arXiv subjects

Luc Rouppe van der Voort

Publications and source records attributed to Luc Rouppe van der Voort.

At least 19 recordsLinked to original sources

arXiv2026

Evidence for plasmoid formation related to a hot UV burst in the solar atmosphere

Magnetic reconnection in the solar atmosphere drives energetic phenomena such as Ellerman bombs, UV bursts, and surges. We used coordinated observations from the Swedish 1-m Solar Telescope (SST), IRIS, SDO, and the Hinode/X-ray Telescope (XRT) to analyze an active region with strong magnetic flux emergence. This region displayed long duration UV burst activity, occurrence of multiple Ellerman bombs, and recurrent ejection of surges. Chromospheric imaging show sthe presence of a dome-like structure that overarched the region where magnetic flux was emerging. The curved chromospheric fibrils were closely associated with bright and similarly curved extensions from a UV burst in the IRIS 1400 A channel. We observed an episode of formation of a bright sheet along the curved fibrils in H-beta that subsequently fragmented into plasmoid-like blobs with peak emission at a Doppler offset of +43 km/s. IRIS Si IV lines in the same region show complex and non-Gaussian line profiles that display asymmetric extensions to high Doppler offsets. Some profiles have clear components with Doppler offsets in excess of 150 km/s on both sides of the nominal line center. The optically thin SDO/AIA channels, including 94 A, show intermittent and repeated occurrences of the burst. Emission measure and filter ratio analysis indicate that the burst is multi-thermal and can attain temperatures beyond 1 MK. AIA and Hinode/XRT observations reveal a localized EUV and soft-X-ray counterpart of the burst/dome system, supporting the presence of intrinsically multithermal plasma that reaches coronal, and likely multi-MK, temperatures.

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arXiv2026

The umbrella effect: Magnetic canopy topology modulates spatially averaged chromospheric three-minute power

Three-minute ($\sim$5 mHz) oscillations are a prominent signature of wave propagation in the solar chromosphere, but their measured power can vary substantially between full-disc observations and small, high-resolution fields of view. We investigate whether chromospheric magnetic canopy topology, particularly its lateral continuity and areal filling, modulates field-of-view-averaged three-minute power. We analyse H$α$ and Ca II 8542 Å spectral imaging from the Swedish 1-m Solar Telescope, comparing canopy-poor quiet-Sun scenes with canopy-dominated active-region environments, complemented by radiative-MHD Bifrost simulations of canopy-poor and canopy-rich atmospheres. We compute field-of-view-averaged power spectra from fixed-wavelength intensities and bisector-derived velocities in the observations, and from synthetic intensities, velocities, and temperature proxies in the simulations. We also quantify the relation between integrated 3-5 mHz excess power and canopy filling factor, using HMI-based magnetic-field extrapolations for the observations. In both observations and simulations, canopy-poor scenes show clear 3-5 mHz power enhancements, whereas canopy-dominated scenes show strongly reduced measured power. The trend is present in H$α$ and Ca II 8542 Å intensities and is supported by complementary velocity and temperature proxies. Integrated 3-5 mHz excess power decreases with increasing canopy filling factor. We conclude that chromospheric field-of-view-averaged three-minute power is a topology-weighted observable: dense, laterally continuous magnetic canopies can strongly reduce the measured large-scale three-minute signature without requiring the absence of local wave activity. This provides a practical criterion for interpreting chromospheric oscillation power as a proxy for atmospheric wave energy in the Sun and in magnetised cool-star atmospheres.

