arXiv subjects

M. Stangalini

Publications and source records attributed to M. Stangalini.

At least 19 recordsLinked to original sources

arXiv2026

Spectropolarimetric analysis of waves linked to FIP

High-resolution spectropolarimetry has opened new avenues for understanding how chromospheric waves shape coronal plasma composition. All modeling efforts so far highlight wave activity and, in particular, the ponderomotive force associated to Alfv\'enic perturbations, as a key ingredient. Over recent years, studies based on spectropolarimetric measurements in the solar chromosphere have identified magnetic perturbations associated to waves linked to regions in the corona with enhanced FIP bias. Building on this established framework, the present work explores the diagnostic potential of Stokes V amplitude asymmetries as an alternative diagnostic tool for investigating wave behavior relevant to compositional fractionation processes.

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arXiv2026

Can instrumental effects obscure the true photospheric wave spectrum?

Optical aberrations and instrument resolution can affect the observed morphological properties of features in the solar atmosphere. However, little work has been done to study the effects of spatial resolution on the dynamical processes occurring in the Sun's atmosphere. In this work, owing to the availability of high-resolution observations of a magnetic pore captured with the Interferometric BIdimensional Spectrometer mounted at the Dunn Solar Telescope, we studied the impact of the diffraction limit and the sampling of an instrument on line-of-sight Doppler velocity oscillations. We reported a noticeable shift in the dominant frequency band from $5$ to $3$ mHz, as both the angular and detector resolutions of the instruments were degraded. We argue that the observed behaviour is a result of the increased contamination of straylight from neighbouring quiet Sun regions, masking the true behaviour of umbral oscillations. These results suggest that the wave energy contributions reported in the literature and based on low-resolution instrumentation may be fundamentally underestimated. As we move into the era of high-resolution instrumentation such as DKIST and MUSE, this work will offer a critical baseline for interpreting new observations, especially in terms of distinguishing true dynamic behaviours from artefacts introduced by instrument-related limitations.

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arXiv2025

First detection of acoustic-like flux in the middle solar corona

Waves are thought to play a significant role in the heating of the solar atmosphere and the acceleration of the wind. Among the many types of waves observed in the Sun, the so-called p-modes with a 3 mHz frequency peak dominate the lower atmosphere. In the presence of magnetic fields, these waves can be converted into magnetohydrodynamic modes, which then leak into the corona through magnetic conduits. High-resolution off-limb observations have indeed revealed signatures of ubiquitous and global 3 mHz oscillations in the corona, although limited to low heights and to incompressible modes. We present high-cadence, high-resolution observations of the corona in the range 1.7 - 3.6 solar radii taken in broad-band 580-640 nm visible light by the Metis coronagraph aboard Solar Orbiter. These observations were designed to investigate density fluctuations in the middle corona. The data were acquired over several days in March 2022, October 2022, and for two days in April 2023. We selected representative regions of the corona on three sample dates. Analysis of the data in those regions revealed the presence of periodic density fluctuations. By examining several time-distance diagrams, we determined the main properties (apparent propagation speed, amplitude) of those fluctuations. We also show power spectra in selected locations in order to determine the dominant frequencies. We found wave-like, compressible fluctuations of low amplitude - of the order of 0.1 % of the background - in several large-scale regions in the corona at least up to 2.5 solar radii. We also found that the apparent propagation speeds of these perturbations typically fall in the range 150 - 450 km/s. A power spectrum analysis of time series revealed an excess power in the range 2-7 mHz, often with peaks at 3 or 5 mHz, i.e. in a range consistent with p-mode frequencies of the lower solar atmosphere.

