arXiv subjects

Sami K. Solanki

Publications and source records attributed to Sami K. Solanki.

At least 37 records · Page 2Linked to original sources

arXiv2026

The rotation-magnetism relationship in solar-type stars. Constraining magnetic flux emergence rates

The rotation-activity relationship of G-type stars results from surface magnetic fields emerging from the interior. How the magnetic flux and its emergence rate scale with rotation rate are not well understood, both observationally and theoretically. We aim to constrain the emerging magnetic flux as a function of the rotation rate in solar-type stars by numerical simulations compared to empirical constraints set by direct measurements of stellar magnetic fields. We used our flux emergence and transport (FEAT) model for stars with a range of power-law slopes for the dependence of the emerging flux on rotation. Complementing this with a heuristic account of the main flux components, we modelled the resulting mean unsigned field strength as a function of the rotation rate. We compared the results with the Zeeman-intensification measurements and spectropolarimetric data of solar-type stars. Deviations of the model from observations of G stars correlate strongly with stellar metallicity (r=0.83) and effective temperature (r=-0.76), with a combined coefficient of 0.90, reflecting the dependence of magnetic activity on these two parameters. Correcting for these effects with multilinear regression, we find that magnetic flux emergence rates must scale steeply with rotation (power-law exponent of ~1.9) to reproduce observed field strengths, significantly exceeding the estimates in the literature. We provide correction factors for metallicity and temperature for measurements of early-G-type stellar magnetic fields. Stellar magnetic flux emergence rates scale steeply with rotation, requiring active-region fields to dominate the total surface flux on rapid rotators, whereas small-scale-dynamo fields dominate for slow rotators such as the Sun. Metallicity significantly influences the rotation-magnetism relationship, necessitating sample-dependent corrections for accurate stellar dynamo modelling.

astro-ph.SR↗

arXiv2026

Sunrise III: Instrument, mission, data, and first results

Sunrise III is a stratospheric balloon-borne solar observatory with a 1-m diameter telescope and three post-focus instruments, along with an image stabilisation system, all within a protective gondola. It samples the lower solar atmosphere, from the solar surface to the middle chromosphere, at a resolution approaching 50~km on the Sun. Sunrise III flew successfully for 6.5 days suspended from a zero-pressure stratospheric balloon from northern Sweden to north-western Canada in July 2024, gathering around 200 TB of data. The present issue of ApJL focuses on the first scientific results from the data collected during that flight. This paper introduces this Focus Issue, providing a very brief overview of the capabilities of the instrumentation, the flight and of the gathered data. Challenges for the measurements, data reduction and interpretation are also briefly touched upon. The paper ends with an overview of the first set of science results obtained from these data, as presented in the current Focus Issue.

astro-ph.SR↗

arXiv2026

Quantifying the effect of passband on observations in the Ca II K line

Full-disk observations of the Sun in the Ca II K line have been carried out since the late 19th century at various observatories worldwide. These long-term records of solar activity are crucial for reducing discrepancies among solar irradiance reconstructions and for advancing our understanding of the solar dynamo. To construct a consistent composite record, data from different observatories must be cross-calibrated to account for variations in spectral passband and spatial resolution, which are the primary sources of discrepancies between archives. In this study, we use high spectral and spatial resolution observations in the Ca II K line from the state-of-the-art Sunrise III mission to emulate different passbands and derive empirical contrast-contrast relationships between them. We find that these relationships are well described by a power law and provide coefficients for different combinations of passband widths in the range 0.1--9 Angstroms and spatial resolutions between 1 arcsec and 6 arcsec. Applying such a relationship to observations from two major Ca II K archives demonstrates its potential to improve their cross-calibration. The results provide a foundation for the construction of a consistent, century-long time series of solar activity from historical and modern Ca II K observations.