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arXiv2026

Bifrost spectral inversions, Fast non-LTE solar chromospheric diagnostics from 3D simulations

Modern solar observatories produce vast amounts of detailed spectral data. To infer atmospheric parameters from these data, in particular for the solar chromosphere, requires immense computational resources. This problem is becoming more acute as spatial resolution improves, and faster methods are much needed to make sense of the data. Archive-based inversions provide a cost-effective way to infer atmospheric parameters from solar spectra, and involve building databases of synthetic spectra and atmospheric models. We extend this idea to a $k$-nearest neighbour inversion with non-LTE chromospheric spectra from a three-dimensional radiative-magnetohydrodynamic simulation. Our database consists of 150 million \theline\ line profiles from a magnetically quiet simulation. We validate our method on a different simulation, and then apply the database inversion on high-cadence observations taken with the Swedish 1-m Solar Telescope and find excellent agreement between the observed and fitted spectra for almost the entire field of view. We recover the chromospheric temperature and line-of-sight velocity, and calculate the uncertainty in their inversions. The gas temperature has small uncertainties over a broad range of optical depths, and we recover a physically consistent solution over a broad range of optical depths, even beyond the line sensitivity region. Compared with the traditional inversion method, our approach is four orders of magnitude faster. Limitations and future improvements of the method are discussed.

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arXiv2026

Investigating white-light flare mechanisms via the Paschen jump using high-resolution continuum observations from the Swedish 1-m Solar Telescope

The continuum is understood to contain a large portion of the energy emitted by a solar flare. The optical continuum, known as white light (WL), is particularly relevant since it may be observed by ground-based instruments. We measured the WL enhancements short- and longward of the Paschen jump in order to gain insights into the possible mechanism(s) behind the creation of these increases in our two case studies. We took measurements from the Swedish 1-m Solar Telescope of the pseudo-continuum around the Ca II 8542 Å line as well as the true continuum around the K I 7699 Å and the Fe I 6173 Å line, providing us with observations on both sides of the Paschen jump. We observe WL enhancements of over 40% against the dark (pen-) umbral background in both flares. The WL excess in flare 1 is co-temporal with the derivative of the GOES soft X-ray and hard X-ray (HXR) measurements from the Advanced Space-based Solar Observatory (ASO-S), and the flare is compatible with the Neupert effect. For flare 2, a preceding smaller flare may be the cause of the temporal discrepancy. Signatures of chromospheric evaporation and condensation are found in the WL area for both flares. The ratio of intensities blueward and redward of the Paschen jump in flare 1 is below one for most WL pixels. This is in disagreement with the accepted WL formation mechanisms. We believe this is a consequence of the Ca II 8542 Å pseudo-continuum being affected by line wing opacity changes. The co-temporality of WL and HXR enhancements suggests that the WL emission enhancements in flare 1 (and parts of flare 2) are a result of direct electron precipitation. We conclude that more reliable continuum measurements free of any nearby line influence are necessary in order to obtain conclusive evidence for the formation mechanism(s) behind optical continuum enhancements from such analysis as presented in this work.

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arXiv2026

The Influence of Opacity on Inferred MHD Wave Signatures in the Lower Solar Atmosphere

Magnetohydrodynamic wave activity in small-scale magnetic structures, such as solar pores, provides key insights into energy transport in the lower solar atmosphere. This study presents high-resolution observations of ten solar pores contained within a 43 x 43 Mm$^2$ field of view, obtained with the CRISP instrument at the Swedish 1-m Solar Telescope. We investigate the temporal behaviour of the line-of-sight velocity (vlos) and magnetic field (blos) oscillations within the pore structures. Using SIR inversions, we analyse the oscillatory signals at multiple optical depths (log $τ$ levels) to assess how variations in geometric height (z) and temperature relate to the observed blos fluctuations. Our results reveal that higher-frequency oscillations (> 6 mHz) exhibit strong coherences with in-phase fluctuations between blos and z across atmospheric layers, consistent with upward-propagating magneto-acoustic waves. In contrast, coherent lower-frequency oscillations display significant phase differences, which may arise from opacity effects contaminating the inversion response. These findings highlight the importance of accounting for opacity effects when interpreting magnetic oscillations, with direct implications for forthcoming high-precision magnetic diagnostics from facilities such as DKIST.