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arXiv2025

Umbral oscillations in the photosphere. A comprehensive statistical study

It is well-known that the global acoustic oscillations of the Sun's atmosphere can excite resonance modes within large-scale magnetic concentrations. These structures are conduits of energy between the different layers of the solar atmosphere, and understanding their dynamics can explain the processes behind coronal heating and solar wind acceleration. In this work, we studied the Doppler velocity spectrum of more than a thousand large-scale magnetic structures (i.e., sunspots) in the solar photosphere that crossed near the disk centre of the Sun. We exploited the excellent stability and seeing-free conditions of the Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO) to cover nearly seven years of observations, providing the most comprehensive statistical analysis of its kind. Here, we show that the power spectra of the umbra of sunspots in the photosphere is remarkably different from the one of quiet-Sun regions, with both exhibiting a primary peak at 3.3 mHz, but the sunspot umbrae also displaying a closely packed series of secondary peaks in the $4-6$~mHz band. Understanding the origin of such peaks is a challenging task. Here, we explore several possible explanations for the observed oscillations, all pointing toward a potential resonant interaction within these structures and an unknown driver. Our observational result provides further insight into the magnetic connectivity between the different layers of the dynamic atmosphere of the Sun.

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arXiv2025

ASR: Archival Solar flaRes catalogue

Solar flares result from the rapid conversion of stored magnetic energy within the Sun's corona. These energy releases are associated with coronal magnetic loops, which are rooted in dense photospheric plasma and are passively transported by surface advection. Their emissions cover a wide range of wavelengths, with soft X-rays being the primary diagnostic for the past fifty years. Despite the efforts of multiple authors, we are still far from a complete theory, capable of explaining the observed statistical and individual properties of flares. Here, we exploit the availability of stable and long-term soft x-ray measurements from NASA's GOES mission to build a new solar flare catalogue, with a novel approach to linking sympathetic events. Furthermore, for the most energetic events since 2010, we have also provided a method to identify the origin of the observed flare and eventual link to the photospheric active region by exploiting the array of instruments onboard NASA's Solar Dynamic Observatory. Our catalogue provides a robust resource for studying space weather events and training machine learning models to develop a reliable early warning system for the onset of eruptive events in the solar atmosphere.

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arXiv2025

The dynamics of small-scale magnetic fields modulated by the solar cycle

In addition to sunspots, which represent the most easily visualized manifestation of solar magnetism, cutting-edge observations of the solar atmosphere have uncovered a plethora of magnetic flux tubes, down to the resolving power of modern high-resolution telescopes (a few tens of km), revealing how the Sun is a fully magnetized star. These magnetic elements are advected and buffeted by ambient plasma flows and turbulent convection, resulting in perturbations of the flux tubes that make them natural conduits for channeling wave energy into the upper layers of the Sun's atmosphere and significantly contributing to the acceleration of the solar wind. Today, data acquired by the Helioseismic and Magnetic Imager (HMI) onboard NASA's Solar Dynamics Observatory (SDO), have made it possible to study the dynamics of small-scale magnetic fields over long timescales. Here, for the first time, we present the discovery of a modulation in the dynamical behavior of small-scale magnetic concentrations in the photosphere over temporal scales consistent with the solar activity cycle (i.e. 11 years), which has only been made possible by the long observing lifetime of the SDO/HMI spacecraft. Furthermore, a temporal varying polarization of their perturbations is also found on similar timescales. This demonstrates how the small-scale dynamics of magnetic fields are also affected by the global dynamo. These discoveries were realized through automated tracking of magnetic fields in the solar photosphere across 11 continuous years, resulting in the most extended statistical analyses of its kind so far, with more than 31 million magnetic concentrations examined.