astro-ph.SR↗

arXiv2026

Diversity of morphology of type II spicules in MURaM-ChE simulations

Spicules are ubiquitous, small-scale features in the solar atmosphere, exhibiting a jet-like appearance most clearly identified by their apparent motion in off-limb observations. While they are often interpreted as narrow, thread-like structures, their true three-dimensional (3D) structure remains unknown. We aim to uncover the 3D morphology and dynamics of fast-evolving spicules (type II) using a MURaM-ChE simulation. We use a H$α$ proxy that has been developed using non-equilibrium (NE) hydrogen populations in MURaM-ChE. The proxy, modelled as an escape probability, is synthesised to isolate on-disc as well as off-limb H$α$ wing features. The 3D structure of these features is investigated using the 3D information on opacity in H$α$. We identify type II spicules, with unique 3D morphologies: the dominant being thread-like and slab-like. The appearance of spicules as slabs or threads is a function of time and Doppler velocity. The spicules extending above the spicule-forest (2Mm -3Mm above the surface) tend to be located at quasi-separatrix layers (QSLs). We find that the spatially resolved contributions to the opacity of spicules are often similar for spicules synthesised in the horizontal direction, and their on-disk rapid blueshifted excursion (RBE) synthesised in the vertical direction at the same Doppler velocity of 37km/s. This confirms that RBEs are indeed the on-disc counterparts of spicules. Furthermore, our analysis indicates that cross-field motions can significantly contribute to spicule dynamics. Spicules exhibit a range of morphologies, including both slab-like and thread-like structures. Their observed appearance depends strongly on line-of-sight projection and Doppler sampling. Spicules are preferentially located at QSLs, highlighting the role of magnetic topology in driving spicular dynamics.

astro-ph.SR↗

arXiv2026

A single power law for the TRAPPIST-1 flare distribution across four orders of magnitude in energy

TRAPPIST-1 is an ultra-cool dwarf that flares frequently. These flares shape the surrounding planets' high-energy irradiation environments, with consequences for atmospheric chemistry and escape, and they can contaminate transmission spectroscopy of those planets. A quantitative flare-frequency distribution (FFD) spanning the full energy range is therefore essential for both interpreting JWST spectra and modeling the planets' irradiation histories. Here we present a unified FFD over four orders of magnitude in energy by jointly analyzing $\approx$87\,hr of JWST/NIRISS and JWST/NIRSpec time-series spectroscopy together with $\approx$74\,days of \textit{Kepler}/K2 photometry. To enable a consistent comparison across these heterogeneous datasets, we convert all events to energies in the TESS bandpass. For the Kepler-to-TESS conversion we adopt a cooler flare continuum appropriate for ultra-cool dwarfs ($T_{\rm flare}=3500$\,K). After correcting for flare-detection sensitivities, the combined JWST+K2 cumulative FFD is consistent with a single power law, $N(\ge E_\mathrm{TESS})\propto E_\mathrm{TESS}^{-β}$, with $β=0.753$ over $E_{\rm TESS}\simeq10^{29}$-$10^{33}$\,erg. The slope of the distribution indicates that the time-averaged flare energy budget is dominated by rare, high-energy events rather than by the more numerous low-energy flares. Moreover, we found that strong flares with energies $E_\mathrm{TESS} > 10^{32}$~erg occur once every 25 days, about an order of magnitude more frequently than inferred from previous TRAPPIST-1/analog FFD estimates. This elevated rate of energetic flares has important implications for atmospheric escape, photochemistry, and habitability assessments of the TRAPPIST-1 planets.

astro-ph.SR↗

arXiv2026

Self-consistent numerical simulations for the formation and dynamics of solar prominences

Solar prominences are cool and dense plasma structures floating in the hot solar corona. They are ubiquitous features in the solar atmosphere, but their formation mechanism is still unclear. Here we perform comprehensive fully three-dimensional numerical simulations of prominence formation including the physics necessary to describe all atmospheric layers of the sun. With appropriate initial conditions for the magnetic field, solar prominences form self-consistently in the simulations. The formation starts by the random ejection of a dense plasma seed from the chromosphere into the corona. Subsequently, the prominence is built up by a combination of plasma injections from the chromosphere and condensation of inflowing coronal plasma. The prominence properties qualitatively match those of observed prominences. Our findings demonstrate the importance of the dynamics at and below the solar surface in the formation and evolution of solar prominences. This suggests that subsurface dynamics should also be considered in the study of prominence eruptions, which can be associated with coronal mass ejections.