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arXiv2025

Shock-induced magnetic reconnection driving Ellerman bomb emission and a spicule

The mechanism that forms dynamic type II spicules has remained elusive for many years. Their dynamical behaviour has long been linked to magnetic reconnection, yet no conclusive evidence has been provided. However, one recent observational study found signs of reconnection, as traced by Ellerman bombs (EBs), at the footpoints of many spicules. The triggering of EBs is generally linked to reconnection due to flux emergence and convective motions in the photosphere. We aim to explore whether we can connect EBs to type II spicules, and to what extent we can use EBs as an observational proxy to probe reconnection in this dynamic. We also aim to provide further insight into the mechanisms that trigger EBs. We used a simulation run with the radiative magnetohydrodynamics code Bifrost to track spicules and study the physical processes behind their formation. To detect EBs and classify the spicules, we synthesised the H-alpha line using the multilevel radiative transfer code RH1.5D. We also traced shocks and current sheets to decipher the origin of EBs and spicules. We selected one type II spicule with a strong EB near its footpoint and studied their formation in detail. A magnetoacoustic shock advects the magnetic field lines towards an oppositely directed ambient field, creating a current sheet. The current sheet accelerates dense plasma via a whiplash effect generated by reconnection into the inclined ambient field, launching the spicule. Several EB profiles trace shock- and magnetic-reconnection-induced dynamics during this process at the spicule footpoint. We present a new EB triggering mechanism in which a shock-induced current sheet reconnects, triggering an EB in the lower solar atmosphere. The shock-induced current sheet generates the launch of a type II spicule via reconnection outflows. These results provide a physical origin for the observed connection between EBs and spicules.

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arXiv2025

Science Requirement Document (SRD) for the European Solar Telescope (EST) (3rd Edition, December 2025)

The European Strategy Forum on Research Infrastructures (ESFRI) included the European Solar Telescope (EST) as an ESFRI Project in its 2016 Roadmap and confirmed this status in 2021. During the Preparatory Phase (2017 to 2022), the EST Science Advisory Group (SAG) was established in 2017. Its first task was to revise the Science Requirements Document (SRD), originally formulated in 2011. This second edition of the SRD was published in December 2019 (Schlichenmaier et al. 2019, 2019arXiv191208650S). Since 2019, the EST Project Office has advanced the telescope design and developed the Science Instrumentation Suite (SIS). All telescope subsystems and the SIS have now passed their Design Reviews. Aligned with the discussions and resolutions of the SAG, the SIS comprises three categories of first-generation instruments: (1) Tunable Imaging Spectropolarimeters coupled with Fixed Band Imagers (TIS/FBIs), employing large-aperture Fabry-Pérot etalons; (2) Integral Field Spectropolarimeters based on microlens arrays (IFS-M); (3) The near-infrared spectropolarimeter EMBER (spectropolariMeter Based on slicEr-mirrors for the near-infraRed), using image-slicing technology. This configuration enables simultaneous observations of the same two-dimensional solar region across a broad wavelength range (380-2200 nm), giving EST a uniquely powerful multi-wavelength observing capability. This has made it necessary to revise the Observing Programmes so that they accurately reflect the capabilities of the SIS and ensure that all science objectives can be met. This third edition of the EST SRD integrates the outcomes of numerous SAG meetings held between 2020 and 2025 and incorporates all design developments up to 2025. It updates the Observing Programmes accordingly and assesses their alignment with the scientific objectives defined by the SAG.

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arXiv2025

Active chromospheric fibril singularity: Coordinated observations from Solar Orbiter, SST, and IRIS

The fine structures of the solar chromosphere, driven by photospheric motions, play a crucial role in the dynamics of solar magnetic fields. Many have been already identified such as fibrils, filament feet, and arch filament systems. Still, high resolution observations show a wealth of structures that remain elusive. We have observed a puzzling, unprecedented chromospheric fibril singularity in close vicinity of a blow-out solar jet and a flaring loop. We aim to understand the magnetic nature of this singularity and the cause of its activity using coordinated high-resolution multi-wavelengths observations. We aligned datasets from Solar Orbiter, SST, IRIS, and SDO. We re-projected the Solar Orbiter datasets to match the perspective of the Earth-based instruments. We performed potential field extrapolations from Solar Orbiter/PHI data. We analysed the spatial and temporal evolution of the plasma structures and their link with the surface magnetic field. This leads us to derive a model and scenario for the observed structures which we explain in a general schematic representation. We have discovered a new feature, a singularity in the chromospheric fibril pattern. It is formed in a weak magnetic field corridor between two flux concentrations of equal sign, at the base of a vertically inverted-Y shape field line pattern. In this specific case some activity develops along the structure. Firstly a flaring loop at one end, secondly a blow-out jet at the other end, where a coronal null-point was present and associated with a chromospheric saddle point being located onto the fibril singularity. The observations sugge