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arXiv2025

Metis Observations of Alfv\'{e}nic Outflows Driven by Interchange Reconnection in a Pseudostreamer

This study presents observations of a large pseudostreamer solar eruption and, in particular, the post-eruption relaxation phase, as captured by Metis onboard the Solar Orbiter on October 12, 2022, during its perihelion passage. Utilizing total brightness data, we observe the outward propagation of helical features up to 3 solar radii along a radial column that appears to correspond to the stalk of the pseudostreamer. The helical structures persisted for more than 3 hours following a jet-like coronal mass ejection associated with a polar crown prominence eruption. A notable trend is revealed: the inclination of these features decreases as their polar angle and height increase. Additionally, we measured their helix pitch. Despite a 2-minute time cadence limiting direct correspondence among filamentary structures in consecutive frames, we find that the Metis helical structure may be interpreted as a consequence of twist (nonlinear torsional Alfv\'{e}n waves) and plasma liberated by interchange reconnection. A comparison was performed of the helix parameters as outlined by fine-scale outflow features with those obtained from synthetic white-light images derived from the high-resolution magnetohydrodynamics simulation of interchange reconnection in a pseudostreamer topology by Wyper et al. (2022). A remarkable similarity between the simulation-derived images and the observations was found. We conjecture that these Metis observations may represent the upper end in spatial and energy scale of the interchange reconnection process that has been proposed recently as the origin of the Alfv\'{e}nic solar wind.

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arXiv2024

SoFT: Detecting and Tracking Magnetic Structures in the Solar Photosphere

In this work, we present SoFT: Solar Feature Tracking, a novel feature-tracking tool developed in Python and designed to detect, identify, and track magnetic elements in the solar atmosphere. It relies on a watershed segmentation algorithm to effectively detect magnetic clumps within magnetograms, which are then associated across successive frames to follow the motion of magnetic structures in the photosphere. Here, we study its reliability in detecting and tracking features under different noise conditions starting with real-world data observed with SDO/HMI and followed with simulation data obtained from the Bifrost numerical code to better replicate the movements and shape of actual magnetic structures observed in the Sun's atmosphere within a controlled noise environment.

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arXiv2024

Unexpected frequency of horizontal oscillations of magnetic structures in the solar photosphere

It is well known that the dominant frequency of oscillations in the solar photosphere is $\approx$3 mHz, which is the result of global resonant modes pertaining to the whole stellar structure. However, analyses of the horizontal motions of nearly 1 million photospheric magnetic elements spanning the entirety of solar cycle 24 have revealed an unexpected dominant frequency, $\approx$5 mHz, a frequency typically synonymous with the chromosphere. Given the distinctly different physical properties of the magnetic elements examined in our statistical sample, when compared to largely quiescent solar plasma where $\approx$3 mHz frequencies are omnipresent, we argue that the dominant $\approx$5 mHz frequency is not caused by the buffeting of magnetic elements, but instead is due to the nature of the underlying oscillatory driver itself. This novel result was obtained by exploiting the unmatched spatial and temporal coverage of magnetograms acquired by the Helioseismic and Magnetic Imager (HMI) on board NASA's Solar Dynamics Observatory (SDO). Our findings provide a timely avenue for future exploration of the magnetic connectivity between sub-photospheric, photospheric, and chromospheric layers of the Sun's dynamic atmosphere.

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arXiv2024

Unveiling the True Nature of Plasma Dynamics from the Reference Frame of a Super-penumbral Fibril

The magnetic geometry of the solar atmosphere, combined with projection effects, makes it difficult to accurately map the propagation of ubiquitous waves in fibrillar structures. These waves are of interest due to their ability to carry energy into the chromosphere and deposit it through damping and dissipation mechanisms. To this end, the Interferometric Bidimensional Spectrometer (IBIS) at the Dunn Solar Telescope was employed to capture high resolution H$\alpha$ spectral scans of a sunspot, with the transverse oscillations of a prominent super-penumbral fibril examined in depth. The oscillations are re-projected from the helioprojective-cartesian frame to a new frame of reference oriented along the average fibril axis through non-linear force-free field extrapolations. The fibril was found to be carrying an elliptically polarised, propagating kink oscillation with a period of $430$ s and a phase velocity of $69\pm4$ km s$^{-1}$. The oscillation is damped as it propagates away from the sunspot with a damping length of approximately $9.2$ Mm, resulting in the energy flux decreasing at a rate on the order of $460$ W m$^{-2}$/Mm. The H$\alpha$ line width is examined and found to increase with distance from the sunspot; a potential sign of a temperature increase. Different linear and non-linear mechanisms are investigated for the damping of the wave energy flux, but a first-order approximation of their combined effects is insufficient to recreate the observed damping length by a factor of at least $3$. It is anticipated that the re-projection methodology demonstrated in this study will aid with future studies of transverse waves within fibrillar structures.