astro-ph.SR↗

arXiv2026

Unraveling the Secrets of the lower Solar Atmosphere: One year of Operation of the Solar Ultraviolet Imaging Telescope (SUIT) on board Aditya-L1

The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard Aditya--L1, the first solar space observatory of the Indian Space Research Organization (ISRO), India, launched on September 2, 2023. SUIT is designed to image the Sun in the 200--400 nm wavelength band in eight narrowband and three broadband filters. SUIT's science goals start with observing the solar atmosphere and large-scale continuum variations, the physics of solar flares in the NUV region, and many more. The paper elucidates the functioning of the instrument, software packages developed for easier calibration, analysis, and feedback, calibration routines, and the regular maintenance activity of SUIT during the first year of its operation. The paper also presents the various operations undergone by, numerous program sequences orchestrated to achieve the science requirements, and highlights some remarkable observations made during the first year of observations with SUIT.

astro-ph.SR↗

arXiv2026

Sunrise III: The Wavefront Correction System

This paper describes the wave-front correction and image stabilisation system (CWS) developed for the Sunrise III balloon-borne telescope, and provides information about its performance as measured during the integration into the telescope and during the 2024 science flight. The fast image stabilisation is done by a correlation tracker (CT) and a fast tip-tilt mirror, low order aberrations such as defocus and coma are measured by a six-element Shack-Hartmann wavefront sensor (WFS) and corrected by an active telescope secondary mirror for automated focus and manual coma correction. The CWS is specified to deliver a stabilised image with a precision of 0.005 arcsec (rms). The autofocus adjustment is specified to maintain a focus stability of 0.01 waves in the focal plane of the CWS.

astro-ph.IM↗

arXiv2026

A stray light analysis for SO/PHI-HRT and an updated comparison of the inferred magnetic field with SDO/HMI

Context. The High Resolution Telescope of the Polarimetric and Helioseismic Imager on Solar Orbiter (SO/PHI-HRT) operates in an extreme observational environment, observing the Sun as close as $0.28$ au. The high thermal load and large illuminating field puts high demands on the instrument in terms of both imaging performance and false light control. Aims. To characterise the amount of stray light (false light) within SO/PHI-HRT, apply a correction, and re-compare the data products with the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory (SDO/HMI). Methods. We analyse solar limb profiles and a Mercury transit to quantify the amount of stray light and add a correction term when partially reconstructing the SO/PHI-HRT images. For the comparison with SDO/HMI we use data from the 2023 March Solar Orbiter inferior conjunction and compare the magnetic fields on a pixel-by-pixel basis. Results. Increased continuum intensity contrast in the quiet Sun, and darker intensity levels are found in strong magnetic features. Consequently, much stronger fields are inferred in these features. Comparing the stray light corrected data with that from the standard SDO/HMI data products results in a much closer agreement across all vector magnetic field components, particularly when the cadence and noise levels are identical. In most solar features, SO/PHI-HRT infers stronger fields than the SDO/HMI line-of-sight magnetograms. Compared to the vector magnetic field from SDO/HMI the two are very well aligned, with only slight differences in the strongest field regions (where $|\mathbf{B}|>1600$ G or $|\mathbf{B_{\text{LOS}}}|>1300$ G).

astro-ph.SR↗

arXiv2026

Simulations of facular magnetic fields on cool stars I: Main sequence stars with solar metallicity

