Research Highlights
ARTICLES | IIA

High-resolution helioseismology observations with the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) provide a unique three-dimensional view of the solar interior structure and dynamics, revealing a tremendous complexity of the physical processes inside the Sun. We present an overview of the results of the HMI helioseismology program and discuss their implications for modern theoretical models and simulations of the solar interior.

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Context. Spicules are elongated, jet-like structures that populate the solar chromosphere and are rooted in the lower solar atmosphere. In recent years, high-resolution observations and advanced numerical simulations have provided insights into their properties, structures, and dynamics. However, the formation mechanism of spicules, particularly the more dynamic type II spicules, which are primarily found in the quiet Sun and coronal holes, remains elusive. Aims. This study explores whether quiet Sun Ellerman bombs (QSEBs), which are ubiquitous small-scale magnetic reconnection events in the lower atmosphere, are linked to the formation of type II spicules. Methods. We analysed a high-quality 40-minute time sequence acquired with the Swedish 1-m Solar Telescope. Hβ data were used to observe QSEBs and spicules, while spectropolarimetric measurements in the photospheric Fe I 6173 Å line provided line-of-sight magnetic field information. We employed k-means clustering to automatically detect QSEBs and explored their potential association with spicules. Results. We identified 80 clear cases in which spicules occurred soon after the QSEB onset and not later than 30 s after the ending of the QSEBs. In all these instances, the events involved type II spicules, rapidly fading from the images. The footpoints of the spicules seemed to be rooted in QSEBs, where the onset of QSEBs often preceded the formation of the associated spicules. In addition to these clear cases, we found around 500 other events that hinted at a connection but with some ambiguities. The combined clear and ambiguous cases constitute 34% of the total detected QSEBs and a smaller percentage of the spicules in our dataset. Conclusions. Our findings suggest that a fraction of the type II spicules originate from QSEBs, supporting magnetic reconnection as a potential driving mechanism. In this context, QSEBs and spicules represent the conversion of magnetic energy into thermal and kinetic energy, respectively. We suggest that an observational programme including multiple Balmer lines would likely detect more unambiguous connections between QSEBs and spicules.

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The gravitational wave (GW) interferometers Laser Interferometer Space Antenna (LISA) and the Einstein Telescope (ET) are expected to be functional in the next decade(s), possibly around the same time. They will operate over different frequency ranges, with similar integrated sensitivities to the amplitude of a stochastic GW background (SGWB). We investigate the synergies between these two detectors, in terms of a multiband detection of a cosmological SGWB characterized by a large amplitude, and a broad frequency spectrum. We develop the notion of integrated sensitivity and propose a novel signal-to-noise ratio optimal for characterization of the geometrical properties of the interferometer systems of LISA and the ET operating simultaneously. By investigating various examples of SGWBs, such as those arising from cosmological phase transition, cosmic string, and primordial inflation, we show that LISA and the ET operating together will have the opportunity to assess more effectively the characteristics of the GW spectrum produced by the same cosmological source, but at separate frequency scales. Moreover, the two experiments in tandem can be sensitive to features of early Universe cosmic expansion before big bang nucleosynthesis (BBN), which affects the SGWB frequency profile and which would not be possible to detect otherwise, since two different frequency ranges correspond to two different pre-BBN (or postinflationary) epochs. Besides considering the GW spectrum, we additionally undertake a preliminary study of the sensitivity of LISA and the ET to soft limits of higher-order tensor correlation functions. Given that these experiments operate at different frequency bands, their synergy constitutes an ideal direct probe of squeezed limits of higher-order GW correlators, which cannot be measured operating with a single instrument only.

