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The gamma-ray-emitting binary stellar system LS I +61° 303 was observed with the Imaging X-ray Polarimetry Explorer on two successive orbits over orbital phases of 0.74─1.05. Polarization is detected at a significance of 4.2σ with an average polarization degree of 13.1% ± 3.0% in the 2─8 keV band after background subtraction. This is the second detection of polarization of the X-ray synchrotron emission from a gamma-ray binary and, again, suggests that the magnetic field in the particle acceleration region has a significant ordered component. The orbital motion on the sky of LS I +61° 303 is not well determined, which leads to ambiguity in interpretation of the X-ray electric vector position angle (EVPA) measurement. Use of orbital elements determined via radial velocity measurements combined with radio imaging of variable nebular emission suggests an offset between the X-ray EVPA and the compact object-massive star axis on the order of ∼30°. Such an offset could be produced by Coriolis forces due to binary motion. Use of two different sets orbital elements determined via optical polarimetry suggest either no offset or a perpendicular orientation but require an unexpectedly high inclination, while another set based on the same data but modeled assuming a moderate inclination suggests an offset of 15°. Use of orbital elements derived from modeling of the keV/TeV light curves suggests good alignment between the X-ray EVPA and the compact object-massive star axis. Such alignment was found for the gamma-ray binary PSR B1259-63. If the same physical situation holds for LS I +61° 303, that would favor the orbital elements derived from the keV/TeV light curves.

The mechanisms triggering solar flares and driving coronal heating occur across wide temperature ranges on small spatial scales and short timescales, making them difficult to observe with current instrumentation. The upcoming SOLAR-C mission, launching in the late 2020s, will provide unprecedented plasma diagnostic capability with its high-throughput extreme-ultraviolet (EUV) spectroscopic telescope (EUVST), capable of ~0.2 arcsec/pix spatial sampling (~0.4 arcsec resolution), continuous temperature coverage from 0.02-15 MK, and exposure times down to 0.5 seconds. We present forward modelling of the spectrograph's short wavelength camera (170-210 Å; SOLAR-C/EUVST-SW) and its response to log T~6.2 coronal plasma in a three-dimensional MHD-simulated pre-flare active region. We compare this performance to that of the previous-generation EUV Imaging Spectrometer (EIS) on Hinode (SOLAR-B). Our results demonstrate that SOLAR-C/EUVST can distinguish individual flux tubes in simulated active region loops which Hinode/EIS cannot resolve. In simulated pre-flare plasma, SOLAR-C/EUVST captures sharp velocity gradients between adjacent upflowing and downflowing plasma which Hinode/EIS is unable to resolve. Doppler velocity measurement accuracy will reach better than 1 km/s in active regions. We show that this next-generation spectrograph can be expected to directly observe processes potentially related to flare triggering, such as plasma flows from low-altitude reconnection linked to emerging flux, and determine whether active region loops consist of a small number of strands or the hundreds predicted by magnetic reconnection-induced nanoflare heating models.

Deciphering the physical properties of flares is crucial for understanding the nature of one of the most energetic events on the Sun. We analyze high spatial and spectral resolution observations of a C-class flare, focusing on a complex light bridge in the active region. The observations show Stokes profiles with opposite Doppler shifts and asymmetries between photospheric and chromospheric spectral lines, and Ca II infrared spectral lines in emission, tracing areas of extreme atmospheric heating. Inversion results are consistent with these spectral features, showing regions of opposite-sign plasma flows between the photosphere and chromosphere, adjacent to areas with extreme chromospheric temperatures. The latter areas show relatively weak line-of-sight velocities despite the intense heating. The magnetic field configuration shows abrupt changes with height in the regions with opposite-sign flows, while remaining nearly constant with height in the areas of extreme heating. These findings point to a scenario of small-scale magnetic reconnection in the photosphere, suggesting a flare triggering mechanism based on an avalanche of many small reconnection events.

In May 2024, multiple X-class solar flares, full-halo coronal mass ejections (CMEs), severe geomagnetic disturbances, and severe ionospheric negative storms were observed, and these space weather events caused some social impacts. In this study, we analyzed the performance of space weather forecasts for these extreme space weather events, focusing on maximum forecast levels; X-class solar flares, geomagnetic disturbances with K ≥ 7, and ionospheric storms with I-scale = I3, using multi verification indices such as proportion correct (accuracy), probability of detection (discrimination), false-alarm ratio (reliability), frequency bias (bias), and equitable threat score (skill). As a result, NICT's forecasts were evaluated as having a strong discrimination ability with moderate reliability for X-class solar flare, and high accuracy with subject to discrimination ability for I3 ionospheric storms, while for K ≥ 7 geomagnetic disturbances forecast, nearly perfect performance had been achieved by using solar wind simulation (SUSANOO-CME) incorporating multiple earth-directing CMEs. Comparative analyses with forecasts issued by other regional warning centers (RWCs) of the International Space Environment Service (ISES) indicated that RWC Japan showed the highest discrimination capability for X-class solar flares, while RWC USA showed the highest skill with zero false alarms. Perfect skill for K ≥ 7 geomagnetic disturbances were archived by RWC Japan, together with RWC Australia. Although continued efforts to improve forecast performance are still significant subject, we anticipate that the evaluation presented here will support appropriate implementation of effective measures to mitigate space weather risks to modern social infrastructure.