Stellar convection in the presence of magnetic field affects the emergent intensity, as well as the structure and evolution of cool main-sequence dwarfs. We aim to understand the effect of faculae-like field strengths on near-surface stellar convection using 3D radiative MHD simulations of near-surface magneto-convection. We compare simulations of F, G, K and M main-sequence stars with a small-scale dynamo (SSD) to faculae-like spatially averaged field strengths (from 100 to 500 G). We focus on the effect of imposed magnetic field on the thermodynamic stratification and velocities, along with the bolometric intensity and surface field strength. Imposed magnetic fields result in reduced average density and gas pressure near the surface compared to the SSD simulations. The temperature stratification also shows a dip at and just below the stellar surface. The changes in average bolometric intensity are within a percent, with different trends with field strength for different stellar types. In addition, the convective velocities are reduced. The magnitude of changes in thermodynamic quantities are related to field strength as well as the stellar $T_{\rm eff}$. Faculae-strength magnetic fields modify the near surface convection by reducing gas pressure and density as well as suppressing convection in regions with strong field concentrations. The strength of these effects depends on the stellar type.

astro-ph.SR↗

arXiv2026

Cyclic sunspot activity during the first millennium CE as reconstructed from radiocarbon

Context. Solar activity, dominated by the 11-year cyclic evolution, has been observed directly since 1610. Before that, indirect cosmogenic proxy data are used to reconstruct it over millennia. Recently, the precision of radiocarbon measurements has improved sufficiently to allow reconstructing solar activity over millennia. Aims. The first detailed reconstruction of solar activity, represented by annual sunspot numbers, is presented for 1-969 CE. Methods. The reconstruction of sunspot numbers from D14C was performed using a physics-based method involving several steps: using a carbon-cycle box model, the 14C production rate, corrected for the geomagnetic shielding, was computed from the measured data; The open solar magnetic flux was computed using a model of the heliospheric cosmic-ray modulation; Sunspot numbers were calculated using a model of the evolution of the Sun's magnetic field. The Markov Chain Monte Carlo approach was used to account for different sources of uncertainty. Results. Annual sunspot numbers were reconstructed for the first millennium CE. This period includes one extreme solar event of 774 CE and one Grand solar minimum of 650-730 CE. We could identify 91 solar cycles, of which 26 were well-defined, while 24 and 41 were reasonably and poorly defined, respectively. The mean cycle length was 10.6 years, but the lengths of individual cycles vary between 8 and 15 years. The existence of empirical Waldmeier's relations remains inconclusive. No significant periodicities were found beyond the 11-year cycle. Conclusions. This work fills the gap in the solar cycle statistics between the previously reconstructed first millennium BCE and the second millennium CE, providing vital constraints for the solar dynamo and irradiance models. A consistent 3-millennium-long reconstruction of sunspot numbers, based on a composite multi-proxy cosmogenic record, is pending.

astro-ph.SR↗

arXiv2025

Statistical properties of spicules in MURaM-ChE

Numerical simulations of the solar chromosphere have progressed towards reproducing spicules, which are transient features observed at the solar limb using spectral lines such as H$α$, Ca II H&K, or Mg II h&k. Two types of spicules, referred to as types I and II, have been identified in observations and studied in previous numerical works. The statistics of type II spicules in 3D numerical simulations have not yet been studied. We aim to compare the statistics of properties such as lengths, lifetimes, widths, heights, inclinations, and maximum velocities of self-consistently formed spicules in a MURaM-ChE simulation with observations. We employ a H$α$ proxy to identify fine-scale structures at the solar limb resembling spicules in a simulation of an enhanced network region. We track the evolution of 58 such features found in a 21-minute time sequence, and compare their dynamical and morphological properties with those derived from quiet-Sun observations using the Solar Optical Telescope (SOT) onboard the Hinode mission in the Ca II H spectral line. Previous studies have shown that spicules show very similar properties in Ca II H and H$α$. The spicule-like structures found in the simulation have statistical properties which are broadly consistent with those observed with Hinode/SOT. In particular, we find evidence for the self-consistent formation of both type I and type II spicules within the simulation, even in the absence of ambipolar diffusion. We also investigate the properties of rapid blueshifted and redshifted excursions (RBEs and RREs) in the simulation in relation to the spicules.