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Inflationary models that involve bursts of particle production generate bump-like features in the primordial power spectrum of density perturbations. These features influence the evolution of density fluctuations, leaving their unique signatures in cosmological observations. A detailed investigation of such signatures would help constrain physical processes during inflation. With this motivation, the goal of this paper is two-fold. First, we conduct a detailed analysis of the effects of bump-like primordial features on the sky-averaged 21 cm signal. Using semi-numerical simulations, we demonstrate that the primordial features can significantly alter the ionization history and the global 21 cm profile, making them a promising probe of inflationary models. We found a special scale (namely, the turnover wavenumber, kturn) at which the effect of primordial bump-like features on the global 21 cm profile vanishes. Also, we found that the behaviour of the primordial features on the global profile and ionization history are quite opposite for k > kturn and k < kturn. We trace the root cause of these behaviours to the effects of primordial features on the halo mass function at high redshifts. Furthermore, we discuss the degeneracy between the astrophysical parameters and the primordial features in detail. Secondly, for a fixed set of astrophysical parameters, we derive upper limits on the amplitude of bump-like features in the range 10-1 < k [ Mpc^-1] < 102 using current limits on optical depth to reionization from CMB data by Planck.

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The spectra of coronal mass ejections (CMEs) in the low corona play a crucial role in understanding their origins and physical mechanisms and enhancing space weather forecasting. However, capturing these spectra faces significant challenges. This paper introduces a scheme of a multislit spectrometer design with five slits, acquiring the global spectra of the solar corona simultaneously with a focus on the spectra of CMEs in the low corona. The chosen wavelength range of the spectrometer (170–180 Å) includes four extreme ultraviolet emission lines (Fe x 174.53 Å, Fe ix 171.07 Å, Fe x 175.26 Å, Fe x 177.24 Å), which provides information on the plasma velocity, density, and temperature. Utilizing a numerical simulation of the global corona for both the on-disk and the off-limb scenarios, we focus on resolving the ambiguity associated with various Doppler velocity components of CMEs, particularly for a fast CME in the low corona. A new application of our decomposition technique is adopted, enabling the successful identification of multiple discrete CME velocity components. Our findings demonstrate a strong correlation between the synthetic model spectra and the inverted results, indicating the robustness of our decomposition method and its significant potential for global monitoring of the solar corona, including CMEs.

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The Solar Ultraviolet Imaging Telescope (SUIT) on board the Aditya-L1 mission observes the Sun in the 200 – 400 nm wavelength range. This paper presents the results of various on ground and on-board tests and their comparison with the specifications. Moreover, we also present the scheme for data calibration. We demonstrate that the test results are compliant with the specified numbers, except the spatial resolution. Such discrepancy will limit the photometric measurements only, at a scale of 2.2 instead of 1.4 as originally envisioned.The results obtained here show that SUIT observations open a new window for solar observations.

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Murthy et al. (2025) (hereafter Paper I) have recently reported the discovery of unexpectedly bright diffuse extreme-ultraviolet radiation at high latitudes in both the Northern and Southern Galactic Hemispheres. After correction for extinction by the total interstellar dust in the direction of each observation, the spectra are nearly identical, suggesting that the radiation has a unique source and likely originates in the halo of our galaxy. The observed spectrum extends down to 912 Å, the interstellar hydrogen absorption edge. Radiation even slightly short of that edge would, if ubiquitous, be sufficient to explain the high degree of ionization in our galaxy and throughout the universe. We hypothesize that this newly discovered radiation originates in the slow decay of dark matter. The intensity of the radiation implies that the decay cannot be via the weak interaction, suggesting the existence of a new, even weaker fundamental interaction, consistent with the exceedingly long decay lifetime required.

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The arrival of a series of coronal mass ejections (CMEs) at the Earth resulted in a great geomagnetic storm on 10 May 2024, the strongest storm in the last two decades. Aims. We investigated the kinematic and thermal evolution of the successive CMEs to understand their interaction en route to Earth. We attempted to find the dynamic, thermodynamic, and magnetic field signatures of CME-CME interactions. Our focus was to compare the thermal state of CMEs near the Sun and in their post-interaction phase at 1 AU. Methods. The 3D kinematics of six identified Earth-directed CMEs were determined using the graduated cylindrical shell (GCS) model. The flux rope internal state (FRIS) model was implemented to estimate the CMEs' polytropic index and temperature evolution from their measured kinematics. The thermal states of the interacting CMEs were examined using in situ observations from the Wind spacecraft at 1 AU. Result Our study determined the interaction heights of selected CMEs and confirmed their interactions that led to the formation of complex ejecta identified at 1 AU. The plasma, magnetic field, and thermal characteristics of magnetic ejecta (MEs) within the complex ejecta and other substructures, such as interaction regions within two MEs and double flux rope-like structures within a single ME, show possible signatures of CME-CME interaction in in situ observations. The FRIS-model-derived thermal states of individual CMEs reveal their diverse thermal evolution near the Sun, with all CMEs transitioning to an isothermal state at 6–9 R⊙ except for CME4, which was in an adiabatic state due to a lower expansion rate. The electrons of the complex ejecta at 1 AU are in a predominant heat-release state, while the ions show a bimodal distribution of thermal states. On comparing the characteristics of CMEs near the Sun and at 1 AU, we suggest that such a one-to-one comparison is difficult due to the CME-CME interactions significantly influencing the CMEs' post-interaction characteristics.