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arXiv2025

Fine details in solar flare ribbons: Statistical insights from observations with the Swedish 1-m Solar Telescope

Flare ribbons serve as chromospheric footprints of energy deposition resulting from particle acceleration during magnetic reconnection. Their fine-scale structure provides a valuable tool for probing the dynamics of the flare reconnection process. Our goal is to investigate the fine-scale structure of flare ribbons through multiple observations of flares, utilising data obtained from the Atmospheric Imaging Assembly (AIA) and the Swedish 1-m Solar Telescope (SST). The aligned AIA and SST datasets for the three solar flares were used to examine their overall morphology. The SST datasets were specifically used to identify fine-scale structures within the flare ribbons. For spectroscopic analysis of these fine structures, we applied machine-learning methods (k-means clustering) and Gaussian fitting. Using k-means, we identified elongated features in the flare ribbons, termed as "riblets", which are short-lived and jet-like small-scale structures that extend as plasma columns from the flare ribbons. Riblets are more prominent near the solar limb and represent the ribbon front. Riblet widths are consistent across observations, ranging from 110-310 km (0".15-0".41), while vertical lengths span 620-1220 km (0".83-1".66), with a potential maximum of 2000 km (2".67), after accounting for projection effects. Detailed H-beta spectral analysis reveals that riblets exhibit a single, redshifted emission component, with velocities of 16-21 km s^1, independent of viewing angle. Our high-resolution observations of the three flare ribbons show that they are not continuous structures, but are composed of vertically extended, fine-scale substructures. These irregular features indicate that the reconnection region is not a smooth, laminar current sheet, but rather a fragmented zone filled with magnetic islands, consistent with the theory of patchy reconnection within the coronal current sheet.

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arXiv2025

Small-scale dynamic phenomena associated with interacting fan-spine topologies: quiet-Sun Ellerman bombs, UV brightenings, and chromospheric inverted-Y-shaped jets

QSEBs are small-scale magnetic reconnection events in lower solar atmosphere. Sometimes, they exhibit transition region counterparts, known as UV brightenings. Magnetic field extrapolations suggest that QSEBs can occur at various locations of a fan-spine topology, with UV brightening occurring at null point through a common reconnection process. We aim to understand how complex magnetic configurations like interacting fan-spine topologies can cause small-scale dynamic phenomena in lower atmosphere. QSEBs were detected using k-means clustering on Hbeta observations from Swedish 1-m Solar Telescope (SST). Further, chromospheric inverted-Y-shaped jets were identified in the Hbeta blue wing. Magnetic field topologies were determined through potential field extrapolations from photospheric magnetograms using the Fe I 6173 A line. UV brightenings were detected in IRIS 1400 A SJI. We identify two distinct magnetic configurations associated with QSEBs, UV brightenings, and chromospheric inverted-Y-shaped jets. The first involves a nested fan-spine structure where, due to flux emergence, an inner 3D null forms inside fan surface of an outer 3D null with some overlap. QSEBs occur at two footpoints along the shared fan surface, with 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 numerical simulation. In 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. This study demonstrates through observational and modelling support that 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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arXiv2025

Coronal kink oscillations and photospheric driving: combining SolO/EUI and SST/CRISP high-resolution observations