astro-ph.SR↗

arXiv2025

Probing chromospheric fine structures with a Hα proxy using MURaM-ChE

H$α$ observations of the solar chromosphere reveal dynamic small-scale structures known as spicules at the limb and rapid blue and red shifted excursions (RBEs/RREs) on-disk. We want to understand what drives these dynamic features, their magnetohydrodynamic (MHD) properties and their role in energy and heat transport to the upper solar atmosphere. To do this, we aim to develop a proxy for synthetic H$α$ observations in radiative-MHD simulations to help identify these features. We use the chromospheric extension to the MURaM code (MURaM-ChE) to simulate an enhanced network region. We develop a proxy for H$α$ based on a photon escape probability. This is a Doppler-shifted proxy that we use to identify fine structures in the line wings. We study on-disk features in 3D, obtaining their 3D structure from the absorption coefficient. We validate the H$α$ proxy by comparing it against features detected in the wings of H$α$ synthesized using MULTI3D. We detect numerous small-scale structures rooted at the network patches, similar to observations in H$α$. The dynamics of an example feature (RBE) at a Doppler shift of 37 km/s show that flux emergence and consequent reconnection drive the formation of this feature. Pressure gradient forces build up to drive a flow along the field line carrying the feature, making it a jet. There is strong viscous and resistive heating at the first appearance of the feature associated with the flux emergence. At the same time and location, a heating front appears and propagates along the field lines at speeds comparable to the Alfven velocity. We show that a synthetic observable based on an escape probability is able to reliably identify features observed with the H$α$ spectral line. We demonstrate its applicability by studying the formation, dynamics and properties of an RBE.

astro-ph.SR↗

arXiv2025

3D structures of the base of small-scale recurrent jets revealed by Solar orbiter

Solar jets, characterized by small-scale plasma ejections along open magnetic field lines or the legs of large-scale coronal loops, play a crucial role in the dynamics of the solar atmosphere. Although spectral and EUV images have been widely used to analyze the formation and evolution of jets, the detailed 3D structure at the base of the jet has not been studied in detail due to the limitations in the spatial resolution of observations. Solar Orbiter enables us to investigate the structure of solar jets with much higher spatial and temporal resolutions and from a different angle than from Earth. By combining observations made by instruments onboard Solar Orbiter with data from the SDO, we analyzed recurrent solar jets originating in a mixed-polarity region near an active region. Additionally, we employed potential field and magnetohydrostatic extrapolation techniques to determine the magnetic field topology associated with the jets. The jets display dynamic, multi-strand outflows emanating from compact bright kernels above the magnetic inversion line, with apparent speeds exceeding 100 km/s. Magnetic field evolution reveals continuous flux cancellation at the jet footpoints. Throughout the sequence, base flows are confined within quasi-separatrix layers, with the highest velocities and temperatures located near coronal null points. Over four eruptions, the magnetic topology evolves from a simple fan-spine configuration with a single null to a more complex dome-shaped base containing multiple nulls with separatrix curtain, accompanied by a morphological transition from narrow, well-collimated spire to broader, fragmented outflows. These results provide the first direct observational evidence that dynamic changes in null-point geometry modulate jet morphology and energetics via successive reconnection episodes.