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arXiv2024

Sausage, kink, and fluting MHD wave modes identified in solar magnetic pores by Solar Orbiter/PHI

Solar pores are intense concentrations of magnetic flux that emerge through the Sun's photosphere. When compared to sunspots, they are much smaller in diameter and hence can be impacted and buffeted by neighbouring granular activity to generate significant magnetohydrodynamic (MHD) wave energy flux within their confines. However, observations of solar pores from ground-based telescope facilities may struggle to capture subtle motions synonymous with higher-order MHD wave signatures due to seeing effects produced in the Earth's atmosphere. Hence, we have exploited timely seeing-free and high-quality observations of four small magnetic pores from the Polarimetric and Helioseismic Imager (PHI) on board the Solar Orbiter spacecraft. Through acquisition of data under stable observing conditions, we have been able to measure the area fluctuations and horizontal displacements of the solar pores. Cross correlations between perturbations in intensity, area, line-of-sight velocity, and magnetic fields, coupled with the first-time application of novel Proper Orthogonal Decomposition (POD) techniques on the boundary oscillations, provide a comprehensive diagnosis of the embedded MHD waves as sausage and kink modes. Additionally, the previously elusive m = 2 fluting mode is identified in the most magnetically isolated of the four pores. An important consideration lies in how the identified wave modes contribute towards the transfer of energy into the upper solar atmosphere. We find that the four pores examined have approximately 56%, 72%, 52%, and 34% of their total wave energy associated with the identified sausage modes, and around 23%, 17%, 39%, and 49% to their kink modes, respectively, while the first pore also has around an 11% contribution linked to the fluting mode. This study marks the first-time identification of concurrent sausage, kink, and fluting MHD wave modes in solar magnetic pores.

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arXiv2023

Eruptive events with exceptionally bright emission in HI Ly-alpha observed by the Metis coronagraph

Metis, the coronagraph on board Solar Orbiter, provides for the first time coronagraphic imaging in the ultraviolet HI Ly-alpha line and, simultaneously, in polarized visible light, thus providing a host of information on the properties of CMEs and solar eruptions like their overall dynamics, time evolution, mass content, and outflow propagation velocity in the expanding corona. We analyzed in this work six CMEs observed by Metis between April and October 2021, which are characterized by a very strong HI Ly-alpha emission. We studied in particular the morphology, kinematics, and the temporal and radial evolution of the emission of such events, focusing on the brightest UV features. The kinematics of the eruptive events under consideration were studied by determining the height-time profiles of the brightest parts on the Metis plane of the sky. Furthermore, the 3D position in the heliosphere of the CMEs were determined by employing co-temporal images from two other coronagraphs: LASCO/C2 onboard SOHO, and COR2 onboard STEREO-A. Finally, the radiometrically calibrated Metis images of the bright UV features were analyzed to provide estimates of their volume and density. From the kinematics and radiometric analyses, we obtained indications of the temperatures of the bright UV cores of these events. The analysis of these strong UV-emitting features associated with coronal mass ejections demonstrates the capabilities of the current constellation of space coronagraphs, Metis, LASCO/C2, and COR2, in providing a complete characterization of the structure and dynamics of eruptive events in their propagation phase from their inception up to several solar radii. Furthermore, we show how the unique capabilities of the Metis instrument to observe these events in both HI Ly-alpha line and polarized VL radiation allow plasma diagnostics on the thermal state of these events.

