Yumi Bamba

Understanding when and where a solar flare will occur continues to be an important goal for the heliophysics community, relevant to fundamental research, coordinated flare observations, and space weather applications. In 2024 April, the Focusing Optics X-ray Solar Imager (FOXSI-4) and High-resolution Coronal Imager (Hi-C Flare) carried out NASA’s first coordinated sounding rocket solar flare campaign at the Poker Flat Research Range (PFRR) in Fairbanks, Alaska, for the purpose of performing a triggered launch to observe a large (>C5 class) solar flare with flare-optimized instrumentation. For this campaign, we developed and successfully implemented a novel real-time early flare alert system that leverages low-latency solar imaging and irradiance measurements of the early stages of flares to establish when a large flare is imminent, predict the remaining duration, and identify the flare location in real time. By providing actionable flare nowcasts, this optimized collection of tools/visualizations informed real-time launch decisions and enabled the successful launch of the FOXSI-4 and Hi-C Flare experiments to observe a long-duration M-class flare. We detail the implementation of this early flare alert system, its operational validation during the launch, and the lessons learned to inform future flare monitoring systems and solar flare observing campaigns.

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.

In early May 2024, active regions (ARs) 13663 and 13664 appeared on the solar disk and produced 17 X-class flares. The associated coronal mass ejections caused geomagnetic disturbances, named the Gannon Storm, resulting in significant societal impacts. To investigate why these regions generated successive X-class flares, we conducted detailed analyses of nine X-class flares that occurred near the solar disk center using data from the Solar Dynamics Observatory and post-event analysis based on the κ-scheme, a flare prediction scheme utilizing nonlinear force-free field modeling. Comparative analysis was also performed with ARs 12192 and 12673, which produced multiple X-class flares during the previous solar cycle 24. Our analysis showed that AR 13664 maintained flare-favorable conditions through the emergence of multiple twisted magnetic flux systems and sustained flux injection over 11 days, which continuously increased the releasable magnetic energy (Er) and preserved small critical radii (rc) conducive flare onset reconnection. AR 13663 also produced multiple flares due to strong magnetic shear and a rapid flux injection episode lasting 2.5 days, which led to the rapid growth of flare-triggering magnetic structures. All nine flares originated under the conditions of rc < 1.1 [Mm] and Er > 1.9x1030 [erg], which we propose as new threshold values for X-class flare prediction using the κ-scheme. Additionally, some flares were triggered by small-scale reconnection outside strong shear regions, propagating along the magnetic polarity inversion line like a fuse and activating larger energy release. These findings highlight the importance of monitoring both strong and weak shear regions for effective flare forecasting. This study demonstrates the potential to contribute to the development of a preemptive and stepwise warning system for large flares based on the κ-scheme, by enabling future statistical analyses of a broader set of events—including M-class flares—to derive threshold values corresponding to flare magnitudes.

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

Glancing blows of three interplanetary shocks caused an unexpectedly large magnetic storm on November 4-6, 2023, which was popular for citizen scientists because of the surprising appearance of the crimson-red auroras world-wide in middle latitudes. Based on the analysis of the in-situ interplanetary magnetic field data at DSCOVR and STEREO-A, we show that the multi-step main phase of the magnetic storm is explained by the shock pileup, i.e. slow interplanetary shock was caught up from behind by the fast one, and the multi-step prolonged recovery phase by the remnant structure associated with the shock pileup.

The solar active region NOAA 12887 produced a strong X1.0 flare on 2021 October 28, which exhibits X-shaped flare ribbons and a circle-shaped erupting filament. To understand the eruption process with these characteristics, we conducted a data-constrained magnetohydrodynamics simulation using a nonlinear force-free field of the active region about an hour before the flare as the initial condition. Our simulation reproduces the filament eruption observed in the Hα images of GONG and the 304 Å images of SDO/AIA, and suggests that two mechanisms can possibly contribute to the magnetic eruption. One is the torus instability of the preexisting magnetic flux rope (MFR) and the other is upward pushing by magnetic loops newly formed below the MFR via continuous magnetic reconnection between two sheared magnetic arcades. The presence of this reconnection is evidenced by the SDO/AIA observations of the 1600 Å brightening in the footpoints of the sheared arcades at the flare onset. To clarify which process is more essential for the eruption, we performed an experimental simulation in which the reconnection between the sheared field lines is suppressed. In this case too, the MFR could erupt, but at a much reduced rising speed. We interpret this result as indicating that the eruption is not only driven by the torus instability, but additionally accelerated by newly formed and rising magnetic loops under continuous reconnection.

