arXiv subjects

T. Van Doorsselaere

Publications and source records attributed to T. Van Doorsselaere.

At least 19 recordsLinked to original sources

arXiv2026

A Self-consistent Model for the Generation of Coronal Condensations: The Effects of Turbulent Wave Dissipation

Prominences and coronal rain are two manifestations of coronal condensations whose formation mechanisms remain only partially understood. While previous studies have explored how localised steady or stochastic heating can trigger thermal instability and condensations, the role of wave driven energy transport and dissipation has not been extensively investigated. In the solar atmosphere, Alfven and kink waves are an abundant source of energy capable of contributing both to coronal heating and plasma structuring, making them compelling candidates in the formation processes of the structures we observe in the corona. We aim to explore how Alfven and kink wave energy and their dissipation influence the formation and evolution of condensations in coronal magnetic structures. In particular, we investigate each waves role independently of parameterised external heating mechanisms to isolate whether either wave can trigger condensations. We perform 2.5D simulations using the open source MPI AMRVAC code extended with the newly developed UAWSoM module. This module self consistently evolves Alfven and kink wave energy and their feedback on the plasma through wave pressure and heating terms. We examine the conditions under which condensations form, their morphology, and their dynamical evolution. Our results show that for our range of parameters, in 2.5D MHD, Alfven waves did not trigger the formation of prominences or condensations. We find that if the radius of the fine scale structures supporting the kink waves is of the order of 100 km, then kink waves can trigger coronal rain that differs considerably in its morphology from coronal rain triggered by parameterised heating functions. Through the use of UAWSoM, we have established that Alfven and kink waves leave distinct thermodynamic signatures on coronal loops, given the same wave energy injection.

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arXiv2026

Fast and periodic propagating disturbances along coronal loops detected with EUI on board Solar Orbiter

Recent high-resolution observations from the Solar Orbiter mission can help detect the indirect signatures of heating at the smallest scales. In this work we measure the properties and investigate the physical origin of propagating disturbances (PDs) in the intensity at the smallest resolvable scales with Solar Orbiter/EUI along coronal loops. We used two sequences of EUI/HRIEUV at high spatial (down to 125 km per pixel) and temporal resolutions (5 s of cadence). We placed slits along 13 active region (AR) coronal loops. We measured the plane-of-sky (PoS) velocities and the intensity perturbation damping along the slits of PDs. We also measured the periodicity of PDs in one slit by using a Fourier analysis. We report the detection of PDs that have high PoS velocities ranging between 500 and 2000 km/s (which we call "fast" PDs). They are only visible in the upper part of the coronal loops. The intensity increase associated with these fast PDs is on the order of 4\% to 8\% of the HRIEUV intensity, and we measured little to no damping of their intensity with distance. We also measured a peak in the Fourier power spectra at 2 min above the 95\% confidence limit that is associated with fast PDs. The fast PDs are detected in the same coronal loops as PDs with a lower PoS velocity (70 to 90 kms/s), which we refer to as "slow" PDs. Unlike the fast PDs, these slow PDs are only visible in the lower part of the coronal loop, and they show clear intensity damping. Slow PDs show properties consistent with slow magneto-acoustic modes or upflows. On the other hand, fast PDs cannot be explained by slow magneto-acoustic modes. Instead, they show properties consistent with fast flows induced by magnetic reconnection, current sheet generated by propagating transverse oscillations, and fast magnetohydrodynamics modes or Alfvén waves.