The driving and excitation mechanisms of decay-less kink oscillations in coronal loops remain under debate. We aim to quantify and provide simple observational constraints on the photospheric driving of oscillating coronal loops in a few typical active region configurations: sunspot, plage, pores and enhanced-network regions. We then aim to investigate the possible interplay between photospheric driving and properties of kink oscillations in connected coronal loops. We analyse two unique datasets of the corona and photosphere taken at a high resolution during the first coordinated observation campaign between Solar Orbiter and the Swedish 1-m Solar Telescope (SST). A local correlation tracking method is applied on the SST/CRISP data to quantify the photospheric motions at the base of coronal loops. The same loops are then analysed in the corona by exploiting data from the Extreme Ultraviolet Imager on Solar Orbiter, and by using a wavelet analysis to characterize the kink oscillations. Each photospheric region shows dynamics with an overall increase in strength going from pore, plage, enhanced-network to sunspot regions. Differences are also seen in the kink-mode amplitudes of the corresponding coronal loops. This suggests the photosphere is involved in the driving of coronal kink oscillations. However, the few samples available does not allow to further establish the excitation mechanism yet. Despite oscillating coronal loops being anchored in seemingly "static" strong magnetic field regions as seen from coronal EUV observations, photospheric observations provide evidence for a continuous and significant driving at their base. The precise connection between photospheric driving and coronal kink oscillations remains to be further investigated. This study finally provides critical constraints on photospheric driving that can be tested in existing numerical models of coronal loops.

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arXiv2024

Magnetic Topology of quiet-Sun Ellerman bombs and associated Ultraviolet brightenings

Quiet-Sun Ellerman bombs (QSEBs) are small-scale magnetic reconnection events in the lower atmosphere of the quiet Sun. Recent work has shown that a small percentage of them can occur co-spatially and co-temporally to ultraviolet (UV) brightenings in the transition region. We aim to understand how the magnetic topologies associated with closely occurring QSEBs and UV brightenings can facilitate energy transport and connect these events. We used high-resolution H-beta observations from the Swedish 1-m Solar Telescope (SST) and detected QSEBs using k-means clustering. We obtained the magnetic field topology from potential field extrapolations using spectro-polarimetric data in the photospheric Fe I 6173 A line. To detect UV brightenings, we used coordinated and co-aligned data from the Interface Region Imaging Spectrograph (IRIS) and imposed a threshold of 5 sigma above the median background on the (IRIS) 1400 A slit-jaw image channel. We identify four distinct magnetic configurations that associate QSEBs with UV brightenings, including a simple dipole configuration and more complex fan-spine topologies with a three-dimensional (3D) magnetic null point. In the fan-spine topology, the UV brightenings occur near the 3D null point, while QSEBs can be found close to the footpoints of the outer spine, the inner spine, and the fan surface. We find that the height of the 3D null varies between 0.2 Mm to 2.6 Mm, depending on the magnetic field strength in the region. We note that some QSEBs and UV brightenings, though occurring close to each other, are not topologically connected with the same reconnection process. We find that the energy released during QSEBs falls in the range of 10^23 to 10^24 ergs. This study shows that magnetic connectivity and topological features, like 3D null points, are crucial in linking QSEBs in the lower atmosphere with UV brightenings in the transition region.

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arXiv2024

High-resolution observational analysis of flare ribbon fine structures

Context. Since the mechanism of energy release from solar flares is still not fully understood, the study of fine-scale features developing during flares becomes important for progressing towards a consistent picture of the essential physical mechanisms. Aims. We aim to probe the fine structures in flare ribbons at the chromospheric level using high-resolution observations with imaging and spectral techniques. Methods. We present a GOES C2.4 class solar flare observed with the Swedish 1-m Solar Telescope (SST), the Interface Region Imaging Spectrograph (IRIS), and the Atmospheric Imaging Assembly (AIA). The high-resolution SST observations offer spectroscopic data in the H-alpha, Ca II 8542 Å, and H-beta lines, which we use to analyse the flare ribbon. Results. Within the eastern flare ribbon, chromospheric bright blobs were detected and analysed in Ca II 8542 Å, H-alpha, and H-beta wavelengths. A comparison of blobs in H-beta observations and Si IV 1400 Å has also been performed. These blobs are observed as almost circular structures having widths from 140 km-200 km. The intensity profiles of the blobs show a red wing asymmetry. Conclusions. From the high spatial and temporal resolution H-beta observations, we conclude that the periodicity of the blobs in the flare ribbon, which are near-equally spaced in the range 330-550 km, is likely due to fragmented reconnection processes within a flare current sheet. This supports the theory of a direct link between fine-structure flare ribbons and current sheet tearing. We believe our observations represent the highest resolution evidence of fine-structure flare ribbons to date.

