irregular behavior in some neutron monitor (NM) data streams and evolving background conditions. We therefore performed a post-event reanalysis using a patched version of the operational algorithm to mitigate these issues. Our analysis indicates that GLE77 consisted of multiple peaks and maintained an unusually low rigidity spectral index (as low as 4) for several hours, suggesting that it was the hardest-spectrum GLE since 2000. The analysis also indicates that the estimated global maximum effective dose rate (including the galactic cosmic-ray contribution) briefly exceeded the ICAO Moderate advisory threshold (30 μSv/h) at altitudes of 14 km (~FL460) and above, but the exceedance was short-lived (~10 min) and limited in magnitude and geographic extent. This finding confirms that GLE77 did not warrant major operational responses such as route or altitude changes. Overall, these results validate the operational potential of WASAVIES and identify key upgrades needed to ensure stable, reliable nowcasts for future extreme events.

The potential field source surface (PFSS) method is a widely used magnetic field extrapolation technique in the space weather community. The only free parameter in the PFSS method is the source-surface height ( R _SS ), beyond which all field lines are open. Although R _SS is known to vary with solar activity, there is no consensus on how to determine it for a given surface magnetic field distribution. In this study, we investigate the nature of R _SS using a long-period (2006–2023) dataset, covering two solar minima and one maximum. We adopt ADAPT-GONG magnetograms and determine R _SS by matching the open flux estimated from observations at 1 au with that calculated using the PFSS method. Our analysis reveals that R _SS increases slightly after the solar minima and around the solar maximum, and that it can be characterized by both the mean unsigned photospheric magnetic field strength and the dipolarity parameter f _dip , defined as f dip = B dip 2 / ( B dip 2 + B quad 2 + B oct 2 ) , with B _dip , B _quad , and B _oct denoting the magnitudes of dipolar, quadrupolar, and octupolar components of photospheric radial magnetic field, respectively. Our results suggest that R _SS does not exhibit a simple monotonic dependence on the solar activity and must be determined by properly considering both surface magnetic field strength and global field structure.

BepiColombo, the joint ESA/JAXA mission to Mercury, was launched in October 2018 and is scheduled to arrive at Mercury in November 2026 after an 8-year cruise. Like other planetary missions, its scientific objectives focus mostly on the nominal, orbiting phase of the mission. However, due to the long duration of the cruise phase covering distances between 1.2 and 0.3 AU, the BepiColombo mission has been able to outstandingly contribute to characterise the solar wind and transient events encountered by the spacecraft, as well as planetary environments during the flybys of Earth, Venus, and Mercury, and contribute to the characterisation of the space radiation environment in the inner Solar System and its evolution with solar activity. In this paper, we provide an overview of the cruise observations of BepiColombo, highlighting the most relevant science cases, with the aim of demonstrating the importance of planetary missions to perform cruise observations, to contribute to a broader understanding of Space Weather in the Solar System, and in turn, increase the scientific return of the mission. Graphical Abstract

In 2024 May, the scientific community observed intense solar eruptions that resulted in a great geomagnetic storm and auroral extensions, highlighting the need to document and quantify these events. This study mainly focuses on their quantification. The source active region (AR; NOAA Active Region 13664) evolved from 113 to 2761 millionths of the solar hemisphere between May 4 and 14. NOAA AR 13664’s magnetic free energy surpassed 10³³ erg on May 7, triggering 12 X-class flares on May 8–15. Multiple interplanetary coronal mass ejections (ICMEs) were produced from this AR, accelerating solar energetic particles toward Earth. According to satellite and interplanetary scintillation data, at least four ICMEs erupted from AR 13664, eventually overcoming and combining each other. The shock arrival at 17:05 UT on May 10 significantly compressed the magnetosphere down to ≈5.04 R _E and triggered a deep Forbush Decrease. GOES satellite data and ground-based neutron monitors confirmed a ground-level enhancement from 2 UT to 10 UT on 2024 May 11. The ICMEs induced exceptional geomagnetic storms, peaking at a provisional Dst index of −412 nT at 2 UT on May 11, marking the sixth-largest storm since 1957. The AE and AL indices showed great auroral extensions that located the AE/AL stations into the polar cap. We gathered auroral records at that time and reconstructed the equatorward boundary of the visual auroral oval to 29.°8 invariant latitude. We compared naked-eye and camera auroral visibility, providing critical caveats on their difference. We also confirmed global disturbances of the storm-enhanced density of the ionosphere.

Understanding the mechanism(s) of solar wind acceleration is important in astrophysics and geophysics. A promising model of solar wind acceleration is known as the wave/turbulence-driven (WTD) model, in which Alfvén waves feed energy to the solar wind. In this study, we tested the WTD model with global measurements of wind speed from interplanetary scintillation (IPS) observations. For Carrington rotations in minimal and maximal activity phases, we selected field lines calculated by the potential-field source-surface method in high and midlatitudes and compared the simulated and observed wind velocities. The simulation was performed in a self-consistent manner by solving the magnetohydrodynamic equations from the photosphere to the solar wind. In high-latitude regions, the simulated solar wind velocity agrees better with the IPS observation than with the classical Wang–Sheeley empirical estimation, both in maximal and minimal activity phases. In midlatitude regions, the agreement worsens, possibly because of the inaccuracy of the WTD model and/or the magnetic-field extrapolation. Our results indicate that the high-latitude solar wind is likely to be driven by waves and turbulence and that the physics-based prediction of the solar wind velocity is highly feasible with an improved magnetic-field extrapolation.