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arXiv2025

Expanding stellar horizons with polarized light

The polarization of light is a critically under-utilized, rich source of information in astronomy. For stars in particular, surface magnetism polarization that can be detected and measured with spectro-polarimetry. Many questions about these surface fields remain unanswered due to a lack of dedicated instruments capable of probing weak and strong surface magnetic fields for the entire mass range of stars, from M-dwarfs (and even substellar objects) to massive O-type stars at different evolutionary stages and metallicities. These questions range from the origin of these fields to their true incidence rate throughout the stellar population and the dependence on metallicity. Magnetic fields, although currently often excluded from stellar evolution models, play an important role in stellar evolution. Connecting the surface fields to internal fields through asteroseismology will instigate a new era of understanding stellar evolution and the transport of angular momentum and chemical elements throughout stellar interiors, also impacting our understanding of star-planet interactions and stellar remnants. Polarimetry is also an under-utilized tool to observationally constrain the mode identification of nonradial oscillations, which lies at the basis of accurate asteroseismic parameter estimation at percentage-level for stellar radii, masses, ages, internal rotation, and magnetic field strengths. Combining strong constraints on mode identification and surface magnetic properties through the acquisition of time-resolved, high-resolution and high-signal-to-noise (S/N) spectro-polarimetry and spectroscopy promises to bring leaps forward in our understanding of stellar structure, particularly when combined with long-term space photometric data from past, current, and future missions.

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arXiv2025

Uniturbulence and Alfvén Wave Solar Model in MPI-AMRVAC

The coronal heating problem remains a fundamental challenge in solar physics. While AWSoM-type models (Alfvén Wave Solar Model) have proven highly successful in reproducing the large-scale structure of the solar corona, they inherently neglect contributions from additional wave modes that arise when the effects of transverse structuring is fully incorporated into the magnetohydrodynamic (MHD) equations. In this paper, we compare the roles of kink wave- and Alfvén wave-driven heating in sustaining a region of the solar atmosphere, using newly developed physics and radiative cooling modules within MPI-AMRVAC. We extend the existing MHD physics module in MPI-AMRVAC by incorporating additional Alfvén and kink wave energy contributions to the MHD equations. We examine their roles in heating the solar atmosphere and driving the solar wind. To validate our approach, we compare numerical results from Python-based simulations with those obtained using the UAWSoM module in MPI-AMRVAC. Furthermore, we assess the heating efficiency of kink waves relative to that of pure Alfvén waves through two parameter studies: (1) exploring how different Alfvén wave reflection rates impact the simulated atmosphere, and (2) varying the relative magnitudes of Alfvén and kink wave energy injections. Finally, we present results from a larger-scale domain, sustained entirely by kink wave-driven heating. Our results show that kink wave-driven (UAWSoM) models are able to sustain a stable atmosphere without requiring any artificial background heating terms, unlike traditional Alfvén-only models. We attribute this to the increased heating rate associated with kink waves compared with Alfvén waves, given the same energy injection. Kink waves can sustain a model plasma with temperature and density values representative of coronal conditions without resorting to ad hoc heating terms.

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arXiv2025

Transverse waves observed in a fibril with the MiHI prototype

Context. Fine-scale structures of the solar chromosphere, particularly fibrils, are known to host various types of magnetohydrodynamic (MHD) waves that can transport energy to the corona. In particular, absorption features observed in the Hα channel have been widely detected that exhibit transverse oscillations. Aims. We aimed to detect a high-frequency transverse oscillation in fibrils. Methods. We conducted a case study on a high-frequency transverse oscillation in a chromospheric fibril. A chromospheric fibril was observed on 24 August 2018, in the Hα spectral line, with the prototype Microlensed Hyperspectral Imager (MiHI) at the Swedish 1- meter Solar Telescope. The MiHI instrument is an integral field spectrograph capable of achieving ultra-high resolution simultaneously in the spatial, temporal, and spectral domains. Results. The detected oscillation characteristics include a period of 15 s and a displacement amplitude of 42 km. Using the bisector method, we derived Doppler velocities and determined that the polarisation of the oscillation was elliptical. Conclusions. The energy contained in the oscillation ranges from 390 to 2300 W/m2, which is not sufficient to balance radiative losses of the chromosphere.