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arXiv2024

High-resolution observations of recurrent jets from an arch filament system

Solar jets are collimated plasma ejections along magnetic field lines observed in hot (EUV jets) and cool (chromospheric surges) temperature diagnostics. Their trigger mechanisms and the relationship between hot and cool jets are still not completely understood. We aim to investigate the generation of a sequence of active region solar jets and their evolution from the photospheric to the coronal heights. Using the synergy of high spatial and temporal resolution observations by the SST, along with the SDO, we analyze a sequence of solar jets originating in a mixed polarity region between the leading and following sunspots of an active region. We use a NFFF extrapolation technique for deriving the magnetic field topology of the active region. A mixed polarity region is formed over a long period (24 hours) with persistent magnetic flux emergence. This region has been observed as an arch filament system (AFS) in chromospheric SST observations. In this region, negative polarities surrounded by positive polarities create a fan-surface with a null point at a height of 6 Mm detected in the NFFF extrapolation. SST observations in H-beta spectral line reveal a large flux rope over the AFS and moving from the North to South, causing successive EUV and cool jets to move in the East-West direction and later towards the South along the long open loops. The high resolution SST observations (0.038 arcsec per pixel) resolve the dark area observed at the jet base and reveal the existence of an AFS with an extended cool jet which may be the result of a peeling-like mechanism of the AFS. Based on the combined analysis of SST and AIA observations along with extrapolated magnetic topology, it is suggested that the magnetic reconnection site may move southward by approximately 20 Mm until it reaches a region where the open magnetic field lines are oriented North-South.

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arXiv2024

Transition region response to Quiet Sun Ellerman Bombs

Quiet Sun Ellerman Bombs (QSEBs) are key indicators of small-scale photospheric magnetic reconnection events. Recent high-resolution observations have shown that they are ubiquitous and that large numbers of QSEBs can be found in the quiet Sun. We aim to understand the impact of QSEBs on the upper solar atmosphere by analysing their spatial and temporal relationship with the UV brightenings observed in transition region diagnostics. We analyse high-resolution H-beta observations from the Swedish 1-m Solar Telescope and utilise k-means clustering to detect 1423 QSEBs in a 51 min time series. We use coordinated and co-aligned observations from the Interface Region Imaging Spectrograph (IRIS) to search for corresponding signatures in the 1400 A slit-jaw image (SJI) channel and in the Si IV 1394 A and Mg II 2798.8 A triplet spectral lines. We identify UV brightenings from SJI 1400 using a threshold of 5$σ$ above the median background. We focused on 453 long-lived QSEBs ($>1$ min) and found 67 cases of UV brightenings from SJI 1400 occurring near the QSEBs, both temporally and spatially. Temporal analysis of these events indicates that QSEBs start before UV brightenings in 57 % of cases, while UV brightenings lead in 36 % of instances. The majority of the UV brightenings occur within 1000 km from the QSEBs in the direction of the solar limb. We also identify 21 QSEBs covered by the IRIS slit, with 4 of them showing emissions in both or one of the Si IV 1394 A and Mg II 2798.8 A triplet lines, at distances within 500 km from the QSEBs in the limb direction. We conclude that a small fraction (15 %) of the long-lived QSEBs contribute to localized heating observable in transition region diagnostics, indicating a minimal role in the global heating of the upper solar atmosphere.