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arXiv2023

The temporal and spatial evolution of MHD wave modes in sunspots

Through their lifetime sunspots undergo a change in their area and shape and, as they decay, they fragment into smaller structures. Here, for the first time we analyze the spatial structure of magnetohydrodynamic (MHD) slow body and fast surface modes in observed umbrae as their cross-sectional shape changes. The Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) techniques were used to analyze 3 and 6 hours SDO/HMI time series of Doppler velocities at the photospheric level of approximately circular and elliptically shaped sunspots. Each time series were divided equally into time intervals, to evidence the change of the sunspots' shape. To identify physical wave modes, the POD/DMD modes were cross-correlated with a slow body mode model using the exact shape of the umbra, whereas the shape obtained by applying a threshold level of the mean intensity for every time interval. Our results show that the spatial structure of MHD modes are affected, even by apparently small changes of the umbral shape, especially in the case of the higher-order modes. For the datasets used in our study, the optimal time intervals to consider the influence of the change in the shape on the observed MHD modes is 37 - 60 minutes. The choice of these intervals is crucial to properly quantify the energy contribution of each wave mode to the power spectrum.

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arXiv2023

Spectropolarimetric investigation of magnetohydrodynamic wave modes in the photosphere: First results from PHI on board Solar Orbiter

In November 2021, Solar Orbiter started its nominal mission phase. The remote-sensing instruments on board the spacecraft acquired scientific data during three observing windows surrounding the perihelion of the first orbit of this phase. The aim of the analysis is the detection of magnetohydrodynamic (MHD) wave modes in an active region by exploiting the capabilities of spectropolarimetric measurements. The High Resolution Telescope (HRT) of the Polarimetric and Helioseismic Imager (SO/PHI) on board the Solar Orbiter acquired a high-cadence data set of an active region. This is studied in the paper. B-$\omega$ and phase-difference analyses are applied on line-of-sight velocity and circular polarization maps and other averaged quantities. We find that several MHD modes at different frequencies are excited in all analysed structures. The leading sunspot shows a linear dependence of the phase lag on the angle between the magnetic field and the line of sight of the observer in its penumbra. The magnetic pore exhibits global resonances at several frequencies, which are also excited by different wave modes. The SO/PHI measurements clearly confirm the presence of magnetic and velocity oscillations that are compatible with one or more MHD wave modes in pores and a sunspot. Improvements in modelling are still necessary to interpret the relation between the fluctuations of different diagnostics.

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arXiv2023

Observational Evidence of S-Web Source of the Slow Solar Wind

From 2022 March 18-21, active region (AR) 12967 was tracked simultaneously by Solar Orbiter (SO) at 0.35 au and Hinode/EIS at Earth. During this period, strong blue-shifted plasma upflows were observed along a thin, dark corridor of open field originating at the AR's leading polarity and continuing towards the southern extension of the northern polar coronal hole. A potential field source surface (PFSS) model shows large lateral expansion of the open magnetic field along the corridor. Squashing factor Q-maps of the large scale topology further confirm super-radial expansion in support of the S-Web theory for the slow wind. The thin corridor of upflows is identified as the source region of a slow solar wind stream characterised by approx. 300 km s-1 velocities, low proton temperatures of approx. 5 eV, extremely high density over 100 cm-3, and a short interval of moderate Alfvenicity accompanied by switchback events. When connectivity changes from the corridor to the eastern side of the AR, the in situ plasma parameters of the slow wind indicate a distinctly different source region. These observations provide strong evidence that the narrow open field corridors, forming part of the S-Web, produce extreme properties in their associated solar wind streams.