astro-ph.SR↗

arXiv2025

Flares on TRAPPIST-1 reveal the spectrum of magnetic features on its surface

TRAPPIST-1 is an M8 dwarf hosting seven known exoplanets and is currently one of the most frequently observed targets of the James Webb Space Telescope (JWST). However, it is notoriously active, and its surface is believed to be covered by magnetic features that contaminate the planetary transmission spectra. The radiative spectra of these magnetic features are needed to clean transmission spectra, but they currently remain unknown. Here, we develop a new approach for measuring these spectra using time-resolved JWST/NIRISS observations. We detect a persistent post-flare enhancement in the spectral flux of TRAPPIST-1. Our analysis rules out lingering flare decay as the cause of the flux enhancement and, thus, points to structural changes on the stellar surface induced by flares. We suggest that the flaring event triggers the disappearance of (part of) a dark magnetic feature, producing a net brightening. This suggestion is motivated by solar data: flare-induced disappearance of magnetic features on the solar surface has been directly detected in high spatial resolution images, and our analysis shows that this process produces changes in solar brightness very similar to those we observe on TRAPPIST-1. The proposed explanation for the flux enhancement enables, to our knowledge, the first measurement of the spectrum of a magnetic feature on an M8 dwarf. Our analysis indicates that the disappearing magnetic feature is cooler than the TRAPPIST-1 photosphere, but by at most a few hundred kelvins.

astro-ph.EP↗

arXiv2025

Sunrise III: Overview of Observatory and Instruments

In July 2024, Sunrise completed its third successful science flight. The Sunrise III observatory had been upgraded significantly after the two previous successful flights in 2009 and 2013. Three completely new instruments focus on the small-scale physical processes and their complex interaction from the deepest observable layers in the photosphere up to chromospheric heights. Previously poorly explored spectral regions and lines are exploited to paint a three-dimensional picture of the solar atmosphere with unprecedented completeness and level of detail. The full polarimetric information is captured by all three instruments to reveal the interaction between the magnetic fields and the hydrodynamic processes. Two slit-based spectropolarimeters, the Sunrise UV Spectropolarimeter and Imager (SUSI) and the Sunrise Chromospheric Infrared spectro-Polarimeter (SCIP), focus on the near-ultraviolet and the near-infrared regions respectively, and the imaging spectropolarimeter Tunable Magnetograph (TuMag) simultaneously obtains maps of the full field-of-view of $46 \times 46$ Mm$^2$ in the photosphere and the chromosphere in the visible. The instruments are operated in an orchestrated mode, benefiting from a new Image Stabilization and Light Distribution unit (ISLiD), with the Correlating Wavefront Sensor (CWS) providing the autofocus control and an image stability with a root-mean-square value smaller than 0.005''. A new gondola was constructed to significantly improve the telescope pointing stability, required to achieve uninterrupted observations over many hours. Sunrise III was launched successfully on July 10, 2024, from the Esrange Space Center near Kiruna (Sweden). It reached the landing site between the Mackenzie River and the Great Bear Lake in Canada after a flight duration of 6.5 days. In this paper, we give an overview of the Sunrise III observatory and its instruments.

astro-ph.IM↗

arXiv2025

Structure and Dynamics of the Sun's Interior Revealed by Helioseismic and Magnetic Imager

High-resolution helioseismology observations with the Helioseismic and Magnetic Imager (HMI) onboard 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.

astro-ph.SR↗

arXiv2025

X-class flare on Dec 31, 2023, observed by the Solar Ultraviolet Imaging Telescope on board Aditya-L1

We present the multi-wavelength study of the ejection of a plasma blob from the limb flare SOL2023-12-31T21:36:00 from NOAA 13536 observed by the Solar Ultraviolet Imaging Telescope (SUIT) on board Aditya-L1. We use SUIT observations along with those from Atmospheric Imaging Assembly (AIA) on board SDO and Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter to infer the kinematics and thermal nature of the ejected blob and its connection to the associated flare. The observations show that the flare was comprised of two eruptions. The blob was ejected during the first eruption and later accelerated to velocities over 1500 km/s measured at a maximum projected height of ~ 178 Mm from the Sun's surface. The acceleration of the ejected plasma blob is co-temporal with the bursty appearance of the hard X-ray light curve recorded by STIX. Radio spectrogram observations from STEREO-A/WAVES and RSTN reveal type III bursts at the same time, indicative of magnetic reconnection. DEM analysis using AIA observations suggests the plasma blob is comprised of cooler and denser plasma in comparison to the ambient corona. To the best of our knowledge, this is the first observation of such a plasma blob in the NUV, providing crucial measurements for eruption thermodynamics.

astro-ph.SR↗