Ayumi Asai

We report imaging spectroscopic observations of an M1.4 solar flare obtained during a coordinated observation between the infrared spectropolarimeter SCIP onboard the S unrise iii balloon mission and Domeless Solar Telescope (DST) at Hida Observatory, Kyoto University. The flare that occurred on 2024 July 13 in NOAA Active Region 13738 exhibited a compact flare kernel located within a sunspot umbra. SCIP performed rapid slit-scan observations over a field of view of 58″ × 58″ around the umbra with a cadence of 40 s, covering infrared chromospheric and upper-photospheric lines including Ca ii 8498/8542 Å and K i D ₁ . At the same time, DST observed a wider surrounding region with a cadence of 25 s in H α , Ca ii 8542 Å, and Na i D ₁ /D ₂ . Clear flare-related brightenings are detected in all chromospheric lines observed by SCIP and DST, while no significant enhancement is found in photospheric lines. The high spatial resolution of SCIP reveals fine substructures within the kernel on spatial scales of order 1000 km, which appear smeared in ground-based observations. The spectral profiles exhibit temporally and spatially varying Doppler shifts and line broadenings, indicating complex, fine-scale plasma motions in the chromosphere. These results suggest that the observed red asymmetry arises from the temporal succession of multiple fine-scale kernels, as revealed by SCIP, rather than from a single continuous process.

In this Letter, we present the direct observations of a mode conversion from a large-amplitude transverse propagating kink wave to antiphase large-amplitude longitudinal oscillations (LALOs) in a quiescent prominence observed on 2014 February 25. This event was simultaneously recorded by the Solar Dynamics Observatory/Atmospheric Imaging Assembly and the Behind Solar Terrestrial Relation Observatory/Extreme Ultraviolet Imager. The transverse oscillation was triggered when a global extreme-ultraviolet wave associated with an X4.9 flare swept over the northern end of the prominence. For the first time, a clearly visible, coherent large-amplitude propagating kink wave was observed in the prominence, traveling southward along the prominence spine with a phase speed of ∼380 km s ⁻¹ . The propagating wave reached its largest initial amplitude (∼15 Mm) and period (∼24 minutes) near the spine center. After one cycle of the transverse oscillation, antiphase LALOs were detected simultaneously near both ends of the prominence, exhibiting similar physical parameters (period ∼79 minutes, initial displacement amplitude ∼10 Mm, and velocity amplitude ∼12 km s ⁻¹ ). Their strict antiphase relation, supported by quantitative order-of-magnitude consistency, demonstrates that the LALOs were directly triggered by a nonlinear magnetic pressure gradient (ponderomotive effect) induced by the large-amplitude propagating kink wave. These results provide the first direct evidence of the large-amplitude transverse-to-longitudinal wave-mode conversion and reveal the nonlinear coupling mechanism in a prominence.

Recent Sun-as-a-star studies have shown that postflare loops can manifest as a secondary peak in the H α light curve, suggesting that stellar postflare loops are detectable. To understand what determines the timing of such a secondary peak in the H α light curve associated with postflare loops, we must quantitatively identify the key physical processes controlling the appearance of H α postflare loops. Previous case studies have indicated that the timing of the appearance of H α postflare loops is likely governed by radiative cooling. However, the statistical characteristics of the timing of H α postflare loops' appearance remain insufficiently investigated. In this study, we statistically investigated the timing of the appearance of H α postflare loops to quantify their cooling processes. As a result, we found a negative correlation between the time difference between the soft X-ray peak and the appearance of the H α postflare loops (Δ t ) and the soft X-ray peak flux ( F _X ). This relationship is consistent with the theoretical scaling between radiative cooling timescale ( τ _rad ) and F _X , where τ rad ∝ F X − 1 / 2 . This statistical result indicates that the timing of the appearance of H α postflare loops relative to the soft X-ray peak is primarily controlled by radiative cooling. Furthermore, we examined the dependence of the scaling law on flare spatial scales ( L ). Consequently, we demonstrated that the spatial scale of unresolved stellar flares can be estimated using the following scaling law: L ∝ F X 1 / 3 Δ t 2 / 3 . Our results are useful for interpreting secondary peaks in the H α data of stellar flares and provide a new method to estimate the spatial scale of unresolved stellar flares.

Prominence seismology, applied to large-amplitude longitudinal oscillations, is used to indirectly diagnose the geometry and strength of the magnetic fields inside the prominence. In this paper, combining imaging and spectroscopic data, the magnetic field configuration of a quiescent prominence is revealed by large-amplitude longitudinal oscillations observed in end view on 2023 December 4. In particular, the prominence oscillation involved blueshift velocities in Dopplergrams and horizontal motions in extreme-ultraviolet images. Originally, the prominence oscillation was triggered by the collision and heating of an adjoining hot structure associated with two coronal jets. The oscillation involved two groups of signals with similar oscillatory parameters, a three-dimensional (3D) initial amplitude of ∼40 Mm and a 3D velocity amplitude of ∼48 km s ⁻¹ , both lasting for ∼4 cycles with a period of ∼77 minutes, with a phase difference of ∼ π /8. The angle between the 3D velocities and the prominence axis ranges from 10 ^∘ to 30 ^∘ . Two methods, utilizing time–distance diagrams and velocity fields, are employed to calculate the curvature radius of magnetic dips supporting the prominence materials. Both methods yield similar value ranges and trends from the bottom to the top of magnetic dips, with the curvature radius increasing from ∼90 Mm to ∼220 Mm, then decreasing to ∼10 Mm, with transverse magnetic field strength ≥25 Gauss. From this, the realistic 3D geometry of the prominence magnetic dips is determined to be sinusoidal. To the best of our knowledge, we present the first accurate calculation of the 3D curvature radius and geometry of the prominence magnetic dips based on longitudinal oscillatory motions.