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arXiv2025

The effect of thermal misbalance on magnetohydrodynamic modes in coronal magnetic cylinders

This study investigates the dispersion of magnetohydrodynamic waves influenced by thermal misbalance in a cylindrical configuration with a finite axial magnetic field within solar coronal plasmas. Specifically, it examines how thermal misbalance, characterized by two distinct timescales directly linked to the cooling and heating functions, influences the dispersion relation. This investigation is a key approach for understanding non-adiabatic effects on the behaviour of these waves. Our findings reveal that the effect of thermal misbalance on fast sausage and kink modes, consistent with previous studies on slabs, is small but slightly more pronounced than previously thought. The impact is smaller at long-wavelength limits but increases at shorter wavelengths, leading to higher damping rates. This minor effect on fast modes occurs despite the complex interaction of thermal misbalance terms within the dispersion relation, even at low-frequency limits defined by the characteristic timescales. Additionally, a very small amplification is observed, indicating a suppressed damping state for the long-wavelength fundamental fast kink mode. In contrast, slow magnetoacoustic modes are significantly affected by thermal misbalance, with the cusp frequency shifting slightly to lower values, which is significant for smaller longitudinal wavenumbers. This thermal misbalance likely accounts for the substantial attenuation observed in the propagation of slow magnetoacoustic waves within the solar atmosphere. The long-wavelength limit leads to an analytical expression that accurately describes the frequency shifts in slow modes due to misbalance, closely aligning with both numerical and observational results.

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arXiv2024

Modelling the propagation of slow magneto-acoustic waves in a multi-stranded coronal loop

We study the propagation properties of slow magneto-acoustic waves in a multi-thermal coronal loop using a 3D MHD model, for the first time. A bundle of 33 vertical cylinders, each of 100{\,}km radius, randomly distributed over a circular region of radius 1{\,}Mm is considered to represent the coronal loop. The slow waves are driven by perturbing the vertical velocity ($v_z$) at the base of the loop. We apply forward modelling to the simulation results to generate synthetic images in the coronal channels of SDO/AIA. Furthermore, we add appropriate data noise to enable direct comparison with the real observations. It is found that the synthetic images at the instrument resolution show non-cospatial features in different temperature channels in agreement with previous observations. Time-distance maps are constructed from the synthetic data to study the propagation properties. The results indicate that the oscillations are only visible in specific channels depending on the temperature range of plasma existing within the loop. Additionally, the propagation speed of slow waves is also found to be sensitive to the available temperature range. Overall, we propose that the cross-field thermal properties of coronal structures can be inferred using a combination of numerical simulations and observations of slow magneto-acoustic waves.

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arXiv2024

Observations of Fan-Spine Topology by Solar Orbiter/EUI: Rotational Motions and Indications of Alfvén Waves

Torsional Alfvén waves do not produce any intensity variation and are, therefore, challenging to observe with imaging instruments. Previously, Alfvén wave observations were reported throughout all the layers of the solar atmosphere using spectral imaging. We present an observation of a torsional Alfvén wave detected in an inverted y-shape structure observed with the HRIEUV telescope of the EUI instrument onboard Solar Orbiter in its 174 Å channel. The feature consists of two footpoints connected through short loops and a spine with a length of 30 Mm originating from one of the footpoints. In the current work, we also make use of the simultaneous observations from two other instruments onboard Solar Orbiter. The first one is PHI that is used to derive the magnetic configuration of the observed feature. The second one is SPICE that provided observations of intensity maps in different lines including Ne VIII and C III lines. We also address the issues of the SPICE point spread function and its influence on the Doppler maps via performed forward modeling analysis. The difference movie shows clear signatures of propagating rotational motions in the spine. Doppler maps obtained with SPICE show strong signal in the spine region. Under the assumption that the recovered point spread function is mostly correct, synthesized raster images confirm that this signal is predominantly physical. We conclude that the presented observations are compatible with an interpretation of either propagating torsional Alfvén waves in a low coronal structure or untwisting of a flux rope. This is the first time we see signatures of propagating torsional motion in corona as observed by the three instruments onboard Solar Orbiter.