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arXiv2024

Generic low-atmosphere signatures of swirled-anemone jets

Solar jets are collimated plasma flows moving along magnetic field lines and accelerated at low altitude following magnetic reconnection. Several of them originate from anemone-shaped low-lying arcades and the most impulsive ones tend to be relatively wider and display untwisting motions. We aim to establish typical behaviours and observational signatures in the low atmosphere that can occur in response to the coronal development of such impulsive jets. We analysed an observed solar jet associated with a circular flare ribbon, using high-resolution observations from SST coordinated with IRIS and SDO. We related specifically identified features with those developing in a generic 3D line-tied numerical simulation of reconnection driven jets, performed with the ARMS code. We identified three features in the SST observations: the formation of a hook along the circular ribbon, the gradual widening of the jet through the apparent displacement of its kinked edge towards, and not away from the presumed reconnection site, and the falling back of some of the jet plasma towards a footpoint offset from that of the jet itself. The 3D numerical simulation naturally accounts for these features which were not imposed a priori. Our analyses allow to interpret them in the context of the 3D geometry of the asymmetric swirled anemone loops and their sequences of reconnection with ambient coronal loops. Given the relatively-simple conditions in which the observed jet occurred, together with the generic nature of the simulation that comprised minimum assumptions, we predict that the specific features that we identified and interpreted are probably typical of every impulsive jet

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arXiv2023

High Resolution Observations of the Low Atmospheric Response to Small Coronal Heating Events in an Active Region Core

High resolution spectral observations of the lower solar atmosphere (chromosphere and transition region) during coronal heating events, in combination with predictions from models of impulsively heated loops, provide powerful diagnostics of the properties of the heating in active region cores. Here we analyze the first coordinated observations of such events with the Interface Region Imaging Spectrograph (IRIS) and the CHROMospheric Imaging Spectrometer (CHROMIS), at the Swedish 1-m Solar Telescope (SST), which provided extremely high spatial resolution and revealed chromospheric brightenings with spatial dimensions down to ~150km. We use machine learning methods (k-means clustering) and find significant coherence in the spatial and temporal properties of the chromospheric spectra, suggesting, in turn, coherence in the spatial and temporal distribution of the coronal heating. The comparison of IRIS and CHROMIS spectra with simulations suggest that both non-thermal electrons with low energy (low-energy cutoff ~5keV) and direct heating in the corona transported by thermal conduction contribute to the heating of the low atmosphere. This is consistent with growing evidence that non-thermal electrons are not uncommon in small heating events (nano- to micro-flares), and that their properties can be constrained by chromospheric and transition region spectral observations.

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arXiv2023

Comparative clustering analysis of Ca II 854.2 nm spectral profiles from simulations and observations

We aim to compare and contrast the typical shapes of synthetic Ca II 854.2 nm spectra found in Bifrost simulations having different magnetic activity with the spectral shapes found in a quiet Sun observation from the Swedish 1-m Solar Telescope (SST). We use clustering techniques to extract the typical Ca II 854.2 nm profile shapes synthesized from Bifrost simulations with varying amounts of magnetic activity. We degrade the synthetic profiles to observational conditions and repeat the clustering, and we compare our synthetic results with actual observations. While the mean spectra for our high resolution simulations compare reasonably well with the observations, we find that there are considerable differences between the clusters of observed and synthetic intensity profiles, even after the synthetic profiles have been degraded. The typical absorption profiles from the simulations are both narrower and display a steeper transition from the inner wings to the line core. Furthermore, even in our most quiescent simulation we find a far larger fraction of profiles with local emission around the core, or other exotic profile shapes, than in the observations. Looking into the atmospheric structure for a selected set of synthetic clusters, we find distinct differences in the temperature stratification for the clusters most and least similar to the observations. The narrow and steep profiles are associated with either weak gradients in temperature, or temperatures rising to a local maximum in the line wing forming region before sinking to a minimum in the line core forming region. The profiles that display less steep transitions show extended temperature gradients that are steeper in the range $-3 \lesssim \log τ_{5000} \lesssim -1$.

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