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We present the discovery of a peculiar central stellar structure in the collisional ring galaxy AM0644-741 using HST imaging and MUSE integral field unit (IFU) data. We identified two Sérsic components with a Sérsic index of 1.72 (inner part) and 1.11 (outer part) in the HST F814W band optical image using GALFIT. We utilized the MUSE data cube to construct stellar line-of-sight velocity (VLOS), velocity dispersion (σLOS), h3 and h4 velocity moments, and stellar population age maps using the GIST pipeline for further investigating both Sérsic components, which have a difference of ∼60° in their position angle. The inner component, with an effective radius of ∼1 kpc, shows a strong anticorrelation between VLOS/σLOS and h3, indicating the presence of a rotating stellar structure. In addition, the inner component shows a higher velocity dispersion (average values reaching up to ∼240 km s‑1) along with disky isophotes and a stronger Mg b line strength, which all together highlight a peculiar dynamical state of AM0644-741's central region. Our analysis suggests that the recent encounter has had a smaller impact on the stellar orbits within the inner component. In contrast, it has specifically affected the stellar orbits of the progenitor's outer disk when forming the star-forming ring. The Baldwin, Phillips and Terlevich (BPT) analysis of the unresolved nuclear source shows a low-ionization nuclear emission-line region (LINER) type ionization, hinting at active galactic nucleus (AGN) activity in the galaxy. Our study projects the dynamical evolution of collisional systems and provides scope for simulations to explore the central region in greater detail.

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Quiet-Sun Ellerman bombs (QSEBs) are small-scale magnetic reconnection events in the lower solar atmosphere. Sometimes, they exhibit transition region counterparts, known as ultraviolet (UV) brightenings. Magnetic field extrapolations suggest that QSEBs can occur at various locations of a fan-spine topology, with UV brightening occurring at the magnetic null point through a common reconnection process. Aims. We aim to understand how more complex magnetic field configurations such as interacting fan-spine topologies can cause small-scale dynamic phenomena in the lower atmosphere. Methods. QSEBs were detected using k-means clustering on Hβ observations from the Swedish 1-m Solar Telescope (SST). Further, chromospheric inverted-Y-shaped jets were identified in the Hβ blue wing. Magnetic field topologies were determined through potential field extrapolations from photospheric magnetograms derived from spectro-polarimetric observations in the Fe i 6173 Å line. UV brightenings were detected in IRIS 1400 Å slit-jaw images. Results. We identify two distinct magnetic configurations associated with QSEBs, UV brightenings, and chromospheric inverted-Yshaped jets. The first involves a nested fan-spine structure where, due to flux emergence, an inner 3D null forms inside the fan surface of an outer 3D null with some overlap. The QSEBs occur at two footpoints along the shared fan surface, with the UV brightening located near the outer 3D null point. The jet originates close to the two QSEBs and follows the path of high squashing factor, Q. We discuss a comparable scenario using a 2D numerical experiment with the Bifrost code. In the second case, two adjacent fan-spine topologies share fan footpoints at a common positive polarity patch, with the QSEB, along with a chromospheric inverted-Y-shaped jet, occurring at the intersection having high Q values. The width of the jets in our examples is about 0.003, and the height varies between 100–200. The width of the cusp measures between 100–200. Conclusions. This study demonstrates through observational and modelling support that small-scale dynamic phenomena, such as associated QSEBs, UV brightenings, and chromospheric inverted-Y-shaped jets, share a common origin driven by magnetic reconnection between interacting fan-spine topologies

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