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arXiv2023

Slow wind belt in the quiet solar corona

The slow solar wind belt in the quiet corona, observed with the Metis coronagraph on board Solar Orbiter on May 15, 2020, during the activity minimum of the cycle 24, in a field of view extending from 3.8 $R_\odot$ to 7.0 $R_\odot$, is formed by a slow and dense wind stream running along the coronal current sheet, accelerating in the radial direction and reaching at 6.8 $R_\odot$ a speed within 150 km s$^{-1}$ and 190 km s$^{-1}$, depending on the assumptions on the velocity distribution of the neutral hydrogen atoms in the coronal plasma. The slow stream is separated by thin regions of high velocity shear from faster streams, almost symmetric relative to the current sheet, with peak velocity within 175 km s$^{-1}$ and 230 km s$^{-1}$ at the same coronal level. The density-velocity structure of the slow wind zone is discussed in terms of the expansion factor of the open magnetic field lines that is known to be related to the speed of the quasi-steady solar wind, and in relation to the presence of a web of quasi separatrix layers, S-web, the potential sites of reconnection that release coronal plasma into the wind. The parameters characterizing the coronal magnetic field lines are derived from 3D MHD model calculations. The S-web is found to coincide with the latitudinal region where the slow wind is observed in the outer corona and is surrounded by thin layers of open field lines expanding in a non-monotonic way.

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arXiv2023

In-flight validation of Metis Visible-light Polarimeter Coronagraph on board Solar Orbiter

Context. The Metis coronagraph is one of the remote-sensing instruments of the ESA/NASA Solar Orbiter mission. Metis is aimed at the study of the solar atmosphere and solar wind by simultaneously acquiring images of the solar corona at two different wavelengths; visible-light (VL) within a band ranging from 580 nm to 640 nm, and in the HI Ly-alpha 121.6 +/- 10 nm ultraviolet (UV) light. The visible-light channel includes a polarimeter with electro-optically modulating Liquid Crystal Variable Retarders (LCVRs) to measure the linearly polarized brightness of the K-corona to derive the electron density. Aims. In this paper, we present the first in-flight validation results of the Metis polarimetric channel together with a comparison to the on-ground calibrations. It is the validation of the first use in deep space (with hard radiation environment) of an electro-optical device: a liquid crystal-based polarimeter. Methods. We used the orientation of the K-corona's linear polarization vector during the spacecraft roll maneuvers for the in-flight calibration. Results. The first in-flight validation of the Metis coronagraph on-board Solar Orbiter shows a good agreement with the on-ground measurements. It confirms the expected visible-light channel polarimetric performance. A final comparison between the first pB obtained by Metis with the polarized brightness (pB) obtained by the space-based coronagraph LASCO and the ground-based coronagraph KCor shows the consistency of the Metis calibrated results.

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arXiv2022

The Propagation of Coherent Waves Across Multiple Solar Magnetic Pores

Solar pores are efficient magnetic conduits for propagating magnetohydrodynamic wave energy into the outer regions of the solar atmosphere. Pore observations often contain isolated and/or unconnected structures, preventing the statistical examination of wave activity as a function of atmospheric height. Here, using high resolution observations acquired by the Dunn Solar Telescope, we examine photospheric and chromospheric wave signatures from a unique collection of magnetic pores originating from the same decaying sunspot. Wavelet analysis of high cadence photospheric imaging reveals the ubiquitous presence of slow sausage mode oscillations, coherent across all photospheric pores through comparisons of intensity and area fluctuations, producing statistically significant in-phase relationships. The universal nature of these waves allowed an investigation of whether the wave activity remained coherent as they propagate. Utilizing bi-sector Doppler velocity analysis of the Ca II 8542 Å line, alongside comparisons of the modeled spectral response function, we find fine-scale 5 mHz power amplification as the waves propagate into the chromosphere. Phase angles approaching zero degrees between co-spatial bi-sectors spanning different line depths indicate standing sausage modes following reflection against the transition region boundary. Fourier analysis of chromospheric velocities between neighboring pores reveals the annihilation of the wave coherency observed in the photosphere, with examination of the intensity and velocity signals from individual pores indicating they behave as fractured wave guides, rather than monolithic structures. Importantly, this work highlights that wave morphology with atmospheric height is highly complex, with vast differences observed at chromospheric layers, despite equivalent wave modes being introduced into similar pores in the photosphere.

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