The solar active region 12673 produced two successive X-class flares (X2.2 and X9.3) approximately 3 hr apart in 2017 September. The X9.3 flare was the largest recorded solar flare in Solar Cycle 24. In this study we perform a data-constrained magnetohydrodynamic simulation taking into account the observed photospheric magnetic field to reveal the initiation and dynamics of the X2.2 and X9.3 flares. According to our simulation, the X2.2 flare is first triggered by magnetic reconnection at a local site where at the photosphere the negative polarity intrudes into the opposite-polarity region. This magnetic reconnection expels the innermost field lines upward, beneath which the magnetic flux rope is formed through continuous reconnection with external twisted field lines. Continuous magnetic reconnection after the X2.2 flare enhances the magnetic flux rope, which is lifted up and eventually erupts via the torus instability. This gives rise to the X9.3 flare.

Predicting large solar flares: The sudden release of magnetic energy on the Sun drives powerful solar flares, which are difficult to predict. Kusano et al. derived physics-based thresholds for the onset of large solar flares and show how they can be predicted from routine solar observations (see the Perspective by Veronig). They tested their method using observations of the Sun from 2008 to 2019. In most cases, the method correctly identifies which regions will produce large flares within the next 20 hours, although there are some false positives and false negatives. The method also provides the exact location where each flare will begin and limits on how powerful it will be. Accurate predictions of solar flares could improve forecasts of space weather conditions around Earth. Science , this issue p. 587 ; see also p. 504

The largest X9.3 solar flare in solar cycle 24 and the preceding X2.2 flare occurred on September 6, 2017, in the solar active region NOAA 12673. This study aims to understand the onset mechanism of these flares via analysis of multiple observational datasets from the Hinode and Solar Dynamics Observatory and results from a non-linear force-free field extrapolation. The most noticeable feature is the intrusion of a major negative-polarity region, appearing similar to a peninsula, oriented northwest into a neighboring opposite-polarity region. We also observe proxies of magnetic reconnection caused by related to the intrusion of the negative peninsula: rapid changes of the magnetic field around the intruding negative peninsula; precursor brightening at the tip of the negative peninsula, including a cusp-shaped brightening that shows a transient but significant downflow (~100 km/s) at a leg of the cusp; a dark tube-like structure that appears to be a magnetic flux rope that erupted with the X9.3 flare; and coronal brightening along the dark tube-like structure that appears to represent the electric current generated under the flux rope. Based on these observational features, we propose that (1) the intrusion of the negative peninsula was critical in promoting the push-mode magnetic reconnection that forms and grows a twisted magnetic flux rope that erupted with the X2.2 flare, (2) the continuing intrusion progressing even beyond the X2.2 flare is further promoted to disrupt the equilibrium that leads the reinforcement of the magnetic flux rope that erupted with the X9.3 flare.

The largest X9.3 solar flare in solar cycle 24 and the preceding X2.2 flare occurred on 2017 September 6, in the solar active region NOAA 12673. This study aims to understand the onset mechanism of these flares via analysis of multiple observational data sets from the Hinode and Solar Dynamics Observatory and results from a nonlinear force-free field extrapolation. The most noticeable feature is the intrusion of a major negative-polarity region, appearing similar to a peninsula, oriented northwest into a neighboring opposite-polarity region. We also observe proxies of magnetic reconnection related to the intrusion of the negative peninsula: rapid changes of the magnetic field around the intruding negative peninsula; precursor brightening at the tip of the negative peninsula, including a cusp-shaped brightening that shows a transient but significant downflow (∼100 km s⁻¹) at a leg of the cusp; a dark tube-like structure that appears to be a magnetic flux rope that erupted with the X9.3 flare; and coronal brightening along the dark tube-like structure that appears to represent the electric current generated under the flux rope. Based on these observational features, we propose that (1) the intrusion of the negative peninsula was critical in promoting the push-mode magnetic reconnection that forms and grows a twisted magnetic flux rope that erupted with the X2.2 flare, and (2) the continuing intrusion progressing even beyond the X2.2 flare is further promoted to disrupt the equilibrium that leads the reinforcement of the magnetic flux rope that erupted with the X9.3 flare.