Prominence seismology, applied to the large-amplitude longitudinal oscillation, is used to indirectly diagnose the geometry and strength of the magnetic fields inside the prominence. In this paper, combining imaging and spectroscopic data, the magnetic field configuration of a quiescent prominence is revealed by large-amplitude longitudinal oscillations observed in end view on 2023 December 4. Particularly, the prominence oscillation involved blueshift velocities in Dopplergrams and horizontal motions in extreme-ultraviolet (EUV) images. Originally, the prominence oscillation was triggered by the collision and heating of an adjoining hot structure associated with two coronal jets. The oscillation involved two groups of signals with similar oscillatory parameters, a three-dimensional (3D) initial amplitude of 40 Mm and a 3D velocity amplitude of 48 km/s, both lasting for 4 cycles with a period of 77 minutes, with a phase difference of pi/4. While the angle between 3D velocities and the prominence axis ranges from 10 to 30. Two methods, utilizing time-distance diagrams and velocity fields, are employed to calculate the curvature radius of magnetic dips supporting the prominence materials. Both methods yield similar value ranges and trends from the bottom to the top of magnetic dips, with the curvature radius increasing from 90 Mm to 220 Mm, then decreasing to 10 Mm, with transverse magnetic field strength 25 Gauss. From this, the realistic 3D geometry of the prominence magnetic dips is determined to be sinusoidal. To the best of our knowledge, we present the first accurate calculation of the 3D curvature radius and geometry of the prominence magnetic dips based on longitudinal oscillatory motions.

The efficient release of magnetic energy in astrophysical plasmas, such as during solar flares, can in principle be achieved through magnetic diffusion, at a rate determined by the associated electric field. However, attempts at measuring electric fields in the solar atmosphere are scarce, and none exist for sites where the magnetic energy is presumably released. Here, we present observations of an energetic event using the National Science Foundation’s Daniel K. Inouye Solar Telescope, where we detect the polarization signature of electric fields associated with magnetic diffusion. We measure the linear and circular polarization across the hydrogen Hε Balmer line at 397 nm at the site of a brightening event in the solar chromosphere. Our spectro-polarimetric modeling demonstrates that the observed polarization signals can only be explained by the presence of electric fields, providing conclusive evidence of magnetic diffusion, and opening a new window for the quantitative study of this mechanism in space plasmas.

Context. Solar filament eruptions usually appear to occur in association with the sudden explosive release of magnetic energy accumulated in long-lived arched magnetic structures. The released energy occasionally drives fast-filament eruptions that can be the source regions of coronal mass ejections. A quantitative analysis of high-speed filament eruptions is thus essential to help elucidate the formation and early acceleration of coronal mass ejections. Aims. The goal of this paper is to investigate the dynamic processes of a fast-filament eruption by using unprecedented high-resolution full-disk Hα imaging spectroscopy observations. Methods. The whole process of the eruption was captured in a wide spectral window of the Hα line (±9.0 Å), which allowed for the detection of highly Doppler-shifted plasma. By applying the “cloud model” and obtaining two-dimensional optical thickness spectra, we derived the Doppler velocity; the true eruption profiles (height, velocity, and acceleration); and the trajectory of the filament eruption in 3D space. Results. The Doppler velocity maps show that the filament was predominantly blueshifted. During the main and final process of the eruption, strongly blueshifted materials manifest, traveling with velocities exceeding 250 km s⁻¹. The spectral analysis further revealed that the erupting filament is made of multiple components, some of which were Doppler-shifted approximately to −300 km s⁻¹. We found that the filament eruption attains a maximum true velocity and acceleration of about 600 km s⁻¹ and 2.5 km s⁻², respectively, and its propagation direction deviates from the radial direction. On the other hand, downflows manifested as redshifted plasma close to the footpoints of the erupting filament move with velocities of 45–125 km s⁻¹. We interpret these redshifted signatures as draining material and therefore as mass loss of the filament, which has implications for the dynamic and the acceleration process of the eruption. Furthermore, we have estimated the total mass of the Hα filament, resulting in ∼5.4 × 10¹⁵ g.

Postflare loops are loop-like plasmas observed during the decay phase of solar flares, and they are expected to exist for stellar flares. However, it is unclear how postflare loops are observed in stellar flares’ cases. To clarify behaviors of postflare loops in spatially integrated data, we performed the Sun-as-a-star analysis of the X1.6 flare that occurred on 2023 August 5, using GOES X-ray flux (∼10⁷ K), extreme ultraviolet (EUV) images taken by Atmospheric Imaging Assembly on board the Solar Dynamic Observatory (≥10^4.9 K), and Hα data taken by Solar Dynamics Doppler Imager on board the Solar Magnetic Activity Research Telescope at Hida Observatory, Kyoto University (∼10⁴ K). As a result, we found that this flare showed signatures corresponding to the important dynamics of the postflare loops even in the spatially integrated data: (1) The Hα light curve showed two distinct peaks corresponding to the flare ribbons and the postflare loops. The plasma cooling in the postflare loops generated different peak times in soft X-rays, EUV, and Hα light curves. (2) Downflows were confirmed as simultaneous redshifted/blueshifted absorptions in the Hα spectra. (3) The apparent rise of postflare loops was recognized as a slowing of the decay for the Hα light curve. These results are the key to investigating stellar postflare loops with spatially integrated data. We also discuss the dependence of our results on flare locations and their possible applications to stellar observations.