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arXiv2023

Modulation depth of the gyrosynchrotron emission as identifier of fundamental sausage modes

We study the intensity, the modulation depth and the mean modulation depth of the gyrosynchrotron (GS) radiation as a function of the frequency and the line of sight (LOS) in fast sausage modes. By solving the 2.5D MHD ideal equations of a straight coronal loop considering the chromosphere and with typical flaring plasma parameters we analyse the wavelet transform of the density and the GS emission for different radio frequencies and different spatial resolutions, given impulsive and general perturbations with energies in the microflare range. A wavelet analysis performed over the GS radiation emission showed that a fast fundamental sausage mode of 7s with a first harmonic mode of 3s developed, for all the initial energy perturbations used. For both the high spatial resolution (central pixel integration) and the low spatial resolution (entire loop integration), the larger the radio frequency, the larger the modulation depth. However, high and low resolution integrations differ in that, the larger the LOS angle with respect to the loop axis, results in a larger and smaller modulation depth, respectively. Fast MHD modes triggered by instantaneous energy depositions of the order of a microflare energy are able to reproduce deep intensity modulation depths in radio emission as observed in solar events. As the trends of the GS emission obtained by Reznikova, Antolin, and Van Doorsselaere (2014), for a linear and forced oscillation, remain present when analysing a more general context, considering the chromosphere and where the sausage mode is triggered by a impulsive, nonlinear perturbation, it seems that the behaviour found can be used as observational identifiers of the presence of sausage modes with respect to other quasi-periodic pulsation features. It can be inferred from this that finite-amplitude sausage modes have the potential to generate the observed deep modulation depths.

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arXiv2022

Observational and numerical characterization of a recurrent arc-shaped front propagating along a coronal fan

Recurrent, arc-shaped intensity disturbances were detected by EUV channels in an active region. The fronts were observed to propagate along a coronal loop bundle rooted in a small area within a sunspot umbra. Previous works have linked these intensity disturbances to slow magnetoacoustic waves that propagate from the lower atmosphere to the corona along the magnetic field. The slow magnetoacoustic waves propagate at the local cusp speed. However, the measured propagation speeds from the intensity images are usually smaller as they are subject to projection effects due to the inclination of the magnetic field with respect to the line-of-sight. Here, we aim to understand the effect of projection by comparing observed speeds with those from a numerical model. Using multi-wavelength data we determine the periods present in the observations at different heights of the solar atmosphere through Fourier analysis. We calculate the plane-of-sky speeds along one of the loops from the cross-correlation time lags obtained as a function of distance along the loop. We perform a 2D ideal MHD simulation of an active region embedded in a stratified atmosphere. We drive slow waves from the photosphere with a 3 minutes periodicity. Synthetic time-distance maps are generated from the forward-modelled intensities in coronal wavelengths and the projected propagation speeds are calculated. The intensity disturbances show a dominant period between [2-3] minutes at different heights of the atmosphere. The apparent propagation speeds calculated for coronal channels exhibit an accelerated pattern with values increasing from 40 to 120 km/s as the distance along the loop rises. The propagation speeds obtained from the synthetic time-distance maps also exhibit accelerated profiles within a similar range of speeds. We conclude that the accelerated propagation in our observations is due to the projection effect.

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arXiv2021

The dynamics and observability of circularly polarized kink waves

Context. Kink waves are routinely observed in coronal loops. Resonant absorption is a well-accepted mechanism that extracts energy from kink waves. Nonlinear kink waves are know to be affected by the Kelvin-Helmholtz instability. However, all previous numerical studies consider linearly polarized kink waves. Aims. We study the properties of circularly polarized kink waves on straight plasma cylinders, for both standing and propagating waves, and compare them to the properties of linearly polarized kink waves. Methods. We use the code MPI-AMRVAC to solve the full 3D Magnetohydrodynamic (MHD) equations for a straight magnetic cylinder, excited by both standing and propagating circularly polarized kink (m = 1) modes. Results. The damping due to resonant absorption is independent of the polarization state. The morphology or appearance of the induced resonant flow is different for the two polarizations, however, there are essentially no differences in the forward-modeled Doppler signals. For nonlinear oscillations, the growth rate of small scales is determined by the total energy of the oscillation rather than the perturbation amplitude. We discuss possible implications and seismological relevance.

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arXiv2021

MHD Waves in open coronal structures

Modern observatories have revealed the ubiquitous presence of magnetohydrodynamic waves in the solar corona. The propagating waves (in contrast to the standing waves) are usually originated in the lower solar atmosphere which makes them particularly relevant to coronal heating. Furthermore, open coronal structures are believed to be the source regions of solar wind, therefore, the detection of MHD waves in these structures is also pertinent to the acceleration of solar wind. Besides, the advanced capabilities of the current generation telescopes have allowed us to extract important coronal properties through MHD seismology. The recent progress made in the detection, origin, and damping of both propagating slow mangetoacoustic waves and kink (Alfvénic) waves is presented in this review article especially in the context of open coronal structures. Where appropriate, we give an overview on associated theoretical modelling studies. A few of the important seismological applications of these waves are discussed. The possible role of Aflvénic waves in the acceleration of solar wind is also touched upon.

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arXiv2021

Compressive oscillations in hot coronal loops: Are sloshing oscillations and standing slow waves independent?

Employing high-resolution EUV imaging observations from SDO/AIA, we analyse a compressive plasma oscillation in a hot coronal loop triggered by a C-class flare near one of its foot points as first studied by Kumar et al. We investigate the oscillation properties in both the 131{\,}Å and 94{\,}Å channels and find that what appears as a pure sloshing oscillation in the 131{\,}Å channel actually transforms into a standing wave in the 94{\,}Å channel at a later time. This is the first clear evidence of such transformation confirming the results of a recent numerical study which suggests that these two oscillations are not independent phenomena. We introduce a new analytical expression to properly fit the sloshing phase of an oscillation and extract the oscillation properties. For the AIA 131{\,}Å channel, the obtained oscillation period and damping time are 608$\pm$4{\,}s and 431$\pm$20{\,}s, respectively during the sloshing phase. The corresponding values for the AIA 94{\,}Å channel are 617$\pm$3{\,}s and 828$\pm$50{\,}s. During the standing phase that is observed only in the AIA 94{\,}Å channel, the oscillation period and damping time have increased to 791$\pm$5{\,}s and 1598$\pm$138{\,}s, respectively. The plasma temperature obtained from the DEM analysis indicates substantial cooling of the plasma during the oscillation. Considering this, we show that the observed oscillation properties and the associated changes are compatible with damping due to thermal conduction. We further demonstrate that the absence of a standing phase in the 131{\,}Å channel is a consequence of cooling plasma besides the faster decay of oscillation in this channel.

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arXiv2020

Magnetohydrodynamic Fast Sausage Waves in the Solar Corona

Characterized by cyclic axisymmetric perturbations to both the magnetic and fluid parameters, magnetohydrodynamic fast sausage modes (FSMs) have proven useful for solar coronal seismology given their strong dispersion. This review starts by summarizing the dispersive properties of the FSMs in the canonical configuration where the equilibrium quantities are transversely structured in a step fashion. With this preparation we then review the recent theoretical studies on coronal FSMs, showing that the canonical dispersion features have been better understood physically, and further exploited seismologically. In addition, we show that departures from the canonical equilibrium configuration have led to qualitatively different dispersion features, thereby substantially broadening the range of observations that FSMs can be invoked to account for. We also summarize the advances in forward modeling studies, emphasizing the intricacies in interpreting observed oscillatory signals in terms of FSMs. All these advances notwithstanding, we offer a list of aspects that remain to be better addressed, with the physical connection of coronal FSMs to the quasi-periodic pulsations in solar flares particularly noteworthy.

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arXiv2020

Resonant absorption: transformation of compressive motions into vortical motions

This paper investigates the changes in spatial properties when magnetohydrodynamic (MHD) waves undergo resonant damping in the Alfvén continuum. The analysis is carried out for a 1D cylindrical pressure-less plasma with a straight magnetic field. The effect of the damping on the spatial wave variables is determined by using complex frequencies that arise as a result of the resonant damping. Compression and vorticity are used to characterise the spatial evolution of the MHD wave. The most striking result is the huge spatial variation in the vorticity component parallel to the magnetic field. Parallel vorticity vanishes in the uniform part of the equilibrium. However, when the MHD wave moves into the non-uniform part, parallel vorticity explodes to values that are orders of magnitude higher than those attained by the transverse components in planes normal to the straight magnetic field. In the non-uniform part of the equilibrium plasma, the MHD wave is controlled by parallel vorticity and resembles an Alfvén wave, with the unfamiliar property that it has pressure variations even in the linear regime.

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arXiv2020

Coronal Density and Temperature Profiles Calculated by Forward Modeling EUV Emission Observed by SDO/AIA

We present a model for the intensity of optically thin EUV emission for a plasma atmosphere. We apply our model to the solar corona as observed using the six optically thin EUV channels of the SDO/AIA instrument. The emissivity of the plasma is calculated from the density and temperature using CHIANTI tables and the intensity is then determined by integration along the line of sight. We consider several different profiles for the radial density and temperature profiles, each of which are constrained by the observational data alone with no further physical assumptions. We demonstrate the method first by applying it to a quiet region of the corona, and then use it as the background component of a model including coronal holes, allowing the plasma densities and temperatures inside and outside the hole to be estimated. We compare our results with differential emission measure (DEM) inversions. More accurate estimates for the coronal density and temperature profiles have the potential to help constrain plasma properties such as the magnetic field strength when used in combination with methods such as seismology.

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arXiv2019

Excitation of decay-less transverse oscillations of coronal loops by random motions

The relatively large-amplitude decaying regime of transverse oscillations of coronal loops has been known for two decades and interpreted in terms of MHD kink modes of cylindrical plasma waveguides. Recent observational analysis has revealed the so-called decay-less small-amplitude oscillations, with a multi-harmonic structure being detected. Several models have been proposed to explain them. In particular, decay-less oscillations have been described in terms of standing kink waves driven with continuous monoperiodic motions of loop footpoints, in terms of a simple oscillator model of forced oscillations due to harmonic external force, and as a self-oscillatory process due to the interaction of a loop with quasi-steady flows. However, an alternative mechanism is needed to explain the simultaneous excitation of several longitudinal harmonics of the oscillation. We study the mechanism of random excitation of decay-less transverse oscillations of coronal loops. With a spatially one-dimensional and time-dependent analytical model taking into account effects of the wave damping and kink speed variation along the loop, we consider transverse loop oscillations driven by random motions of footpoints. The footpoint motions are modelled by broad-band coloured noise. We have found the excitation of loop eigenmodes and analysed their frequency ratios as well as the spatial structure of the oscillations along the loop. The obtained results successfully reproduce the observed properties of decay-less oscillations. In particular, excitation of eigenmodes of a loop as a resonator can explain the observed quasi-monochromatic nature of decay-less oscillations and generation of multiple harmonics detected recently. We propose the mechanism that can interpret decay-less transverse oscillations of coronal loops in terms of kink waves randomly driven at the loop footpoints.

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arXiv2019

The Temperature-dependent Damping of Propagating Slow Magnetoacoustic Waves

The rapid damping of slow magnetoacoustic waves in the solar corona has been extensively studied in previous years. Most studies suggest that thermal conduction is a dominant contributor to this damping, albeit with a few exceptions. Employing extreme-ultraviolet (EUV) imaging data from SDO/AIA, we measure the damping lengths of propagating slow magnetoacoustic waves observed in several fan-like loop structures using two independent methods. The dependence of the damping length on temperature has been studied for the first time. The results do not indicate any apparent decrease in damping length with temperature, which is in contrast to the existing viewpoint. Comparing with the corresponding theoretical values calculated from damping due to thermal conduction, it is inferred that thermal conduction is suppressed in hotter loops. An alternative interpretation that suggests thermal conduction is not the dominant damping mechanism, even for short period waves in warm active region loops, is also presented.

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